*The Japan Research and Development Center of Metals' National Project
on*

Light for the 21^st^ Century:

The Development of Compound Semiconductors for High Efficiency
Optoelectronic Conversion

Year 2000 Report of Results[^1]

**[Background]{.underline}**

According to the international agreement, COP, energy efficiency is
becoming an important consideration for consumer products. The electric
power consumed for lighting is about 20% of the total electric power
consumed in Japan. Appropriate effort is therefore required to develop
energy-efficient lighting equipment.

The Japan Research and Development Center of Metals (JRCM) has organized
a 5-year R&D project (1998-2002) to develop energy-efficient lighting
equipment using light-emitting diodes (LEDs), with the participation of
thirteen member companies and universities, as required by the New
Energy and Industrial Technology Development Organization (NEDO).

The current project targets an energy efficiency twice that of
traditional fluorescent lamps, through the use of long-life, thin,
lightweight, GaN-based high-efficiency blue and ultraviolet LEDs.

This lighting equipment would have the following advantages:

1.  Less electricity consumption due to high optoelectronic conversion
    efficiency.

2.  Ease of miniaturization due to the small size of the light source
    \-- the lighting equipment would be smaller, thinner and lighter.

3.  Long life.

4.  Simple structure \-- no special devices would be needed to control
    the lighting equipment, and the number of components in the
    equipment would be reduced.

5.  High reliability due to the use of all-solid-state devices without
    any gases or filaments \-- very reliable against mechanical shock.

    Table of Contents

[0 Executive Summary 10](#executive-summary)

[0.1 Fundamental Studies on Light Emission Mechanisms
10](#fundamental-studies-on-light-emission-mechanisms)

[0.2 Improvements of Epitaxial Growth Processes for Blue and UV LEDs
10](#improvements-of-epitaxial-growth-processes-for-blue-and-uv-leds)

[0.3 Substrates for Homoepitaxy and their Characterization
11](#substrates-for-homoepitaxy-and-their-characterization)

[0.4 Technologies for Phosphor-Based White LEDs and Basic Properties of
LED Lighting Sources
12](#technologies-for-phosphor-based-white-leds-and-basic-properties-of-led-lighting-sources)

[1 Summary, Plan, and Research 14](#summary-plan-and-research)

[1.1 Details of Research and Development
14](#details-of-research-and-development)

[1.2 Plans and Actions Surrounding Each Research Challenge
14](#plans-and-actions-surrounding-each-research-challenge)

[1.2.1 Research & Development Plans and Objectives; Overseas Trends
14](#research-development-plans-and-objectives-overseas-trends)

[1.2.2 White Diodes and Luminescence Characteristics
16](#white-diodes-and-luminescence-characteristics)

[1.3 Cooperative Project 19](#cooperative-project)

[1.4 Participation in "21^st^ Century Dream Technology Exhibition"
19](#participation-in-21st-century-dream-technology-exhibition)

[1.5 Report on Technology Trends from MRS 2000 Fall Meeting
19](#report-on-technology-trends-from-mrs-2000-fall-meeting)

[1.5.1 Overview of 2000 Fall Meeting 20](#overview-of-2000-fall-meeting)

[1.6 Patent Search 24](#patent-search)

[1.6.1 New Open Applications and Registrations
24](#new-open-applications-and-registrations)

[1.7 Conclusions 29](#conclusions)

[1.8 Future Course 30](#future-course)

[1.9 References 30](#references)

[2 Basic Research Related to Light Emission Mechanisms
31](#basic-research-related-to-light-emission-mechanisms)

[2.1 Achievements in 1998\~1999 31](#achievements-in-19981999)

[2.2 Summary of Research and Development in 2000
32](#summary-of-research-and-development-in-2000)

[2.3 Excitation Spectroscopy Measurements of Temperature Dependence of
Absorption and Emission
32](#excitation-spectroscopy-measurements-of-temperature-dependence-of-absorption-and-emission)

[2.3.1 Methods and means 32](#methods-and-means)

[2.3.2 Results and Discussion 33](#results-and-discussion)

[2.4 Growth of GaN Epitaxial Thin Films by MBE Method and Their
Evaluation
45](#growth-of-gan-epitaxial-thin-films-by-mbe-method-and-their-evaluation)

[2.4.1 Methods for MBE Growth of GaN Epitaxial Thin Films and their
Evaluation
45](#methods-for-mbe-growth-of-gan-epitaxial-thin-films-and-their-evaluation)

[2.4.2 Results and Discussion 48](#results-and-discussion-1)

[2.5 Summary 55](#summary)

[2.6 Future Plans 56](#future-plans)

[2.7 References 57](#references-1)

[3 Clarification of Light Emission Mechanisms and Physical Properties of
GaN-based Semiconductors
58](#clarification-of-light-emission-mechanisms-and-physical-properties-of-gan-based-semiconductors)

[3.1 Achievements in 1998\~1999 58](#achievements-in-19981999-1)

[3.2 Summary of Research and Development in 2000
59](#summary-of-research-and-development-in-2000-1)

[3.2.1 Examination of light emission mechanism of GaInN/GaN multiple
quantum well LEDs with different In compositions
59](#examination-of-light-emission-mechanism-of-gainngan-multiple-quantum-well-leds-with-different-in-compositions)

[3.2.2 Examination of emission lifetime characteristics of LEDs of
different luminescence efficiencies
59](#examination-of-emission-lifetime-characteristics-of-leds-of-different-luminescence-efficiencies)

[3.2.3 Examination of in-plane distribution of light emission
60](#examination-of-in-plane-distribution-of-light-emission)

[3.2.4 Reverse bias PL characteristics of Si-doped quantum well LEDs
60](#reverse-bias-pl-characteristics-of-si-doped-quantum-well-leds)

[3.3 Methods and Means 60](#methods-and-means-1)

[3.3.1 PL and Time Resolved PL Measurement System
60](#pl-and-time-resolved-pl-measurement-system)

[3.3.2 Reverse Biased PL and Time-Resolved PL Measurements of LED
Samples
60](#reverse-biased-pl-and-time-resolved-pl-measurements-of-led-samples)

[3.3.3 High Spatial Resolution PL Mapping System Utilizing a Near-Field
Optical Probe
61](#high-spatial-resolution-pl-mapping-system-utilizing-a-near-field-optical-probe)

[3.3.4 Method of Interference-Free Measurement of Emission Spectra
61](#method-of-interference-free-measurement-of-emission-spectra)

[3.4 Results and Discussion 64](#results-and-discussion-2)

[3.4.1 Investigation of Emission Characteristics of GaInN/GaN Quantum
Well Structure LEDs with Different In Compositions
64](#investigation-of-emission-characteristics-of-gainngan-quantum-well-structure-leds-with-different-in-compositions)

[3.4.2 Investigation of Emission Lifetime Characteristics of LEDs with
Different Emission Efficiencies
69](#investigation-of-emission-lifetime-characteristics-of-leds-with-different-emission-efficiencies)

[3.4.3 Investigation of In-Plane Distribution of Light Emission
72](#investigation-of-in-plane-distribution-of-light-emission)

[3.4.4 Reverse Bias PL Characteristics of Si-Doped Quantum Well LEDs
73](#reverse-bias-pl-characteristics-of-si-doped-quantum-well-leds-1)

[3.5 Summary 79](#summary-1)

[3.6 Future plans 80](#future-plans-1)

[3.7 References 80](#references-2)

[4 Basic Research Concerning GaN Single Crystals Grown by the Solution
Growth Method
82](#basic-research-concerning-gan-single-crystals-grown-by-the-solution-growth-method)

[4.1 Achievements in 1998 and 1999 82](#achievements-in-1998-and-1999)

[4.2 Summary of Research and Development in 2000
83](#summary-of-research-and-development-in-2000-2)

[4.3 Methods and Means 83](#methods-and-means-2)

[4.3.1 Principles of the pressure-controlled solution growth method
83](#principles-of-the-pressure-controlled-solution-growth-method)

[4.3.2 Laboratory equipment 84](#laboratory-equipment)

[4.3.3 GaN single crystal growth method
87](#gan-single-crystal-growth-method)

[4.3.4 Crystal evaluation method 88](#crystal-evaluation-method)

[4.4 Experimental Results and Discussion
89](#experimental-results-and-discussion)

[4.4.1 Results of GaN single crystal growth experiments
89](#results-of-gan-single-crystal-growth-experiments)

[4.4.2 Results of crystal evaluation 93](#results-of-crystal-evaluation)

[4.5 Summary 98](#summary-2)

[4.6 Future Plans 99](#future-plans-2)

[4.7 References 100](#references-3)

[5 Research Concerning Substrate Surface Morphology
102](#research-concerning-substrate-surface-morphology)

[5.1 Achievements in 1998 \~1999 102](#achievements-in-1998-1999)

[5.2 Summary of Research and Development in 2000
103](#summary-of-research-and-development-in-2000-3)

[5.3 Introduction of High Precision Processing Technology for Sapphire
Substrates
103](#introduction-of-high-precision-processing-technology-for-sapphire-substrates)

[5.3.1 Methods and Means 103](#methods-and-means-3)

[5.3.2 Results and Discussion 104](#results-and-discussion-3)

[5.4 Introduction of GaN Substrate Precision Processing Technology
106](#introduction-of-gan-substrate-precision-processing-technology)

[5.4.1 Methods and Means 106](#methods-and-means-4)

[5.4.2 Results and discussion 107](#results-and-discussion-4)

[5.5 Summary 108](#summary-3)

[5.6 Plans for the Future 108](#plans-for-the-future)

[5.7 References 108](#references-4)

[6 Research and Development of GaNAsP-based Multicolored Light Emitting
Materials Using MOCVD
110](#research-and-development-of-ganasp-based-multicolored-light-emitting-materials-using-mocvd)

[6.1 Achievements in 1998 -- 1999 110](#achievements-in-1998-1999-1)

[6.2 Summary of Research and Development of 2000
111](#summary-of-research-and-development-of-2000)

[6.3 Development of GaN~1-x~P~x~ Multicolored Light Emitting Materials
through Laser-Assisted MOCVD
112](#development-of-gan1-xpx-multicolored-light-emitting-materials-through-laser-assisted-mocvd)

[6.3.1 Methods and Means 112](#methods-and-means-5)

[6.3.2 Results and Considerations for Growth of GaNP
113](#results-and-considerations-for-growth-of-ganp)

[6.4 GaNP Growth (DMHy/PH~3~/TMG) through Nonradiated MOCVD
121](#ganp-growth-dmhyph3tmg-through-nonradiated-mocvd)

[6.4.1 Methods and Means 121](#methods-and-means-6)

[6.4.2 Results and Observations 122](#results-and-observations)

[6.5 GaNP LEDs 124](#ganp-leds)

[6.5.1 Methods and Means 124](#methods-and-means-7)

[6.5.2 Results and Observations 125](#results-and-observations-1)

[6.6 Summary 126](#summary-4)

[6.7 Future Course 126](#future-course-1)

[6.8 References 127](#references-5)

[7 Development of MOCVD-grown Low-Defect GaN Thin Film Epi-Substrates
128](#development-of-mocvd-grown-low-defect-gan-thin-film-epi-substrates)

[7.1 Achievements in 1998 and 1999
128](#achievements-in-1998-and-1999-1)

[7.2 Summary of Research and Development in 2000
129](#summary-of-research-and-development-in-2000-4)

[7.3 Investigations into Dislocation Defect Reduction by ELOG Growth
129](#investigations-into-dislocation-defect-reduction-by-elog-growth)

[7.3.1 Methods and Means (Investigation into ELOG Growth for Reduced
Dislocation Density)
131](#methods-and-means-investigation-into-elog-growth-for-reduced-dislocation-density)

[7.3.2 Results and discussion (investigation of ELOG growth-based
dislocation defect reduction)
135](#results-and-discussion-investigation-of-elog-growth-based-dislocation-defect-reduction)

[7.4 Evaluation of ELOG-grown substrates
142](#evaluation-of-elog-grown-substrates)

[7.4.1 Methods and Means 143](#methods-and-means-8)

[7.4.2 Results and Discussion 145](#results-and-discussion-5)

[7.5 Summary 154](#summary-5)

[7.6 Future plans 154](#future-plans-3)

[7.7 References 155](#references-6)

[8 Development of GaN Epitaxial Substrates Based on Low Pressure Gas
Phase Technique
156](#development-of-gan-epitaxial-substrates-based-on-low-pressure-gas-phase-technique)

[8.1 Achievements in 1998 and 1999
156](#achievements-in-1998-and-1999-2)

[8.2 Summary of Research & Development in 2000
157](#summary-of-research-development-in-2000)

[8.2.1 Research on GaN-related transport and reaction phenomena
157](#research-on-gan-related-transport-and-reaction-phenomena)

[8.2.2 Research on single crystal growth techniques
158](#research-on-single-crystal-growth-techniques)

[8.3 Microwave-based GaN Synthesis Experiments
158](#microwave-based-gan-synthesis-experiments)

[8.3.1 Methods and means 158](#methods-and-means-9)

[8.3.2 Results and Discussion 162](#results-and-discussion-6)

[8.4 GaN Crystal Growth Method Utilizing Novel Plasma Generation
Technique
170](#gan-crystal-growth-method-utilizing-novel-plasma-generation-technique)

[8.4.1 Methods and means 170](#methods-and-means-10)

[8.4.2 Results and discussion 171](#results-and-discussion-7)

[8.5 Summary 175](#summary-6)

[8.6 Future Plans 176](#future-plans-4)

[9 Development of LED Epi Process Technology
177](#development-of-led-epi-process-technology)

[9.1 Achievements in 1998 and 1999
177](#achievements-in-1998-and-1999-3)

[9.1.1 Achievements in 1998 177](#achievements-in-1998)

[9.1.2 Achievements in 1999 177](#achievements-in-1999)

[9.2 Summary of Research and Development in 2000
178](#summary-of-research-and-development-in-2000-5)

[9.3 Fabrication of AlGaN/GaInN UV MQW Structures
179](#fabrication-of-algangainn-uv-mqw-structures)

[9.3.1 Methods and means 179](#methods-and-means-11)

[9.3.2 Results and discussion 181](#results-and-discussion-8)

[9.4 Study of AlGaN cladding layers with high AlN molar fraction
183](#study-of-algan-cladding-layers-with-high-aln-molar-fraction)

[9.4.1 Methods and means 184](#methods-and-means-12)

[9.4.2 Results and discussion 184](#results-and-discussion-9)

[9.5 Fabrication of UV LED Structures
185](#fabrication-of-uv-led-structures)

[9.5.1 Methods and means 185](#methods-and-means-13)

[9.5.2 Results and discussion 186](#results-and-discussion-10)

[9.6 Summary 186](#summary-7)

[9.6.1 Fabrication of AlGaN/GaInN UV MQW structures
186](#fabrication-of-algangainn-uv-mqw-structures-1)

[9.6.2 Study of AlGaN cladding layer with high AlN molar fraction
187](#study-of-algan-cladding-layer-with-high-aln-molar-fraction)

[9.6.3 Fabrication of UV LED element structures
187](#fabrication-of-uv-led-element-structures)

[9.7 Future Plans 187](#future-plans-5)

[9.7.1 Non-radiative recombination rate reduction
187](#non-radiative-recombination-rate-reduction)

[9.7.2 AlGaInN emitting layer (high InN molar fraction)
187](#algainn-emitting-layer-high-inn-molar-fraction)

[9.7.3 ELO technique 188](#elo-technique)

[9.7.4 Large-size MOCVD apparatus 188](#large-size-mocvd-apparatus)

[9.7.5 Application of SiC substrates
188](#application-of-sic-substrates)

[9.8 References 188](#references-7)

[10 Development of High-Output UV Light-Emitting Components
189](#development-of-high-output-uv-light-emitting-components)

[10.1 Achievements in 1998 and 1999
189](#achievements-in-1998-and-1999-4)

[10.1.1 Achievements in 1998 189](#achievements-in-1998-1)

[10.1.2 Achievements in 1999 189](#achievements-in-1999-1)

[10.2 Summary of Research and Development in 2000
190](#summary-of-research-and-development-in-2000-6)

[10.3 Building of prototypes of UV-emitting components
190](#building-of-prototypes-of-uv-emitting-components)

[10.3.1 Experimental method 191](#experimental-method)

[10.3.2 Results and discussion 191](#results-and-discussion-11)

[10.3.3 Building of white LED prototypes
194](#building-of-white-led-prototypes)

[10.4 Investigations into new dislocation reduction techniques
194](#investigations-into-new-dislocation-reduction-techniques)

[10.4.1 LEPS technique 195](#leps-technique)

[10.4.2 Experimental (LEPS-GaN growth)
195](#experimental-leps-gan-growth)

[10.4.3 Results and discussion 196](#results-and-discussion-12)

[10.5 Application of LEPS technique to LEDs
198](#application-of-leps-technique-to-leds)

[10.5.1 Experimental methods 198](#experimental-methods-1)

[10.5.2 Results and discussion 198](#results-and-discussion-13)

[10.6 Building of white LED prototypes utilizing blue LEDs
200](#building-of-white-led-prototypes-utilizing-blue-leds)

[10.6.1 Experimental method 201](#experimental-method-1)

[10.6.2 Results and discussion 201](#results-and-discussion-14)

[10.7 Summary 201](#summary-8)

[10.8 Future Plans 202](#future-plans-6)

[10.9 References 202](#references-8)

[11 Development of High-efficiency Phosphors
204](#development-of-high-efficiency-phosphors)

[11.1 Achievements in 1998 and 1999
204](#achievements-in-1998-and-1999-5)

[11.1.1 Review of phosphors and selection of model phosphors
204](#review-of-phosphors-and-selection-of-model-phosphors)

[11.1.2 Improvements in characteristics of red phosphors
204](#improvements-in-characteristics-of-red-phosphors)

[11.1.3 Introduction of equipment for phosphor firing
204](#introduction-of-equipment-for-phosphor-firing)

[11.1.4 Precise measurements and evaluation of phosphor emission
characteristics
204](#precise-measurements-and-evaluation-of-phosphor-emission-characteristics)

[11.2 Summary of Research and Development in 2000
205](#summary-of-research-and-development-in-2000-7)

[11.3 Quantum Efficiency Measurements
205](#quantum-efficiency-measurements)

[11.3.1 Methods and means 205](#methods-and-means-14)

[11.3.2 Results and discussion 207](#results-and-discussion-15)

[11.4 Evaluation of Temperature Characteristics
211](#evaluation-of-temperature-characteristics)

[11.4.1 Methods and means 211](#methods-and-means-15)

[11.4.2 Results and discussion 211](#results-and-discussion-16)

[11.5 Summary 212](#summary-9)

[11.6 Future Plans 212](#future-plans-7)

[11.6.1 Quantum efficiency improvement
212](#quantum-efficiency-improvement)

[11.6.2 Methods of synthesis 213](#methods-of-synthesis)

[11.6.3 Other 213](#other)

[11.7 References 213](#references-9)

[12 Electrode Attachment, Packaging, and White LED Development
214](#electrode-attachment-packaging-and-white-led-development)

[12.1 Achievements in 1998\~1999 214](#achievements-in-19981999-2)

[12.1.1 Achievements in 1998 214](#achievements-in-1998-2)

[12.1.2 Achievements in 1999 214](#achievements-in-1999-2)

[12.2 Summary of Research and Development in 2000
215](#summary-of-research-and-development-in-2000-8)

[12.3 Development of electrode forming techniques
216](#development-of-electrode-forming-techniques-2)

[12.3.1 Methods and Means 216](#methods-and-means-16)

[12.3.2 Results and Discussion 216](#results-and-discussion-17)

[12.4 Investigations into increasing light extraction efficiency (edge
tilting technology)
223](#investigations-into-increasing-light-extraction-efficiency-edge-tilting-technology)

[12.4.1 Methods and means 223](#methods-and-means-17)

[12.4.2 Results and discussion 225](#results-and-discussion-18)

[12.5 White LED development 229](#white-led-development)

[12.5.1 Study of mixed white light production and emission
characteristics of RGB model phosphors
229](#study-of-mixed-white-light-production-and-emission-characteristics-of-rgb-model-phosphors)

[12.5.2 Study of phosphor layer forming techniques and transmitted UV
light control
231](#study-of-phosphor-layer-forming-techniques-and-transmitted-uv-light-control)

[12.5.3 Evaluation of emission characteristics and color rendering of a
white LED lamp
234](#evaluation-of-emission-characteristics-and-color-rendering-of-a-white-led-lamp)

[12.6 Development of Sealing Resins for UV LEDs
239](#development-of-sealing-resins-for-uv-leds)

[12.6.1 Methods and means 239](#methods-and-means-21)

[12.6.2 Results and discussion 240](#results-and-discussion-22)

[12.7 Summary 243](#summary-10)

[12.7.1 Electrode forming techniques 243](#electrode-forming-techniques)

[12.7.2 Improvements in light extraction efficiency
243](#improvements-in-light-extraction-efficiency-2)

[12.7.3 Development of white LEDs 244](#development-of-white-leds-1)

[12.7.4 Development of sealing resins for UV LEDs
244](#development-of-sealing-resins-for-uv-leds-1)

[12.8 Plans for the future 244](#plans-for-the-future-1)

[12.9 References 245](#references-10)

[13 Development of LED-based Light-Source Devices for Illumination
246](#development-of-led-based-light-source-devices-for-illumination)

[13.1 Achievements in 1998-1999 246](#achievements-in-1998-1999-2)

[13.2 Summary of Research and Development in 2000
246](#summary-of-research-and-development-in-2000-9)

[13.3 Study of phosphor conversion-based LED light source devices by
optical simulation
247](#study-of-phosphor-conversion-based-led-light-source-devices-by-optical-simulation)

[13.3.1 Methods and means (simulation-based study of light source
devices)
248](#methods-and-means-simulation-based-study-of-light-source-devices)

[13.3.2 Results and discussion (simulation-based study of light source
devices)
250](#results-and-discussion-simulation-based-study-of-light-source-devices)

[13.4 Correlated color temperature alteration techniques and color
rendering property evaluation of mixed light from white, red, and green
LEDs
254](#correlated-color-temperature-alteration-techniques-and-color-rendering-property-evaluation-of-mixed-light-from-white-red-and-green-leds)

[13.4.1 Methods and means (correlated color temperature alteration
technique & color rendering property evaluation)
255](#methods-and-means-correlated-color-temperature-alteration-technique-color-rendering-property-evaluation)

[13.4.2 Results and discussion (correlated color temperature alteration
technique & color rendering property evaluation)
255](#results-and-discussion-correlated-color-temperature-alteration-technique-color-rendering-property-evaluation)

[13.5 Study of environmental compatibility evaluation of LED light
source devices
257](#study-of-environmental-compatibility-evaluation-of-led-light-source-devices)

[13.5.1 Methods and means (environmental compatibility evaluation study)
258](#methods-and-means-environmental-compatibility-evaluation-study)

[13.5.2 Results and discussion (environmental compatibility evaluation
research)
259](#results-and-discussion-environmental-compatibility-evaluation-research)

[13.6 Summary 263](#summary-11)

[13.7 Future plans 263](#future-plans-8)

[13.8 References 264](#references-11)

[14 Research and development of light-emitting-region conversion
illumination systems
265](#research-and-development-of-light-emitting-region-conversion-illumination-systems)

[14.1 Achievements in 1998\~1999 265](#achievements-in-19981999-3)

[14.2 Summary of research and development in 2000
266](#summary-of-research-and-development-in-2000-10)

[14.3 Verification of improved efficiency of coupling elements
267](#verification-of-improved-efficiency-of-coupling-elements)

[14.3.1 Methods and means 268](#methods-and-means-22)

[14.3.2 Results and discussion 270](#results-and-discussion-23)

[14.4 Conceptual design of directionality conversion system
277](#conceptual-design-of-directionality-conversion-system)

[14.4.1 Methods and means (conceptual design of directionality
conversion system)
277](#methods-and-means-conceptual-design-of-directionality-conversion-system)

[14.4.2 Results and discussion (conceptual design of directionality
conversion system)
279](#results-and-discussion-conceptual-design-of-directionality-conversion-system)

[14.5 Summary 280](#summary-12)

[14.6 Plans for the future 281](#plans-for-the-future-2)

[14.7 References 282](#references-12)

[15 Lighting Fixture Development Based on Novel LED Concepts
283](#lighting-fixture-development-based-on-novel-led-concepts)

[15.1 Achievements in 1998\~1999 283](#achievements-in-19981999-4)

[15.1.1 Achievements in 1998 283](#achievements-in-1998-3)

[15.1.2 Achievements in 1999 283](#achievements-in-1999-3)

[15.1.3 Discussion of Achievements in 1998\~1999
283](#discussion-of-achievements-in-19981999)

[15.2 Summary of Research and Development in 2000
284](#summary-of-research-and-development-in-2000-11)

[15.3 Prototyping and Evaluation of LED Table Lamp
285](#prototyping-and-evaluation-of-led-table-lamp)

[15.3.1 Methods and Means 285](#methods-and-means-23)

[15.3.2 Results and Discussion 285](#results-and-discussion-24)

[15.4 Prototyping and Evaluation of LED Ceiling Downlight
287](#prototyping-and-evaluation-of-led-ceiling-downlight)

[15.4.1 Methods and Means 287](#methods-and-means-24)

[15.4.2 Results and Discussion 287](#results-and-discussion-25)

[15.5 Conclusions 289](#conclusions-1)

[15.6 Agenda for the Future 290](#agenda-for-the-future)

[16 Conclusions and Future Course 292](#conclusions-and-future-course)

[16.1 Planning and Investigations 292](#planning-and-investigations)

[16.2 Basic Research on Light Emission Mechanisms
292](#basic-research-on-light-emission-mechanisms)

[16.3 Properties of GaN-based Semiconductors and Discovery of Light
Emission Mechanisms
294](#properties-of-gan-based-semiconductors-and-discovery-of-light-emission-mechanisms)

[16.4 Basic Research on Bulk GaN Single Crystals by Solution Growth
Method
295](#basic-research-on-bulk-gan-single-crystals-by-solution-growth-method)

[16.5 Research into Crystalline Substrate Surfaces
297](#research-into-crystalline-substrate-surfaces)

[16.6 Research and Development of GaNAsP-based Multicolored Light
Emitting Materials by MOCVD
297](#research-and-development-of-ganasp-based-multicolored-light-emitting-materials-by-mocvd)

[16.7 Development of Low Defect Density GaN Thin Film Epitaxial
Substrates through MOCVD
298](#development-of-low-defect-density-gan-thin-film-epitaxial-substrates-through-mocvd)

[16.8 Development of GaN Epitaxial Substrates through Low Pressure Vapor
Phase Method
299](#development-of-gan-epitaxial-substrates-through-low-pressure-vapor-phase-method)

[16.9 Development of LED Epitaxial Process Technologies
300](#development-of-led-epitaxial-process-technologies)

[16.10 Development of High Output Ultraviolet Emitting Elements
301](#development-of-high-output-ultraviolet-emitting-elements)

[16.11 Development of Highly Efficient Phosphor Substances
302](#development-of-highly-efficient-phosphor-substances)

[16.11.1 Improving Quantum Efficiency
302](#improving-quantum-efficiency)

[16.11.2 Synthesis Method 303](#synthesis-method)

[16.11.3 Other 303](#other-1)

[16.12 Development of Lead Bonding Methods, Packaging Technologies, and
White LEDs
303](#development-of-lead-bonding-methods-packaging-technologies-and-white-leds)

[16.12.1 Electrode Formation Technology
303](#electrode-formation-technology)

[16.12.2 Improvement in Light Extraction Efficiency
303](#improvement-in-light-extraction-efficiency)

[16.12.3 Development of White LEDs 304](#development-of-white-leds-2)

[16.12.4 Development of Resin Sealants for UV LED
304](#development-of-resin-sealants-for-uv-led)

[16.13 Development of Light Emitting Diode Light Source Devices for
Illumination
305](#development-of-light-emitting-diode-light-source-devices-for-illumination)

[16.14 Research and Development of Zone Conversion Illumination Systems
305](#research-and-development-of-zone-conversion-illumination-systems)

[16.14.1 Robust Design 306](#robust-design)

[16.14.2 Fundamental Specifications 307](#fundamental-specifications)

[16.15 Development of Illumination Devices Based on New LED Concepts
307](#development-of-illumination-devices-based-on-new-led-concepts)

[17 List of Research Publications and Articles
309](#list-of-research-publications-and-articles)

# Executive Summary

This is the summary of the "Light for the 21^st^ Century" National
Project, based on white light-emitting diodes (LEDs), which has been
carried out through a cooperation between universities and industry in
the fiscal year 2000. The following results in four research areas have
been obtained.

## Fundamental Studies on Light Emission Mechanisms

In order to clarify the light emission mechanism in InGaN-based
semiconductors, time-resolved photoluminescence (PL) and
photoluminescence excitation (PLE) spectroscopy measurements have been
carried out. PLE spectroscopy enabled us to observe clear structures
associated with the absorption of InGaN ternary alloys up to room
temperature for the first time. It was found that the Stokes shift was
independent of temperature, and that the two observed emission
components derived from states populated by carrier relaxation from the
same excited state. The experimental value of the Stokes shift was in
good agreement with a theoretical value based on the electron-phonon
interaction. It has therefore been proposed that coupled electron/phonon
polaron states contribute to the highly efficient radiative
recombination processes in this material system. The obtained results
seem to be important for understanding high-efficiency recombination
mechanisms in ultraviolet (UV) LEDs.

We have studied carrier recombination processes in InGaN/GaN multiple
quantum wells (MQWs) through systematic measurements of the luminescence
of LED samples under various conditions. From carrier lifetime
measurements over a wide range (7-540K) of temperatures, carrier
localization and delocalization characteristics were consistently
observed for LED samples with varying In composition. Through a
comparison with simulations, we found that the localization energy depth
increases with increasing In composition, which prevents the transfer of
carriers to non-radiative recombination centers. Non-radiative
recombination centers, which are probably dominated by point defects in
InGaN, also increase with increasing In composition.

Characteristics associated with more deeply localized states were
observed for an LED with higher quantum efficiency based on the
temperature dependence of the carrier decay time. By using a near-field
PL mapping system with high spatial resolution, the luminescence
intensity and wavelength distribution in the plane of the QW (quantum
well) of a typical blue LED were observed. The spatial scale of this
distribution was estimated to be 0.3-0.5 μm.

The piezoelectric field strength in a 2 nm InGaN QW was studied using PL
measurements under reverse bias, taking into account the measured
depletion length of the diode, and was estimated to be E~piezo~ = 1.48
MV/cm for x=0.23, close to the published values. Bias-dependent
measurements on LEDs suggest that Si doping is important both for
modifying the overall band structure of the device and for free-carrier
screening of the internal electric field in the QWs.

## Improvements of Epitaxial Growth Processes for Blue and UV LEDs

A MQW structure consisting of AlGaN barrier and InGaN well layers for UV
LEDs has been studied. By increasing the growth temperature of the AlGaN
barrier layers, the surface morphology of the MQW layers clearly showed
steps formed by a step-flow growth mode, and the interface between well
and barrier layers was sharp. An AlGaN/InGaN MQW LED was then fabricated
on a sapphire substrate. The peak wavelength of the emission spectrum
was 390 nm at room temperature. At a forward current of 20 mA, the
forward bias voltage and the output power were 3.7 V and about 1 mW,
respectively.

We have observed that low-level doping of Si in barrier layers in a MQW
increased the output power of the UV LED.

A new GaN growth process called LEPS (Lateral Epitaxy on a Patterned
Substrate) was developed. It has been found that the LEPS process is
effective in decreasing the dislocation density and for improving the
output power of UV LEDs. When the LEPS UV LED die was operated at a
forward bias current of 20 mA at room temperature, the emission
wavelength, the output power and the external quantum efficiency were
measured to be 382 nm, 2.5 mW and 3.7%, respectively.

We have developed a new technique for GaNP growth. A laser-assisted
MOCVD method was applied to GaNP growth. It is anticipated that, through
ArF laser irradiation, source gases will be decomposed at lower
temperatures, resulting in GaNP with a high P composition. Using this
method, we obtained GaNP with 10% P composition. This composition is the
highest reported value for MOCVD-grown GaNP. A GaN~0.9~P~0.1~/GaN single
quantum well (SQW) LED was fabricated and bright blue emission at 425 nm
was observed for the first time.

## Substrates for Homoepitaxy and their Characterization

We have investigated the effect of nitrogen pressure ramp rate on the
size and morphology of GaN bulk single crystals grown by the
pressure-controlled solution growth (PC-SG) method. A GaN single crystal
with a surface area of 334 mm^2^, which is the largest GaN single
crystal ever grown by the solution growth method, was obtained by
natural nucleation without any seeds. To characterize the GaN bulk
crystals, improvements were first made to the lapping and polishing
techniques. Next, the GaN crystals were lapped and polished to achieve
flat surfaces. Finally, these high quality flat surfaces were
characterized by XRD, AFM, TEM and PL. The resulting data indicated that
the crystals with good morphology also had good crystallinity. It was
found that reductions in nitrogen pressure ramp rate and temperature
fluctuations are necessary to grow larger GaN bulk single crystals with
good morphology and good crystallinity. We have also studied the crystal
growth of GaN on a hetero seed by the PC-SG method. A GaN crystal with a
diameter of 47 mm with a normal axis in the (0001) direction, could be
grown on a 2-inch sapphire substrate.

Homoepitaxial growth of GaN thin films on GaN bulk single crystals
prepared by pressure-controlled solution growth (PC-SG) method has been
carried out using molecular beam epitaxy. A correlation between the
lattice-polarity of bulk GaN substrates, the lattice-polarity of
homoepitaxial GaN layers, and their optical properties, was determined.
The luminescence intensity of homoepitaxial layers has been greatly
improved as compared to that of heteroepitaxial layers on sapphire
substrates. Relatively lower minimum yields (χ~min~) of
polarity-controlled GaN epilayers as measured by Rutherford
backscattering have indicated a high crystalline quality.

We have succeeded in growing GaN crystals using a new vapor phase growth
technology which allows high-yield low-cost substrates for GaN based
LEDs. We have constructed an innovative semi-closed system where metal
gallium is in a quartz ampoule and nitrogen gas activated by microwaves
is introduced to synthesize GaN on a substrate. A high growth rate of
100 μm/h was achieved by adjusting the growth conditions. We have
indications of the possibility of maximum growth rates up to 1 mm/h by
using high-frequency-discharge nitrogen plasma. By optimizing the growth
conditions (microwave power, nitrogen pressure and growth temperature),
we have obtained a thick GaN crystalline film on a sapphire substrate of
10x4 mm^2^ area. The surface showed good uniformity. However, our best
results were obtained using a GaN substrate. The grown surface showed
6-fold symmetric grain boundaries and the full width at half maximum
(FWHM) of the x-ray rocking curve was about 360 arcsec.

By the newly developed FACELO (Facet Controlled ELOG) method, we
successfully lowered the etch pit density (EPD) to as low as 10^4^-10^6^
cm^2^, the lowest level in the world as far as we know. On a GaN
template with low EPD grown by optimizing the nuclei density, we carried
out FACELO growth using a two-step process: ELOG growth of inclined
{1[1]{.underline}22} facets for the first step, followed by growth of
vertical {11[2]{.underline}0} facets for the second step. A trial of
growth on 2 inch sapphire was carried out; however, the overgrowth over
the masks remained incomplete and only a rough surface morphology was
obtained. The effective reduction of the dislocation density was
attributed to the following phenomena: (1) bending of the a+c type
threading dislocations (TDs) toward the \<11[2]{.underline}0\> direction
due to the inclined facet and termination of the laterally propagating
dislocations due to the void surface, (2) bending of the a type TDs
parallel to the mask stripe, (3) generation of new dislocations above
the coalescence line at the top of voids, and (4) the blocking of the
penetration towards the surface due to a SiO~2~ mask layer by the second
FACELO (vertical facet) process. From μ-PL spectroscopic observations,
it was suggested that the degree of impurity inclusions into regrown GaN
layers in the FACELO process depends on the type of facet, in the order
{11[2]{.underline}0} \> {11[2]{.underline}2} \> {0001}.

From time-resolved PL spectroscopy measurements it was confirmed that in
comparison to ELOG GaN films grown with an SiO~2~ mask, ELOG-GaN films
grown with a W mask exhibit a longer free exciton lifetime.

## Technologies for Phosphor-Based White LEDs and Basic Properties of LED Lighting Sources

We have pointed out that it is important to measure precisely the
absorption, internal and external quantum efficiency of red, green and
blue phosphor materials. It is expected using the best candidate
phosphors that the emission efficiency of the red phosphor will be
higher than that of the usual one.

In the area of electrode formation technology, 70% reflectivity and
2x10^-5^ ohm-cm^2^ contact resisitivity in the n-type electrode were
obtained by optimizing the conditions for Ti/Al contact formation. For
the formation of the p-type electrode, Rh and Pt, which are electrode
materials with lower contact resistance compared to Ni/Ag, were
explored.

Chip shaping was studied in order to improve light extraction
efficiency. It was proposed that etching the end face to produce
inclined facets was effective in directing the light, which would
otherwise be absorbed internally, through the substrate for extraction.
Inclined facets were formed by the dry etching method.

For white LED device development, application technologies for phosphors
have been examined. Both basic evaluation of typical RGB phosphors and
simulation of mixed RGB white phosphors have been carried out.
Transmission of ultraviolet radiation through the phosphor layer can be
controlled by the phosphor layer fabrication method. Luminescence and
color rendering properties of the white LED lamp have been investigated.
The ultraviolet radiation was reflected over a phosphor layer mounted on
the UV LED chip, and an optical thin film filter, into which the visible
light can penetrate, was formed. It was confirmed that the transmission
intensity of ultraviolet radiation was decreased from 31% to 13% and
that the luminous flux of all white LED lamps can be improved up to 30%
by coating the side edge of the UV LED chip with the phosphor.

We have examined the luminescence properties of reflection type light
sources which are composed of blue LEDs and YAG phosphor by using
optical computer simulation. It was found that the computer simulation
results agree well with the properties of the actual light source. This
fact suggests that light sources can in principle be designed by using
this simulation method.

Furthermore, we have examined the changes of correlated color
temperatures of white LEDs formed from combining red and green LEDs, and
evaluated color rendering properties for each condition. We examined
light source properties for five correlated color temperatures by
controlling the intensities of the three LEDs. In particular, a high
color rendering index (Ra=95) was obtained under daylight conditions. We
psychophysically measured the effects of visual stress induced by the
light sources composed of LEDs using the method of magnitude estimation.
The results indicate that the glare ("mabushisa") and discomfort
("fukaikan") sensations are closely correlated to one another and that
magnitude estimates exponentially increase as a function of illuminance.

The light efficiency was evaluated in order to design light guides for
mass-production. The parameters (the deformation of the pattern, the LED
location, etc.) were chosen, and the relationship between each parameter
and light efficiency was simulated to evaluate the allowed variability
of these parameters. Moreover, in order to reduce the influence of
fabrication errors, new methods (the trapezoidal pattern shape, the
corner LED setting, etc.) were proposed.

The effect of the directivity conversion method utilizing a prism sheet
was evaluated for high-directivity application, for example, a footlight
and a spotlight. High directivity was realized by utilizing a prism
sheet on a 4 inch light guide. The uniformity of the light intensity was
also evaluated. It is confirmed that the uniformity is not affected by
using a prism sheet.

We fabricated lighting fixtures using white LEDs which have the
highest-level luminous efficacy (15 lm/W) in commercially available
devices. We also studied the utility of chip-type LEDs and tried to
clarify potential benefits of downsizing while still providing
reasonable lighting characteristics. Through the prototyping of LED
table lamps and LED down-ceiling lights, it is suggested that a drastic
energy saving can be achieved by downsizing. In addition, it is
necessary to investigate the flexibility in designing LED light sources
and the benefits of utilizing LEDs were discussed from the point of view
of installation.

# Summary, Plan, and Research

## Details of Research and Development

At the 3rd Session of the Conference of the Parties to the United
Nations Framework Convention on Climate Change (COP3) held in 1997 Japan
was called upon to reduce emissions of greenhouse effect gases by 6%
(compared to 1990 levels). To combat global warming, Japan's promotion
of energy-saving measures in commercial, civilian, and transportation
sectors has become an urgent task \[1\]. Development of energy-saving
technologies for lighting is especially important since it represents
over 20% of the energy consumed by the public. In 1998, the Ministry of
International Trade and Industry (presently the Ministry of Economy,
Trade and Industry) launched a national project entitled "Development of
Compound Semiconductors for High Efficiency Lighting" (commonly referred
to as "Light for the 21^st^ Century") for the purpose of developing
practical applications of white LED light sources that exceed the energy
efficiency of conventional incandescent and fluorescent lamps \[2, 3\].

Over the years since the project started, the term LED (light-emitting
diode) "semiconductor lighting" has become firmly established \[4\].
Most notably, with the practical application of blue LEDs using InGaN
nitride semiconductors, white LEDs having a luminous efficiency of
better than 20 lm/W have been developed by combining yellow phosphors
(YAG:Ce) and high luminance blue LEDs capable of over 10 candela. As
such, white LEDs are receiving worldwide attention as a next-generation
energy-saving light source \[5\]. What's more, innovative semiconductor
structures have been developed for UV LEDs that emit light via the
short-wavelength ultraviolet region, raising expectations for practical
uses of white LEDs as an illumination source \[6\].

This report discusses primary research achievements over the three years
of the project. This Chapter outlines objectives and plans of the
research, characteristics of white LED light sources, and the project's
cooperative arrangement and policies. Chapters 2 through 15 present
fourteen research themes, and Chapter 16 sums up the achievements of
2000 and future policy.

## Plans and Actions Surrounding Each Research Challenge

### Research & Development Plans and Objectives; Overseas Trends

By making highly efficient white LEDs practical, the national project
"Light for the 21^st^ Century" aims to promote energy savings, to aid in
the prevention of global warming, and to create new lighting
applications and culture. As a result, the effect of energy savings
achieved in 2010 through the use of white LED illumination is estimated
at a market penetration of 13% with an equivalence of 83,000 kl of crude
oil.

At present, the following four R&D programs are underway to make white
LEDs practical.

(1) Discovery of light emitting mechanisms of mixed crystal
    semiconductors and chemical compounds based on AlN, GaN, InN for UV
    LEDs.

(2) Development of substrates for homoepitaxy.

(3) Improvement of epitaxial growth technologies for blue and UV LEDs
    and development of UV LEDs of high luminosity.

(4) Development of highly efficient RGB phosphors and discovery of
    practical uses of white LEDs as an illumination system.

As shown in figure 1.2.1-(1), a number of high performance products
using UV LEDs already exist in 2001, based on growth of high quality
semiconductor crystals and development of highly efficient LEDs and
evaluation of materials. We hope to achieve white LEDs with an
efficiency of 80 -- 100 lm/W by about 2003 and then 120 lm/W by 2010,
thus paving the way for lighting fixtures that eventually exceed the
efficiency of fluorescent lamps.

![](media/image1.png){width="5.998611111111111in"
height="4.227083333333334in"}

Figure 1.2.1-(1): Research on Semiconductor Lighting under the "Light
for the 21st Century" Project.

1998: 1) high-speed laser spectrometer; 2) chemical agent (gas), raw
materials, substrates; 3) X-ray analysis device; 4) bulk single
crystals; 5) introduction of luminescence center - epitaxy, doping; 6)
CL, STM, TEM evaluation

1999: 1) MOCVD for LEDs; 2) heteroepitaxy; 3) evaluation of surface and
boundary face

2000: 1) LED wafers; 2) MOCVD for homoepitaxy, MOCVD for LEDs; 3)
homoepitaxy

2002: 1) LED wafers

2003: (1) LED elements; (2) white, red, blue, green, white, UV, UV

2006: (1) development of new phosphors; (2) lighting sources for
illumination; (3) industrialization

At present, semiconductor manufacturers and primary light fixture
manufacturers in America and Germany are establishing joint ventures
(GELcore, LumiLeds, Osram Opto, Cree Lighting) and aggressively pursuing
research and development of white LED lighting technologies and
applications. Japan's national project is influencing the overseas
lighting industry, starting with the U.S. and its Solid State Lighting
Initiative of 2001, followed by Taiwan which is likewise establishing
projects to find practical uses for white LEDs.

![](media/image2.png){width="5.990277777777778in"
height="4.611111111111111in"}

Figure 1.2.1-(2): Evolution by Year of White LED Luminous Efficiency and
Future Predictions (bold line(s) represents this project).

Axes: Year; Luminous Efficiency (lm/W).

Labels: Blue LED + YAG; Company N - white LED; Company T (Toyoda Gosei)-
UV LED + RGB; luminous efficiency of an incandescent lamp; white
illumination by combining red, green, and blue LEDs; Taguchi research
lab (Yamaguchi University); Company L (LumiLeds) estimate; Company N
estimate; Company O (Osram Opto) estimate; Company L estimate; luminous
efficiency of a fluorescent lamp; Goal of Light for 21^st^ Century
project; Company L - red, orange; \* Ultimate goal of Light for 21^st^
Century project.

### White Diodes and Luminescence Characteristics

A light emitting diode emits light when a current is applied in the
forward direction to the p-n junction of a semiconductor, injecting a
negative electron and a positive hole into the active layer and causing
a radiation recombination process. Usually a double heterojunction or a
quantum well structure is used for light and charge confinement.

In general, the luminous efficiency (wallplug efficiency) of an LED is
found through the following equation and is based on three independent
efficiency factors.

\(1\)
$\eta_{\text{wp}} = \eta_{v} \cdot \eta_{i} \cdot \eta_{\text{ext}}$

In this equation, η~wp~ is the input to output wallplug efficiency, η~v~
is the potential efficiency, η~i~ is the internal quantum efficiency,
and η~ext~ is the light extraction efficiency.

As shown in Figure 1.2.2-(1), using a wavelength λ~0~ (UV) of a UV LED,
the luminous efficiency that can be gained from a white LED by exciting
the three primary color phosphors is given in the following equation
\[3\].

\(2\) ![](media/image3.wmf){width="3.5416666666666665in"
height="0.6111111111111112in"}

Here, P is the output from the UV LED, η~ph~ the efficiency of the
phosphor, η~uvph~ the conversion efficiency, F~ph~(λ) the phosphor's
emission spectrum, and κ(λ) the visibility coeffecient. Assuming
η~uvph~ equals 95% and η~ph~ equals 90%, a 30 mW LED will achieve about
100 lm/W.

Within the Light for the 21^st^ Century project, output light of 15.6 mW
(at 20 mA) and 38 mW (at 50 mA) has been achieved at 382 nm with an
external quantum efficiency of 24% (at 20 mA) \[6\].

![](media/image4.png){width="2.9583333333333335in"
height="3.7083333333333335in"}To obtain a whiteness with high color
rendering index (Ra \>= 85) using an LED, there are basically two
methods (single chip and multi-chip), as shown in Table 1.2.2. The first
(single-chip) method involves exciting phosphors using LEDs radiating
blue or UV light as the excitation source. The second (multi-chip)
method is to simultaneously use two or three different colors of LEDs,
such as red, green and blue, or blue and yellow. The second method
presents challenges for practical use, such as differences in drive
voltage, luminous output, element life and thermal characteristics among
LEDs. In contrast, the first method employs only one type of element and
the drive circuit design is quite simple.

Figure 1.2.2-(1): Conceptual Diagram of White LED by Combining UV LED
and Phosphors and the Relationship of Efficiency.

Labels: White light; Phosphor; UV LED; Substrate.

Within the first (single-chip) method, there are two approaches \[8\].
The first approach excites a yellow phosphor using a blue LED. The
second approach uses a UV LED to excite RGB phosphors. The second
approach was proposed for the first time under the Light for the 21^st^
Century project and was developed as a new white LED light source for
illumination.

+----------+----------+---------------------+--------------------------+
| Method   | Ex       | Light Emitting      | Principles of Light      |
|          | citation | Material and        | Emission                 |
|          | Source   | Fluorescent         |                          |
|          |          | Characteristics     |                          |
+----------+----------+---------------------+--------------------------+
| Single   | Blue LED | InGaN/YAG,          | Blue light excites       |
| Chip     |          | ZnS-based           | yellow phosphor          |
+----------+----------+---------------------+--------------------------+
|          | UV LED   | InGaN/RGB phosphors | RGB phosphors excited by |
|          |          |                     | UV light in same manner  |
|          |          |                     | as in fluorescent lamps  |
+----------+----------+---------------------+--------------------------+
| Mu       | Blue LED | InGaN, GaP, AlInGaP | Two complementary colors |
| lti-Chip |          |                     | placed in a single       |
|          | Yellow   |                     | package.                 |
|          | LED      |                     |                          |
+----------+----------+---------------------+--------------------------+
|          |          |                     |                          |
+----------+----------+---------------------+--------------------------+
|          |          |                     |                          |
+----------+----------+---------------------+--------------------------+
|          |          |                     |                          |
+----------+----------+---------------------+--------------------------+
|          | Blue LED | InGaN, AlInGaP,     | Three primary color LEDs |
|          |          | AlGaAs              | placed in a single       |
|          | Green    |                     | package.                 |
|          | LED      |                     |                          |
|          |          |                     |                          |
|          | Red LED  |                     |                          |
+----------+----------+---------------------+--------------------------+

![](media/image5.png){width="2.6944444444444446in"
height="3.888888888888889in"}\
Figure 1.2.2-(2) shows the dependence of the injection current of the
emission spectrum of a white phosphor made by combining red (Y~2~O~2~S:
Eu^3+^), green (ZnS: Cu, Al), and blue (Sr, Ca, Ba,
Mg)~10~(PO~4~)~6~Cl~2~:Eu^2+^ phosphors using a UV LED (light wavelength
of 371 nm, output of 5 mW (at 20 mA), and external quantum efficiency of
5%) as an excitation source.

The peaks are at 626, 528, and 447 nm respectively \[9\]. Even if the
current is increased, no significant change is seen in the emission
spectrum. The luminosity of each tends to increase linearly with an
increase in current. The average color rendering index is Ra = 83.

The white LED light source used to excite the RGB phosphors can emit
various colors of light according to the particular combination of
phosphors, making their range of application for illumination purposes
broad. Nonetheless, the luminous flux of each LED is weak (1 lm), making
it necessary to arrange multiple LEDs to serve as a source of
illumination.

Figure 1.2.2-(2): Changes in the emission spectrum with a rise in
injection current of RGB white phosphors.

Axes: Luminosity (free scale); Wavelength (nm)

To develop LED-based illumination, optimized design of a device system
including light distribution from multiple LED integrated elements is
necessary. As such, models to calculate the ideal luminous intensity and
illumination distribution of multi-point LED light sources are being
studied \[10\].

## Cooperative Project

This project is promoted by a joint research body established by 13
companies and 4 universities (Yamaguchi University, Mie University,
Tokyo University, and Chiba University) under the purview of the Japan
Research Center for Metal-based Materials (JRCM). In 2001, prototype UV
LEDs were developed whose characteristics surpass those exhibited by
LEDs of leading manufacturers. In the coming year, it is expected that
further research will yield additional improvements in lighting
efficiencies.

## Participation in "21^st^ Century Dream Technology Exhibition"

Nihon Keizai Shimbun hosted "The Exhibition of Dream Technologies for
the 21^st^ Century" at the Tokyo International Exhibition Center for 17
days from July 21 through August 6, 2001. This project was introduced as
part of the NEDO exhibit to show the promise of LED lighting. The
exhibit showed the principles of LEDs, the potential of LEDs as
illumination sources, and displayed prototype LEDs and LED-based
lighting fixtures.

![](media/image6.png){width="5.5569444444444445in"
height="3.941666666666667in"}

Figure 1.4: Exhibition Booth at the Exhibition of Dream Technologies for
the 21st Century

## Report on Technology Trends from MRS 2000 Fall Meeting

We attended the Material Research Society (MRS) 2000 Fall meeting held
in Boston from November 25 through December 1, 2000 to research market
trends, primarily from the GaN symposium.

The MRS is a non-profit organization established in 1973 among
governmental, academic, and industry participants to share knowledge on
new materials and to promote exchange of information through the
publication of related books. The Fall meeting was the second major
meeting for GaN materials.

### Overview of 2000 Fall Meeting

Forty-one symposia - some large, some small - were held at 3 sites: the
Hynes Convention Center, the Sheraton Hotel, and the Marriott Hotel.
Among these, the symposia on GaN-based and SiC-based materials were of
primary interest while other symposiums were of tangential relevance.
There were 2,703 talks, 1,556 poster exhibits, and 3,639 research
announcements.

Other than a few symposia like that on GaN-based materials, most of the
names of the symposia were different than the previous year, making it
quite difficult to identify relevance. Based on the limited number of
symposia viewed, it appeared that the number of attendees of each
lecture was proportional to the number of research announcements. The
great success of the symposium on organic optoelectronic materials was
especially impressive.

#### GaN-based Materials Symposium

Similar to last year, the symposium on GaN-based materials was the
largest, with a total of 271 announcements, including lectures and
poster exhibits. Announcements on GaN-based materials could also be seen
here and there at other symposiums.

Announcements involved research bodies from 25 countries comprised of
331 universities, 109 commercial firms, 45 governmental research labs
and 10 military research labs, for a total of 495 organizations. The
U.S., Japan, Germany, France, and South Korea were at the top.

Announcements on GaN-based materials worthy of attention are listed
below.

#### Nanomaterials

Among the 7 symposiums obviously covering nanomaterials, there were 841
announcements. Still, there were a large number of announcements at
other symposiums that could be categorized under nanomaterials. Among
the numerous applications of nanomaterials expected to be in
semiconductor devices and displays, 9 announcements on hydrogen
occlusion via a nanotube gained much attention.

#### Interesting Announcements from Symposium on GaN and Related Alloys

Among the announcements made on GaN-based compounds and semiconductors,
those relevant to this project are listed below, categorized as:
substrates, UV light emission, elemental structures and processes, and
LEDs.

**[Substrates]{.underline}**

"Structure and Optical Properties of GaN Bulk Crystals Grown from the
Liquid Phase", M. Albrecht, Erlangen-Nuernberg University (with Ioffe
Inst. and TDI)

-   While melting GaN, LPE growth at 1000°C, less than 2 barometric
    pressure on a 50 mm GaN crystal substrate.

-   At present, achieved oriented polycrystals. Maximum length 15 mm,
    diameter 20 mm, dimension of crystal grain \<= 3 mm x 1.5 mm.

"Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation
and Substrate Preparation", C. Rojo, Crystal IS (with Rensselaer
Polytechnic Inst.)

-   AlN single crystals a promising substrate for growth of GaN mixed
    crystals, especially Al-rich ones.

-   Sublimation. Transport by a temperature gradient in Al + N~2~ state.
    2000°C Growth rate of 0.9 mm/h.

-   Diameter of 15 mm, grain size of 10 mm x 6 mm, poly portion also
    present.

-   Yellow or colorless. With CL, strong emission from a deep level
    which is about 0.8 times the band edge energy.

-   AlN film formed on substrate. Various tests and HEMT prototype
    performed.

"Characteristics of FIELO-GaN Grown by Hydride Vapor Phase Epitaxy", A.
Usui, NEC

-   FIELO-GaN grown film by HVPE exhibits a dislocation density of 10^6^
    cm^-2^, PL decay time of 1.3 ns at its longest, which is
    significantly longer than that by conventional MOVPE.

-   Achieved φ35 mm GaN substrate by depositing 60 to 400 μm GaN film on
    a φ2˝ sapphire substrate and then cooling it to allow thermal
    expansion to naturally cause separation between substrate and film.

-   Created InGaN MQW blue LD on FIELO-GaN.

Several additional announcements (Air Force Research Lab. + University
Massachussetts + Samsung + Wright State Univeristy + University of
Dayton; Tech. University of Munich; and others.) covered formation of
independent GaN film by HVPE, GaN film growth thereon and its
characteristics.

**[UV Light Emission]{.underline}**

"UV Emitter Based on High-Efficiency GaN/AlxGa1-xN Multi-Quantum Wells",
M. Iwaya, Meijo University (Meijodai)

-   Luminosity distribution is observed at the light emission surface of
    an LED where the region of high luminosity is found to be at the
    region of low dislocation density.

-   PL luminescence characteristics close to GaN obtained at room
    temperature via low dislocation density and Si doping.

"Current Injection UV-Emission from InAlGaN Multi-Quantum Well Light
Emitting Diodes", A. Kinoshita, Riken (with Waseda University)

-   230 nm PL observed on InAlGaN at room temperature.

-   Substrate is CREE (0001) SiC.

-   MQW, thick luminous layer, add SL layer on top (above and below) the
    active layer.

-   Peak wavelength during current injection found to be 360 nm with MQW
    and 340 nm with thick light emitting layer.

"Improved Optical Quality of BAlGaN/AlN MQW Structure Grown on 6H-SiC
Substrate by Controlling Residual Strain Using Multi-Buffer Layer",
Hideo Kawanishi, Kogakuin.

-   B 5-13% possible with BAlGaN on 6H-SiC.

-   Light emission wavelength of 248.1 nm with
    B~0.07~Al~0.52~Ga~0.41~N/AlN MQW

-   Growth temperature of 1460 / 1200ºC

-   Conductivity still unobtainable

**[Elemental Structures and Processes]{.underline}**

"Transfer of Nitride Device Heterostructures from Sapphire Growth
Substrates to Silicon by Metal Bonding and Laser Lift-Off", T. Sands, UC
Berkeley

-   GaN element separated from sapphire substrate by irradiating with an
    excimer laser

-   The substrate/nitride boundary area reaches 1000ºC, but most of the
    nitride portion remains at 400ºC.

-   Grow In film on the electrodes to make a In/Pd/GaN/sapphire
    substrate, then attach it to a Pd/Si substrate by heating to 200ºC,
    causing it to bond via a Pd-In reaction. Then separate the sapphire
    substrate via laser irradiation. This leaves a GaN/electrode/inner
    layer/Si structure.

-   No changes in LED characteristics before and after separation (lift
    off).

"Integration of In~x~Ga~l-x~N Laser Diodes with Dissimilar Substrates by
Laser Lift-Off", W.S. Wong, Xerox

-   Apply Si or Cu atop an LD structure (GaN/sapphire substrate), then
    separate the sapphire substrate using laser lift off. Heat
    dissipation when using Cu is good.

-   Even if the Al~2~0~3~-GaN boundary area reaches 900ºC, the GaN
    beyond 2µm from the boundary area reaches only 200ºC.

-   By removing the Si or Cu temporarily applied, it is possible to
    create an "LD film". It is also good to attach this atop a diamond
    base.

"Group-III Nitride Quantum Heterostructures Emitting in the Whole
Visible Range", N. Grandjean, CNRS

-   Creation of quantum dots by employing island structures produced
    naturally during MBE.

-   Island height (3 nm to 5.5 nm) supported by PL light emission
    wavelength (2.6 eV to 1.9 eV).

-   Unclear whether the wavelength change mechanism is due to quantum
    size or Stark effects.

-   It is possible to achieve white LEDs.

"Drastic Reduction of Threading Dislocation Density of AlGaN on SiC by
Using Highly-Si-Incorporated AlGaN Superlattice", M. Ainoya, Riken (with
Waseda University)

-   AlGaN containing 1.2 x 10^20^/cm^3^ Si

-   The thermal dislocation density (TDD) changes from 10^10^/cm^2^ to
    10^8^/cm^2^ with (AlGaN 80 nm + AlGaN:Si 20 nm) x 6.

-   Dislocations bend within the Si doping layer.

-   Same effects observed with both SiC substrates and sapphire
    substrates.

"Growth and Characterization of the GaN Underlying Layer Used in
Blue-Violet GaN-Based Laser Diodes on Sapphire", Kenji Funato, Sony

-   Dislocation reduced by improving underlayer, life extended (1000 h
    or more).

-   Relaxation of crystals increases dislocation and threshold current
    rises.

-   Growth pressure was 0.9 to 1.6 atm. Raising the pressure tends to
    restrain relaxation.

-   PL strength was reduced remarkably to better than EPD 1.2 x
    10^7^cm^-2^.

-   With lines of 2 μm and spaces of 10 μm, the dislocation density of
    the wing equaled mix \< 10^5^cm^-2^ and edge \< 10^6^cm^-2^.

-   LD life: the life of ELO is twice that of sapphire at low output,
    but the difference grows smaller at higher output.

-   The output with ELO is 1.21 W/mA (74mA). With sapphire it is 0.92
    W/mA (90mA).

-   The minimum dislocation density on sapphire is 4 x 10^8^cm^-2^. With
    ELO it is below 1 x 10^6^cm^-2^.

"Gallium Nitride Films on Liquid Precursors", M. Puchinger,
Max-Planck-Inst. (with UCSB)

-   GaN crystals obtained by spin coating liquid precursors onto the
    substrate and then heating within N~2~.

-   At present, only multi crystals are obtained with thick film, but
    single crystals of GaN film (\<= 20 nm) can be obtained on sapphire
    even if facet exists.

**[LEDs]{.underline}**

"Development of High Power Nitride LEDs for Semiconductor Lighting", S.
A. Stockman, Lumileds

-   The present maximum levels are 100 lm/W (600 nm yellow) for AlInGaP
    and 80 lm/W (540 nm yellow green) for InGaN.

-   Due to the heat resistance of epoxy resins, an artillery shell shape
    is thought to limit light output to \<= 0.1 W. (currently 5 to 10
    mW). On the other hand, 1 W per element is desired immediately.

-   Direct bonding to a copper block holds promise. Concept of 18 LEDs
    mounted to a square substrate that is 2 to 2.5\". Some thinking of
    enlargening the chip size and then forming into a flipchip or
    multi-chip module.

-   With an increase in chip size, comb-shaped electrodes will be
    needed. A prototype is being worked on.

-   However, there is data suggesting that the efficiency quickly
    degrades as the chip size grows larger. Therefore, LumiLeds is
    aiming for a segregated chip plus a multi-chip approach.

-   \<While demonstrating a blue LED\> Compared to a conventional LED,
    it outputs 20 times the light at 15 times the current. There are
    also white LEDs. These use red and green phosphors but the
    efficiency is still low.

-   \<Application\> LCD backlight, indoor lighting, road and tunnel
    lighting, etc. 10W device is hoped for.

-   The most important problem to solve is that of heat.

## Patent Search

Registrations and open applications for patents and new uses dealing
with GaN-based LEDs from the years 1998 and 1999 have been entered into
a database, categorized according to their importance, and then studied.
New applications for the year 2000 will be entered into the database.
The content of those in dispute has been researched.

### New Open Applications and Registrations

#### New Data Supplement

A patent search was performed to locate patents covering important
technological problems pertinent to this project. The contents of these
patents have been read from a CD-ROM published by the patent office and
added to a database. During the period spanning from December 5, 1999 to
November 13, 2000, there were 477 new filings and patent awards relating
to this project. This is broken down further as follows:

New patent filings: 402

New patent awards: 79

Among these, as shown in Table 1.6.1-(1), four had already been awarded
when filed. While these were new filings, they also constituted new
patent awards. Care must be taken because it cannot be determined by
checking public disclosure (patent filings) only that they were already
awarded.

  ------------------- ----------------- ------------- --------------------
  Public Disclosure   Date of Disclosre Award Number  Date of Award
  Number                                              

  2000-105321         April 11, 2000    2954203       July 16, 1999

  2000-114597         April 21, 2000    3019085       January 7, 2000

  2000-114664         April 21, 2000    3031415       February 10, 2000

  2000-196196         July 14, 2000     2995187       October 22, 1999
  ------------------- ----------------- ------------- --------------------

Table 1.6.1-(1): Patents Already Awarded upon Public Disclosure

#### Grant Classifications

Table 1.6.1-(2) shows the number of registered patents and open filings
according to grant classification of the 477 registered patents and open
applications.

The number of patents relating to MOCVD-based crystal growth and buffer
layers exceeded all others, followed by element isolation and cleavage,
substrate materials, active layers, current constriction layers to
create lasers and stripes, etching, and dopant-related patents. This
study found patents relating to phosphors and combined white light
emission, but none had reached the award stage.

  ---------------- ---------------------------------------- --------------- ----------------
  Classification   Classification Description               New             New
  Number                                                    Applica-tions   Registra-tions

  A01              Substrate Material: Sapphire             11              1

  A02              Substrate Material: GaN                  8               

  A03              Substrate Material: Other                16              4

  B01              Layer Structure: Dopant                  10              1

  B02              Layer Structure: Buffer layer            51              7

  B03              Layer Structure: Contact layer           11              

  B04              Layer Structure: Clad layer              7               

  B05              Layer Structure: Active layer            12              5

  B06              Layer Structure: Current constriction    10              6
                   layer                                                    

  B07              Layer Structure: Barrier layer           3               

  B08              Layer Structure: Heterostructure         2               1

  B09              Layer Structure: Quantum well structure  9               2

  B10              Layer Structure: MIS structure           2               

  B11              Layer Structure: Ridge structure         6               

  B12              Layer Structure: Carrier concentration   7               1

  B13              Layer Structure: Stripe                  10              

  B14              Layer Structure: Other                                   

  C01              Electrode: Ohmic                         9               2

  C02              Electrode: P-structure                   4               1

  C03              Electrode: N-structure                   1               2

  C04              Electrode: Shape                         3               4

  C05              Electrode: Material                                      2

  C06              Electrode: Wire bonding                                  

  C07              Electrode: Other                         1               

  D01              Manufacturing Technology: MOCVD, MOVPE   45              15

  D02              Manufacturing Technology: MBE, PVD       4               4

  D03              Manufacturing Technology: Heat           7               1
                   processing                                               

  D04              Manufacturing Technology: Etching        12              2

  D05              Manufacturing Technology: Isolation,     25              1
                   Cleavage                                                 

  D06              Manufacturing Technology: Sealant        2               

  D07              Manufacturing Technology: Bonding        3               2

  D08              Manufacturing Technology: Other          4               1

  E01              Element Structure: Single                                

  E02              Element Structure: Multiple              4               1

  E03              Element Structure: Direction of light    7               1
                   extraction                                               

  E04              Element Structure: Combination with      12              1
                   phosphors                                                

  E05              Element Structure: Phosphor material                     

  E06              Element Structure: Other                 1               

  F01              Element Morphology: LED                  6               

  F02              Element Morphology: LD                   5               

  F03              Element Morphology: Other                6               3

  G01              Mount Structure: Package                 9               1

  G02              Mount Structure: Lead frame              1               

  G03              Mount Structure: Flipchip                3               

  G04              Mount Structure: Face down               3               

  G05              Mount Structure: Other                   4               

  H01              Characteristics: High                                    

  H02              Characteristics: Blue light, UV light                    

  H03              Characteristics: White light,            10              
                   multi-colored light                                      

  H04              Characteristics: Surface luminescence    2               

  H05              Characteristics: Crystallization                         

  H06              Characteristics: Other (heat resistance, 6               
                   etc.)                                                    
  ---------------- ---------------------------------------- --------------- ----------------

Table 1.6.1-(2) Number of Patents based on Grant Classification.

#### Examples of Important New Patent Registrations and Applications

Tables 1.6.1-(3) through 1.6.1-(6) below show examples of patent
registrations and applications pertaining to buffer layer structures.

  --------- ------------- -------------------------------------------------
  Number    Claimant      Description

  302687    Toyoda Gosei  Low temperature growth (400 -- 900 degrees) AlN
                          Wursted-structure buffer layer (film thickness of
                          10 -- 15 nm).

  3091593   Nichia        MOCVD using a GaAlN buffer layer on sapphire +
            Chemical      GaN formed substrate.

  3036495   Toyoda Gosei  Stripe or lattice AlGaN buffer layer (on Si
                          substrate).

  3028809   Sanken        TiN buffer layer.
            Electric      
  --------- ------------- -------------------------------------------------

Table 1.6.1-(3): New Patent Registrations on Buffer Layer Structure

  ------------- ------------------ ---------------------------------------
  Number        Claimant           Description

  2000-261037   Sharp              Super lattice formation on n-type clad
                                   layer.

  2000-068594   Nichia Chemical    Super lattice buffer layer.

  2000-040842   Toyoda Gosei       Multi-crystal buffer layer.

  2000-040843   Toyoda Gosei       Two GaN buffer layers on sapphire
                                   substrate.
  ------------- ------------------ ---------------------------------------

Table 1.6.1-(4): Open Applications (those relating to growth methods for
layer structures).

  ------------- ------------------ ---------------------------------------
  Number        Claimant           Description

  2000-077336   Sony               Ion implantation stripe mask usage

  2000-164929   Sony               Specific crystal facet growth

  2000-021789   Toshiba            SiO~2~ stripe mask usage

  2000-101194   Sanyo              SiO~2~ stripe mask + surface polishing

  2000-124500   Toshiba            Growth of crystals on a cyclically
                                   irregular surface

  2000-058918   Murata             Striping of an N-type electrode
                Manfacturing       

  2000-068609   Ricoh              Two stage stripe masking

  2000-156348   Nichia Chemical    Growth on substrates with height
                                   differences

  2000-106348   Matsushita         Growth on porous substrates
                Electric           

  2000-174343   Sharp              Cross layer stripe mask usage
  ------------- ------------------ ---------------------------------------

Table 1.6.1-(5): Open Applications Founded on Same Concept as Patent
3036495 (lateral growth by stripe and substrate surface irregularities).

  ------------- ------------------ ---------------------------------------
  Number        Claimant           Description

  2000-036617   Murata             ZnO buffer layer (on Si substrate)
                Manufacturing      

  2000-058451   Showa Denko        Phosphorus boron (BP) buffer layer

  2000-133843   Fuji Electric      Fluoride thin film buffer layer

  2000-188260   Samsung Electric   Boron nitride (BN) buffer layer

  2000-031537   Matsushita         GaN buffer layer containing arsenic
                Electric           

  2000-031539   Hewlett-Packard    MgZnCd-doped boundary layer
  ------------- ------------------ ---------------------------------------

Table 1.6.1-(6): Buffer Layers Based on Non-Conventional Materials.

#### Distribution of Applicants by Technology Classification

Table 1.6.1-(7) shows the distribution of patent applicants and
assignees of new published unexamined patent applications (Kokai) and
registered patents by technical field.

  ---------------- -------------------------------------- ------------ -----------
  Classification   Classification Description             Number of    Number of
  Number                                                  Applicants   Assignees

  A01              Substrate Material: Sapphire           10           1

  A02              Substrate Material: GaN                5            

  A03              Substrate Material: Other              10           4

  B01              Layer Structure: Dopant                6            1

  B02              Layer Structure: Buffer layer          25           10

  B03              Layer Structure: Contact layer         8            

  B04              Layer Structure: Clad layer            4            

  B05              Layer Structure: Active layer          11           5

  B06              Layer Structure: Current constriction  8            5
                   layer                                               

  B07              Layer Structure: Barrier layer         3            

  B08              Layer Structure: Heterostructure       2            1

  B09              Layer Structure: Quantum well          8            2
                   structure                                           

  B10              Layer Structure: MIS structure         4            

  B11              Layer Structure: Ridge structure       5            

  B12              Layer Structure: Carrier concentration 11           1

  B13              Layer Structure: Stripe                8            

  B14              Layer Structure: Other                              

  C01              Electrode: Ohmic                       10           2

  C02              Electrode: P-structure                 4            1

  C03              Electrode: N-structure                 1            1

  C04              Electrode: Shape                       3            4

  C05              Electrode: Material                                 2

  C06              Electrode: Wire bonding                             

  C07              Electrode: Other                       1            

  D01              Manufacturing Technology: MOCVD, MOVPE 20           14

  D02              Manufacturing Technology: MBE, PVD     4            4

  D03              Manufacturing Technology: Heat         5            1
                   processing                                          

  D04              Manufacturing Technology: Etching      13           2

  D05              Manufacturing Technology: Isolation,   12           1
                   Cleavage                                            

  D06              Manufacturing Technology: Sealant      2            

  D07              Manufacturing Technology: Bonding      3            2
  ---------------- -------------------------------------- ------------ -----------

[]{.mark}

  -------------- --------------------------------------- ----------- -----------
  [D08]{.mark}   [Manufacturing Technology:              []{.mark}   []{.mark}
                 Other]{.mark}                                       

  [E01]{.mark}   [Element Structure: Single]{.mark}      []{.mark}   []{.mark}

  [E02]{.mark}   [Element Structure: Multiple]{.mark}    []{.mark}   []{.mark}

  [E03]{.mark}   [Element Structure: Direction of light  []{.mark}   []{.mark}
                 extraction]{.mark}                                  

  [E04]{.mark}   [Element Structure: Combination with    []{.mark}   []{.mark}
                 phosphors]{.mark}                                   

  [E05]{.mark}   [Element Structure: Phosphor            []{.mark}   []{.mark}
                 material]{.mark}                                    

  [E06]{.mark}   [Element Structure: Other]{.mark}       []{.mark}   []{.mark}

  [F01]{.mark}   [Element Morphology: LED]{.mark}        []{.mark}   []{.mark}

  [F02]{.mark}   [Element Morphology: LD]{.mark}         []{.mark}   []{.mark}

  [F03]{.mark}   [Element Morphology: Other]{.mark}      []{.mark}   []{.mark}

  [G01]{.mark}   [Mount Structure: Package]{.mark}       []{.mark}   []{.mark}

  [G02]{.mark}   [Mount Structure: Lead frame]{.mark}    []{.mark}   []{.mark}

  [G03]{.mark}   [Mount Structure: Flipchip]{.mark}      []{.mark}   []{.mark}

  [G04]{.mark}   [Mount Structure: Face down]{.mark}     []{.mark}   []{.mark}

  [G05]{.mark}   [Mount Structure: Other]{.mark}         []{.mark}   []{.mark}

  [H01]{.mark}   [Characteristics: High]{.mark}          []{.mark}   []{.mark}

  [H02]{.mark}   [Characteristics: Blue light, UV        []{.mark}   []{.mark}
                 light]{.mark}                                       

  [H03]{.mark}   [Characteristics: White light,          []{.mark}   []{.mark}
                 multi-colored light]{.mark}                         

  [H04]{.mark}   [Characteristics: Surface               []{.mark}   []{.mark}
                 luminescence]{.mark}                                

  [H05]{.mark}   [Characteristics:                       []{.mark}   []{.mark}
                 Crystallization]{.mark}                             

  [H06]{.mark}   [Characteristics: Other (heat           []{.mark}   []{.mark}
                 resistance, etc.)]{.mark}                           
  -------------- --------------------------------------- ----------- -----------

[Patent Disputes]{.mark}

[Blue highlighting indicates missing pages.]{.mark}

Table 1.6.1-(7): Number of Participants by Technology Classification.

## Conclusions

The 21st Century is certainly the era of technology innovation with
semiconductor lighting and this project is playing a leadership role.
Differing from conventional visible light LEDs, white LEDs have strong
needs as a general lighting source. Nonetheless, since such applications
need a high volume of light (several tens to several thousands lm) in
future new and innovative technologies by researchers and engineers of
semiconductor technology, light emission element technology, and
illumination engineering in collaboration with lighting and interior
designers is critical to the realization of white LED lighting systems.
It is hoped that further technology innovation in white LEDs for
illumination will not only create a new lighting culture through LED
applications, but will also build a safe societal foundation which
produces less environmental waste.

## Future Course

Research is progressing as planned according to the flow shown in Figure
1.2.1-(1). The light emission mechanism of the UV LED developed this
year has been uncovered and defect densities have been reduced,
contributing to improvements in external quantum efficiency. From here,
we plan to pursue improvements in the precision of the epitaxial growth
process and to strengthen the cooperation among project participants so
that the evaluation of film quality in growth layers and end
characteristics can lead to improvements in LED structures.
Additionally, with regard to white LED lighting, we plan to lead the
world in proposing concrete applications and standardization of LED
lighting fixtures through lighting system section meetings.

## References

\[1\] Public Notice by Ministry of International Trade and Industry
"Basic Plan for Development of Compound Semiconductors for
High-Efficiency Optoelectronic Conversion (Light for the 21^st^
Century)", April 1, 1998 34

\[2\] "Development of Compound Semiconductors for High-Efficiency
Optoelectronic Conversion, 'Light for the 21^st^ Century' Project
Pamphlet", New Energy and Industrial Technology Development Organization
(NEDO), Published March 1999.

\[3\] Proposal "Development of Compound Semiconductors for
High-Efficiency Optoelectronic Conversion", Japan Research and
Development Center for Metals (JRCM) \[Light for 21^st^ Century research
book\], "Development of Compound Semiconductors for High-Efficiency
Optoelectronic Conversion (Light for the 21^st^ Century)" 1998 Report of
Results and 1999 Report of Results, Japan Research and Development
Center for Metals, March 1999 and March 2000.

\[4\] T. Taguchi, "White LED and Lighting Systems", O plus E, No. 8
(2000) 1028.

\[5\] T. Taguchi, "Lighting in the 21^st^ Century with LEDs", Nikkan
Display March Issue (2000) 1.

\[6\] Nikkan Kogyo Newspaper, April 6, 2001.

\[7\] T. Whitaker, "Compound Semiconductor", 4 (2000) 34.

\[8\] T. Taguchi, "The 21^st^ Century Lighting Project Based on LED",
Proc. of Int. Display Workship, (2000) 817.

\[9\] K. Murakami, T. Taguchi and M. Yoshino, "White Illumination
Characteristics of ZnS-based Phosphor Materials Excited by InGaN-based
Ultraviolet Light-Emitting Diode", Proc. of SPIE, Display Tech. III 4079
(2000) 112.

\[10\] Uchida, Taguchi, "Evaluating Characteristics of White LED Light
Sources with Multipoint Light Source Illumination Calculation Method",
Collection of Papers from 2000 Electric Society Basic Materials Common
Department Lecture (2000) 188.

# Basic Research Related to Light Emission Mechanisms

## Achievements in 1998\~1999

In order to find out more about the peculiar physical properties of
GaN-based materials and to obtain knowledge helpful in improving the
luminous efficiency of white LEDs, our research in 1998-1999 was done in
close cooperation with other Joint Research Groups. Specifically, using
high-quality GaN and In~x~Ga~1-x~N epitaxial films, we conducted basic
research related to the elucidation of light emission mechanisms and to
hetero/homoepitaxial interface control technology.

As a result, we discovered that InGaN luminescence spectra were made up
of two components. Judging by the results of detailed emission
spectroscopy measurements, the light emission mechanism of InGaN could
not be explained by previously discussed localized excitons and
quantum-dot-bound excitons, which was indicative of contribution by free
electrons. Time-resolved spectroscopy results confirmed the presence of
localized carriers in mixed InGaN crystals, which spurred us to conduct
investigations into mechanisms capable of satisfying both results. Based
on considerations related to the temperature dependence of the decay
time constant of the two components measured by time-resolved
spectroscopy, a model involving polarons was introduced and shown to
explain the light emission measured from InGaN mixed crystal thin films.
It was found that the model, which took into consideration the fact that
nitride semiconductors exhibit an extremely strong electron-lattice
interaction, could also provide a non-contradictory explanation for
previously obtained measurements showing peculiar temperature- and
magnetic-field-dependences of the luminescence spectra.

Furthermore, based on the reverse bias dependence of the decay time
constant of the two emission components, we discovered that the
shielding effect of the piezoelectric field did not contribute to the
light emission mechanism of InGaN mixed crystal thin films.

Using homoepitaxially grown high-quality GaN thin films, we carried out
time-resolved spectroscopy of light emission due to exciton
recombination. By measuring the excitation energy density dependence of
time-decay characteristics, we observed that the dominant emission line
of the exciton system, which manifests itself at 3.471 eV, undergoes a
transition from neutral-donor-bound-exciton emission to exciton molecule
emission. The radiative recombination lifetime of the exciton molecules
was estimated to be approximately 45 ps, and the radiative recombination
lifetime of exciton molecules was estimated to be approximately 940 ps.
In addition, based on the spectrum of the exciton molecule emission, the
exciton molecule binding energy was estimated to be 6.8 meV.

Growth of GaN crystals was carried out using the RF-MBE method and the
NH~3~ gas source MBE method.

In the RF-MBE method, the pre-growth treatment conditions for both
sapphire substrates and MOCVD-grown GaN substrates were optimized, and
the treatment conditions necessary for obtaining high-quality GaN
epitaxial thin films were clarified. Using a sulfuric/phosphoric
etchant, we observed atomic steps on the surface of the sapphire
substrate at an etchant temperature of 110ºC. In addition, it was shown
that etching with buffered hydrofluoric acid was an efficient way of
cleaning the GaN substrate surface. Optimum treatment conditions and the
importance of factors such as substrate selection and substrate surface
treatment for obtaining higher quality were demonstrated. In addition,
InGaN was grown and, based on the results of X-ray diffraction
measurements and Auger electron spectroscopy measurements, it was
confirmed that In had been incorporated into the crystal.

In the NH~3~ gas source MBE growth method, we obtained a GaN thin film
that had a Ga polar face, was flat at the atomic layer level (RMS = 5
Å), and exhibited outstandingly excellent emission characteristics.

## Summary of Research and Development in 2000

Various experiments were carried out within the framework of this study
for the explanation of the physico-optical properties of GaN compound
semiconductors and high-quality GaN compound semiconductor thin film
growth.

In order to clarify the light emission mechanism, we investigated the
correlation between the excitation spectrum and the two emission
components observed in the luminescence spectra of In~x~Ga~1-x~N. Our
results were: we observed a distinct absorption peak between low and
room temperature in the excitation spectrum of In~x~Ga~1−x~N; and at the
same time, the Stokes shift value was practically constant with respect
to temperature for the high-energy emission component. Such results
could not be explained by an emission model based on excitons localized
at the minima of the potential fluctuations in quantum dots caused by In
composition inhomogeneity, which had been reported previously. However,
they could be accounted for in a non-contradictory manner by a light
emission mechanism model including the polaron concept, which we had
proposed in our previous research work.

We received substrates from the Bulk Growth Group and conducted basic
research concerned with homoepitaxial growth to clarify the correlation
between the crystal polarity of GaN bulk single crystals used for the
substrates and the crystal polarity of the epitaxial thin films. In
addition, it should be noted that emission characteristics were greatly
improved by homoepitaxial growth.

Single polarity control of GaN thin films was implemented using the
NH~3~ GS-MBE method, and the crystalline properties of the resultant GaN
epitaxial thin films were evaluated using RBS analysis, which confirmed
their extremely high crystalline properties.

## Excitation Spectroscopy Measurements of Temperature Dependence of Absorption and Emission

### Methods and means

In the course of this study, emission mechanism models proposed prior to
last year were investigated by carrying out measurements of the
physico-optical properties of InGaN thin films using selective
excitation spectroscopy.

Excitation spectroscopy is a method in which wavelength-variable lasers,
such as dye lasers, are used as light sources to selectively excite
carriers having a resonance level at a specific wavelength, and the
emission caused by the carriers is observed. By using this method of
measurement, it is possible to investigate dependence on various
excitation energies of a particular emission.

In general, the method for measuring excitation spectra is called
photoluminescence excitation spectroscopy (Photoluminescence Excitation:
PLE) and is used to vary the wavelengths of the exciting light. Sources
of white light, such as Xe lamps etc., are typically used as excitation
light sources for such measurements, during which, after rendering the
light monochromic, specimens are irradiated with the light and the PL
spectra of the specimens are observed. However, the intensity of the
excited light in this method is extremely low, and, as a result, the
emission intensity becomes extremely low, too. This is why in the
present study we used a powerful dye laser. The PLE method is
characterized by a dramatic increase in the amount of generated
information because PL spectra are obtained as two functions of
excitation wavelength and emission wavelength. The use of a high-power
dye laser permits measurement of excitation light intensity and
temperature dependences as well, which makes the method a powerful means
for optical evaluation.

![](media/image7.png){width="3.9027777777777777in" height="2.5in"}

Figure 2.3.1-(1) Excitation Spectroscopy Measurement System.

FIG. 2.3.1-(1) shows the excitation spectroscopy measurement system used
in the present study. The system is basically the same as a PL
measurement system. A dye laser pumped by a Xe-Cl excimer laser
(wavelength: 308 nm, pulse width: 20 ns, repetition rate: 100 Hz) was
used as an excitation light source. Because the power of the dye laser
varied depending on the wavelength, adjustments were carried out using a
power meter and a variable ND filter to maintain the excitation power
density constant during measurement.

The specimens used for the measurements were undoped InGaN mixed crystal
thin films with a thickness of 0.1 *μ*m grown by metal-organic chemical
vapor deposition (Metalorganic Chemical Vapor Deposition: MOCVD) on a
sapphire (0001) substrate with a 3-*μ*m GaN buffer layer in between,
where the In composition ranged from 4 and 14%.

### Results and Discussion

#### Excitation energy dependence of PL spectra

This section addresses the temperature dependence obtained when varying
the excitation energy of a single In~0.08~Ga~0.92~N thin film. These
measurements differ from previous measurements in which a He-Cd laser
(325 nm) and an Ar laser (257 nm) were used as excitation light sources,
and for which the GaN which was used as a buffer layer was excited
simultaneously.

![](media/image8.png){width="4.402777777777778in"
height="3.3333333333333335in"}

Figure 2.3.2-(1) Temperature dependence of PL spectra. (a) Excitation at
3.54 eV. (b) Excitation at 3.38 eV.

Here, we would like to discuss the difference between the emission for
excitation using light of an energy higher than that of the GaN bandgap,
and for excitation using light of an energy lower than that of the GaN
bandgap.

FIG. 2.3.2-(1) shows the temperature dependence of PL spectra obtained
by excitation at an energy of 3.54 eV (above the E~g~ of GaN) using a
dye laser as the excitation light source. Although using higher power
for excitation made it more difficult to detect, we were able to confirm
the presence of two components to the emission in all of the spectra. In
the same manner as before, we used Gaussian fitting for the two
components and plot the temperature dependence of their peak positions
in FIG. 2.3.2-(2), together with the overall PL peak position.

![](media/image9.png){width="5.981944444444444in"
height="2.745138888888889in"}

Figure 2.3.2-(2) Temperature dependence of emission peak positions. (a)
Excitation at 3.54 eV. (b) Excitation at 3.38 eV.

The principal characteristics apparent from the figures are:

\(1\) At temperatures below 200K, the temperature dependence of the PL
peak position is different for the two emission components.

\(2\) For the two different excitation energies, the peak positions of
the two emission components exhibit practically the same temperature
dependences, as in the case of measurements conducted previously using a
He-Cd laser (325 nm) and an Ar^+^ laser (257 nm).

![](media/image10.png){width="3.25in" height="4.930555555555555in"}

Figure 2.3.2-(3) Comparison of PL spectra.

Next, we would like to examine the difference in emission intensity in
the case of varying excitation energy. As far as luminous efficiency is
concerned, considerable differences are observed, which is why we
plotted PL spectra for the two cases together in FIG. 2.3.2-(3). At each
temperature, normalization was carried out at maximum emission intensity
during 3.54 eV excitation. Characteristics apparent from the figure are:

\(3\) At low temperatures, varying the excitation energy from 3.54 eV to
3.38 eV resulted in a decrease in the emission intensity of the
higher-energy component.

\(4\) When the temperature is raised, the emission intensity of both
components decreases during 3.38 eV excitation in comparison with 3.54
eV excitation.

\(1\) and (2) result from the contribution of the two components to
emission from In~0.08~Ga~0.92~N. The difference between the intensities
of the higher-energy and lower-energy components influences the peak
positions of the PL spectra in In~0.08~Ga~0.92~N. These results can also
serve as a confirmation of the existence of the two components. In
addition, under the conditions used, the difference between the two
spectra was the difference between the excitation energy being above the
GaN bandgap and below it, and results (3) and (4) show that carriers
excited in the vicinity of the GaN band edge contributed to the
luminescence of In~0.08~Ga~0.92~N. In addition, it is evident from (4)
that the higher the temperature, the higher the amount of contribution
of the carriers excited at the GaN band edge to the luminescence of
In~0.08~Ga~0.92~N. This is believed to be due to the fact that carriers
become prone to diffusion at high temperatures.

In this manner, the contribution of the GaN-excited carriers to the
luminescence of In~0.08~Ga~0.92~N is evident from the temperature
dependence results. Therefore, next, we will examine the dependence of
the luminescence on excitation light intensity. FIG. 2.3.2-(4) shows the
excitation power density dependence of the emission intensity at 100K
and 250K. First of all, it should be noted that when comparison by
excitation energy was carried out at each temperature, with varying
excitation energy at both 100K and 250K, there were differences in terms
of absolute emission intensity, but the amount of increase in the
emission intensity versus the excitation power density was practically
the same. Furthermore, when comparison by temperature was carried out,
the amount of increase in the emission intensity versus the excitation
power density was larger at 250K than at 100K. This result was identical
to the temperature variation of the excitation power density dependence
of the integrated emission intensity observed in the previous section.

![](media/image11.png){width="3.75in" height="3.5972222222222223in"}

Figure 2.3.2-(4) Excitation power density dependence in case of 3.54 eV
excitation and 3.38 eV excitation.

From the above results, it is evident that although there were no
significant changes in the temperature characteristics of the two
emission components in In~0.08~Ga~0.92~N no matter whether the GaN of
the buffer layer was simultaneously excited during the excitation of
In~0.08~Ga~0.92~N or not, considerable changes in emission intensity did
occur. In particular, it is evident that when the temperature reached
approximately room temperature, transition of carriers from GaN greatly
increased the emission intensity of In~0.08~Ga~0.92~N, with the GaN
layer playing an important role as a carrier-supplying source. This fact
means that when quantum well structures etc. are actually used as
devices, the contribution of carriers from GaN used as a barrier layer
can be a means for increasing the emission intensity of In~x~Ga~1−x~N,
which is regarded as an important issue in device fabrication.

#### Excitation spectra of InxGa1−xN mixed semiconductors

There is a considerable energy difference (Stokes shift) between
absorption and emission in In~x~Ga~1−x~N. In other words, carriers
excited at the absorption edge contribute to radiative recombination
after undergoing a considerable energy relaxation from their energy
position. The gigantic Stokes shift observed in In~x~Ga~1−x~N has long
been a huge mystery. This is also another reason for the general
acceptance of the theory that claims that the origin of In~x~Ga~1−x~N
emission is attributable to localized excitons \[1\].

Thus, for a more detailed discussion of the characteristic emission of
In~x~Ga~1−x~N \[2\], which had been discovered prior to last year, we
investigated the Stokes shift of In~x~Ga~1−x~N using selective
excitation spectroscopy.

![](media/image12.wmf){width="6.0in" height="3.198611111111111in"}

Left: Figure 2.3.2-(5) Excitation spectrum of In0.08Ga0.92N.

Right: Figure 2.3.2-(6) Enlarged view of the vicinity of GaN band edge
in excitation spectrum of In0.08Ga0.92N.

An excitation spectrum (PLE) of an In~0.08~Ga~0.92~N thin film at 4K is
shown in FIG. 2.3.2-(5). The detection position is the peak position of
the PL spectrum of In~0.08~Ga~0.92~N. In~0.08~Ga~0.92~N has roughly two
main absorption peaks in energy positions in the vicinity of 3.5 eV
(GaN) and in the vicinity of 3.3 eV (In~0.08~Ga~0.92~N).

An enlarged view of the vicinity of 3.5 eV is shown in FIG. 2.3.2-(6). A
reflectance spectrum of the specimen is also shown in FIG. 2.3.2-(6).
The excitation spectrum correlated with the reflectance spectrum in that
the position of the three peaks that can be seen in the vicinity of 3.5
eV in the excitation spectrum was determined by the free excitons A, B,
and C of GaN. There is one interesting experimental result, illustrated
in FIG. 2.3.2-(7) and FIG. 2.3.2-(8), which is concerned with the issue
of whether the GaN was that of the buffer layer or was the GaN present
in In~0.08~Ga~0.92~N. FIG. 2.3.2-(7) shows the temperature dependence of
reflectance spectrum (Reflectance) and PL spectrum in the vicinity of
GaN observed in the In~0.08~Ga~0.92~N thin film, and FIG. 2.3.2-(8)
shows the PL spectrum, reflectance spectrum, and excitation spectrum at
100K plotted together.

![](media/image13.png){width="2.9166666666666665in"
height="4.069444444444445in"}

Left: Figure 2.3.2-(7) Temperature dependence of exciton emission of GaN
in In0.08Ga0.92N thin film.

Right: Figure 2.3.2-(8) Enlarged view of the vicinity of GaN band edge
at 100K.

It can be seen from FIG. 2.3.2-(7) that the emission line of the free
exciton A of GaN seen at 4K is made up of two emission lines. Of these
two emission lines, the one corresponding to the reflectance spectrum is
on the lower energy side. Examination of FIG. 2.3.2-(7) also reveals
that of of these two lines, the one corresponding to PLE is on the
higher energy side. If we take into consideration the fact that it is
the surface of the In~0.08~Ga~0.92~N thin film that is observed in
reflectance spectrum measurements, we can speculate that, out of these
two emission lines, the one on the lower energy side is probably the GaN
of the In~0.08~Ga~0.92~N mixed crystal, and the one on the higher energy
side is probably the free exciton A emission line originating from the
GaN of the buffer layer. Therefore, it can be assumed that the
absorption peak observed in the excitation spectrum this time was
generated by the GaN of the buffer layer. Although at the present moment
we still cannot leave the realm of speculation and more detailed
measurements are necessary, at least it has been firmly established that
free excitons excited in GaN contribute to the emission of
In~0.08~Ga~0.92~N in one form or another.

Next, let us take a look at the absorption peaks in the vicinity of 3.3
eV. FIG. 2.3.2-(9) is an enlarged view of the vicinity of 3.3 eV in FIG.
2.3.2-(5). Examination of this figure reveals that judging by the drop
in the intensity of the excitation spectrum at the boundary of about 3.3
eV, this peak is the absorption peak of In~0.08~Ga~0.92~N. Although
there have been reports regarding In~x~Ga~1−x~N excitation spectra
before \[3\], such a beautiful absorption peak was observed for the
first time only in the present study. The fact that a peak was observed
in the excitation spectrum means that at this energy position there
exists some kind of resonance level. For this reason, in order to
investigate the relationship between this resonance level and the
two-component emission, we examined the excitation energy dependence at
100K, where the two components are clearly observed.

![](media/image15.png){width="2.8381944444444445in"
height="3.167361111111111in"}Left: Figure 2.3.2-(9) Excitation spectrum
of In~0.08~Ga~0.92~N.

Right: Figure 2.3.2-(10) Excitation spectrum of two components

![](media/image17.png){width="6.00625in" height="3.7006944444444443in"}

Figure 2.3.2-(11) Excitation energy dependence of PL spectra. (a)
Higher-energy side relative to peak position (b) Lower-energy side
relative to peak position

The excitation energy dependence of the emission intensity of the two
components at such time is plotted in FIG. 2.3.2-(10). From this figure,
it is evident that the absorption peaks of the two components are
located practically in the same energy position. Changes in the PL
spectrum at excitation energy above and below the absorption peak
position are shown in FIG. 2.3.2-(11).

Here, (a) is the excitation energy dependence on the higher energy side
relative to the absorption peak position, and (b) is the excitation
energy dependence on the lower energy side relative to the absorption
peak position. For either one of the two emission components, prior to
and subsequent to the absorption peak position, the emission intensity
first increases and then drops, but on the higher-energy side, the
amount of increase is larger, and the amount of decrease is larger, too.

![](media/image18.wmf){width="4.783333333333333in"
height="5.428472222222222in"}

Figure 2.3.2-(12) Excitation energy dependence of spectra in the
vicinity of GaN band edge.

In addition, as shown in FIG. 2.3.2-(12), during absorption in the
vicinity of the GaN band edge, the amount of decrease in the emission
intensity was larger for the high-energy component when the excitation
energy changed from above the band edge to below the band edge.
Furthermore, the positions of the absorption peaks at the GaN band edge
associated with the two emission components were equal, just as they are
for the analogous absorption peaks at the In~0.08~Ga~0.92~N band edge.
It is evident from the above results that carriers contributing to the
high-energy component and carriers contributing to the low-energy
component were generated in the same absorption position. Thus, one may
infer that excited carriers first undergo energy relaxation from their
excitation energy position to a high energy level, and from there they
undergo a transition to a low energy level.

Next, when we carried out measurements of the temperature dependence of
In~0.08~Ga~0.92~N excitation spectrum, a distinct absorption peak was
observed in the excitation spectrum in the vicinity of room temperature.
The peak is shown in FIG. 2.3.2-(13). Inspection of the figure reveals
that while no significant shift is observed in the excitation spectrum
in the temperature range from low temperatures to 100K, there is a
gradual red shift taking place in the excitation spectrum as soon as the
temperature exceeds 100K.

![](media/image19.png){width="3.5180555555555557in" height="5.25in"}The
shift of the excitation spectrum is shown in FIG. 2.3.2-(14). The figure
shows that the position of the peak in the excitation spectrum varies
with temperature practically in parallel with the position of the
emission peak of the high-energy component, and, as a result, the Stokes
shift with respect to the high-energy component is a constant value of
approximately 50 meV at all temperatures. Based on time-resolved
spectroscopy results, it is evident that there are no substantial
changes in the emission position of the high-energy component in the
temperature range from low temperatures to 100K. As is well known for
other compound semiconductors, in a system where there is a contribution
from localized excitons, when the temperature is increased, the energy
difference between the absorption spectrum and the PL spectrum (the
Stokes shift) becomes smaller due to exciton delocalization. From this
point of view, we can say that the large Stokes shift observed in
In0.08Ga0.92N is not caused by localized excitons. Therefore, to
understand the origin of the high-energy component luminescence, it
should be taken into account that the Stokes shift is large and that it
can be maintained all the way to room temperature.

Figure 2.3.2-(13) Temperature dependence of PL spectra and excitation
spectra.

![](media/image20.png){width="5.99375in" height="6.930555555555555in"}

Figure 2.3.2-(14) Temperature dependence of the peak position of PL
spectra and excitation spectra.

Based on the results of magnetic field dependence and time-resolved
spectroscopy measurements of PL spectra conducted prior to last year,
and taking into consideration the facts that

-   the emission of the high-energy component in In~0.08~Ga~0.92~N is
    caused by free carriers,

-   the high-energy component has an extremely short lifetime (about 30
    ps) \[4\], \[5\], and

-   there is an extremely strong interaction with LO phonons in GaN
    \[6\],

it is believed that electrons in a polaron state contribute to emission
in the high-energy component, and that the amount of shift expected
based on the electron-phonon interaction is α~e~ηω~l~ = 44.2 meV (α~e~:
Fröhlich coupling constant, ηω~l~: LO phonon energy). Thus, there is a
very good agreement with 50 meV, the size of the Stokes shift obtained
in the present series of experiments, thereby allowing for a
non-contradictory explanation of the light emission mechanism model
including the concept of the polaron, which has been suggested by us.

Furthermore, there is another fact that is evident from the insert shown
in FIG. 2.3.2-(14). Because the excitation spectrum provides information
about the excitation state of In~0.08~Ga~0.92~N, we can say that the
position of the peak of the excitation spectrum practically represents
the bandgap of In~0.08~Ga~0.92~N. Thus, it can be seen that the
temperature dependence of the bandgap of In~0.08~Ga~0.92~N is weaker in
comparison with GaN.

![](media/image21.wmf){width="3.527083333333333in"
height="4.929861111111111in"}

Figure 2.3.2-(15) Comparison of absorption spectra and excitation
spectra at 77K and 290K.

So far we have described excitation spectra only, and next, we will
examine a comparison between absorption spectra and excitation spectra.
FIG. 2.3.2-(15) shows absorption spectra (ABS) and excitation spectra
(PLE) at 77K and 290K. The positions of the absorption spectra and those
of the excitation spectra at these two temperatures coincide, and, of
course, it is a firmly established fact that light absorbed by the
In~0.08~Ga~0.92~N sample excites carriers in the crystal and the
carriers participate in the radiative recombination of the
In~0.08~Ga~0.92~N.

![](media/image22.wmf){width="4.25in" height="5.458333333333333in"}

Figure 2.3.2-(16) Dependence of PL spectra and excitation spectra on In
composition.

The above discussion centered exclusively on samples with an In
composition of 8%. Now, we will examine changes in the excitation
spectra and PL spectra occurring when the In composition is different.
FIG. 2.3.2-(16) shows PL spectra and excitation spectra obtained when
the In composition is varied from 4% to 14%. The temperature at which
the measurements were carried out was 4K. When the content of In is
increased from pure GaN, the position of the peaks of both PL spectra
and excitation spectra shifts towards lower energies. This is due to the
change in the bandgap occurring during transition from a GaN state to an
InN state occurring with the increase in the In composition. The
position of the peaks of the PL spectra and excitation spectra at
various compositions is depicted in FIG. 2.3.2-(17).

The bowing parameter C of In~x~Ga~1−x~N, obtained from a formula used to
determine the bandgap energy of a mixed crystal semiconductor by the
peak position of the excitation spectrum:

*E~g~ =* x*E~gAC~ +* (1−x)*E~g,BC~ − C*x(1−x) (2.3.2-(1))

was approximately 1.22 eV. Here, the bandgap of GaN was assumed to be
3.5 eV and the bandgap of InN was assumed to be 2.0 eV. The bowing
parameter turned out to be extremely close to the value of 1.00 eV
reported in the past. Now, the dependence of the Stokes shift on the In
composition is shown in FIG. 2.3.2-(18). With increase of the In
composition the amount of the Stokes shift increased as well. From these
results it was evident that as In increased, the state from which there
was a high probability of radiative recombination became deeper, and the
process of relaxation of carriers from an excited state to an emitting
energy state took a longer time.

![](media/image23.png){width="5.998611111111111in"
height="2.6638888888888888in"}

Left: Figure 2.3.2-(17) Dependence of the peak position of PL spectra
and excitation spectra on In composition.

Right: Figure 2.3.2-(18) Dependence of the Stokes shift on In
composition.

## Growth of GaN Epitaxial Thin Films by MBE Method and Their Evaluation

###  Methods for MBE Growth of GaN Epitaxial Thin Films and their Evaluation

#### RF-MBE Method and Homoepitaxial Growth

Homoepitaxial growth was carried out using the RF-MBE method on bulk GaN
single crystal c-face substrates grown by the pressure-controlled
solution growth (PC-SG) technique. The GaN bulk single crystal c-face
substrates possessed Ga polarity in the \[0001\] direction and N
polarity in the \[000-1\] direction. The different polarities resulted
in different chemical stabilities and crystal growth character, and thus
it was necessary to control the polarity of bulk GaN and evaluate the
results of crystal growth on the respective faces. Therefore, a
substrate was cleaved and crystal growth was carried out on the N-polar
face and on the Ga-polar face. The determination of the polar faces of
the GaN bulk single crystal substrates was based on the degree of
activity with respect to alkali etching \[7\]. The bulk single crystals
had a smooth face and a face with a macro-stepped structure, and it was
determined that the smooth face was an N-polar face while the stepped
face was a Ga-polar face.

Prior to sending the GaN bulk single crystal substrates to a growth
chamber, they were subjected to etching with aqua regia for 10 minutes
in order to eliminate impurities adhering to the surface. After
increasing the substrate temperature to 870ºC in a nitrogen atmosphere
and conducting heat treatment for 30 minutes, growth of homoepitaxial
GaN thin films was carried out for 4 hours at a growth temperature of
870 ºC, with the Knudsen cell temperature set to 970ºC, the nitrogen
flow rate set to 1.5 sccm, the RF plasma output set to 350W, and the ion
polarization voltage set to 500V. Upon termination of growth, the
temperature was gradually lowered in a nitrogen atmosphere.

#### NH~3~ GS-MBE Method and Heteroepitaxial Growth on Sapphire

A detailed investigation was carried out into the influence exerted on
the crystal polarity by the nitriding treatment conducted in the initial
stage of heteroepitaxial growth on sapphire substrates by the NH~3~
GS-MBE method. FIG. 2.4.1-(1) shows the sequence of growth steps under
the NH~3~ GS-MBE method. A phosphoric/sulfuric etchant was used for
surface treatment of the sapphire substrates prior to growth. First of
all, the sapphire substrates were subjected to a heat treatment at a
substrate temperature of 1100ºC (measured using a thermocouple) for 10
minutes, followed by a nitriding treatment. Experiments were carried out
by varying the nitridation time as a parameter.

![](media/image24.png){width="5.985416666666667in" height="2.48125in"}

Figure 2.4.1-(1) Growth time graph under NH~3~ GS-MBE method.

Y-axis label: Substrate temperature (ºC)

Other labels, left to right: Heat treatment 10 min; Nitridation; GaN
thin film growth 1 hrs; Lowering temperature (40ºC/min); RHEED
observations.

After nitridation, GaN thin film growth was carried out at a substrate
temperature of 1000ºC. The lowering of the temperature upon termination
of growth was carried out in an ammonia atmosphere and in vacuo while
observing reflection high-energy electron diffraction (reflection
high-energy electron diffraction: RHEED) images at a substrate
temperature of about 200\~300ºC for the purpose of polarity
determination. In addition, in order to determine the effects produced
by an AlN buffer layer, evaluation was also conducted with respect to
samples obtained by depositing an AlN buffer layer at 1000ºC after
nitridation and then carrying out the deposition of the GaN thin film.

#### Evaluation of Crystalline Properties using Rutherford Back Scattering

A schematic diagram of a Rutherford back scattering analyzer (Rutherford
back scattering: RBS) used in the experiments is shown in FIG.
2.4.1-(2). In addition to ordinary RBS measurements utilizing a
semiconductor detector and high energy ions of 350\~990 keV (in the case
of monovalent helium ions) and 0.70\~1.98 MeV (in the case of divalent
helium ions), the analyzer possesses analytical capabilities, whereby it
is capable of performing RBS measurements of the region closest to the
surface (approx. 500\~800 Å) with an ion beam of 350 keV\~450 keV at
high resolution (of the order of several Å). Therefore, in regions with
extremely thin surfaces, using its resolution of several Å units, the
analyzer permits measurement of elemental composition and film thickness
in the depth direction while at the same time allowing for obtaining
elemental depth profiles extending to deeper regions. Furthermore,
because it possesses a micro-beam focusing function allowing it to focus
a high energy beam to a size of 5 *μ*m, it permits local analysis of
sample surface as well.

![](media/image25.png){width="5.9944444444444445in"
height="2.972916666666667in"}

Figure 2.4.1-(2) Schematic diagram of RBS analyzer

Keys:

(Top line, 4 items, left to righ) High-resolution RBS detection system;
Pump (for cooling system); Gate valve; High-voltage power supply tank

(Line below, 2 items) Molecular turbo-pump; Molecular turbo-pump

(Line below, 1 item) Goniometer

(Line below, 1 item) Cooling system cold head

(Line below, 2 items) E×B filter; Accelerating tube, ion source

(Line below, 3 items) Load lock chamber; scanning electrode; beam slit

(Line below, 2 items0 Ordinary RBS detector; Q lens

The analyzer is made up of an ion source/accelerator, beam optics, a
sample chamber, and a high-resolution RBS detection system.

The ion source/accelerator is a rotary disk-type high-voltage generator
model accelerator, which is made up of an ion source and a high-voltage
generator consisting of a drive motor and a laminated disk and is
contained inside a sealed tank filled with SF6 gas.The beam optics are
made up of an object slit (beam slit), an E×B type mass separation
filter, a microprobe scanning electrode, and a twin microprobe-forming
quadrupole magnetic lens (Q lens).

The sample stage consists of a sample chamber, a goniometer used for
sample position determination, an ordinary RBS detector, a
high-resolution RBS detection system, and a sample-changing apparatus.
The goniometer possesses degrees of freedom determined by two
translation axes and two rotational axes, which permits measurement in
various sample positions. The cooling of samples to 20K is also
possible.

He ions from the ion source are accelerated to a high energy of 350
keV\~990 keV (for monovalent ions), and, after mass separation by the
E×B filter and the Q lens, are directed at the sample. At such time,
back-scattered ions are detected by the semiconductor detector or the
high-resolution RBS detection system.

If the sample is a single crystal, then the incident ions are scattered
only by the atomic nuclei of the region closest to the surface when the
ions coincide with a specified crystal axis, and, for this reason,
penetrate into the depth of the sample in the manner shown in FIG.
2.4.1-(3). This is called channeling. If measurements are conducted
under channeling conditions, then ion backscattering occurs when there
is imperfection in the lattice positions inside the crystal (atom C in
the figure), and it is possible to carry out a detailed analysis
regarding the lattice positions of impurities and the perfection of the
crystal. The minimum ion yield χ~min~, which is used as an indicator of
crystal properties, is defined by the following formula, wherein the RBS
signal intensity under channeling conditions is designated as H and the
RBS signal under random conditions (an average value of the RBS signal
in non-channeling conditions) is designated as H~0~:

χ~min~ = (H/H~0~) × 100 (%) (2.4.1-(1))

In other words, the smaller χ~min~, the more regular the arrangement of
the crystal. In the present series of experiments, the evaluation of the
crystal properties of GaN was carried out by measuring χ~min~.

![](media/image26.png){width="3.5555555555555554in"
height="2.9444444444444446in"}

Figure 2.4.1-(3) The principle of channeling.

(Top three items, left to right) Rutherford backscattering from
interstitial atoms; incident primary ions; ion channeling

(Below, on the left): Rutherford backscattering

(Below, two items, left to right): Angle of incidence θ; region closest
to surface of sample.

(Box, top to bottom): A atoms, B atoms, C atoms

### Results and Discussion

#### RF-MBE Method and Homoepitaxial growth

FIG. 2.4.2-(1) shows RHEED patterns, in the \<11-20\> direction, of a
homoepitaxial GaN thin film grown on the N-polar face and Ga-polar face
prior to growth (immediately prior to sending samples to the growth
chamber), upon termination of growth, and after lowering the
temperature. The fact that the RHEED pattern of the N-polar face prior
to growth has a spotty appearance suggests that there are atom-level
irregularities on the smooth N-polar face (FIG. 2.4.2-(1) (a)). The fact
that the RHEED pattern changed its appearance from spotty to streaky
after carrying out homoepitaxial growth on the surface shows that the
flatness of the surface was restored (FIG. 2.4.2-(1) (b)). Then, judging
by the fact that the RHEED pattern (1×1) symmetry upon termination of
growth changed to (3×3) as a result of surface rearrangement upon
lowering the temperature, is believed to be due to the growth of a
homoepitaxial GaN thin film with an N-polar face (FIG. 2.4.2-(1) (c))
\[7\].

![](media/image27.png){width="3.9027777777777777in"
height="5.541666666666667in"}

Figure 2.4.2-(1) RHEED patterns in \<11-20\> direction upon termination
of growth of GaN bulk, homoepitaxial thin film and after lowering
temperature

\[Keys, FIG. 2.4.2-(1)\]

(Across top, left to right): N-polar face; Ga-polar face

(Left hand side, top to bottom): Prior to growth; Upon termination of
growth; After lowering temperature.

In contrast, the fact that the RHEED pattern obtained from the Ga-polar
face prior to growth is streaky is believed to reflect the flatness of
the macro-stepped terrace (FIG. 2.4.2-(1) (d)). The RHEED pattern that
exhibited (1×1) structure upon termination of growth (FIG. 2.4.2-(1)
(e)) was still unchanged, exhibiting (1×1) structure after lowering the
temperature (FIG. 2.4.2-(1) (f)). For this reason, it is believed that a
homoepitaxial GaN thin film had grown with a Ga-polarity.

Furthermore, the crystal polarity of these homoepitaxial GaN thin films
was determined using the difference in reaction to an aqueous solution
of NaOH \[7\]. A 1.8M aqueous solution of NaOH was used to conduct
etching for 3 minutes, as a result of which the homoepitaxial GaN thin
film grown on the N-polar face was etched away by the aqueous solution
of NaOH and there was a certain deterioration in surface morphology,
while the homoepitaxial GaN thin film grown on the Ga-polar face
exhibited no conspicuous differences in the characteristics of its
surface prior to and after the etching. This suggests that the
homoepitaxial GaN thin film grown on the N-polar face possesses
N-polarity while the homoepitaxial GaN thin film grown on the Ga-polar
face possesses Ga-polarity. This supports the above-described RHEED
observation results, and confirms that the homoepitaxial GaN thin film
grows with the same polarity as the GaN bulk single crystal used as the
substrate.

Next, the surface of the homoepitaxial GaN thin film was examined using
a scanning electron microscope (SEM) and an atomic force microscope
(AFM). SEM images of the surface and AFM images of the surface are shown
in FIG. 2.4.2-(2).

![](media/image28.png){width="3.9444444444444446in"
height="2.8333333333333335in"}

Figure 2.4.2-(2) SEM images and AFM images of the surface of a
homoepitaxial GaN thin film grown on the N-polar (smooth) face (on the
left) and on the Ga-polar (stepped) face (on the right) of a bulk GaN
substrate.

\[Keys, FIG. 2.4.2-(2)\]

(Across top, left to right): N-polar face; Ga-polar face

(Left hand side, top to bottom): SEM images of surface; AFM images of
surface

It can be confirmed that in the case of a homoepitaxial GaN thin film
grown on the N-polar face, which is the smooth face of the bulk GaN, the
surface assumed a stepped shape. Conversely, in the case of the
homoepitaxial GaN thin film grown on the Ga-polar face, which had a
stepped shape, the surface became practically flat. The height of the
stepped structure of the homoepitaxial GaN thin film grown on the
N-polar face was about 1.5\~10.9 nm, and because this corresponds to a
3\~21 monolayers (MLs), it was obvious that the structure was not the
same as atomic layer steps observed on MOCVD-grown GaN thin films. Thus,
this type of stepped surface morphology is believed to be due to the
fact that the GaN thin film was grown three-dimensionally. On the other
hand, because in the case of the homoepitaxial GaN thin film grown on
the Ga-polar face the stepped surface morphology could not be confirmed,
it is believed that the GaN thin film had been grown two-dimensionally.
These results suggest that, in the same manner as with epitaxial growth
on sapphire substrates, as a result of different polarity, there occur
changes in the growth mode, which exert influence on the surface
morphology.

FIG. 2.4.2-(3) shows PL spectra obtained at 4.2K for the N-polar face
and Ga-polar face of the GaN bulk single crystal and of the
homoepitaxial GaN thin films grown thereon. Conspicuous differences are
visible in the PL spectra obtained when the N-polar side and Ga-polar
side of the GaN bulk single crystal were excited, so that both deep
level luminescence and luminescence in the vicinity of the band edge are
observed in the PL spectra obtained for the N-polar side while only deep
level luminescence is observed and no luminescence in the vicinity of
the band edge is visible in the PL spectra obtained for the Ga-polar
side. This is believed to be due to the difference in the surface
properties of the GaN bulk single crystal. On the other hand, it is
evident from the PL spectra obtained for the homoepitaxial thin films
grown on the N- and Ga-polar faces of the GaN bulk single crystal
substrates that neutral donor-bound exciton edge luminescence with a
strong emission intensity and a narrow emission half-width is dominant
and deep level luminescence is suppressed.

![](media/image29.png){width="5.997222222222222in"
height="3.338888888888889in"}

Figure 2.4.2-(3) PL spectra obtained at 4.2K for (a) the N-polar face
and (b) Ga-polar face of the GaN bulk single crystal and of the
homoepitaxial GaN thin films grown thereon. (a) N-polar face (b)
Ga-polar face.

It was found that the emission half-width of neutral donor-bound
excitons was 1.5 meV for the film grown on the N-polar face and 4.3 meV
for the film grown on the Ga-polar face, which represented a
considerable improvement in terms of the emission characteristics of the
substrate.

Inspection of the peak positions of the exciton emission of the
homoepitaxial GaN thin films reveals that for homoepitaxial GaN thin
film on the N-polar face and on the Ga-polar face, the positions were
3.4702 eV and 3.4715 eV, respectively. The position of the emission peak
of neutral donor-bound excitons in an unstrained state, in which no
strain due to compression or stretching is present, is 3.4706 eV.
Therefore, there is practically no strain in the homoepitaxial GaN thin
films grown on the N-polar face and on the Ga-polar face. Some emission
peak shift is believed to be due to the fact that the thickness of the
GaN bulk single crystal is relatively small, at about 20 μm, and some
warping occurs during the growth of the single crystal.

In addition, it was found that in comparison with heteroepitaxial growth
on a sapphire substrate there were considerable improvements, such as
the fact that the emission half-width was reduced by more than 1/2, the
emission intensity became stronger by two orders of magnitude, and the
emission intensity ratio between band edge emission/deep level emission
increased by 1\~2 orders of magnitude, etc.

#### NH~3~ GS-MBE Method and Heteroepitaxial Growth on Sapphire Substrates

FIG. 2.4.2-(4) shows the dependence of the X-ray rocking curve
half-width of the GaN thin film on nitridation time. This dependence
suggests that changes definitely occurred on the sapphire surface as a
result of nitridation. Next, a RHEED image of the sapphire surface that
has been subjected to nitridation with NH~3~ is shown in FIG. 2.4.2-(5).
In the RHEED image, in addition to a streak pattern originating from the
sapphire substrate (Al~2~O~3~), inside it, there appears to be an image
with a narrow streak interval. In addition, it was confirmed that when
nitridation was carried out over an extended period of time, there was a
gradual approach to the streak interval of AlN. This confirms that, in
the same manner as when RF nitrogen plasma is used, a process of
substitution with AlN took place on the surface of the sapphire
substrate as a result of exposure to NH~3~ gas.

![](media/image30.png){width="5.991666666666666in"
height="2.3986111111111112in"}

Left: Figure 2.4.2-(4) Dependence of the X-ray locking curve half-width
of the GaN thin film on nitridation time.

Right: Figure 2.4.2-(5) RHEED pattern of the sapphire substrate obtained
by nitridation with NH~3.~

Next, we investigated the influence exerted by the nitridation of the
sapphire substrate by means of exposure to NH~3~ gas on the polarity of
the crystalline thin film that grows on it. It is known that when
nitridation is carried out using NH~3~ gas under the metal-organic
chemical vapor deposition (MOCVD) method, in the same manner as in the
case of nitridation using the RF-MBE technique, the epitaxial thin film
grown thereafter has an N-polar face. By contrast, under the NH~3~
GS-MBE method, an N-polar face is obtained when direct epitaxial thin
film growth is carried out on a sapphire substrate, but there have been
reports that a Ga-polar face is obtained when the sapphire substrate
surface is nitrided with NH~3~ gas, and a definite conclusion has not
yet been reached \[9\].

![](media/image31.png){width="5.988888888888889in"
height="2.426388888888889in"}

Figure 2.4.2-(6) RHEED patterns obtained upon termination of growth
under NH~3~ atmosphere and in vacuo after lowering the temperature to
300ºC.

![](media/image32.png){width="5.990972222222222in"
height="2.407638888888889in"}

Figure 2.4.2-(7) SEM images of the surface prior to and after NaOH
etching of a GaN thin film directly grown after nitridation.

In this series of experiments, it was evident that the sapphire
substrate surface had been definitely nitrided, and, for this reason,
the polarity of the GaN epitaxial thin film was determined based on the
degree of activity with respect to alkali etching and surface
reorganization in the RHEED images. FIG. 2.4.2-(6) shows a RHEED pattern
obtained after lowering the temperature after growing a GaN epitaxial
thin film on top of a nitrided sapphire. The pattern obtained when the
temperature was lowered under a NH~3~ gas atmosphere exhibited a (1×1)
structure, whereas the one obtained when it was lowered in vacuum
exhibited a (3×3) structure. Furthermore, FIG. 2.4.2-(7) shows SEM
images obtained prior to and after etching the sample with NaOH. As is
evident from the SEM images, the GaN epitaxial thin film grown on the
nitrided sapphire was extremely active with respect to alkali etching.
Based on these results, it is believed that the GaN epitaxial thin film
grown on the nitrided sapphire has an N-polar face. The (1×1) structure
exhibited by the RHEED pattern obtained under a NH~3~ atmosphere is
believed to be due to the fact that there was no adhered layer of Ga on
top of the N-polar face and surface reorganization due to attached Ga
atoms did not occur because the temperature was lowered in the presence
of active nitrogen. The same phenomenon has been observed in the RF-MBE
method when the temperature was lowered while performing irradiation
with nitrogen plasma \[10\].

As a result of the investigation, it was found that regardless of the
presence or absence of nitridation and the degree of nitridation, under
the growth conditions used in the present series of experiments, GaN
epitaxial thin films grown by the NH~3~ GS-MBE method had only an
N-polar face.

Furthermore, we examined the polarity of the GaN epitaxial thin films
with an AlN buffer layer. FIG. 2.4.2-(8) shows SEM images obtained prior
to and after alkali etching of a GaN epitaxial thin film, in which an
AlN buffer was used. Both when nitridation was and was not conducted,
the etching of the GaN thin film surface did not take place, based on
which, it is evident that using an AlN buffer layer makes it possible to
exercise polarity control from an N-polar face to a Ga-polar face in a
simple manner.

![](media/image33.png){width="5.986111111111111in"
height="3.811111111111111in"}

Figure 2.4.2-(8) SEM images of the surface prior to and after NaOH
etching of a GaN thin film grown using an AlN buffer layer (top: without
nitriding treatment, bottom: with nitriding treatment).

\[Keys, FIG. 2.4.2-(8)\]

(Right hand side, top to bottom): Nitridation 0 min; Nitridation 60 min.

Finally, evaluation of the crystal properties of GaN epitaxial thin
films grown under the NH~3~ GS-MBE method using nitridation and an AlN
buffer layer was carried out with the help of RBS analysis. FIG.
2.4.2-(9) shows RBS spectra of a GaN epitaxial thin film grown under the
NH~3~ GS-MBE method. In addition, for reference purposes, FIG.
2.4.2-(10) shows RBS spectra of a GaN epitaxial thin film grown using
the MOCVD method. The random spectrum is an RBS spectrum measured
without channeling, and the channeled spectrum is an RBS spectrum
measured under channeling conditions. During measurement, the \<0001\>
axis was used as the channeling axis. In the random spectrum, the signal
intensity (H~0~) is high because ions are subjected to backscattering by
atoms in the crystal, but in the channeled spectrum, as is also shown in
FIG. 2.4.1-(3), ions penetrate into the crystal without being scattered
by the atoms and therefore, the signal intensity (H) becomes weaker.

Here, we obtained the minimum ion yield χ~min~, which is used as an
indicator of crystal properties, for GaN thin films grown using the
NH~3~ GS-MBE method and the MOCVD method. As a result, the χ~min~ of GaN
epitaxial thin films grown using nitriding-based and AlN buffer
layer-based NH~3~ GS-MBE were 1.5%, and films grown by MOCVD were 1.7%,
respectively. Based on this, it was found that the crystal properties of
both films, as viewed from the \<0001\> axis direction, were practically
identical. In addition, in comparison with Si single crystal wafers,
which have a χ~min~ of about 2%, or GaAs single crystal wafers, which
have a χ~min~ of several %, the value of χ~min~ of GaN epitaxial thin
films is equivalent or lower, which means that in this crystal the
crystal defect density is extremely high and the deviation from the
crystal lattice positions of the atoms making up the crystal is
extremely small.

![](media/image34.png){width="2.9583333333333335in"
height="2.513888888888889in"}

Left: Figure 2.4.2-(9) RBS spectrum of GaN grown using NH~3~ GS-MBE
(channeling axis: \[0001\]).

Right: Figure 2.4.2-(10) RBS spectrum of GaN grown using MOCVD
(channeling axis: \[0001\]).

## Summary

 When PL measurements were carried out using excitation energies higher
and lower than the bandgap of GaN for the two emission components
obtained for In~0.08~Ga~0.92~N thin films, no significant variations
were observed in the emission peak positions of the two components of
In~0.08~Ga~0.92~N. However, it was found that when GaN was not excited,
the intensity of emission decreased and as the temperature became
higher, the role of GaN as a carrier-supplying source became more
important.

 Excitation spectrum measurements for In~0.08~Ga~0.92~N were carried
out and distinct absorption peaks of In~0.08~Ga~0.92~N were observed in
the temperature range from 4K to room temperature. In addition, we
discovered that free excitons A, B, and C produced by GaN at 4K
contributed to the emission of In~0.08~Ga~0.92~N. This suggested the
possibility that the GaN was primarily a buffer layer. The absorption
positions of the two emission components emanating from the
In~0.08~Ga~0.92~N thin film were identical, from which it was inferred
that excited carriers underwent energy relaxation from the excitation
energy position first to a higher energy level and from there performed
a transition to a lower energy level. Because when the temperature
dependence of the excitation spectra was measured, the peak positions
did not exhibit significant changes depending on temperature, it was
believed that the bandgap of In~0.08~Ga~0.92~N itself did not exhibit a
peculiar temperature dependency.

 Among the two emission components, the emission peak position and the
excitation spectrum peak position of the high-energy component shifted
practically in parallel in response to temperature changes, and the
Stokes shift of the high-energy component exhibited no temperature
dependence, its value being approximately 50 meV across all
temperatures. It is known that in a system where there is contribution
from localized excitons, when the temperature is increased, the energy
difference between the absorption spectrum and the PL spectrum (the
Stokes shift) becomes smaller due to exciton delocalization, and based
on this, the large Stokes shift observed in In~0.08~Ga~0.92~N is not due
to localized excitons. Thus, to understand the origin of the high-energy
component luminescence, it should be taken into account that the Stokes
shift is large and can be maintained all the way to room temperature.
Considering the emission mechanism model including the polaron concept,
which was proposed by us in research conducted prior to last year, the
amount of Stokes shift, α~e~ηω~l~ = 44.2 meV (α~e~: Fröhlich coupling
constant, ηω~l~: LO phonon energy), which is expected based on
electron-phonon interaction, is in extremely good agreement with the
value obtained in the present series of experiments and can explain it
in a non-contradictory fashion.

 Because the peak positions of the absorption spectrum and the
excitation spectrum were located practically in the same energy
position, it was evident that, in the same manner as with ordinary
semiconductor materials, the light absorbed by the In~0.08~Ga~0.92~N
sample excited carriers inside the crystal and the carriers contributed
in the radiative recombination of In~0.08~Ga~0.92~N.

 Excitation spectrum measurements were carried out for InGaN with an In
composition of 4\~14%. The smaller the In composition became, the more
distinctly was the absorption peak observed in the excitation spectrum.
In addition, the larger the In composition became, the larger the Stokes
shift was.

 Homoepitaxial growth by the RF-MBE method on GaN bulk single-crystal
substrates grown by the pressure-controlled solution growth technique
was investigated. As a result, epitaxial thin films that represented a
continuation of the polarity of the GaN bulk single crystals used for
the substrates were obtained. In addition, it was confirmed that when
the N-polar face of the GaN bulk single crystal was used as the
substrate, the GaN epitaxial thin films grown thereon grew
three-dimensionally, whereas GaN epitaxial thin films grown on the
Ga-polar face grew two-dimensionally. Results of PL measurements of
these homoepitaxial thin films showed that an extremely sharp and strong
emission line due to neutral donor-bound excitons was dominant, and
practically no deep level luminescence was observed. In addition, it was
found that in comparison with heteroepitaxial growth on sapphire
substrates, there were considerable improvements, such as the fact that
the emission half-width was reduced by more than 1/2, the emission
intensity became stronger by two orders of magnitude, and the emission
intensity ratio between band edge emission/deep level emission increased
by 1\~2 orders of magnitude, etc.

 Regarding control over the initial-stage growth process under the
NH~3~ GS-MBE method, we clarified the influence exerted by an AlN buffer
layer and sapphire substrate surface nitridation using NH~3~ gas on the
crystal properties. It was found that for GaN epitaxial thin films grown
on sapphire substrates, N-polar face growth was dominant regardless of
the presence or absence of nitridation of the sapphire substrate
surface. It was confirmed that, by contrast, insertion of an AlN buffer
layer prior to GaN epitaxial thin film growth resulted in obtaining a
single Ga-polar face regardless of the presence or absence of
nitridation. The ability to control single polarity faces is useful in
high-quality crystal growth by the NH~3~ GS-MBE method. Furthermore, as
a result of RBS analysis-based evaluation of the crystal properties of
the GaN epitaxial thin films grown by the NH~3~ GS-MBE method using an
AlN buffer layer, it was also found that the crystallinity of the
resultant epitaxial films was extremely high.

## Future Plans

In research conducted up until now, based on measurement results, we
have been able to develop some ideas on the origin of the high-energy
component and the mutual transitions of carriers between the two
components. The specific tasks and plans for the future are described
below.

 No experimental results have been obtained that would allow us to make
definite conclusions regarding the low-energy component among the two
components. However, based on numerous experimental results, the
low-energy component is expected to exert important influence on the
emission characteristics of In~x~Ga~1−x~N. At present, it has not been
confirmed yet whether the low-energy component, which is observed at low
temperatures, continues to exist at high temperatures. In addition,
judging from the fact that the intensity of the LO phonon replica
changes when the In composition is varied, the low-energy component is
expected to strongly depend on the In composition. In the future, we are
preparing to elucidate the origin of the low-energy component by
conducting more detailed measurements using samples with different In
compositions.

 In comparison with the high-energy component, the low-energy component
has a wider linewidth and its dependence on external forces, such as
excitation intensity, temperature, etc. is difficult to see. In
addition, in samples with a wide linewidth, there is a considerable
chance of the localization effect in In~x~Ga~1−x~N obscuring the
authentic emission characteristics of In~x~Ga~1−x~N. In this connection,
we will use crystals of higher quality produced by the Joint Research
Organization.

 When we examined the emission of In~x~Ga~1−x~N, there was an
unmistakable contribution of GaN to the intensity of the emission. In
particular, the higher the temperature becomes, the more important the
carriers originating from GaN become, and we are planning a more
detailed examination of this mechanism. This presents much interest in
terms of device fabrication.

 We are planning to improve the crystal properties of GaN thin films
obtained by homoepitaxial growth, in particular, their surface flatness,
which was insufficient this time. As for the Ga-polar face, which has
step-shaped irregularities on the surface, we will have to try to bury
the irregularities of the substrate by increasing the growth time,
increasing the diffusion length of the starting material, etc. As for
the N-polar face, in order to control the stepped three-dimensional
structure formed during homoepitaxial growth, we will examine the
possibility of introducing shutter control techniques, etc. In addition,
we will simultaneously continue our research regarding homoepitaxial
growth based on the NH~3~ GS-MBE method, which allows for easily
obtaining films with excellent emission characteristics. We will grow
high-quality InGaN mixed crystals on excellent homoepitaxial GaN buffer
layers.

 We will conduct a detailed investigation into the structural (defect
density, surface properties) and optical characteristics of
homoepitaxial thin films. In addition, in order to clarify the
relationship between the crystal properties and emission
characteristics, in the future we are preparing to conduct more detailed
RBS measurements, X-ray diffraction measurements, etc. regarding GaN
bulk single crystals and GaN epitaxial thin films. In particular, as far
as RBS measurements are concerned, we will conduct more detailed
measurements using channeling axes other than the \<0001\> axis
evaluated this time.

## References

\[1\] K. P. O'Donnel, R. W. Martin, and P. G. Middleton: Phys. Rev.
Lett. 82 (1999) 237.

\[2\] H. Kudo, H. Ishibashi, R. Zheng, Y. Yamada, and T. Taguchi: phys.
stat. sol. (b) 216 (1999) 163.

\[3\] S. Chichibu, T. Sota, K. Wada, and S. Nakamura: J. Vac. Sci.
Technol. B16 (1998) 2204.

\[4\] H. Kudo, H. Ishibashi, R. Zheng, Y. Yamada, T. Taguchi, S.
Nakamura, and G. Shinomiya: J. Lumin. 87-89 (2000) 1199.

\[5\] H. Kudo, T. Tanabe, H. Ishibashi, R. Zheng, Y. Yamada, and T.
Taguchi: phys. stat. sol. (a) 180 (2000) 27.

\[6\] R. Zheng, T. Taguchi, and M. Matsuura: J. Appl. Phys. 87 (2000)
2526.

\[7\] A. R. Smith, R. M. Feenstra, D. W. Greve, M. S. Shin, M.
Skowronski, J. Neugebauer, and J. E. Northrup: Appl. Phys. Lett. 72
(1998) 2114.

\[8\] S. Kurai, A. Kawabe, T. Sugita, S. Kubo, Y. Yamada, T. Taguchi,
and S. Sakai: Jpn. J. Appl. Phys. 38 (1999) L102.

\[9\] S. Sonoda, S. Shimizu, X. Q. Shen, S. Hara, and H. Okumura: Jpn.
J. Appl. Phys. 39 (2000) L202.

\[10\] S. Kurai, S. Kubo, T. Sugita, A. Kawabe, Y. Yamada, T. Taguchi:
Technical Report of Institute of Electronics, Information and
Communication Engineers 98 (384) (1998) 33.

# Clarification of Light Emission Mechanisms and Physical Properties of GaN-based Semiconductors

## Achievements in 1998\~1999

Generally speaking, the material used for the active layer of
nitride-based light-emitting devices is GaInN, a ternary mixed crystal,
and active research has been conducted by numerous researchers in order
to establish the origin of its highly efficient luminescence. Roughly
three models have been proposed that attempt to explain the highly
efficient luminescence of GaInN in spite of a high density of defects
\[1\].

\(1\) A model in which carriers are captured into extremely small
regions of high In concentration (quantum dots) present in the GaInN
active layer \[2\].

\(2\) A model in which carriers are localized at potential fluctuations
generated by inhomogeneities in the In composition in the GaInN active
layer \[3\].

\(3\) A model in which electrons enter a polaron state as a result of a
strong interaction between the electron system and the lattice and are
captured into the lattice \[4\].

In each of these models, capture or spatial localization of carriers
contributes to the high-efficiency luminescence. However, up till now no
definite conclusions have been reached regarding the details of the
process, including its origin, although active discussions have been
conducted by scientific societies and other organizations in Japan and
overseas. Furthermore, at present, all of the available reports attempt
only to understand emission characteristics and do some modeling and
none of them suggests ways to practically implement high-efficiency
light-emitting elements.

The origin of carrier localization is believed to be the key to the
practical realization of high-efficiency LEDs. In order to implement
super-efficient blue-violet LEDs with an energy conversion efficiency of
40% and an internal quantum efficiency of 90%, the most important task
at the present moment is to clarify the light emission mechanism of
GaN-based light-emitting elements and, based on that, to map out paths
towards practical implementation of high-efficiency elements.

Thus, in order to clarify the light emission mechanism of GaN-based
compound semiconductors, the present study was carried out with a
view 1) to clarify the mechanism of light emission in GaInN-based
light-emitting layers, and 2) to clarify the relationship between
defects, dislocations, and the mechanism of light emission.

[The main results obtained in 1998-1999 are summarized
below.]{.underline}

By investigating the effects of adding Si (donor) to GaInN/GaN quantum
wells, it was found that piezoelectric effects exert considerable
influence on the light emission characteristics, and these influences
can be practically eliminated by the addition of donors to quantum wells
in the amount of approximately 10^18^ cm^-3^. The piezoelectric field
reduces carrier recombination inside the quantum wells, and in devices
operating at low carrier densities, such as LEDs, the elimination of the
piezoelectric field is very important. On the other hand, it has been
pointed out that since the addition of a high concentration of Si to
quantum well layer brings about an increase in the threshold value of
the induced stimulated emission, the crystal properties of GaInN
deteriorate \[5\].

We have conducted a systematic investigation into emission
characteristics of GaInN/GaN quantum well LEDs with different In
concentrations over a wide temperature range (7\~540K) and discovered
(a) that the dependence of the emission lifetime on temperature was
different in three different temperature regions, (b) that these three
temperature regions were observed in samples with different In
compositions, and, furthermore, (c) that the temperature at which
carriers underwent a transition from a localized state to a delocalized
state increased with the increase in the In composition. Based on that,
it was found that the localization energy of the carriers increased with
an increase in the In composition ratio.

## Summary of Research and Development in 2000

Just as in 1999, in 2000 we continued our investigations concerning the
emission characteristics of GaInN/GaN quantum well structures in
connection with crystal growth parameters or structure. It is believed
that the physical phenomena involved in the mechanism of light emission
vary depending on the device structure and crystal growth conditions,
such as the In composition and the density of the Si dopant impurities
in the active layer of the GaInN. For this reason, we prepared a
systematic series of samples used for measurements, in which parameters
were varied depending on the intended purpose, and subjected them to
evaluation and investigation.

Below, we provide a summary of the objectives and results obtained in
the four areas of research included in the present report.

### Examination of light emission mechanism of GaInN/GaN multiple quantum well LEDs with different In compositions

The effect of spatial localization of carriers in GaInN is believed to
be of crucial importance to the understanding of the mechanism of light
emission. The emission characteristics (in particular, carrier
localization effects) of GaInN are believed to be strongly dependent on
the In composition, and a systematic evaluation of samples with
different In compositions is important for the clarification of the
origin of carrier localization. However, reports published so far have
provided evaluations for a narrow range of In compositions and described
different device structures. For this reason, our investigation focused
primarily on the localization state of carriers while varying the In
composition of the GaInN active layer in the LED structures. As a
result, based on the temperature dependence of the emission lifetime
over a wide range of temperatures (7\~540K) and on simulation using a
simplified localization model, we discovered that, with the increase in
the In composition, the localization energy increases, and, on the other
hand, the density of non-radiative centers increases as well.

### Examination of emission lifetime characteristics of LEDs of different luminescence efficiencies

Based on the examination of the carrier localization state carried out
in 1), we evaluated differences in the emission lifetime characteristics
of LED samples with different luminescence efficiencies at the same
emission wavelengths and investigated the relationship between the
luminescence efficiency and the carrier localization state. Based on
these results, we found that, in samples exhibiting a high-efficiency,
carriers are localized deeper and the transition to non-radiative
recombination centers is suppressed, thereby improving the efficiency of
luminescence.

### Examination of in-plane distribution of light emission

We used a high spatial resolution near-field probe PL system, introduced
in 2000, in order to evaluate the in-plane distribution of light
emission spectra of GaInN/GaN quantum well LEDs and summarized its
usefulness based on the obtained basic data.

### Reverse bias PL characteristics of Si-doped quantum well LEDs

The addition of Si to GaInN/GaN quantum wells is believed to be
efficient in the elimination of piezoelectric effects. However, the
addition of donors to the active layer also affects the LED band
structure, and these influences must be taken into consideration. In the
present study, we investigated the influence exerted on the device
characteristics by the addition of donors to the active layer using
reverse biased PL and time resolved PL characteristics of LED samples.

## Methods and Means

### PL and Time Resolved PL Measurement System

The PL measurements, for which we conducted measurements of the
temperature dependence and PL excitation power density dependence at
room temperature or low temperatures, as well as measurements of the
carrier lifetime by time-resolved PL and measurements of its temperature
dependence, etc., were carried out in cooperation with Yamaguchi
University. As concerns the excitation light sources used for the
measurement of the excitation power density, for weak excitation we used
a He-Cd laser (325 nm, cw, 2 mW), and for high density excitation a
Xe-Cl excimer laser (308 nm, pulse width: 2.5 ns, repetition rate: 70
Hz, peak power: 91 kW, average power: 16 mW) and a Nd YAG laser (355 nm,
pulse width: 5 ns, repetition rate: 10 Hz). For the time-resolved PL
measurements, we used the third harmonic of an Ar laser-excited
mode-locked titanium sapphire laser (266 nm, pulse width: 200 fs,
repetition rate: 250 kHz, peak power: 1.6 MW, average power: 80 mW) to
produce the excitation light and a multichannel spectrometer and a
streak camera for detection, with the measurements conducted using
three-dimensional data made up of wavelength, time, and luminescence
intensity. In order to measure the emission characteristics over a wide
temperature range, for temperature dependence measurements, we used a
temperature-variable cryostat for the range 7\~300K and a heater for the
range 300\~540K.

### Reverse Biased PL and Time-Resolved PL Measurements of LED Samples

Measurements of light emission characteristics under an external
electric field applied to quantum wells were carried out by measuring PL
in a state, wherein a reverse bias was applied to the LEDs. We used the
second harmonic (400 nm) of a mode-locked titanium sapphire laser to
selectivity excite quantum wells in LEDs emitting in the blue-green
region. For ordinary PL measurements, we employed a spectrometer and a
CCD detector, and for time-resolved PL measurements we used a
spectrometer and a streak camera system. In order to quantitatively
evaluate the intensity of the internal electric field in quantum wells
in experiments in which a reverse bias is applied, it is important to
estimate the externally applied electric field acting on the quantum
wells. To estimate the externally applied electric field, we used an LCR
meter to measure the pn junction capacitance of the LED samples, to
which the reverse bias was applied, estimated the width of the depletion
layer, and assumed that the externally applied voltage was applied to
the entire depletion layer. In addition, we investigated the
contribution of carriers generated by photo-excitation to the
photocurrent and to radiative recombination by measuring the
photocurrent in conjunction with luminescence spectra (emission peak
energy, luminescence intensity).

### High Spatial Resolution PL Mapping System Utilizing a Near-Field Optical Probe

The evaluation of the in-plane distribution of light emission was
carried out using a high spatial resolution PL mapping system utilizing
a near-field optical probe, which was introduced in 2000. In the system,
a semiconductor sample is excited by the near-field light of a laser
originating from a microscopic opening (100-500 nm) at the tip of an
optical fiber probe, and emission is detected via the same probe
(illumination-collection mode). It was estimated that the spatial
resolution was about 250 nm when using a probe with an opening of 500
nm. Generally speaking, the measurement of the in-plane distribution of
light emission is based on the use of CL (cathodo-luminescence) and
micro PL systems. Because in the case of CL the lateral diffusion length
of primary electrons is approximately 1 *μ*m, the spatial resolution
power is limited by the lateral diffusion width. In addition, in samples
of a multilayer structure, such as LEDs, the interpretation of the
emission distribution data is made difficult by the fact that carriers
are excited in all the layers from the surface to a depth of
approximately 1 *μ*m. On the other hand, in a micro PL system, the light
of the laser is focused with a microscope object lens, and although a
high spatial resolution power is indeed obtained, the theoretical
spatial resolution power does not exceed the diffraction limit (\~λ/NA).
Because as compared with these conventional methods, near-field probe PL
possesses a higher spatial resolution and can selectivity excite quantum
wells in the device structure, it is believed to facilitate the
interpretation of mapping and other data.

![](media/image36.png){width="3.5694444444444446in"
height="2.451388888888889in"}FIG. 3.3.3-(1) shows a schematic diagram of
the system. The laser used for excitation (325 nm or 441 nm) was coupled
to a fiber, and the quantum wells of the LED structures were excited
using near-field light from a microscopic opening at the tip of the
optical fiber. Emission was collected using the same microscopic opening
in the optical fiber and the measurement of the spectra was carried out
using a monochromator and a CCD detector. Using feedback (shear force
control), the distance between the tip of the fiber and the sample was
controlled so as to maintain a constant distance, and, from the voltage
applied to the piezoelectric elements along the Z axis, it was possible
to simultaneously obtain information on surface irregularities, such as
information obtained by AFM. In addition, the newly introduced system
was equipped with a cryostat used for low temperature (5K) measurements.

Figure 3.3.3-(1) Block diagram of near-field probe PL system.

### Method of Interference-Free Measurement of Emission Spectra

Due to the wide emission half-width of GaInN (which is about 120 meV in
the blue region at 470 nm) and the difference in the indexes of
refraction between the GaN epilayer and the sapphire typically used for
the substrate, during the optical evaluation of GaInN-based
light-emitting elements and materials, fringes caused by Fabry-Perot
interference often appear in the emission spectra. The fringes resulting
from interference quantitatively influence the half-width and peak
position of the spectra, and complicate the interpretation of the
emission process. Although in principle it is possible to eliminate the
influence of the interference by subjecting the measured spectra to
numerical processing, it is in fact quite difficult to remove the
interference fringes completely.

![](media/image37.png){width="3.9375in"
height="2.8958333333333335in"}Figure 3.3.4-(1) Conceptual diagram of
Fabry-Perot interference effect in GaN-based light emitting elements on
sapphire.

Here, we will describe the method used in the present study for
interference-free measurement of spectra. FIG. 3.3.4-(1) is a typical
diagram showing the origin of interference in the PL and EL spectra of
GaN-based light-emitting elements on sapphire substrates. Because of the
difference in the indexes of refraction between the sapphire and the
GaN, light emitted from the active layer undergoes repeated multiple
reflection in the epilayer of the GaN, and the emission components
guided to the detection system are represented by a direct component
E~1~ originating from the active layer and component E~2~ reflected at
the interface of GaN/sapphire. Here, with E representing the electric
field of the light, the respective components are given by the following
formulae:

*E*~1~(λ) = *A*~0~(*ё*)*T*exp *j*(*ωt* + *kr*~1~) (1)

*E*~2~(λ) = *A*~0~(λ)*T*exp *j*(*ωt* + *kr*~2~) (2)

(Where *A*~0~(λ) represents the actual emission spectrum of the active
layer, R represents the reflection coefficient at the GaN/sapphire
interface, and T represents the transmission coefficient of the GaN/air
interface.) In addition, the optical path difference between components
E~1~ and E~2~ is given by:

$r_{1} - r_{2} = \frac{2d}{\text{cos}\theta}$ (3)

In this case, light intensity in a direction orientated at an angle *θ*
to the center of the optical axis is given by:

$I_{\theta}(\lambda) = \left| E_{1} + E_{2} \right|^{2} = A_{0}(\lambda)^{2}T^{2}\left\{ 1 + R^{2} + 2R\text{cos}\left( \frac{2k_{0}\text{nd}}{\text{cos}\theta} \right) \right\}$
(4)

Because the light directed by the condenser lens into the detection
system is given by an integral over a certain angle to the central axis,
the actually measured spectrum is represented in the following manner as
a dependence of the light intensity on the wavelength:

$I(\lambda) = \int_{0}^{\theta_{1}}{I_{\theta}(\lambda)d\theta = A_{0}(\lambda)^{2}T^{2}\left\{ \theta_{1} + R^{2}\theta_{1} + 2R\int_{0}^{\theta_{1}}{\text{cos}\left( \frac{2k_{0}\text{nd}}{\text{cos}\theta} \right)d\theta} \right\}}$
(5)

Now, based on the actual emission spectrum (a) of FIG. 3.3.4-(2), the
computation of formula (5) in case (b), in which the detected emission
component corresponds to a relatively narrow angle range of *θ*~1~ = 10°
(N.A. = 0.1), and in case (c), wherein the emission is collected over a
wide angle of *θ*~1~ = 60 ° (N.A. = 0.8), gives us the lower portion of
FIG. 3.3.4-(2). The index of refraction used here for the GaN and the
sapphire was, assuming that d = 2.8 *μ*m, the value shown in FIG.
3.3.4-(1). Case (b), wherein the light was detected only over a narrow
angle range from the central axis, reveals a spectrum influenced by
interference, with the interference period being 14 nm, which is in
agreement with the value calculated from ∆λ = λ^2^/*2nd*. On the other
hand, when the light is collected over a wide angular range, and the
N.A. of the detection system is relatively large, it is possible to
reproduce the original emission spectrum so that it is free from the
influence of interference.

![](media/image38.png){width="3.8645833333333335in"
height="3.9479166666666665in"}

Figure 3.3.4-(2) (a) Original spectrum used for calculations, and
calculation results in case of (b) N.A.= 0.1, and (c) N.A.= 0.8.

![](media/image39.png){width="3.2083333333333335in"
height="3.1041666666666665in"}As a method of increasing the N.A. of the
detection system, it is possible to use a lens with a large N.A. (0.5 or
more). However, during typical PL measurements, it is necessary to focus
the laser used for excitation with a lens and irradiate the sample with
its light, and in addition, because the sample has to be placed in a
cryostat etc. during low temperature measurements, there has to be a
certain distance between the sample and the condenser lens, which
usually makes it difficult to increase the N.A. of the condenser lens on
the detector side. In addition, one might consider condensing the
emitted light over a wide angle range using an integrating sphere and
then guiding it to the detection system with a lens. In this case,
however, the intensity of the light guided to the detection system is
noticeably decreased, which renders the method impractical.

Figure 3.3.4-(3) PL measurement system utilizing a reflector cup.

Thus, in order to eliminate the interference effect by increasing the
N.A. of the detection system, we used a reflector cup such as the one
shown in FIG. 3.3.4-(3). The reflector cup has a bottom face diameter of
8 mm and sidewalls inclined at an angle of approximately 20°, which
allows it to guide a wide-angle component of the emission from the
sample into the **aperture** of the condenser lens. In order to obtain a
flat and high reflectance ratio in the desired wavelength region, the
cup was electroplated with silver. In addition, the cup could be easily
placed into the cryostat for temperature dependence measurements.

![](media/image40.png){width="3.6875in"
height="3.8020833333333335in"}FIG. 3.3.4-(4) shows a typical example of
PL measurements carried out on a blue LED sample. The figure shows
emission spectra obtained in the case of (a) using an ordinary condenser
lens with an N.A.= 0.1, and in the case of (b) conducting measurements
using a reflector cup. In the case of ordinary measurements, the
influence of interference on the spectrum is obvious and it is difficult
to distinguish the original emission peak from interference. On the
other hand, it is believed that in the case (b), in which a reflector
cup was used, there is no influence from interference and the spectrum
under measurement is the original spectrum. It can be seen that the
spectrum of the emission is composed of a main peak with a central
wavelength of 465 nm and a half-width of 150 meV and various peaks on
the high-energy side (425 nm). With the exception of certain
measurements, such as time resolved measurements, etc., emission
spectrum measurements in the present study have been carried out by the
method of FIG. 3.3.4-(3) in order to eliminate the influence of
interference. A patent is pending on the emission spectrum measurement
method shown above (Emission spectrum measurement apparatus and
measurement method, Japanese Patent Application JPA 2001-011214).

Figure 3.3.4-(4) (a) Measurement results obtained using a conventional
measurement system, (b) Measurement results obtained using a reflector
cup.

## Results and Discussion

### Investigation of Emission Characteristics of GaInN/GaN Quantum Well Structure LEDs with Different In Compositions

#### Sample structure

Crystal growth was conducted by the MOVPE (Metalorganic Vapor Phase
Epitaxy) method, and LED structures were fabricated on c-plane sapphire.
The active layer was composed of Ga~1-xI~N~x~N/GaN multiple quantum
wells, with no doping in the quantum well layer and Si added to the
barrier layer. The concentration of the Si dopant was approximately
2\~3×10^18^ cm^-3^. Based on the cross-sectional TEM observations, the
thickness of the well layer was approximately 2 nm, and the thickness of
the barrier layer was approximately 7.5 nm. In order to systematically
investigate changes in the emission characteristics associated with the
changes in the In composition of the GaInN active layer, we varied the
In composition alone to prepare 3 samples with (a) x = 0.06 (λp = 410
nm), (b) x = 0.18 (λp = 445 nm), and (c) x=0.22 (λp = 475 nm). Beyond
the In composition of the quantum wells, the three samples were
identical in terms of the LED structure and crystal growth conditions.
The emission wavelength shown here is the emission peak wavelength
obtained when an electric current (20 mA/cm^2^) was injected into the
LEDs.

#### Comparison of LED efficiencies

![](media/image41.png){width="3.9583333333333335in"
height="3.1875in"}FIG. 3.4.1-(1) shows a comparison of the efficiencies
obtained when an electrical current (20 A/cm^2^) was injected into the
three LEDs. The values shown here are relative values of the external
quantum efficiency normalized to the efficiency of the LED with x = 0.18
(b). Because the highest efficiency among the three samples was obtained
at x = 0.18 (445 nm) (b), it is believed that under the growth
conditions used in the experiment, in the wavelength range from 410 nm
to 175 nm, the peak efficiency is achieved by an LED with an emission
wavelength of approximately 450 nm. The differences in the efficiencies
obtained at this injection current correspond to the differences in the
PL emission intensity, and are believed to be due to differences in the
internal quantum efficiencies of GaInN quantum wells with different In
compositions.

Figure 3.4.1-(1) Relative external quantum efficiencies of LED samples
with different In compositions ((a) x = 0.06, 410 nm, (b) x = 0.18, 445
nm, (c) x = 0.22, 475 nm). Normalized to efficiency at 445 nm.

#### Temperature dependence of emission lifetime

Data on the PL excitation intensity dependence (emission peak energy
shift, spectrum half-width) and time resolved PL measurements at room
temperature were summarized in the 1999 Achievement Report.

Next, in order to investigate the localization state of carriers in each
sample in detail we examined the temperature dependence of the emission
lifetime. Measurements were carried out over a wide temperature range,
from 7K to 540K. FIG. 3.4.1-(2) shows the dependence of the emission
lifetime on the emission energy at various temperatures for sample (a)
with x = 0.06. In this case, we defined the emission lifetime as the
time required for the emission intensity to reach 1/e of the peak value,
which appears to be somewhat different from the decay lifetime obtained
by fitting. In particular, we believe that there will be different
values in the decay waveform, which cannot be expressed using **a**
single exponential function. However, here we are most interested in
temperature-dependent emission lifetime changes and this won\'t present
a problem for a discussion of the trend of the changes.

![](media/image42.png){width="3.5680555555555555in"
height="2.904861111111111in"}Figure 3.4.1-(2) Dependence of emission
lifetime on the emission energy at various temperatures in the range of
from 7K to 300K for LED (a) with x = 0.06 (410 nm).

As can be seen from the figure, the dependence of the emission lifetime
on the emission energy exhibits a characteristic behavior depending on
the temperature. It is easier to understand if it is plotted in the same
manner as in FIG. 3.4.1-(3). FIG. 3.4.1-(3) shows the temperature
dependence of the emission lifetime of the three LED samples at various
emission energies of the emission spectrum ranging from low energies to
high energies.

First of all, referring to figure (a) with x = 0.06,
temperature-dependent changes in the emission lifetime can be divided
into three regions: I, II, and III. In region I (7K\~90K), the
difference between the emission lifetime on the high-energy side and low
energy side of the emission spectrum increases with increasing
temperature. In region II (120K\~240K), the large difference between the
emission lifetime on the high-energy side and low energy side decreases
with increasing temperature and the curve of emission lifetime versus
emission energy becomes almost flat. In region III (240K\~540K), the
emission lifetime, whose curve is now almost flat, rapidly becomes very
short as the temperature increases.

![](media/image43.png){width="5.982638888888889in"
height="3.3506944444444446in"}

Figure 3.4.1-(3) Dependence of emission lifetime on temperature at
various emission energies in the low energy portion and high energy
portion of the emission spectrum for various LEDs. (a) x = 0.06 (410
nm), (b) x = 0.18 (445 nm), (c) x = 0.22 (475 nm).

It is believed that the temperature-dependent changes in the emission
decay time can be explained by changes in the localization state of
carriers. Although Si was added to the barrier layers of these LED
samples in the amount of approximately 2\~3×10^18^ cm^-3^ and the
influence of the piezoelectric effects in the quantum wells could be
considered moderated to a certain degree, it is believed that the
influence of the piezoelectric effects was still affecting the emission
lifetime differences between the samples with different In compositions.
However, because the temperature dependence of the piezoelectric effects
is believed to be negligibly small, we think that the trend of the
temperature dependence of the emission lifetime can be discussed by
primarily taking into consideration the changes in the localization
state of carriers.

![](media/image44.png){width="3.59375in" height="3.0in"}One possible
model is illustrated in FIG. 3.4.1-(4), which is explained hereinbelow.
First of all, in region I, as the temperature increases from low
temperatures, in the localization level distribution inside the band,
carriers possessing particularly high energy levels get delocalized
first, and the percentage of carriers captured into non-radiative
recombination centers increases. Thus, at first, high-energy lifetime
becomes shorter as the temperature rises. Furthermore, in region II,
which is characterized by an elevated temperature, carriers in a deep
localized state are thermally excited and undergo a transition to a
high-energy level. As a result, observation shows that, conversely, the
lifetime on the high-energy side tends to increase. In region III, the
lifetime becomes shorter for all emission energies, which is believed to
indicate that almost all carriers that were in a localized state have
been delocalized by thermal excitation and the proportion of carriers
captured into non-radiative recombination centers such as dislocations
and point defects rapidly increases.

Figure 3.4.1-(4) Conceptual diagram of model used for explanation of
emission lifetime changes in temperature regions I, II, III.

As concerns samples with different In compositions, in case of (b) with
x = 0.18 and (c) with x = 0.22, which are shown in FIG. 3.4.1-(3),
basically the same three temperature regions are observed and it is
believed that they can be explained using the same model. Here, the
discontinuity of the data observed at the temperature of 300K is due to
a difference in the excitation intensity associated with different
experimental conditions in the temperature region below 300K and above
300K. As can be seen from FIG. 3.4.1-(3), the temperatures corresponding
to the boundaries between the regions increase as the In composition
increases. In particular, the temperature that marks the transition from
region II to region III is believed to be the temperature at which a
transition from a localized state of carriers to a delocalized state
takes place. If we designate this temperature as T~d~, then in case of
(a) T~d~ = 240K, and in case of (b) with x = 0.18 and (c) with x = 0.22
the value of T~d~ increases to approximately 450K\~510K.

Next, in order to investigate the physical parameters governing the
changes in the temperature dependence of the emission lifetime shown in
FIG. 3.4.1-(3), we calculated the temperature dependence of the carrier
lifetime from a rate equation based on a simple model assuming a density
N~def~ of non-radiative recombination centers and an energy depth E~loc~
of localized level in the band, as shown in FIG. 3.4.1-(5). First of
all, we used the values of FIG. 3.4.1-(3) to plot the temperature
dependence of the emission lifetime of the three samples at the emission
peak energy only in FIG. 3.4.1-(6). In case of (a) with x = 0.06, the
emission lifetime rapidly decreased starting in the vicinity of 240K,
whereas samples (b) and (c), which had different In compositions,
exhibited a flat characteristic up to 400K. What we would like to point
out here is that when (b) with x = 0.18 and (c) with x = 0.22 were
compared, the degree of decrease in emission lifetime at temperatures
above T~d~ was conspicuously larger for (c) with x = 0.22.

![](media/image45.png){width="5.990972222222222in"
height="2.7430555555555554in"}

Left: Figure 3.4.1-(5) Simulation model based on presence of
non-radiative recombination centers and localization levels inside band.

Right: Figure 3.4.1-(6) Temperature dependence of emission lifetime at
emission peak energy for different LEDs.

An explanation of the simulation results is provided next. However, here
we will focus our attention only on the energy depth E~loc~ of the
localized level and the density N~def~ of the non-radiative
recombination centers, while substituting values obtained from
literature or values considered appropriate for other parameters used in
the rate equation. Here, we investigated the contribution of the two
parameters, E~loc~ and N~def~, to temperature-dependent changes in the
emission lifetime based on a comparison between the data obtained by
measurement and the results of calculation of the temperature dependence
of the carrier lifetime associated with changes in E~loc~ and N~def~,
and did not quantitatively estimate the parameters by fitting etc.

FIG. 3.4.1-(7) represents results of calculation of the carrier lifetime
at the localized level in the case where E~loc~ was varied and N~def~
remained constant at N~def~ = 1×10^18^ cm^-3^. From the figure, it is
evident that as the energy depth of the localized level increases, the
temperature at which a rapid decrease in the emission lifetime starts
increases. The temperature at which the decrease in the emission
lifetime starts corresponds to the temperature T~d~ (the temperature
marking the boundary between region II and region III) defined in FIG.
3.4.1-(3), which leads us to believe that the energy depth of carrier
localization increases with the increase in the In composition.

In addition, FIG. 3.4.1-(8) shows the results of carrier lifetime
calculations, in which N~def~ was varied at a constant E~loc~ = 150 meV.
From this figure, is evident that as N~def~ increases, the tendency
towards emission lifetime decay associated with a temperature increase
in the region above T~d~ increases. Based on the fact that, as shown in
FIG. 3.4.1-(6), the tendency towards emission lifetime decay at
temperatures higher than 400K is greater for sample (c) with x = 0.22
than for sample (b) with x = 0.18, it is believed that the density of
non-radiative centers in sample (c) with x = 0.22 is increased. Because
the crystal growth conditions of the three samples are practically the
same with the exception of the In composition, the threading dislocation
density of the crystals is a constant. Therefore, the increase in the
non-radiative recombination center density is believed to be due to an
increase in the point defect density associated with the increase in the
In composition.

![](media/image46.png){width="5.9875in" height="2.3875in"}

Left: Figure 3.4.1-(7) Temperature dependence of emission lifetime
obtained by varying localization level energy depth (E~loc~) at constant
non-radiative recombination center density (N~def~ = 1×10^18^ cm^−3^).

Right: Figure 3.4.1-(8) Temperature dependence of emission lifetime
obtained by varying non-radiative recombination center density (N~def~)
at constant localization level energy depth (E~loc~ = 150 meV).

The fact that sample (b) with x = 0.18 exhibits the maximum value at the
LED efficiency shown in FIG. 3.4.1-(1) and the result that its emission
lifetime remains practically constant all the way to the high
temperature region are in agreement from a qualitative standpoint.
Generally speaking, it is believed that if the In composition increases
and the energy of carrier localization increases, the percentage of
carriers captured into defects decreases and the quantum efficiency
increases, but because point defects exhibit a tendency to increase with
increasing In composition, in case of sample (c) with x = 0.18 the
efficiency drops in comparison with sample (b) with x = 0.18. Based on
the above, it is believed that the localization energy and the point
defect density act as the controlling parameters of the quantum
efficiency of the LEDs.

### Investigation of Emission Lifetime Characteristics of LEDs with Different Emission Efficiencies

In 3.4.1 we discussed the emission lifetime characteristics and the
carrier localization state. Here, based on these results and the results
of measurements of the temperature dependence of the emission lifetime
for LED samples with different emission efficiencies, we will discuss
the influence of the carrier localization state on the emission
efficiency.

Two LED samples (Sample A and Sample B) were prepared, which had an
emission wavelength of 470 nm, an identical quantum well structure, and
different emission efficiencies. Their basic structure was identical to
the structure of the samples investigated in 3.4.1, the quantum well
layer thickness being approximately 2 nm, the barrier layer thickness
being approximately 7.5 nm, and the concentration of Si added to the
barrier layers being approximately 2\~3×10^18^ cm^−3^. The difference
between Samples A and B consisted in the n-type layers that preceded the
growth of the active layers.

The current-voltage characteristics (I-V characteristics) of the two
LEDs are shown in FIG. 3.4.2-(1). Because there are no significant
differences between the I-V characteristics of Samples A and B, the
differences in the structure of the n-type layers obviously do not
influence their electrical transmission characteristics. FIG. 3.4.2-(2)
shows the relative external quantum efficiencies of Samples A and B as
functions of the injection current density. The quantum efficiencies
shown in the figure are used to compare the relative differences between
the two samples, with the values indicated being different from the
external quantum efficiencies obtained when measurements are carried out
using ordinary resin moldings. In both cases the position of the peaks
of efficiency is at approximately 40\~50 A/cm^2^, with the efficiency of
sample B being approximately 20% higher in comparison with sample A at
the position of the efficiency peaks. In addition, the efficiency of
sample B is approximately twice as high as that of sample A in the low
current region starting at 5 A/cm^2^.

![](media/image47.png){width="5.985416666666667in"
height="2.1534722222222222in"}

Left: Figure 3.4.2-(1) Voltage-current density characteristics of
Samples A and B.

Right: Figure 3.4.2-(2) External quantum efficiency-current density
characteristics of Samples A and B.

FIG. 3.4.2-(3) shows the dependence of the PL spectrum peak position on
the excitation intensity under weak excitation (300 W/cm^2^) using a
He-Cd laser and strong excitation (10k\~2×10^4^ kW/cm^2^) using a Xe-Cl
laser. In this excitation intensity range, the emission peak of sample A
undergoes a five-nm shift towards shorter wavelengths while a
considerably larger peak shift of about 10 nm is observed in case of
sample B.

![](media/image48.png){width="4.03125in" height="3.6354166666666665in"}

Figure 3.4.2-(3) Dependence of PL emission peak wavelength on excitation
intensity for sample A and sample B.

Next, we carried out measurements of the temperature dependence of the
emission lifetime characteristics based on time-resolved PL
measurements. FIG. 3.4.2-(4) shows the dependence of the emission
lifetime of samples A and B at room temperature on the emission energy.
In both cases the emission lifetime is shorter in the high-energy
portion and longer in the low-energy portion, and it is expected that
carriers undergo relaxation (localization) from the high-energy side to
the low-energy side. In particular, in case of sample B, the energy
dependence of the emission lifetime is more noticeable, with the
emission lifetime on the low-energy side being as long as 25 ns.

![](media/image49.png){width="5.9847222222222225in"
height="3.0340277777777778in"}

Figure 3.4.2-(4) Dependency of emission lifetime on emission energy at
room temperature for sample A and sample B.

Next, FIG. 3.4.2-(5) shows the dependence of the emission lifetime on
temperature (7K\~300K) at various emission energies of the emission
spectrum ranging from the low-energy side to the high-energy side. The
energy interval between the curves in figure is 10 meV.

First of all, in the case of sample A, in the range between low
temperatures and approximately 150K, the energy dependency of the
emission lifetime increases, and when the temperature increases further,
the energy dependence decreases. On the other hand, in case of sample B,
with the exception of the data discontinuity at 120K, the energy
dependency of the emission lifetime increases practically monotonically.
Therefore, it is evident that, in this temperature range, both
temperature regions I and II defined in FIG. 3.4.1-(3) are observed in
case of sample A and only region I is observed in case of sample B. The
model discussed in Section 3.4.1 points to a considerable energy depth
of carrier localization in case of sample B.

This phenomenon qualitatively corresponds to the large peak shift
observed in sample B. Although the mechanism whereby the difference in
the n-type underlayer structures of sample A and B exerts influence on
the localization energy is unclear, it is believed that in the highly
efficient sample B, from the standpoint of energy, carriers are
localized deeper, and their transition to non-radiative recombination
centers is suppressed.

![](media/image50.png){width="5.989583333333333in"
height="2.6680555555555556in"}

Figure 3.4.2-(5) Dependency of emission lifetime on temperature at
various emission energies ranging from low-energy portion to high-energy
portion of light emission for sample A and sample B.

### Investigation of In-Plane Distribution of Light Emission

In 2000 we introduced a near-field probe PL system, shown in FIG.
3.3.3-(1), in order to measure the in-plane distribution of light
emission with high spatial resolution. Here, in order to obtain basic
data, we carried out measurements using typical multiple quantum well
LEDs with an emission wavelength of 460 nm. For excitation, we used a
He-Cd laser (441 nm), which allowed for selective excitation of the
quantum well active layer in the LED samples. The PL mapping
measurements were conducted at room temperature using 20×20 points in a
4 *μ*m×4 *μ*m region.

![](media/image51.png){width="3.375in" height="3.875in"}Figure 3.4.3-(1)
(a) Distribution of PL emission intensity, and (b) distribution of PL
peak wavelength in a 4 μm×4 μm region of a blue LED.

The emission intensity distribution (a) and the emission peak wavelength
distribution (b), which were calculated from the PL spectra obtained in
each measurement position, are shown in FIG. 3.4.3-(1). First of all, we
examined the in-plane distribution of the emission intensity shown in
(a). It was confirmed that the instability of the system at the time of
measurement was insignificant and it is believed that the distribution
measured clearly showed the in-plane distribution of the emission
intensity. The intensity distribution was observed as light and dark
regions with a size of approximately 0.3\~0.5 *μ*m. The respective
spectra in positions A, B, and C inside the measurement region are shown
in FIG. 3.4.3-(2). The figure shows that, in comparison with the
position of maximum intensity, the PL intensity in low-level positions
constituted approximately 25%.

Also, in case of the peak wavelength distribution (b), a distribution of
approximately 8 nm could be seen in-plane, and it could be appreciated
from a comparison with the emission intensity distribution (a) that in
regions showing emission at long wavelengths the intensity of light
emission tended to be higher. These results indicated that there was a
clear in-plane distribution of light emission of a submicron size.

![](media/image52.png){width="3.28125in"
height="3.0416666666666665in"}The data shown here are preliminary data,
and in order to correctly interpret the measurement data, it is
necessary to accumulate further data including confirmation of
reproducibility. At present, we are conducting evaluation of samples
with different efficiencies or wavelengths and carrying out detailed
investigations into the relationship between the in-plane distribution
of light emission and the efficiency of LEDs.

Figure 3.4.3-(2) Emission spectra in various positions. "A," "B," and
"C" correspond to points shown in FIG. 3.4.3-(1).

### Reverse Bias PL Characteristics of Si-Doped Quantum Well LEDs

#### Sample structure

As mentioned in the 1999 Report, it was found that the addition of Si to
quantum wells was important in the elimination of the internal electric
field applied to quantum wells \[5\]. However, because in the case of
LED structures the addition of Si to quantum wells exerts influence on
the band structure of the pn junction, it is necessary to investigate
the influence of Si doping with account taken of the device
characteristics. Thus, we conducted an investigation into the effects of
the addition of silicon to quantum wells in LED structures.

The samples were multiple quantum well LEDs with an emission center
wavelength of approximately 510 nm, a quantum well width of 2.5 nm, a
barrier layer width of 15 nm, and a quantum well number of four. The In
composition of the quantum well layer was estimated to be x=0.23. We
prepared sample C, in which the concentration of silicon added to the
barrier layer was 4×10^17^ cm^−3^, and sample D with a concentration of
4×10^18^ cm^−3^. The quantum well layer was not doped. In addition, the
concentration of Si (donors) in the n-type layers of the LEDs was
approximately 5×10^18^ cm^−3^ and the concentration of Mg (acceptors) in
the p-type layer was approximately 5×10^19^ cm^−3^ cm_sup-3.

#### Measurement of depletion layer width

In order to estimate the external electric field applied to the quantum
wells under reverse bias, we estimated the depletion layer width from
measurements of the depletion layer capacitance. FIG. 3.4.4-(1) shows
the results (a) of measurements of the depletion layer capacitance under
reverse bias for samples C and D. In addition, (b) shows the depletion
layer width estimated from the depletion layer capacitance based on the
formula:

$L = \varepsilon_{0}\varepsilon\frac{S}{C}$ (6)

Here, the area of the pn junction was S = 9×10^-4^ cm^2^, and the value
for GaN, ε=8.9, was used as the dielectic constant. The results show
that the higher the donor concentration of the quantum wells, the
narrower the depletion layer width, and, as a result, the higher the
junction capacitance.

![](media/image53.png){width="5.986805555555556in"
height="3.6444444444444444in"}

Figure 3.4.4-(1) (a) Changes in pn junction depletion layer capacitance,
and (b) changes in depletion layer width under reverse bias in LED
samples C and D.

The depletion layer width of samples C and D under zero bias was,
respectively, approximately 82 nm and 47 nm. In the case of sample C the
width was practically equal to the multiple quantum well active layer
width of 85 nm, which shows that the quantum well region had been
practically depleted. On the other hand, because in case of sample D the
depletion layer constituted approximately 50% of the quantum well
region, we expected half of the quantum wells to depleted, and half of
them to be n-type.

#### PL measurements under reverse bias

Changes in the PL spectrum associated with the application of a reverse
bias are shown in FIG. 3.4.4-(2). The wavelength of the laser used for
excitation was 400 nm, which was the condition of selective excitation
for the quantum wells of the sample. The measurements were carried out
at 77 K. Spectrum measurements were carried out under conditions
preventing the influence of the interference. A shift towards the
high-energy side of the spectrum associated with the applied bias is
clearly observed in the case of sample C. On the other hand, in the case
of sample D, the measurements were carried out under the same
conditions, but a large spectral width was observed at 0V and multiple
peaks appeared as the applied bias increased. However, no noticeable
peak shift was observed. Changes in the emission peak energy associated
with the applied external electric field are shown in FIG. 3.4.4-(3).
Here, the bias voltage is applied to the depletion layer width estimated
in FIG. 3.4.4-(1) and the x-axis of the diagram represents the external
electric field applied to the quantum well layer. In addition, as far as
the bias of the external electric field is concerned, the forward
direction of the pn junction (from the surface to the substrate) was
considered as positive. In other words, the gradient of the applied
external electric field was the same as the gradient of the internal
electric field.

![](media/image54.png){width="6.010416666666667in" height="4.0in"}

Figure 3.4.4-(2) Dependence of PL spectra one reverse bias at 77 K. (a)
Sample C (4×10^17^ cm^-3^), (b) Sample D (4×10^18^ cm^-3^).

In the case of sample C, the emission peak exhibited a shift of
approximately 110 meV towards the high-energy side when the external
electric field ranged from zero to approximately −0.4 MV/cm, whereas in
case of sample D there was a shift of approximately 25 meV towards the
low-energy side when the external electric field ranged from zero to
approximately −0.7 MV/cm. Figure 3.4.4-(4) shows a conceptual diagram of
changes in the band structure associated with the application of an
external electric field in sample C. Because in sample C the emission
peak shifts towards the high-energy side following the application of an
external electric field of the same direction as the internal electric
field, it is believed that the band in the quantum wells biased by the
internal electric field was close to a flat band. This phenomenon is in
good agreement with the previous report \[7\], in which it was indicated
that the piezoelectric field due to the strain within the quantum wells
of GaInN was oriented in a direction opposite to the direction of the
internal electric field (from the surface to the substrate).

Next, in order to estimate the piezoelectric field present in the
quantum wells, we performed fitting to transition energy obtained by
calculation. Results obtained for sample C are shown in FIG. 3.4.4-(6).
However, as for calculations of the transition energy associated with
the application of the bias, at present we are conducting further
detailed investigations and the results presented here are merely
preliminary results. For fitting, we used an In composition of x = 0.23,
and the estimated piezoelectric constant was a value constituting 40% of
the value reported by Bernardini *et al*. \[8\]. Based on this, the
piezoelectric field in the quantum wells was determined to be
approximately 1.48 MV/cm. Here, the estimated value is close to the
values reported previously \[7, 9, 10\].

![](media/image55.png){width="5.995138888888889in"
height="2.5527777777777776in"}

Left: Figure 3.4.4-(3) PL peak energy shift versus applied electric
field for LED sample C and D.

Right: Figure 3.4.4-(4) Schematic diagram illustrating changes in band
structure associated with application of external electric field in
quantum wells of sample C with low Si doping.

On the other hand, in case of sample D with a high silicon doping
concentration, as expected based on the measurements of the depletion
layer width, the electric field in almost half of the quantum wells
under zero bias is canceled by free electrons. However, it is believed
that in the remaining half of the quantum wells depletion takes place
and the band is biased by the internal electric field. It is believed
that these quantum wells possess different emission transition energies,
and as a result, under zero bias, as shown in FIG. 3.4.4-(2), a wide
spectrum was observed. When the reverse bias is applied, in quantum
wells that have been depleted under zero bias, the transition energy
becomes higher due to the cancellation of the internal electric field
and in those quantum wells that have not been depleted (in which the
internal electric field has been screened by free electrons) depletion
takes place with the increase in the reverse bias and the emission
transmission energy becomes lower. Therefore, it is believed that there
was practically no change in the overall emission peak energy.

![](media/image56.png){width="3.1875in" height="3.0833333333333335in"}

Figure 3.4.4-(5) Results of fitting of transition energy versus external
electric field for quantum wells of sample C with low Si doping.

FIG. 3.4.4-(6) shows the PL intensity (a), photocurrent (b), and dark
current (c) observed when a reverse bias is applied to the two samples.
Based on the band structure model shown in FIG. 3.4.4-(4), it was
expected that in case of sample C the internal electric field would be
canceled and the PL intensity would increase with the increase in the
reverse bias. However, in the same manner as in the previously reported
results \[7\], the PL intensity decreased with the increase in the
applied reverse bias. It is believed that this decrease in the PL
intensity can be understood based on an increase in the photocurrent,
where carriers excited inside the quantum wells are caused to drift by
the electric field and the proportion of the carriers contributing to
the photocurrent increases. In addition, the increase in the dark
current associated with the reverse bias in the entire photocurrent is
negligibly small.

![](media/image57.png){width="5.988194444444445in"
height="2.8916666666666666in"}

Figure 3.4.4-(6) Changes in (a) PL intensity, (b) photocurrent, (c) dark
current versus applied reverse bias for LED samples C and D.

#### Time resolved PL measurements under bias

Subsequently, we conducted measurements of the PL lifetime associated
with the application of the reverse bias. The measurements were
conducted at 77K using an excitation wavelength of 400 nm and an
excitation intensity of 3.5 *μ*J/cm^2^. FIG. 3.4.4-(7) shows changes in
the time-resolved spectrum of the PL peak and FIG. 3.4.4-(8) shows the
emission lifetime. The emission lifetime of sample C under zero bias was
as long as 52 ns and became as short as approximately 10 ns when a bias
of −10 volts was applied. On the other hand, when a bias or −10 volts
was applied in case of sample D, the emission lifetime, which had been
approximately 10 ns under zero bias, reached approximately 14 ns, that
is, it became slightly longer.

![](media/image58.png){width="5.988888888888889in"
height="3.209722222222222in"}

Figure 3.4.4-(7) Dependence of time resolved spectra on reverse bias for
(a) sample C and (b) sample D.

![](media/image59.png){width="3.1145833333333335in"
height="3.0520833333333335in"}It is believed that the factors that
contribute to the changes in the carrier lifetime associated with the
application of the reverse bias are primarily (1) changes in the
emission lifetime associated with the changes in that internal electric
field in quantum wells, (2) the increase in the photocurrent due to
carrier leakage from inside the quantum wells, and (3) the dependence of
the non-radiative recombination process on the bias. As shown in FIG.
3.4.4-(6), there are no extreme differences between the bias-dependent
changes in the PL intensity and the photocurrent of the two samples,
and, for this reason, it is believed that the conspicuous differences in
the PL lifetime changes shown in FIG. 3.4.4-(8) are governed by the
contribution of (1) the internal electric field (changes in the emission
lifetime due to the biasing of the band).

The above described PL measurements under bias permits the conclusion
that the addition of silicon to the active layer is important in
canceling the internal electric field but because it affects the
widening of the depletion layer in the pn junction, the determination of
its optimal value is extremely important.

Figure 3.4.4-(8) Dependence of PL lifetime on reverse bias for (a)
sample C and (b) sample D.

## Summary

Just as in 1999, in 2000 we continued systematic investigations
concerning the emission characteristics of GaInN/GaN quantum well
structures in connection with the crystal growth parameters or
structure. The results are summarized below.

1\) Based on the investigations of the emission properties of GaInN
conducted by numerous research organizations, it is believed that the
key to the elucidation of the emission mechanism consists in the effect
of spatial localization of carriers in GaInN. The emission
characteristics (in particular, carrier localization effects) of GaInN
are believed to be strongly dependent on the In composition, and for
this purpose it is important to conduct a systematic evaluation of
samples with different In compositions. For this reason, we conducted a
systematic investigation into light emission characteristics of
GaInN/GaN quantum well LEDs with different In compositions (emission
wavelengths) over a wide temperature range (7K\~540K). As a result, it
was shown that the temperature dependence of the emission lifetime was
split into three temperature regions and this temperature dependence
could be explained by changes in the localization state of the carriers.
In addition, based on simulation using a simplified localization level
model, it was found that, with increasing In composition, the
localization energy of the carriers increases, and, on the other hand,
the point defect density increases as well. Based on a correlation
between the In composition-induced differences in the temperature
dependences of the emission lifetime and the quantum efficiencies of the
LEDs, it is believed that the localization energy and the point defect
density act as the controlling parameters of LED efficiency.

2\) In order to examine the relationship between the carrier
localization characteristics and the quantum efficiency, we conducted PL
and time-resolved PL measurements on two LED samples with identical
emission wavelengths and different quantum efficiencies. As a result, it
was found that in the sample that had a high quantum efficiency there
was a large shift of the PL peak towards the high-energy side associated
with the excitation intensity, a considerable dependence of the emission
lifetime on the energy, and a long emission lifetime at the emission
peak. Based on these experimental results, it is believed that in the
sample that had a high quantum efficiency, carriers were in a deeper
localization state, and their transition to non-radiative recombination
centers was suppressed. It is believed that the deep localization state
of the carriers is the main reason why the sample exhibited the high
quantum efficiency.

3\) In order to measure the in-plane distribution of emission, this year
we introduced a near-field probe-based PL system. First of all, we
collected basic data using typical multiple quantum well LED samples
with a wavelength of 460 nm. As a result, we found that an emission
intensity distribution with a size of about 0.3\~0.5 *μ*m could be
observed in a 4 *μ*m × 4 *μ*m measurement region and the PL intensity in
low-level positions was approximately 25% less than in the position of
maximum intensity. The data provided here are preliminary data, and it
is necessary to accumulate further data, including confirmation of
reproducibility, in order to correctly interpret the data obtained by
the measurements. Currently, we are conducting detailed investigations.

4\) As concerns the effects of Si-doping in the quantum well active
layer, based on reverse bias PL and time resolved PL measurements, we
conducted evaluation from the standpoint of the quantum well band
structure, including the piezoelectric effects. We measured the
capacitance of the depletion layer under bias and, based on the
calculated value of the depletion layer width, estimated the external
electric field applied to the quantum wells and measured spectral
changes depending on the applied external electric field. As a result,
in the sample with low Si doping (4×10^17^ cm^−3^), we observed a shift
of the emission peak towards the high-energy side associated with the
application of the external electric field, a long emission lifetime (52
nm) under zero bias, and a decrease in the emission lifetime associated
with the application of the electric field. Based on the experimental
results concerning the peak shift and the results of calculation of the
transition energy, it was estimated that for an In composition of x =
0.23 the piezoelectric field applied to the quantum wells was about 1.48
MV/cm. On the other hand, in the sample that had a higher concentration
of Si dopant (4×10^18^ cm^−3^), there was observed a slight shift of the
emission peak towards the low-energy side. In addition, because the
emission lifetime was approximately 10\~14 ns irrespective of the bias,
the electric field generated by the piezoelectric effect was believed to
be canceled by the carriers generated by the donor impurities. The
addition of Si to the active layer is important for canceling the
internal electric field, but because it affects the spread of the
depletion layer in the pn junction, the determination of its optimal
value is important for highly efficient current injection.

## Future plans

In 2001, we will continue investigations conducted in the past year and
use selective excitation PL to examine the Stokes shift and the depth of
the localization level or the spatial distribution of quantum wells in a
two-dimensional plane, as well as investigate the origin of carrier
localization. Furthermore, we will examine the relationship between the
LED sample structure, crystal growth conditions and carrier localization
characteristics, and investigate ways to obtain higher efficiencies.

Also, in the last part of the project, based on the findings obtained up
till now, we will examine the relationship between the crystal growth
parameters and emission characteristics of light-emitting layers in the
near ultra-violet region, which is the goal of the current project, and
investigate methods for obtaining high efficiencies. To this end, we
will conduct active exchange of information and samples between the
Epitaxial Group and Yamaguchi University. In particular, by comparing
the crystals obtained in the course of the current project with
ultraviolet LEDs from other companies, we will try to clarify the
parameters that contribute to high-efficiency emission in the
ultraviolet region.

Specifically, we well conduct our research by focusing it on:

-   differences due to heteroepitaxy and homoepitaxy in the influence of
    crystal dislocations and crystal strain on the emission
    characteristics (efficiency) and differences in the In doping
    effects

-   effects of doping with impurities in the UV emission active layer

-   optimization of the doping profile based on simulation

to obtain findings useful in the optimization of UV LED structures.

## References

\[1\] S.D. Lester, F. A. Ponce, M. G. Craford and D. A. Steigerward,
Appl. Phys. Lett., 66 (1995) 1249.

\[2\] Y. Narukawa, Y. Kawakami, Sz. Fujita, and S. Nakamura, Phys. Rev.
B55, R1938 (11997); Y. Narukawa, Y. Kawakami, M. Funato, Sz. Fujita, Sg.
Fujita and S. Nakamura, Appl. Phys. Lett. 70 981 (1997).

\[3\] S. Chichibu, T. Azuhata, T. Sota,and S. Nakamura, Appl. Phys.
Lett. 70, 2822 (1997); S. Chichibu, T. Azuhata, T. Sota and S. Nakamura,
Appl. Phys. Lett. 69, 4188 (1996).

\[4\] T. Taguchi, H. Kudo, H. Ishibashi, R. Zheng, and Y. Yamada,
Conference on Solid State Devices and Materials, Tokyo, 1999, present
patent. 194\~195.

\[5\] S. Watanabe, N. Yamada, Y. Yamada, T. Taguchi, T. Takeuchi, H.
Amano, and I. Akasaki, Phys. Stat. Sol. (b) 216, 335 (1999).

\[6\] S. Watanabe, M. S. Minsky, N. Yamada, T. Takeuchi, R. Schneider,
C. Sasaki, M. Iwata, Y. Yamada and T. Taguchi: Proc. IWN2000, IPAP Conf.
Series 1 532 (2000).

\[7\] T. Takeuchi, C. Wetzel, S. Yamaguchi, H. Sakai, H. Amano, and I.
Akasaki, Appl. Phys. Lett., 73, 1691 (1998).

\[8\] F. Bernardini, V. Fiorentini and D. Vanderbilt, Phys. Rev. B56,
10024 (1997).

\[9\] C. Wetzel, S. Nitta, T. Takeuchi, S. Yamaguchi, H. Amano and I.
Akasaki, MRS, MIJ-NSR Volume 3, Article 31 (1998).

\[10\] S. Chichibu, T. Azuhata, T. Sota, T. Nukai and S. Nakamura, Proc.
IWN2000, IPAP Conf. Series 1 528 (2000).

# Basic Research Concerning GaN Single Crystals Grown by the Solution Growth Method

## Achievements in 1998 and 1999

High-pressure solution growth (HP-SG: High-Pressure Solution Growth) is
a crystal growth method which permits growth of bulk GaN single crystals
of the highest quality and allows for obtaining bulk GaN single crystals
of crystal quality so high that the density of dislocations in the
crystals is lower than 10^4^ cm^-2^ \[1-3\]. In addition, it is known
that homoepitaxial thin films grown on such crystals, as well as
light-emitting diodes (LED: Light Emitting Diode) fabricated using such
crystals possess extremely superior characteristics in comparison with
heteroepitaxial thin films grown on sapphire substrates \[4,5\].
However, mass production based on the HP-SG method presents difficulties
because under large temperature gradients of 50\~150°C/cm, growing a
crystal requires six to ten days. Another problem is that the size of
the crystal is 104 mm^2^ at the maximum, which is small for industrial
use.

In the course of the present study we developed a new
pressure-controlled solution growth method (PC-SG: Pressure-Controlled
Solution Growth) with a view to develop a technology for inexpensively
mass producing high-quality large bulk GaN single crystal substrates
possessing a crystal quality equal to or higher than that obtained by
the HP-SG method, and started research concerning the growth of bulk GaN
single crystals based on this method. The evaluation of the
characteristics of the resultant bulk GaN single crystals was carried
out in cooperation with the Taguchi Laboratory at Yamaguchi University.

In 1998, we carried out investigations regarding the crystal growth
process and crystal growth conditions using an ultrahigh HIP apparatus
and successfully grew bulk GaN single crystals with a surface diameter
of 7 mm, which showed that it was possible to grow bulk GaN single
crystals by the PC-SG method. We found that the resultant GaN single
crystals possessed excellent crystal characteristics. In addition, we
investigated the specifications of the newly introduced ultrahigh
pressure crystal growth apparatus.

In 1999, in order to substantiate the effectiveness of the PC-SG method,
we obtained basic data regarding single crystal growth conditions and
investigated factors exerting influence on the speed of growth. We
examined the mechanism of growth of GaN crystals and showed that the
process of diffusion of nitrogen atoms in the Ga melt is the
rate-determining process. Based on these research results, we conducted
bulk GaN single crystal growth and obtained GaN single crystals with a
diameter of 12 mm, which exceeded the target for 2001 (10 nm on the side
or larger). These bulk GaN single crystals exhibited structural
characteristics superior to those of GaN single crystals grown
heteroepitaxially by the metal organic chemical vapor deposition method
(MOCVD: Metal Organic Chemical Vapor Deposition), etc., and optical
characteristics exceeding those of GaN single crystals grown by the
HP-SG method. These results demonstrated the effectiveness of the PC-SG
method as a method that can be used for growing bulk GaN single
crystals. In addition, we installed an ultrahigh pressure growth
apparatus at the Japan Ultrahigh Temperature Materials Research Center
(Inc.) and completed preparation for full-scale research starting from
2001.

## Summary of Research and Development in 2000

In 2000, we conducted research aimed at the clarification of effects
produced by the degree of supersaturation of nitrogen in the Ga melt on
the growth of bulk GaN single crystals under the PC-SG method. We also
investigated the effects produced by seed crystals, crystal growth time,
equilibrium pressure, and pressure increase rate used when the pressure
of nitrogen is increased in excess of the equilibrium pressure to the
final crystal growth pressure, in order to gain an understanding of the
conditions required for growing large bulk GaN single crystals of
excellent crystal quality.

By using a lower pressure increase rate in the ultrahigh HIP apparatus,
we obtained large bulk GaN single crystals of excellent surface
condition. GaN single crystals with a size of 334 mm^2^ (which
corresponds to a diameter of 20.6 mm) were obtained in the ultrahigh
pressure crystal growth apparatus at a pressure increase rate of 69
MPa/h. These data represent the top size of GaN single crystals grown
under the solution growth method. Although GaN single crystals of
excellent surface condition were obtained at a pressure increase rate of
9 MPa/h, the size of the crystals was approximately 18 nm, and the
effect of GaN single crystal enlargement by means of reducing the
pressure increase rate was not observed. Based on the examination on the
results and the relationship between the nucleation density of the GaN
crystals and temperature fluctuations, it was found that a reduction in
the pressure increase rate was an efficient measure for obtaining bulk
GaN single crystals of excellent surface condition and that, in order to
obtain large bulk GaN single crystals, it was necessary to reduce
temperature fluctuations and try to achieve a reduction in the
nucleation density of the GaN crystals.

In addition, we also evaluated the characteristics of the resultant GaN
bulk single crystals and examined the relationship between the surface
condition of the bulk GaN single crystals and crystal quality. It was
found that differences in the surface condition of GaN single crystals
reflected differences in the state of crystal growth, so that single
crystals exhibiting excellent surface condition possessed excellent
crystal characteristics.

## Methods and Means

### Principles of the pressure-controlled solution growth method

The solution growth (SG: Solution Growth) method is a method of
depositing crystals by bringing a solute dissolved in a solvent into a
supersaturated state. In the case of GaN, where the atmosphere
(nitrogen) itself is the solute, as shown in FIG. 4.3.1, a
supersaturated state can be attained by applying nitrogen overpressure
exceeding the equilibrium pressure. The PC-SG method was developed with
account taken of this phenomenon. Whereas in the HP-SG method the
driving force of crystal growth is the concentration differential
associated with the temperature gradient, under the PC-SG method, the
driving force of crystal growth is the concentration differential
associated with a pressure differential.

![](media/image60.png){width="3.28125in" height="4.135416666666667in"}

Figure 4.3.1 Comparison of high pressure solution growth (HP-SG) method
and pressure-controlled solution growth (PC-SG) method.

\[Keys for FIG. 4.3.1\]

(HP-SG method) (PC-SG method)

(Y-axis) Melt depth; (Y-axis) Melt depth; (above arrows) Nitrogen gas;
(X-axis) Temperature; (X-axis) Temperature

(Bottom figure): (Y-axis) Pressure; (X-axis) Temperature; (Box) PC-SG
method

Under the PC-SG method, a supersaturated state is obtained by raising
the temperature of the entire system to a temperature T~1~ and holding
it at this temperature, followed by raising the pressure of the system
from P~1~ to P~3~ and changing the state of the system from a→b→c.
Continuous crystal growth is possible because the nitrogen required for
crystal growth can be continuously supplied doing crystal growth by
maintaining the temperature at T~1~ and continuously increasing and
maintaining the nitrogen pressure. In this method, it is important to
maintain a constant concentration of nitrogen in the Ga melt and it is
necessary to precisely control the temperature of the system and the
nitrogen pressure.

### Laboratory equipment

#### Ultrahigh pressure HIP apparatus

Before the ultrahigh crystal growth apparatus introduced in 1999 was put
into operation, we used an ultrahigh pressure HIP apparatus for GaN
single crystal growth experiments. A conceptual diagram of the apparatus
and the heating equipment is shown in FIG. 4.3.2-(1). According to the
principle of operation of the apparatus, after increasing the pressure
to 175 MPa with the help of a compressor, the reactor pressure is raised
by using cubic expansion caused by gas compression and heating produced
by pushing in the lower lid. The reactor pressure is obtained by
calculation from the hydraulic pressure used for pushing in the lower
lid and the ratio between the hydraulic push-in area and the lower lid
push-in area. The apparatus is capable of increasing the pressure up to
980 MPa, with the pressure controlled with a precision of ±0.1% (1 MPa).
The heating equipment, which is made up of top and bottom graphite
heaters, two W-5%Re/W-26%Re thermocouples and a temperature regulator,
is capable of increasing the temperature to a maximum temperature of
2000°C.

![](media/image61.png){width="4.94375in" height="2.6381944444444443in"}

Figure 4.3.2-(1) Conceptual diagram of ultrahigh pressure HIP apparatus
and heating equipment.

#### Ultrahigh pressure crystal growth apparatus

The apparatus was designed and manufactured based on data obtained in
the course of GaN solution growth experiments conducted using the
ultrahigh pressure HIP apparatus in order to make growth of large GaN
single crystals possible. A general view and a conceptual diagram of the
apparatus are shown in FIG. 4.3.2-(2), with its main specifications
listed in Table 4.3.2. The maximum pressure used in the apparatus is
1000 MPa, with the pressure controlled with a precision of ±0.1% (1
MPa). The maximum temperature used in the apparatus was 1600°C.

![](media/image62.png){width="4.8180555555555555in"
height="3.654861111111111in"}

Figure 4.3.2-(2) (a) General view, and (b) conceptual diagram of the
ultrahigh pressure crystal growth apparatus.

\(a\) General view of the ultrahigh pressure crystal growth apparatus.

\(b\) Conceptual diagram of the ultrahigh pressure crystal growth
apparatus.

+----------------------+-----------------------+-----------------------+
| Apparatus            | Ultra-High Pressure   | Ultra-High Pressure   |
|                      | Crystal Growth        | HIP Apparatus         |
| Characteristics      | Apparatus             |                       |
|                      |                       | (used in the past)    |
|                      | (newly designed)      |                       |
+----------------------+-----------------------+-----------------------+
| Max. pressure        | 1,000 Mpa             | 980 Mpa               |
|                      |                       |                       |
|                      | {9,869 atm}           | {9,672 atm}           |
|                      |                       |                       |
|                      | {10,240 kgf/cm^2^}    | {10,000 kgf/cm^2^}    |
+----------------------+-----------------------+-----------------------+
| Max. temperature     | 1600ºC                | 2000ºC                |
+----------------------+-----------------------+-----------------------+
| Inner dia. of        | 130 mm                | 100 mm                |
| pressure vessel      |                       |                       |
+----------------------+-----------------------+-----------------------+
| Continuous operation | (Estimate) ≥ 14 days  | Practically ≤ 2 days  |
| time                 |                       |                       |
+----------------------+-----------------------+-----------------------+
| Process chamber      | 70 mm                 | 50 mm                 |
| space Dia.           |                       |                       |
|                      | Max. 150 mm           | Max. 100 mm           |
| Height               |                       |                       |
+----------------------+-----------------------+-----------------------+
| Max. outside dia. of | 60\~64 mm             | 40 mm                 |
| susceptor            |                       |                       |
|                      | 52\~56 mm             | 38 mm                 |
| Max. inner dia. of   |                       |                       |
| crucible             |                       |                       |
+----------------------+-----------------------+-----------------------+

Table 4.3.2 Main specifications of the ultrahigh pressure crystal growth
apparatus. \[Note\] The maximum internal diameter of the crucible
corresponds to the maximum external diameter of the substrate in the
case of using a **hetero seed substrate**.

#### Multiple stage crucible system

![](media/image63.png){width="2.386111111111111in"
height="2.59375in"}Under the PC-SG method, GaN crystal growth is made
possible even when the temperature gradient in the Ga melt is
practically zero \[6\]. In addition, because the diffusion of nitrogen
in the Ga melt is a rate-determining factor, a shallower Ga melt is more
advantageous for GaN crystal growth \[6,7\]. We have proposed a
multi-stage crucible system (FIG. 4.3.2-(3)) as a method that allows us
to make the most of these characteristics of the PC-SG method. The
method offers the possibility of high productivity because the crystal
growth conditions inside each of the crucibles are individually adjusted
without imparting a temperature gradient. In addition, in this system,
on the bottom of the crucibles, there are provided structures in which
hetero seeds can be placed, which permits use of the system for crystal
growth experiments utilizing seed crystals.

Figure 4.3.2-(3) Conceptual diagram of the multi-stage crucible system.
(a) Graphite susceptor, (b) graphite crucible, (c) pBN tube, (d) pBN
mask, (e) the Ga melt, (f) hetero seed.

### GaN single crystal growth method

#### Charging of source material

Source material charging conditions are shown in Table 4.3.3. 6N-Ga is
charged to pBN crucibles and a predetermined number of crucibles are
stacked on a graphite susceptor and placed inside the apparatus.
Nitrogen gas is used for atmosphere substitution inside the furnace,
whereupon the temperature is raised to 1475°C and the pressure is raised
to 400 Mpa, and the state (state depicted in (a) of FIG. 4.3.1) is
maintained for certain time.

+----------------------+-----------------------+-----------------------+
| Apparatus            | Ultra-high Pressure   | Ultra-high Pressure   |
|                      | HIP Apparatus         | Crystal Growth        |
| Characteristics      |                       | Apparatus             |
+----------------------+-----------------------+-----------------------+
| 6N-Ga weight         | 50g                   | 80g                   |
+----------------------+-----------------------+-----------------------+
| pBN crucible dia.    | 38 mm                 | 52 mm                 |
+----------------------+-----------------------+-----------------------+
| pBN crucible number  | 5\~6                  | 10\~9                 |
+----------------------+-----------------------+-----------------------+

Table 4.3.3 Source material charging conditions. \[Note\] The number of
the pBN crucibles is the number of crucibles that can be stacked in the
multi-stage crucible system.

#### Equilibrium pressure measurement

In the GaN equilibrium pressure measurement experiments, the pressure
was raised from 400 MPa to 590, 730, 830, and 980 MPa while maintaining
a temperature of 1475°C, and, after maintaining the state (state (c) in
FIG. 4.3.1) for 16 hours, the system was cooled down to room temperature
over a period of 1.5\~2 hours.

The amount of increase in the weight of the pBN crucibles charged with
Ga was due to the increased amount of nitrogen resulting from the growth
of the GaN crystal. Gallium adhered to the GaN crystals was removed by
etching with aqua regia (HCl:HNO~3~ = 1:1).

#### GaN single crystal growth experiments using ultrahigh pressure HIP apparatus

Because the PC-SG method is a method, in which a supersaturated state is
obtained by applying overpressure, it is believed that crystal growth is
influenced not only by the nitrogen pressure during crystal growth, but
also by the rate of increase of the pressure. For this reason, in the
course of the GaN single crystal growth experiments, starting from a
state characterized by a temperature of 1475°C and a pressure of 400
MPa, the pressure was increased to 980 MPa at a pressure increase rate
of 8, 12, 49, and 293 MPa/h while maintaining the temperature constant.
Seed crystals were not used, and crystal growth was based on natural
nucleation. The temperature gradient in the direction of growth was not
higher than 1°C/cm. After maintaining the state for a certain time, the
system was cooled to room temperature over a period of 1.5\~2 hours.

#### GaN single crystal growth experiments using ultrahigh pressure crystal growth apparatus

After putting the ultrahigh pressure crystal growth apparatus into
operation, we conducted GaN single crystal growth experiments by the
PC-SG method using the multi-stage crucible system. Starting from a
state characterized by a temperature of 1475°C and a pressure of 400
MPa, we increased the pressure to 1000 MPa using a pressure increase
rate of 9.1, 34, and 69 MPa/h while maintaining the temperature
constant. The pressure gradient in the direction of growth was not
higher than 0.5°C/cm. After maintaining this state for certain time, the
system was cooled down to room temperature over a period of 1.5\~2
hours.

In order to investigate the relationship between the crystal growth time
and the size of the GaN single crystal, we conducted crystal growth by
varying the crystal growth time. The pressure increase rate was 34
MPa/h.

In order to investigate the effects of seed crystals on the enlargement
of the GaN single crystals, in parallel to the above described
experiments, we conducted GaN single crystal growth experiments by
placing graphite plates and c-plane sapphire substrates with a diameter
of two inches on the bottom of the crucibles.

### Crystal evaluation method

The GaN single crystals obtained under the PC-SG method were evaluated
using the method described below.

#### X-ray diffraction

Using a four-crystal x-ray diffractometer (X\'Pert-MRD from PHILIPS), we
measured the x-ray diffraction peak of the GaN crystals' (0002) plane in
order to evaluate crystal quality based on its half-width. We used
monochromatic x-rays of CuK~α1~ from a Ge (220) four-crystal
monochromator. The measurements were carried out at an electron beam
accelerating voltage of 45 kV and a beam current of 40 mA using a Cu
target.

#### Atomic force microscopy (AFM: Atomic Force Microscopy) observations

In order to investigate the process of crystal growth in GaN single
crystals, we carried out AFM observations of as-grown GaN single
crystals using a Nano Scope III analyzer. The head used was a J.-type
head and the probe was an NCH-10V (125 Wide). The observations were
carried out in a **tapping mode** at an applied voltage of 1.3 volts.

#### Transmission electron microscopy (TEM: Transmission Electron Microscopy) observations

In order to investigate dislocations and crystal defects in the GaN
single crystals, we conducted planar TEM observations. After subjecting
the surface of the samples to ion milling to obtain a thickness allowing
for a sufficient penetration of the electron beam, we conducted
observations using a TEM microscope, the "H-9000" from Hitachi. The
accelerating voltage of the electron beam was 300 kV and the observed
magnification was 9,000\~3,000,000 times (magnification accuracy: ±10%).

#### Photoluminescence (PL: Photoluminescence) measurements

In order to evaluate the optical characteristics of the GaN single
crystals, we measured the PL spectra of the GaN single crystals at 4.2 K
and 300 K. A He-Cd laser (CW, oscillation wavelength: 325 nm, output: 10
mW) was used as the excitation light source. After reflecting the
excitation light with mirrors and adjusting its intensity with filters
it was focused with a quartz lens on a sample in a He dewar. PL from the
sample was focused on a spectrometer with a quartz lens, with its
intensity measured using image pickup elements (CCD: Charge Coupled
Device). For the measurement of the temperature dependence, which was
carried out in the temperature range of from 5K to 300K, we used a
temperature-variable cryostat.

#### Electrical characteristics

Based on Hall effect measurements, we obtained the carrier density of
the GaN crystals. The samples used for measurements were obtained by
cutting GaN crystals grown on sapphire substrates placed on the bottom
of the crucibles into squares 10 mm on a side.

## Experimental Results and Discussion

### Results of GaN single crystal growth experiments

#### Results of equilibrium pressure measurement experiments

![](media/image64.png){width="3.9166666666666665in"
height="2.5833333333333335in"}The results of GaN equilibrium pressure
measurements are shown in FIG. 4.4.1-(1). The equilibrium pressure of
GaN at a heater temperature of 1475°C was 590\~730 MPa. Therefore,
overpressure at a heater temperature of 1475°C and a crystal growth
pressure of 980 MPa was approximately 300 MPa.

Figure 4.4.1-(1) Results of GaN equilibrium pressure measurements.

#### Results of GaN single crystal growth experiments using ultrahigh pressure H.I.D. apparatus

![](media/image65.png){width="4.0680555555555555in"
height="2.9291666666666667in"}The relationship between the pressure
increase rate (∆P/∆t) and the surface area of a GaN single crystal
produced in the vicinity of the surface of the Ga melt via natural
nucleation, obtained using the ultrahigh pressure HIP apparatus, is
shown in FIG. 4.4.1-(2). It was found that there was a strong
correlation between ∆P/∆t and the surface of the single crystal, and
large GaN single crystals were obtained at 49 MPa/h. A GaN single
crystal possessing a surface area of 120 mm^2^, which exceeded the
single crystal surface of 104 mm^2^ grown by UNIPRESS in Poland, was
obtained at a pressure increase rate of 12 MPa/h \[3\]. The thickness of
the single crystals (a)\~(d) was 20\~30 micrometers.

Figure 4.4.1-(2) Relationship between pressure increase rate (∆P/∆t) and
surface area of a GaN single crystal grown in the vicinity of Ga melt
surface by natural nucleation.

Photographs of crystals (a)\~(d) of FIG. 4.4.1-(2) are shown in FIG.
4.4.1-(3). It has been found that the pressure increase rate ∆P/∆t
exerts influence not only on the size of the GaN single crystals, but
also on their surface condition. In other words, the contrast of the
hexagon-forming facets becomes weaker in single crystals (a), (b), and
(c) with a decrease in ∆P/∆t, and in case of single crystal D the
contrast disappears altogether and a smooth surface is obtained.
Furthermore, speaking of the lines
![](media/image66.png){width="4.291666666666667in"
height="4.895833333333333in"}extending from the center of the GaN single
crystal (which are formed by the joining of the facets): in the case of
single crystal (a) they are considerably distorted (and the shape of the
hexagon is significantly changed in the process of growth), in the case
of single crystals (b) and (c) they are practically rectilinear (the
growth takes place with the hexagon-like shape maintained), and in the
case of single crystal (d), they appear only on the periphery of the
crystal.

Figure 4.4.1-(3) Relationship between pressure increase rate (∆P/∆t) and
the surface condition of GaN single crystals produced in the vicinity of
Ga melt surface by natural nucleation: (a) 293 MPa/h, (b) 49 MPa/h, (c)
12 MPa/h, (d) 8 MPa/h.

#### Results of GaN single crystal growth experiments using ultrahigh pressure crystal growth apparatus

The relationship between the pressure increase rate (∆P/∆t) and GaN
single crystals produced in the vicinity of the Ga melt surface by
natural nucleation, obtained using the ultrahigh pressure crystal growth
apparatus, is shown in FIG. 4.4.1-(4). A GaN single crystal possessing a
surface area of 334 mm^2^ (which corresponded to a diameter of 20.6 mm)
was obtained at a ∆P/∆t of 69 MPa/h. As far as the size of the GaN
single crystals is concerned, for the solution growth method these are
top data. However, unlike the results obtained using the super high
pressure HIP apparatus, no increase in the surface of the single crystal
due to a decrease in ∆P/∆t was observed. On the other hand, the surface
condition of the crystal improved with decreasing pressure increase
rate, and single crystals possessing an excellent crystal surface were
obtained at 9.1 MPa/h. The thickness of single crystals (e)\~(g) was
20\~30 micrometers.

At a pressure increase rate of 34 MPa/h, we extended the time, during
which the system was maintained at the crystal growth temperature of
1475°C and a pressure of 1000 MPa, from 16 hours 254 hours in order to
investigate the effects of the extension on the size of the crystal, but
observed no effect. This means that the size of the crystal is
determined by the process of raising the pressure up to the final
pressure. In addition, no increase was observed in crystal thickness due
to the extension of the hold time. This has to be confirmed by further
extending the hold time.

![](media/image67.png){width="5.995138888888889in"
height="2.6729166666666666in"}

Figure 4.4.1-(4) Relationship between pressure increase rate (∆P/∆t) and
surface area of a GaN single crystal produced in the vicinity of Ga melt
surface by natural nucleation.

The results of investigation regarding the effects of seed crystals on
the enlargement of GaN single crystals are shown in FIG. 4.4.1-(5). In
all cases, GaN single crystals produced in the vicinity of the surface
of the Ga melt by natural nucleation exhibited a greater tendency
towards enlargement when c-plane sapphire substrates were placed on the
bottom of the crucibles than when graphite plates were placed on the
bottom. All of the GaN single crystals shown in FIG. 4.4.1-(4) are
crystals grown by placing a sapphire substrate on the bottom of the
crucibles.

![](media/image68.png){width="5.995138888888889in"
height="2.1305555555555555in"}

Figure 4.4.1-(5) GaN crystals produced in the vicinity of Ga melt
surface by natural nucleation when seed crystals (a) and (b) were placed
on the bottom of the crucibles.

\(a\) C-plane just sapphire substrate

\(b\) Graphite plate

On the other hand, apart from GaN single crystals produced in the
vicinity of the Ga melt surface by natural nucleation, films of GaN
crystals also grew on the sapphire substrates placed on the bottom of
the crucibles (FIG. 4.4.1-(6)). XRD data showed that the growth face of
these crystals was in the (0001) plane, oriented along the c-axis. As
can be seen from the electron microscope photographs shown in figure,
the GaN crystal was a columnar crystal, which was also confirmed by the
fact that there were a large number of holes remaining after polishing.
Because the growth mode of the GaN crystals is strongly dependent on the
crystal growth conditions, such as temperature and pressure, it is
believed that the growth mode can be switched from columnar
(three-dimensional) growth to planar (two dimensional) growth by
adjusting the conditions. If GaN crystals of specified quality could be
grown by this method, then reproducibility and mass production could be
dramatically improved.

![](media/image69.png){width="4.28125in" height="2.6041666666666665in"}

Figure 4.4.1-(6) Photographs of GaN crystals grown on sapphire
substrates placed at bottom of crucibles.

#### Discussion of crystal growth experiment results

Regardless of whether the ultrahigh pressure HIP apparatus or the
ultrahigh pressure crystal growth apparatus was used, the pressure
increase rate exerted influence on the surface condition of the
crystals, and crystals possessing excellent crystal surfaces were
obtained at a ∆P/∆t of 10 MPa/h or lower. On the other hand, the size of
the GaN single crystals exhibited a dependence on the equipment used. In
the case of the ultrahigh pressure HIP apparatus, no increase in the
single crystal surface area was observed as a result of a decrease in
∆P/∆t, but in the case of the ultrahigh pressure crystal growth
apparatus, there was no increase in the single crystal surface area and
there was even a slight decrease in the surface area. A discussion of
the above-mentioned phenomena is presented hereinbelow.

A strong contrast between the facets and the distorted lines formed by
the joining of the facets are believed to be due to the fact that the
rate of increase in nitrogen pressure, in other words, the rate of
increase in the degree of supersaturation, is greater than the rate, at
which nitrogen is supplied in the form of GaN crystal. In other words,
because crystallization takes place at the point in time when the degree
of nitrogen supersaturation exceeds a certain level, the shape of the
GaN crystal reflects the distribution of supersaturation at that point,
so that some of the facets become longer, some of the facets become
shorter, and the lines formed along the joints between the facets become
distorted. In addition, it is believed that in the newly grown crystal
portions and already grown crystal portions, the thickness of the
crystal becomes uneven and the contrast between the facets becomes more
pronounced. On the other hand, when the increase rate of the nitrogen
pressure is small, the degree of nitrogen supersaturation is maintained
below a certain level, with nitrogen continuously supplied to the
crystal as GaN, and, for this reason, crystals with excellent surface
condition can grow without developing in a stepwise fashion. This
explanation is supported by the results of the AFM observations of
Section 4.4.2 (2).

Because the size of the crystal is determined by the nucleus density of
the crystal, the fact that a large (small) single crystal is obtained
means that the density of the generated nuclei has been low (high).
Thus, it is believed that there is no direct relationship between the
pressure increase rate, in other words, the rate of increase in the
level of nitrogen supersaturation, and the nucleus density of the
crystal. In general, the nucleus density strongly correlates with
temperature fluctuations, with the nucleus density of the crystal
increasing under conditions characterized by significant temperature
fluctuations. This is also true for the present study, and it is
believed that the fact that large (small) single crystals are grown is
due to a low (high) nucleus density of the crystals. Evidence in support
of this point is shown hereinbelow.

In accordance with the PC-SG method, the temperature was maintained
constant and the applied pressure was increased, following the increase
in the pressure, the heater power required for maintaining the
temperature increased as well. Because under super high pressures of
700\~1000 MPa nitrogen changes from a gas to a near-liquid state, the
equalization of temperature due to gas mixing is made more difficult.
Because the flow rate of nitrogen in a liquid state is slower and mixing
is made more difficult, the temperature of the entire system can easily
become inhomogeneous, and precise control of the temperature becomes an
extremely difficult practical problem. In particular, in case of the
super high pressure HIP apparatus, whose heating system is relatively
small, the distribution of the discharged heat becomes inhomogeneous due
to the small size of the heater, and temperature fluctuations are liable
to increase. This tendency is even more noticeable when the applied
nitrogen pressure is increased, and, in the super high pressure HIP
apparatus, the larger the pressure increase rate, the larger the
temperature fluctuations (full width), which may reach 4.0\~5.0°C. When
∆P/∆t is small, the power change rate of the heater is also small, and
for this reason, the temperature fluctuations are also small, reaching
approximately 2.0\~4.0 °C. In the case of the super high pressure
crystal growth apparatus, as shown in Table 4.3.2, the volume of the
heating system is approximately three times larger (approximately two
times larger based on the diameter ratio, and 1.5 times larger based on
the height ratio), but the supplied electric power is increased only
approximately 1.5 times. The temperature fluctuations are typically
within 1.0\~2.0°C, which represents a tremendous improvement in
comparison with the super high pressure HIP apparatus. However, the size
of the temperature fluctuations varies for each run of the experiment
regardless of the pressure increase rate ∆P/∆t. Temperature fluctuation
obtained from temperature charts used during the growth of GaN single
crystals (e)\~(g), which are shown in FIG. 4.4.1-(4), was 0.3\~0.9 °C in
case (e), 0.5 \~1.5°C in case (f), and 1.0\~2.8°C in case (g).

From the above-mentioned facts it is evident that the size of the GaN
single crystal is strongly correlated with the temperature fluctuations,
and a reduction in temperature fluctuations is efficient in growing
large GaN single crystals. In order to achieve the goal set for 2002
(single crystals with a diameter of 1 inch or larger) it is necessary to
further reduce the temperature fluctuations in the super high pressure
crystal growth apparatus and reduce them to at least 0.3\~0.9°C or
lower.

The reasons, due to which the GaN single crystals grown in the vicinity
of the Ga melt surface became larger when sapphire substrates were
placed on the bottom of the crucibles than when graphite plates were
placed in the crucibles, are still unclear, but it is certain that the
nucleus density of the GaN single crystals decreased when the sapphire
substrates were placed on the bottom of the crucibles. It is believed
that because in this case the nitrogen in the Ga melt was spent both on
GaN growth in the vicinity of the melt surface and on GaN growth on the
sapphire substrates, the pressure increase rate was effectively reduced.
If we assume that this is true, then reducing the pressure increase rate
is expected to exert no adverse influence on increasing the size of GaN
single crystals.

### Results of crystal evaluation

Differences in the surface condition of GaN single crystals reflect
differences in the state of crystal growth, and single crystals
possessing excellent surface condition are expected to exhibit excellent
crystal properties. We evaluated GaN single crystals (a) and (d) shown
in FIG. 4.4.1-(3) using x-ray diffraction, AFM observations, TEM
observations, and PL in order to investigate the effects of the pressure
increase rate on the crystal quality.

#### X-ray diffraction results

An x-ray locking curve of reflection from the (0002) plane obtained by
four crystal x-ray diffraction is shown in FIG. 4.4.2-(1). It was found
that the half width of the (0002) plane reflection peak of single
crystal (a), which contained low-angle (approximately 170 arcsec) grain
boundaries, was 80 arcsec, with the crystal exhibiting excellent crystal
properties. In case of single crystal (d), no distinct small-angle grain
boundaries were observed, but its half-width increased to 120 arcsec.

![](media/image70.png){width="3.0in" height="3.3020833333333335in"}

Figure 4.4.2-(1) Rocking curve obtained by x-ray diffraction (ω/2θ).

A discussion of the increase in the half-width is provided below. As
shown by a detailed comparison of the peak shapes, the peak of single
crystal (d) appears to be composed of two superimposed peaks of
approximately the same x-ray intensity. This indicates the presence of
small angle grain boundaries in single crystal (d) and a decrease in
this angle. The angle, as estimated from the locking curve, is
approximately 60 arcsec, with the peak half width being approximately 80
arcsec, on par with single crystal (a). Based on the x-ray incident area
(width: 0.5 mm, height: 0.5 mm), the size of the grain boundaries is
expected to be approximately 0.1\~0.5 mm or higher. The presence of
small-angle grain boundaries in undoped GaN crystals grown by the HP-SG
method has been also reported, with the angle being approximately 60
arcsec \[3\].

Thus, so as to get rid of the influence due to the small-angle grain
boundaries, the locking curve of single crystal (d) was measured by
setting the width of the x-ray slit to 0.05 mm and its height was set to
0.5 mm. The results are shown in FIG. 4.4.2-(2). The tilt angle
determined from the locking curve profile for the (0002) plane was 43
arcsec, and the twist angle obtained from measurements in the (10-10)
plane was 18 arcsec. Polowski *et al*. have reported a tilt angle of
18\~25 arcsec and twist angle of \~20 arcsec as optimum values for
undoped bulk GaN single crystals \[3\]. The fact that the tilt angle of
the GaN single crystal (d) obtained in the present study was slightly
larger is most likely due to a significant bowing of the substrate. On
the other hand, the twist angles are practically the same and we can say
that both crystals possess identical crystal properties.

In addition, Polowski *et al*. have reported that the crystal properties
of GaN single crystals are considerably improved by the addition of Mg
(0.5% of Ga by weight) \[3\], and we can expect to obtain the same
effects under the present method as well.

![](media/image71.wmf){width="5.972916666666666in"
height="2.452777777777778in"}

Figure 4.4.2-(2) Rocking curves obtained by x-ray diffraction.

#### AFM observation results

The front and back faces of the GaN single crystals exhibit patterns so
different that the difference can be seen during visual examination. In
case of single crystal (a), a strong contrast, such as the one shown in
FIG. 4.4.1-(3), can be observed on one of the faces, and the other face
is smooth. In case of single crystal (d), some contrast, albeit
extremely faint, is observed on one of the faces, and the other face is
smooth.

AFM observation results for GaN single crystals (a) and (d) are shown in
FIG. 4.4.2-(3) and FIG. 4.4.2-(4). To distinguish observation planes,
the smooth face will be called the smooth face, and the other face (the
face where contrast is visible) will be called the rough face. The
results obtained during AFM observations were the same as during the
visual observation. Namely, steps corresponding to a strong contrast
were observed in case of single crystal (a) on the rough face. The
height of the steps was about 100 nm or so from the line profile. The
interval between the steps was not less than 20 *μ*m. Insular growth
(size: 20 *μ*m in diameter, height: 30\~70 nm) was observed in both
single crystals (a) and (d) as a feature of the rough face.

On the other hand, step-flow growth was observed in both single crystals
(a) and (d) on the smooth side. In case of single crystal (a), however,
the step width is larger (1\~2 *μ*m) and the steps are higher (2\~5 nm).
In case of single crystal (d), the step width is narrower (\~0.7 *μ*m),
and the steps are lower (0.3\~0.5 nm). In addition, there are
considerable differences in the flatness of the steps (RMS: Root Mean
Square), with that of single crystal (a) being 0.579 nm and that of
single crystal (d) being 0.138 nm. Moreover, the RMS of single crystal
(d) is a value that also includes step-related fluctuations. Thus, it
was found that the smooth face of single crystal (d) possesses flatness
at **a** monatomic-layer-high level. Its flatness is equivalent to the
flatness of a GaN crystal grown by the MOCVD method or exceeds its
level.

![](media/image72.png){width="4.739583333333333in" height="4.625in"}

Figure 4.4.2-(3) AFM observation results for rough faces of GaN single
crystals (top: single crystal (d), bottom: single crystal (a)).

![](media/image73.png){width="4.8125in" height="3.4125in"}

Figure 4.4.2-(4) AFM observation results for smooth faces of GaN single
crystals (top: single crystal (d), bottom: single crystal (a)).

\[Note\] Bright dots in figures (in particular, in case of single
crystal (a)) result from contamination (particles) adhered during
washing and drying.

Differences in the characteristics of single crystals (a) and (b)
reflect differences in crystal growth and support the discussion of the
differences in growth provided in Section 4.4.1(4). Namely, the model
says that in case of single crystal (a), the GaN crystal grows in a
discontinuous (stepwise) manner, and in case of single crystal (d)
crystal growth proceeds in a continuous manner.

#### TEM observation results

As reported in last year's Research Achievement Report regarding single
crystal (a) shown in FIG. 4.4.1-(3), crystal defects were not found
during low-magnification TEM observations either. Therefore, the defect
density of undoped GaN crystals grown by the PC-SG method was low, with
the defect density of GaN single crystals in the observation field
estimated to be not more than 10^5^ cm^-2^. The defect density of single
crystal (d) can be expected to be of the same magnitude as that of
single crystal (a).

#### Results of PL observation

![](media/image74.png){width="3.0569444444444445in"
height="2.1909722222222223in"}PL spectra of GaN single crystals (a) and
(d) at 4.2K are shown in FIG. 4.4.2-(5). In addition to band-edge
emission at 3.5 eV, peaks were also observed from a deep level at 2.3
eV, which is called the yellow band. In both single crystals (a) and
(d), the band edge is exhibits excellent PL characteristics stronger
than those of the yellow band. It is evident, however, that in terms of
the PL intensity ratio of the band edge and the yellow band as well as
relative PL intensity at band edge, single crystal (d) possesses
considerably superior PL characteristics. These characteristics are more
pronounced in PL spectra at room temperature (FIG. 4.4.2-(6)), and in
GaN single crystals possessing crystal surfaces such as those of single
crystal (a), sometimes it is impossible to measure the PL spectrum. In
addition, the yellow band is stronger than the band edge in the
room-temperature PL spectra of GaN crystals grown by the HP-SG method,
which is not a good PL characteristic \[9, 10\].

Left: Figure 4.4.2-(5) PL spectra of bulk GaN single crystals (a) and
(d) at 4.2K.

Right: Figure 4.4.2-(6) Room-temperature PL spectra of bulk GaN single
crystals (a) and (d).

Discussions have been going on for a long time about the origin of the
yellow band at 2.3 eV, with popular explanations including one
attributing its origin to impurities, such as carbon, etc., and another
one putting it down to crystal defects. As is evident from the
relationship between the crystal quality and the PL spectrum, the yellow
band is sensitive to crystal properties, and might possibly represent
emission related to crystal defects.

The temperature dependence of the PL intensity of the band edge emission
of single crystal (d), which exhibited excellent PL characteristics, is
shown in FIG. 4.4.2-(7). Although the PL intensity of the band-edge
emission is known to decrease by 3\~4 orders of magnitude in GaN
crystals grown by the MOCVD method, it decreased by only one order of
magnitude in the crystals grown by the
![](media/image76.png){width="4.083333333333333in"
height="3.1666666666666665in"}PC-SG method. This means that emission
centers are effectively present at room temperature. The reason for that
is believed to be a lower crystal defect density in GaN crystals grown
by the PC-SG method in comparison with GaN crystals grown by the MOCVD
method.

Figure 4.4.2-(7) Temperature dependence of PL intensity of band edge
emission of bulk GaN single crystals (d) having excellent crystal
surface.

#### Results of electrical characteristic evaluation

GaN grown on sapphire substrates was subjected to measurement of the
hole effect at 300K. As a result, it was found that the crystals, which
belonged to the n-type, had a carried density of 1.3\~1.9×10^19^ cm^-3^,
and a mobility of 36\~67 cm^2^/V⋅s. Although the data do not provide
direct information about bulk GaN crystals, bulk crystals are expected
to possess equivalent characteristics. In addition, the values obtained
by measurement were in good agreement with values for crystals grown by
the HP-CG method \[11\], as well as exhibited good agreement with
mobility and carrier density obtained from the half value of the
band-edge emission peak obtained in the course of the PL measurements
\[12\].

## Summary

The effects of seed crystals, crystal growth time, nitrogen pressure
increase rate, and equilibrium pressure were subjected to investigation
using the ultra-high HIP apparatus and the ultra-high crystal growth
apparatus introduced in 1999 in order to clarify the effects produced by
the degree of supersaturation of nitrogen in the Ga melt under the PC-GS
method on bulk GaN single crystal growth and to determine conditions for
growing large bulk GaN single crystals of excellent crystal quality. In
addition, we evaluated the characteristics of the resultant GaN bulk
single crystals to study the relationship between the surface condition
and crystal quality of GaN bulk single crystals. The results are
summarized below.

\(1\) The equilibrium pressure of GaN at a heater temperature of 1475ºC
was 590\~730 MPa, and overpressure at a crystal growth pressure of 980
MPa was approximately 300 MPa.

\(2\) GaN single crystals of excellent surface condition with a size of
120 mm^2^ were obtained by reducing the increase rate of nitrogen
pressure in the ultra-high HIP apparatus. This size exceeded the size of
104 mm^2^ of GaN single crystals grown by the HP-SG method.

\(3\) GaN single crystals with a size of 334 mm^2^ (which corresponds to
a diameter of 20.6 mm) were obtained in the ultra-high crystal growth
apparatus at a nitrogen pressure increase rate of 69 MPa/h. These data
represent the top size of GaN single crystals grown under the solution
growth method. Although GaN single crystals of excellent surface
condition were obtained at a pressure increase rate of 9 MPa/h, the size
of the crystals was approximately 18 mm in diameter, and there was no
GaN single crystal enlargement effect due to reduction in the pressure
increase rate. At a pressure increase rate of 9 MPa/h, GaN single
crystals with excellent surface condition were obtained, but the size of
the crystal was 18 mm, and there was no GaN single crystal enlargement
effect due to the reduction in the pressure increase rate.

\(4\) Based on (2) and (3), it was found that a reduction in the
increase rate of the nitrogen pressure was effective in terms of
obtaining bulk GaN single crystals of excellent surface conditions.

\(5\) Based on (2), (3), and examination of the relationship between the
nucleation density of GaN crystals and temperature fluctuations, it was
found that in order to grow large bulk GaN single crystals, it was
necessary to try to reduced the temperature fluctuations and to reduce
the nucleation density of GaN crystals.

\(6\) The relationship between the size of the GaN single crystals
produced in the vicinity of the Ga melt surface by natural nucleation
and the seed crystals placed on the bottom of the crucibles showed that
larger GaN single crystals were obtained in the case of c-plane just
sapphire substrates than in the case of graphite plates.

\(7\) Extending the hold time at the final crystal growth pressure had
no effects on enlargement of GaN single crystals and suggested that the
size of the crystals was determined by the process of raising the
pressure to the final pressure.

\(8\) Based on the results of evaluation of the characteristics of the
resultant GaN bulk single crystals, it was found that differences in the
surface condition of the GaN single crystals reflected differences in
the state of crystal growth, with single crystals of excellent surface
condition possessing excellent crystal properties.

\(a\) The tilt angle value of 43 arcsec and twist angle value of 18
arcsec obtained from a four-crystal X-ray locking curve profile show
that the crystals were at an equivalent level as compared with undoped
GaN single crystals grown by the HP-SG method and possessed superior
crystal properties.

\(b\) AFM observations showed that they had monatomic layer-level
flatness.

\(c\) PL spectrum measurements at 4.2K and room temperature showed that
they had excellent PL characteristics, with the PL intensity at band
edge being stronger than that of the yellow band.

## Future Plans

Prospects and future plans aimed at achieving the goals of the final
year of the present study (GaN single crystal size: not less than 25 mm
in diameter, defect (dislocation) density: not more than 10^5^ cm^-2^)
are described hereinbelow.

The crystal defect density of bulk GaN single crystals grown by the
PC-SG method, as determined by TEM observations, is below 10^5^ cm^-2^,
and we can expect to reach our goal. However, when the crystal defect
density exceeds 10^6^ cm^-2^, accurate evaluation of the defect density
by TEM observations is practically impossible, and it is necessary to
conduct the evaluation by means of etch pitting. In addition, as far as
crystal properties other than crystal defect density is concerned, it
was found that bulk GaN single crystals grown by the PC-SG method had
excellent crystal properties, in particular, superior PL
characteristics. However, it has not been confirmed whether crystals
possessing such superior crystal properties are also superior as
substrates for epitaxial growth. Therefore, issues that have to be
confirmed or resolved in the course of the present study in the future
include (1) confirmation of performance as a homoepitaxial substrate,
and (2) increasing the size of bulk GaN single crystals.

As far as (1) is concerned, the evaluation of homoepitaxial growth on
bulk GaN single crystal substrates grown in the present study based on
cooperation between the Taguchi Lab at Yamaguchi University and
organizations participating in the present project started in at the end
of 2000. Because the evaluation of bulk crystal characteristics requires
a certain size and number of specimens, we will have to be able to
fabricate single crystals with a size of at least 10\~15 mm in diameter
with good reproducibility. In addition, because handling presents
difficulties at the current crystal thickness (20\~30 *μ*m), we are
planning to investigate the crystal growth process and the limiting
factors of crystal thickness. Investigations into polarity control are
necessary as well, and we are planning to confirm the Mg doping-related
effects. The crystals will be passed on to organizations in charge of
epitaxial growth technology development and LED development and we
expect that their use for epitaxial substrates will be made possible as
a result of feedback from the results of their evaluation.

As far as (2) is concerned, a GaN single crystal with a surface area of
334 mm^2^ (which corresponds to a diameter of 20.6 mm), i.e. the highest
value in terms of size for GaN single crystals grown by the solution
growth method, was obtained using the PC-SG method utilizing a multiple
stage crucible system in an ultra-high pressure crystal growth
apparatus. This result holds much significance, giving us the confidence
that the attainment of the goal set for the final year (single crystal
size: not less than 25 mm) is quite possible. However, making this goal
a reality requires yet another breakthrough or improvement in the level
of the technology. In this connection, as discussed in Section 4.4.1(4),
it is believed that reducing temperature fluctuations will be most
effective, and we are planning to implement modifications to the
ultra-high pressure crystal growth apparatus and heating equipment in
the coming year. Improvements in terms of temperature fluctuations are
expected to not only enable enlargement of single crystals due to
crystal nucleus density reduction, but also to contribute to an
improvement in reproducibility resulting from the stabilization of the
temperature environment.

Based on the above-described technological research combined with this
year's research results, we expect to be able to reproducibly fabricate
large bulk GaN single crystals with a diameter exceeding 25 mm and
excellent crystal surfaces, i.e. excellent crystal properties.

## References

\[1\] S. Porowski, I. Grzegory, and J. Jun: High Pressure Chemical
Synthesis, eds. J. Jurczak and B. Baranowski (Elsevier, Amsterdam, 1989)
Chap. 3, p. 21.

\[2\] S. Porowski and I. Grzegory: J. Cryst. Growth 178 (1997) 174.

\[3\] S. Porowski: MRS Insternet J. Nitride Semicond. Res. 4S1 (1999) G.
1.3

\[4\] S. Porowski: Extended Abstracts of the 1999 Int. Conf. On Solid
State Devices and Materials, p. 58, Tokyo, Japan, September 1999.

\[5\] M. Kamp, V. Schwegler, C. Kirchner, H. Y. Chung, C. Wang, M.
Seyboth, M. Scherer and K. J. Ebeling: 3^rd^ Int. Symp. On Blue Laser
and Light Emitting Diodes, Berlin, Germany, No. WeC2, March 2000.

\[6\] T. Inoue, Y. Seki, O. Oda, S. Kurai, Y. Yamada, and T. Taguchi:
Jpn. J. Appl. Phys. 39 (2000) 2394.

\[7\] S. Krukowski: Proc. 15^th^ Melt Growth Meeting, Sendai, 1999
(Tohoku Univ., Sendai, 1999) p. 279.

\[8\] J. Karpinski, J. Jun, and S. Porowski: J. Cryst. Growth 66 (1984)
1.

\[9\] P. Perlin, J. Camasel, W. Knapp, T. Talefcio, J. C. Chervin, T.
Suski, I. Grzegory, and S. Porowski, Appl. Phys. Lett. 67 (1995) 2524.

\[10\] A. Pelzmann, C. Kirchner, M. Mayer, V. Schwegler, M. Schauler, M.
Kamp, K. J. Ebeling, I. Grzegory, M. Leszczynski, G. Nowak, and S.
Porowski: J. Cryst. Growth 189/190 (1998) 167.

\[11\] K. Pakula, A. Wysmolek, K. P. Korona, J. M. Baranowski, R.
Stepniewski, I. Grzegory, M. Bockowski, J. Jun, S. Krukowski, M.
Wroblewski, and S. Porowski, Solid State Communications 97 (1996) 919.

\[12\] E. Iliopoulos, D. Doppalapudi, H. M. Ng, and T. D. Moustakas,
Appl. Phys. Lett. 73 (1999) 375.

# Research Concerning Substrate Surface Morphology

## Achievements in 1998 \~1999

It was found that the surface morphology of sapphire substrates exerted
considerable influence on the physical properties of GaN and on the
characteristics of LEDs when GaN was heteroepitaxially grown on sapphire
substrates for the purpose of fabricating light-emitting elements for
LEDs.

Objectives of the present study included extreme abrasive modification
of sapphire substrates to understand the influence exerted upon the
physical properties of GaN heteroepitaxially grown thereon and
introduction of basic technologies to be used for drastically improving
the physical properties of GaN. Furthermore, yet another objective was
to establish a technology for polishing GaN substrates by applying the
polishing technology used for sapphire substrates.

In terms of practical results, we have been able to achieve a surface
roughness of Ry ≤ 1 nm, a face inclination of ≤ 0.05° with respect to
the designated axis, and a flatness of ≤ 1 *μ*m, which exceeded the
surface precision goal for sapphire substrates (face roughness: Ry ≤ 2
nm, face inclination: 0.1° with respect to the designated axis,
flatness: ≤ 1 *μ*m). Here, Ry is the same parameter as the one
previously designated by Rmax. In particular, surface roughness
measurements were taken within a 1 *μ*m by 1 *μ*m field of view, and the
roughness was practically monoatomic (Ry ≤ 0.3 nm); if we take into
consideration the fact that it was an off-orientation substrate, we can
say that it was almost absolutely flat.

Although single-point measurement in the central portion of the
substrate was used for *c*-axis orientation, it was confirmed it was
possible to fabricate off-orientation substrates inclined at an angle of
±0.05° with respect to any arbitrary axial direction. As for variation
in the axis orientation on the surface of the substrate, five-point
measurement results showed that a ±0.02° precision could be maintained.

As for variation in the thickness of the substrate, five-point
measurements showed that an excellent parallelism of not more than 1
*μ*m was achieved.

Although certain practical problems were indeed encountered, such as
imperfections called "latent flaws" and contamination due to inadequate
cleaning, it was confirmed that several methods allowing for improvement
were available, such as, respectively, enhanced MCP (Mechanical Chemical
Polish) and changing the cleaning equipment from an ultrasonic
multi-tank cleaning machine to a single wafer cleaning machine.

The polishing of GaN substrates was carried out using the same
technology as that used for the polishing of sapphire substrates. We
determined the polishing method, selected the polishing equipment (a
High-Press lapping machine), and installed it in August of 1999.

We carried out polishing tests on GaN substrates and confirmed that
polishing was possible; in the future, we are going to work on practical
improvements.

As far as a transition to the mass production of sapphire substrates is
concerned, we have implemented improvements in important processes such
as axis orientation alignment, polishing, latent flaw removal, and
cleaning, and gathered data related to the transition to mass
production.

## Summary of Research and Development in 2000

As for substrate surface precision, in 1998 and 1999 we were able to
achieve the target precision (surface roughness: Ry ≤ 2nm, face
inclination: ≤ 0.1° with respect to the designated axis, flatness: ≤ 1
*μ*m).

In 2000, we studied the influence exerted by sapphire substrates on the
physical properties of GaN by epitaxially growing GaN on substrates
whose precision we could ensure. The method we used consisted in
fabricating *c*-plane substrates with various off-angles (five levels:
0°, 0.15°, 0.3°, 0.5°, and 1°) and off-directions (two levels: *a*-axis
direction and *m*-axis direction) and investigating the results of
epitaxial growth. As a result, it was found that excellent results in
terms of epitaxial growth could be obtained on *c*-plane substrates
inclined at an off-angle of 0.15° with respect to the *m*-axis
direction.

In addition, along with using enhanced MCP to solve the practical
problem of latent flaws, which reveal themselves during epitaxial
growth, we eliminated the problem of the influence exerted by substrate
contamination on epitaxially grown films by using an enhanced cleaning
process (switching from multi-tank ultrasonic cleaning to single-wafer
cleaning).

Regarding the polishing of GaN substrates, we conducted polishing
experiments using the newly introduced polishing equipment. Currently we
are still in the stage of conducting polishing experiments and results
concerning GaN substrate evaluation have not been obtained yet, but, at
the same time, we are also conducting experiments aimed at obtaining
superior surface morphology by reforming GaN surface.

## Introduction of High Precision Processing Technology for Sapphire Substrates

Objectives of the present stage in research and development include
introduction of a processing technology intended for the finishing of
the surface morphology of sapphire substrates used in LEDs to a
consistent precision and a study of the influence exerted by the surface
morphology of the substrates on the physical properties of the
heteroepitaxially grown GaN.

The target surface precision of 2-inch sapphire substrates was as
follows:

-   Surface roughness Ry ≤ 2 nm, Face inclination, with respect to
    designated axis ≤ 0.1

-   Flatness ≤ 1 *μ*m

In addition, as concerns the evaluation of substrates obtained by
finishing the surface morphology of sapphire substrates to a
predetermined degree of precision, we have carried out such evaluation
using heteroepitaxial growth on GaN by means of MBE in a joint study
with Yamaguchi University.

### Methods and Means

In 1998 and 1999, in order to increase the precision of processing in
accordance with the new evaluation criteria used for high precision
sapphire substrates, we developed a quick method for increasing
processing precision, in which we followed prior-art processing steps
while varying the processing speed and time, and making the most of
already established know-how (maintenance and control of polishing
plate, selection of abrasive materials, etc.). Furthermore, we tried to
achieve improvements not only in terms of substrate surface precision,
but also in terms of eliminating latent flaws in the substrates,
enhancing the cleaning process, stabilizing the axial orientation of the
substrates, and controlling bowing and thickness.

This year's objectives included establishing evaluation methods and
achieving further progress in terms of the above mentioned topics. The
specific objectives we tried to achieve included research on surface
roughness, axis orientation, orientation flat orientation, bowing,
parallelism, latent flaws, cleaning, surface reforming, and substrate
evaluation methods \[1,2,3\].

In order to maintain a constant surface roughness, we carried out
measurements, inspection, and adjustment in order to maintain the degree
of flatness of the polishing plates within 1 *μ*m.

Along with establishing a method permitting the measurement of the
orientation flat orientation in a simple manner using x-rays, we
conducted a series of whole number measurements to verify its precision.

Because the direction of bowing occurring at room temperature after
growth is different from the direction during epitaxial growth, the
standards preferably used for bowing are: 5 *μ*m or less, concave
epi-surface (cupping down). However, the regulation of bowing is
difficult, and it is preferable to regulate the bowing of sapphire
substrates in accordance with the relative importance of bowing during
epitaxial growth and bowing at room temperature after growth. Regulation
of bowing is possible by adjusting the polishing roughness of the front
and back sides of sapphire substrates, and experiments are underway to
investigate this technique.

As far as parallelism is concerned, 1 *μ*m or less, as determined by
five-point measurements, is a very good degree of parallelism, and, for
this reason, parallelism has not been selected as a research topic for
this year.

Latent flaws were eliminated by enhancing MCP. Furthermore, inspection
based on the use of special microscopes was used for checking substrates
for latent flaws \[4\].

As far as cleaning was concerned, along with confirming the limitations
of the previously used ultrasonic multi-tank cleaning equipment, we
tested a single-wafer cleaning machine and confirmed its effectiveness
\[5\].

### Results and Discussion

#### Surface roughness

In terms of surface roughness, as described in connection with the
results obtained in 1998 and 1999, surface irregularities were reduced
to 0.3 nm or less. This is believed to be due to the fact that, with
monoatomic irregularities, as depicted in FIG. 5.3.2-(1) (schematic
representation), which shows an off-angle substrate surface, atoms are
shaved off along the off-angle (along the direction of polishing in the
figure). Incidentally, it was confirmed that when the steps were formed
by heat treating an off-orientation substrate, the ratio of the step
width to the step height was proportionate to the off-angle.

FIG. 5.3.2-(2) shows an AFM image of a stepped substrate with a 0.05°
off-angle, in which the number of steps was confirmed to be practically
the same as the number determined by calculation.

![](media/image77.png){width="4.083333333333333in"
height="1.0833333333333333in"}

Figure 5.3.2-(1) Off-angle substrate surface (schematic representation)
\[Keys for FIG. 5.3.2-(1)\] (Arrow) Direction of polishing.

![](media/image78.png){width="3.5625in"
height="1.1666666666666667in"}Quite naturally, in order to maintain the
precision of the surface, it is important to guarantee the precision of
the polishing plate, and this is why we took all the necessary
measurements in order to guarantee a flatness of not more than 1 *μ*m
across the entire surface. In particular, we enhanced the polishing
plate adjustment in the MCP stage.

FIG. 5.3.2-(2) AFM image of stepped substrate.

#### Axis orientation

As for axis orientation control points, first of all, they are used for
determining the plane orientation of the *c*-plane; at the present
moment, substrates with an off-orientation of approximately 0.2° with
respect to the *m*-axis direction have become mainstream. Experiments
have been conducted using substrates with an off-orientation of up to 1º
with respect to the *m*-axis direction (five levels: 0°, 0.15°, 0.3°,
0.5°, and 1°) and an off-orientation of up to 1º with respect to the
*a*-axis direction, but results better than those afforded by substrates
with an off-orientation of 0.2° with respect to the *m*-axis have not
been obtained.

Another important issue is the angle of the orientation flat. At the
present moment, the angle is controlled to ±0.3° with respect to the
*a*-axis direction, but for some applications it is necessary to control
the angle to ±0.1 °. At the present moment, inspection and grading are
necessary in order to control the angle to ±0.1°, because variations
caused by chamfering etc. during post-processing after initial
adjustment cannot be controlled to within 0.1°.

#### Bowing

The currently used bowing standard for 2-inch substrates is: 5 *μ*m or
less, concave (down). This is a practical standard. Namely, it has been
found that because the coefficient of thermal expansion of GaN is
smaller than that of the sapphire substrate when GaN is epitaxially
grown thereon, bowing occurs on the sapphire substrate side (an
originally concave surface becomes convex), and the bowing of the
sapphire substrate has come to be used as the benchmark for determining
the bowing produced after GaN growth.

In the present study we accumulated know-how concerning bowing control
methods. Namely, because the surface roughness of the front and back
sides of a substrate is an important bowing factor, we created a
polishing method, in which the roughness of the front and back sides of
a substrate is varied in order to control bowing, and, in addition,
experiments are currently underway, in which we are investigating
methods that can be used for varying the thickness of substrates.

#### Latent flaws

Imperfections sometimes occur in epitaxially grown GaN layers. These
imperfections are called latent flaws because they are difficult to
detect by inspection under an ordinary optical microscope after
polishing a substrate. Because the presence of such flaws can be now
determined using special microscopes, such as the "Magic-Mirror
microscope" etc., it has become possible to prevent latent flaws from
escaping detection in the technological process, thereby permitting
fabrication of substrates free from flaws.

#### Contamination

The operation of the multi-tank ultrasonic cleaning system employed in
the past was followed by cleaning with a mixed solution of concentrated
sulfuric acid and hydrogen peroxide (acid cleaning) intended for the
removal of organic contamination. Although a considerable reduction in
the degree of organic contamination was indeed obtained as a result of
such cleaning, it was found that there was practically no decrease in
the level of metallic elements (for example, Fe). Notwithstanding the
fact that cleaning is now performed via the re-circulating filtration of
the solution in the ultrasonic tanks, metallic elements are not removed
and remain in the solution, and because multiple wafers are cleaned at
the same time, metallic contamination accumulates.

In order to get rid of the metallic elements, it is necessary to
constantly use a clean cleaning solution, but the above-mentioned
multi-tank ultrasonic cleaning system has limitations as far as such a
requirement is concerned, because the system needs a considerable amount
of cleaning solution. In this connection, we tested a single-wafer
cleaning system, and found that the amount of metallic elements could be
reduced to below 1/10 without increasing the amount of the cleaning
solution being used (it was possible, for example, to reduce the amount
of Fe to 10^10^/cm^2^, as determined by total reflection fluorescent
x-ray spectroscopy), and we are now designing equipment based on the
experimental apparatus in order to further confirm its efficiency.

## Introduction of GaN Substrate Precision Processing Technology

Based on literature data concerning the physical properties \[6\] of GaN
and actual processing results \[7\], for the precision processing of GaN
substrates, we used a rough grinding machine and a finishing machine,
which were introduced in 1999, followed by MCP with KOH as an etchant.
The process, which consists of a series of steps including rough
grinding, finishing, and MCP, is expected to provide a surface roughness
of not more than 1 nm.

### Methods and Means

For the precision polishing of GaN substrates it is necessary to achieve
a surface roughness of not more than 1 *μ*m by carrying out polishing as
a series of steps including rough grinding, finishing, and MCP.
Furthermore, it is essential to maintain the flatness of the surface of
the polishing plate, and a device called a biaxial facing apparatus is
used to constantly control the surface precision of the polishing plate
to not more than 1 *μ*m.

The rough grinding machine required special specifications for a cooling
system for the polishing plate in order to avoid thermal deformation of
the polishing plate, and forced driving (using a device that forcibly
moved the workpiece) was used as the processing mode.

The finishing machine and the MCP apparatus were both desktop polishing
machines, with the surface precision of their polishing plates
maintained at not more than 1 *μ*m using the above described biaxial
facing apparatus. Controlling their precision makes it possible to
guarantee that the surface precision of the polished workpieces is on
the order of nm.

A problem encountered in the process of polishing is the residual stress
remaining between the GaN layer and the sapphire layer, which makes
achieving a perfectly polished state rather difficult. The best method
is probably not to continue directly polishing the GaN layer, but to
provide a sapphire layer around its periphery in order to achieve a
uniform and constant load with respect to the GaN layer. In practice,
the target polished state can be achieved by carrying out the polishing
operation several times.

### Results and discussion

The experimental polishing of GaN substrates has previously been carried
out by Japanese universities and companies, but unlike in the present
project, they reported results regarding experimental polishing of GaN
substrates manufactured by HVPE purchased in the United States.

![](media/image79.png){width="3.0625in"
height="2.3958333333333335in"}FIG. 5.4.2-(1) is an AFM image of an
as-grown GaN substrate showing the surface of the GaN substrate.
As-grown GaN crystals on the surface appeared to be of a columnar shape,
and the flatness of the surface was rather low, at approximately 50
*μ*m, which precluded its use as a GaN substrate without additional
processing.

In order to produce a flat surface by polishing, we removed
approximately five *μ*m of the GaN layer, which had a thickness of 15
*μ*m. As a result, the flatness of the surface was improved, as shown in
FIG. 5.4.2-(2), which is an AFM image of the GaN substrate after
polishing.

FIG. 5.4.2-(1) AFM image of as-grown GaN substrate.

![](media/image80.png){width="2.875in"
height="2.1458333333333335in"}When an enlarged photograph of the
substrate was taken in order to determine whether the substrate met
requirements for practical use, it was confirmed that, as shown in the
enlarged AFM image of the GaN substrate after polishing, there were
numerous large pits visible in the enlarged image, which made the
surface condition worse than that found in ordinary MOCVD substrates.

Left: FIG. 5.4.2-(2) AFM image of GaN substrate after polishing.

Right: FIG. 5.4.2-(3) Enlarged AFM image of GaN substrate after
polishing.

In addition, even when the thickness of the layer removed by polishing
was increased from five *μ*m to 10 *μ*m, the AFM images did not change.
This fact indicates that it was impossible to reduce crystal
imperfections by polishing. Our objectives for the next year include
research aimed at determining the effects produced when epitaxial growth
is carried out on flat surfaces improved by polishing, as well as
methods of polishing that make surface improvement possible.

## Summary

As far as the technology for high precision polishing of sapphire
substrate surfaces is concerned, using MCP as the last stage of the
ordinary polishing process, we have been able to completely eliminate
surface flaws, achieving a surface roughness of not more than 0.3 nm. In
addition, we conducted measurements using AFM.

Additionally, special microscopes, such as Magic-Mirror microscopes
etc., permit definitive determination of flaws and contamination, which
allows for evaluation at the substrate stage. As for cleaning, by
switching from the multi-tank ultrasonic cleaning system to a
single-wafer cleaning system, we have been able to reduce metallic
element contamination to under 1/10. Analysis was conducted using the
total reflection fluorescent x-ray method.

Additionally, the precision of the *c*-plane orientation was controlled
to within ±0.05° or less with respect to the designated axis (an
off-angle of 0.15° relative to the *m*-axis direction is considered
standard), and users have told us that there has been a decrease in the
variation in epitaxial growth results due to variation in the off-angle
of the substrates.

As far as the polishing of GaN substrates is concerned, experimental
polishing was conducted using polishing equipment introduced in the
course of the present study (High-Press lapping machine). Although
polishing was possible, the crystal surface of the GaN used in this
study was coarse, and the surface roughness target could not be met.

## Plans for the Future

In a joint study conducted in cooperation with the Yamaguchi University,
we will evaluate MBE-based heteroepitaxial growth on GaN to try to
obtain better substrates. Specifically, we will try to reduce the number
of imperfections in GaN epitaxial films by maintaining a high level of
substrate surface precision and by varying surface characteristics, such
as step-forming, etc.

In addition, by polishing the surface of GaN substrates, we will try to
reduce the surface roughness to 1 nm or less, and will investigate
methods for achieving a substantial reduction in the amount of
imperfections on GaN surfaces.

## References

\[1\] 1998 Annual book of ASTM standards, Section 10, Electrical
Insulation and Electronics.

\[2\] 1997 SEMI Standards, Vol. 5, Material traceability FPD.

\[3\] Techniques for fabrication and evaluation of ultra-precise surface
interfaces, by M. Matsunaga, Y. Sakurai, J. Chikawa (or: Sengawa),
Science Forum Co., Ltd.

\[4\] Techniques for fabricating and evaluating perfect wafer surfaces,
by H. Tsuya, Science Forum Co., Ltd.

\[5\] Ultra-precise wafer surface control techniques, by K. Matsushita,
S. Fukaya, H. Tsuya, S. Takasu, Science Forum Co., Ltd.

\[6\] Hardness and fractured roughness of bulk single crystal gallium
nitride, M. D. Drory, J. W. Ager, T. Suski, I. Grzegory, and S.
Porowski, Appl. Phys. Lett., Vol. 69, No. 26, 4044, December 1996.

\[7\] Chemical polishing of bulk and epitaxial GaN, J. L. Weyher, S.
Muller, I. Grzegory, S. Porowski, Journal of Crystal Growth,182 (1997)
17\~22.

# Research and Development of GaNAsP-based Multicolored Light Emitting Materials Using MOCVD

## Achievements in 1998 -- 1999

A large degree of band gap bowing is expected from III-V-N type mixed
crystals. For example, the band gap of GaN~1-x~P~x~,, which is created
by adding P to GaN, does not decrease monotonically as one moves from
GaN to GaP. Rather, it curves convexly downwards and the band gap takes
on a negative value near x=0.5, as suggested by band structure
calculations. By taking advantage of this characteristic and regulating
the x of GaN~1-x~P~x~ to about 10%, the primary colors red, green, and
blue can be formed from a united family of materials with promise for
photonic devices.

In 1998, an experiment was conducted using MOCVD (metal organic chemical
vapor deposition) in which a high concentration of As was added to GaN
to form GaNAs with a high As component. The photonic characteristics of
this composition were then studied. Additionally, GaNP was grown using
MOCVD with DMHy (Dimethylhydrazine) and PH~3~ (Phosphine) as raw
materials. And, a new laser-assisted MOCVD method was also investigated
to raise the P concentration.

SIMS (Secondary Ion Mass Spectrometer) analysis of films grown by MOCVD
using DMHy with a constant V/III ratio but varying growth temperature
confirmed that it is possible to mix in phosphorus on the order of
several percent by setting the growth temperature low. Yet, we were
unable to observe peak shifts in the band edge of the GaN caused by the
addition of phosphorus (P).

In 1999, laser-assisted MOCVD was further studied and GaNP films were
grown. Laser exposure was employed because it breaks down the source
gases via light energy and results in more efficient crystal growth. To
increase the effect of the laser exposure, an ArF laser was used, as
well as TBP (tertiarybutyl phosphine) as the phosphorus source since it
easily decomposes at the ArF excitation wavelength. At a growth
temperature of 850º C, a crystal with a high P composition was attained.
PL (photoluminescence) measurements showed that we were able to achieve
GaN~1-x~P~x~ with a 100 meV shift in the wavelength of the band edge as
compared to GaN.

The x composition was estimated from an analysis using EDX (energy
dispersive X-ray spectrometer), SIMS, and X-ray diffraction. Since the
surface of the specimen had large irregularities, the x values varied
greatly and could not be established. Nevertheless, the existence of P
was clear and it is thought that a GaNP film was achieved. The SIMS
analysis showed a P composition of about 0.02 but EDX showed a
significantly higher composition. What's more, the degree of red shift
was greater than that reported in previous announcements, and we believe
that we have confirmed band bowing in GaNP, although not quantitatively.
Thus, the red shift of the PL spectrum of GaNP using laser-assisted
MOCVD has been shown to be two to three times that of conventional
MOCVD, making laser-assisted MOCVD a promising crystal growth
technology.

While not possible with InGaN, the large band gap bowing effect of
GaN~1-x~P~x~-based materials makes it possible to emit red, green, and
blue light by changing the ratio of mixed crystals. Determining the
level of potential held by these materials is now the main objective of
research.

Our aim is to grow a GaN~1-x~P~x~ crystal with a high P composition and
through photonic measurements like photoluminescence, establish a larger
peak shift toward the long wavelength from that of GaN. Moreover, we aim
to fabricate a GaNP LED and show light emission by current injection.

## Summary of Research and Development of 2000

GaN~1-x~P~x~ with a high P composition was grown using two growth
methods: laser-assisted MOCVD and MOCVD without photon excitation. The
photonic characteristics were then evaluated. We also created a
GaN~1-x~P~x~ SQW (single quantum well), fabricated an LED and then
investigated its light emission characteristics.

Together with Tokyo University's Onabe Research Lab, GaNP was grown
using MOCVD at Tokyo University's leading Science Research Center. GaNP
was also grown at low temperature using MOCVD with photon excitation.
Uncovering the photonic characteristics was performed jointly by
Yamaguchi University's Taguchi Research Lab and Tokyo University's Onabe
Research Lab.

To be more concrete, growth of GaNP using laser-assisted MOCVD was
performed through exposure with an ArF laser using TMG
(Trimethylgallium), NH~3~ (Ammonia) and TBP as source gases. For MOCVD
without photon excitation, DMHy, PH~3~, and TMG were used. We studied
the conditions needed to attain high quality GaNP by changing growth
parameters. We further evaluated the crystals' optical properties using
PL measurements, SEM (secondary electron microscopy) for crystal
observation, X-ray diffraction for crystal evaluation, and SIMS for
composition analysis. We also studied electrical properties of LEDs and
light emission characteristics through EL (electroluminescence).

Due to the large band gap bowing of III-V-N-type crystals, it is
possible to achieve a material uniting the three primary colors red,
green, and blue by varying slightly the P or As crystal mix ratio. These
materials hold promise as photonic device materials. To uncover the
potential of these new materials, this project pressed forward
development beginning in 1998, growing and evaluating GaNAs and GaNP
crystals through MOCVD methods. For GaNAs, 0.2% As was added to GaN and
the peak shift of band edge light was observed through photoluminescence
measurement, but we were unable to mix a sufficient quantity of As.
Therefore, we began development of GaNP in earnest in 1999.

In order to increase the incorporation of P, we began a new crystal
growth method using laser-assisted MOCVD. This method involves exposing
the substrate surface to an ArF laser during growth, thereby decomposing
the source gases at low temperature and raising the incorporation of P.
By using this method, we were able to attain a mixed crystal with a P
composition of about 10%. We also fabricated a GaNP and GaN SQW (single
quantum well) and were first to realize blue light emission with a GaNP
LED.

## Development of GaN~1-x~P~x~ Multicolored Light Emitting Materials through Laser-Assisted MOCVD

### Methods and Means

Figure 6.3.1-(1) shows a diagram of the crystal growth platform used in
this research. TMG was used as the Ga source, NH3 as the N source, and
TBP as the additive P source. TMG and TBP are maintained in a
thermostatic chamber at a constant temperature and then pulled as source
gases at a constant volumetric flow via a MFC (mass flow controller).

Thereafter, acceleration is achieved by mixing in a large amount of
oxygen gas and then introducing it into the reactor. Heat by a graphite
heater is applied to the substrate used for growth and the gases are
supplied to the susceptor. In contrast, NH3 is delivered from above via
a separate system and is mixed in directly above the substrate. The
growth temperature is calibrated using both a thermocouple integrated
into the heater and a radiant heat thermometer which monitors the
substrate surface.

![](media/image82.png){width="4.884027777777778in"
height="2.348611111111111in"}Figure 6.3.1-(1): Laser-Assisted MOCVD
Platform.

The primary feature of this crystal growth platform is that it is MOCVD
with photon excitation. An ArF laser (LPX200 by Lambda Physik) is used
for excitation, introducing from outside the growth chamber and scanned
horizontally 2 mm above the substrate surface. The laser wavelength is
193 nm and the output is 100 mJ, modulated at a pulse repetition rate of
100 Hz.

The main benefit of photon-excitation lies in the elementary process of
source gas decomposition, accelerating normal heat decomposition with
decomposition caused by photon excitation. This elementary process is as
follows.

Light absorption by TMG reaches its peak at around 195 nm and at 170ºC
has an absorption cross section of 1.3 x 10^-16^ cm^2^mol^-1^.
Decomposition begins around 250 nm and as it passes from a singlet state
to a triplet state due to excitation through light absorption, the
metal-carbon bond breaks \[4\]. At a wavelength of 280 nm the following
reaction begins:

Ga(CH~3~)~3~ → CH~3~ + Ga(CH~3~)~2~

Ga(CH~3~)~2~ → C~2~H~6~ + Ga

The following reaction begins at a wavelength of 240 nm \[5, 6\]:

Ga(CH~3~)~3~ → 2CH~3~ + GaCH~3~

Absorption of light with NH~3~ begins at 210 nm and peaks at 190 nm. An
absorption value of 500 to 600 cm^-1^ has been reported \[7\]. The
wavelength of the excitation laser used for growth is 193 nm so it
covers the absorption band of TMG and NH3 and promises to facilitate
source gas decomposition.

### Results and Considerations for Growth of GaNP

#### The effect of photon excitation

A low temperature GaN buffer layer of 50 nm was grown on a sapphire
substrate (0001), then another 1 µm of GaN was grown on top of that.
This was then used as the substrate for growing GaNP. When growing GaNP,
the surface morphology worsens as the temperature is lowered to the
level where islands easily form. GaNP was grown between 850ºC and 950ºC.
Near 900ºC we were able to attain a smooth surface. Figure 6.3.2-(1)
shows the TEM (transmission electron microscopy) photo showing the cross
section of the GaN/GaNP interface. GaNP is shown in a large domain. The
crystal structure was found to be hexagonal through SAD (selected area
diffraction) measurements.

![](media/image83.png){width="5.992361111111111in"
height="2.123611111111111in"}

Left: Figure 6.3.2-(1): Cross Section TEM Photo of GaN/GaNP Boundary.

Right: Figure 6.3.2-(2): Effect of Laser Exposure.

Next, GaNP was grown on a substrate with a GaN buffer layer doped with
Mg and a comparison was made between laser-assisted and
non-laser-assisted methods. The GaNP was grown at a temperature of 900ºC
with a growth duration of 60 minutes, then annealed at 1050ºC for 1
hour. The PL spectrum of the specimen is shown in Figure 6.3.2-(2).

The specimen which underwent laser exposure has a clear peak and an
unmistakable difference in intensity that is several tens of times that
of the specimen with no laser exposure. Yet, because the growth style
differs when it is subjected to laser exposure and when it is not, we
cannot state that the intensity difference is an effect of the laser
exposure. Further study is necessary. When growing GaNP, the P
composition is an important element. We verified the P composition
dependency while growing GaNP crystals with TBP as the P source gas.
They were grown at a temperature of 900ºC for 60 minutes and annealed at
1050ºC for 30 minutes. The TBP flow rate was at 4 levels: 0, 0.02, 0.06,
and 0.15 sccm.

![](media/image84.png){width="5.983333333333333in"
height="2.0729166666666665in"}

Left: Figure 6.3.2-(3): PL spectra at various flow rates.

Right: Figure 6.3.2-(4): PL peak position as a function of TBP flow
rate.

Figure 6.3.2-(3) shows the PL intensity of each flow rate level at 5K.
The GaNP peak can be seen as a small shoulder at the GaN edge emission
peak.

Figure 6.3.2-(4) plots the GaNP PL position as a function of flow rate.
A condition of saturation can be seen at a flow rate of 0.06 sccm. The
amount of shift in the PL peak position becomes about 0.5% and, when one
estimates the P composition of GaNP from results by Iwata and others
\[8\], we find that saturation begins at a very low P composition.

#### Thoughts on Optical Evaluation of GaNP (Role of P~N~ in GaNP)

Among the III-V and II-VI compound semiconductors, the behavior of
substitutional charged impurities within a semiconductor has been
researched most with GaP. It is well known that, among substitutional
charged impurities of GaP, specific atoms, N or Bi for example, are
bound exciton defects \[9\].

The electronegativity of an N atom is quite large at 3.0 compared to
1.64 for a P atom. For this reason, an N atom strongly traps nearby
electrons and the potential of these trapped electrons attracts positive
holes to form excitons. Additionally, the electronegativity of a Bi atom
is small at 1.24 relative to a P atom, trapping positive holes and
attracting electrons to form excitons. Such impurities appear
electrically neutral except in their immediate vicinity, and therefore
at short distances their potential operates to create deep levels.

Likewise with GaN, differences in the electronegativity between P or As
atoms and the N atoms of the mother crystal result in trapped positive
holes, causing electrons to be drawn by Coulomb potential in the same
fashion as Bi atoms in GaP. The resulting neutral donor type
exciton-binding defects are easy to observe.

Among research on the introduction of substitional charged impurities,
such as P and As atoms, into GaN, there are a relatively large number of
papers on As, but very few on P. Ion implantation is one method of
introducing P that was reported by Pankove et al \[10\] many years ago.
More recent research has been performed by Jadwisienczak et al \[11\],
in which the light emission of a P isoelectronic trap peaks at 2.88 to
2.9 eV. On another front, Ogino et al \[12\] pursued a mixed crystal of
P and GaN, producing a reaction between a Ga melt and NH~3~ to create a
powder which was then measured for photoluminescence. In this experiment
too, the P atom serving as an isoelectronic trap showed a peak at 2.88
to 2.9 eV.

If P~N~ in GaNP works as a neutral donor-type isoelectronic trap, it is
thought that the conductivity of the host crystal should exert a large
influence. Thus we grew GaNP with a P composition of 0.1% to 1% on
p-type and n-type non-doped substrates. Then, through PL measurements,
we confirmed the behavior of the P atom as a bound exciton defect.

#### Evaluation of GaNP Grown on GaN Epitaxial Sapphire Substrate

GaNP was grown on a GaN epitaxial sapphire substrate. Three GaN
epitaxial substrates were used: one which was not intentionally doped,
one doped with Mg, and another doped with Si. The quantity of each
dopant was found through SIMS measurements to be 1.8 x 10^19^ cm^-3^ for
the p-type substrate and 3.1 x 10^18^ cm^-3^ for the n-type substrate.
The true carrier concentration is unknown since we did not perform Hall
measurements, but it is thought that the activation rate of Mg is about
1% and of Si is about 10%, so we estimate that a carrier concentration
of about 1 x 10^17^ cm^-3^ exists on both p and n-type substrates. GaNP
was grown by MOCVD with photon excitation at a temperature between 850ºC
and 950ºC. The grown GaNP epitaxial layers were 0.5 to 1 µm thick and
the P compositions were estimated to be 0.5 to 1% based on SIMS and
X-ray analysis, although establishing this quantitatively was difficult
because the surface morphology of the GaNP was very poor. Moreover, SIMS
measurement detected dopants of each substrate in the GaNP epitaxial
layer, with Mg on the p-type substrate and Si on the n-type substrate.
Even though no intentional doping was carried out, it is thought that
p-like and n-like GaNP epitaxial layers formed by an auto-doping effect.

PL measurements were performed at 77K and at room temperature. As the
source for photon excitation we used a He-Cd laser with a wavelength of
325 nm and an output of 50 mW. The light intensity density at the
specimens was about 1 Wcm^-2^. The PL measurement of the GaNP epitaxial
film was, of course, effected by the underlying GaN substrate. To
measure the photoluminescence of the underlying GaN substrate for
comparison, we used three methods: (1) measure the substrate prior to
GaNP growth, (2) measure after growth, and (3) turn over the
experimental material and expose the GaN to photon excitation from the
side of the sapphire substrate. We did this thinking that it is possible
that the GaN of the substrate receives residual heat (heat history) from
the growth of GaNP and is somehow transformed. However, the results show
no significant difference between the PL measurement of the pre-growth
substrate and that measured from the back side of the substrate after
regrowth of GaNP.

![](media/image85.png){width="5.986111111111111in"
height="2.077777777777778in"}

Left: Figure 6.3.2-(5): Spectrum (77K) of GaNP on Undoped Substrate.

Right: Figure 6.3.2-(6): Spectrum (77K) of GaNP on p-type Substrate.

Figures 6.3.2-(5), (6) and (8) show the PL spectra at 77K of GaNP films
grown on the undoped substrate, the p-type substrate, and the n-type
substrate. Each figure shows the PL spectra of both the GaNP and the
underlying GaN for the purpose of comparison. The GaNP spectra from both
the undoped substrate and the p-type substrate are similar, but the GaNP
of the n-type substrate is quite different in both shape and intensity.
The GaNP of the p-type substrate shows strong exciton luminescence and
phonon replica. The spectrum shape is similar to the substrate but shows
a subtle difference in the peak near 3.27 to 3.25 eV.

![](media/image86.png){width="5.9840277777777775in" height="2.09375in"}

Left: Figure 6.3.2-(7): Spectrum (room temperature) of GaNP on n-type
Substrate.

Right: Figure 6.3.2-(8): Spectrum (77K) of GaNP on n-type Substrate.

The shape of the GaNP on the undoped substrate clearly differs from the
substrate, but closely resembles the GaNP on the p-type substrate. On
the other hand, the GaNP on the n-type substrate shows only broad light
emission with no exciton luminescence and is very similar to the
substrate photoluminescence. Also, the intensity is two orders of
magnitude weaker than that of the undoped or p-type substrates.

![](media/image87.png){width="2.9652777777777777in"
height="2.313888888888889in"}Figure 6.3.2-(8) shows the PL spectrum of
GaNP on an n-type substrate at 77K. Figure 6.3.2-(9) shows the PL
spectrum of GaNP on a p-type substrate at room temperature. The PL
spectra of both the GaNP and the underlying GaN are plotted together for
comparison. The PL spectra of GaNP on the n-type substrate shows little
difference between 77K and room temperature. The PL intensity of GaNP on
the p-type substrate at room temperature is weaker by four orders of
magnitude compared to that at 77K. The phonon replica in GaNP is
remarkable compared to that at 77K (shown in Figure 6.3.2-(6)).

Figure 6.3.2-(9): Spectrum (room temperature) of GaNP on p-type
Substrate.

These results, as viewed from the substrate's conductivity, are as
follows.

Exciton luminescence is predominant in GaNP on a p-type substrate in
which holes are the majority carrier, and its intensity is strongly
dependent on temperature. In GaNP on a n-type substrate in which holes
are the minority carrier, exciton luminescence is not seen and the
intensity shows little dependence on temperature. On the other hand,
with GaNP on the undoped substrate -- which is thought to be n-type --
the PL spectrum is similar to that of the GaNP on the p-type substrate.

These results can be explained quantitatively as follows.

P~N~ as a neutral donor works as a hole trap only with the existence of
a reasonable concentration of positively charged holes. With GaNP on a
p-type substrate, holes are the majority carrier so P~N~ can work
effectively as a neutral donor. Because electrons are the majority
carrier in GaNP on an n-type substrate, only generated holes exist
nearby P~N~ during photon excitation and P~N~ does not work as an
effective neutral donor due to competition with the non-radiating center
of inactivated Si atoms and the numerous electrons surrounding P~N~.
Although GaNP on the undoped substrate is n-type, the quantity of
electrons differs greatly compared to the n-type substrate and so the
behavior resembles that of GaNP on a p-type substrate.

#### Evaluation of Photonic Properties of GaNP with Different Substrates (Strong Mutual Interaction with Lattice)

The phonon replica is peculiar to GaNP on a p-type substrate. In
particular, as the measurement temperature rises the PL intensity falls,
yet mutual phonon replica can be clearly observed. Figure 6.3.2-(10)
shows the PL measurement results from 7K to room temperature. The peak
energy and peak number have been separated for each replica and are
shown in Figure 6.3.2-(11) as a Gaussian distribution. The spacing
between each peak is 80 meV and the intensity distribution has been fit
using LO phonon scattering.

The peak PL intensity can be found by

$I(E) = \sum_{}^{}{\text{exp}( - S)\frac{S^{i}}{i!} \times \frac{1}{1 + (E - E_{0} + \text{iE}_{\text{LO}}\frac{)^{2}}{\gamma^{2}}}}$

$S = \frac{I_{1}}{I_{0}}$

and the relationship to the phonon-exciton coupling can be derived. In
Figure 6.3.2-(11) the zero phonon line is unclear so the intensity of
each phonon replica was approximated using a Poisson distribution to
derive \[14\] the phonon-exciton coupling coefficient of S = 1.5. This
value is quite large compared to the value of S = 0.2 calculated from
the phonon replica observed by InGaN electroluminescence and we see that
it has a strong localized center.

In this way, we observed that P~N~ in GaNP has the characteristics of an
effective neutral donor when holes are the majority carrier and has a
localized center that strongly interacts with the lattice.

![](media/image88.png){width="5.985416666666667in"
height="2.1131944444444444in"}

Left: Figure 6.3.2-(10): Spectrum of GaNP on p-Type Substrate.

Right: Figure 6.3.2-(11): Phonon Replica Peak Separation.

#### Temperature Dependence of GaNP Photonic Characteristics

To research the photonic characteristics of GaNP, GaNP with differing P
compositions was grown and then PL measurements were performed. The
specimen was grown on an undoped GaN substrate at a growth temperature
of 900ºC for 60 minutes with TBP flow rates of 0.1 sccm and 0.2 sccm,
and then annealed for 60 minutes at 1050ºC. Among the substrates there
was some variation in the GaNP epitaxial film but the strongest PL
intensity was used for comparison. The observed temperature dependence
of the PL spectra is shown in Figures 6.3.2-(12) and (13).

![](media/image89.png){width="5.983333333333333in"
height="1.8368055555555556in"}

Left: Figure 6.3.2-(12): Temperature Dependency of PL Spectrum of GaNP
(TBP: 0.2 sccm).

Right: Figure 6.3.2-(13): Temperature Dependency of PL Spectrum of GaNP
(TBP: 0.1 sccm).

The peak around 3.476 eV is the band emission of GaN and shows no large
variation with regard to temperature between 5K and 40K, but thereafter
gradually shows red shifting. Figure 6.3.2-(14) shows the temperature
dependence of this peak. Shown by the solid line in this figure, the
measured values represented by the black squares were fit using the
Varshni empirical relationship of the W. Shan model \[15\] (α : 8.32 x
10-4 eV/K, β : 835.6 K), confirming the GaN band emission.

On the other hand, the fact that the peak position in the vicinity of
3.35 eV varies according to the GaNP composition leads us to believe
that it is GaNP luminescence. This temperature dependence, unlike the
band emission of GaN, is shown as a red shift relative to a rise in
temperature from 5K.

![](media/image90.png){width="5.998611111111111in"
height="2.2020833333333334in"}

Figure 6.3.2-(14): Temperature Dependency of GaN Band Emission.

Next, the dependence on excitation power using the same specimen was
measured using PL measurement. The results are shown in Figure
6.3.2-(15). The temperature dependence of the GaNP peak was measured by
setting the excitation light intensity to 0.5 mW for weak excitation,
and to 4.5mW for strong excitation. The results are shown in Figure
6.3.2-(16).

![](media/image91.png){width="5.999305555555556in"
height="2.217361111111111in"}

Left: Figure 6.3.2-(15): Relationship between PL Peak and Excitation
Light Intensity.

Right: Figure 6.3.2-(16): Relationship among PL Peak and Excitation
Light Intensity and Temperature.

#### Discussion

The PL spectrum of GaNP -- unlike the band emission of GaN -- appears to
experience a red shift proportional to the temperature. A dependence
like this can also be seen in the PL emission band tailing of
semiconductors doped with both high quantities of n and p impurities.

In semiconductors containing a high quantity of both n and p impurities,
tailing occurs at the end of each band, both conduction and valence
bands. The peak wavelength during recombination between the free
electrons neighboring the Fermi level within the semiconductor and holes
bound to the tailing valence band is expressed by the following equation
\[16, 17, 18\].

$$\omega \approx E_{g} + \mu_{n} - \sqrt{2}\gamma - \varepsilon_{p}^{o}$$

Here, E~g~ represents the band gap energy when undoped, μ~n~ represents
the Fermi level, γ is the rms value of the potential fluctuation caused
by impurities, and ε -- found by the following equation -- is the
equivalent thermal depth of bound holes produced by potential
fluctuation.

$\varepsilon_{p} = T\text{ln}\left\lbrack N_{v}\left\lbrack p + n\theta(\varepsilon) \right\rbrack^{- 1} \right\rbrack$

In this equation, T is the absolute temperature, N~v~ is the effective
state density of the valence band, and p and n represent the hole and
electron densities, respectively. θ is the ratio of the recombination
coefficient between free electrons and localized holes and the binding
coefficient caused by the localization center of the free holes. In this
case, the temperature dependence of the PL spectrum peak wavelength is
defined primarily by ε and results in a negative factor with regard to
temperature (experiencing red shift).

On the other hand, the photoluminescence excitation intensity and the
density of excited carriers (p +n) within the semiconductor have a
positive correlation, therefore if we tentatively assume that the
correlation is linear, we should find from the equation that exp(ε/T) ∝
1/(excitation intensity).

If we recalculate the variable in Figure 6.3.2-(15) we arrive at Figure
6.3.2-(17) with a sectionally linear slope. In reality the excitation
intensity and carrier occurrence is not linear and since the Fermi level
changes, the explanation is not this simple. Moreover, in Figure
6.3.2-(17) the peak position temperature dependency factor in relation
to photoluminescence excitation intensity is larger when GaNP has a high
P composition than when it has a low P composition. The difference in P
composition in the GaNP can be explained qualitatively by noting that
when the composition is increased by varying the carrier binding and
recombination state, it results in a weaker photoluminescence intensity
during recombination, θ becomes smaller and the apparent ε grows larger.

![](media/image92.png){width="3.0229166666666667in"
height="2.4069444444444446in"}Figure 6.3.2-(17): exp(ε/T) ∝1/(Excitation
Intensity).

Further study is required in order to apply the analogy of band tail
transition of high doped semiconductors to GaNP, as shown above. From
here, let's consider the concentration and distribution of P atoms in
GaNP as seen from the PL spectra.

The PL spectra of the GaNP can be categorized into two cases -- when the
P composition is extremely low, and when the density of P atoms is near
1%.

As reported above, in the GaNP specimens created with ion implantation
of P atoms, there is a P density of about 1 x 10^17^ to 1 x 10^18^
cm^-3^ and nearly all of the P atoms are isolated, showing light
emission as an isoelectronic trap around 2.97 eV \[10, 11\].

In contrast, when the P atom density rises to 1 x 10^19^ to 1 x 10^20^,
the P atoms are no longer isolated and show characteristics of bulk
GaNP, exhibiting a photoluminescence peak near the area of band emission
of GaN. It has been reported that as the P atom density increases
further, photoluminescence becomes difficult around 2% and at 8%
separation into GaN-rich GaP and GaP-rich GaN occurs \[8\].

In what fashion are P atoms distributed within the GaNP crystal? If we
assume a totally random distribution and zincblende lattice, the number
of nearest neighbor and next nearest neighbor atoms to a particular P
atom are 12 and 22, respectively. Therefore, taking x as the P atom
density, the number of isolated P atoms (with only N nearest neighbors)
and the number of nearest-neighbor P atom pairs (with only N
next-nearest neighbors) can be calculated \[19\] to be Ns = x(1-x)^12^
and Np = x^2^(1-x)^22^, respectively.

According to this, the probability of existence for an opposing P atom
is about 1% of the isolated P atoms, even if density x becomes 10%,
which means there is an overwhelmingly high number of isolated P atoms.

This contradicts the reality that even in GaNP with a density of about
0.1%, the photoluminescence of the bulk GaNP is stronger than the
photoluminescence by the isoelectronic trap made up of isolated P atoms.

Now let's look at crystal structures with arsenic or simple phosphorus.
Arsenic and phosphorus have a coordination number of 3 and is the most
stable polymorphism. Among the bonds positioned at the vertex of the
simple cubic lattice, three are deviant, thereby skewing the entire
structure. With black phosphorus, it bonds in two locations at an angle
of 102º and one location at 96.5º.

Figure 6.3.2-(18) shows the number and distance of neighboring atoms
within arsenic and phosphorus crystalline structures. The first
most-proximate atom is comprised of four atoms in a pyramidal formation.
Figure 6.3.2-(19) shows the most stable structure of a monovalent 3s or
3p valence electron bound molecule, as predicted by first principle
calculations. According to this, it is estimated that initially there
are only isolated P atoms, but gradually a pyramid of four P atoms
forms, the pyramids themselves form into clusters of six, and then the
six pyramidal clusters form into clusters of twelve \[20\].

We can't imagine that the distribution of P atoms within the GaN host
crystal is exactly as pictured above, but it can be easily imagined that
if a large Ga-P bond is present among the short, strong bonds of GaN,
the Ga-N bond will break once and the next P atom is easily accepted
near the Ga-P bond, unlike near other Ga-N bonds. In other words, pairs
of P atoms are formed and gradually cluster into near-pyramidal forms.

The photoluminescence spectra of GaNP clearly show that it exhibits the
properties of the bulk GaNP even though the P density is comparatively
low. It is surmised that this is because clusters form from the time
that P atom density is low, rather than following a random distribution.
It is believed that this clustering has a spatial composition
distribution and behaves in a way similar to the band tailing produced
by potential fluctuations in heavily doped semiconductors

![](media/image93.png){width="5.995833333333334in" height="2.50625in"}

Left: Figure 6.3.2-(18): Histogram of Neighboring Atoms of Black
Phosphorus and Arsenic.

Right: Figure 6.3.2-(19): First Principle Calculations (H.S. Lim)\[21\].

## GaNP Growth (DMHy/PH~3~/TMG) through Nonradiated MOCVD

### Methods and Means

Growth of GaNP on a sapphire substrate by MOVPE was examined. The growth
procedure consisted of growing a high quality GaN layer at 1000ºC with
the introduction of an AlN buffer layer at a low temperature of 850ºC.
Then GaNP was grown epitaxially at 850ºC. The source gases for Ga, Al,
N, and P were TMG, TMA, DMHy and PH~3~, respectively. The growth
pressure was 160 Torr. The temperature of the thermostatic chambers
holding the organic source materials were set to 0ºC for TMG, +15ºC for
DMHy, and +20ºC for TMA. The saturated vapor pressures were 66.8 mmHg,
about 95 mmHG, and 9.36 mmHG, respectively. The PH~3~ was diluted 10% in
hydrogen.

Oxide film was first removed from the sapphire surface through heat
treatment at 1050ºC. Thereafter the temperature was reduced to 850ºC and
an AlN buffer layer was grown (TMA: 9.1 sccm. DMHy: 220 sccm). AlN is
used to relieve the large difference between the lattice constants of
sapphire and GaN and becomes a crystalline nucleus, making growth
thereafter of GaN a smooth process. GaN with a layer thickness of about
1.2 µm is then grown for 25 minutes and used as a substrate (TMG: 7.6
sccm. DMHy: 40 sccm). After lowering the temperature to 850ºC, a layer
of GaNP is grown for 20 minutes. The rate of flow for each source gas
during growth of the GaNP layer was 29.8 µmol/min (7.6 sccm) for TMG,
220 µmol/min (40 sccm) for DMHy, and 135 to 450 µmol/min (30 to 100
sccm) for PH~3~.

### Results and Observations

![](media/image94.png){width="2.4430555555555555in"
height="1.604861111111111in"}The surface of all specimens fabricated
according to the growth conditions noted above turned yellowish and the
growth layer of the film was extremely rough and difficult to see.
Figure 6.4.2-(1) shows an SEM image of the surface. The GaN surface is
formed by multiple grains, as can be seen in Figure 6.4.2-(1). It is
thought that because the flow rate of PH~3~ is quite high, the GaNP
layer grown on the surface is not N-rich, rather it is P-rich or simply
a GaNP layer. If GaP-rich, elimination is possible through etching via
aqueous treatment, thus this treatment was performed for 15 minutes.
Consequently, after etching, the layer that had existed on the specimen
surface was eliminated, so the characteristics of the surface after
etching was also examined.

Figure 6.4.2-(1): SEM Image of GaNP Mixed Crystal Surface.

![](media/image95.png){width="2.8895833333333334in"
height="4.75in"}Figure 6.4.2-(2) shows the results of X-ray diffraction
measurement (002) of the specimen before and after etching of the
GaP-rich surface layer. The peak shown at about 41.7º is caused by the
surface (006) of the sapphire substrate while the peak at about 34.7º is
due primarily to the surface (002) of the underlying GaN layer. At high
resolution, the remaining peaks are difficult to verify since their
intensity is very weak, so a low resolution X-ray diffraction
measurement was employed.

With low resolution measurement, the peak width at half height becomes
extremely broad, allowing verification of the AlN buffer peak around
36º. Also, an X-ray diffraction peak from the GaP surface (111) was
confirmed at about 28.3º (calculated to be 28.334º) with the pre-etched
specimen only. This tells us that while the crystallization is extremely
poor, crystal growth of GaP did occur.

Additionally, although the diffraction peak from GaP (111) on the etched
specimen disappeared, another peak appeared instead at around 32.4º. The
source of this peak is unknown, but since it was found after etching and
because it is not seen with the GaN growth, it is believed to be a
diffraction peak from the initial GaNP growth layer.

Figure 6.4.2-(2): (0002) X-Ray diffraction spectra prior to and after
aqua regia etching of GaNP. (Top to bottom: high-resolution measurement
after etching, and low-resolution measurements prior to and after
etching.) Keys (Top to bottom): High-resolution scan, after etching;
Low-resolution scan prior to etching; Low-resolution scan after etching.

Figure 6.4.2-(3) shows the photoluminescence spectra before and after
etching. Because a GaP-rich layer is thickly applied, a GaNP peak near
3.5 eV and a GaN peak is not observed. Instead broad emissions near 1.9
eV and 2.25 eV are found. Since these emissions disappear after etching
it is thought to be emissions from the GaP-rich layer. The peak at 1.9
eV appears at the same location as seen in the PL measurement when a
four dimensional mixed crystal was evaluated so it is thought to be an
emission from the GaPN mixed crystal. If this peak position is from
GaPN, the N concentration equates to approximately 3%. Also, the peak at
2.25 eV is thought to be due to reflection from the GaP growth.

![](media/image96.png){width="5.992361111111111in"
height="1.6645833333333333in"}

Figure 6.4.2-(3): PL Spectra Before and After Etching (left: before,
right: after).

The peak near 3.5 eV on the etched specimen is an emission observed from
a location different from the GaN PL peak position. This peak shifts
systematically as the flow rate of PH~3~ is varied and no emission from
the substrate's GaN can be seen. Since, even after etching, something is
growing on the GaN, this is thought to be a result of inhibition of
emission from the GaN buffer layer of the substrate.

Next, GaP was grown on GaN with a PH~3~ flow rate of 100 sccm.
Observation of the photoluminescence spectrum of the surface after
etching showed no shifting of the PL peak with only emission from the
GaN.

Next, the PH~3~ flow rate dependency of the PL peak energy was examined
by varying the PH~3~ flow rate from 30 sccm to 100 sccm at 850ºC. The
findings are shown in Figure 6.4.2-(4), illustrating a PL peak shift
dependent on the PH~3~ flow rate. At 875ºC and 835ºC, band edge emission
occurs at 3.498 eV and 3.496 eV.

Figure 6.4.2-(5) shows test values for PL peak energy of a GaNP mixed
crystal fabricated by MBE and the theoretical equation for band gap. The
composition was determined from the band edge PL peak energy by
referencing the band gap bowing of 9.3 eV that was derived
experimentally from a GaNP mixed crystal up to 8.2%. As a result, we
found a P density of 0.6% at 100 sccm.

From the results above, growth of GaNP mixed crystals on a sapphire
(0001) substrate under conditions of 850ºC with a PH~3~ flow rate from
30 to 100 sccm resulted in a GaP-rich layer having a very rough surface.
Since the GaP-rich layer was eliminated by aqueous etching, neither
X-ray diffraction or PL measurements could confirm that it was GaP-rich.
Additionally, emissions from the GaNP were confirmed from the surface
after etching.

By estimating the P density from the PL peak energy of GaNP, we found
that we were capable of fabricating GaNP mixed crystals with a P density
of approximately 0.25, 0.5, and 0.6%.

![](media/image97.png){width="5.9847222222222225in"
height="3.040277777777778in"}

Left: Figure 6.4.2-(4): PH~3~ Flow Rate Dependency of Band Edge PL Peak
Energy.

Right: Figure 6.4.2-(5): Band Gap Bowing of GaNP Mixed Crystal.

## GaNP LEDs

### Methods and Means

Using laser-assisted MOCVD, we were able to grow GaNP with a high P
composition (up to 10%) on the N-rich side. We examined the
photoluminescence characteristics of these mixed crystals and confirmed
a maximum shift in the band edge emission peak by 0.2 eV from the GaN
band edge toward the long wavelength. Based on these results, we
fabricated a prototype SQW GaNP LED.

First, a Mg-doped GaN film was grown 2 µm thick on a sapphire substrate.
Then, using laser-assisted MOCVD, undoped n-GaNP was grown to a
thickness of 1 µm at 950ºC using TMG, TBP, and NH~3~ while exposed with
an ArF laser at 30 nm. The photoluminescence characteristics of the
resulting crystals were examined. Figure 6.5.1-(1) shows the PL
spectrum. We observed a sharp peak near 410 nm, a sub peak at 450 nm,
and a broad peak around 575 nm. This sample was used to fabricate the
LED structure. After depositing SiO~2~ on the GaN epitaxial surface,
photo-resist was used to create the pattern and then portions of the
n-GaN and GaNP layers were removed through dry etching to form p-type
electrodes. ECR (electron cyclotron resonance) plasma etching with Ar (7
sccm) / H~2~ (15 sccm / CH~4~ (5 sccm) combined gas was used for dry
etching. Subsequent to etching, an LED was fabricated through vapor
deposition of Al/Ti/Au for the n-type electrode and Pt/Au for the p-type
electrode.

![](media/image98.png){width="5.997222222222222in"
height="2.6909722222222223in"}

Left: Figure 6.5.1-(1): PL Spectrum.

Right: Figure 6.5.2-(1): I-V Characteristics of LEDs.

### Results and Observations

Figure 6.5.2-(1) shows the I-V characteristics of the fabricated LED.

Onset characteristics of the forward current were poor and it is
believed that this is because the n-GaN layer is undoped, resulting in a
high resistance.

![](media/image99.png){width="2.8256944444444443in"
height="2.1277777777777778in"}We plan on improving the equipment so it
is possible to dope with Si as an n-type impurity. The LED's ability to
withstand voltage in the reverse direction is greater than 20 V. We next
performed EL measurements of the GaNP LED. Figure 6.5.2-(2) shows the EL
spectrum. It shows a sharp peak around 425 nm and a broad peak around
590 nm. The emission color is a visible blue. An image of the LED
emission is shown in Figure 6.5.2-(3). Deep level emission around 590 nm
is thought to be caused by crystal defects in the GaNP. When a GaNP LED
is fabricated with the GaNP layer thickness at 100 nm, the broad
emission at 590 nm is stronger than the peak at 420 nm, as shown in
Figure 6.5.2-(4), and the color of the emission becomes a bluish white.
By thinning the emission layer and raising the crystallinity, deep level
emission is suppressed and it is thought that this is because the band
edge emission has become stronger.

The results above show that GaNP, apart from InGaN-based materials,
holds promise as a new blue light emitting diode material.

Figure 6.5.2-(2): EL Spectrum (with a thin GaNP film).

![](media/image100.png){width="2.7909722222222224in"
height="1.9305555555555556in"}

Left: Figure 6.5.2-(3): Image of Blue Light Emission from LED.

Right: ![](media/image101.png){width="2.8375in"
height="2.1395833333333334in"}Figure 6.5.2-(4): EL Spectrum (with a
thick GaNP film).

## Summary

Growth of GaNP with a high P composition was examined using two methods:
MOCVD using DMHy and PH~3~ source gases, and laser-assisted MOCVD.

Through an experiment with MOCVD using DMHy and PH~3~, and keeping the
V/III ratio of the source gases constant while varying the growth
temperature, SIMS analysis found 0.5% P was combined when the
temperature was set low. Among the optical characteristics we observed a
peak shift in the band edge caused by the addition of P to GaN with
respect to the PH~3~ flow rate.

Additionally, GaNP was grown by a new laser-assisted MOCVD method. The
results show the the source gases were photodecomposed at low
temperature and SIMS analysis found a P addition of about 10%. Using TBP
-- which is easily decomposed -- as the P source, we were able to attain
a crystal with a high P composition at a growth temperature of 850ºC. PL
measurements showed a shift of 0.2 eV in the band edge peak in the long
wavelength direction compared to GaN. Moreover, while inadequate from a
quantitative perspective, we observed a large red shift compared to
previous reports and believe we have confirmed band bowing in GaNP. The
degree of red shift shown in the photoluminescence spectra from our GaNP
specimens is two to three times that of conventional MOVPE.
Laser-assisted MOCVD has thus been shown to have high potential.

It is necessary to move forward with more emphasis on quantitative
research in 2001. With the creation of a prototype GaNP SQW LED, we were
the first to prove blue and blue white light emission by current
injection. This shows that, at a minimum, GaNP LEDs hold promise as a
blue LED.

## Future Course

In 2001, we intend to work with Professor Onabe of Tokyo University in
growing GaNP (As) crystals and finding the optimum growth conditions
under laser-assisted MOCVD by raising the intensity of the laser and
improving the equipment itself.

Tokyo University will examine crystal growth of GaNAs or GaNP by gas
source MBE which has a stronger non-equilibrium state, large band gap
bowing, and a high expectation for wave length shifting. We will also
work on optimizing the design of equipment used for laser-assisted
MOCVD. In other words, we will optimize the growth conditions to achieve
GaNP with a high P composition by making modifications to the substrate
susceptor, heater construction, and gas inlets.

We will raise the photoluminescence intensity of the LED by further
studying GaNP multiple quantum well structures and epitaxial structures.
To understand the light emission mechanism of GaNP we will continue to
research in detail photonic characteristics through photoluminescence
measurements. We will use selective epitaxial growth to increase the
brightness of the LED and to improve the crystal quality. Stated
differently, selective epitaxial growth, where epitaxial growth is
performed locally and at specified locations of the substrate surface
instead of across the entire surface, is thought to be more effective in
attaining high quality. Evaluation of photonic characteristics and the
light emission mechanism will be researched together with Professor
Taguchi of Yamaguchi University. Lastly, we will work on raising the P
composition and discovering the optimum growth structures and conditions
for high-brightness luminescence.

## References

\[1\] J. Kikawa, S. Yoshida, and Y. Itoh, Workshop on Nitride Semicond.
IPAP Cof. Ser. 1 (2000) 429.

\[2\] J. Kikawa, S. Yoshida, and Y. Itoh, J. Crystal Growth (2000) May
in Press.

\[3\] J. Kikawa, S. Yoshida, and Y. Itoh, Mat. Res. Soc. Symp. Proc.
(2001) G2.2, in press.

\[4\] Fundamentals of the Photon Excitation Process, K. Takahashi (1994)
Kogyochosakai.

\[5\] H. Okabe, J. Appl. Phys. 69, 1730 (1991).

\[6\] Th. Beuermann, Chemicals, 3, 230 (1988).

\[7\] K. Watanabe, J. Chem. Phys., 22, 1564 (1954).

\[8\] Kakuya Iwata, Molecular Beam Crystal Growth of III-V Nitride
Semiconductors, Osaka University Academic Dissertation (1998).

\[9\] P. J. Dean, Prog. Crystal. Growth Charact. 1982, Vol. 5, pp.
89-174.

\[10\] J. I. Pankove and J. A. Hutchby, J. Appl. Phys. 47 (1976) 5387.

\[11\] W. M. Jadwisienczak and H. J. Lozykowski, Mat. Res. Soc. Symp.
Proc. Vol. 482 (1998) 1033.

\[12\] T. Ogino and M. Aoki, Jpn. J. Appl. Phys. 18 (1979) 1049.

\[13\] C. Onodera and T. Taguchi, J. Crystal Growth, 101 (1990) 502.

\[14\] T. H. Kell, Phys, Rev. 140 (1965) A601.

\[15\] Development of Compound Semiconductors for High-Efficiency
Optoelectronic Conversion (Light for the 21^st^ Century) Report of
Results, p17, Japan Research and Development Center for Metals, (March,
2000).

\[16\] A. P. Levanyuk and V. V. Osipov, Sov. Phys. Usp, 24, 187 (1981).

\[17\] V.V. Osipov, T. I. Soboleva and M. G. Foigel, Sov. Phys.
Semicond., 13, (1979) 319.

\[18\] A. P. Levanyuk and V. V. Osipov, Sov. Phys. Semicond., 7, 751
(1973).

\[19\] T. Fukushima and S. Shinoyama, Jpn. J. Appl. Phys. 12 (1973) 549.

\[20\] Bonds and Structures of Molecules and Solids, D. Pettifor
(translated by M. Aoki and S. Nishiya), GIHODO SHUPPAN Co., Ltd. (1997).

# Development of MOCVD-grown Low-Defect GaN Thin Film Epi-Substrates

## Achievements in 1998 and 1999

Development of GaN-on-Sapphire substrates used for the homoepitaxial
growth of high-efficiency UV-emitting LEDs was continued. Although the
currently used technique, which is based on MOCVD, is capable of easily
and inexpensively producing large surface area substrates and numerous
practical expectations are pinned on it, it has certain problems, such
as the high density of dislocation defects and substrate bowing. In this
project, our goal is to implement 2-inch substrates of practical value,
in which the density of dislocations would be reduced from the current
10^9^ cm^-2^ to not more than 10^4^ cm^-2^.

Prior to 1999, we conducted basic research concerning the reduction of
dislocation density and systematically investigated the influence of the
growth temperature, pressure, growth atmosphere, mask material, crystal
directions, etc. in 10-mm substrates using ELOG (Epitaxial Lateral
Overgrowth), which is a well-known powerful technique. In terms of
dislocation density, we reached a level of 10^6^ cm^-2^. Our main
achievements are listed below.

\(1\) It was discovered that low-defect GaN was obtained by ELOG growth
if we changed the ordinary SiO~2~ mask to a W mask. It was confirmed by
X-ray diffraction, TEM observations, and PL observations that it was a
high-quality crystal that had an insignificant amount of *c*-axis
wobble, in other words, no low-angle tilt grain boundaries, and, in
addition, no dislocations caused thereby. Based on this, we assumed that
although active hydrogen generated by the contact reaction between the W
mask and ammonia etched away GaN in the vicinity of the W mask, creating
gaps, it did not tilt the laterally growing GaN on the mask.

\(2\) A novel "facet-controlled ELOG technique" ("FACELO" technique), a
type of ELOG, was developed. It was discovered that the morphology of
the facets growing in the window areas was dependent on the growth
temperature and pressure and could change to {11[2]{.underline}0}
rectangular facets as well as to {11[2]{.underline}2} facets.
Furthermore, it was determined that because the propagation behavior of
dislocations was different depending on the morphology of the facets, by
efficiently controlling the facet morphology, we could expect to achieve
a reduction in the density of dislocations without the dislocations
reaching the top face of the crystal.

\(3\) Simulations of substrate bowing were carried out to obtain a
relationship between the amount of bowing and the thickness of the GaN
film, etc. With the target amount of bowing in 2-inch substrates being
50 *μ*m or less, it was found that for the present the objective in
terms of GaN film thickness should be 10 *μ*m.

\(4\) In order to conduct observations of dislocations in GaN crystals,
we developed a FIB (Focused Ion Beam processing) - TEM observation
technique, which has been demonstrated to be an effective technique for
microscopic observation of dislocations. In addition, for the
measurement of dislocation density, we investigated the GPD (Growth Pit
Density) technique in comparison with the conventional EPD (Etch Pit
Density) technique and determined characteristics under both observation
techniques, as well as the fact that density according to both
techniques was identical.

\(5\) In order to investigate the effects of dislocation reduction on
the luminescence efficiency of LEDs, we fabricated blue LED structures
consisting of AlGaN/InGaN/GaN single quantum wells on substrates. When
using samples with a dislocation density of 10^8^ cm^-2^ \~ 10^9^ cm^-2^
on ordinary GaN-on-sapphire substrates without ELOG, the PL intensity
tended to increase with decreasing dislocation density. However, in case
of EL emission in HVPE samples, whose dislocation density had been
reduced to 2 × 10^8^ cm^-2^, no conspicuous difference was confirmed.

## Summary of Research and Development in 2000

Sapphire is commonly utilized in LED substrates used for GaN growth.
However, because there is a considerable difference in lattice constants
and thermal expansion coefficients between sapphire and GaN, even after
developing and optimizing the technology of inserting low-temperature
buffer layers of GaN and AlN, the threading dislocation density is still
extremely high, at 10^8^\~10^10^ cm^-2^ \[1,2\], which is believed to be
an important factor that significantly reduces the efficiency of
luminescence in ultraviolet LEDs with an emission wavelength of not more
than 400 nm.

As in 1999, we continued to focus our investigations on the ELOG
technique in order to implement practical 2-inch low-dislocation-density
GaN-on-sapphire substrates with a dislocation density of not more than
10^4^ cm^-2^ for use in epitaxial growth of high-efficiency UV-emitting
LEDs. We used the MOCVD equipment introduced in 1999, trying to achieve
a dislocation density of 10^5^ cm^-2^\~10^4^ cm^-2^ by investigating the
core technologies of dislocation reduction while further expanding the
use of the newly developed FACELO (Facet Controlled ELO:
facet-controlled ELOG) technique or effectively combining and
integrating it with other core technologies. Furthermore, although at
present we are conducting investigations using 10-mm substrates, we have
started investigations aimed at reproducing dislocation reduction in
2-inch substrates. A thorough understanding of the mechanism of
dislocation reduction is necessary for this purpose as well, and in this
connection we are trying to make the most of the analysis of information
concerning GaN crystal micro-regions. We decided to concentrate our
efforts on limited-field TEM, micro photoluminescence, etc.

The following results were obtained in 2000.

\(1\) We arrived at a level of dislocation density of 10^6^
cm^-2^\~10^4^ cm^-2^. Dislocation density was successively reduced by
means of carrying out FACELO growth on underlying GaN crystals with a
dislocation density of 10^7^ cm^-2^, in which the initial nucleus
density was controlled, by using a two step process consisting of
{11[2]{.underline}2} FACELO and then {11[2]{.underline}0} FACELO, which
allowed us to obtain low dislocation density Ga-on-sapphire substrates
which, to the best of our knowledge, had the best quality in the world.
We have started investigations into 2-inch substrates, but we are still
dealing with substrates of inadequate surface condition due to
incomplete burial.

\(2\) TEM observations etc. confirmed that the crucial point in the
reduction of dislocations by means of FACELO was the fact that
dislocations were bent by the {11[2]{.underline}2} triangular facets in
a direction normal to the *c*-axis and propagation could be terminated
by the void formed in the central portion of the mask, and the fact that
new dislocations originating from the top edge of the void could be
prevented from propagating all the way to the surface by the
{11[2]{.underline}0} rectangular facets.

## Investigations into Dislocation Defect Reduction by ELOG Growth

The selective lateral growth (Epitaxial Lateral Overgrowth: ELOG)
technique, in which a mask of insulating material, such as SiO~2~, is
patterned to the desired shape on a GaN substrate and GaN is then
selectively grown in the window areas, has been attracting attention as
a dislocation reduction technique. GaN ELOG started in 1997 with thick
film growth by the HVPE method, which permitted reduction of the
threading dislocation density to a level of 10^7^ cm^-2^ \[3\]. In
addition, as far as the mechanism of dislocation reduction is concerned,
it was discovered that it was related to a phenomenon in which
dislocations were bent by the {1[1]{.underline}01} facets formed by the
selective growth \[4\]. Subsequently, ELO was used in the MOCVD method
by Nam *et al*. \[5\] and Nakamura *et al*. \[6\], and although
dislocations propagated and remained in the window areas, considerable
reductions over the mask were reported.

In the course of the present project, it has been reported prior to last
year \[7\] that the morphology of MOCVD ELO facets could be controlled
by controlling the growth temperature and pressure \[7\]. Subsequently,
the "facet-controlled ELOG" (facet controlled ELO: FACELO) was proposed,
which is a technique for controlling the propagation of dislocations by
controlling the morphology of facets formed by selective growth \[8\].
FIG. 7.3 shows a model of two varieties of FACELO used for dislocation
propagation control.

![](media/image102.png){width="5.993055555555555in" height="3.2625in"}

FIG. 7.3 Control of propagation of dislocations and facet morphology by
FACELO.

This year, we continued our investigations into dislocation density
reduction by FACELO in cooperation with Mie University in order to
establish a technology for the fabrication of low-dislocation-density
GaN. In addition, with a view to further reduce the density of
dislocations by using the FACELO method, we achieved further evolution
in research and realized a defect density of 10^5^ cm^-2^ and are
planning to achieve a defect density on the order of 10^4^ cm^-2^ or
less.

In {11[2]{.underline}2} FACELO, we were able to reduce the dislocation
density to a level of 10^6^ cm^-2^. Dislocations remaining after
{11[2]{.underline}2} FACELO are dislocations rectilinearly arranged in
the center of the mask, namely, dislocations generated in the portion
where the ELO-GaN coalesces above the void and propagating to the
surface. For this reason, we carried out 2-step FACELO growth in an
attempt to further reduce the dislocation density by blocking the
dislocations remaining after the first step of {11[2]{.underline}2}
FACELO growth with a film grown in a second step of {11[2]{.underline}0}
FACELO growth.

In order to investigate the reduction of density dislocations in 2-inch
substrates, we examined the suitability of the MOCVD equipment
introduced in 1999 and decided to keep the growth conditions expected to
influence the ELOG process under strict control. In addition, in a
parallel investigation, we initiated research concerning facet
morphology control using the HVPE apparatus installed at the Mie
University in 2000 in order to understand the mechanism of dislocation
reduction in comparison with the MOCVD method.

### Methods and Means (Investigation into ELOG Growth for Reduced Dislocation Density)

#### Investigations into facet-controlled ELOG growth

Samples obtained by growing approximately 4 *μ*m of GaN on sapphire
(0001), with a low-temperature GaN buffer layer interposed in between,
were used as the underlying substrates. A SiO~2~ film with a thickness
of about 80 nm was deposited on the underlying substrates, and
photolithography was used to form three striped patterns with a
mask/window ratio of 7/3, 5/5, and 3/3 *μ*m. The stripes were formed in
parallel to \<1[1]{.underline}00\>, where the velocity of lateral growth
by ELOG is high and which permits the use of the facet control
technology. The TMG of the source material and the flow rate of NH~3~
were 46 *μ*mol/min and 1.0\~1.5 slm, with H~2~ used as the carrier gas.
In order to form the target ELO-GaN facets, the growth temperature and
pressure were varied within the range of from 950\~1050ºC and 80\~500
Torr. In addition, approximately 250 nm of InGaN was grown at a growth
temperature of 800ºC. Growth pits formed by InGaN thin film growth have
been reported to be in a one-to-one correspondence with threading
dislocations (*a* and *a*+*c*) \[9\]. The FACELO growth process is shown
in FIG. 7.3.1-(1).

![](media/image103.png){width="4.032638888888889in"
height="2.582638888888889in"}

FIG. 7.3.1-(1) FACELO growth process

\[Keys, FIG. 7.3.1-(1)\]

(Across top): First-stage ELO Second-stage ELO

(Middle, right) InGaN growth

(Bottom) FACELO-GaN growth

In the case of two-step double FACELO, after conducting a
{1[1]{.underline}22} FACELO as a first step of FACELO, we tested two
methods for use as a second step \[10, 11\]. The corresponding models
and mask pattern alignment are shown in FIG. 7.3.1-(2).

![](media/image104.png){width="4.477083333333334in"
height="4.723611111111111in"}

FIG. 7.3.1-(2) Models and mask pattern alignment of two-step double
FACELO

\[Keys, FIG. 7.3.1-(2)\]

(Underneath two drawings at top) Models (cross sections)

(Underneath two drawings at bottom) Mask alignment

(Left) Model (B on B)

(Right) Model (A on B)

Models of dislocation propagation are shown in FIG. 7.3.1-(3) and FIG.
7.3.1-(4). In case of growth based on Model (B on B),
{11[2]{.underline}2} FACELO according to model B, which is shown in FIG.
7.3.1-(2) (first-stage ELOG using a growth pressure of 500 Torr, a
growth temperature of 950ºC, a growth time of 45 min; second stage ELOG
using a growth pressure of 500 Torr, a growth temperature of 1050ºC, and
a growth time of 105 min), was carried out by forming a mask along the
\<1[1]{.underline}00\> direction, and, after that, a mask stripe was
patterned along the \<10[1]{.underline}0\> direction. In other words,
the second mask was formed by rotating it 60º with respect to the first
mask, and then a second-stage FACELO was conducted under the same growth
conditions as the first-stage FACELO. In this case, the second mask was
simply geometrically rotated 60º, and mask formation presented no
difficulties.

![](media/image105.png){width="5.993055555555555in" height="2.725in"}

FIG. 7.3.1-(3) Model of dislocation propagation in 2-step double FACELO
model (B on B)

\[Keys, FIG. 7.3.1-(3)\]

(Underneath figure on the left) When first and second masks are aligned

(Same on the right) When first and second masks are not aligned

![](media/image106.png){width="3.40625in"
height="2.4270833333333335in"}In case of model (A on B),
{11[2]{.underline}2} FACELO based on model B (first-stage ELOG using a
growth pressure of 500 Torr, a growth temperature of 950ºC, a growth
time of 45 min; second stage ELOG using a growth pressure of 500 Torr, a
growth temperature of 1050ºC, and a growth time of 105 min) was carried
out, whereupon a stripe mask was formed so that the second mask was
positioned exactly above the first mask, that is, in parallel to
\<1[1]{.underline}00\>. Then, second-stage FACELO was carried out using
model A-based {11[2]{.underline}0} FACELO (using a growth pressure of
300 Torr, a growth temperature of 1050ºC, and a growth time of 60 min).
In this case, our goal was to get rid of dislocations in the
second-stage FACELO by using the second mask to terminate the
rectilinearly arranged dislocations of the first-stage FACELO.

FIG. 7.3.1-(4) Model of dislocation propagation in 2-step double FACELO
model (A on B)

#### Research concerning 2-inch substrates

In the course of our investigations into 2-inch substrates, first of
all, we examined the influence of the growth temperature, the V/III
ratio (ratio of Group V material to Group III material), and other
growth parameters on growth at atmospheric pressure. The substrates used
in the study were obtained by growing approximately 3 *μ*m of GaN on the
C-plane of a sapphire substrate and then using SiO~2~ to form on its
surface a stripe-shaped mask pattern in the \<1[1]{.underline}00\>
direction of GaN. Trimethylgallium (TMGa) and ammonia (NH~3~) were used
as the source materials, with hydrogen used as the carrier gas.

Prior to the ELOG investigation, we checked the uniformity of film
thickness across the surface and confirmed that, as
![](media/image107.png){width="4.569444444444445in"
height="3.6277777777777778in"}shown in FIG. 7.3.1-(5), the distribution
of the film thickness was maintained within ±2% within a 30-mm radius
from the center. The mask pattern used for the ELOG investigation is
shown in Table 7.3.1.

FIG. 7.3.1-(5) Uniformity across surface of 2-inch substrate

\[Keys, FIG. 7.3.1-(5)\]

(X-axis) Position, mm

(Y-axis) Film thickness, μm.

  ----------------------- ----------------------- -----------------------
                          SiO~2~ mask width (L)   Window width (S)

  Pattern                2 *μ*m                  8 *μ*m

  Pattern                5 *μ*m                  5 *μ*m

  Pattern                2 *μ*m                  2 *μ*m
  ----------------------- ----------------------- -----------------------

Table 7.3.1 Mask patterns

#### Investigation of HVPE-based ELOG

The HVPE apparatus introduced in the present study was a horizontal
reactor. The electric furnace was a resistance-heating furnace divided
into 7 zones, with the upstream 4 zones used as a gallium chloride
(GaCl) production region, and the downstream 3 zones used as a growth
region. GaCl supplied to the growth region was produced by reacting
metallic Ga with a purity of 6N with HCl at about 850ºC. In addition,
NH~3~ was used as a Group V source material. N~2~ and H~2~, in mixed gas
form, were used as the carrier gas. The susceptor permits rotation for
uniform film growth on the substrate.

On the substrates used for the ELOG of GaN, the SiO~2~ masks formed
stripe patterns with a mask/window ratio of 5/5 *μ*m in the
\<1[1]{.underline}00\> direction and \<11[2]{.underline}0\> direction of
the GaN. The carrier gas N~2~ and the source gas NH~3~ were supplied
from the start of the temperature rise. NH~3~ was introduced in order to
suppress desorption of nitrogen from the GaN substrates at elevated
temperatures. When deposition was conducted in a mixed atmosphere of
N~2~ and H~2~ upon reaching the growth temperature, H~2~ was introduced
in the carrier gas and GaN growth was initiated by simultaneously
introducing HCl. In addition, NH~3~ was supplied until a temperature of
500ºC was reached in order to control desorption during the lowering of
the temperature. The rotation velocity of the susceptor was 10 rpm, the
growth temperature was 1070ºC, and the total gas flow rate was 2.5 slm
(with an NH~3~-based total of 2.0 slm, and an HCl-based total of 0.5
slm).

### Results and discussion (investigation of ELOG growth-based dislocation defect reduction)

#### Investigations of facet-controlled ELOG

![](media/image108.png){width="3.3055555555555554in"
height="4.104166666666667in"}To carry out {11[2]{.underline}0} FACELO,
which forms dislocation-free regions above the mask, as shown in Model
(A) illustrated in FIG. 7.3, a rectangular morphology, in which
{11[2]{.underline}0} constituted the side faces, was created during the
first-stage ELOG. Then, during the second-stage ELOG, the lateral growth
velocity was increased in order to bury the mask. During the experiment,
the first-stage ELOG was carried out on substrates with a mask/window
ratio of 7/3 *μ*m using a growth pressure of 80 Torr, a growth
temperature of 950ºC, and a growth time of 30 minutes, and the
second-stage ELOG was conducted using a growth pressure of 500 Torr, a
growth temperature of 1050ºC, and a growth time of 90 minutes. FIG.
7.3.2-(1) shows an SEM image of a sample. As shown in the surface SEM
image, dislocations are distributed largely in the window areas, and
their amount is greatly decreased in the mask areas. In addition, in the
center of the mask areas, the dislocations are arranged in a rectilinear
fashion. The dislocations are believed to be due to a lattice plane
shift during the coalescence of the ELO-GaN and to the influence of the
strain generated thereby. The average dislocation density, including the
window areas and the mask areas of the sample, was 6.0 × 10^7^ cm^-2^,
which constituted approximately 1/3 of the dislocation density of the
underlying GaN.

FIG. 7.3.2-(1) {11[2]{.underline}0} FACELO-GaN (Mask/Window = 7 μm/3 μm)

\[Keys, FIG. 7.3.2-(1)\]

(Top diagram, on the left) Surface image.

(Bottom diagram, same) Bird's eye view.

It is believed that in case of {11[2]{.underline}2} FACELO based on
Model B, as a result of the formation of the {11[2]{.underline}2} plane
during the first-stage ELOG, dislocations can be bent to switch from a
direction normal, to a direction parallel, to the (0001) plane. Then the
bent dislocations coalesce together and disappear or propagate to the
surface when the ELOG layer coalesces in the center of the mask during
the second-stage ELOG. For this reason, the dislocations concentrate in
the central portion of the mask. In the experiments, the first-stage
ELOG was conducted at a growth pressure of 500 Torr and a growth
temperature of 950ºC, and in the second-stage ELOG, the growth pressure
of 500 Torr was left unchanged while the growth temperature was raised
to 1050ºC.

![](media/image109.png){width="2.816666666666667in" height="3.5in"}FIG.
7.3.2-(2) shows an SEM image of a sample obtained by conducting
{11[2]{.underline}2} FACELO (first-stage ELOG: 45 min, second-stage
ELOG: 105 min) on a substrate with a mask/window ratio of 5/5 *μ*m. As
can be confirmed by examining the cross-section SEM image, a void with a
height of 5\~7 *μ*m was formed in the coalescence portion of the ELOG
layer. As is also evident from the surface SEM image, the dislocations
were arranged in a rectilinear fashion in the center of the mask area
only, with the remaining region (region with a width of 10 *μ*m) being
practically dislocation-free.

FIG. 7.3.2-(2) {1122} FACELO GaN

\[Keys, FIG. 7.3.2-(2)\]

(Top figure, on the left) Surface image.

(Bottom figure, same) Cross-section.

The bending of the dislocations by the {11[2]{.underline}2} plane is
expected based on the fact that dislocations do not propagate to the
surface above the window areas in the cross-section SEM image, and, as
will be described later, this is also confirmed by TEM observations.
Then, since the amount of dislocations arranged in the center of the
mask area is considerably reduced, it is believed that the dislocations
are terminated by the void. The dislocation density of the sample was 6
\~10×10^6^ cm^-2^, which points to a reduction by more than an order of
magnitude in comparison with the dislocation density of 2\~4×10^8^
cm^-2^ of the underlying GaN.

![](media/image110.png){width="2.775in" height="3.4375in"}Results of the
double FACELO are shown below. First of all, SEM images of the two-step
FACELO-GaN based on Model (B on B) are shown in FIG. 7.3.2-(3).
Partially dislocation-free regions are confirmed based on the
distribution of InGaN growth pits shown in the surface SEM images, but
there remained dislocations believed to be produced in the coalescence
portion of the second-step FACELO. The dislocation density of the sample
was 2\~10×10^6^ cm^-2^, which means there was a reduction by about 1/2
in comparison with the dislocation density of 6\~10×10^6^ cm^-2^ of the
one-step FACELO; however, a reduction in the amount of dislocations
located in the ELOG coalescence portion is critical for achieving
further reductions in the dislocation density.

FIG. 7.3.2-(3) Surface and cross-sectional SEM images of second
FACELO-GaN based on Model (B on B)

\[Keys, FIG. 7.3.2-(3)\]

(Top figure, inside) Surface image.

(Bottom figure, inside) Cross-sectional view.

(Bottom figure, right, top to bottom) InGaN; Second FACELO; -GaN; First
FACELO; -GaN; Underlying GaN

Next, a cross-section image of the two-step FACELO-GaN based on Model (A
on B) is shown in FIG. 7.3.2-(4). It can be seen that the second mask is
placed directly above the first mask. As a result, as shown in the
model, the dislocations remaining in the coalescence portion in the
first FACELO are terminated by the second mask, and because the first
window areas are dislocation-free, there are no dislocations propagating
to the second window areas. As a result, there are no dislocations on
the surface.

![](media/image111.png){width="5.988194444444445in"
height="2.5527777777777776in"}

FIG. 7.3.2-(4) Bird's eye view SEM image of second FACELO-GaN based on
Model (A on B)

\[Keys, FIG. 7.3.2-(4)\]

(Right, top to bottom) Surface; Second FACELO-GaN; First; FACELO; -GaN;
Underlying GaN

![](media/image112.png){width="3.3854166666666665in"
height="2.1979166666666665in"}A surface SEM image is shown in FIG.
7.3.2-(5). The surface area of the image is 4.4×10^6^ cm^-2^, with
absolutely no dislocations found within this region. In other words, one
can say that the dislocation density is not more than 2×10^5^ cm^-2^.
Dislocations arranged in a rectilinear fashion were observed in the
portions where the second mask did not coincide with the first mask. In
addition, in certain areas, we confirmed the presence of dislocations
generated in the coalescence portion of the second FACELO, and it is
believed that further optimization of growth conditions is necessary.

FIG. 7.3.2-(5) Surface SEM of second FACELO-GaN based on Model (A on B).

As shown above, our purpose was to achieve a reduction in the amount of
dislocations based on an ELOG technique (FACELO) intended for
controlling the propagation of dislocations by means of controlling the
facet morphology generated by selective growth. In case of the
{11[2]{.underline}0} FACELO shown in Model A, the density of
dislocations is determined by the ratio of the size of the mask and the
window, and if the wobble of the *c*-axis of the crystal is taken into
consideration, the ratio cannot be large. However, the advantage is that
burial can be carried out using a thin ELO layer. On the other hand, a
dislocation density reduction down to a level of 10^4^ cm^-2^ proved
possible in case of the {11[2]{.underline}2} FACELO based on Model B.
Rectilinearly arranged dislocations remained only in the coalescence
portion in the center of the mask, and there were no dislocations in the
window areas. In order to achieve a dislocation density on the order of
10^4^ cm^-2^ or less, which is the goal of the present project, we
repeated FACELO in two steps. In the {11[2]{.underline}2} FACELO,
dislocations rectilinearly arranged in the center of the mask were
terminated by a second mask. The realized dislocation density was not
more than 2×10^5^ cm^-2^, which nearly met the target value. In the
future, we are planning to achieve a further reduction in the density of
defects by means of further optimizing mask material, morphology, and
growth conditions along with conducting research into the application of
this technology to 2-inch substrates of practical value.

#### ![](media/image113.png){width="3.595138888888889in" height="8.622222222222222in"}Investigations of 2-inch substrates

FIG. 7.3.2-(6) shows results concerning the differences in the SiO~2~
mask width (L) and window width (S) depending on the growth temperature.

FIG. 7.3.2-(6) Differences in the growth pattern depending on changes in
growth temperature

\[Keys, FIG. 7.3.2-(6)\]

\(a\) Pattern , 1130ºC (b) Pattern , 1130ºC

\(c\) Pattern , 1110ºC (d) Pattern , 1110ºC

\(e\) Pattern , 1080ºC (f) Pattern , 1080ºC

\(g\) Pattern , 1060ºC (h) Pattern , 1060ºC

\(i\) Pattern , 1040ºC (j) Pattern , 1040ºC

At elevated growth temperatures of 1130ºC and 1110ºC, the action of
etching was clearly visible, and the presence of no-growth regions was
confirmed in patterns with a small window width. When the growth
temperature was decreased from 1130ºC to 1110ºC, the etching action was
reduced. When the growth temperature was further decreased to 1080ºC, in
pattern , voids appeared above the mask, growth from the window areas
proceeded in a reverse mesa shape, and a flat C-plane appeared on the
top face. In pattern , growth occurred along the window areas. At
1060ºC, in case of pattern , the voids above the mask became smaller,
forming a near-square cross-section. The adjacent crystal above SiO~2~
was in contact therewith, but no coalescence took place. In case of
pattern , the entire cross section assumed the same shape, with a
C-plane observed on the top face and planes, believed to be
{11[2]{.underline}2} planes, observed on the inclined faces. At 1040ºC,
in pattern , no voids were generated above the SiO~2~ mask. C-planes
appeared on the top face, and inclined faces, believed to be
{11[2]{.underline}2} planes, became wider than at 1060ºC. In pattern ,
the inclined faces were clearly visible, and the flat region of the top
face became narrower. These phenomena, as described above, coincide with
the tendency of {11[2]{.underline}2} planes to emerge easily during low
temperature growth.

![](media/image114.png){width="4.708333333333333in"
height="5.71875in"}FIG. 7.3.2-(7) shows results obtained by growth
conducted by varying only the amount of the supplied ammonia in order to
study the influence of the V/III ratio. In the case of pattern , when
the amount of the supplied ammonia was 3 slm, the cross section was
triangular in shape, but when it was set to 1 slm, a flat surface
appeared on the top face and the proportion of the inclined faces
decreased, resulting in a cross section close to a square in shape (a,
b). In pattern , the coalescence of the triangular crystals started at
3 slm, leaving behind residual V-shaped trenches. At 1 slm, a flat
surface appeared on the stripe-shaped top portion, with trenches between
crystals becoming shallower. However, there was no coalescence of
crystals (c, d).

FIG. 7.3.2-(7) Differences in growth pattern depending on amount of
supplied ammonia

\[Keys, FIG. 7.3.2-(7)\]

\(a\) Pattern , NH3: 3slm (b) Pattern , NH3: 1slm

\(c\) Pattern , NH3: 3slm (d) Pattern , NH3: 1slm

\(e\) Pattern , NH3: 1slm (f) Pattern , NH3: 0.75slm

As shown in (e) and (f) of FIG. 7.3.2-(7), when there were slightly
inclined faces along with a flat surface on the top face (amount of
supplied ammonia: 1 slm), after reducing the amount of supplied ammonia
to 0.75 slm the inclined faces disappeared, producing voids between
adjacent crystals.

The results above coincide with the tendency of inclined faces to grow
and produce a near-triangular cross section when the V/III ratio is
increased and to produce near-square cross section when the V/III ratio
is reduced.

![](media/image115.png){width="4.097222222222222in"
height="3.1666666666666665in"}FIG. 7.3.2-(8) shows results obtained by
conducting growth under the conditions, under which triangular-shaped
crystals are obtained in (c) of FIG. 7.3.2-(7) using pattern , and then
conducting growth under conditions, under which a flat surface is formed
on the top face and trenches become shallower, as shown in (d). Although
it could be confirmed that the flat surface occupied most of the face,
coalescence was insufficient, and the condition of the surface was poor.
It was confirmed that pits were generated above the SiO~2~, mask where
the coalescence of the crystals takes place. The pits were arranged in
the same \<1[1]{.underline}00\> direction as the direction of the
stripes.

FIG. 7.3.2-(8) Results of second ELOG

\[Keys, FIG. 7.3.2-(8)\]

(Top) \<1100\> direction

(Right0 SiO2 mask.

Based on the above, we started investigations into 2-inch
low-dislocation substrates. The current results indicate insufficient
burial. We have conducted investigations into the influence exerted by
growth temperature, V/III ratio, and other parameters during ELOG growth
under normal pressure. When the growth temperature is high, a flat
surface forms easily on the top portion, and when the growth temperature
is low, inclined faces tend to form easily. The growth pattern changes
greatly with slight changes in the growth temperature (about 20ºC). When
the V/III ratio is large, inclined faces are formed and
triangular-shaped crystals grow, whereas when the V/III ratio is small,
a flat surface is formed on the top face and square-shaped crystals are
obtained. Thus, it is concluded that the amount of supplied ammonia also
exerted a strong influence on the growth pattern.

In the present study, observations showed that adjacent crystals did
come in contact with each other above the SiO~2~ mask, but it is
believed that there was no coalescence. In addition, even if the
coalescence did occur, V-shaped trenches and pits were generated, and a
flat surface over the entire face was not obtained. This is believed to
be due to the fact that the lateral growth velocity on the SiO~2~ mask
is small and the crystal axis is slightly inclined, and we believe that
the lateral growth velocity has to be increased. In the future, we are
planning to conduct investigations including growth parameters such as
growth pressure and atmosphere, which are believed to exert influence on
lateral growth.

#### Investigation of HVPE-based ELOG

In order to investigate the effects of the carrier gas on the morphology
of facets produced by selective lateral growth, we studied morphological
changes produced by variations in the proportion of H~2~ in the carrier
gas. Cross sectional views and perspective view SEM images of crystals
obtained by conducting growth for three minutes on substrates with a
mask/window ratio of 5 *μ*m/5 *μ*m in the \<1[1]{.underline}00\>
direction of the GaN using a carrier gas with a H~2~/(N~2~+H~2~) ratio
of 0, 0.5, and 0.9 are shown in FIG. 7.3.2-(9).

![](media/image116.png){width="4.375in"
height="6.916666666666667in"}When H~2~ was not used, in other words,
when the ratio H~2~/(N~2~+H~2~) was 0, only {11[2]{.underline}2} and
(0001) facets emerged. These planes were identical to the facet planes
formed in the initial stage of selective growth by the MOCVD method. By
contrast, when the H~2~/(N~2~+H~2~) ratio was 0.5, {11[2]{.underline}2}
facets emerged on the side faces and, closer to the mask, there emerged
75\~77º faces, and so far as we know, these 75\~77º faces have not been
previously reported. In terms of the index of these faces, the most
likely candidate is {33[6]{.underline}2}. At a H~2~/(N~2~+H~2~) ratio of
0.9, the proportion of {33[6]{.underline}2} facets became even larger.
These morphological changes are believed to be due to a decrease in the
lateral growth velocity caused by using H~2~ in the carrier gas. On the
other hand, when using a mask in the \<11[2]{.underline}0\> direction,
the side faces were {1[1]{.underline}01} facets regardless of the ratio
of H~2~ in the carrier gas.

FIG. 7.3.2-(9) Surface morphology depending on carrier gas ratios

\[Keys, FIG. 7.3.2-(9)\]

(Across top, left to right) Cross sectional view, perspective view

![](media/image117.png){width="3.25in" height="6.6875in"}In order to
study how facet morphology changes over time during selective lateral
growth, we observed changes in crystal morphology over time using a
H~2~/(N~2~+H~2~) ratio of 0.5. The \<1[1]{.underline}00\> direction was
used for the SiO~2~ mask. Cross sectional views and bird's eye view SEM
images of crystals obtained by conducting growth for 1, 3, 5, and 10
minutes are shown in FIG. 7.3.2-(10). In the case of 1-minute growth,
{11[2]{.underline}2} emerged on the side faces. By contrast, when growth
was conducted for 3 minutes, {11[2]{.underline}2} emerged on the side
faces and {33[6]{.underline}2} below. At a growth time of 5 minutes, the
{11[2]{.underline}2} and the {33[6]{.underline}2} in the lower portion
remained unchanged, and coalescence of the ELOG crystal was starting to
show. At 10 minutes, the coalescence was complete, and the SiO~2~ mask
was buried. A smooth crystal with a thickness of 40\~60 *μ*m was
obtained by conducting growth for 60 minutes. In order to investigate
the effects of the mixed carrier gas, X-ray rocking curve measurements
were carried out for crystals obtained by 60-min ELOG. The *c*-axis of
the crystal on the mask had been reported to wobble when using a SiO~2~
mask with a stripe pattern. (0004) diffraction X-ray rocking curves were
measured in two cases, i.e. when the incident X-rays were oriented in
the horizontal direction (φ=0º) and in the vertical direction (φ=90º)
with respect to the mask. The half-width in the case of φ=90º was larger
than that in case of φ=0º, and it was believed that the *c*-axis of the
crystal was tilted. However, the half-width at φ=0º was 216 arcsec, an
indication of the high quality of the crystal.

FIG. 7.3.2-(10) Surface morphology depending on growth time

\[Keys, FIG. 7.3.2-(10)\]

(Across top, left to right) Cross sectional view, perspective view

\(a\) Growth time: 1 minute

\(b\) Growth time: 3 minutes

\(c\) Growth time: 5 minutes

\(d\) Growth time: 10 minutes

As mentioned above, we obtained and installed HVPE equipment and
investigated growth conditions in a study for comparison with the MOCVD
method. We investigated ELOG-GaN obtained by using a mixed carrier gas
of H~2~ and N~2~ by focusing on the study of changes in facet
morphology. When a mask with stripes arranged in the
\<1[1]{.underline}00\> direction was used, the addition of H~2~ resulted
in {11[2]{.underline}2} and (0001) at a growth time of 1 minute, and
when growth was conducted for 3 minutes, {33[6]{.underline}2} emerged,
with complete burial achieved at 10 minutes. The emergence of
{33[6]{.underline}2} was due to the decrease in the lateral growth
velocity due to the addition of H~2~. In the case of a
\<11[2]{.underline}0\>-direction mask, only {1[1]{.underline}01} facets
emerged on the side faces.

In the future, we are going to study the relationship between the facet
morphology and the propagation of dislocations and investigate optimum
growth conditions.

## Evaluation of ELOG-grown substrates

![](media/image118.png){width="3.9375in"
height="3.2708333333333335in"}As mentioned in the previous section,
which described investigations of ELOG growth, we developed a novel
FACELO technique, which is a technology for controlling the direction of
propagation of dislocation lines by means of controlling facets in order
to reduce the density of dislocations. By using SEM observations of
growth pits of FACELO-GaN, we compared the distribution of dislocations
in different samples grown by FACELO under different conditions and
proposed a model of the dislocation reduction mechanism. FIG. 7.4 shows
typical growth models of FACELO-GaN. In model A, {11[2]{.underline}0}
facets are produced during growth in the first stage and the lateral
growth velocity is increased in the second stage. Under such conditions,
dislocation lines do not bend during the FACELO process, with multiple
dislocation lines reaching the surface and corresponding growth pits
observed on the surface. In model B, {11[2]{.underline}2} facets are
produced during growth in the first stage and the lateral growth
velocity is increased in the second stage. In such a case, the
dislocation lines bend towards the mask region**(s)** and are terminated
by the void produced above the mask. Only dislocations produced in the
junction portion at the top of the void reach the surface, with growth
pits appearing to be arranged only in parallel to the stripes of the
mask. In order to ascertain the adequacy of this model, we carried out
observations of dislocation lines using TEM. Furthermore, analysis of
Burger's vectors was also conducted in order to study the nature of the
dislocations.

FIG. 7.4 FACELO-GaN growth models

It is known that in ELO-grown samples, depending on the mask material,
there may be differences in the physical properties of the material
above the mask pattern (wing portions) and above the opening portions
\[13-15\]. When LED structures are grown on top of ELOG-GaN crystals, it
is important to control this difference in characteristics. In order to
study the positional relationship with respect to the mask pattern, in
this study, to conduct this evaluation, we used the newly introduced
micro-photoluminescence (micro-PL) technique because of its superior
position analysis capabilities. Furthermore, the influence of the
material of the SiO~2~ mask and the W mask on the ELOG-GaN crystal was
evaluated at the Yamaguchi University by means of time resolved
photoluminescence.

### Methods and Means

#### TEM of substrates grown by facet-controlled ELOG

The samples used for evaluation were grown by ELOG in the following
manner. A low-temperature buffer and an underlying GaN layer were grown
on (0001) sapphire substrates by MOVPE. SiO~2~ was vapor deposited
thereon by RF sputtering, and a stripe-shaped mask pattern was formed
along the \<1[1]{.underline}00\> direction of the GaN by
photolithography. Two types of FACELO-GaN layers were grown. One of them
was obtained by using samples corresponding to Model A (Sample A), on
which a mask with width of 7 *μ*m was grown with a periodicity of 10
*μ*m by conducting first-stage growth at 80 Torr and 1000°C for 30
minutes and second-stage growth at 500 Torr and 1050°C for 90 minutes.
Finally, in order to produce growth pits, InGaN was grown thereon.
Another type of layer was obtained by using samples corresponding to
Model B (Sample B) and growing a mask with a width of 5 *μ*m with a
periodicity of 10 *μ*m at 500 Torr and 950°C for 45 minutes in the first
stage, at 500 Torr and 1025°C for 105 minutes in the second stage, and
then conducting additional growth at 1070°C for 30 minutes. InGaN was
grown as a final step. For TEM analysis, we prepared cross sectional
samples with a thickness of approximately 100 nm by using the Focused
Ion Beam technique and analyzed them using the JEM-4000FX from NEC at an
accelerating voltage of 350 kV.

#### Micro-photoluminescence of facet-controlled ELOG-GaN

Samples (1)\~(4), which were used for evaluation, were obtained by
varying the conditions of ELOG growth on GaN, on which SiO~2~ masks had
been patterned to produce a striped shape in the \<1[1]{.underline}00\>
direction.

 Sample illustrating process of burial starting from triangular facets

 Sample illustrating process of burial starting from square facets

 Sample illustrating burial by one-step regrowth

 Sample obtained by forming triangular facets and then completing
burial using square facets

 and  are samples grown at atmospheric pressure by Sumitomo Chemical
Co., Ltd., whereas  and  are samples grown at reduced pressure (\~0.4
atmospheres) by Mie University. The pitch of the mask pattern was 10
*μ*m in either case, but the width of the mask pattern was 2 *μ*m in
case of  and  and 5 *μ*m in case of  and . A micro-PL apparatus from
Atago Bussan Co., Ltd. was used. A He-Ne laser was used as an excitation
light source, with the position analysis resolution being approximately
one *μ*m.

#### Time resolved photoluminescence of ELOG-GaN

ELOG-GaN samples utilizing W- and SiO~2~ masks were subjected to time
resolved PL measurements using a femtosecond laser and PL measurements
using a He-Cd laser in order to study the respective excitation density
dependences and temperature dependences. The time resolved PL
measurement system is shown in FIG. 7.4.1.

![](media/image119.png){width="5.991666666666666in"
height="4.4215277777777775in"}

FIG. 7.4.1 Time resolved spectroscopy measurement system

\[Keys, FIG. 7.4.1\]

Wavelength: 267 nm

Pulse width: 200 fs

Repetition rate: 250 kHz

The third harmonic of a titanium-sapphire laser was used as the
excitation light with an excitation wavelength of 267 nm, a pulse width
of 200 fs, and a repetition rate of 250 kHz. The light emitted from the
samples was detected using a streak camera attached to a spectrometer,
and three-dimensional data were collected, including light intensity,
light wavelength, and time. The time resolution capability was 10 ps or
less.

### Results and Discussion

#### TEM of facet-controlled ELOG substrates

TEM images of {11[2]{.underline}0} square-facet FACELO samples and
{11[2]{.underline}2} triangular-facet FACELO samples are shown,
respectively, in FIG. 7.4.2-(1a) and FIG. 7.4.2-(1b). The figures
inserted in the corners are SEM images of growth pits. The dark circular
structures were produced during the fabrication of the cross sections
and do not correspond to structures of the samples themselves.

![](media/image120.png){width="5.99375in" height="3.6243055555555554in"}

![](media/image121.png){width="5.975694444444445in"
height="3.982638888888889in"}

Top: FIG. 7.4.2-(1a) TEM of {1120} square-facet FACELO-GaN substrate.

Bottom: FIG. 7.4.2-(1b) TEM image of {1122} triangular-facet FACELO-GaN
substrate.

In Sample A, the dislocation lines of the mask region are terminated by
the mask, but the numerous dislocation lines of the window region reach
the surface. On the other hand, in Sample B, the multiple dislocation
lines of the window region are bent in the direction of the mask region
and terminated by the void in the top portion of the mask. The remaining
dislocation lines disappear at the {11[2]{.underline}2} facet comprising
the edge portion of the mask. These are bent in a direction parallel to
the stripes of the mask (direction normal to the surface of the paper in
the figure) and are believed to extend outside the sample, which was
thinned for TEM observations. Only the dislocation lines generated at
the top edge of the void reached all the way to the surface.

Next, in order to study the relationship between the nature of the
dislocations and the direction of bending, we carried out a Burger\'s
vector analysis. FIG. 7.4.2-(2) is a TEM image of the vicinity of the
edge of the mask in Sample B, where (a) is a bright field image, and (b)
and (c) are dark field images obtained using (11[2]{.underline}0) and
(0002) diffraction waves, respectively. Dislocation lines bent in the
direction of the mask region are observed in both dark field images,
whereas dislocation lines bent in the direction of the stripes of the
mask are only observed in the image in which (11[2]{.underline}0)
diffraction waves were used. Based on these results, it is evident that
mixed dislocations are bent in the direction of the mask region and are
terminated by the void, whereas edge dislocations are bent in a
direction parallel to the stripes of the mask.

![](media/image122.png){width="5.990972222222222in"
height="2.4569444444444444in"}

FIG. 7.4.2-(2) TEM image of the vicinity of the mask edge (Sample B).
(a) Bright field image; (b), (c) Dark field images

FIG. 7.4.2-(3) shows dark field images of dislocation lines generated at
the top edge of the void. Judging by the contrast of the images, it was
determined that both mixed dislocations and edge dislocations were
present among the dislocation lines generated at the top edge of the
void and both types of dislocations generated growth pits.

![](media/image123.png){width="5.99375in" height="2.2090277777777776in"}

FIG. 7.4.2-(3) TEM images of the top edge of the void (dark field
images)

FIG. 7.4.2-(4) shows a three-dimensional sketch of the dislocation
reduction mechanism in FACELO-GaN, as confirmed in the present study. It
is noteworthy that reduction of dislocations in FACELO-GaN requires
growing only 10 *μ*m.

![](media/image124.png){width="3.2916666666666665in"
height="3.5277777777777777in"}

FIG. 7.4.2-(4) Dislocation propagation mechanism in FACELO-GaN

#### Micro-photoluminescence of facet-controlled ELOG-GaN

FIG. 7.4.2-(5)\~FIG. 7.4.2-(8) show two-dimensional maps of peak
intensity in cross-sections made in a direction normal to the stripes of
the samples. In the figures, the bottom side represents the substrate,
and the top side represents the surface of the crystal. In all cases,
measurements were conducted with a pitch of 1 *μ*m and mapping was
subjected to smoothing. Results obtained for each sample are described
below.

 A sample illustrating the process of burial starting from triangular
facets is shown in FIG. 7.4.2-(5). It exhibits a characteristic peak
intensity distribution, where the peak intensity is stronger on the
facet-forming slanted faces ({11[2]{.underline}2} faces) and weaker
above the opening portions, with an intensity ratio of approximately one
order of magnitude. The same distribution has been reported for
observations of cross sections in HVPE samples using cathodoluminescence
(CL), and is believed to be due to the generation of defects on the
faces, where facets are formed. Also, it has been confirmed by micro-PL
that the peak wavelength is longer and the half-width becomes wider in
positions of high peak intensity, and these results are believed to
indicate an increase in the incorporation of defects or impurities in
this portion.

![](media/image125.png){width="5.9944444444444445in"
height="3.3361111111111112in"}

FIG. 7.4.2-(5) Cross sectional PL intensity mapping for a sample
illustrating process of burial starting from triangular facets

\[Keys, FIG. 7.4.2-(5)\]

(Right, top to bottom) Regrowth layer; Buffer layer

(Across bottom) Opening Opening Opening Opening

-   An example illustrating the process of burial starting from square
    facets is shown in FIG. 7.4.2-(6). In this sample, above the mask
    pattern, there are high-intensity portions rectilinear in the
    direction of growth. Based on the fact that this is a square-facet
    sample, it is evident that these portions of high peak intensity are
    interfaces ({11[2]{.underline}0} faces) of growth in the lateral
    direction. The fact that portions of large half-width and portions
    of large peak wavelength coincide with the portions of high peak
    intensity and there is considerable incorporation of defects or
    impurities at the facet growth interfaces is identical to the
    results of .

 A sample illustrating burial by one-step regrowth is shown in FIG.
7.4.2-(7). There is high peak intensity at the growth interfaces of the
square facets, and the peak wavelength and half-width exhibit basically
the same distribution as in . At the very surface, the peak intensity
declines, and, after facets from the two edges coalesce above the mask,
in the same manner as in other areas, the incorporation of defects or
impurities is reduced.

![](media/image126.png){width="5.995833333333334in"
height="3.3055555555555554in"}

FIG. 7.4.2-(6) Cross sectional PL intensity mapping of sample
illustrating process of burial starting from formation of square facets

\[Keys, FIG. 7.4.2-(6)\]

(Right, top to bottom) Overgrowth layer; Underlayer

(Across bottom) Opening Opening

![](media/image127.png){width="5.998611111111111in"
height="3.3180555555555555in"}

FIG. 7.4.2-(7) Cross sectional PL intensity mapping of sample
illustrating process of burial starting from square facets in one step

\[Keys, FIG. 7.4.2-(7)\]

(Right, top to bottom) Overgrowth layer; Underlayer

(Across bottom) Opening Opening Opening

A sample obtained by completing burial using square facets upon
formation of triangular facets is shown in FIG. 7.4.2-(8). This sample
also exhibits practically the same distribution as in . Namely, the
peak intensity is strong only in the coalescence region of the square
facets. In the sample of , which illustrates the process of triangular
facet burial, a distribution corresponding to the growth interfaces of
the triangular facets was visible, but in this sample it was not clearly
observed, and the incorporation of defects and impurities at the growth
interfaces of the vertical facets is believed to be greater than at the
interfaces of the triangular facets.

![](media/image128.png){width="5.993055555555555in" height="3.29375in"}

FIG. 7.4.2-(8) Cross sectional PL intensity mapping of sample
illustrating two-step burial with triangular and square facets

\[Keys, FIG. 7.4.2-(8)\]

(Right, top to bottom) Overgrowth layer; Underlayer

(Across bottom) Opening Opening Opening

To sum up all of the above, it was found that the distribution of PL
spectra related to the growth mechanism could be obtained by means of
the micro-PL technique. It is believed that the overgrowth interfaces of
the facets contain defects and impurities, and, as far as the tendency
relative to the facet surfaces is concerned, for *c*-plane,
incorporation of defects and impurities is the least of all, for
{11[2]{.underline}2} it is larger, and for {11[2]{.underline}0} it is
largest. In the future, in order to study the fine structure of the
spectrum, we will conduct measurements at low temperatures.

#### Time resolved photoluminescence of ELOG-GaN

The 3.4786-eV neutral donor bound exciton emission line (I~2~) is
dominant in the PL spectra of the ELOG-GaN samples, and emission lines
corresponding to the free exciton resonance energy of the reflection
spectra are observed at 3.4850 eV (Ex^A^) and 3.4924 eV (Ex^B^). In
addition, the free exciton emission becomes dominant with temperature
and is observed at room temperature.

FIG. 7.4.2-(9) and FIG. 7.4.2-(10) show the dependence of
time-integrated emission spectra on the excitation energy density in
ELOG-GaN with W masks and ELOG-GaN with SiO~2~-masks, respectively.

![](media/image129.png){width="5.986111111111111in"
height="3.3944444444444444in"}

FIG. 7.4.2-(9) Time integrated emission spectra of ELOG-GaN using W
masks

![](media/image130.png){width="5.989583333333333in"
height="3.1909722222222223in"}

FIG. 7.4.2-(10) Time integrated emission spectra of ELOG-GaN using
SiO2-masks

In ELOG-GaN with W masks, for Ex^A^, which is clearly observed at the
excitation energy density of (a), the relative intensity ratio with
respect to (D^0^,X) decreases with increasing excitation energy density,
and practically no peak can be observed in (f). This is believed to be
due to the fact that exciton emission in the vicinity of the (D^0^,X)
energy position becomes more pronounced with increasing excitation
energy density. The same can be said about ELOG-GaN using SiO~2~ masks,
but because of the large half-width at (D^0^,X), it was not observed as
markedly as in the case of ELOG-GaN with W masks.

In addition, plots obtained by plotting the peak intensity against the
excitation energy density are shown on the right-hand side of the
respective figures. The characteristic dependence of exciton emission,
in accordance with which there is a super-linear increase as a function
of the excitation energy density in the vicinity of the (D^0^,X) energy
position, can be observed in this figure as well. However, it was found
that its slope was 1.50 for ELOG-GaN with SiO~2~ masks and 1.75 for
ELOG-GaN with W masks, with the W mask ELOG-GaN exhibiting a stronger
increase.

FIG. 7.4.2-(11) shows time-decay spectra of free exciton emission. Both
for the W mask and SiO~2~-mask samples, the emission lifetime became
longer with increasing excitation energy density, with the emission
lifetime saturating at the excitation energy density of (d)\~(f) and
becoming practically constant. In the W mask ELOG-GaN, the emission
lifetime is 42 ps in (a) and reaches 143 ps in (f), and in the
SiO~2~-mask ELOG-GaN, the emission lifetime is 48 ps in (a) and 117 ps
in (f).

![](media/image131.png){width="5.993055555555555in"
height="3.4409722222222223in"}

FIG. 7.4.2-(11) Dependence of free exciton emission decay time spectra
on excitation energy density for W mask (left) and SiO~2~ mask (right)
ELOG.

The tendency of the excitation lifetime to increase with excitation
energy density is believed to be due to the fact that, as the excitation
energy density increases, the impurity level becomes more saturated,
and, as a result, carrier transport related to the impurity level is
suppressed. It is believed that, in the low excitation energy density
region of (a), carriers are trapped by the impurity level at an early
stage, and, as a result, the emission lifetime becomes shorter, while in
(f), in the region of excitation energy density where the impurity level
is saturated, the transport of carriers to the impurity level is
suppressed, and, as a result, the emission lifetime becomes longer. In
addition, the fact that the emission lifetime in (f) is 143 ps for the W
mask ELOG-GaN and much shorter, 117 ps, for SiO~2~-mask ELOG-GaN, is
believed to be due to the fact that SiO~2~-mask ELOG-GaN is strongly
influenced by non-radiative recombination centers, while in W mask
ELOG-GaN samples the non-radiative recombination centers are evidently
suppressed.

The same tendency was observed in the time decay spectra of the exciton
emission. As for emission components, in regions (a)\~(c), neutral donor
bound exciton emission saturates, and exciton emission becomes dominant.
The emission lifetime becomes longer with increasing excitation energy
density, saturating and becoming practically constant in regions
(d)\~(f). In addition, the emission lifetime at excitation energy (f) is
134 ps for the SiO~2~-mask ELOG-GaN and 192 ps for the W mask ELOG-GaN,
with the emission lifetime of excitons of the W mask ELOG-GaN being
longer, which indicates that the non-radiative recombination centers of
the W mask ELOG-GaN are suppressed.

Based on the above-described results, it became evident that W mask
ELOG-GaN has better crystal properties than SiO~2~-mask ELOG-GaN, and
that in such samples the non-radiative recombination centers are
suppressed. Thus, it was confirmed that the material of the mask exerts
an influence on the optical characteristics of ELOG-GaN.

## Summary

By using the newly developed FACELO technique, we were able to reduce
the dislocation density to a level of 10^6^ cm^-2^\~10^4^ cm^-2^. The
dislocation density was successively reduced by controlling the initial
nucleus density and carrying out two-step FACELO growth on a underlying
low-dislocation GaN crystal (10 mm on a side) in two steps including
{11[2]{.underline}2} FACELO and then {11[2]{.underline}0} FACELO. So far
as we know, the resulting GaN-on-sapphire substrates have the lowest
dislocation density in the world. We have determined that a crucial
point in dislocation reduction is that dislocations with
{11[2]{.underline}2} triangular facets are bent in a horizontal
direction with respect to the *c*-axis and propagation is terminated by
the void formed in the central portion of the mask. In addition, new
dislocations originating from the top edge of the void and reaching the
surface, where they remain in a linear pattern, can be efficiently
prevented from reaching the surface by {11[2]{.underline}0} rectangular
facets. We have started investigations into 2-inch substrates, but
because of insufficient burial we are still dealing with substrates of
insufficient surface condition. A parallel effort has been going into
accumulation of data on the basic technology of ELOG growth by means of
HVPE.

Based on analysis of dislocation lines in FACELO-GaN by TEM, it has been
determined that mixed dislocations are bent towards the mask regions and
terminated by the void, whereas edge dislocations are bent in a
direction parallel to the stripes of the mask. It has been determined
that, based on this mechanism, dislocation lines generated at the
interface of the substrate and GaN do not reach the surface, and the
dislocation density is greatly reduced.

It has been determined that a PL spectrum distribution related to the
growth mechanism is obtained using the micro-PL technique. A
considerable amount of defects and impurities is believed to be
contained at the facet regrowth interface. The tendency to incorporate
impurities via facet surfaces is believed to be least pronounced in the
case of the *c*-plane, more pronounced in the case of the
{11[2]{.underline}2} planes, and strongest in the case of the
{11[2]{.underline}0} planes.

Results of time-resolved spectroscopy measurements have confirmed that,
in comparison with ELOG-GaN with a SiO~2~ mask, ELOG-GaN with a W mask
has a longer free exciton emission life and its non-radiative
recombination process is suppressed.

## Future plans

In our pursuit of core technologies of dislocation defect reduction in
10-mm substrates, we have been able to accumulate a large number of
technologies by developing a novel FACELO method, using W masks, etc.
The dislocation density is now 10^6^\~10^4^ cm^-2^, which is practically
at the target level. However, in our investigations into 2-inch
substrates, for which we changed the MOCVD growth equipment, we have
been unable to reproduce the current FACELO results, with the current
post-burial surface morphology remaining inadequate.

In the future, in order to realize 2-inch GaN-on-sapphire substrates of
practical value with a dislocation density of not more than 10^4^
cm^-2^, which is the target of the current project, we will focus our
efforts on the study of 2-inch substrates. To this end, along with
conducting basic research aimed at further dislocation density
reduction, we will try to reduce dislocations by efficiently combining
the core technologies accumulated up until now. In order to achieve an
efficient combination, it will be necessary to further elucidate the
mechanism of dislocation propagation or extinction using TEM
observations. We believe that the characteristic mechanism of the MOCVD
method will also become apparent from a comparative study of the HVPE
method. In addition, in connection with the different crystal quality of
GaN on the mask and above the window areas in ELOG-GaN, we are going to
focus on the evaluation of crystal quality in micro-regions by means of
micro-photoluminescence, time resolved photoluminescence,
cathodoluminescence, etc.

We plan to develop high-quality GaN-on-sapphire substrates of practical
value in a timely fashion by applying the findings obtained above in our
investigations to 2-inch substrates. The fabricated substrates will be
supplied to the UV LED Epi-Group as substrates for homoepitaxy or to the
Substrate Group as high-grade seed crystals for confirmation of the
effects of the low-dislocation substrates.

## References

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2^nd^ International Symposium on Blue Laser and Light Emitting Diodes,
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# Development of GaN Epitaxial Substrates Based on Low Pressure Gas Phase Technique

## Achievements in 1998 and 1999

In the future, in the process of transition to industrial production,
the cost of manufacture per unit of luminance of white illumination LEDs
will have to be made equivalent to that of already existing means of
illumination. In addition to being plagued by poor efficiency of
electricity-to-light conversion, the GaN LEDs currently available on the
commercial market are expensive in spite of the use of sapphire
substrates, which are very close to industrial production at the present
moment. Thus, in the course of the present project, it is necessary to
seek increased efficiency and improvements in the reliability of white
illumination LEDs by practically implementing GaN substrates, and, on
the other hand, to investigate possible low-cost fabrication methods
with a view to start industrial production in the future.

As far as current techniques of gas phase growth of GaN are concerned,
methods that have been reported to achieve successful synthesis include
MOCVD, the sandwich technique, as well as HVPE and other open-tube
growth methods; all of the above methods, however, make use of expensive
source materials, most of which are discharged from the system without
contributing to the reaction. In addition, because of the high volume of
the discharged toxic exhaust gases and by-products, it is necessary to
install expensive environmental protection equipment and prevent
environmental damage by frequently renewing the chemicals. Because the
existing GaN gas phase growth methods, which have already allowed us to
achieve high levels of quality, are open-tube type methods, it is
difficult in principle to realize additional cost reductions in terms of
source material expenses, equipment costs, and running costs. Thus, it
is believed that transition to industrial production in case of
fabrication methods used for illumination LED substrates will present
serious difficulties. Therefore, with account taken of the future cost
reduction needs, in the course of the present project we have been
trying to develop GaN substrates for epitaxial growth using a new low
pressure gas phase technique.

In 1998, no bulk crystals were obtained by using the low pressure gas
phase technique, and, in order to decide on the preferable system for
GaN growth, we conducted investigations into the processes of crystal
growth (sublimation, transport, and reactions of source materials) in
the low pressure gas phase technique. As a result, as shown below, it
was determined that the application of the open-tube system created
serious obstacles, and we concluded that it was more practical to use
GaN growth based on the low pressure gas phase technique under a
pseudo-open tube system, which is characterized by supplying source
materials from outside the system.

\(i\) In order to obtain basic data regarding the reaction of Ga and
N~2~, measurements were carried out to determine the GaN dissociation
pressure and the presence/absence of GaN synthesis in the vicinity of
1000ºC in an equilibrium system, and, as a result, it was experimentally
confirmed that the dissociation pressure at 940ºC was 3.6 × 10^5^ Pa,
which was almost the same as the value expected based on data from
literature. In addition, it was also confirmed that the rate of
dissociation of N from GaN was extremely high.

\(ii\) Based on thermodynamic investigations and theoretical equations
of Stephan flow, we performed growth velocity calculations for direct
synthesis from Ga and N~2~, and concluded that improvements in the
growth velocity of up to two orders of magnitude, with a maximum of 6
mm/h, were possible if trace amounts of carrier gases such as HCl were
used.

\(iii\) Preliminary experiments confirmed that if nitrogen was activated
with microwaves etc., it reacted with the Ga vapor, thereby making
synthesis of GaN possible.

In 1999, our investigations into applicable systems to be used with the
low pressure gas phase technique were narrowed down to the pseudo-open
tube system, which is characterized by supplying source gases from
outside the system and studies were conducted regarding GaN reactions
involving the N~2~ plasma created by microwave excitation. As a result,

\(iv\) using sapphire as seed crystals, we carried out growth in regions
where a high concentration of plasma was created by adjusting the
microwave output, and, based on the optimization of pressure,
temperature, etc., achieved a relatively high rate of synthesis of 100
*μ*m/h for GaN in microcrystal powder form, which was more than 20 times
the prior-art level. In addition, depending on the conditions used, we
were able to obtain hexagonal-shaped single crystals as small as 30
*μ*m.

\(v\) In addition to GaN, an EDX analysis of SEM images of the resultant
powdery material did not reveal any impurity peaks, and X-ray
diffraction confirmed its orientation along the *c*-axis on the sapphire
substrate.

\(vi\) Based on the findings obtained in the course of the
above-described basic experiments, we designed and built a "Gas Phase
Bulk Growth Furnace" intended for the systematic development of bulk GaN
crystals.

## Summary of Research & Development in 2000

A high quality (low dislocation density) and low cost are essential
conditions for the development of illumination LED substrates, which
constitutes the ultimate goal of the present project. Thus, it is
believed that the pseudo-open tube design, which has a high source
material yield, is preferable for bulk growth from the gas phase. In
2000, we have been optimizing the growth conditions of the pseudo-open
tube system with a view to grow GaN single crystals with a diameter of
several mm to 10 mm and in order to investigate the possibility of using
the grown GaN single crystals as LED substrates. Specific problems
included:  improvement in the rate of GaN synthesis of up to 100 *μ*m/h
or more.  Clarification of the conditions of monocrystallization. 
Determination of the growth conditions that should be used in the bulk
growth furnace.  Conducting evaluation of crystals as substrates for UV
LEDs.

### Research on GaN-related transport and reaction phenomena

The growth velocity of bulk single crystals is an important factor in
the fabrication of such crystals. Generally speaking, the velocity of
crystal growth from the gas phase is slower in comparison with that of
growth from the liquid phase, and from the standpoint of its practical
application in the future, it is necessary to achieve a good
understanding of the various possibilities of further improvement. In
1999, we attained a GaN rate of synthesis of 100 *μ*m/h and demonstrated
that such rates were possible. Thus, based on the obtained findings, we
have been trying to further improve the rate of synthesis and achieve
even higher crystal growth velocities.

In this system, reactions are believed to take place instantaneously,
and transport is believed to act as the rate-determining factor. For
this reason, we have been trying to optimize the temperature and
pressure conditions and the structure of the reaction system to improve
the rate of transport. In addition, we have been investigating
high-frequency wave-based plasma as a method for generating activated
nitrogen. Because the use of this method makes it easier to control the
position of the plasma and allows for increasing the activation ratio by
raising the pressure of nitrogen, it offers the possibility of raising
the rate of the GaN synthesis reaction. Moreover, because it has the
advantage of requiring equipment that is smaller in size and less
expensive in comparison with the microwave system, it is believed that
the study of growth based on this system will be necessary from the
standpoint of our future cost-reduction efforts.

In 2000, our achievements have included the determination of the
essential conditions necessary for further crystal property improvements
at the maximum rate of synthesis of 100 *μ*m/h under the microwave
system and the experimental confirmation of the possibility of
improvement of up to 1 mm/h under the high-frequency wave system.

### Research on single crystal growth techniques

In 2000, we have been using existing furnace equipment in order to
continue investigations into the conditions of monocrystallization, in
particular, conditions such as the substrate temperature and the
pressure of the supplied nitrogen. As a parallel effort, we have studied
monocrystallization within a wider range of controllable conditions
using the bulk growth furnace that was introduced in 1999. In addition,
with a view to eventually grow large 10-mm GaN single crystals, we have
been using GaN single crystals as seed crystals, which are indispensable
for monocrystallization. In addition, in order to make sure the crystals
can be used as LED substrates, we have been conducting evaluation of
crystal properties and optical characteristics. In 2000, our
achievements have included confirmation of growth of a dense film-like
GaN crystal oriented along the *c*-axis within a 10 × 4 mm area on a
sapphire substrate, and, furthermore, successful synthesis of crystals
exhibiting no surface roughness and possessing a highly symmetric
surface morphology along with and an X-ray rocking curve half-width of 6
minutes.

## Microwave-based GaN Synthesis Experiments

### Methods and means

#### Avenues of research

In order to stably synthesize bulk single crystals from a gas phase, it
is necessary to build a system capable of realizing diffusion-controlled
constant mass transport. Diffusion-controlled gas transport phenomena in
closed-tube systems are described by the so-called Stephan flow, in
which the difference in vapor pressure between the source materials and
the synthesis section acts as the driving force, and were investigated
during the first year of the study. In a pseudo-open tube system, in
which nitrogen is introduced from outside, it is important to control
the temperature of the seed crystal and the Ga source material, which
determines the vapor pressure difference, as well as the stability of
the N~2~ flow rate. To this end, we have put into operation the gas
phase bulk growth furnace (introduced last year), and we currently
working to establish its controllability characteristics.

In addition, as regards the phenomenon of transport of reactive radicals
activated by the plasma state, in addition to the above-mentioned
diffusion-controlled mass transport, the reactive radicals, when
charged, are acted upon by the electric field and drift-controlled. In
this connection, we are working to better appreciate the respective
influence exerted in the case of microwave excitation, where the
electric field is presumed to be zero when integrated over time, and in
the case of high-frequency wave pulse excitation, which has been adopted
as a new method of activation.

As regards microwave excitation, the vapor pressure of Ga in the
vicinity of 1000ºC is low, approximately 1 Pa, and nitrogen is supplied
such that a constant pressure is maintained within the range of
efficient Ga transport rates, in other words, at a pressure of about
several hundred Pa. For this reason, the difference between the vapor
pressure in the outlet section and in the reaction zone is usually
larger in comparison with the difference between the vapor pressure of
Ga and the Ga source material section, and, as result, the nitrogen
transport rate does not determine the rate of synthesis. Because in this
system synthesis is carried out in the above-mentioned pressure range,
the mean free path of the plasma is short, and, as a result, a diffuse
plasma undergoes repeated generation and extinction. The density
distribution of the plasma determines the quantity of the synthesized
GaN and the region of synthesis. In this connection, we have been trying
to determine the position of the substrate based on experimental
findings regarding the density distribution, which we obtained the year
before by plasma spectroscopy measurements.

Although it has been confirmed that, due to the extremely high
activation energy of GaN in the vicinity of 1000ºC, in a thermal
equilibrium state dissociation does not proceed, in a non-equilibrium
plasma state the energy of the plasma may impart dissociation energy to
the surface and cause decomposition, and, therefore, it is essential to
have a clear understanding of this state in order to proceed. In other
words, when other parameters are predetermined, maximal values are
expected to be present in the relationship between the microwave output
and the rate of synthesis and crystal properties, and we will proceed to
further investigate these values.

Additionally, although sapphire is commonly used as the seed crystal
substrate, it is necessary to carry out observations of the growth mode
and surface morphology obtained with various substrates to have a clear
picture of the range of applicability of the present method of
synthesis. In addition, in the process of carrying out the
above-mentioned experiments, it is important to regulate the
relationship between the problems to be solved, parameters, and core
technologies, as shown in FIG. 8.3.1-(1), to obtain a clear picture of
their mutual influence while strengthening the understanding of the most
important factors. In the present chapter, we describe the results
obtained by the experimental study of Ga crucible temperature
dependence, pressure dependence, microwave output dependence, substrate
temperature dependence, and substrate seed crystal
dependence.![](media/image132.png){width="5.993055555555555in"
height="3.290277777777778in"}

FIG. 8.3.1-(1) Problems to be solved and core technologies associated
with low pressure gas phase technique

#### Experimental methods

Experiments were carried out using the gas phase bulk growth furnace
designed and manufactured last year. As far as source material supply
was concerned, we designed the experimental system so that Ga and N~2~
gases were used as the supply sources and no by-products were generated,
just as in the sublimated state in a closed-tube system. Ga was placed
in a crucible and diffusion was carried out by using as the driving
force the vapor pressure difference inside the reaction tube due to the
temperature distribution created by the heater. In accordance with the
principles of the gas phase method, nitrogen was supplied from outside
in such a manner that a constant pressure was maintained and diffused
onto the seed crystals in an activated plasma state. We set up an
experimental system such as the one shown in FIG. 8.3.1-(2).

Because microwaves leaked outside via the thermocouple as an antenna
when the plasma was ignited, we conducted temperature measurements by
inserting the thermocouple in the center of the reaction tube after
turning off the microwave output. For this reason, the temperature of
the substrate during synthesis was expected to be several degrees higher
than the measured values.

After taking the synthesized crystals out, they were cleaned with aqua
regia and evaluated using Nomarski microscope observations, X-ray
measurements, electron microscope observations, photoluminescence, etc.

![](media/image133.png){width="5.988888888888889in"
height="3.0756944444444443in"}

FIG. 8.3.1-(2) Experimental system for GaN synthesis using microwaves

[(i) Ga crucible temperature]{.underline}

In order to form a GaN single crystal, first of all, it was necessary to
set the temperature of the Ga crucible to the range in which a Ga
transport amount necessary for GaN crystal formation could be obtained.
Because in the present system Ga was diffused in an inert N~2~ gas
having a sufficiently short mean free path, the Stephan flow model,
which takes into consideration advection and which is the principle of
the closed-tube method, is applicable in this case. Assuming that all of
the Ga reacts in the vicinity of the substrate, and designating the
diffusion coefficient of Ga as *D*, the total pressure as *P*, the Ga
vapor pressure on the surface of the crucible as *P*~Ga~, and the
distance between Ga and the substrate as *L*, the Ga transport amount,
*J*~Ga~, is expressed by relationship (8.3.1), which is derived from
Stephan flow.

*J*~Ga~ = (2*D* ⋅ *P* ⋅ *P*~Ga~)/{*RT*L ⋅ (2*P*−*P*~Ga~)} (8.3.1)

In addition, because the synthesis is carried out at a constant
pressure, (8.3.2) holds true.

*J*~Ga~ ∝ (rate of synthesis) ∝ (quantity of consumed N~2~) (8.3.2)

Based on the formulas and on the Ga vapor pressure curve, the quantity
of consumed N~2~ is expected to display a sensitive positive correlation
with the temperature of the Ga crucible.

The synthesis experiments were carried out under the following
conditions with a view to establish a suitable Ga temperature range
based on the quantity of consumed N~2~ and the crystal surface
morphology, as observed under a Nomarski microscope. As for N~2~
consumption, the current study utilized sapphire substrates with a
diameter of 16 mm, and it was estimated at 61 *μ*m if all of the 10-cc
N~2~ gas was consumed in the synthesis of GaN on the substrate. The
total pressure was 200 Pa, the microwave output was 300 W, and the
temperature was:

 Ga temperature of 1000ºC − substrate temperature of 800ºC

 Ga temperature of 1070ºC − substrate temperature of 1000ºC

 Ga temperature of 1100ºC − substrate temperature of 1000ºC

[(ii) Pressure dependence]{.underline}

The pressure inside the reaction tube was varied to investigate the
crystalline properties and the rate of synthesis. Based on formula
(8.3.1), the rate of synthesis can be expected to improve when the total
pressure *P* is low. However, in a state wherein the pressure of Ga
vapor from GaN is higher than the saturated vapor pressure of Ga alone,
a non-equilibrium reaction is stimulated, and, for this reason, GaN
including Ga droplets may be synthesized if the V/III ratio is small. In
addition, when the total pressure *P* is high, a decline in the quantity
of transported Ga occurs, and GaN micropowder can be produced. With
account taken of the above, experiments were conducted by varying the
pressure of the supplied nitrogen under the following conditions.

 Microwave output: 300 W, Ga temperature: 1080ºC, substrate
temperature: 800ºC, total pressure: 100 Pa

 Same as above, total pressure: 200 Pa

 Same as above, total pressure: 500 Pa

 Microwave output: 300 W, Ga temperature: 1100ºC, substrate
temperature: 1000ºC, total pressure: 100 Pa

 Same as above, total pressure: 200 Pa

In this series of experiments, in addition to the quantity of consumed
N~2~ and Nomarski micrographs, we conducted X-ray (0002) ω-mode
measurements to compare crystal properties based on the half-width.

[(iii) Microwave output dependence]{.underline}

In the above-mentioned pressure and microwave output ranges, the
generated nitrogen plasma consists mainly of the so-called 2^nd^
positive system of N~2~, i.e. active radicals in the form of nitrogen
molecules that are positive ions or are in a neutral excited state
displaying the most intensive emission at 337 nm. Radicals etc., which
are in the most highly reactive dissociation state in the nitrogen
plasma, have not been observed under any conditions because of the short
mean free path. As regards the distribution of the emission intensity,
when the microwave output is changed, the intensity ratio does not
change, only the absolute values change. In other words, in the reaction
taking place on the substrate, only the density of the arriving active
radicals changes, and it is necessary to keep the ratio of V/III and the
Ga within an appropriate range. Based on the above considerations, we
varied the microwave output to investigate the correlation with the
quantity of synthesized material and crystal properties.

The experiments were conducted under the following conditions: a
pressure of 200 Pa, a substrate temperature of 1000ºC, a Ga temperature
of 1100ºC, and a microwave output of, respectively,  200 W,  300 W,
400 W, and  600 W.

[(iv) Substrate temperature studies]{.underline}

During ordinary gas phase growth, the temperature of the substrate is
one of the parameters exerting the strongest influence on
monocrystallization, and it is believed to have a strong correlation
with initial nucleation in the present growth method as well. This year,
in order to conduct the experiments with high precision, we made
improvements to the furnace and at present we are trying to determine
the conditions necessary for transition to higher substrate
temperatures. While it is believed that higher temperatures are
preferable in order to activate surface migration, on the other hand, if
sufficient Ga supply amounts are not provided at higher temperatures,
decomposition of GaN will take place, which is why it is believed that
further fine-tuning of these parameters needs to be done.

[(v) Seed crystal dependence]{.underline}

Monocrystallization on a sapphire substrate has not been realized yet,
the reason being either insufficient control over initial nucleation or
insufficiently clear understanding of the crucial factors of the
synthesis method itself. Seed crystals were modified into GaN
substrates, the influence of surface treatment on which was expected to
be relatively weak, and synthesis experiments were carried out in order
to determine at the earliest possible stage whether synthesis of GaN
single crystals was possible by the present method.

### Results and Discussion

#### Confirmation of performance of gas phase bulk growth furnace and crystalline GaN synthesis experiments

We completed the commissioning of the "Gas Phase Bulk Growth Furnace" a
single-wafer crystal growth furnace designed and manufactured the year
before. Its main specifications are shown in FIG. 8.3.2-(1). Whereas the
basic experimental furnace was of the horizontal type, which permitted
easy arrangement of jigs inside the furnace, the gas phase bulk growth
furnace was of a vertical design in order to achieve uniformity of
temperature distribution in the radial direction as well as a reduction
in convection. In addition, the degree of freedom and controllability of
each parameter was greatly improved in comparison with the existing
furnace.

An essential condition for metallic Ga transport in the low-pressure gas
phase technique consists in achieving a higher degree of ultimate
vacuum. Because the equilibrium vapor pressure of Ga at 1000ºC is low,
about 1 Pa, and because it is prone to form Ga~2~O~3~ by reacting with
residual oxygen in the gas phase, usually, the level of the residual
oxygen is reduced while increasing the mean free path by using MBE and
other ultra-high vacuum devices. On the other hand, although there was
concern regarding oxidation in the basic experimental furnace because
under the total pressure of 10^1^\~10^3^ Pa the mean free path in the
low pressure gas phase technique is on the order of *μ*m, considerable
improvements have been achieved with respect to this aspect.

In addition, by combining the microwave generator with an automatic
matching device, output fluctuations were reduced from the previous 20%
to ±3%, enabling effective production of active radicals at low pressure
and low output. Additionally, by using a PID flow rate controller, the
pressure fluctuation range was stabilized below the lower limit of
measurement, enabling synthesis with low convection. Based on the
confirmation of above-described basic performance characteristics we
were able to conduct synthesis experiments with high controllability.

![](media/image134.png){width="5.983333333333333in"
height="2.9298611111111112in"}

FIG. 8.3.2-(1) Main specifications and conceptual diagram of gas phase
bulk growth furnace

Next, we used the device to conduct GaN crystallization by varying the
microwave output, substrate position, Ga crucible position, temperature,
and other conditions. As a result, at a substrate temperature of
approximately 1000ºC, an adequate state was achieved in terms of the Ga
transport amount and nitrogen plasma density and crystalline, albeit
polycrystalline, GaN was successfully synthesized on a sapphire
substrate.

On the other hand, we confirmed the phenomenon of GaN decomposition at a
rate of several *μ*m/h when synthesized GaN/sapphire was placed in a
region characterized by a low Ga transport amount and a high density of
plasma. Decomposition did not occur if the GaN was simply placed in a
region of elevated temperature, and in that case no changes in surface
morphology were noticed either. This makes the optimization of the ratio
between the plasma density and Ga transport amount important, and, in
case of a predetermined Ga supply amount**,** the amount synthesized and
the flatness of the surface morphology are believed to assume extreme
values through tweaking the plasma density. By taking the above results
into consideration, we proceeded with experiments intended to study in
detail the correlation between the parameters below and crystal
properties.

#### Ga crucible temperature dependence

The results are shown in FIG. 8.3.2-(2) and FIG. 8.3.2-(3). There is a
sensitive dependence between the temperature and the quantity of
consumed N~2~, which increases monotonically. The quantity of consumed
N~2~  at a Ga temperature of 1000ºC is just 2.4 cc/h, with powder-like
GaN sparsely adhered to the surface of the sapphire, as shown in FIG.
8.3.2-(3). Although another issue to be considered in connection with
crystalline properties is the temperature of the substrate, the sparsely
synthesized GaN shows that the absolute quantity of transported Ga is
insufficient, and it has been found that under the RF-MBE method, which
is also based on a non-equilibrium reaction utilizing plasma, there is a
suitable region on the higher temperature side as compared with
temperatures of 900\~1000ºC. By contrast, a Ga temperature of  1070ºC
and  1100ºC resulted in a 10-cc/h N~2~ consumption, with GaN
crystallizing in the form of a dense film over the entire surface of the
substrate. Although there were fluctuations in the amount synthesized
across the surface, on the average it was 30 *μ*m/h. This value was
insufficient to proceed with investigations into monocrystallization,
but further improvements in the rate of synthesis can be expected if the
temperature is raised even higher. However, because based on formula
(8.3.1) the quantity of transported Ga also depends on the total
pressure and the distance to the substrate, improvements are achievable
by varying these parameters. Based on the above, it was determined that
the preferred Ga crucible temperature under this system was 1100ºC, or,
in case other conditions were predetermined, it had to be set to a
higher temperature.

![](media/image135.png){width="2.8645833333333335in" height="2.3125in"}

FIG.8.3.2-(2) Dependence of N~2~ consumption on Ga temperature

\[Keys, FIG.8.3.2-(2)\]

(X-axis) Ga crucible temperature (ºC)

(Y-axis) Quantity of consumed nitrogen (cc/h)

![](media/image136.png){width="5.992361111111111in"
height="1.9833333333333334in"}

FIG.8.3.2-(3) Ga temperature and surface morphology

\[Keys, FIG.8.3.2-(3)\]

 Ga temperature 1000ºC

 Ga temperature 1070ºC

 Ga temperature 1100ºC

(Across bottom) GaN powder Sapphire substrate Dense monocrystalline GaN
film

#### Pressure dependence

The results are shown in FIG. 8.3.2-(4) and FIG. 8.3.2-(5). As far as
the rate of Ga transport was concerned, as shown on the left side of
FIG. 8.3.2-(4), the quantity of consumed nitrogen decreased
monotonically relative to the pressure at a constant temperature, and,
notwithstanding the large difference in vapor pressure between the Ga
crucible and the substrate at 500 Pa in , the transport amount remained
small and GaN did not crystallize. In addition, when the substrate
temperature was low at a constant pressure, the amount of consumed
nitrogen rapidly increased. The above-mentioned phenomena were in good
agreement with formula (8.3.1), showing that transport in the system was
diffusion-controlled.

As far as crystal properties were concerned, according to Nomarski
micrographs of FIG. 8.3.2-(5), the surface of the crystal in , which
was obtained at 800ºC and a pressure of 100 Pa, was covered with Ga
droplets, and, for this reason, despite cleaning with aqua regia,
dendritic microcrystals were obtained due a large amount of admixed Ga.
The picture of the crystal in , which was obtained at 1000ºC and 100
Pa, was taken prior to the aqua regia cleaning, and likewise, there were
some Ga droplets observed; however, the transport of Ga was reduced in
comparison with the 800ºC case and GaN was synthesized in the form of a
film, with the half-width, as determined by X-ray (0002) *ω*-mode
measurements, being 15 minutes. In this manner, at low pressure the
amount of transported Ga was too large, producing Ga droplets, which
affected crystal properties.

Next, in case of  and , where the total pressure had been set to 200
Pa, a dense film of GaN crystal was synthesized. According to the X-ray
measurement results shown on the right-hand side of FIG. 8.3.2-(4), in
each case, the half-width was considerably improved, at 8 minutes, which
was believed to be due to the V/III ratio approaching the right value.
In addition, at 500 Pa in case of , due to the insufficient Ga
transport amount, there was practically no synthesis on the substrate,
and no (0002) peaks were obtained as a result of X-ray measurements. We
concluded that the amount of transported Ga and crystal properties were
extremely sensitive with respect to the total pressure, with changes
being particularly large at low pressure. It was determined that the
preferable range for excellent crystal properties and experiment
controllability was approximately between 150\~300 Pa.

![](media/image137.png){width="5.988888888888889in"
height="2.6951388888888888in"}

FIG. 8.3.2-(4) Dependence of nitrogen consumption and X-ray half-width
on pressure

\[Keys, FIG. 8.3.2-(4)\]

(Figure on the left)

(X-axis) Pressure (Pa)

(Y-axis) Amount of consumed nitrogen (cc/h)

(Inside, top) Substrate temperature: 800ºC

(Inside, bottom) Substrate temperature: 1000ºC

(Figure on the right)

(X-axis) Pressure (Pa)

(Y-axis) X-ray half-width (min)

(Inside, top) GaN was in powder form in case of \*.

![](media/image138.png){width="5.995833333333334in"
height="4.613888888888889in"}

FIG. 8.3.2-(5) Synthesis pressure and surface morphology

\[Keys, FIG. 8.3.2-(5)\]

 Substrate temperature: 800ºC

Total pressure: 100 Pa

(Underneath figure) Dendritic GaN crystals (after removal of Ga)

 Substrate temperature: 800ºC

Total pressure: 200 Pa

(Underneath figure) Dense GaN film

 Substrate temperature: 800ºC

Total pressure: 500 Pa

(Underneath figure) GaN microcrystals Sapphire substrate

 Substrate temperature 1000ºC

Total pressure: 100 Pa

(Underneath figure) Ga droplets

 Substrate temperature: 1000ºC

Total pressure: 200 Pa

(Underneath figure) Dense GaN film

#### Microwave output dependence

The results are shown in FIG. 8.3.2-(6) and FIG. 8.3.2-(7). The amount
of consumed nitrogen, in other words, the amount synthesized, showed a
monotonic increase relative to the plasma output. Because in (2) and (3)
the fluctuations of the area accessible to plasma were small, the GaN
synthesis reaction proceeded with the Ga supply acting as the
rate-determining factor; however, because in the present experiment the
range accessible to plasma varied depending on the changes in the
microwave output, changes were accordingly observed in the amount of
consumed N~2~.

As for the crystal properties of GaN, the X-ray half width reached its
minimum at 400 W.

Then, in case of  200 W, a powder was formed, as shown in the Nomarski
micrographs, and X-ray ω-mode measurements were not taken. The reason
for the formation of the powder was supposed to be the insignificant
amount synthesized and the inability of GaN to bond with other nuclei on
the surface. Also, in case of  and , dense microcrystalline films with
a narrow X-ray half-width were obtained. At  600 W, the X-ray
half-width was the widest, and although the surface was covered with a
dense crystalline film, surface irregularities were still visible. This
was believed to be due either to an increase in the V/III ratio and
damage to GaN on the growth surface or to certain changes in the
reaction mechanism, but the exact reason was not known. In this manner,
we discovered that tweaking the microwave output resulted in extreme
values with respect to crystal properties if other conditions were
fixed.

![](media/image139.png){width="5.997916666666667in"
height="2.661111111111111in"}

FIG. 8.3.2-(6) Dependence of N~2~ consumption and X-ray half-width on
microwave output

\[Keys, FIG. 8.3.2-(6)\]

(Figure on the left)

(X-axis) Microwave output (W)

(Y-axis) Amount of consumed nitrogen (cc/h)

(Figure on the right)

(X-axis) Microwave output (W)

(Y-axis) X-ray half-width (min)

(Across top) \*In case of  200 W, powder was formed, rendering
measurement impossible

![](media/image140.png){width="5.990277777777778in"
height="5.834027777777778in"}

FIG. 8.3.2-(7) Microwave output and surface morphology

\[Keys, FIG. 8.3.2-(7)\]

 200 W, (underneath figure) GaN powder

 300 W, dense GaN film

 400 W, dense GaN film

 600 W, dense GaN film

#### Studies of substrate temperature

Although substrate temperature conditions could not be completely
controlled and a clear picture of its precise temperature at all times
could not be obtained, FIG. 8.3.2-(8) (on the left) shows an SEM
photograph of the surface of a dense film obtained when the substrate
temperature was maintained on the high-temperature side, and, for
comparison, a photograph of a representative dense film (on the right).
The film formed as a result of the increased temperature was produced by
the binding of GaN crystals possessing a 6-fold rotational symmetry, and
it was assumed that a change in growth mode had occurred. The X-ray
spectrum of FIG. 8.3.2-(9) clearly shows that the resultant crystals
possessed excellent orientation properties.

![](media/image141.png){width="3.4145833333333333in"
height="1.336111111111111in"}

Left: FIG. 8.3.2-(8) SEM photographs of dense microcrystalline film of
GaN (on sapphire substrate) prior to and after adjustment of temperature

![](media/image142.png){width="2.2368055555555557in"
height="1.8701388888888888in"}Right: FIG. 8.3.2-(9) X-ray measurements
of dense microcrystalline film of GaN synthesized on sapphire substrate

#### Seed crystal dependence

In the course of the above-described experiments we gained an
understanding of the necessary conditions of GaN crystal formation and
thus were able to move beyond the powder-like forms available until last
year and successfully synthesized dense films of excellent orientation
properties. However, monocrystallization on sapphire substrates was not
achieved, and we switched to using GaN substrates, the influence of
surface treatment etc. on which was supposed to be relatively weak, as
seed crystals, and conducted corresponding synthesis experiments. The
results are shown in FIG. 8.3.2-(10) and FIG. FIG. 8.3.2-(11). The SEM
photographs revealed clear rectilinear grain boundaries, with no surface
irregularities visible anywhere on the entire 5-mm surface. In addition,
excellent crystal properties were shown by X-ray diffraction
measurements. The growth rate obtained was 40 *μ*m, which was 1 order of
magnitude higher than in case of growth in other open-tube systems
utilizing a nitrogen plasma, and bulk crystal growth was therefore
within the range of practical applications. Thus, the concerns that we
had regarding this method of growth, such as whether the use of plasma
itself would block the monocrystallization of GaN, etc., were
eliminated, demonstrating that the growth of large single crystals by
means of controlling initial nucleation was indeed possible.

![](media/image143.png){width="5.992361111111111in"
height="2.5652777777777778in"}

FIG. 8.3.2-(10) SEM photographs of crystals where GaN was used in seed
crystals

In this manner, by synthesizing GaN with microwaves, we managed to carry
out the crystallization of GaN, which had been previously obtained in
microcrystalline powder form, and were able to synthesize dense films of
excellent orientation properties, and, furthermore, were able to
synthesize crystals without surface irregularities on GaN substrates. In
the future, we are planning to continue investigations into the
substrate temperature, growth velocity, and processes etc. of growth
with a view to synthesize large GaN single crystals by controlling the
initial nucleation.

![](media/image144.png){width="5.9944444444444445in"
height="2.0729166666666665in"}

FIG. 8.3.2-(11) X-ray measurement results for crystals where GaN was
used in seed crystals

## GaN Crystal Growth Method Utilizing Novel Plasma Generation Technique

### Methods and means

Among the methods of N~2~ activation, in addition to the use of
microwaves, there are methods that involve the generation of
non-equilibrium plasma via application of a high-frequency electric
field, DC electric field, etc. In the present chapter, we describe our
attempts to synthesize GaN single crystals using N~2~ plasma based on
the use of an pulsed type high-frequency power supply other than
microwaves. Impulse-type high-frequency wave-based techniques generate
plasma via application of positive and negative pulse voltage and are
characterized by  a high plasma density,  discharge at atmospheric
pressure,  simple plasma control,  a higher degree of freedom in terms
of electrode design, and  small size, light weight, etc. For this
reason, they permit generation of plasma in various shapes and in
various locations, and, due to the fact that they allow for setting the
pressure of the reaction system to a higher pressure than in the case of
microwaves, they are expected to permit an increase in the velocity of
growth. On the downside, however, there is some possibility of discharge
occurring in unexpected places because of the high pressure. For this
reason, in a narrow reaction system, it is necessary to design the
insulation very thoroughly to form a stable plasma.

FIG. 8.4.1 shows a high-frequency pulsed discharge. The GaN growth
experiments were carried out at an applied voltage of not more than 4
kV. The pressure was 200 Pa and the temperature 1000ºC. In addition, a
whitish film was generated on the surface when a sapphire substrate was
placed inside. X-ray diffraction measurements revealed a polycrystalline
GaN peak and SEM observation results confirmed that 2 *μ*m hexagonal
plate-shaped microcrystals had been grown. In this manner, it was
confirmed that GaN readily reacted in a plasma produced by using a
high-frequency pulsed power supply. However, neither the growth
velocity, nor the crystal size could come close to results obtained by
using conventional microwave plasma.

![](media/image145.png){width="1.9583333333333333in"
height="4.25in"}Figure 8.4.1

Next, in order to explore the possibility of accelerating the rate of
the GaN synthesis reaction in the high-frequency plasma, we conducted
experiments in a system, in which Ga and N~2~ were violently reacted. In
the system, Ga was supplied not by diffusion transport, but rather by
means of a direct, albeit violent, contact between the N~2~ gas and the
Ga. The experiment was conducted by applying the principle of the
so-called DC sputter by making use of the polarity that the plasma power
supply possessed. The experiments were carried out at a pressure of 200
Pa and a temperature of 1000ºC in a vertical furnace. As a result, solid
matter, which covered the Ga crucible, filled the inside of the reaction
tube, as shown in FIG. 8.4.2. It was found that because the reaction was
conducted at an N~2~ flow rate of 7 sccm, if GaN deposition on the
crucible continued, its rate could reach approximately 1.2 mm/h. The
material produced was in fact a mixture of GaN microcrystals and Ga, and
if the crucible continued to be filled in that manner, as a result of
the subsequent decrease in the reaction rate, GaN single crystals could
hardly grow in that state. However, one significant achievement was that
showed that in a GaN synthesis reaction conducted using Ga and
plasma-activated nitrogen, it was possible to achieve growth rates
exceeding 1 mm/h, which had been impossible to reach using reactions
based on thermal equilibrium.

![](media/image146.png){width="3.0in" height="3.0in"}From the
above-described results, it was evident that in order to realize growth
of GaN single crystals, in the next step, it would be necessary to
create a zone of high-density plasma by adjusting the high-frequency
output, and continue the optimization of the temperature, pressure, and
other parameters in the zone, as well as to conduct experiments and
investigations into the reaction of GaN synthesis using the source
material temperature, substrate temperature, relative position of the
substrate and Ga crucible, N~2~ pressure, etc. as parameters.

![](media/image145.png){width="1.9583333333333333in"
height="4.25in"}Figure 8.4.2

### Results and discussion

#### Temperature dependence

As was described above, although the GaN reaction was confirmed, we
could only obtain microcrystals, which called for the optimization of
the growth conditions. First of all, we conducted GaN growth experiments
with the temperature used as a parameter by placing only Ga inside the
reaction tube and maintaining the pressure and high-frequency output
constant.

![](media/image147.png){width="3.9583333333333335in" height="2.75in"}

FIG. 8.4.3 Relationship between growth temperature and N~2~ flow rate

\[Keys, FIG. 8.4.3\]

(X-axis) Temperature (ºC).

(Y-axis) N~2~ flow rate.

![](media/image148.png){width="2.6458333333333335in"
height="1.9791666666666667in"}FIG. 8.4.3 shows, as an indicator of the
reaction rate, the relationship between the growth rate and the N~2~
flow rate, which was plotted along the vertical axis. However, the term
"temperature" here refers to the temperature of the sidewalls of the
reaction tube, and not to the internal reaction temperature. From the
figure, it is evident that the synthesis of GaN took place at the
highest rate when growth was conducted in the range of from 800 to 900
degrees. The low rate exhibited at lower temperatures was believed to be
due to the low growth temperature and the low amount of evaporated Ga.
On the one hand, it is believed that the slowing down of the reaction at
more elevated temperatures is attributable to the fact that in the
region, where plasma existed, dissociation became stronger than the
reaction, which made growth impossible, and, in the low temperature
portion, no activated nitrogen existed because of the remoteness from
plasma, and therefore, no reaction occurred. SEM images of a crystal
surface generated on the inner walls of the reaction tube (quartz tube)
after the experiment are shown in FIG. 8.4.4. It was found that it was
GaN in the form of a dense film.

FIG. 8.4.4 SEM image

#### Optimization of substrate position

Considering that growth of films has been successfully realized even on
amorphous substrates, such as quartz, it could be expected that it was
possible to realize monocrystallization if the orientation substrate was
arranged properly. Therefore, we carried out GaN single crystal growth
experiments using sapphire substrates. Judging by the results of the
growth experiments, in which microwave plasma was utilized, for the
crystallization to take place, Ga and N~2~ had to be supplied in a
well-balanced proportion. Moreover, it was necessary to guarantee good
Ga transport by making the temperature of the Ga higher than that of the
substrate and to make sure the substrate was set to a temperature
suitable for growth. In addition, in a situation where the Ga and the
substrate were in a wide range of plasma, the Ga, simultaneously with
evaporation, could come in contact with activated nitrogen, resulting in
the production of GaN in the form of powder, and, for this reason, it
was necessary to make sure that growth on the substrate was enhanced by
setting the distance between the Ga and the substrate to a length not
shorter than the average free path.

In this manner, with account taken of the conditions necessary for
guiding reactive radicals to the substrate, we conducted various
experiments by varying the pressure, temperature, and other conditions
in a substrate position we considered the best, and, as a result, at 300
Pa, 900ºC, a distance of 20 mm between Ga and the substrate, and 300 W,
in part of a 16-mm sapphire substrate, we grew a lump-shaped solid,
which separated from the substrate when etched with aqua regia,
producing a 3×6 mm crystal such as the one shown in FIG. 8.4.5. The
resultant crystal was a thin plate with an apparent transparency of
approximately 0.1\~0.2 mm and a rough surface. FIG. 8.4.6 is an enlarged
photograph of the crystal surface. It shows that the surface was made up
of crystal grains with a size of several 10 to 200 *μ*m, with hexahedral
crystals visible here and there. The surface was considerably rough,
with ridges and valleys all over it.

![](media/image149.png){width="5.99375in" height="2.74375in"}

Left: FIG. 8.4.5 Lump-shaped crystal.

Right: FIG. 8.4.6 Micrograph of the surface.

The results of X-ray diffraction measurements are shown in FIG. 8.4.7.
*ω*-2*θ* scanning yielded a (0002) orientation peak only, which showed
that the crystal was oriented along the *c*-axis. Incidentally, when the
rocking curve of the (0002) peak was measured, it was split into several
peaks, as shown in FIG. 8.4.8, clearly indicating its polycrystalline
nature. From the shift of the peaks, it was evident that in each crystal
the *c*-axis was inclined approximately 20 minutes. FIG. 8.4.9 is a pole
figure obtained by measurement in the (10-12) plane. (0002) diffraction
appears in the vicinity of 45º, with the tilt component widened in the
radial direction and the mosaic component in the direction of rotation.
Diffraction in the (0002) plane appeared as spots in some places on a
weak ring, with each of the *φ* angles (Psi) shifted, matching the
inclination of the rocking curve. In addition, it was found that the
6-fold rotational symmetry was destroyed, and, with the in-plane
orientation in considerable disorder, growth proceeded with crystal
grain boundaries remaining independent.

![](media/image150.png){width="4.45625in" height="3.2819444444444446in"}

Left: FIG. 8.4.7 ω-2θ spectrum.

Right: FIG. 8.4.8 Rocking curve.

![](media/image151.png){width="4.385416666666667in"
height="3.1354166666666665in"}

FIG. 8.4.9 Pole figure of GaN crystal.

FIG. 8.4.10 shows room-temperature PL mapping results for the crystal.
The excitation light source was a Hd-Cd laser with a generation
wavelength of 325 nm and an intensity of 8 mW. Si was used as a
detector. The mapping was carried out by means of a 1-mm pitch scan
across a measurement range of from 340 nm to 700 nm. The vicinity of the
center, where the PL intensity was high, was the location, from which
the GaN polycrystals emitted light. There was a strong emission from the
absorption edge in the GaN crystal, but its wavelength was somewhat low,
at 371 nm, which was believed to be due to large residual strain. The
fact that the ratio of the intensity of the absorption edge emission and
the intensity of broad emission on the lower energy side varied
depending on the location was believed to be due not only to the
different growth orientation of the polycrystals, but also to the fact
that the crystal structure, as well as the defect and impurity
distribution, of the crystal grains were not uniform. In addition, the
peak observed at 410 nm was caused by the underlying glass used to
secure the sample. The reason why polycrystals were grown was believed
to be due to the lack of satisfactory growth along the surface due to
the high reaction rate of GaN and the insufficient diffusion of atoms on
the substrate. Thus, to achieve monocrystallization, it is necessary to
raise the temperature of the substrate to stimulate crystallization. In
addition, the reason for peeling was believed to be bowing generated by
mismatch with the sapphire substrate and a difference in coefficients of
thermal expansion. Thus, to avoid it, the use of improved substrate
crystals, in other words, SiC substrates and GaN substrates, is believed
necessary.

![](media/image152.png){width="5.992361111111111in"
height="3.7069444444444444in"}

Figure 8.4.10.

As described above, as a result of growing GaN crystals using
high-frequency plasma, it was confirmed that a GaN synthesis reaction
based on nitrogen activation took place in the same manner as under the
microwave system, and experiments, in which a forced reaction with Ga
was carried out, demonstrated that growth rates as high as 1 mm/h were
possible. Furthermore, as a result of conducting growth experiments by
placing sapphire substrates in the vicinity of the Ga source material,
we were able to grow mm-sized, albeit polycrystalline, crystals. In the
future, in order to conduct growth characterized by both
monocrystallization and higher reaction rates, we are planning to carry
out growth experiments at an increased rate of activation, which cannot
be easily carried out under the microwave system, by generating plasma
at a high density in a narrow region in the vicinity of the substrate by
raising the pressure and narrowing down the plasma discharge area.
Furthermore, another important task is to apply the obtained results to
the microwave system to further optimize the growth conditions.

## Summary

As a result of conducting further optimization of the pressure,
temperature, etc. in the region of high-density plasma and carrying out
investigations into the process of GaN synthesis by using the source
material temperature, substrate temperature, relative position of the
substrate and the Ga crucible, and the N~2~ pressure as parameters, we
were able to effect transition from micropowder to a dense film-shaped
GaN crystal oriented along the *c*-axis over a 10×4 mm^2^ area on a
sapphire substrate, as confirmed by the SEM photographs shown in FIG.
8.3.2-(8), with the above crystal displaying a strong (0002) single peak
without a sapphire peak, as shown by the X-ray diffraction spectrum of
FIG. 8.3.2-(9). Furthermore, we were able to synthesize a crystal
oriented along the *c*-axis that displayed a surface morphology of
excellent symmetry without surface irregularities, as shown in the SEM
photographs of FIG. 8.3.2-(10).

In addition, we experimentally established the correlation of the
maximum rate of synthesis of 100 *μ*m/h with various parameters, which
gave us the necessary degree of freedom in terms of improving the
crystal properties. Furthermore, with regard to the rate of synthesis,
varying the growth conditions, such as the partial pressure of reactive
radicals and the method of activation, allowed us to confirm that
improvement of up to a maximum level of 1 mm/h was feasible under the
gas phase growth method for compound semiconductors, albeit in
micro-crystal form.

## Future Plans

Recent research concerning GaN substrates has seen considerable progress
in terms of the development of GaN single crystals in thick film form
based on the hydride gas phase technique (HVPE) and bulk single crystals
based on the high-pressure solution technique, as well as the expanded
use of SiC substrates aimed at increasing substrate quality and
diameter. However, at present we are facing numerous problems due to a
large number of threading dislocations and the mosaic-shaped low tilt
angle grain boundaries characteristic of hexagonal crystals. In
addition, there are numerous problems arising in connection with
supplying crystals for illumination LEDs at a low cost. Based on
theoretical investigations, the possible rate of synthesis of GaN by the
low-pressure gas phase technique is expected to be 1\~6 mm/h, and if we
are able to achieve a growth rate of 1 mm/h for direct GaN synthesis
using the pseudo-open tube system and establish a production technology
permitting fabrication of large size substrates and high growth rates,
the cost of substrates for epitaxial growth will be dramatically
reduced, allowing for low cost equivalent to that of the currently used
GaAs substrates.

In the future, we are planning to grow large GaN bulk single crystals
with a diameter of not less than 10 mm by moving ahead with our
investigations into a wide range of conditions based on the already
obtained findings and exploring the possibility of using the grown GaN
single crystals as LED substrates. One important specific problem is how
best to control the stoichiometry of Ga and N~2~. For this, it will be
necessary to carry out further optimization by conducting theoretical
studies and systematic experiments concerning single crystal growth
conditions such as temperature and pressure. In addition, we will work
on another promising technique for improving the rate of growth, i.e.
the addition of reactive gases, such as H~2~ and HCl. Furthermore, yet
another goal will be to grow low dislocation density crystals with
reduced defect generation by using SiC and GaN crystals etc. as
substrates or by carrying out substrate surface processing and low
supersaturated growth during the initial stage of growth.

# Development of LED Epi Process Technology

## Achievements in 1998 and 1999

The study of emitting layers, crystal dislocation density reduction, and
ways to increase the concentration of p-type layers are considered
important issues in the development of high-efficiency UV light-emitting
diodes (LEDs) possessing an external quantum efficiency of not less than
40%. In this study, we carried out crystal growth using the molecular
beam epitaxy (MBE) technique and the metalorganic chemical vapor
deposition (MOCVD) technique. In 1998, we conducted investigations into
the fabrication of blue LEDs by the MBE technique, and in 1999, we
conducted investigations into MOCVD-grown high-brightness blue LEDs and
into ways of shortening the emission wavelength of UV LEDs.

### Achievements in 1998

#### Epi-growth based on MBE technique

We carried out epitaxial growth using the MBE technique, which permits
high p-doping upon growth without heat treatment, fabrication of quantum
well structures of excellent interface sharpness, and fabrication of
ultra-thin films necessary for improvement in element characteristics.
Examination of p-type GaN confirmed that it had a p-type carrier
concentration of 1×10^18^ cm^-3^. Emission spectra with an emission
wavelength of 420 nm and a half-width of 20 nm were observed in LED
structures obtained by superimposing an n-type GaN layer, GaInN multiple
quantum well (MQW) emitting layers, and a p-type GaN layer. However, the
emission intensity was lower than in LEDs fabricated by the MOCVD
technique by 1 or 2 orders of magnitude.

#### Introduction of small-size MOCVD apparatus

We evaluated the performance characteristics of the apparatus using
growth of GaInN and undoped GaN. The in-plane distribution of the film
thickness of the undoped GaN was confirmed to be within 4%. The *ω*-scan
X-ray diffraction half-width was about 370 arcsec in the case of (0002)
diffraction and 540 arcsec in the case of asymmetric plane
(10[1]{.underline}2) diffraction, and, according to Hall measurements,
the electron concentration was 1.7×10^17^ cm^-3^ and the mobility was
560 cm^2^/Vs. In addition, the non-uniformity of GaInN film composition
was around 4%, which resulted in a film of excellent in-plane
uniformity.

### Achievements in 1999

#### Commissioning of small-size MOCVD apparatus

To confirm the proper functioning of the MOCVD apparatus introduced in
1998, we conducted studies of blue LED structures. It was confirmed that
blue LEDs with an emission wavelength of 475 nm and a luminous intensity
equivalent to 2 candelas were obtained from LED structures, in which
MQWs consisting of GaN barrier layers/GaInN well layers were used as
emitting layers.

#### Studies of UV LED structures

We conducted investigations into the shortening of the emission
wavelength using GaN/GaInN MQW structures suitable for use in blue LED
structures. Although the shortening of the wavelength was achieved by
raising the growth temperature, the intensity of photoluminescence (PL)
from the UV MQW (emission wavelength: 388 nm) decreased by 1/2. It was
believed that factors involved in the decreased emission intensity
included the weaker confinement of the carriers to the well layers as a
result of the shortening of the wavelength, which prevented their
functioning as a quantum structure, and the weakening of the effects of
In that contribute to the PL emission \[1\].\
Fabrication of LED structures demonstrated that their peak wavelength
was 398 nm and their emission output was 0.5 mW, which was lower in
comparison with blue LEDs.

#### Dislocation density reduction by ELO technique

We investigated the ELO technique for the purpose of reducing the amount
of dislocations in the GaN layer. It was found that the tilt of the
(0001) axis, which created problems in case of ordinary SiO~2~ films,
could be prevented by using plasma CVD-grown SiN~x~ as the mask
material. We introduced a dislocation density evaluation method based on
observations of etch pits caused by sulfuric and phosphoric acid. The
dislocation density of ELO-GaN, as evaluated by the same method, was
1×10^8^ cm^-2^ in the windows and 2×10^7^ cm^-2^ on the mask. The study
confirmed that since the density of dislocations of GaN before ELO was
5×10^8^ cm^-2^, the dislocation density reduction was 1/5 in the windows
and 1/25 on the mask.

#### High-concentration p-type

We carried out investigations into the growth of p-type GaN possessing a
high carrier concentration with a view to improve the piezoelectric
efficiency of LED elements. By optimizing the growth temperature, the
amount of supplied Mg source material, and the conditions of activation
annealing, we were able to obtain p-type GaN with a concentration of
positive holes as high as 1×10^18^ cm^-3^.

#### Introduction of new facilities

An MOCVD apparatus for simultaneous processing of six 2-inch substrates
was introduced in March 2000. The proper functioning of the apparatus
was evaluated using GaN and GaInN single quantum well structure (SQW)
growth. It was confirmed that the in-plane distribution of film
thickness in the GaN layer was within 4% at 15 points on the 2-inch
wafer surface and was within 4% across the six wafers. The carrier
concentration was 1×10^17^ cm^-3^ and the degree of mobility was 640
cm^2^/Vsec. As a result of a PL evaluation using GaInN SQW structures,
it was confirmed that the peak wavelength distribution was within 5% at
25 points on the wafers and within 5% across the six wafers, which was
indicative of excellent uniformity.

## Summary of Research and Development in 2000

During the present project, we developed UV LEDs with an emission
wavelength of not longer than 400 nm and white LEDs utilizing
fluorescent materials in order to build illumination light sources with
an energy efficiency exceeding that of fluorescent lamps. When GaN/GaInN
MQW structures typically used in blue LEDs were used for UV LEDs, the
emission output turned out to be lower in comparison with blue LEDs. It
is believed that, as a result of decreasing the molar fraction of InN of
GaInN in the well layers for the purpose of shortening the wavelength,
the well bandgaps approached the bandgap of the GaN barrier layers,
which made it impossible to efficiently confine electrons in the well
layers, and the MQW stopped functioning as a quantum structure, causing
a decrease in the emission intensity. For this reason, AlGaN cladding
layers and AlGaN barrier layers with a large bandgap are essential in
terms of efficiently confining electrons to the well layers in UV LEDs.

This year, our most important task has been to boost the efficiency of
UV LEDs. In order to achieve increased efficiency, it is necessary to
optimize the MQW growth conditions and the structure of the emitting
layers for UV emission. The present study demonstrates that MQW
structures of excellent interface sharpness, as well as excellent UV
emission characteristics, can be obtained by improving the growth
process and optimizing the structure of UV MQWs. Here, explanations will
be provided regarding the characteristics of the elements of UV LEDs and
investigations into AlGaN cladding layers with high AlN molar fractions,
which are essential for efficient LEDs.

## Fabrication of AlGaN/GaInN UV MQW Structures

In the case of GaN/GaInN MQW structures, in which GaN is used in the
barrier layers and GaInN is used in the well layers, a decrease in the
PL intensity occurs in the UV region \[1\]. This is believed to be due
to the fact that in the UV region the InN molar fraction of the well
layers decreases, the difference between the bandgaps of the well layers
and the barrier layers becomes smaller, and, for this reason, the effect
of carrier confinement weakens and the emission intensity declines.
Thus, in order to efficiently confine carriers to the well layers, it is
necessary to use AlGaN with a large bandgap to secure the energy
difference relative to GaInN well layers with low InN molar fractions.
In connection with this, we are investigating  ways to improve the
emission efficiency in the UV region using AlGaN barrier layers, and 
ways to optimize the growth conditions in order to fabricate
high-quality MQWs.

### Methods and means

All of the samples were fabricated by MOCVD. (0001) sapphire was used
for the substrates. After depositing a low-temperature buffer GaN layer
on a sapphire substrate, undoped GaN was grown at an elevated
temperature. The pressure used for the high-temperature growth of GaN
was 20 kPa. The AlGaN/GaInN MQW structure utilizing the AlGaN barrier
layers investigated in the present experiment is shown in FIG.
9.3.1-(1). The thicknesses of the AlGaN barrier and the GaInN well
layers were, respectively, 8 nm and 2 nm, with the AlGaN barrier layers
doped with Si and the GaInN well layers left undoped. The growth
sequence of the MQW is shown in FIG. 9.3.1-(2). The growth temperature
was 860ºC in the case of the GaInN layers, and, in the case of the AlGaN
layers, we studied two cases, namely,  growth at 860ºC and  growth at
1100ºC. The mixed crystal composition of the AlGaN barrier layers and
GaInN well layers, as determined by X-ray diffraction, showed that the
molar fraction of AlN was 12% and the molar fraction of InN was 5%.

The surface condition of the MQW layers was examined under an atomic
force microscope (AFM), and transmission electron microscope (TEM)
observations were carried out for the purpose of structure evaluation.
PL measurements were done using a He-Cd laser with a wavelength of 325
nm as the excitation light source in order to evaluate the optical
characteristics. Furthermore, the dependence of the PL intensity on
temperature was investigated by varying the sample temperature from 10
to 300 K.

![](media/image153.png){width="5.9944444444444445in"
height="2.9451388888888888in"}

FIG. 9.3.1-(1) Schematic diagram of AlGaN/GaInN MQW structure

\[Keys, FIG. 9.3.1-(1)\]

(On the right) AlGaN: Si 8 nm/GaInN 2 nm; 5-pair MQW

(Bottom layer) Sapphire (0001)

In process , both the AlGaN barrier layers and the GaInN well layers
were grown at 860ºC, and in process  the AlGaN barrier layers were
grown at 1100ºC while the GaInN layers were grown at 860ºC.

![](media/image154.png){width="3.5729166666666665in"
height="3.5972222222222223in"}During the growth of the MQW,
trimethylaluminum (TMAl), triethylgallium (TEGa), and trimethylindium
(TMIn) were used as Group III source materials, with ammonia (NH~3~)
used as the Group V source material and monosilane (SiH~4~) used as the
dopant material.

FIG. 9.3.1-(2) MQW growth sequence

\[Keys, FIG. 9.3.1-(2)\]

(X-axis) Time.

(Y-axis0 Temperature \[ºC\].

(Inside, left to right) AlGaN barrier layer; GaInN well layer; AlGaN
barrier layer.

### Results and discussion

#### AFM observations of surface condition

![](media/image155.png){width="3.3291666666666666in"
height="3.8534722222222224in"}The surface condition of MQW layers
obtained during the fabrication of the MQW in accordance with the growth
sequence of FIG. 9.3.1-(2) in case , wherein both the AlGaN barrier
layers and the GaInN well layers were grown at 860ºC, and in case ,
wherein the AlGaN barrier layers were grown at 1100ºC and the GaInN
layers were grown at 860ºC, is shown in FIG. 9.3.2-(1). In case , a lot
of pits were generated, whereas in case  there were almost no pits, and
the surface condition was excellent.

FIG. 9.3.2-(1) AFM images of MQW layers (2 μm × 2 μm)

Left: actual image, right: derivative image.

\[Keys, FIG. 9.3.2-(1)\]

\(a\) (case ) AlGaN/GaInN MQW with both AlGaN layers and GaInN layers
grown at 860ºC

\(b\) (case ) AlGaN/GaInN MQW with AlGaN layers grown at 1100ºC and
GaInN layers grown at 860ºC

#### TEM observations

The quality of the interfaces in the MQW were evaluated using
cross-sectional TEM. FIG. 9.3.2-(2) (a) is a cross-sectional TEM image
of an AlGaN/GaInN MQW structure with an excellent surface condition,
which was obtained when the AlGaN layers were grown at an elevated
temperature. From the results of (1), it is evident that in order to
obtain MQW layers of excellent surface condition it is necessary to grow
the AlGaN layers at an elevated temperature. On the other hand, in order
to incorporate In during the growth of GaInN, it is necessary to
maintain a low growth temperature. Although it was believed that as a
result of the temperature rise during the growth of AlGaN, the GaInN
layers were subject to thermal damage, it was found that in FIG.
9.3.2-(2) (a) there was no visible damage to the MQW layers and
excellent hetero-interfaces were formed. For comparison, a
cross-sectional TEM image of a high-brightness blue LED with a GaN (7
nm)/GaInN (3 nm) MQW structure is shown in (b) of the same figure. There
was no marked difference in terms of the interface sharpness between the
two figures. From this, we concluded that UV MQWs utilizing AlGaN
barrier layers had structures that compared very well with those of
high-brightness blue MQWs utilizing GaN barrier layers.

![](media/image156.png){width="2.7284722222222224in"
height="3.8715277777777777in"}FIG. 9.3.2-(2) Cross-sectional TEM images
captured in the vicinity of MQWs

\[Keys, FIG. 9.3.2-(2)\]

\(a\) AlGaN/GaInN UV MQW structure

\(b\) GaN/GaInN high-brightness blue LED

#### Evaluation of optical characteristics using PL

To evaluate the optical characteristics of the MQW structures, we
carried out PL measurements. FIG. 9.3.2-(3) shows a PL spectrum obtained
from an AlGaN/GaInN MQW structure with AlGaN barrier layers grown at an
elevated temperature. For comparison, a PL spectrum of a high-brightness
blue MQW is shown with a dotted line in the same figure. UV emission
with a peak wavelength of 385 nm was observed. The emission intensity
was approximately the same as that obtained in the case of the blue MQW
structure. In the case of MQW structures utilizing GaN barrier layers,
the PL intensity decreases as the emission wavelength is shortened (FIG.
9.3.2-(4)) \[1\]. This is believed to be due to the fact that, as
described above, as a result of a reduction in the InN molar fraction of
the well layers for the purpose of shortening the wavelength, the
bandgaps of the MQW layers approach the bandgap of the GaN barrier
layers and stop functioning as quantum structures, resulting in a
decrease in intensity. On the other hand, it is believed that when the
AlGaN barrier layers are used, it is possible to efficiently confine
electrons to the well layers because the energy difference with respect
to the well layers can be reduced, thereby suppressing the decrease in
the PL intensity.

![](media/image157.png){width="3.592361111111111in"
height="2.6708333333333334in"}

FIG. 9.3.2-(3) PL spectra obtained from UV MQW and blue MQW structures

\[Keys, FIG. 9.3.2-(3)\]

(Peak on left) AlGaN/GaInN MQW; peak wavelength: 385 nm

(Peak on right) GaN/GaInN MQW; peak wavelength: 448 nm

FIG. 9.3.2-(5) shows the dependence of the PL intensity on the
temperature, which was obtained by varying the sample temperature from
10 K to 300 K. Results obtained for an AlGaN/GaInN MQW with AlGaN grown
at an elevated temperature and, for comparison, a GaN/GaInN blue MQW,
are shown in the same figure. It was found that as the temperature
increased, the PL intensity decreased (temperature-induced extinction).
This is believed to be due to the fact that as the temperature rises,
non-radiative centers, such as crystal defects, etc., become activated,
which leads to extinction. The ratio of the PL intensities at 300 K and
at 10 K (I~300\ K~/I~10\ K~) was 33% for the UV MQW and 23% for the blue
MQW. In addition, when AlGaN was grown at 860ºC, the ratio was 22%. In
this connection, it is believed that growing the AlGaN barrier layers at
1100ºC improves the crystal properties, and, as a result, the PL
intensity becomes higher than that of the blue MQW and the non-radiative
centers are suppressed.

![](media/image158.png){width="2.938888888888889in"
height="2.048611111111111in"}

Left: FIG. 9.3.2-(4) Relationship between PL peak wavelength and PL
intensity

Right: FIG. 9.3.2-(5) Dependence of PL intensity of UV MQW on
temperature

![](media/image160.png){width="3.109722222222222in"
height="2.2319444444444443in"}FIG. 9.3.2-(6) shows the peak wavelength
and the PL intensity ratio I~300\ K~/I~10\ K~. When GaN barrier layers
were used, a decrease in the PL intensity ratio was observed in the UV
region below 400 nm, with the I~300\ K~/I~10\ K~ ratio at a peak
wavelength of 389 nm being approximately 10%; the ratio, however, was
almost tripled, to 33%, when AlGaN barrier layers were used. This
demonstrated that AlGaN/GaInN MQWs can effectively increase efficiency
in the UV region.

FIG. 9.3.2-(6) Relationship between PL peak wavelength and PL intensity
ratio

\[Keys, FIG. 9.3.2-(6)\]

(X-axis) PL peak wavelength \[nm\].

(Y-axis) PL intensity ratio (I~300\ K~/I~10\ K~) \[%\]

## Study of AlGaN cladding layers with high AlN molar fraction

As a result of the decreased InN molar fractions of the GaInN used in
the emitting layer, in order to efficiently confine carriers to the well
layers, it is essential to use not only the AlGaN barrier layers
described in 9.3, but also a AlGaN cladding layer with a high AlN molar
fraction. The AlN molar fraction of the AlGaN cladding layer currently
used in UV LEDs is about 20% \[2\]. However, as the AlN molar fraction
increases, the growth of p-type AlGaN is rendered more difficult, and
problems arise in terms of p-type conductivity control. In addition, the
risk of parasitic reactions between NH~3~ and TMAl, which is used as the
Al source material, presents a problem in the commonly used
normal-pressure MOCVD technique, which makes growth at high AlN molar
fractions difficult. To grow AlGaN, it is important to suppress the
parasitic reactions. The MOCVD apparatus used in the present study
allows for growth at a reduced pressure, which is believed to be
advantageous in terms of suppressing the parasitic reactions and growing
AlGaN with a high AlN molar fraction. In this connection, we have been
investigating the growth of single thin films of AlGaN with a high AlN
molar fraction (an AlN molar fraction of about 20%) for use in UV LEDs.

### Methods and means

Single thin films of AlGaN were grown on undoped GaN layers. The
thicknesses of the AlGaN layers was 0.3 *μ*m. The conditions, under
which the AlGaN layers were grown are shown in Table 9.4.1. The growth
pressure used for growing the GaN was 20 kPa and the total gas flow rate
was 10 slm. In the case of AlGaN growth, at a growth pressure of 20 kPa,
the AlN molar fraction was high, about 15%, regardless of the TMAl gas
composition, which made high-concentration growth difficult. For this
reason, in this study we investigated AlGaN growth at 10 kPa, i.e. a
pressure lower than the growth pressure used for GaN. The AlN molar
fraction of the AlGaN layer was obtained using X-ray diffraction in a
2θ/*ω* scan.

  ----------------------------------- -----------------------------------
  Growth temperature                  1180ºC

  Growth pressure                     10 kPa

  TMGa molar flow rate                7.8 × 10^−5^ mol/min

  TMAl molar flow rate                2.6\~7.8 × 10^−5^ mol/min

  NH~3~ molar flow rate               1 × 10^−1^ mol/min

  Total gas flow rate                 10\~18 slm
  ----------------------------------- -----------------------------------

Table 9.4.1 AlGaN Growth Conditions

### Results and discussion

FIG. 9.4.2-(1) shows the relationship between the TMAl gas composition
and the AlN molar fraction under a total gas flow rate of 10 slm. At a
TMAl gas composition of 25%, the AlN molar fraction was 12%, and at 50%,
the AlN molar fraction was 15%. These results show that an increased
concentration is difficult to achieve by simply increasing the TMAl gas
composition. This is believed to be due to the fact that, at a total gas
flow rate of 10 slm, the high flow velocity results in a decrease in the
incorporation of Al into the solid phase.

FIG. 9.4.2-(2) shows the relationship between the total gas flow rate
and the AlN molar fraction. In the case of a total gas flow rate of 14
slm, the AlN molar fraction at the TMAl gas composition of 25% was 22%,
and in case of a total gas flow rate of 18 slm, the AlN molar fraction
at a TMAl gas composition of 50% was 37%, obtaining an AlGaN layer with
an AlN molar fraction of approximately 20%, which is necessary for UV
LEDs.

The relationship between the flow velocity during the growth of AlGaN
and the AlN molar fraction is shown in FIG. 9.4.2-(3). To suppress
parasitic reactions and obtain AlGaN layers with an AlN molar fraction
exceeding 20%, it is necessary to increase the flow velocity to 100
cm/sec or higher.

![](media/image161.png){width="2.9875in" height="2.1951388888888888in"}

Left: FIG. 9.4.2-(1) Relationship between TMAl gas composition and AlN
molar fraction (total gas flow rate: 10 L)

Right: FIG. 9.4.2-(2) Relationship between total gas flow rate and AlN
molar fraction

\[Keys, FIG. 9.4.2-(2)\]: TMA gas composition 50%; TMA gas composition
25%

![](media/image163.png){width="3.25625in"
height="2.3291666666666666in"}The results above show that an increase in
the flow velocity achieved by increasing the total gas flow rate 1.5\~2
times and a reduction in the growth temperature during AlGaN growth by
1/2 in comparison with the growth conditions used in case of GaN (20→10
kPa) provides for efficient growth with a high AlN molar fraction.

FIG. 9.4.2-(3) Relationship between flow velocity during AlGaN growth
and AlN molar fraction

\[Keys, FIG. 9.4.2-(3)\]: (X-axis) Flow velocity \[cm/sec\].

## Fabrication of UV LED Structures

This section describes the fabrication, emission characteristics, and
electrical characteristics of UV LED structures obtained based on
results of the investigations described in 9.3 and 9.4.

### Methods and means

A UV LED structure is shown in FIG. 9.5.1. An undoped GaN layer, an
n-type GaN:Si layer, an AlGaN/GaInN 5-pair MQW, a p-type
Al~0.2~Ga~0.8~N:Mg cladding layer, and a p-type GaN:Mg layer were
deposited on a sapphire substrate. The carrier concentration of the
p-type AlGaN:Mg was 1 × 10^17^ cm^-3^. Al was used as the n-type
electrode, and Ni/Au was used as the p-type electrode. The alloying of
the p-side electrode was conducted for 10 min at a pressure of 1
atmosphere and a temperature of 540ºC in a 5000 sccm stream of mixed gas
consisting of oxygen and argon. The emission spectrum of the
electroluminescence (EL) of the LED element was measured using a
wavelength measurement apparatus with an auto-prober. The wavelength
range was 300\~800 nm. The current/voltage characteristic of the LED
element was measured using a DC current/voltage source monitor.

![](media/image164.png){width="3.303472222222222in"
height="2.4472222222222224in"}FIG. 9.5.1 UV LED Structure

\[Keys, FIG. 9.5.1\]

(Top) P-electrode.

(Middle, right) N-electrode.

(Lowest layer) Sapphire (0001).

### Results and discussion

FIG. 9.5.2-(1) shows the EL spectrum. It was confirmed that UV emission
at a wavelength of 390 nm occurred after current injection. The
operating voltage at 20 mA was 3.7V (FIG. 9.5.2-(2)). The emission
power, converted to lamp-equivalent output, corresponded to 1 mW
(measured using an auto-probe), with the output of the UV LED being
lower in comparison with that of the blue LED. In case of PL induced by
light excitation, no PL intensity decrease due to the shortening of the
wavelength was observed; however, with current injection, the EL
intensity decreased, which was believed to be due to the formation of
non-radiative centers. It is necessary to improve the crystal quality
and reduce the non-radiative recombination rate by carrying out further
optimization of the growth conditions.

![](media/image165.png){width="2.9145833333333333in"
height="2.1465277777777776in"}

Left: FIG. 9.5.2-(1) Current injection emission spectrum of UV LED.

\[Keys, FIG. 9.5.2-(1)\]: (X-axis) Wavelength \[nm\]; (Y-axis) Intensity
\[a.u.\].

Right: FIG. 9.5.2-(2) I-V characteristic of UV LED.

## Summary

### Fabrication of AlGaN/GaInN UV MQW structures

We investigated AlGaN/GaInN MQW emitting layers designed for use in the
emitting layers of UV LEDs. MQW structures of excellent surface
condition and interface sharpness were obtained by growing AlGaN barrier
layers at an elevated temperature of 1100ºC. PL measurements showed that
excellent UV emission characteristics were obtained, with a peak
wavelength of 385 nm and an emission intensity equivalent to that of the
PL from a blue MQW. As a result of investigating the ratio of PL
intensities (I~300\ K~/I~10\ K~), it was established that the ratio
reached 33% in UV MQWs utilizing AlGaN barrier layers, which was higher
than the 23% in the case of blue QWs utilizing GaN barrier layers. By
using AlGaN barrier layers, we were able to suppress the decrease in the
PL intensity ratio in the UV region of GaN barrier layer MQWs.

### Study of AlGaN cladding layer with high AlN molar fraction

We investigated the growth of a AlGaN cladding layer with a high AlN
molar fraction for use in UV LEDs. It was found that parameters such as
growth pressure and total gas flow rate could effectively influence the
growth of the AlGaN with a high AlN molar fraction. As a result, a AlGaN
layer with AlN molar fractions exceeding 20% was obtained.

### Fabrication of UV LED element structures

We fabricated LED structures utilizing AlGaN/GaInN MQW as emitting
layers. The presence of 390-nm UV current injection emission was
confirmed. The operating voltage at 20 mA was 3.7V. As converted to lamp
output, the emission output corresponded to 1 mW, which was lower in
comparison with that of blue LEDs, and, therefore, called for further
investigations into the optimization of the growth conditions in order
to increase the efficiency.

## Future Plans

In the future, we are planning to carry out further optimization of the
growth conditions and establish an epi-technology for high-efficiency UV
LEDs. The topics for future research are shown below.

### Non-radiative recombination rate reduction

Further optimization of the growth conditions, and, especially, an
increase in the quality of the MQWs, are necessary in order to reduce
the non-radiative recombination rate, which causes the decreased output
of UV LEDs. In addition, we would like to study the influence of the
introduction of a superlattice cladding layer in terms of improving the
crystal properties of MQW layers and in terms of the LED emission
brightness.

In addition, from the I-V characteristic we concluded that the cause of
the non-radiative recombination rate was in the interface (pn junction)
between the MQW layer and the p-layer, and we believe that it is
important to investigate p-AlGaN cladding layers as well. It is
necessary not only to improve the interface sharpness of the pn
junction, but also to continue investigating the growth of
high-carrier-concentration p-AlGaN in order to improve electrical
characteristics.

### AlGaInN emitting layer (high InN molar fraction)

In the UV region, the InN molar fraction of the GaInN emitting layer
decreases. For this reason, the effects of In weaken, and the PL
intensity decreases. It is believed that enhanced In effects can be
expected in case of an AlGaInN emitting layer, because a high InN molar
fraction can be achieved. 360-nm UV current injection emission from LED
structures utilizing MQW structures made of quaternary mixed crystals
such as AlGaInN has been observed \[3\]. In previous investigations into
AlGaInN emitting layers their intensity has been lower by one order of
magnitude as compared with blue MQWs. The introduction of Al in the
emitting layer results in a deterioration of the surface flatness,
bringing about a decrease in the PL intensity. By optimizing the growth
conditions, we are planning to increase the quality of the AlGaInN layer
and achieve high-efficiency UV emission.

### ELO technique

Regarding the ELO technique, we will work on the optimization of the
regrowth conditions and further reduction of dislocations. We will
fabricate UV LED structures on ELO-GaN substrates to confirm the
dislocation reduction effect.

### Large-size MOCVD apparatus

We would like to make use of multiple wafer growth in order to clarify
the effects of the substrate on the LED emission efficiency. The density
of dislocations, the amount of strain applied to the emitting layers,
and control of the substrate surface condition are believed to exert
considerable influence on the emission efficiency. A large MOCVD
apparatus permits comparison, under the same growth conditions, of
substrates exhibiting different dislocation densities, differences in
the degree of lattice matching, and differences in the methods of
surface processing. We would like to investigate the effects produced on
the efficiency of LED emission by different dislocation densities of
ELO-GaN substrates and GaN substrates, different degrees of lattice
matching of SiC substrates, by sapphire substrates, as well as by
differences in the corresponding pre-processing conditions. Furthermore,
an in-situ observation system based on the surface reflectivity of
visible light could be a powerful tool in controlling the condition of
the substrate surface immediately prior to growth.

### Application of SiC substrates

The use of SiC substrates offers certain advantages in terms of LED
fabrication, such as a smaller degree of lattice mismatch in comparison
with sapphire substrates, electrical conductivity of the substrate, etc.
We would like to confirm the influence exerted by the use of SiC
substrates on LED characteristics.

## References

\[1\] High-efficiency photoelectric conversion compound semiconductor
research activity report (1999), Research and Development Center for
Metals, p. 157.

\[2\] T. Mukai, M. Yamada, and S. Nakamura: Jpn. J. Appl. Phys. 37
(1998) L1358.

\[3\] A. Kinoshita, H. Hirayama, M. Ainoya, Y. Yamahi, A. Hirata, Y.
Aoyagi: Materials of Scientific Symposium No. 61 of Applied Physics
Society, 7a-L-7, p. 332.

# Development of High-Output UV Light-Emitting Components

## Achievements in 1998 and 1999

In 1998 and 1999, as a result of carrying out fundamental investigations
of UV light-emitting components using existing MOVPE (Metalorganic Vapor
Phase Epitaxy) crystal growth equipment, as well as fundamental
investigations of the ELO (Epitaxially Lateral Overgrowth) technique,
which offers a promising technology for dislocation reduction, we
obtained the information necessary to determine our approach to
component design.

### Achievements in 1998

Based on the blue LED (Light Emitting Diode) fabrication technology,
which our company had been developing and maintaining in-house as a
proprietary technology, we conducted investigations into the fundamental
film-forming conditions necessary for In~x~Ga~1−x~N (0≤x≤0.12,
wavelength: 362\~400 nm), which is used in the emitting layer of UV
LEDs, and carried out evaluation of its characteristics with special
emphasis on PL (Photo-Luminescence) measurements.  The PL intensity of
In~x~Ga~1−x~N single films (0 ≤ x ≤ 0.12, 50 nm, n = 2 × 10^18^ cm^-3^)
peaked in the vicinity of In composition x = 0.05 (emission wavelength:
378 nm).  We investigated the dependence of the PL intensity of
In~x~Ga~1−x~N (0 ≤ x ≤ 0.12, w = 2 nm \~ 10 nm, n = 2 × 10^18^
cm^−3^)/AlGaN-SQW (Single Quantum Well) structures on *x* and well width
(*w*) and strengthened our grasp on the PL characteristics that are
strongly dependent on *x* and *w*. It was found that a well width of 3
nm was the optimal value for all tested *x*. The PL intensity decreased
with decreasing In composition, and the elucidation of the causes and
development of countermeasures against this phenomenon were put on our
agenda for the next year.  We verified the dislocation reduction
effects afforded by the ELO technique and found that although the
dislocation reduction effects did exist, there were various problems,
which included substrate bowing, crystal axis (*c*-axis) wobble, etc.

### Achievements in 1999

Using the InGaN-SQW-UV LED as a basic structure, we  investigated the
dependence of the optical characteristics (temperature characteristics
of PL) of InGaN-SQW structures on the In composition,  built prototypes
of UV LEDs,  examined the dependence of the emission intensity on the
dislocation density,  investigated the ELO technique, and  designed
and introduced a novel MOVPE apparatus (property of NEDO) \[*NEDO: New
Energy and Industrial Technology Development Organization − trans*.\].

 By evaluating the PL temperature characteristics of InGaN-SQW
structures, it was confirmed that as the In composition decreased, the
intensity of room-temperature PL decreased and the temperature
extinction of the PL intensity increased.  Results of UV LED
prototyping showed a decrease in the emission output associated with the
shortening of the emission wavelength.  Examination of the relationship
between the density of dislocation-related pits on the surface of
InGaN-MQW (Multi-Quantum Well) structures and the PL intensity confirmed
that there was a negative correlation between the density of
dislocations and the PL intensity. Thus, we concluded that , , and 
in the 1998 achievements section were due to the powerful influence of
dislocations and other defects in the UV emission wavelength region, and
determined that dislocation defect reduction was essential to the
practical implementation of high-output UV LEDs and that it was
important to optimize crystal growth conditions (especially the
conditions of the initial stage of crystal growth) in order to determine
the conditions necessary for reducing the number of dislocation defects.

 With a view to reduce the warping of the substrate and the wobble of
the crystal axis, which present serious problems in ELO-GaN layers, we
conducted investigations into the use of fine-striped masks in the ELO
technique. As a result, by setting the ratio of masked/open width to 0.5
*μ*m/0.5 *μ*m, we were able to fabricate thin ELO-GaN and successfully
reduced the *c*-axis wobble. Although we did achieve a dislocation
density reduction as a result of introducing ELO-GaN, it did not amount
to a substantial improvement in the optical characteristics of the UV
InGaN-SQW structures formed thereon, which attested to the need to
develop novel ELO techniques. In addition, we  completed the design and
commissioning of a novel MOVPE apparatus and started research aimed at
determining the appropriate crystal growth conditions.

Based on the above-described achievements, our top-priority development
tasks for 2000 include  optimization of the initial stage of crystal
growth and crystal growth conditions of the emitting layer, 
development of dislocation defect reduction techniques, such as ELO,
etc.,  development of component structures suitable for
short-wavelength LEDs.

## Summary of Research and Development in 2000

Based on the previously set tasks, we conducted the following
investigations.

-   Optimization of the amount of Si used for the doping of InGaN-MQW
    structures

-   Investigation into new dislocation density reduction techniques
    based on problems that had been uncovered in ELO

-   Building of UV LED prototypes by applying the results of the
    above-described investigations

-   Building of blue LED prototypes for the purpose of comparison with
    the UV LEDs

-   Building of white LED prototypes by applying the results obtained by
    building the UV and blue LED prototypes

As a result of the above-mentioned investigations, it was found that the
emission output of UV LEDs could be improved by adding trace amounts of
Si to the barrier layers.

In addition, we proposed a technique called LEPS (Lateral Epitaxy on the
Patterned Substrate) as a novel technique for dislocation reduction. As
a result, it has been confirmed that this method is extremely effective
in improving the output of UV LEDs and in terms of dislocation
reduction.

## Building of prototypes of UV-emitting components

When we conducted preliminary studies of MQW layers last year, we
obtained results indicating that their emission characteristics could be
improved by Si doping. In this report we present the results of our
investigations into the influence exerted on the optical characteristics
and device characteristics by the Si doping of the barrier layers of UV
LEDs.

### Experimental method

The structure of the samples used in the experiments is shown in FIG.
10.3.1. The samples were fabricated using a vertical-type atmospheric
pressure MOVPE apparatus. *C*-plane sapphire was used for the substrate,
and, after subjecting it to thermal cleaning treatment in a hydrogen
atmosphere at approximately 1150ºC, a GaN buffer layer (thickness t = 27
nm) was grown thereon at a temperature of 450ºC. A Si-doped n-GaN layer
(4 *μ*m), a Si-doped n-Al~y~Ga~1-y~N layer (y = 0.1, 50 nm), an MQW
structure (number of wells = 4), which was made up of In~x~Ga~1-x~N (x =
0.08, 3 nm) well layers and Si-doped n-GaN (6 nm) barrier layers, a
Mg-doped p-Al~y~Ga~1−y~N layer (y = 0.1, 50 nm), and a Mg-doped p-GaN
contact layer (100 nm) were successively grown thereon.

The amount of Si used to dope the barrier layers was varied (2.4 ×
10^18^ cm^-3^, 8 × 10^17^ cm^-3^, 2 × 10^17^ cm^-3^, and no doping).
Upon completion of growth, some of the p-type layer of the wafer was
removed by dry etching to expose the n-GaN layer and an n-electrode was
formed thereon. In addition, after forming a p-electrode in the p-GaN
contact layer, device isolation was carried out, resulting in an LED
chip.

Using a PL apparatus installed at Yamaguchi University, the fabricated
samples were subjected to optical measurements in order to examine their
PL spectra under excitation (using a He-Cd laser with an excitation
density of 25 mW/cm^2^) and the temperature dependence of the PL
intensity across a temperature range from 4K to room temperature.

![](media/image167.png){width="4.083333333333333in" height="2.25in"}

FIG. 10.3.1 Structure of LED prototype

\[Keys, FIG. 10.3.1\]

(Top to bottom) p-GaN:Mg layer, 100 nm; p-Al~y~Ga~1−y~N layer, 50 nm;
MQW layers, InGaN 3 nm/GaN:Si 6 nm, 4 cycles; n-Al~y~Ga~1−y~N:Si layer,
50 nm; n-GaN:Si , 4 μm; Low-temperature GaN buffer layer, 27 nm.

### Results and discussion

#### Results and discussion of optical evaluation by PL

The PL spectra of the UV LED prototypes obtained at 4 K are shown in
FIG. 10.3.2-(1), and changes in the PL emission intensity of the active
layer across the range of from 4 K to room temperature are shown in FIG.
10.3.2-(2).

![](media/image168.png){width="3.0in" height="2.9305555555555554in"}

Left: FIG. 10.3.2-(1) Comparison of PL spectra (active layer) obtained
at 4 K

\[Keys, FIG. 10.3.1-(1)\]: (Inside, bottom left) No doping.

Right: FIG. 10.3.2-(2) Temperature dependence (active layer) of PL
emission intensity.

\[Keys, FIG. 10.3.1-(2)\]: No doping

Mg-related emission was observed in the vicinity of 3.2 eV in undoped
samples, where the barrier layers were not subjected to Si doping. On
the other hand, this emission was not observed in Si-doped samples. In
this connection, it is believed that the Si doping of the barrier layers
produces suppressing effects on the diffusion of Mg from the p-type
layer into the emitting layer.

Examination of the temperature dependence of the PL emission intensity
showed that, in comparison with the Si-doped samples, samples that had
not been doped with Si exhibited a weaker intensity in all temperature
zones. This is believed to be due to the formation of non-radiative
recombination centers as a result of Mg diffusion in the emitting layer.
In addition, as a result of subjecting the barrier layers to high-level
doping, the PL emission intensity at low temperatures was high, but, as
a result of a considerable temperature extinction, the PL emission
intensity at room temperature decreased. Examination of the full width
half maximum (FWHM) of the PL spectra of FIG. 10.3.2-(1) showed that the
FWHM widened when the amount of Si used for doping was increased. Based
on this, we believe that excessive Si doping brings about a
deterioration in crystal properties.

#### LED prototyping results and discussion

The relationship between the LED chip output and the amount of Si used
for the doping of the barrier layers is shown in FIG. 10.3.2-(3).

![](media/image170.png){width="3.5243055555555554in"
height="2.3048611111111112in"}

FIG. 10.3.2-(3) Dependence of LED chip output on amount of Si used for
doping (of barrier layers)

\[Keys, FIG. 10.3.2-(3)\]

(X-axis) Amount of Si used for doping (of barrier layers) (cm^−3^)

(X-axis, point of origin) No doping.

(Y-axis) LED output (a. u.).

![](media/image171.png){width="3.317361111111111in"
height="3.390277777777778in"}The output peaked at a Si doping amount of
2 × 10^17^ cm^-3^. The sample was mounted on a TO-18 STEM *\[scanning
transmission electron microscope − trans.\]* stage and an integrating
sphere was used to measure the output, which was 1.2 mW (at 20 mA).
Although the emission output did increase as the amount of Si used for
doping was decreased, but with no doping at all, there was a
considerable drop in the output. This was in good agreement with the
results of the PL-based optical characteristic evaluation and was
believed to be due to the fact that absence of doping caused a decrease
in the emission output due to the Mg diffusion from the p-layer into the
emitting layer, with the emission output dropping because of
deterioration in crystal quality when the carrier concentration was
increased.

A current value dependence of the EL spectrum of an LED chip doped with
Si at a level of 2 × 10^17^ cm^-3^ is shown in FIG. 10.3.2-(4). The blue
shift, which is observed in blue LEDs, was not observed as the current
value was increased. On the other hand, when the current value was
increased to 20 mA and higher, we observed a red shift, which was
believed to be due to the shrinking of the bandgap caused by heat. These
results are believed to be due to the fact that because in UV LEDs the
In composition of the InGaN well layers is low, In composition
fluctuations have practically no effect, and no blue shift is generated
as a result of the increase in the current value.

FIG. 10.3.2-(4) Dependence of EL spectrum on current value.

\[Keys, FIG. 10.3.2-(4)\]

(X-axis) EL wavelength (nm).

### Building of white LED prototypes

UV epi-wafers manufactured by our company were processed by Stanley
Electric Co., Ltd., who used them to fabricate LED chips subsequently
die-bonded in a flip chip fashion on a TO-18 STEM stage \[1\] and
subjected to measurements. In addition, a prototype of a white LED lamp
was build \[2\] from the UV LED chips based on the technique of RGB
(red/green/blue) fluorescent material excitation.

Results obtained for LED chips die bonded using face-up mounting by our
company and die bonded using flip-chip mounting by Stanley Electric Co.,
Ltd. are shown in Table 10.3.1.

  ----------------------------------- -----------------------------------
  Die Bonding Technique               LED Output (mW)

  Face up                             1.2

  Flip chip                           2.9
  ----------------------------------- -----------------------------------

Table 10.3.1 Differences in LED output due to die bonding techniques
(TO-18: bare chip: at 20 mA)

Thus, by using flip-chip mounting, the LED output was increased
approximately 2.4 times, obtaining an output of 2.9 mW and an external
quantum efficiency of 4.3%. This improvement in LED output was believed
to be due to improvement in light extraction efficiency and corroborated
the double increase effects that have been reported \[1\]. In addition,
although in the present study we did not do any resin sealing, we
believe that in the future the output will be improved even more by
means of resin sealing \[1\].

The luminous efficacy of the white LED lamp obtained by exciting the RGB
fluorescent materials was 1.7 lm/W.

## Investigations into new dislocation reduction techniques

In addition to the ELO technique and the Pendeo-Epitaxy technique \[3,
4\], recent efforts aimed at dislocation density reduction by means of
selective growth have included investigations into the air-bridge
technique \[5\] and facet-controlled ELO \[6\]. In case of ordinary ELO
growth, it is necessary to grow a GaN layer as an underlying layer and
carry out the operations of mask-forming and ELO growth, and, therefore,
growth has to be carried out twice. In addition, there is the problem of
the auto-doping Si contamination of the growth layer by the mask layer
material (SiO~2~) and the problem that selective growth of AlGaN cannot
not be carried out by reacting TMA (trimethylaluminum) and SiO~2~. In
this study, we attempted to further reduce the dislocation density of
GaN by mask-less one-step growth by the LEPS technique, in which the
substrate is processed to impart it with a groove-and-ridge topography
and create preferential conditions for lateral growth from the ridges.

### LEPS technique

Schematic diagrams illustrating the commonly used ELO technique and the
LEPS technique are shown in FIG. 10.4.1.

![](media/image172.png){width="5.995833333333334in"
height="2.854861111111111in"}

FIG. 10.4.1 Schematic representation of ELO and LEPS techniques

\[Keys, FIG. 10.4.1\]

(Left column) Sapphire substrate; Mask; GaN layer; GaN layer; (a) ELO
technique

(Right column) Sapphire substrate processed to impart groove-and-ridge
topography; GaN layer; (b) LEPS technique

While the ELO technique requires crystal growth to be carried out twice
or more times, the same effects are believed to be attainable by the
LEPS technique, where growth has to be conducted only once.

### Experimental (LEPS-GaN growth)

The structure of a processed substrate used for growth is shown in FIG.
10.4.2. A ridge-and-groove stripe with 2-*μ*m ridges and 2-*μ*m grooves
with a depth of 2 *μ*m, whose length direction coincided with
\<11[2]{.underline}0\> of the GaN, was formed on a *c*-plane sapphire
substrate. The ridge-and-groove pattern was formed by dry etching using
an ICP (Inductive Coupled Plasma)-type RIE (Reactive Ion Etching)
apparatus introduced the year before.

GaN was grown in an atmospheric pressure MOVPE apparatus, with GaN
buffer layers grown at low temperatures, and GaN layers grown at
elevated temperatures. The shape of the fabricated samples was examined
using scanning electron microscopy (SEM). In addition, dislocation
behavior was observed using transmission electron microscopy (TEM) and
dislocation density distribution was studied by observing pits formed as
a result of dry etching.

![](media/image173.png){width="3.817361111111111in"
height="2.317361111111111in"}

FIG. 10.4.2 Structure of processed substrate.

\[Keys, FIG. 10.4.2\]

(Underneath image) Ridge/groove/height: 2 μm/2 μm/2 μm

### Results and discussion

Results obtained by varying the growth time and observing the process of
growth by means of cross-sectional SEM are shown in FIG. 10.4.3-(1).

![](media/image174.png){width="4.414583333333334in"
height="3.7194444444444446in"}

FIG. 10.4.3-(1) Cross-sectional SEM photos of LEPS-GaN

\[Keys, FIG. 10.4.3-(1)\]

(Top row) After growing for 15 min After growing for 30 min

(Bottom photo) After growing for 60 min

Although the growth of GaN takes place both in the ridge and groove
portions, it was confirmed that film originating from the ridge portions
grew in the lateral direction. The shape formed in the process of growth
is enclosed by a {0001} face and a {11[2]{.underline}2} face, which is a
stable face. As the growth continues, the laterally grown film
coalesces, forming a flat film made up of {0001} only. As a result of
the above-described lateral growth, the supply of source materials to
the groove portions is shut off, so that growth can no longer continue
and hollow portions are formed therein.

TEM observations were carried out in order to examine dislocation
behavior. A cross-sectional TEM image is shown in FIG. 10.4.3-(2).
Whereas in the portions above the ridges dislocations generated at the
substrate/GaN interface extended upwards, no dislocations were observed
in the portions above the grooves, where growth in the lateral direction
took place. On the other hand, generation of new dislocations was
observed in locations where the laterally grown film coalesced. This
means that while in the portions above the ridges dislocations propagate
upwards, in the portions above the grooves, where growth in the lateral
direction takes place, a reduction in the density of dislocations can be
achieved. Because the {11[2]{.underline}2} face was revealed in the
process of growth, it is believed that the propagation and reduction of
these dislocations occur in accordance with the same mechanism as the
one reported in case of FIELO \[7\].

![](media/image175.png){width="4.719444444444444in" height="3.0in"}FIG.
10.4.3-(2) Cross-sectional TEM image of LEPS-GaN.

\[Keys, FIG. 10.4.3-(2)\]

(Left, top to bottom) No dislocations observed in lateral growth
portions; High-density of dislocations observed above sapphire;
(Directly under image) Dislocations observed in central sections of
concave portions

Next, in order to check the dislocation density of the resultant film,
we carried out observations of pits (which corresponded to certain
dislocations) detected during dry etching. An SEM image of the film is
shown in FIG. 10.4.3-(3).

It was confirmed that while pits corresponding to dislocations were
visible above the ridges, the number of pits in the locations of lateral
growth above the grooves was considerably reduced.

![](media/image176.png){width="4.70625in" height="2.2819444444444446in"}

FIG. 10.4.3-(3) Pits produced by dry etching on an LEPS-GaN film

\[Keys, FIG. 10.4.3-(3)\]

(Bottom, left): Sapphire substrate.

## Application of LEPS technique to LEDs

The LEPS technique had been confirmed as an effective method of
dislocation density reduction. For this reason, we decided to see if it
could be applied to UV LEDs, whose emission output was believed to be
amenable to improvement by dislocation density reduction.

### Experimental methods

The structure of the fabricated UV LEDs is shown in FIG. 10.5.1.

![](media/image177.png){width="3.634027777777778in" height="3.4875in"}

FIG. 10.5.1 Structure of LED prototypes

\[Keys, FIG. 10.5.1\]

(Top to bottom) p-GaN contact layer

p-AlGaN cladding layer

InGaN-MQW active layer

n-AlGaN cladding layer

n-GaN contact layer

Buffer layer

Sapphire substrate processed to impart ridge-and-groove topography

First of all, ridge-and-groove stripes with 2-*μ*m ridges and 2-*μ*m
grooves with a depth of 2 *μ*m, whose length direction coincided with
\<11-20\> of the GaN, were formed on a *c*-plane sapphire substrate. The
growth of LED structures on processed substrates was carried out based
on the structure shown in FIG. 10.3.1. Also, the amount of Si used for
the doping of the barrier layers was 2 × 10^17^ cm^-3^. LED wafers were
obtained by etching away part of the p-type layer and emitting layer,
and then forming n- and p-type electrodes. After component isolation,
the LED wafers were mounted by ordinary die bonding on a TO-18 STEM
stage and subjected to measurements. The evaluation of dislocation
density was carried out by means of cathodoluminescence (CL). LED chips
were characterized by measuring the current value dependence of the
output. Samples, in which ordinary substrates without ridge-and-groove
processing were used, were fabricated and compared.

### Results and discussion

CL observation results obtained for a sample that had undergone the
ridge-and-groove processing are shown in FIG. 10.5.2-(1).

Because the emission spectrum intensity of the emitting layer during the
CL measurements in the present study was extremely weak and thus made CL
observations very difficult, to conduct measurements, the wavelength was
set to a wavelength of 366 nm, at which the emission of the n-GaN
contact layer takes place.

![](media/image178.png){width="4.853472222222222in"
height="4.085416666666666in"}FIG. 10.5.2-(1) CL (366 nm) image of UV
LED: comparison of processed and unprocessed portions.

\[Keys, FIG. 10.5.2-(1)\]

(Top) Unprocessed and processed portions (2000x)

(Bottom, left) Unprocessed portion (8000x)

(Bottom, right) Processed portion (8000x)

Dark spots corresponding to dislocations in the amount of approximately
1 × 10^9^ spots/cm^2^ were observed in the unprocessed portion. Above
the ridges, the number of spots was approximately the same. Above the
center of the grooves it was 3 × 10^8^ spots/cm^2^ and on the order of
10^7^ spots/cm^2^ in other portions of the groove.

The spots observed in the central portion of the grooves were believed
to originate from newly generated dislocations produced by coalescence.
The distribution shown in the CL image was in good agreement with the
behavior of dislocations shown in the cross-sectional TEM of FIG. 10.4.4
and was identical to the distribution obtained under ELO \[8\].

The LED chips were mounted on the TO-18 STEM stage and emission output
measurements were carried out using an integrating sphere. The LED
output and dislocation density (CL dark spots) results were summarized
in Table 10.5.2 for LEDs fabricated on ordinary substrates and for LEDs
fabricated by the LEPS technique.

  ----------------------- ------------------------ -----------------------
                          Ordinary growth          LEPS growth

  Dislocation density     1 × 10^9^                6 × 10^8^
  (cm^−2^)                                         

  Output (mW)             0.9                      2.5
  ----------------------- ------------------------ -----------------------

Table 10.5.2 Comparison of UV LED Characteristics (wavelength: 380 nm)
(\*Dislocation density of samples fabricated by LEPS is an average
value.)

The emission output was almost tripled by applying the LEPS technique.
Such a considerable improvement in the emission output in comparison
with that reported for ELO-grown UV LEDs \[9\] strengthens the case for
the superiority of the LEPS technique.

FIG. 10.5.2-(2) shows the current value dependences of the external
quantum efficiency and output of LEDs grown on ordinary substrates and
LEDs grown on LEPS substrates.

FIG. 10.5.2-(2) Current value dependence of efficiency and output.

\[Keys, FIG. 10.5.2-(2)\]

![](media/image179.png){width="3.4027777777777777in"
height="2.24375in"}(X-axis) Current (mA).

(Y-axis, left) External quantum efficiency (%).

(Y-axis, right) Chip output (mW).

 Chip output: regular growth

 Chip output: LEPS growth

 External quantum efficiency: ordinary growth

 External quantum efficiency: LEPS growth

It was found that in both LEDs the current at which the external quantum
efficiency peaked was 50 mA, a value considerably higher in comparison
with values reported for blue and green LEDs \[10\]. This is believed to
be due to the trapping of injected carriers in non-radiative centers in
the micro-current region, and it is believed that in UV diodes, as
compared with blue diodes, the influence of the non-radiative centers,
such as dislocations, etc., is stronger. It is believed that this
attests to the need to reduce the density of dislocations in UV LEDs.

Considering the differences between the substrates, we can see that
LEPS-grown LEDs exhibit an improved buildup in the low current region.
This is believed to be due to a decrease in the number of non-radiative
recombination centers as a result of the dislocation density reduction.

The external quantum efficiency of LEPS-grown UV LEDs at 20 mA was 3.7%.
It can be expected to increase to up to 8.9% if we add the approximately
2.4-fold improvement in light extraction efficiency afforded by the use
of flip chip mounting, as described in 10.3.4. Furthermore, it is
reasonable to expect lamps with efficiencies exceeding 10% to be
fabricated if we also consider the light extraction efficiency
improvement effects produced by resin sealing \[1\].

In the present study, the dislocation density was still just 6 ×
10^8^/cm^2^. It is believe that in the future we can expect the output
to be improved and external quantum efficiencies to exceed 20% as a
result of achieving further reductions in the dislocation density by
optimizing the growth conditions and improving the LEPS technique.

## Building of white LED prototypes utilizing blue LEDs

In addition to using a combination of RGB fluorescent materials and UV
LEDs, which has been suggested in the course of the project, white LEDs
can be fabricated using blue LEDs + yellow fluorescent materials. In
this study, for the purpose of comparison with white LEDs fabricated
using UV LEDs, we fabricated blue LED prototypes and white LEDs
utilizing the blue LEDs.

### Experimental method

With the exception of increasing the In composition of the emitting
layer in order to increase the wavelength, LED wafers were fabricated by
the same method as in Section 10.3. The epi-wafers were used by Stanley
Electric Co., Ltd. to make chips, made into lamps, and combined with
yellow fluorescent materials to fabricate white LEDs.

### Results and discussion

The current value dependence of the external quantum efficiency of
resin-sealed blue LED lamps is shown in FIG. 10.6.1, and the
characteristics of the lamps are shown in Table 10.6.1.

![](media/image180.png){width="4.524305555555555in"
height="2.7319444444444443in"}

FIG. 10.6.1 Current dependence of external quantum efficiency and output
of blue LED lamps

\[Keys, FIG. 10.6.1\]

(X-axis) Current value (mA)

(Y-axis, left) External quantum efficiency.

(Y-axis, right) Chip output (mW).

  ----------------- ------------------ ------------------ -----------------
  Emission          FWHM               Lamp Output        External Quantum
  wavelength                                              Efficiency

  468.6 nm          22.9 nm            9.11 mW            17.1%
  ----------------- ------------------ ------------------ -----------------

Table 10.6.1 Characteristics of blue LED lamps (at 20 mA)

The value of the electric current at which the external quantum
efficiency reached a maximum was 10 mA, which was a lower current value
in comparison with the value (50 mA) for UV LEDs. This is believed to be
due to the fact that, in comparison with UV LEDs, the influence of
dislocations and other non-radiative centers in blue LEDs is unlikely to
be stronger. As concerns the LED characteristics obtained at 20 mA, a
lamp output of 9.1 mW and a high luminous efficacy with an external
quantum efficiency of 17.1% were obtained. The efficacy of the white LED
lamps fabricated using the chips was 12 lm/W.

## Summary

As a result of studies conducted to determine the correct amount of Si
to be used for the doping of the barrier layers in UV LEDs, we obtained
an output of 1.2 mW (at 20 mA) at a Si doping level of approximately 2 ×
10^17^ cm^-3^. It is believed that there is a coexistence of an effect
in which Mg diffusion is suppressed by Si doping on the emitting layer,
and an effect whereby Si doping decreases the crystal quality. Using the
chips to fabricate flip chip-type LEDs resulted in an approximately
2.4-fold improvement in the output. Furthermore, as a result of building
a prototype of a white LED lamp utilizing RGB fluorescent materials, we
obtained a luminous efficacy of 1.7 lm/W.

To achieve a reduction in the dislocation density of GaN by mask-less
one-step growth, we used a technique (LEPS technique), in which the
substrate was processed to impart it with a groove-and-ridge topography
and create preferential conditions for lateral growth from the ridges.
As a result, we were able to significantly reduce the dislocation
density in the lateral growth portions. In addition, as a result of
applying the LEPS technique to UV LEDs, we obtained an output of 2.5 mW,
which was approximately 3 times higher in comparison with growth on
ordinary substrates. This demonstrated that the LEPS technique was an
efficient method of dislocation density reduction and emission output
improvement.

For the purpose of comparison with UV LEDs, we fabricated a blue LED
lamp prototype and a white LED lamp, with the blue LED lamp displaying
an output of 9.1 mW and an external quantum efficiency of 17.1%. In
addition, the efficacy of the white LED was 12 lm/W.

## Future Plans

In this study, we were able to substantiate the ability of the LEPS
technique to provide dislocation density reduction and improvements in
terms of the UV emission output. However, even in the low dislocation
region the dislocation density, as observed by CL, is still on the order
of 10^7^/cm^2^, or 6 × 10^8^/cm^2^ on the average. In the future, we are
planning to go ahead with investigations into the LEPS technique so as
to achieve further reductions in the density of dislocations. The use of
the LEPS technique has allowed us to improve the external quantum
efficiency of UV LEDs at 20 mA to 3.7%. We believe that in the future it
will be possible to realize UV LED lamps with an external quantum
efficiency exceeding 20% by further optimizing the growth conditions and
component design and by improving the light extraction efficiency by
means of flip chip mounting and resin sealing. In the future, we are
planning to develop our cooperation with the Optical Source Device Group
and continue further development of high-output UV LED lamps and
white-light lamps using RGB fluorescent materials.

Furthermore, we are planning to conduct investigations into the
application to UV LEDs of new substrates, such as the low-dislocation
GaN substrates fabricated by the enterprises of the Substrate Group,
which will be replacing the currently used sapphire substrates. We are
planning to achieve lower dislocation densities than those achievable in
current LEPS-GaN and realize UV LEDs of higher outputs. In addition, we
are planning to examine the relationship between the output improvements
due to dislocation density reduction and the wavelength and realize
light-emitting components with the highest possible output in the UV
region.

## References

\[1\] 1999 Achievement Report for Light for the 21^st^ Century Project,
12.4, p. 217.

\[2\] 1999 Achievement Report for Light for the 21^st^ Century Project,
12.4, p. 222.

\[3\] A. Usui, H. Sunakawa, A. Sakai, and A. Yamaguchi: Jpn. J. Appl.
Phys. 36 (1997) L899

\[4\] K. Linthicum, T. Gehrke, D. Thomson, E. Carlson, P. Rajagopal, T.
Smith, D. Batchelor, and R. Davis: Appl. Phys. Lett. 75 (1999) 196.

\[5\] I. Kidoguchi, A. Ishibashi, G. Sugahara, A. Tsujimura, and Y. Ban:
Jpn. J. Appl. Phys. 39 (2000) L453.

\[6\] Mizutani et al.: Preprints of Applied Physics Society, Spring of
2000, 28p-YQ-5.

\[7\] For example, A. Sakai, H. Sunakawa, and A. Usui: Appl. Phys. Lett.
71 (1997) p. 2259.

\[8\] For example, S. Nagahama, N. Iwasa, M. Senoh, T. Matsushita, Y.
Sugimoto, H. Kiyoku, T. Kozaki, M. Sano, H. Matsumura, H. Umemoto, K.
Chodo, and T. Mukai: Jpn. J. Appl. Phys. 39 (2000) L647.

\[9\] T. Mukai, and S. Nakamura: Jpn. J. Appl. Phys. 38 (1999) p. 5735.

\[10\] T. Mukai, M. Yamada, and S. Nakamura: Jpn. J. Appl. Phys. 38
(1999) p. 3976.

# Development of High-efficiency Phosphors

## Achievements in 1998 and 1999

The "Light for the 21^st^ Century" Project, whose objective is the use
of light from white LEDs for illumination purposes, requires phosphors
permitting efficient conversion of UV light emitted by GaN-based LEDs
into visible light. Because various other characteristics are required
of the phosphors in addition to the conversion efficiency, we have been
studying methods of examination and evaluation of such characteristics
while at the same time striving to select suitable phosphors from among
the existing phosphors and develop new materials with a view to achieve
a quantum efficiency of ≥ 90%. The results obtained in 1998 and 1999 are
described below.

### Review of phosphors and selection of model phosphors

Based on data accumulated from literature, patent searches, and
measurement of available samples, to obtain high color rendering
3-wavelength-type fluorescent lamps, we decided to proceed based on the
principle of producing white light by means of mixing phosphors emitting
in the blue, green, and red regions of the long wavelength UV. We
selected 5 different kinds of model phosphors in order to further
investigate their luminance and lifetime characteristics under various
packaging conditions.

### Improvements in characteristics of red phosphors

Among the blue, green, and red model phosphors, the luminance of the red
materials is the lowest, which decreases the overall luminance obtained
when colors are mixed to produce white light. Hence, we started our
research by narrowing down the development objective to red phosphors.
The model red phosphor we used was europium-activated yttrium oxysulfide
(Y~2~O~2~S:Eu), which is utilized as a red phosphor in TVs. To improve
its practical characteristics, numerous studies ranging from
manufacturing methods to the emission theory had been conducted on the
material, but it had never been the subject of research related to UV
excitation. Based on information on electron-beam excitation, we carried
out synthesis experiments by adding various chemical elements and
substituting them for portions of the host composition. As a result, a
tendency towards increased intensity in the 360\~380 nm region of the
excitation spectrum was observed with several elements, such as Ga, Lu,
Sc, Mn, Mg, etc., but these effects were insufficient.

### Introduction of equipment for phosphor firing

The basic characteristics of phosphors are dependent primarily on a
combination of raw material selection, firing temperature, firing time,
and firing atmosphere. To be able to investigate the widest possible
range of synthesis conditions for the red phosphors and other phosphors,
we conducted in-depth investigations into the necessary raw materials,
furnace construction, and specifications and then designed and installed
an experimental furnace.

### Precise measurements and evaluation of phosphor emission characteristics

To obtain improved quantum efficiency, we examined available literature,
and, at the same time, gained practical experience in system design by
trial and error, which finally allowed us to introduce precise
measurement and evaluation equipment in 1999. Using NBS standard
phosphors as test samples, we conducted material makeup verification,
and the results were confirmed to be relatively satisfactory.

## Summary of Research and Development in 2000

The light sources produced in the course of the "Light for the 21^st^
Century" Project are evaluated by the total efficiency of the LEDs,
which serve as the excitation light sources, the phosphors, and the
lighting fixtures, whereas the efficiency of the phosphors themselves is
evaluated by the quantum efficiency. In the project, a target value was
set for the quantum efficiency of the phosphors. The quantum efficiency
of phosphors, unlike that of LEDs, is seldom subjected to measurements.
The measurements are limited to optical techniques and precise
measurements require considerable time and labor, and, in addition to
that, the methods of measurements cannot be considered fully established
and there is considerable disagreement between previously reported
values. However, the value of the quantum efficiency is very significant
in that it makes it possible to confirm the current level and see
possibilities via comparison with target values (theoretical values) of
phosphor efficiency. In addition, because phosphors are usually white
and have a considerably reduced capacity for absorption of excitation
light in the long wavelength UV \~ blue region, it is believed that an
important task will be to calculate the efficiency of emission by
precisely separating and measuring absorption and emission. With account
taken of the above, in 1999 we introduced a system for the precise
measurement and evaluation of phosphor emission characteristics and
completed preliminary evaluation of its functional properties.

This year, our research objectives have included a search for
high-efficiency phosphors by re-evaluating conventional phosphors from a
quantum efficiency standpoint and applying the measurement data to the
development of new high-efficiency phosphors. In the present report, we
focus first of all on the method of quantum efficiency measurement, on
simple and straightforward verification of measurement values, as well
as on discovering problems and application to actual measurements, and
then discuss the relationship of the current phosphor measurement
results with respect to the target being sought, endorse a novel
candidate for a second model green phosphor, and discuss the results of
experiments aimed at improving the efficiency of red phosphors.

## Quantum Efficiency Measurements

### Methods and means

#### Definition of quantum efficiency

Quantum efficiency is the proportion, in which the excitation light is
converted into emission of light, and is represented by the following
formula:

\[Internal\] Quantum efficiency = number of photons converted into
emission / number of photons absorbed by the phosphor

The formula does not take into consideration the energy loss due to the
Stokes shift. If we exclude from consideration certain rare cases of
special emission processes, such as double photon emission, avalanche
emission, etc., the ideal quantum efficiency will be 1. There have been
reports \[1, 2, 3\] describing phosphor measurement results where the
actual efficiency values were close to 1.

The proportion of photons of the excitation light absorbed by the
phosphor is an important characteristic in quantum efficiency
measurements. Albeit related to the intrinsic physical properties of the
phosphor, the value varies greatly depending on the way the phosphor is
used, such as positioning with respect to the excitation light source,
film thickness in case of using it in the form of a coating film, etc.
The coefficient of absorption is defined by the following formula:

Coefficient of absorption = (number of photons used for irradiation −
number of reflected photons) / number of photons used for irradiation

Another definition of quantum efficiency is used for external quantum
efficiency, which is expressed by the following formula:

External quantum efficiency = number of photons converted into emission
/ number of photons used to irradiate the phosphor

In case of absolute evaluation of the external quantum efficiency as a
phosphor characteristic, a problem arises in connection with the
coefficient of absorption. In the present report, it is assumed that the
term "quantum efficiency" stands for "internal quantum efficiency," and,
in order to avoid confusion, the term "emission efficiency" is used in
the sense of "external quantum efficiency." Because improvements in
quantum efficiency depend on the phosphors used and improvements in the
efficiency of emission have to be studied by fine-tuning the
characteristics of the phosphors and packaging conditions, in the future
we will proceed by paying particular attention to three characteristics.

#### Summary of instrumentation equipment

Light of a specified wavelength was extracted from a xenon lamp using a
spectrometer and guided to a 4-inch integrating sphere via a condenser
lens and a mirror. Phosphor powder was charged in a cell and placed on
the bottom of the integrating sphere. All of the luminous energy of the
phosphor emitted under the action of the excitation light was guided to
an instantaneous spectrophotometer MCPD-2000 via an optical fiber. The
number of photons absorbed by the phosphor was calculated by placing a
white plate of known reflectance in the same position as the phosphor
sample, performing identical measurements, and then using the data
regarding the difference between the two spectra in the excitation light
wavelength region; in addition, the number of emitted photons was
calculated from the emission spectrum in the visible region (400\~780
nm). A formula we used for the calculations, a description of the
instrumentation equipment, and a flow-chart are provided below.

$\text{Quantum}\ \text{efficiency}_{\eta}\  = \ \frac{\int_{}^{}{\lambda\  \cdot \ P\ (\lambda\ )d\lambda}}{\int_{}^{}{\lambda \cdot \ \lbrack E(}\lambda)\  - \ R(\lambda)\rbrack d\lambda}$,
where

E(λ): Excitation spectrum used to irradiate phosphor

R(λ): Reflection spectrum of excitation light

P(λ): Emission spectrum

![](media/image181.png){width="3.65625in" height="2.46875in"}FIG. 11.3.1
Measurement flowchart. \[Keys, FIG. 11.3.1\] (Spatial arrangement of
keys approximately as in original; "\*" a key in a dotted-line box,
"\*\*" a key in a solid-line box)

Printer\* CRT\* Scan controller\*\* Motor unit\*\* Xenon lamp power
supply\*

Computer\* GPIB Excitation Monochromator\* Xenon lamp housing\*

Fluorescence

MCPD-2000\* Integrating sphere

GPIB Sample

Slave computer\*\*

When excitation light was introduced into the MCPD-2000 via the optical
fiber, decay occurred on the short wavelength side at 250 nm or less. In
addition, on the long-wavelength side, the coefficient of absorption of
the phosphor tended to zero. In either case, in calculation, error was
increased because the denominator was close to zero. Although there is a
limit in terms of precision as far as measurements over a wide range are
concerned, in the wavelength region constituting the subject of the
present study this problem did not occur. Up till now there have been
very few examples of data publications concerning quantum efficiency
measurements, and, in particular, there have been no examples of data
obtained by means of long wavelength UV excitation. Thus, in the present
study, we had to establish the reliability of the data on our own.
Although during the previous year we had carried out measurements of
reference samples using short-wavelength excitation and reported results
almost identical to those obtained previously, we still cannot say that
the confirmation of precision for each wavelength and the verification
of the method were satisfactory. However, the pursuit of measurement
accuracy involves time-consuming investigations into a lot of problems
which by themselves can constitute separate research topics. Because the
original goal was to develop phosphors and measurements were just an
instrument in the process, we proceeded with our research without making
changes to the currently used measurement methods and values while at
the same time intending to perfect the measurement system by the time
the project was completed by accumulating, verifying, and correcting
errors in the data obtained in the future.

### Results and discussion

#### Measurement results

BAM (BaMgAl~10~O~17~:Eu) phosphors are the most likely candidates to be
used as blue phosphors in the future and have found use in lamps and
plasma displays. Up until now, excitation spectrum results obtained by
conducting measurements with a fluorescence spectrophotometer have
confirmed a decrease in the emission efficiency on the long wavelength
side, as shown in FIG. 11.3.2-(1), but it has been unclear whether that
was due to a decrease in the absorption efficiency or to a decrease in
the conversion efficiency. The results of emission characteristic
measurements obtained in the present study are shown in FIG. 11.3.2-(2)
on the right. The coefficient of absorption exhibited a slight increase
between 250 nm and 300 nm, after which it abruptly decreased on the long
wavelength side. The quantum efficiency decreased up to 300 nm or so,
and then there were almost no changes at longer wavelengths. As a result
of this, the emission efficiency decreased starting from 250 nm and
dropped by half at 370 nm.

![](media/image182.png){width="5.986805555555556in"
height="2.6770833333333335in"}

Left: FIG. 11.3.2-(1) BAM excitation spectrum

Right: FIG. 11.3.2-(2) BAM emission characteristics

\[Keys, FIG. 11.3.2-(2)\]

(X-axis) Excitation wavelength (nm)

(Box, top to bottom) Coefficient of absorption

Quantum efficiency

Emission efficiency

Data concerning the quantum efficiency of BaMgAl~10~O~17~:Eu under short
wavelength UV excitation reported up till now are shown in Table
11.3.2-(1).

  --- ------------ ------------------------ ------------------ ------------
      Measured by  Excitation wavelength    Quantum efficiency References
                   (nm)                                        

  1   A            254                      0.84               \(1\)

  2   B            254                      1.03               \(2\)

  3   A            147                      0.67               \(1\)

  4   Present      254                      0.98               
      study                                                    

  5   Present      370                      0.68               
      study                                                    
  --- ------------ ------------------------ ------------------ ------------

Table 11.3.2-(1) Reported values of quantum efficiency

The differences between the measurement values in the table above are
believed to be due to differences in the samples.

The samples in the present study were randomly selected. At 254 nm the
value came close to 1, and dropped below 0.7 at 370 nm. This reduction
was much greater than initially expected, and in the future it will be
necessary to analyze its mechanism and consider ways to improve the
efficiency.

#### Measurement results for blue and green phosphors

Reported here are characteristic results selected from the results we
obtained for red, green, and blue emission of model phosphors. Below,
the phosphors discussed in the present report are represented by
abbreviations listed in the following table.

  --------------- ------------------------------------- ----------------- ----------------
  Emission color  Composition                           Name              Notes
                                                        (abbreviation)    

  Blue            BaMgAl~10~O~17~:Eu                    BAM               Primary model

                  (Sr,Ca,Ba,Mg)~10~(PO~4~)~6~Cl~2~:Eu   SCA               Primary model

                  Ca~2~B~5~O~9~Cl:Eu                    CCB               

  Green           ZnS:Cu,Al                             P22-GN            Primary model

                  BaMgAl~10~O~17~:Eu,Mn                 LP-G3             

  Orange          (Y,Gd)~3~Al~5~O~12:~Ce                P46Y              Primary model

  Red             Y~2~O~2~S:Eu                          P22-RE3           Primary model
  --------------- ------------------------------------- ----------------- ----------------

Table 11.3.2-(2) Composition and names of phosphors

CCB phosphor was developed for use in lamps, in the same manner as the
other two kinds of blue model phosphors. The results obtained by
measuring their respective quantum, absorption, and emission
characteristics are as shown in FIGs. 11.3.2-(4), (6), (8). While there
was absolutely no difference in terms of quantum efficiency, the
absorption coefficient of CCB was approximately 10% higher, and its
emission efficiency was also better.

As far green phosphors were concerned, P22-GN, which had been subjected
to the same measurements, did not exhibit any degradation problems when
used in a LED; however, generally speaking, atmosphere exposure test
results had shown that it could in no way be considered stable. LP-G3
was another candidate used as insurance in case P22-GN exhibited
degradation problems. LP-G3 has a BAM composition and exhibits a
narrower emission spectrum than P22-GN with two peaks, as shown in FIG.
11.3.2-(3). Since the start of the project, its luminance at 254 nm has
been improved.

![](media/image183.png){width="3.9270833333333335in" height="2.28125in"}

FIG. 11.3.2-(3) Green phosphor spectra

\[Keys, FIG. 11.3.2-(3)\]

(X-axis) Wavelength (nm).

(Y-axis) Illegible

(Across top) Emission spectra

The results of the measurements are shown in FIGs. 11.3.2-(5), -(7), and
-(9).

When excitation was carried out at 370 nm, the quantum efficiency of
LP-G3 was higher than that of P22-GN and exhibited almost the same
characteristics as in case of blue BAM. However, because the coefficient
of absorption of P22-GN was higher, approximately the same
characteristics were obtained in terms of the efficiency of emission.
LP-G3 was recommended as a second model green phosphor.

![](media/image184.png){width="5.981944444444444in"
height="6.102777777777778in"}

FIG. 11.3.2-(4) Blue phosphor absorption spectrum

FIG. 11.3.2-(5) Green phosphor absorption spectrum

FIG. 11.3.2-(6) Blue phosphor quantum efficiency

FIG. 11.3.2-(7) Green phosphor quantum efficiency

FIG. 11.3.2-(8) Blue phosphor emission efficiency

FIG. 11.3.2-(9) Green phosphor emission efficiency

\[Keys, FIGs. 11.3.2-(4), (5)\]: (X-axis) Excitation wavelength (nm);
(Y-axis) Absorption coefficient

\[Keys, FIGs. 11.3.2-(6), (7), (8), (9)\]: (X-axis) Excitation
wavelength (nm); (Y-axis) Efficiency.

#### Simulation of white-color-producing color mixing

We estimated the brightness obtained in synthesizing white light by
combining the red, green, and blue phosphors used in the present study.
The color temperature of white light was 6700K,
CIE(x,y)=(0.3089,0.3252). We used computation software we developed
ourselves.

  ----------- --------- ------ -------- ---------- ---------- ----------------
  Phosphors                    Mixing                         
                               Ratios                         

  Green       Red       Blue   Green    Red        Blue       Relative
                                                              luminance

  P22-GN      P22-RE3   BAM    22.8%    55.8%      21.3%      100.0%

  LP-G3       P22-RE3   BAM    18.5%    65.0%      16.5%      88.7%

  P22-GN      P22-RE3   CCB    32.7%    53.7%      13.6%      107.8%

  P22-GN      P22-RE3   SCA    25.7%    54.4%      19.9%      105.2%

  LP-G3       P22-RE3   CCB    24.1%    66.4%      9.6%       90.7%

  LP-G3       P22-RE3   SCA    20.2%    64.8%      15.0%      91.4%
  ----------- --------- ------ -------- ---------- ---------- ----------------

Table 11.3.2-(3) Mixing ratios and luminance estimates of white
phosphors

As can be seen from the simulation results, in comparison with BAM,
admixture of CCB and SCA produces a higher luminance. The CCB mixing
ratio becomes smaller. In case of the green color, the mixing ratio of
P22-GN is higher than that of LP-G3, and the white color luminance is
higher. When a combination of GN-CCB was used, the brightness results
obtained were approximately 8% better than in case of using the model
phosphor. In the future we will examine the corresponding color
rendering properties.

#### Study of red phosphors

We are currently evaluating and analyzing the relationship between the
absorption efficiency and quantum efficiency under various phosphor
composition conditions. Because we have obtained samples that are close
to 150\~200% in terms of emission efficiency, we will continue our
investigations.

## Evaluation of Temperature Characteristics

### Methods and means

Considering the possible forms in which phosphors will be incorporated
in LEDs and used for illumination in the future, it becomes apparent
that it will be very important to be able to maintain excellent emission
within a wide range of temperature conditions, taking into consideration
generation of heat in case of densely packed LED assemblies, outdoor
illumination in cold regions, etc. We are building a phosphor
temperature characteristic evaluation system for investigating changes
in the phosphor emission characteristics by controlling the ambient
temperature and will use it in selecting phosphors and improving
temperature characteristics.

### Results and discussion

#### Equipment specifications

The specifications of the installed system include the following.

-   Temperature control range (−20\~250ºC)

-   Excitation light sources (black lights, sterilizing lamps, xenon
    spectroscopic lights)

-   Light source monitoring function

-   Computation software

-   Measurement parameters (luminance, emission spectrum, temperature)

#### Measurements

Operation confirmation and final acceptance will be completed.

In 2001, we will measure the temperature characteristics of the
phosphors and create a database.

## Summary

We have decided on the method of phosphor quantum efficiency
measurements. The measurements involve a process of separate evaluation
of three characteristics: the coefficient of absorption, the quantum
efficiency, and the emission efficiency, and, as a result of
establishing feedback between the obtained information and experiments
aimed at improving the composition of the red phosphors, we have managed
to improve their emission efficiency by 150\~200%. In addition, some
green and blue phosphors exhibiting a higher quantum efficiency than
that of the primary models have also been discovered. However, it has
been found that even in the best samples, the quantum efficiency of the
current phosphors, including not only red phosphors, but also blue and
green ones, as shown in the table below, is at the level of about 60%,
which is still far from the target 90%.

+----------------------+-----------------------+-----------------------+
| Emission color       | Composition           | Quantum Efficiency    |
+----------------------+-----------------------+-----------------------+
| Blue                 | BaMgAl~10~O~17~:Eu    | 62%                   |
|                      |                       |                       |
|                      | (Sr,Ca,Ba,Mg)~        |                       |
|                      | 10~(PO~4~)~6~Cl~2~:Eu |                       |
+----------------------+-----------------------+-----------------------+
| Green                | BaMgAl~10~O~17~:Eu,Mn | 64%                   |
+----------------------+-----------------------+-----------------------+
| Red                  | Y~2~O~2~S:Eu          | 60%                   |
+----------------------+-----------------------+-----------------------+
| Mixed white          |                       | \>60%                 |
+----------------------+-----------------------+-----------------------+

Table 11.5 Current quantum efficiency

## Future Plans

### Quantum efficiency improvement

We will continue our investigations by repeating the cycle depicted in
the following figure. Specifically, we will focus on search for
sensitizers capable of energy transfer to the Eu^3+^ emission centers of
the red phosphor Ln~2~O~2~S:Eu to improve its quantum and emission
efficiency. The results will be carried over to other green and blue
phosphors.

![](media/image185.png){width="4.115972222222222in"
height="2.1395833333333334in"}FIG. 11.6.-(1) Methods of quantum
efficiency improvement

\[Keys, FIG. 11.6.-(1)\]

(Left column, top to bottom) Phosphor development → Collecting data on
existing phosphors, including: quantum efficiency of various phosphors,
coefficients of absorption, excitation wavelength & intensity dependence
→ Analysis, including: classification by matrix, activating agent, type
of emission → Improvement of existing and design of new materials

(Right column, top to bottom) Establishing methods of quantum efficiency
measurement for powders, including: standardization, increasing
reliability → Study of quantum efficiency measurement under actual
packaging conditions, including: evaluation of coating film structure.

### Methods of synthesis

We will focus on the study of firing in various kinds of atmosphere and
expanding our research by selecting conditions different from those used
before. Because we are still a long way from the target quantum
efficiency value, we need new approaches and will explore possibilities
offered by the use of thin films and ultra-fine microparticles, which
have been said to improve the quantum efficiency.

### Other

In anticipation of problems associated with practical applications, we
will consider the following issues.

-   Setting up a system for analysis of temperature characteristic
    measurements; instrumentation, evaluation.

-   Design and introduction of degradation characteristic acceleration
    test equipment

-   Mixed white color simulation

-   Elucidation of the emission mechanism, joint research in cooperation
    with Yamaguchi University

## References

\[1\] Okubo, et al.: Journal of the Illuminating Engineering Institute
of Japan, 83-2, pp. 87\~93 (1999).

\[2\] Narita, Kazuo: Journal of the Illuminating Engineering Institute
of Japan, 69-2, pp. 15\~19 (1985).

\[3\] Bril, A.: Luminescence of Organic and Inorganic Materials. Pp.
479\~493 (1962).

# Electrode Attachment, Packaging, and White LED Development

## Achievements in 1998\~1999

### Achievements in 1998

#### Development of electrode forming techniques

In order to decrease resistance losses due to consumption by the
electrodes, it is necessary to decrease the contact losses between the
electrodes and GaN. First of all, we investigated methods of forming
transparent electrodes and obtained a contact resistance of 3×10^−3^
Ωcm^2^ and a 50% electrode transmittance by subjecting Ni 1 nm/Au 10 nm
to heat treatment in an oxygen atmosphere. In addition, in the Mg/Au
system, we obtained a contact resistance of 1×10^−3^ Ωcm^2^ and a 70%
electrode transmittance, but this involved problems with electrode
handling.

#### Improvements in light extraction efficiency

Light emitted from ordinary LEDs constitutes 30% of the light emitted
from the light-emitting layer. The remaining 70% is confined inside the
chip and ultimately turns into heat. Therefore, it is important to
increase the light extraction efficiency. In order to suppress the
internal reflection inside the chip, we formed an anti-reflective film
(AR film) on a GaN epitaxial wafer having a thin-film Au/Ni electrode
formed thereon; however, when the transmittance was estimated, it was
found that the gain was only 6% or so. In addition, we conducted a study
of high-reflectance electrodes in flip-chip type LEDs, which offer
considerable advantages in terms of light extraction efficiency
improvements.

#### Development of UV-resistant sealing technology

We studied the characteristics of currently used epoxy resins.

### Achievements in 1999

#### Development of electrode forming techniques

We determined Ni/Ag electrode forming conditions for high-reflectance
p-type electrodes used in flip-chip diodes and were able to obtain a
contact resistance of 3×10^−3^ Ωcm^2^ by means of subjecting a Ni 1
nm/Ag 200 nm structure to a 60-sec alloying treatment at 500ºC in an
electron beam vapor deposition apparatus.

As far as Ni/Rh is concerned, film forming by sputtering could not
produce the necessary ohmic properties of the contact between the
electrode and the p-GaN.

We studied pre-treatment procedures for forming electrode using p-GaN
epitaxial wafers and discovered that contact resistances constituting
1/2 of the prior-art resistances could be obtained in the Ni/Au
structure using ammonia boiling treatment instead of the conventional
treatment with an aqueous solution of HCl or an aqueous solution of HF.

#### Improvements in light extraction efficiency

We fabricated flip-chip type LEDs using UV GaN epitaxial wafers and
confirmed that the light output was doubled in comparison with
conventional transparent electrodes.

#### Development of white LEDs

In order to achieve the target light-generation efficiency, we used the
spectra of model phosphors to estimate the light intensity of UV LEDs.

## Summary of Research and Development in 2000

In the field of electrode forming technology, we studied materials
possessing a higher reflectance and a lower contact resistance than the
conventional materials used for p-type electrodes and n-type electrodes.
In the case of n-type electrodes, we were able to achieve a reflectance
of 70% and a contact resistance of 2×10^−5^ Ωcm^2^ by optimizing the
Ti/Al structure. In addition, we tried new materials, using ZnO/Ni/Ag
for n-type electrodes, and obtained results equivalent to those of
Ti/Al. As for p-type electrodes, we studied Rh, Pt-based electrode
materials and obtained electrode materials possessing contact
resistances lower than that of Ni/Ag.

In an attempt to improve the light extraction efficiency, we conducted
studies of chip shape. Light emitted from the light-emitting section
becomes bottled up in the GaN semiconductor layer, with part of it
emitted below the chip, part from the edge faces of the chip, and part
of it undergoing multiple reflection and absorbed in the semiconductor
layer. It has been suggested that the semiconductor layer etching edge
should be tilted in order to reflect internally absorbed light (light
propagating inside the GaN layer). This year, we developed a technology
for creating an inclined semiconductor layer surface using the dry
etching technique.

In developing white LED devices, we studied phosphor coating techniques
and conducted an evaluation of the basic characteristics of RGB model
phosphors, and, based on the obtained results, conducted simulation of
mixing of RGB phosphors to produce white light. In addition, we
conducted a study of the methods used for forming the phosphor layer and
control of the UV light transmitted via the phosphor layer. Furthermore,
we conducted an evaluation of the color rendering properties and
emission characteristics of a white-color LED lamp prototype. As a
result, by means of forming an optical thin-film filter on top of the
phosphor layer mounted on a UV LED chip such that the UV light was
reflected from the filter while visible light passed though it, and,
furthermore, by coating the edges of the LED chip with a phosphor, we
were able to decrease the amount of transmitted UV light from 31% to 13%
and confirmed a 30% improvement in the total flux of the white LED lamp.

In the development of UV-resistant sealing technology, our goal was to
develop epoxy resins resistant to UV degradation by means of compounding
epoxy resins. This year we investigated degradation behavior by means of
UV irradiation tests, high-temperature tests, etc. As a result, a
possibility of developing a resin with a low level of UV degradation was
discovered by using an acid anhydride-curable epoxy resin composition,
in which we used a phosphorus-based cure promoter and an epoxy monomer
obtained by introducing alicyclic groups into hydrogenated bisphenol A
glycidyl ether.

## Development of electrode forming techniques

### Methods and Means

#### N-type electrodes

[(a) Ti/Al electrodes]{.underline}

In order to study n-type electrode materials, we used an undoped GaN
epitaxial wafer (carrier density: \~1×10^17^ cm^−3^), on which a round
TLM (Transmission Line Model \[1\]) pattern was formed by means of
photolithography. Using electron beam vapor deposition, we formed a Ti
(0, 1, 2, 2.5 nm)/Al 300 nm film. An RTA (Rapid Thermal Annealing)
apparatus was used to conduct alloying at 400ºC\~700ºC for 10 to 120
sec, followed by measuring the contact resistance. Simultaneously, we
formed a thin film on top of a quartz substrate (thickness: 1 mm) to
\[*illegible − trans*.\] of the electrode.

[(b) ZnO/Ni/Ag electrodes]{.underline}

\[*illegible − trans*.\] Sample fabrication techniques are described
below. A round TLM pattern was formed on n-type GaN (carrier density:
\~5×10^18^ cm^−3^) using photolithography, with a 64-nm ZnO thin film
formed at room temperature using the DC arc-discharge ion plating
technique. The carrier density of the ZnO thin film was \~2×10^21^
cm^−3^ and the mobility was \~12.8 cmV^−1^s^−1^. After that, a thin film
of Ni 1 nm/Ag 300 nm was formed thereon, a pattern was formed by
lift-off, and measurement of the contact resistance was carried out.
Subsequently, in order to compare the Ti/Al electrode and the ZnO
electrode, we fabricated LEDs, in which n-type electrode Ti/Al and ZnO
were used to make n-type electrodes, and evaluation of the
characteristics of the n-type electrodes was carried out by measuring
their forward voltage.

#### P-type electrodes

To study p-type electrode materials, we formed electrodes on a p-type
GaN wafer with a hole carrier density of 5×10^18^ cm^−3^ and carried out
the same evaluation as in the case of n-type electrode materials.

### Results and Discussion

#### N-type electrodes

[a) Ti/Al-based electrodes]{.underline}

Results of contact resistance measurement for Ti/Al-based electrodes on
an undoped GaN epitaxial wafer are shown in Table 12.3.2-(1).

  ----------------------- ----------------------- -----------------------
  Electrodes              Alloying Conditions     Contact Resistance
                                                  \[Ωcm^2^\]

  Al alone                \-\-\-\-\-\--           \-\-\-\-\-\--

  Ti 1 nm/Al 300 nm       400\~700ºC, 10\~120 sec Schottky

  Ti 2 nm/Al 300 nm       450ºC, 1 min            2.43×10^−5^

  Ti 25 nm/Al 300 nm      500ºC, 20 sec           4.18×10^−5^
  ----------------------- ----------------------- -----------------------

Table 12.3.2-(1) Contact resistant values for various electrodes

The Al electrode exhibited poor adhesion properties with respect to the
GaN surface, and a working electrode could not be formed. In addition,
according to the results of other studies, electrodes made up of Al
alone could be formed but their contact resistance was very high, at
10^−3^\~10^−4^ Ωcm^2^ \[2\], and it is believed that Al-only electrodes
are unsuitable for application in electronic devices. Also, the alloying
of the electrode made up of Ti 1 nm/Al 300 nm was carried out at
400\~700ºC for 10\~120 sec, but no ohmic contact was obtained.

Sandwiching Ti between Al and GaN makes it possible to decrease the
contact resistance of the electrode. This is believed to be due to the
fact that Ti forms grains by absorbing oxygen from the naturally
oxidized film, which is generated on the GaN surface as a result of the
alloying treatment after electrode film forming, and oxygen-free GaN
becomes exposed and Al comes in contact with it \[3\]. By contrast, it
is believed that in case of Al alone, the effect of removal of the
naturally oxidized film from the surface is absent, which leads to poor
adhesion and an increase in the contact resistance. It is believed that
in case of Ti 1 nm an ohmic contact is impossible to obtain without
completely removing the oxygen from the surface. On the other hand, an
ohmic contact with n-type GaN was formed and a contact resistance of
2.43×10^−5^ Ωcm^2^ was obtained in the Ti/Al electrode with a Ti layer
of 2 nm or more.

Next, FIG. 12.3.2-(1) shows the reflectance of the Ti/Al electrodes. It
was noted that the reflectance of the Ti 2 nm/Al 300 nm electrode
exhibited a tendency to decrease with increasing alloying temperature.
An ohmic contact with GaN cannot be obtained if alloying is not carried
out at a temperature of 450ºC or higher after film forming. In the past,
the alloying treatment was carried out 500ºC for 20 seconds. Under the
above-mentioned conditions, the decrease in the reflectance was
approximately 10%, but conducting the alloying treatment at 450ºC for 1
minute allowed us to lower the decrease in the electrode reflectance to
6%. The decrease in the reflectance after alloying is believed to be
caused by irregular reflection of light by grains formed as a result of
Ti absorbing Al and surface oxygen during heating.

The results confirmed that the Ti 2 nm/Al 300 nm electrode for n-type
GaN simultaneously meets the requirements for a high reflectance (70%)
and a low contact resistance (2.43 × 10^−5^ Ωcm^2^) when the alloying
treatment is conducted for 1 minute at 450ºC.

![](media/image186.png){width="4.179861111111111in"
height="2.448611111111111in"}

FIG. 12.3.2-(1) Ti/Al film reflectance measured from the quartz side

\[Keys, FIG. 12.3.2-(1)\]

(X-axis) Wavelength (nm)

(Y-axis) Reflectance (%)

(Inside, top left) Ti 2 nm/Al 300 nm prior to alloying

(Inside, top right) Ti 1 nm/Al 300 nm prior to alloying

[b) ZnO-based electrodes]{.underline}

  -----------------------------------------------------------------------
  Ag

  Ni

  ZnO

  n-type GaN
  -----------------------------------------------------------------------

FIG. 12.3.2-(2) N-type ZnO-based electrode structure

The Ti/Al electrodes need alloying treatment at a temperature of not
less than 450ºC in order to obtain ohmic contact with respect to GaN.
Therefore, the problem is that the alloying heat treatment of the Ti/Al
electrodes brings about a deterioration in the characteristics of p-type
electrodes. The ZnO-based electrodes investigated in the present study
were transparent electrically conductive films with a transmittance of
80% at a wavelength of 370 nm. We invented a structure, depicted in FIG.
12.3.2-(3), in which the ZnO film serves as an ohmic contact layer for
GaN and the light that propagates from the GaN side and passes through
the ZnO film is reflected by the Ni/Ag electrode, and conducted
appropriate contact resistance and reflectance studies. With account
taken of light absorption, the thickness of Ni, which was used to
suppress the peeling of Ag, was set to 0.3 nm. Optical thin film
simulation results used to estimate the optimum thickness of the ZnO
film for reflectance measurements are shown in FIG. 12.3.2-(3).
Air/Glass 1 mm/ZnO/Ni 0.3 nm/Ag was used as the simulation model.

![](media/image187.png){width="3.329861111111111in"
height="2.1868055555555554in"}

FIG.12.3.2-(3) ZnO film thickness dependence according to results of
optical thin film simulation (Air/Glass/ZnO/Ni/Ag model) Wavelength: 370
nm

As can be seen from FIG. 12.3.2-(3), the reflectance varies depending on
the film thickness as a result of interference in the ZnO layer. Using
the simulation, we obtained the optimum ZnO film thickness, and used ZnO
64 nm/Ni 0.3 nm/Ag 300 nm as the structure of the samples, in which a
thin film was formed on glass for the evaluation of reflectance and
samples, in which a thin film was formed on GaN for the evaluation of
contact resistance. Using the simulation, we obtained the optimum ZnO
film thickness, and decided to used ZnO 64 nm/Ni 0.3 nm/Ag 300 nm as the
structure of samples used for the evaluation reflectance, in which the
thin film was formed on glass, and that of samples used for the
evaluation of contact resistance, in which the thin film was formed on
GaN.

Table 12.3.2-(2) shows the relationship between the alloying
temperature/time and contact resistance in case of using the ZnO/Ni/Ag
electrodes.

  ------------- ----- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
                      Alloying                                                                     
                      time                                                                         
                      \[sec\]                                                                      

                      5          20         40         60         80         100        120        180

  Temperature   800              9.80E-03                                                          
  \[ºC\]                                                                                           

                700              2.62E-03                                                          

                600              2.98E-04              4.77E-04                         Schottky   

                500   2.78E-05   4.25E-05   3.36E-05   5.37E-05   5.58E-05   5.00E-05   Schottky   

                400              1.01E-05              9.78E-06                         1.17E-05   

                300              0.13E-06                                                          1.59E-05

  No alloying                                                                                      
  treatment:                                                                                       
  3.50E-06                                                                                         
  ------------- ----- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------

Table 12.3.2-(2) Contact resistance and alloying temperature \[unit:
Ωcm^2^\]

The contact resistance was the lowest (3.5×10^−6^ Ωcm^2^) in samples
that had not undergone alloying, and, as the alloying temperature rose,
the contact resistance exhibited a tendency to increase.

The reflectance of the ZnO/Ni/Ag electrode is shown in FIG. 12.3.2-(4).
At a wavelength of 370 nm, the reflectance of the Ti/Al film was 67%,
while that of the ZnO/Ni/Ag film was 70%. Although the reflectance of
the ZnO-based electrodes was lower than the calculated value, it was due
to the fact that the quartz substrate, on which the ZnO film was formed,
was taken into consideration in the simulation, resulting in higher
calculated values.

![](media/image188.png){width="4.022222222222222in"
height="2.703472222222222in"}FIG. 12.2.3-(4) Reflectance of Ti/Al film
and ZnO/Ni/Ag film

\[Keys, FIG. 12.3.3-(4)\]

(X-axis) Wavelength (nm)

(Y-axis) Reflectance (%)

In addition, we also studied the I-V characteristic of GaN-based LEDs
using Ti/Al electrodes and ZnO/Ni/Ag electrodes. Based on the optical
thin film simulation (FIG. 12.3.2-(5)), we set the thickness of the ZnO
film formed on n-type GaN to 10 nm. We fabricated LEDs with the n-type
electrode structure and plotted their I-V characteristic in FIG.
12.3.2-(6). To obtain the same electric current density in the device
used at such time as in an ordinary chip, we used a 5×100 mA electric
current. The leading edge voltage of the Ti/Al-based electrode V~f~ =
4.27 V, while that of the ZnO-based electrode was V~f~ = 4.22 V, which
indicates that the ZnO-based electrode holds promise for n-type GaN.

![](media/image189.png){width="2.73125in" height="1.9875in"}

![](media/image190.png){width="2.9298611111111112in"
height="1.8847222222222222in"}Left: FIG. 12.3.2-(5) GaN/ZnO/Ni/Ag model
reflectance simulation

\[Keys, FIG. 12.3.2-(5)\]: (X-axis) ZnO film thickness (nm); (Y-axis)
Reflectance (%)

Right: FIG. 12.3.2-(6) Leading edge voltage in case of using Ti/Al based
and ZnO based electrodes

As far as electrical and optical characteristics and process flexibility
(no alloying treatment) are concerned, the ZnO based electrode is by no
means inferior to the Ti/Al based electrode, but there is the concern
that its characteristics might deteriorate over time because of using Ag
in the reflecting layer. This is a future research topic.

#### P-type electrodes

[a) Rh electrodes]{.underline}

The I-V characteristic for the Rh-GaN electrode is shown in FIG.
12.3.2-(7). An ohmic contact was confirmed without conducting heat
treatment. The contact resistance at such time was 2.48×10^−3^ Ωcm^2^,
which was two orders of magnitude better in comparison with the contact
resistance of the thick-film Ni/Au.

![](media/image191.png){width="2.6486111111111112in"
height="1.8902777777777777in"}

Left: FIG. 12.3.2-(7) I-V characteristics for Rh and Ni/Au electrodes on
p-type GaN

Right: FIG. 12.3.2-(8) Alloying temperature dependence of contact
resistance for Rh electrode on p-type GaN

\[Keys, FIG. 12.3.2-(8)\]: (X-axis) Alloying temperature (ºC); (Y-axis)
Contact resistance (Ωcm^2^)

Next, the heat treatment temperature dependence of the contact
resistance for the Rh electrode is shown in FIG. 12.3.2-(8). Alloying
was conducted in a nitrogen atmosphere for 20 second, followed by rapid
cooling.

It was confirmed that a contact resistance of 2.5×10^−3^ Ωcm^2^ could be
obtained at alloying temperatures of 100ºC or less, but at alloying
temperatures of more than 100ºC the contact resistance increased with
rising temperature. This is believed to be due to a reaction at the
GaN-Rh interface. In addition, a Schottky contact was obtained by
treatment at temperatures higher than 600ºC. For treatment at 300ºC,
which is the temperature used during chip bonding, the contact
resistance increased to 5.7×10^−3^ Ωcm^2^, but even in this case this
electrode appears more promising than Ni/Au. The reflectance of the Rh
electrode is shown in FIG. 12.3.2-(9).

![](media/image193.png){width="3.3847222222222224in"
height="2.1645833333333333in"}

FIG. 12.3.2-(9) Reflectance of Rh electrode

\[Keys, FIG. 12.3.2-(9)\]: (Inside) Prior to Rh heat treatment

At a wavelength of 370 nm, the reflectance of the electrode prior to
heat treatment was 60.3%, while after a heat treatment at 300ºC it rose
to 67%. Although this result is somewhat inferior to the result obtained
for Ni/Ag, the material is still believed to be extremely promising if
we take the contact resistance into consideration as well.

[(b) Rh/Ag electrodes]{.underline}

In order to obtain higher reflectances, we conducted studies of an Rh/Ag
electrode, in which the Rh film thickness was reduced and the film was
used as a contact layer for GaN, with Ag used to produce a high
reflectance electrode.

The thickness of the Rh/Ag electrode was Rh 1 nm/Ag 200 nm. As far as
the contact resistance was concerned, the results were almost the same
as those obtained for the Rh electrode. However, in the same manner as
with the Rh electrode, a rise in the contact resistance with increasing
temperature was also observed. In this connection, using AES (Auger
electron spectroscopy analysis), we confirmed that there were changes
taking place inside the GaN/Rh/Ag sample. We conducted a comparison of
two samples: one at a temperature (a temperature of 600ºC, at which a
Schottky contact is obtained) at which Rh is believed to have diffused
into the GaN film; and one immediately after deposition.

FIG. 12.3.2-(10) shows that, after heat treatment in case of the as-dep.
sample, the Rh profile is rather sharp, whereas after heat treatment at
600ºC, the Rh profile can be confirmed to have diffused into the GaN
film. This phenomenon is believed to bring about an increase in the
contact resistance. Thus, with Rh-based electrodes, it is necessary to
conduct device processing by applying as little heat as possible.

![](media/image194.png){width="5.991666666666666in"
height="2.172222222222222in"}

FIG. 12.3.2-(10) AES analysis of Rh/Ag electrode

[(c) Pt-based electrodes]{.underline}

In case of the Pt-based electrode, in the same manner as with the
Rh-based electrode, we evaluated the contact resistance using the
alloying temperature as a parameter.

As shown in FIG. 12.3.2-(11), when using an electrode with a Pt film
thickness of 200 nm, in the as-deposited state, the obtained contact
resistance was as low as 4.15×10^−4^ Ωcm^2^. However, in the same manner
as with the Rh-based electrode, an increase in the contact resistance
was observed when heat treatment was conducted.

Although the contact resistance obtained after processing at the
temperature necessary for chip bonding (300 ºC, 1 min) increased to
8.4×10^−4^ Ωcm^2^, this is still the lowest contact resistance for
p-type electrodes. However, with Pt electrodes, the reflectance at a
wavelength of 370 nm was just 49.5%, and we could not expect high
reflectances. Thus, in the same manner as with the Rh-based electrode,
we attempted fabrication of a Pt/Ag electrode. In this case, the contact
resistance was inferior to that obtained with Pt alone after 1-min
treatment at 300ºC, but the obtained resistance was 2×10^−3^ Ωcm^2^ and,
in terms of reflectance, the results exceeded 70%. The film structure
used at such time was Pt 1 nm/Ag 200 nm. In case of p-type electrodes,
this electrode is believed to be the most promising for obtaining
electrodes with a low contact resistance and a high reflectance.

![](media/image195.png){width="3.6152777777777776in"
height="2.285416666666667in"}

FIG. 12.3.2-(11) Annealing temperature dependence of contact resistance
for Pt-based electrode on p-type GaN

\[Keys, FIG. 12.3.2-(11)\]

(X-axis) Alloying temperature (ºC)

(Y-axis) Contact resistance (Ωcm^2^).

## Investigations into increasing light extraction efficiency (edge tilting technology)

### Methods and means

Currently, in the course of the "Light for the 21^st^ Century Project,"
we are conducting a study of white LEDs utilizing UV LED elements as
excitation light sources, and we have to take into consideration the
fact that when sealing resin is irradiated with excitation light, the
optical and mechanical characteristics of the sealing resin will
deteriorate. The light emanating from an excitation element is emitted
not only from the light-emitting surface, but also from the edges of the
element, with the emission from the edges being a major factor in the
deterioration of the sealing resin. For this reason, the structure of
the UV light-emitting element serving as the excitation light source has
to be such that the entire element is covered with metal electrodes etc.
so that excitation light does emanate from the edges of the chip. Thus,
light extraction using a reflection horn, which is employed in ordinary
LED lamps for increasing the light extraction efficiency, is impossible,
which makes it difficult to increase the luminescence efficiency of
white LEDs. For this reason, we chose to increase the luminescence
efficiency of white LEDs by means of tilting the edges of the excitation
element 45º to allow the element itself to act as a reflection horn.

![](media/image196.png){width="3.377083333333333in"
height="3.2069444444444444in"}To implement GaN edge tilting, we
considered methods based on the bevel cut technique (FIG. 12.4.1-(1)),
which is one of the dicing techniques, and methods based on the reactive
ion etching technique (Reactive Ion Etching: RIE). As for element edge
tilting using the bevel cut technique, there have been reports \[4\],
according to which the light extraction efficiency of elements was
improved using the bevel cut technique to process the edges of InGaP
devices, and we believe this procedure to be a promising technique for
GaN edge tilting. However, experimental results showed that, because of
the sapphire which is commonly used as a substrate for GaN-based
light-emitting elements, the blades used for dicing were subject to
severe wear (about 1.2 \[*μ*m/line\]) and the technique was not suitable
for forming slanted edges in elements with high controllability. Thus,
in the present study we focused our attention and investigated edge
tilting methods based on the RIE technique.

FIG. 12.4.1-(1) Bevel cut technique (From Disco DIAMOND WHEELS NBC-Z
SERIES Catalog (1999)

\[Keys, FIG. 12.4.1-(1) \]: (Top) Bevel cut.

Although various materials are employed for the RIE mask, one of the
most convenient methods is to use a photoresist mask. Incidentally,
micromachining techniques used to form complicated shapes on the surface
of Si substrates and glass using photoresist masks have attracted
attention in recent years, and the use of a photoresist mask as a RIE
mask for tilting the edge surfaces of GaN looks very promising.

Thus, in order to conduct an in-depth investigation into the use of
photoresist masks as RIE masks for tilting the edges of GaN, we studied
various photoresist mask fabrication conditions.

For the experiments, we prepared two kinds of positive photoresist of
different viscosity. Table 12.4.1-(1) shows the conditions used in the
photoresist experiments.

  ----------------------- ----------------------- -----------------------
  Photoresist             Type-A                  Type-B

  Viscosity               20 mPa                  44 mPa

  Rotational frequency    2000\~5000 rpm          2000\~5000 rpm
  used for coating                                

  Post-bake temperature   110\~140ºC, without     110\~140ºC, without
                          post-bake               post-bake

  Post-bake time          1\~15 min               1\~15 min
  ----------------------- ----------------------- -----------------------

Table 12.4.1-(1) Photoresist used and conditions of experiment

Undoped GaN epitaxial wafers (film thickness: 4 *μ*m) were used as
samples. The photoresists shown in Table 12.4.2-(1) were used for
patterning on the surface of a wafer. Using a parallel plate RIE
apparatus, we conducted a 2-min RIE treatment (etching amount: about 1
*μ*m) using an etching pressure of 7 Pa, an etching gas (Cl~2~) flow
rate of 100 sccm, an etching substrate temperature of 30ºC, and an RF
power of 200 W. Subsequently, we used a scanning electron microscope
(Scanning Electron Microscope: SEM) to examine the shape of the GaN edge
of the sample that was tilted 20º from a vertical position to obtain the
angle between the etched surface and the edge of the sample (see FIG.
12.4.1-(2)) and conducted a comparative study of the tendencies
exhibited under various photoresist conditions. In addition, we also
investigated the GaN orientation dependence of the GaN edge tilt angle.
Furthermore, in order to evaluate reproducibility tendencies, we
prepared multiple samples for several photoresist conditions and
calculated the respective standard deviations.

![](media/image197.png){width="5.129166666666666in"
height="2.1951388888888888in"}

FIG. 12.4.1-(2) Explanatory diagram showing GaN tilted edge angle (FIG.
(a)) and example of SEM microphotograph of GaN tilted edge surface (FIG.
(b)).

\[Keys, FIG. 12.4.1-(2)\]

\(a\) (Left, top) GaN tilted edge surface.; (Left, below) GaN etched
surface.; (Right, top) GaN tilted edge surface angle.; (Right,
underneath) GaN film.; (Right, underneath) Sapphire substrate

.

### Results and discussion

#### Photoresist film thickness dependence

The dependence of the photoresist film thickness for each photoresist on
the rotational frequency of the spinner is shown in FIG. 12.4.2-(1). The
rotational frequency of the spinner was varied between 2000 and 5000
rpm, with the thickness of the film obtained by the post-baking
treatment of the resultant photoresist mask measured using a
contact-type film thickness meter.

FIG. 12.4.2-(1) confirms that the thickness of the photoresist film
obtained using the photoresist of Type-B, which had a higher viscosity,
was greater than that obtained using the photoresist of Type-A. On the
other hand, in case of Type-A photoresist films obtained by varying the
post-bake temperature, there was no difference in the thickness of the
resultant films.

![](media/image198.png){width="3.5493055555555557in"
height="2.670138888888889in"}FIG. 12.4.2-(1) Photoresist film thickness
and rotational frequency of spinner

\[Keys, FIG. 12.4.2-(1)\]

(X-axis) Rotational frequency of spinner.

(Y-axis) Resist film thickness (μm)

(Box)  Type-A post-bake: 120ºC, 1 min

Type-A post-bake: 140ºC, 1 min

 Type-B post-bake: 130ºC, 5 min

Next, the photoresist film thickness dependence of the GaN edge face
angle is shown in FIG. 12.4.2-(2). In addition, the symbols used in the
figure are the same as the ones used in FIG. 12.4.2-(1).

![](media/image199.png){width="3.745138888888889in"
height="2.439583333333333in"}

FIG. 12.4.2-(2) Photoresist film thickness dependence of GaN edge face
angle

\[Keys, FIG. 12.4.2-(2)\]

(X-axis) Photoresist film thickness (μm)

(Y-axis) GaN edge face angle (º )

(Box)  Type-A post-bake: 120ºC, 1 min

Type-A post-bake: 140ºC, 1 min

 Type-B post-bake: 130ºC, 5 min

FIG. 12.4.2-(2) shows that in case of the Type-A photoresist, the GaN
edge face angle tends to become smaller as the thickness of the
photoresist film increases. In addition, although there were no
differences in the photoresist film thickness, as is evident from FIG.
12.4.2-(1), differences were observed in the edge face angle of the GaN
film depending on the post-bake temperature, with the GaN edge face
angle exhibiting a tendency to decrease with increasing post-bake
temperature.

On the other hand, in case of the Type-B photoresist, at any photoresist
film thickness, the GaN edge face angle was about 45º, which was smaller
than the values of the GaN edge face angle obtained for the Type-A
photoresist film. The fact that the resultant GaN edge face angle was
smaller than that of the Type-A photoresist is believed to be due to the
fact that, as compared with the Type-A photoresist,

1\) the Type-B photoresist uses a relatively high post-bake temperature
of 130ºC,

2\) the post-bake time was 5 min, considerably longer than that of
Type-A, etc.

#### Post-bake temperature dependence

FIG. 12.4.2-(3) shows the post-bake temperature dependence of the GaN
edge face angle. The post-bake temperature was studied in the range of
from 110ºC to 140ºC. In addition, we also studied samples that were not
subjected to the post-bake treatment. In FIG. 12.4.2-(3), the round dots
show results from a sample coated with the Type-A photoresist at 2000
rpm and subjected to a post-bake treatment for 1 minute. The square dots
show results from a sample coated with the Type-A photoresist at 3000
rpm and subjected to a post-bake treatment for 1 minute. In addition,
the triangular dots in the figure represent results from a sample coated
with the Type-B photoresist at 3000 rpm and subjected to a post-bake
treatment for 5 minutes.

As can be seen from FIG. 12.4.2-(3), at any rotational frequency of the
spinner, up to a post-bake temperature of 130ºC the Type-A photoresist
produced practically constant GaN edge face angles, which was
approximately 60º in case of samples coated at a rotational frequency of
2000 rpm and approximately 50º in case of samples coated at a rotational
frequency of 3000 rpm. This result is in agreement with the tendency of
the GaN edge face angle to decrease when the rotational frequency of the
spinner decreases, as shown FIG. 12.4.2-(2). In addition, at any
rotational frequency of the spinner, when the post-bake treatment was
carried out at 140ºC, the GaN edge face angle was approximately 46º.

On the other hand, in case of the Type-B photoresist, the GaN edge face
angle was largest in samples that were not subjected to the post-bake
treatment, and as the post-bake treatment temperature increased, the GaN
edge face angle tended to become smaller. Furthermore, a GaN edge face
angle of 45º could be obtained by conducting the post-bake treatment at
130ºC or more.

![](media/image200.png){width="3.1756944444444444in"
height="2.263888888888889in"}

FIG. 12.4.2-(3) Post-bake temperature dependence of GaN edge face angle

\[Keys, FIG. 12.4.2-(3)\]

(X-axis) Post-bake temperature (ºC)

(X-axis, between 0 and 110) No post-bake treatment

(Y-axis) GaN edge face angle (º )

(Box)  Type-A 3000 rpm post-bake 1 min;  Type-A 2000 rpm post-bake 1
min;  Type-B 3000 rpm post-bake 5 min

#### Post-bake time dependence

FIG. 12.4.2-(4) shows the post-bake time dependence of the GaN edge face
angle. The post-bake time dependence was studied within a 1\~15 min time
interval. In FIG. 12.4.2-(4), the round dots show results obtained for
samples fabricated by coating wafers with the Type-A photoresist at 3000
rpm and conducting a post-bake treatment at a constant temperature of
120ºC and the square dots show results obtained for samples fabricated
by coating wafers with the Type-A photoresist at 2000 rpm and conducting
a post-bake treatment at 120ºC. In addition, in FIG. 12.4.2-(4), the
rhombus-shaped dots show results obtained for samples fabricated by
coating wafers with the Type-A photoresist at 2000 rpm and conducting a
post-bake treatment at a constant temperature of 140ºC, and the
triangular dots show results obtained for samples fabricated by coating
wafers with the Type-B photoresist at 3000 rpm and conducting a
post-bake treatment at a temperature of 130ºC.

![](media/image201.png){width="5.9944444444444445in"
height="4.154166666666667in"}

FIG. 12.4.2-(4) Post-bake time dependence of GaN edge face angle

(X-axis) Post-bake time (min)

(Y-axis) GaN edge face angle (º )

(Box)  Type-A 3000 rpm post-bake 120ºC;  Type-A 2000 rpm post-bake
120ºC; Type-A 2000 rpm post-bake 140ºC;  Type-B 3000 rpm post-bake
140ºC

As can be seen from FIG. 12.4.2-(4), in case of the Type-A photoresist,
samples prepared using a spinner rotational frequency of 3000 rpm and a
post-bake temperature of 120ºC exhibited a maximum value of the GaN edge
face angle, with was approximately 65º, when the post-bake treatment was
conducted for 8\~9 min. On the other hand, samples prepared using a
spinner rotational frequency of 2000 rpm, including samples treated at a
post-bake temperature of 120ºC and those treated at 140ºC, exhibited
maximum values of the GaN edge face angle, which was respectively 57º
for samples subjected to post-bake treatment at 120ºC and 43º for
samples subjected to post-bake treatment at 140ºC, when the post-bake
treatment was conducted for about 7 minutes.

Also, it was discovered that in the case of the Type-B photoresist, the
maximum value of the GaN edge face angle, which was approximately
52\~53º, was obtained by conducting the post-bake treatment at 130ºC for
7 minutes. In addition, on the whole, the post-bake time dependence
exhibited fluctuations.

#### Plane orientation dependence in samples with inclined GaN edge faces

We used the Type-A photoresist for the photoresist mask. The patterning
of the Type-A photoresist was carried out at a spinner rotational
frequency of 2000 rpm, and the post-bake treatment was conducted for 1
minute at 120ºC. The RIE treatment was carried out for 2 minutes under
STD. conditions. After that, we used SEM to evaluate the GaN edge face
tilt angle in a direction perpendicular (1-100) and parallel (11-20) to
the orientation flat ((1-100): designated as O.F. below) along the
sapphire c-plane. The results of the evaluation are shown in Table
12.4.2-(1).

+----------------------+-----------------------+-----------------------+
|                      | Parallel to O.F.      | Perpendicular to O.F. |
|                      |                       |                       |
|                      | ((1-100))             | ((11-20))             |
+----------------------+-----------------------+-----------------------+
| Sample A             | 57º                   | 55º                   |
+----------------------+-----------------------+-----------------------+
| Sample B             | 55º                   | 56º                   |
+----------------------+-----------------------+-----------------------+

Table 12.4.2-(1) Plane orientation dependence of GaN edge face angle

As shown in FIG. 12.4.2-(1), there was no significant variation in the
GaN edge face tilt angle neither in the perpendicular, nor in the
parallel direction with respect to the O.F., with the angle being
approximately 55º. This experiment shows that the tilt angle of the GaN
edge faces was unrelated to the crystal plane orientation.

#### GaN edge face reproducibility

We conducted a study of the reproducibility of the GaN edge face angle.
According to the method we used for evaluation, we prepared several GaN
substrates patterned using the Type-A photoresist under three types of
conditions shown in Table 12.4.3-(3) and subjected each sample to RIE
treatment under STD conditions. After that, the photoresist mask was
completely removed using an aqueous solution of sulfuric acid/peroxide,
and the GaN edge face tilt angle was evaluated using SEM. Using the
obtained GaN edge face tilt angles, we obtained standard deviations for
the GaN edge face tilt angles under each type of conditions and
conducted a comparative study. The results of the study are shown in
Table 12.4.2-(3).

  ----------------------------------- ------------- ----------- -----------
  Sample name                         A             B           C

  Spinner rotational frequency        3000          2000        2000

  Post-bake temperature (ºC)          120           120         140

  Post-bake time (min)                1             1           1

  Number of points                    5 points      6 points    7 points

  Average value of GaN edge face      65.2          56.7        44.4
  angle (º )                                                    

  Standard deviation (º )             8.70          2.25        0.79
  ----------------------------------- ------------- ----------- -----------

Table 12.4.2-(3) GaN edge face tilt angle reproducibility

If we compare the value of standard deviation of the GaN edge face tilt
angle obtained under conditions A and the value of the standard
deviation of the GaN edge face tilt angle obtained under conditions B,
as listed in Table 12.4.2-(3), we will see that the value of standard
deviation obtained under conditions B is smaller. This indicates that
when the spinner rotational frequency is decreased, in other words, when
the thickness of the photoresist film is increased, the variation in the
GaN edge face tilt angle tends to decrease.

Furthermore, if we compared conditions B and conditions C, we can see
that the value of the standard deviation of the GaN edge face angle
obtained under conditions C is smaller. This is believed to be due to
the fact that the variation in the GaN edge face tilt angle tends to
become even smaller when the post-bake temperature is increased.

Summing up the above-described results, we can say that the value of
standard deviation under conditions B, where the thickness of the
photoresist film is greater, is smaller than under conditions A, and,
furthermore, the value of standard deviation under conditions C, where
the post-bake temperature is higher, is smaller than under conditions B.
Based on the above, it is believed that a greater thickness of the
photoresist mask film and a higher post-bake temperature are appropriate
for the fabrication of tilted GaN edge faces with high reproducibility.

## White LED development

### Study of mixed white light production and emission characteristics of RGB model phosphors

#### Methods and means

The physical properties of the RGB model phosphors used in the present
development project are shown in Table 12.5.1. A fluorescence
spectrophotometer (Hitachi F-4500) was used to conduct the measurement
of the excitation spectra and emission spectra of the phosphors. Because
in case of emission spectra the central wavelength of the UV LED
emission is not fixed, we conducted measurements within a relatively
wide range in the near-UV region. Based on the resultant emission
spectra of the phosphors, we obtained CIE-based color specification
values (Y~i~, x~i~, y~i~)(i=R,G,B) under the XYZ color specification
system as specified in JIS \[*Japanese Industrial Standards − trans*.\]
\[5\]. In addition, the measurement of the emission spectra at the same
excitation wavelength for each of the phosphors was carried out under
the same spectrometer settings. We used additive mixing \[6\] to obtain
an estimate of the central wavelength of the UV LED emission and a
theoretical Y-stimulus ratio (Y~R~′:Y~G~′:Y~B~′) in the target white
color coordinates from the resultant chromaticity coordinates
(x~i~,y~i~) (i=R,G,B) of the samples. Based on the ratio
((Y~R~′/Y~R~):(Y~G~′/Y~G~):(Y~B~′/Y~B~)) of the theoretical Y-stimulus
ratio (Y~R~′:Y~G~′:Y~B~′) and actual Y-stimulus ratios (Y~R~:Y~G~:Y~B~)
of the phosphors and the specific weights, we obtained white color
mixing ratios for the phosphors.

  ----------- --------- ----------- ---------------- ------------------------------------
  Phosphors   Name      Specific    Average particle Composition
                        weight      size (*μ*m)      

  R           P22-RE3   5.0         5.73             Y~2~O~2~S:Eu

  G           P22-GN4   4.1         6.67             ZnS:Cu,Al

  B           LP-B1     4.3         4.98             (Sr,Ca,BaMg)~10~(PO~4~)~6~Cl~2~:Eu
  ----------- --------- ----------- ---------------- ------------------------------------

Table 12.5.1 Physical properties of RGB model phosphors

#### Results and discussion

FIG. 12.5.1-(1) shows the excitation and emission spectra of the
phosphors.

![](media/image202.png){width="4.373611111111111in"
height="2.571527777777778in"}FIG. 12.5.1-(1) Excitation and emission
spectra of model phosphors

\[Keys, FIG. 12.5.1-(1)\]

(X-axis) Wavelength (nm)

(Y-axis, left) Excitation intensity (a.u.)

(Y-axis, right) Emission intensity (a.u.)

(Inside, top, left) Excitation.

(Inside, top, right) Emission.

![](media/image203.png){width="3.285416666666667in"
height="2.3513888888888888in"}The excitation spectrum intensity was
obtained by normalizing the intensity obtained at an excitation
wavelength of 370 nm to 100. FIG. 12.5.1-(2) shows phosphor mixing
ratios (wt%) vs the central wavelength of UV LED emission used to obtain
the target chromaticity coordinate (x,y)=(0.31,0.31). Using the results
shown in FIG. 12.5.1-(2) as a guide, we fabricated samples of mixed RGB
phosphors. When we obtained their chromaticity coordinates from the
emission spectra, it was confirmed that the resultant values were
practically the same as the target chromaticity coordinates. As FIG.
12.5.1-(2) shows, in the RGB model phosphors used in the present
experiment, the mixing ratio of the R phosphor was higher than the
proportion of the G and B phosphors. This is believed to be due to the
fact that the absolute luminance of the R phosphor is lower in
comparison with the G and B phosphors, and, therefore, when the
excitation wavelength is a short wavelength, even though the luminance
of the R phosphor is increased, its absolute value is small, and thus
the influence exerted on the mixing ratio is insignificant. In addition,
there is a correlation between the Y-stimulus value at a given
excitation wavelength and the excitation spectrum. Thus, it was
confirmed that the phosphor mixing calculations could be simplified by
converting a measured Y-stimulus value at a given excitation wavelength.

FIG. 12.5.1-(2) Phosphor mixing ratio in target chromaticity coordinates
(0.31, 0.31)

\[Keys, FIG. 12.5.1-(2)\]

(X-axis) Central UV LED excitation wavelength (nm)

(Y-axis) Mixing ratio (wt%)

### Study of phosphor layer forming techniques and transmitted UV light control

#### Methods and means

[a. Study of phosphor layer forming techniques]{.underline}

Our method of forming a phosphor layer on a UV LED consisted in using
the flat face of a sapphire substrate of a UV LED chip that had been
assembled using the flip chip technique \[7\]. Here, we studied a
technique, in which we used a phosphor-bearing surface to mount a
phosphor chip obtained by micropatterning a glass substrate, on which a
phosphor layer was formed in advance. An extremely thin glass plate with
a thickness of 0.1 mm, which allowed for micropatterning, was used as
the glass substrate. Spin coating was used for forming a phosphor layer
on the glass substrate. A mixed white phosphor produced by combining RGB
phosphors was used, with the mixing ratio determined in accordance with
the wavelength of the evaluation UV LED, which is described below. In
addition, we used SiO~2­~ inorganic binders to disperse the phosphors and
obtained the optimum compounding ratio from the relationship between the
compounding ratio of the phosphor with respect to the binder and the
resultant coating film adhesive strength in order to maximize the
packing density of the phosphor layer. Phosphor layer film thickness
control was carried out by regulating the spinner rotational frequency,
the phosphor compounding ratio with respect to the binder, and the
slurry density. In the process of phosphor chip micropatterning, we
applied the common LED wafer chip fabrication techniques. The size of
the phosphor chip was 500 *μ*m on a side, with that of the UV LED chip
being 350 *μ*m. The UV LED chip used for evaluation, which was mounted
on a TO-18 STEM using the flip chip assembly technique, had a central
wavelength of 372 nm and an output of 4 mW (I~f~=20mA). The architecture
of a basic white LED lamp combining a UV LED and a phosphor chip is
shown in FIG. 12.5.2-(1). Phosphor edge coating (designated as "EC
treatment" below) was introduced in order to convert the UV light
emitted from the edge faces of the UV LED chip into visible light. The
measurement of the emission characteristics of the basic white LED lamp
was carried out using a spectrometer (the CAS140B from INSTRUMENT
SYSTEMS) with an attached integrating sphere.

![](media/image204.png){width="5.9847222222222225in"
height="2.4881944444444444in"}

Figure 12.5.2-(1) Architecture of basic white LED lamp

\[Keys, 12.5.2-(1)\] (Right, top to bottom) Glass substrate; HR film;
Phosphor layer; Phosphor edge coating; Flip-chip UV LED; Submount; TO-18
STEM

[b. Study of transmitted UV light control]{.underline}

We studied a technique, in which an optical thin film filter (called the
HR film below) possessing the ability to reflect transmitted UV light
and transmit only visible light whose wavelength had undergone
conversion in the phosphor layer is formed between the glass substrate
and the phosphor layer as shown in FIG. 12.5.2-(1) in order to control
the component of the UV light emitted from the UV LED that is not
absorbed by the phosphor layer (transmitted UV light). The basic
architecture of the HR layer consists of alternating layers of two kinds
of thin film (for example, SiO~2~/TiO~2~) of different refractive
indices. We conducted simulation analysis in order to obtain the number
of layers and the film thickness providing a high reflectance in the UV
LED emission wavelength region and a high transmittance in the visible
light region. Using the obtained conditions as a guide, we formed an HR
film on a glass substrate using the vacuum deposition technique. In
addition, assuming that the UV light transmitted through the phosphor
layer would be randomly scattered in the phosphor layer, we evaluated
the UV light incident angle dependence of the UV region reflectance by
varying the cut-off wavelength (defined as the wavelength, at which
reflectance constitutes 50%) representing the boundary between the
reflected wavelength region and the transmitted wavelength region.

#### Results and discussion

[a. Study of phosphor layer forming techniques]{.underline}

FIG. 12.5.2-(2) shows results obtained by mounting phosphor chips with
varying phosphor layer film thickness on an evaluation UV LED chip and
measuring the emission characteristics of the basic white LED lamp. As
can be seen from the results, the visible light intensity of the white
LED peaks in the vicinity of a phosphor layer film thickness of 30\~40
*μ*m, and decreases after the peak with increasing film thickness. In
addition, the transmitted UV light decreases with increasing film
thickness. This is in agreement with common tendencies encountered
during transmission, and implies that the film thickness and phosphor
packing density in this layer forming method are controlled with high
accuracy. It was confirmed that if the phosphor layer on the UV LED chip
edge faces is EC-treated as shown in FIG. 12.5.2-(2), then at the peak
of the visible light intensity, the visible light intensity is increased
15% as compared with that obtained without the EC treatment, and the
amount of transmitted UV light is decreased 9%. Measurements conducted
on EC-treated samples only confirm that with this flip-chip LED
architecture, approximately 13% of the UV light is emitted from the UV
LED chip edge faces. The component emitted from the edge faces of the
LED chips is believed to be converted to visible light and contribute to
an increase in the visible light intensity.

![](media/image205.png){width="3.6798611111111112in"
height="2.354861111111111in"}FIG. 12.5.2-(2) Emission characteristics of
basic white LED lamp with a phosphor chip

\[Keys, FIG. 12.5.2-(2)\]

(X-axis) Phosphor layer film thickness (μm)

(Y-axis, left) Visible light intensity (a.u.)

(Y-axis, right) Transmitted UV light (%)

(Inside, top to bottom) Visible light intensity (EC-treated); Visible
light intensity (no EC treatment); Transmitted UV light (no EC
treatment); Transmitted UV light (EC-treated)

[b. Study of transmitted UV light control]{.underline}

FIG. 12.5.2-(3) shows the reflectance characteristics of glass
substrates with an HR film obtained by varying the cut-off wavelength
defining the UV light reflection region from 380 nm to 440 nm. Results
obtained by irradiating the surface of the HR films with UV light and
measuring the incidence angle dependence of the amount of transmitted UV
light transmitted from the glass surface are shown in FIG. 12.5.2-(4).
As can be seen from FIG. 12.5.2-(4), the incidence angle dependence of
the amount of the transmitted UV light becomes less pronounced when the
cut-off wavelength of the HR film is shifted towards longer wavelengths.
Emission spectra, on the UV LED excitation axis, of samples obtained by
superimposing HR film substrates and substrates spin-coated with a
phosphor layer are shown in FIG. 12.5.2-(5). This figure shows that when
the cut-off wavelength becomes greater than 430 nm, some of the blue
phosphor emission spectrum component is reflected. FIG. 12.5.2-(6) shows
relationships of the amount of the reflected UV light (value obtained by
integration from the incident angle dependence) and the amount of light
converted to visible light by the phosphor versus the cut-off
wavelength. The results show that the optimum cut-off wavelength region
taking into account the UV light reflectance and the amount of visible
light converted by the phosphor lies somewhere between 400 nm and 420
nm.

![](media/image206.png){width="2.754861111111111in"
height="2.1791666666666667in"}

![](media/image207.png){width="2.9451388888888888in"
height="2.098611111111111in"}

Left: FIG. 12.5.2-(3) HR film reflectance characteristics

Right: FIG. 12.5.2-(4) Incidence angle dependence of amount of
transmitted UV light

Left: FIG. 12.5.2-(5) Emission spectra, on UV LED excitation axis, of
samples obtained by superimposing HR film substrates and phosphor-coated
substrates

![](media/image208.png){width="3.0in"
height="2.0652777777777778in"}Right: FIG. 12.5.2-(6) Amount of reflected
UV light and amount of light converted to visible light by the phosphor
versus HR cut-off wavelength

\[Keys, FIG. 12.5.2-(6)\]

(X-axis) HR cut-off wavelength (nm)

(Y-axis, left) Amount of reflected UV light (×100%)

![](media/image209.png){width="2.604861111111111in"
height="2.03125in"}(Y-axis, right) Amount of light converted to visible
light by the phosphor (380 nm = 1)

### Evaluation of emission characteristics and color rendering of a white LED lamp

#### Methods and means

The UV LED chips were made by forming high reflectance electrodes on UV
LED wafers fabricated in the current project (central wavelength of
emission: 382 nm), making chips with a size of 350 *μ*m on a side,
assembling them on electrode wiring submounts using the flip-chip
technique, and then mounting the resultant part on a lead frame and
bonding the electrodes. In the fabrication of the phosphor chips mounted
on the UV LED chips we used RGB model phosphors, with the fabrication of
the chips carried out by forming a 40-*μ*m thick film of mixed phosphor
(standard white phosphor (A)), which corresponded to the central
emission wavelength of the UV LED, on a glass substrate with or without
an HR film (cut-off wavelength: 413 nm) and making chips with a size of
500 *μ*m on a side. An inorganic adhesive was used for mounting the
phosphor chips on the UV LEDs. In addition to samples with/without an HR
film, samples were also prepared, in which the edge faces of the UV LED
chip were either subjected an EC treatment or left untreated, and bullet
type prototype white LED lamps were fabricated by sealing the frame with
resin (FIG. 15.3-(1)). A spectrometer with an attached integrating
sphere was used for conducting emission characteristic measurements.

![](media/image210.png){width="3.428472222222222in"
height="3.2055555555555557in"}

FIG. 12.5.3-(1) White LED lamp architecture

\[Keys, FIG. 12.5.3-(1)\]

(Top to bottom) Sealing resin; Glass substrate; HR film; Phosphor layer;
Phosphor edge coating; Flip-chip UV LED; Submount; Lead frame

In order to investigate the influence exerted on color rendition by
chromaticity coordinates and the emission spectrum shape, in addition to
the above-mentioned standard white phosphor (A), we used a
chromaticity-adjusted white phosphor (B), which was obtained by shifting
the white color coordinates towards the blue side by changing the mixing
ratio of the phosphors, and a comparative white phosphor (C) made of
other phosphor materials with a different spectrum shape to form
phosphor films on glass substrates and conducted evaluation of their
color rendering properties based on the results of measurement of the
emission spectra obtained by UV LED excitation.

#### Results and discussion

FIG. 12.5.3-(2) shows UV-region spectra of white LED lamps with various
peripheral architectures of the UV LED chip. FIG. 12.5.3-(3) shows
visible-region spectra of white LED lamps having a phosphor layer only,
as well as white LED lamps with a phosphor layer + an HR film + EC
treatment. Table 12.5.3-(1) shows characteristics of white LED lamps
with various architectures. As can be seen from FIG. 12.5.3-(2), FIG.
12.5.3-(3), and Table 12.5.3-(1), in comparison with the architecture
having a phosphor layer only, the introduction of the HR film allows for
decreasing the UV light transmitted by the phosphor layer, and, in
addition, subjecting the edge faces of the UV LED chip to an EC
treatment permits conversion of the UV light emitted from the edge faces
of the UV LED to visible light, resulting in an increase in the total
flux of the lamp and in the phosphor conversion efficiency (visible
light flux from the phosphor/LED's UV flux). Based on these results, the
amount of UV light reflected by the HR film is about 35%. Although an HR
film-only architecture is supposed to produce a reflectance of not less
than 80%, in the LED architecture evaluated in the present experiment,
UV light is believed to be emitted not only by the phosphor layer on the
UV LED chip, which caused a reduction in the reflection effect provided
by the HR film in terms of the total amount of transmitted UV light. In
addition, it is believed that because the central wavelength of the UV
LED was not less than 380 nm, a relatively higher value than the value
assumed (370 nm), this increased the UV light incidence angle dependence
of the UV-region reflectance of the HR film. Thus, we found that in
order to make the best of the reflection effects of the HR film, we had
to investigate architectures capable of directing most of the light
emitted from a UV LED to the phosphor layer under the HR film.

![](media/image211.png){width="2.9881944444444444in"
height="2.0569444444444445in"}

Left: FIG. 12.5.3-(2) UV-region spectra of white LED lamps with various
peripheral architectures of UV LED chip

\[Keys, FIG. 12.5.3-(2)\]

(Inside, top to bottom) Phosphor layer only; Phosphor layer + HR film;
Phosphor layer + EC treatment; Phosphor layer + HR film + EC treatment

Right: FIG. 12.5.3-(3) Visible-region spectra of white LED lamps with
various peripheral architectures of UV LED chip

\[Keys, FIG. 12.5.3-(3)\]

(Inside, top to bottom) Phosphor layer + HR film + EC treatment;
Phosphor layer only

  ----------------------------- -------------- ---------------- -----------
  Peripheral architecture of UV Transmitted UV Phosphor         Total flux
  LED chip                      light (%)      conversion       (lm)
                                               efficiency (%)   

  Phosphor layer only           31.4           10.7             0.099

  Phosphor layer + EC treatment 19.5           12.6             0.120

  Phosphor layer + HR film      24.2           10.8             0.100

  Phosphor layer + HR film + EC 12.8           13.8             0.129
  treatment                                                     
  ----------------------------- -------------- ---------------- -----------

Table 12.5.3-(1) Characteristics of white LED lamps with various
peripheral architectures of UV LED chip (I~f~=20 mA)

FIG. 12.5.3-(4) shows results of measurement of UV LED-excited emission
spectra in samples fabricated by forming a film of, respectively,
standard white phosphor (A), chromaticity-adjusted white phosphor (B),
and comparison white phosphor (C) on a glass substrate. Table 12.5.3-(2)
lists the chromaticity coordinates and color rendering evaluation scores
for each of the phosphors. As can be seen from FIG. 12.5.3-(4) and Table
12.5.3-(2), the color rendering properties of the chromaticity-adjusted
white phosphor (B) obtained by increasing the blue component in the
standard white phosphor (A) were greatly improved. In addition, it was
found that the average color rendering evaluation score Ra obtained from
the spectrum of the comparative white phosphor (C) was 43, a relatively
low value as compared with other samples. As can be seen from the shape
of the spectrum of comparative white phosphor (C) shown in FIG.
12.5.3-(4), its components in the vicinity of 480 nm and 580 nm were
practically absent and its JIS test color reproducibility was lower than
that of the model phosphors, which resulted in a decrease in color
rendering properties.

These results show that white color from three-wavelength phosphors
containing many components with a broad spectral distribution, as in the
RGB model phosphors evaluated in the present project, is highly
efficient in terms of improving the color rendering properties of light
sources used for illumination.

![](media/image213.png){width="5.9875in" height="3.6381944444444443in"}

FIG. 12.5.3-(4) Emission spectra of model phosphor samples and
comparative samples

\[Keys, FIG. 12.5.3-(4)\]

(Inside, top to bottom) Standard white phosphor (A)

Chromaticity-adjusted white phosphor (B)

Comparative white phosphor (C)

  ------------------------------ ----------- -------------- --------------
  Samples                        Standard    Chromaticity   Comparative
                                 white       adjusted white white phosphor
                                 phosphor    phosphor (B)   (C)
                                 (A)                        

  Color coordinate x             0.311       0.288          0.298

  y                              0.321       0.331          0.297

  Reference light                D 6710      D 7860         D 8010

  Average color rendering        76          90             43
  evaluation score Ra                                       

  Special color rendering        69          91             26
  evaluation score R~1~                                     

  R~2~                           88          93             81

  R~3~                           85          90             61

  R~4~                           70          92             9

  R~5~                           76          94             40

  R~6~                           85          92             68

  R~7~                           82          89             57

  R~8~                           50          79             −2

  R~9~                           −29         44             −150

  R~10~                          78          88             56

  R~11~                          65          86             −13

  R~12~                          89          83             80

  R~13~                          74          89             40

  R~14~                          89          94             73

  R~15~                          61          90             18

  Type of standard used          JIS Z                      
                                 8726-1990                  
  ------------------------------ ----------- -------------- --------------

Table 12.5.3-(2) Color coordinates and color rendering evaluation scores
of model phosphor samples and comparative samples

## Development of Sealing Resins for UV LEDs

### Methods and means

#### Samples and reagents

We used bisphenol A glycidyl ether (Epicoat 828, from Japan Epoxy Resin
K.K) along with hydrogenated bisphenol A glycidyl ether (YL6663, from
Japan Epoxy Resin K.K.), and 3,4-epoxycyclohexylmethylcarboxylate
(Celoxide 2021 from Daicel Chemical Industries, Ltd.) as epoxy monomers
and methylhexahydrophthalic anhydride (MH-700 from New Japan Chemical
Co., Ltd.) as an acid anhydride.

![](media/image214.png){width="5.998611111111111in"
height="2.2576388888888888in"}

FIG. 12.6.1-(1) Epoxy monomers

\[Keys, FIG. 12.6.1-(1)\]

(Top row) Bisphenol A glycidyl ether, hydrogenated bisphenol A glycidyl
ether

(Bottom row) 3,4-epoxycyclohexylmethylcarboxylate,
methylhexahydrophthalic anhydride

As cure accelerators, we used 2-ethyl-4-methylimidazole (from Shikoku
Corp.), benzyldimethylamine (reagent), tetraphenylphosphonium bromide
(reagent), diazobicyloundeceneoctanoic acid salt (from SAN-APRO
Limited), and tetrabutylphosphonium diethylphosphorodithioate (PX-4ET
from Nippon Chemical Industry Co., Ltd.).

![](media/image215.png){width="5.9875in" height="1.3888888888888888in"}

FIG. 12.6.1-(2) Cure accelerators

\[Keys, FIG. 12.6.1-(2)\]

(Left to right) 2-ethyl-4methylimidazole, benzylmethylamine,
tetraphenylphosphonium bromide, diazobicycloundeceneoctanoic acid salt,
and tetrabutylphosphonium diethylphosphorodithioate.

#### Fabrication of resin plate

![](media/image216.png){width="4.090972222222222in"
height="3.8048611111111112in"}The epoxy monomers, acid anhydride, and
cure accelerators were combined in a predetermined mixing proportion,
charged to jig for resin plate fabrication, which was made of reinforced
glass plates, and cured by heating for a predetermined time at a
predetermined temperature (primary cure). Subsequently, the jig was
taken apart to remove the plate, which was then again heated for a
predetermined time at a predetermined temperature (secondary cure). The
plate was cut into 50 mm × 50 mm × 5 mm (thickness) pieces used as
evaluation plates.

FIG. 12.6.1-(3) Jig for epoxy resin plate fabrication

#### UV irradiation testing

Irradiation with UV light was conducted for 100 hours using a Q-UV
accelerated weathering tester from Q-Panel, in which a UV fluorescent
lamp (λp=340 nm) as the light source. The transmittance and yellow index
before and after irradiation were measured using the UV-3100
spectrophotometer from Shimadzu Corporation.

#### Heat resistance testing

The epoxy plates were placed inside a thermostat maintained at 150ºC,
and the transmittance and yellow index were measured in the manner as
above.

### Results and discussion

In general, not less than about 95% of the components of epoxy resins
used for LED sealing are made up of bisphenol A glycidyl ether or other
epoxy monomers along with the methylhexahydrophthalic anhydride, added
to which are several percent of amine- or phosphorus-based curing
accelerators.

However, these components absorb UV light and oxidize, as shown in FIG.
12.6.2-(1). At such time, the resin assumes a yellowish hue as a result
of production of carbonyl groups and absorption of visible light \[9\].

![](media/image217.png){width="5.988888888888889in"
height="0.8069444444444445in"}

FIG. 12.6.2-(1) UV degradation of bisphenol A glycidyl ether

Therefore, in order to prevent the UV degradation, it is necessary to
develop methods, in which bisphenol A glycidyl ether is not used.

Means for preventing the degradation include allowing all of the UV
light to pass and using UV absorbers to convert UV light into heat
energy and release it.

As far as the former technique is concerned, we know that cure
accelerators that are capable of producing transparent epoxy resins and
do not absorb UV light do not exist. However, because of their low
concentration of less than 1 wt%, total UV absorption by the resin can
be greatly reduced if UV absorption by the primary components is
decreased.

On the other hand, in the second technique, conversion to heat energy
and release based on the use of UV absorbers involves not only the need
to provide high-performance absorbers, but also requires using a
heat-radiating architecture for the LED, which imposes serious
limitations on the design of LED architecture and which is impractical.

For this reason, in the present study, we focused mainly on techniques
permitting UV light to pass through and investigated resin compounds
based on hydrogenated bisphenol A glycidyl ether of FIG. 12.6.1-(1) that
absorb little UV light.

#### Influence of cure accelerators

First of all, we selected cure accelerators for the acid anhydride cure
system. We conducted acid anhydride curing of hydrogenated bisphenol A
glycidyl ether using methylhexahydrophthalic anhydride and investigated
the relationship between the type of cure accelerator and UV
degradation. In addition, is has been found that cure accelerators used
in combination with bisphenol A glycidyl ether allow for obtaining
transparent cured products \[10\].

As shown in Table 12.6.2-(1), compounds providing good initial
characteristics included 2-ethyl-4-methylimidazole,
tetraphenylphosphonium bromide, and tetrabutylphosphonium
diethylphosphorodithioate. However, only tetrabutylphosphonium
diethylphosphorodithioate was the compound exhibiting the least amount
of UV degradation and maintaining a high transmittance after
irradiation. The reason why considerable UV degradation developed when
using tetraphenylphosphonium bromide was probably due to the four
aromatic rings of tetraphenylphosphonium bromide that can easily absorb
UV light.

+---------------------------------+------------+------------+----------+
| Cure accelerators               | Tra        |            | Yellow   |
|                                 | nsmittance |            | index,   |
|                                 | (%, 400    |            |          |
|                                 | nm)        |            | Initial  |
+---------------------------------+------------+------------+----------+
|                                 | Initial    | After      |          |
|                                 |            | i          |          |
|                                 |            | rradiation |          |
+---------------------------------+------------+------------+----------+
| 2-ethyl-4-methylimidazole (5)   | 86.4       | 84.8       | 2.02     |
+---------------------------------+------------+------------+----------+
| Benzyldimethylamine (6)         | 76.5       | 71.3       | 6.12     |
+---------------------------------+------------+------------+----------+
| Tetraphenylphosphonium bromide  | 87.1       | 37.9       | 1.14     |
| (7)                             |            |            |          |
+---------------------------------+------------+------------+----------+
| Diazobicycloundeceneoctanoic    | 73.0       | 71.9       | 2.31     |
| acid salt (8)                   |            |            |          |
+---------------------------------+------------+------------+----------+
| Phosphorus-based cure           | 89.8       | 87.8       | 1.02     |
| accelerator PX-4ET (9)          |            |            |          |
+---------------------------------+------------+------------+----------+

Table 12.6.2-(1) Influence of different types of cure accelerators on UV
degradation of epoxy resins \[YL6663\]:\[MH700\]:\[Cure
accelerator\]=100:90:1

Thus, we decided on tetrabutylphosphonium diethylphosphorodithioate as
the optimal, low UV degradation cure accelerator and carried out the
subsequent experiments.

#### Improvement in heat resistance

One of the reasons for the wide use of bisphenol A glycidyl ether
consists in the high stability of its aromatic rings. The heat
resistance of the sealing resin is indispensable for securing
reliability of semiconductors during soldering and operation. When cured
with acid anhydride, as it is commonly done, its glass transition point
is around 130ºC.

On the other hand, in hydrogenated bisphenol A glycidyl ether, the
stability of the cyclohexane rings is lower than that of the aromatic
rings, and the glass transition point is also lower, in the vicinity of
100ºC. This leads not only to problems in terms of reliability described
above, but also facilitates oxidation of the resin under the action of
heat, and may increase the tendency towards discoloration.

Therefore, with a view to improve the heat resistance, we added CEL2021,
an alicyclic epoxy with a high glass transition point, to the
composition and examined the discoloration properties of the cured
product in a high-temperature weathering test. We found that adding 10
wt% of CEL2021 increased the glass transition point of the cured product
to 130ºC.

![](media/image218.png){width="3.467361111111111in"
height="3.1166666666666667in"}

FIG. 12.6.2-(2) Relationship between concentration of alicyclic epoxy
CEL2021 and thermal discoloration
\[YL6663+CEL2021\]:\[MH700\]:\[PX-4ET\]=100:90:1

\[Keys, FIG. 12.6.2-(2)\]

(Y-axis, left) Transmittance, % (400 nm)

(Y-axis, right) Yellow index

(Inside) Initial transmittance; Transmittance after test ∆
Transmittance, decrease after weathering; Yellow index after test; Ο
Yellow index, increase after weathering (discoloration); Initial yellow
index

As shown in FIG. 12.6.2, as the concentration of CEL2021 relative to
YL6663 grew, the initial discoloration increased, and the transmittance
slightly decreased. After high-temperature weathering, if the
concentration of CEL2021 exceeded 10 wt%, there was a conspicuous
increase in the yellow index and a drop in transmittance.

Therefore, although it is possible to increase the glass transition
point by adding CEL2021, the addition simultaneously leads to an
increase in the initial discoloration and thermal discoloration. At
present, we are studying the relationship between the concentration of
CEL2021 and UV degradation, and believe that the appropriate
concentration of CEL2021 should be within 10 wt%.

## Summary

### Electrode forming techniques

In the development of high-reflectance n-type electrodes, in addition to
obtaining high reflectances by improving the conventional Ti/Al
electrode, our research indicates that the use of electrodes based on a
novel material, ZnO, is also promising.

P-type electrodes studied so fare are shown in FIG. 12.7-(1).

![](media/image219.png){width="4.1819444444444445in"
height="2.571527777777778in"}

FIG. 12.7-(1) Contact resistance and reflectance of p-type electrodes

\[Keys, FIG. 12.7-(1)\]

(Y-axis, left) Reflectance (%)

(Y-axis, right) Contact resistance (Ωcm^2^)

(Box, inside)  Reflectance

Contact resistance

FIG. 12.7-(1) shows that the best electrode was the Pt/Ag electrode. It
was found that the Pt/Ag electrode yielded excellent values of contact
resistance, at 10^−4^ Ωcm^2^, and reflectance, at 71.5%.

### Improvements in light extraction efficiency

We have been conducting investigations into the use of the edge face
tilting method based on dry etching of GaN, which is believed to be an
effective means not only for improving the light extraction efficiency,
but also in terms of phosphor emission efficiency and UV-resistant
sealing. To study specific techniques, we prepared masks of various two
different kinds of photoresist with different viscosities and
investigated various photoresist condition dependences of the GaN edge
face angle.

A tendency towards smaller GaN edge face angles was observed when the
thickness of the photoresist film was increased. In addition, when the
post-bake temperature was increased, a tendency towards smaller GaN edge
face angles was observed, although the thickness of the resultant
photoresist film practically didn't change. Also, as a result of
investigating the post-bake temperature dependence, it was found that in
case of Type-A photoresist GaN edge face angles tended to remain
practically unchanged at post-bake temperatures of up to 130ºC, and
carrying out the post-bake treatment at 140ºC allowed to obtain a tilt
of 45º, the target value of the GaN edge face angle. Additionally, in
case of Type-B photoresist, as the post-bake temperature increased, the
GaN edge face angle exhibited a tendency to decrease, and GaN edge faces
tilted 45º were obtained by means of post-bake treatment at 140ºC.

As a result of studying the GaN face orientation dependence of the GaN
edge face angle, no face orientation dependence of the GaN edge face
angle was observed neither in a direction perpendicular to nor in a
direction parallel to the c-plane sapphire substrate \[1-100\]
direction. In addition, evaluation of the reproducibility of the GaN
edge face angle under various photoresist mask conditions indicated that
the edge faces of GaN could be tilted with good reproducibility if the
photoresist mask was formed using a thick photoresist film and a high
post-bake temperature.

### Development of white LEDs

White LED lamps were fabricated using UV LED chips and subjected to
evaluation. As a result it was confirmed that when an optical thin film
filter transmitting visible light and reflecting UV light was formed on
the phosphor layer mounted on the UV LED chip, and, furthermore, when
the UV LED chip edges were coated with phosphor, the amount of
transmitted UV light in comparison with untreated samples was reduced
from 31% to 13% and the total flux of the white LED lamp was improved
30%. In addition, it was found that in order to make the best of the
optical thin film filter, it was necessary to investigate structures
capable of directing most of the light emitted by a UV LED to the
phosphor layer.

Evaluation of white color rendering using various phosphors conducted
from the standpoint of emission spectrum shape confirmed that color
rendering properties could be improved by regulating phosphors
containing many components with a broad spectral distribution. White LED
lamps combining phosphors with a broad spectral distribution, as in the
RGB model phosphors evaluated in the present report, are of considerable
practical importance as source of light for illumination.

### Development of sealing resins for UV LEDs

As a result of investigating cure accelerators added to mixtures of
epoxy monomers and hydrogenated bisphenol A glycidyl ether, it was
determined that optimal cure accelerators were phosphorus-based cure
accelerators without double bonds. The problem with hydrogenated
bisphenol A glycidyl ether is its low heat resistance, and it was found
that thermal discoloration could be suppressed by adding an alicyclic
epoxy with a high heat resistance.

## Plans for the future

In the area of electrode forming technique development, we will examine
the influence of differences in electrode structure and electrode
materials on the voltage efficiency and emission efficiency of UV LEDs
in order to determine the best type of electrode in terms of device
characteristics. In addition, we will evaluate electrode reliability
based on LED characteristics and continue narrowing down our selection
of electrode materials and electrode architectures.

In the area of light extraction efficiency, we will apply the RIE-based
edge face tilting technology developed this year to the fabrication of
LEDs to confirm the emission efficiency improvement effects. In
addition, we will start development of chip micropatterning techniques
and strive to realize at least 50% of the initial target for light
extraction efficiency.

In the area of UV-resistant sealing technique development, we will not
only continue the irradiation testing currently in progress, but we will
also examine the use of anti-oxidants and UV absorbers in order to
further suppress discoloration. In addition, we plan to investigate
various characteristics of LED sealing resins (viscosity, heat
resistance, water resistance, insulating properties, etc.) and examine
them from the standpoint of reliability.

In terms of white LED development, we plan to confirm the behavior of
visible light conversion efficiency using chips obtained by applying the
edge face tilting technique. In addition, we will conduct investigations
into the structure of the UV light reflecting film corresponding to the
wavelength of the UV LED and further enhance the efficient use of the
excitation light.

## References

\[1\] G. S. Marlow et al., Solid State Elec., 25, 91 (1982).

\[2\] J. S. Foresi, T. D. Moustakas, Appl. Phys. Lett., 62(22), 2859
(1993).

\[3\] Materials of Scientific Symposium No. 61 of Applied Physics
Society (2000) 3a-ZN-2

\[4\] M. R. Krames et al., Appl. Phys. Lett., 16, (1999), 2365.

\[5\] JIS Handbook 33 "Color," JIS Z 8724-1983, p.275-286 (1992).

\[6\] The Color Science Association of Japan, "Color Science Handbook,"
p. 151-152 (1982).

\[7\] Development of high-efficiency photoelectric conversion compound
semiconductors, 1999 Achievement Report, p. 217\~222 (2000).

\[8\] JIS Handbook 33 "Color," JIS Z 8726-1990, p. 222-231 (1992).

\[9\] The Chemical Society of Japan, The Society of Polymer Science,
"New Trends in Polymer Additives," Nikkan Kogyo Shinbunsha (1998)

\[10\] Hirai (H. Kakiuchi), "New Trends in Curing Agents for Epoxy
Resins," p. 183 CMC (1984)

# Development of LED-based Light-Source Devices for Illumination

## Achievements in 1998-1999

Because the per-element optical output of LEDs is lower than that of
conventional light sources when LEDs are used as light sources for
general illumination, it is necessary to try to increase their optical
output by assembling the elements in the form of arrays or in the form
of matrices. In the present study, we conducted an investigation into
increasing the output of LED array light sources and confirmed that
increased LED element temperature exerted considerable influence on the
decrease in the optical output. Thus, as a result of examining various
cooling methods (substrates, heat sinks, coolants) used for LED array
light sources, we found out that the decrease in the optical output
could be suppressed by creating conditions for highly efficient heat
emission from LED elements, which made it possible to achieve increased
light outputs.

On the other hand, in order to study the architecture of UV LED-based
light source devices, we carried out optical measurements of
UV-excitable phosphors capable of conversion to visible light
\[*"down-converting phosphors" − trans*.\] and established optical
simulation techniques for phosphor-converted light produced with the
help of UV LEDs. As a result of comparing the characteristics of
reflection and transmission of phosphor-converted light based on the
simulation, it was conjectured that a reflective architecture making the
most of reflected phosphor light would be efficient in increasing
optical outputs. In this connection, we fabricated prototype models of
light source devices of a transmitting-type architecture and of a
reflective architecture, in which we combined blue LEDs and YAG
phosphors, and subjected their optical output to measurement and
comparison, as a result of which it was confirmed that higher optical
outputs were obtained when using the reflective architecture.

In addition, in environment compatibility evaluation research, which
included physiological and psychological influence exerted on humans by
application of LED array light sources to living spaces, we focused on
the problem of one-dimensional light-emitting surface luminance
distribution in regular arrangements of point LED light sources and
theoretically derived a formula for the estimation of visual discomfort
using psychophysical parameters. In addition, using the estimation
formula, we obtained a correlation for visual discomfort that took into
consideration factors such as the spatial frequency, duty ratio,
contrast ratio, and average luminous intensity and studied its tolerance
values.

## Summary of Research and Development in 2000

Results obtained prior to last year allowed us to confirm that a
reflective architecture, when used for LED light source devices of the
phosphor conversion type, could be efficient in increasing optical
outputs and proposed a model reflective architecture. In this
connection, this year, we conducted optical simulations of LED light
source devices of the model reflective architecture using a phosphor
conversion-based emission system combining blue LEDs and YAG phosphors
and conducted a comparative study using real-life prototype models. It
was confirmed that the results obtained from the simulation were close
to the optical measurement values of the real-life models, which
corroborated the effectiveness of the application of the present
simulation technique to optical control and design optimization.

Subsequently, by combining white LEDs with LEDs emitting light of
different colors, we were able to recreate multicolored light, and,
based on the fact that the color tone could be easily changed by
regulating optical outputs, we studied a correlated color temperature
alteration technique based on combining red and green LEDs with white
LEDs. Furthermore, because color rendering properties are a factor of
considerable importance for illumination light sources, we examined
average color rendering property evaluation scores (Ra). As a result of
investigating the spectral distributions of three kinds of LEDs based on
the method of additive mixture of color stimuli, we obtained correlated
color temperature conditions for incandescent light bulb color, warm
white color, white color, daylight white color, and daylight color. In
addition, it was confirmed that high color rendering properties with a
Ra=95 could be obtained under daylight conditions.

In the area of environment compatibility evaluation, we conducted
psychophysical experiments to determine the psychological influence
exerted on test subjects by matrix-like two-dimensionally arranged LED
array light sources. The verification did not allow us to determine the
relationship of the visual stress to the spatial factors related to the
arrangement of the LED point light sources, but it was confirmed that
there was a strong correlation between "glare" and "discomfort."

## Study of phosphor conversion-based LED light source devices by optical simulation

In recent years, development of high-brightness GaN-based LEDs has made
considerable progress, with white LEDs based on the use of YAG phosphors
attracting attention as a fourth type of illumination light sources in
addition to incandescent lamps, fluorescent lamps, and HIDs. Against
this background, LED-based solid semiconductor light sources, which are
characterized by small size, long useful life, and by being
mercury-free, have aroused even more interest from the standpoint of
possible improvements in energy conversion efficiency, in other words,
better energy savings. At present, the emission efficiency of white LEDs
is 15\~25 lm/W, i.e. its level already exceeds that of incandescent
lamps, and since their commercial introduction in 1996, white light
emitting diodes have made great strides in terms of efficiency
improvements \[1-5\].

One of the objectives of the present project is to develop
high-efficiency 80\~100 lm/W LEDs before the end of 2003 to realize the
goal of next-generation LED-based illumination that is coming to replace
fluorescent lamps. In order to achieve the objective, during the present
project, we have been developing LED light source devices for
illumination using an emission system, in which high-efficiency LEDs
based on GaN with emission wavelengths in the blue to near-UV region
around 400 nm are combined with phosphors capable of efficient
conversion to visible white light.

In the course of this development, we have realized the need to carry
out design optimization and optical control of LED-based light source
devices based on optical simulation. For this reason, last year, we
conducted measurements of optical characteristics of UV-excitable
phosphors capable of conversion to visible light, and, by introducing
the obtained results in our optical simulation, conducted a basic
investigation into simplified device models employing UV LEDs in order
to determine whether a reflective architecture making efficient use of
reflected phosphor light was desirable in terms of obtaining high
optical outputs. On the other hand, in the course of our studies of
phosphor conversion-based LED light source device architectures
combining currently available GaN-type blue LEDs with YAG phosphors, we
fabricated and examined real-life models, which corroborated the fact
that high optical outputs were associated with the use of the reflective
architecture. Therefore, this year, we studied the optical
characteristics of LED light source devices of the reflective
architecture that combined blue LEDs with YAG phosphors by carrying out
actual measurements of optical characteristics of the YAG phosphors and
introducing the resultant values into the optical simulation.

### Methods and means (simulation-based study of light source devices)

First of all, in order to fully understand the characteristics of
current GaN-based blue LEDs, we examined the light distribution
characteristics and spectral distribution of currently available
top-level bullet blue LEDs and, as a result, confirmed that such diodes
emitted blue light with a peak wavelength of 467 nm and an optical
output of 6.3 mW when a forward electric current with a rating of 20 mA
was passed through the diodes. For this reason, in the experiments, in
order to obtain the optical characteristic values of YAG phosphors, we
decided to make use of a spot light source (LA-250Xe from Hayashi
Watch-Works Co., Ltd.), in which a Xenon lamp with an attached optical
filter selectively transmitting light from the long wavelength region in
the vicinity of 470 nm was used as a light source, on the assumption
that the blue light (optical output: 6 mW) obtained from such a spot
light source would be a good approximation of GaN blue LED emission.
FIG. 13.3.1-(1) and FIG. 13.3.1-(2) show, respectively, the light
distribution characteristic and spectral radiant flux of the spot light
source.

![](media/image220.png){width="5.981944444444444in"
height="1.8770833333333334in"}

Left: FIG. 13.3.1-(1) Light distribution characteristic of spot light
source

Right: FIG. 13.3.1-(2) Spectral radiant flux of spot light source

The YAG phosphors used in the experiments were commercially available
products (P46-Y3 from Kasei Optonics, Inc.) whose structure was
represented by the chemical formula Y~3~Al~5~O~12~:Gd, Ce. To prepare
YAG phosphor test samples (called YAG phosphor sheets below), YAG
phosphor powders were introduced into silicone rubber sheets with a
thickness of 0.3 mm so as to obtain a predetermined concentration of
10\~60 wt%. The light distribution characteristics and spectral
distributions of the transmitted light and reflected light resulting
from irradiation of each sheet with light from the spot light source at
an incidence angle varying between 0º to 70º were measured to
investigate the optical characteristic values of the YAG phosphors,
which we planned to introduce into the optical simulation.

The optical simulation was carried out using a ray tracing-type
illumination design software program (Light Tools from Optical Research
Associates), which was based on the Monte Carlo method. FIG. 13.3.1-(3)
shows a reflective LED light source device model used to conduct the
optical simulation. A 120º V-shaped reflecting plate and, above it, a
YAG phosphor sheet, were installed inside a 122×122×36.5 mm box-shaped
fixture, and array substrates having blue LEDs mounted thereon (10 LED
per side) were arranged on both sides of the box at angles of 45º
towards the inside. In this construction, light emitted by the blue LEDs
was directed outside by traveling via the YAG phosphor sheet and
reflecting plate and passing through a 100×100 mm diffuser plate
provided in front of the box. In addition, as concerns the optical
characteristic values (transmittance, reflectance) of the various
constituent materials of the reflecting plates etc. used in this
construction, we subjected the constituent materials of the real-life
prototype models fabricated using commercially available surface-mounted
GaN blue LEDs (rated optical output: 3.6 mW) to measurements using a
C-light source \[*UV-C? − trans*.\] as a reference and employed the
obtained values in the optical simulation. In addition, as far as the
blue LED light source settings used in the simulations are concerned,
1,000 light rays were used as the number of rays (per 1 LED) and the
optical output and light distribution characteristics of the spot light
source were inputted as is because the blue light emitted from the spot
light source when obtaining the optical characteristic values of the YAG
phosphors was assumed to be equivalent to the emission of a GaN-based
blue LED.

![](media/image221.png){width="3.953472222222222in"
height="2.7555555555555555in"}

FIG. 13.3.1-(3) Reflective LED light source device model

The trajectory of a single light ray in the light emitted from a blue
LED is shown in FIG. 13.3.1-(4). The target of the calculations
performed in this simulation was the secondary light, which is usually
crucial to the determination of the amount of illumination. When light
emitted from a blue LED reached the YAG phosphor sheet, it generated
four kinds of light:  wavelength-converted transmitted light (Yt1), 
unconverted transmitted light (Bt1),  wavelength-converted reflected
light (Yr1), and  unconverted reflected light (Br1). The two kinds of
reflected light (Yr1, Br1) were captured as primary light on the
light-receiving surface. On the other hand, the light that passed
through the YAG phosphor sheet (Yt1, Bt1) was reflected by the
reflecting plate and again traveled to the YAG phosphor sheet (Yr2,
Br2), generating wavelength-converted transmitted light (Yt2-1) and
unconverted transmitted light (Yt2-1, Bt2), respectively. Therefore, the
transmitted light (Yt2-1, Yt2-2, Bt2), generated when light reached the
YAG phosphor sheet again, was captured as secondary light on the
light-receiving surface. In addition, most of the calculations in the
simulation were related to the radiant flux (W), with the long
wavelength region of 380\~500 nm used as the blue light component, the
wavelength region of 501\~780 nm used as the yellow light component, and
the blue light component and yellow light component calculated
separately.

![](media/image222.png){width="4.209027777777778in"
height="2.384027777777778in"}

FIG. 13.3.1-(4) Single ray trajectory

\[Keys, FIG. 13.3.1-(4)\]

(Top to bottom) Light receiving surface; Blue LED; YAG phosphor sheet;
Reflecting plate

### Results and discussion (simulation-based study of light source devices)

FIG. 12.3.2-(1) shows the radiant fluxes of the components of the
transmitted and reflected light versus the YAG phosphor content, which
were obtained by irradiating a YAG phosphor sheet with a spot light
source at an incidence angle of 0º. First of all, as far as the blue
transmitted light and yellow transmitted light are concerned, it was
confirmed that, as the content of YAG phosphor increased, the radiant
flux of the blue transmitted light decreased, and, at the same time, as
a result of conversion by the YAG phosphor, there was obtained yellow
transmitted light; however, it was found that the radiant flux of the
yellow transmitted light did not increase with the increase in the
content of the YAG phosphor and remained in a saturated state. In
addition, as the content of the YAG phosphor increased, the color of the
transmitted light readily changed from blue to white to yellow, which
can be seen from the ratio of the radiant flux of the blue transmitted
light to that of the yellow transmitted light. On the other hand, as far
as the blue reflected light and yellow reflected light are concerned, it
was confirmed that as the content of YAG phosphor increased, the radiant
flux of the blue reflected light did not undergo significant changes and
remained almost constant, whereas the radiant flux of the yellow
reflected light continuously increased and reached a saturated state
when the content of the YAG phosphor was approximately 50\~60 wt%. In
addition, it was found that, as the content of the YAG phosphor
increased, the color of the reflected light changed from a bluish white
to white, with white light obtained under all conditions.

![](media/image223.png){width="3.91875in" height="2.441666666666667in"}

FIG. 13.3.2-(1) Relationship between YAG phosphor content and radiant
flux

\[Keys, FIG. 13.3.2-(1)\]

(Box inside, top to bottom) Blue transmitted light

Yellow transmitted light

Blue reflected light

Yellow reflected light

FIG. 13.3.2-(2) shows the luminous flux versus the YAG phosphor content,
obtained as a result of irradiating a YAG phosphor sheet with the spot
light source at an incidence angle of 0º. It was found that as the
content of the YAG phosphor increased, the luminous flux of the
transmitted light tended to gradually decrease, while the luminous flux
of the reflected light tended to undergo a considerable increase. In
addition, it was confirmed that the total luminous flux reached a
saturated state at a YAG phosphor content of 50 wt%. Thus, in order to
study reflective LED light source devices, we made the decision to
introduce the optical characteristic values obtained at the YAG phosphor
content of 50 wt% into the optical simulation.

![](media/image224.png){width="4.558333333333334in"
height="2.813888888888889in"}

FIG. 13.3.2-(2) Relationship between YAG phosphor content and luminous
flux

\[Keys, FIG. 13.3.2-(2)\]

(Box inside, top to bottom): Transmitted light; Reflected light; Total
luminous flux

FIG. 13.3.2-(3) and FIG. 13.3.2-(4) show the light distribution
characteristic and spectral radiant flux of reflected light obtained by
irradiating a YAG phosphor sheet with a YAG phosphor content of 50 wt%
at an incidence angle of 0º. In addition, because in case of measuring
the light distribution characteristic of the reflected light the spot
light source was itself an obstacle, there were some areas, where
measurements could not be done; however, because in case of both
transmitted light and reflected light generated from the YAG phosphor
sheet it was assumed to be a type of diffused light, we obtained the
desired values by using an approximation curve. In addition, for this
reason, the light emitted from the YAG phosphor sheet in the optical
simulation was treated as diffused light as well. From the spectral
radiant flux results shown in FIG. 13.3.2-(4), it was determined that
the radiant flux of the reflected light was 1.68 mW for the blue light
component and 2.81 mW for the yellow light component, with the color of
the emitted light being white, with chromaticity coordinates of x=0.31,
y=0.33. Optical characteristics results obtained in the same manner for
a YAG phosphor sheet with a YAG phosphor content of 50 wt% are shown in
Table 13.3.2-(1).

![](media/image225.png){width="5.988888888888889in"
height="1.8652777777777778in"}

Left: FIG. 13.3.2-(3) Light distribution characteristic of reflected
light from YAG phosphor sheet (50 wt%, 0º)

Right: FIG. 13.3.2-(3) Spectral radiant flux of reflected light from YAG
phosphor sheet (50 wt%, 0º)

+-------+----------+----------+---------+---------+---------+---------+
| Inci  | Tra      |          |         | Re      |         |         |
| dence | nsmitted |          |         | flected |         |         |
| angle | light    |          |         | light   |         |         |
+-------+----------+----------+---------+---------+---------+---------+
|       | Blue     | Yellow   | Chrom   | Blue    | Yellow  | Chrom   |
|       | c        | c        | aticity | co      | co      | aticity |
|       | omponent | omponent | coor    | mponent | mponent | coor    |
|       | (transm  | (transm  | dinates | (refle  | (refle  | dinates |
|       | ittance) | ittance) |         | ctance) | ctance) | (x, y)  |
|       |          |          | (x, y)  |         |         |         |
+-------+----------+----------+---------+---------+---------+---------+
| 0º    | 0.02 mW  | 1.15 mW  | (0.45,  | 1.68 mW | 2.81 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.004)  | (0.191)  |         | (0.280) | (0.469) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 10º   | 0.02 mW  | 1.13 mW  | (0.45,  | 1.79 mW | 2.52 mW | (0.30,  |
|       |          |          | 0.52)   |         |         | 0.32)   |
|       | (0.004)  | (0.189)  |         | (0.298) | (0.420) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 20º   | 0.02 mW  | 1.11 mW  | (0.45,  | 1.66 mW | 2.56 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.004)  | (0.185)  |         | (0.277) | (0.427) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 30º   | 0.02 mW  | 1.09 mW  | (0.45,  | 1.80 mW | 2.83 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.003)  | (0.181)  |         | (0.300) | (0.472) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 40º   | 0.02 mW  | 1.06 mW  | (0.45,  | 1.85 mW | 2.94 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.003)  | (0.177)  |         | (0.308) | (0.490) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 50º   | 0.02 mW  | 1.02 mW  | (0.46,  | 1.63 mW | 2.61 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.003)  | (0.170)  |         | (0.272) | (0.435) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 60º   | 0.02 mW  | 0.93 mW  | (0.46,  | 1.57 mW | 2.53 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.003)  | (0.155)  |         | (0.262) | (0.422) |         |
+-------+----------+----------+---------+---------+---------+---------+
| 70º   | 0.01 mW  | 0.79 mW  | (0.46,  | 1.33 mW | 2.15 mW | (0.31,  |
|       |          |          | 0.52)   |         |         | 0.33)   |
|       | (0.002)  | (0.132)  |         | (0.222) | (0.358) |         |
+-------+----------+----------+---------+---------+---------+---------+

Table 13.3.2-(1) Optical characteristic values of YAG phosphor sheet
(YAG phosphor content: 50 wt%)

Table 13.3.2-(2) shows the radiant flux of a reflective LED light source
device obtained via simulation. In addition, FIG. 13.3.2-(5) shows the
spectral radiant flux obtained based on the optical simulation results.
From the simulation results it was conjectured that the radiant flux of
the blue light component was 11 mW, and that of the yellow light
component was 20 mW, and, upon conversion to luminous flux, the
resultant optical output was approximately 9.1 lm, with the color of the
emission at such time being white, with chromaticity coordinates of
x=0.32 and y=0.34. On the other hand, it was confirmed that the
proportion of the primary light in the resultant total radiant flux
accounted for the most of it, with the proportion of the secondary light
being as small as to be safely ignored. This fact can be interpreted as
a sign of the high reflectance and low transmittance of the YAG phosphor
sheet used in the simulation, indicating that a reflective type
architecture is efficient in terms of obtaining high optical outputs.
However, because it was found that the total radiant flux obtained for
blue light with an optical output of 120 mW (6 mW × 20) was
approximately 31 mW and there were considerable optical energy losses of
more than 70% in the reflective type LED light source device, achieving
further efficiency improvements based on simulation is an important task
for the future.

FIG. 13.3.2-(6) shows the spectral radiant flux of a real-life prototype
model. It was confirmed that in the real-life model, too, its total
radiant flux was low, at 23 mW, and considering that in this case we
used blue LEDs with an optical output of 3.6 mW/diode, the optical
energy losses were also large, at about 70%. In addition, the total
luminous flux of the real-life model was 6.4 lm and the color of the
emission was white, with chromaticity coordinates of x=0.30 and y=0.31,
and if we consider the difference in optical outputs from this
standpoint, this confirms that the results of the simulation were a good
approximation of the results of the real-life model. This provides
evidence in support of the possibility of applying the present
simulation technique to the optical control and design optimization of
LED light source devices.

  ----------------------------------- -----------------------------------
  Components                          Radiant flux

  Blue, reflected, primary (Br1)      11 mW

  Yellow, reflected, primary (Yr1)    20 mW

  Blue, transmitted, secondary (Bt2)  0.011 mW

  Yellow, transmitted, secondary      0.026 mW
  (Yt2-1)                             

  Yellow, transmitted, secondary      0.043 mW
  (Yt2-2)                             

  Blue light radiant flux (Br1+Bt2)   11.011 mW

  Yellow light radiant flux           20.069 mW
  (Yr1+Yt2-1+Yt2-2)                   

  Total radiant flux                  31.080 mW
  (Br1+Bt2+Yr1+Yt2-1+Yt2-2)           
  ----------------------------------- -----------------------------------

Table 13.3.2-(2) Optical simulation results of reflective LED light
source device

![](media/image226.png){width="2.923611111111111in"
height="1.7465277777777777in"}![](media/image227.png){width="2.865972222222222in"
height="1.698611111111111in"}

Left: FIG. 13.3.2-(5) Spectral radiant flux obtained based on optical
simulation results

Right: FIG. 13.3.2-(6) Spectral radiant flux of real-life prototype
model

## Correlated color temperature alteration techniques and color rendering property evaluation of mixed light from white, red, and green LEDs

Sunlight at the height of the day can be separated into red, orange,
yellow, green, blue, indigo, and violet color light with a prism;
however, because the human eye does not possess the capability for
analyzing the components of light, it cannot tell whether the white
light it sees is a mixture of yellow light and blue light or a mixture
of red light, green light, and blue light. However, the human eye is
quite sensitive to colors and perceives even the slightest difference in
color tone, and the difference in color tone exerts considerable
psychological influence on humans. For this reason, the color of light
sources used for illumination is an important factor that not only
affects the atmosphere of the interior and the impression of the space,
but also exerts considerable influence on human perception and emotions.
For example, one of the methods used to represent the color of a light
source is to use the color temperature; in general, however, when the
color temperature is less than 3000 K, the color feels warm, whereas
when it is 5000 K or more, the color produces a cool sensation \[6\].

For light sources such as fluorescent lamps used for general
illumination, the chromaticity range is stipulated by JIS Z 9112, where
it is divided into 5 types of color, such as (starting from the lowest
correlated color temperature (Tcp)): incandescent light-bulb (L:
Tcp=2600\~3150 K), warm white (WW: Tcp=3200\~3700 K), white (W:
Tcp=3900\~4500 K), daylight white (N: Tcp=4600\~5400 K), and daylight
(D: Tcp=5700\~7100 K) \[7\]. Therefore, although appropriately
illuminated spaces can be created depending on the intended use and
individual tastes by using different fluorescent lamps of the above
various light source colors, in such a case, only one light source color
can be selected for one type of fluorescent lamp and the needs of the
user who might want to change the color temperature depending on
seasonal or time-of-the-day variations in a simple and straightforward
manner are not satisfied.

By contrast, LEDs exhibit various emission colors, including the three
primary colors (red, green, blue), which permits full color lighting
based on combining different LEDs and regulating their respective
optical outputs. In addition, there are white LEDs manufactured by
combining phosphors with blue LEDs, and using them together with colored
LEDs allows for creating multi-colored lighting, the color tone of which
can be easily tuned by regulating the respective optical outputs. In
this connection, we studied a correlated color temperature alteration
technique based on using white LEDs made by combining blue LEDs and
phosphors as basic units and combining them with red and green LEDs, the
other two primary colors besides blue. In addition, because color
rendering properties are an important factor in creating high-quality
illumination, we also studied their average color rendering property
evaluation scores (Ra).

### Methods and means (correlated color temperature alteration technique & color rendering property evaluation)

The spectral distributions of the white, red, and green LEDs used in the
experiments are shown in FIG. 13.4.1. The white LEDs were based on a
combination of an InGaN-based blue LED with a peak wavelength of 460 nm
with a YAG phosphor emitting yellow light with a central wavelength of
565 nm, the red LEDs were based on AlGaInP with a peak wavelength of 635
nm, and the green LEDs were based on InGaN with a peak wavelength of 520
nm. In addition, the chromaticity coordinates of the white LEDs used in
the experiments were x=0.31 and y=0.29, with a correlated color
temperature of Tcp=7200 K and an average color rendering score of Ra=85.

![](media/image228.png){width="4.093055555555556in"
height="2.232638888888889in"}FIG. 13.4.1 Spectral distributions of LEDs

\[Keys, FIG. 13.4.1\]

(Box inside, top to bottom): White LED; Red LED; Green LED

We studied the correlated color temperature conditions based on the
method of additive mixture of color stimuli using the spectral
distributions of the LEDs. The values of the spectral distributions of
mixed colors were inputted into an independently developed color
rendering property evaluation score computation program, obtaining the
chromaticity coordinates, correlated color temperature, and average
color rendering property evaluation scores \[8, 9\].

### Results and discussion (correlated color temperature alteration technique & color rendering property evaluation)

The average color rendering property evaluation scores, correlated color
temperatures, chromaticity coordinates, and emission intensity ratios
under various correlated color temperature conditions, which were
obtained based on the method of additive mixture of color stimuli using
the spectral distributions of the white, red, and green LEDs, are shown
in Table 13.4.2. In addition, FIG. 13.4.2-(1) \~ FIG. 13.4.2-(5) show
respective spectral distributions under various correlated color
temperature conditions. The correlated color temperature conditions were
obtained by setting the emission intensity ratio of the green LEDs to
approximately 20% and varying the emission intensity ratio of the white
LEDs and red LEDs. In other words, it was confirmed that arbitrary
correlated color temperatures could be realized by lowering the emission
intensity of the white LED (Tcp=7200 K), which had a high correlated
color temperature, and instead raising the emission intensity of the red
LEDs.

+-------------+-------+-------+-------+----------+---------+-------+
| Light       | Emi   |       |       | Chro     | Cor     | Av    |
| source      | ssion |       |       | maticity | related | erage |
| color       | inte  |       |       | coo      | color   | color |
|             | nsity |       |       | rdinates | temp    | rend  |
|             | ratio |       |       |          | erature | ering |
|             | (%)   |       |       | (x, y)   | Tcp (K) | pro   |
|             |       |       |       |          |         | perty |
|             |       |       |       |          |         | evalu |
|             |       |       |       |          |         | ation |
|             |       |       |       |          |         | score |
|             |       |       |       |          |         | Ra    |
+-------------+-------+-------+-------+----------+---------+-------+
|             | White | Red   | Green |          |         |       |
|             | LED   | LED   | LED   |          |         |       |
+-------------+-------+-------+-------+----------+---------+-------+
| I           | 6.7   | 73.6  | 19.7  | (0.45,   | 2800    | 26    |
| ncandescent |       |       |       | 0.41)    |         |       |
| light-bulb  |       |       |       |          |         |       |
| (L)         |       |       |       |          |         |       |
+-------------+-------+-------+-------+----------+---------+-------+
| Warm white  | 12.4  | 67.1  | 20.5  | (0.41,   | 3370    | 42    |
| (WW)        |       |       |       | 0.39)    |         |       |
+-------------+-------+-------+-------+----------+---------+-------+
| White (W)   | 22.6  | 56.4  | 21.0  | (0.37,   | 4246    | 63    |
|             |       |       |       | 0.37)    |         |       |
+-------------+-------+-------+-------+----------+---------+-------+
| Daylight    | 31.8  | 46.2  | 22.0  | (0.35,   | 5026    | 78    |
| white (N)   |       |       |       | 0.36)    |         |       |
+-------------+-------+-------+-------+----------+---------+-------+
| Daylight    | 53.5  | 26.7  | 19.8  | (0.32,   | 6186    | 95    |
| (D)         |       |       |       | 0.34)    |         |       |
+-------------+-------+-------+-------+----------+---------+-------+

Table 13.4.2 Correlated color temperature conditions

![](media/image229.png){width="2.938888888888889in"
height="1.7805555555555554in"}![](media/image230.png){width="2.946527777777778in"
height="1.7861111111111112in"}

![](media/image231.png){width="2.9472222222222224in"
height="1.7368055555555555in"}![](media/image232.png){width="2.939583333333333in"
height="1.7625in"}

![](media/image233.png){width="2.9305555555555554in"
height="1.8256944444444445in"}Top Left: FIG. 13.4.2-(1) Spectral
distribution of incandescent light bulb (L)

Top Right: FIG. 13.4.2-(2) Spectral distribution of warm white (WW)

Middle Left: FIG. 13.4.2-(3) Spectral distribution of white (W)

Middle Right: FIG. 13.4.2-(4) Spectral distribution of daylight white
(N)

Lower Right: FIG. 13.4.2-(5) Spectral distribution of daylight (D)

On the other hand, the average color rendering property evaluation score
tended to increase with increasing correlated color temperature, with a
particularly high value of Ra=95 obtained for daylight, which exceeded
the Ra=85 of the white LED used in the experiments. In addition,
generally speaking, color rendering properties are high in case of
spectra that are uniform and continuous in the visible light region and
low in case of spectra lacking in uniformity, which is also confirmed by
the spectral distributions under various correlated color temperatures.
However, because 3-wave length type fluorescent lamps, which are
currently in widespread use, possess a spectral characteristic made up
of broad red, green, and blue spectra, and, at the same time, provide
high color rendering properties of Ra=80 or greater, it is expected that
color rendering properties can be improved by rigorously controlling the
emission intensity ratios of the white (blue), red, and green LEDs under
correlated color temperature conditions other than daylight, which was
investigated in the present study.

## Study of environmental compatibility evaluation of LED light source devices

In recent years, high-brightness white LEDs have grabbed the spotlight
and have shown considerable promise to become a fourth type of light
source after incandescent, fluorescent, and HID sources. However, their
luminous flux per single LED is about 1 lm, and in order to be able to
use LEDs for general illumination, it is necessary to obtain higher
optical outputs by integrating them in matrix or array form. For this
reason, when LED light source devices in which high brightness spot
light sources such as LEDs are integrated together for illumination
purposes are used in living spaces, their safety is a condition of
paramount importance, and it is essential to carry out their
environmental compatibility evaluation from the standpoint of visual
stress and other factors of psychological influence.

To this end, last year we focused our research on measuring the
sensation of visual discomfort caused by the one-dimensional
light-emitting surface luminance distribution in regularly arranged
integrated LED light sources using the spatial frequency, brightness
amplitude modulation, and contrast as psychophysical parameters and
theoretically derived a formula for its estimation. Subsequently, using
the estimation formula, we examined tolerance values of visual
discomfort by taking into consideration factors such as the duty ratio,
contrast ratio, and average luminance. However, last year's research did
not include an environmental compatibility evaluation based on
psychophysical testing using real-life integrated LED-based light
sources as stimuli, and estimation using actual test subjects remained
an important task for future studies. Thus, in this year's research, we
employed psychophysical techniques to conduct visual stress estimation
experiments using integrated LED light sources two-dimensionally
arranged in matrix form and found confirmation of the actual
psychological influence exerted by visual stress.

A. J. Wilkins was the first to clearly define the concept of "visual
stress" as a scientific term \[11\]. Wilkins developed a theory of
visual discomfort caused by visual stimuli based on the fact that among
visual stimuli there were stimuli causing discomfort when looked at by
observers (including generally healthy individuals) and such visual
stimuli caused attacks in light-sensitive epileptics and migraine
headache sufferers. In other words, Wilkins proposed that visual stress
was related to the functioning of the brain and when visual stimuli were
subjected to information processing in the brain, they induced attacks
in light-sensitive epileptics and migraine sufferers and caused a
sensation of discomfort in generally healthy individuals.

Up until now, there have been very few examples of research approaching
the problem of visual stress generated by visual stimuli in healthy
individuals from the above-described standpoint. In addition, in current
research related to light-sensitive epileptics and migraine sufferers,
estimation is based mostly on physiological techniques while the number
of examples of research based on psychological approaches is relatively
small. Against this background, researchers from the Hibino Lab at the
Chiyo University approached the problem of visual stress using
psychophysical techniques. As a result, it was found that the method of
magnitude estimation \[12-15\] was effective in the measurement of
visual stress \[16-20\]. Thus, in this study, using the magnitude
estimation method, we attempted an estimation of visual stress caused by
integrated LED sources two-dimensionally arranged in matrix form.

### Methods and means (environmental compatibility evaluation study)

During the experiments, at first we conducted a basic study using
integrated light source samples utilizing bullet-type LEDs of high
directionality characteristics in order to obtain a clear picture of the
fundamental tendencies imparted to human visual perception by spot light
sources such as LEDs, and their characteristics. We prepared two types
of bullet LED sample stimuli, which were made by mounting a matrix of
commercially available bullet-type white LEDs (diameter: 5 mm, angle of
visibility: 20º) arranged at 10 mm intervals (11×11 LEDs) and 15 mm
intervals (16×16 LEDs) in a 150×150 mm central portion of a 200×200 mm
glass-epoxy base plate that was painted white. The sample stimuli were
lit using a DC source, and there were 3 kinds of illumination levels at
a distance of 200 mm from the sample stimuli in a direction
perpendicular to the stimulus: 4500 lx, 2700 lx, and 900 lx. In
addition, sample stimuli were also mounted in a 150×150 mm central area
of a white 450×450 mm holder. On the other hand, a flat surface light
source with two 8-W inverter-type cold-cathode fluorescent lamps (from
Fujicolor: Color Illuminator Pro B4) was used as a reference stimulus
(luminance: 1500 cd/m^2^, correlated color temperature: about 5000 K),
which was also mounted in a 150×150 mm central area of a white 450×450
mm holder.

FIG. 13.5.1 shows a schematic view of the test environment used by test
subjects to evaluate visual stress. All of the test subjects, i.e. nine
students of the Chiyo University, had normal vision (including corrected
vision). The lab where testing was conducted was shaded, and the test
subjects were placed inside a 900×900×1800 mm darkroom inside, with the
stimuli presented only at estimation time. In addition, the viewing
distance was 1200 mm both for the sample stimuli and for the reference
stimulus. Closable 300×300 mm windows were provided in the darkroom,
both in the direction of the sample stimuli and that of the reference
stimulus, with the stimuli shown through the windows to the test
subjects, whose faces were securely positioned.

The magnitude estimation method was used for scaling the magnitude of
the psychological effects produced by the sample stimuli. The test
subjects were instructed to use the magnitude of the effect produced by
the reference stimulus as 100 points and employ it as a reference in
estimating the magnitude of the effects produced by the sample stimuli.
The sample stimuli were presented for 2 seconds in random order, with
three parameters measured at such time: "glare," "discomfort," and
"pleasantness." When possible, we tried to present one set (two kinds of
stimulus samples × three kinds of illumination levels × three parameters
of measurement = 18 estimations) per day, repeating the test a total of
5 times per person.

![](media/image234.png){width="3.9881944444444444in" height="2.6625in"}

FIG. 13.5.1 Schematic view of test environment

\[Keys, FIG. 13.5.1\]

(Drawing on right, top to bottom) Sample stimulus; Reference stimulus

Next, we attempted to determine the degree of "glare" sensation
depending on the directionality characteristics and, in order to confirm
psychological influence due to spatial arrangement factors, we conducted
a study using samples of integrated light sources utilizing
surface-mounted type LEDs with relatively low directionality
characteristics. We prepared three kinds of surface-mounted sample
stimuli, in which commercially available surface-mounted white LEDs (2×3
mm, angle of visibility: 110º) were arranged in a 300×300 mm central
portion of a 350×350 mm glass-epoxy base plate painted white in matrix
form at intervals of 5 mm (60×60 diodes), 10 mm (30×30 diodes), and 20
mm (15×15 diodes). The sample stimuli were lit using a DC source to
provide the illumination levels (200 mm away, in a direction normal to
the stimulus) shown in Table13.5.1 and testing of the visual stress
exerted on the subjects was carried out in the same manner as in the
above-described method. In addition, the magnitude of the reference
stimulus at such time was made closer to that of the sample stimuli by
using a 260×260 mm central area and securing it to a white 450×450 mm
holder.

  ----------------- ----------------- ----------------- -----------------
  Sample stimuli    Illumination                        
                    levels of sample                    
                    stimuli (per 1                      
                    LED)                                

  5 mm interval     20160 lx (5.6 lx) 5040 lx (1.4 lx)  1260 lx (0.35 lx)

  10 mm interval    5040 lx (5.6 lx)  1260 lx (1.4 lx)  315 lx (0.35 lx)

  20 mm interval    1260 lx (5.6 lx)  315 lx (1.4 lx)   78.8 lx (0.35 lx)
  ----------------- ----------------- ----------------- -----------------

Table 13.5.1 Illumination levels of surface mounted-type sample stimuli

### Results and discussion (environmental compatibility evaluation research)

FIG. 13.5.2-(1) shows estimations of the parameters depending on changes
in the illumination of bullet-type sample stimuli. It was found that the
estimations of "glare" and "discomfort" increased with increasing
illumination, whereas the estimations of "pleasantness" tended to
decrease as the illumination level increased. Furthermore, it was found
that the estimations of "glare" were generally higher when the interval
between LEDs was 10 mm than when it was 15 mm. This is believed to be
due to the fact that when the interval was 10 mm, the density of LED
arrangement was higher and the surface area of the light-emitting
portion was larger at the same illumination level, which influenced the
sensation of "glare" felt by the eye. On the other hand, no influence
due to differences between the LED intervals was observed for
"discomfort" and "pleasantness," with both exhibiting the same
tendencies in all cases. This was believed to be strongly related to the
directionality characteristics of the bullet LEDs, with light close to
"beam light" with a visibility angle of 20º being an extremely strong
stimulus for the test subjects, who could not differentiate the
influence of the differences in the LED interval. In this connection, by
switching from integrated light source samples utilizing bullet LEDs
characterized by high directionality characteristics to integrated light
source samples based on surface-mounted LEDs of relatively low
directionality characteristics, we conducted visual stress estimation
experiments in the same manner as above.

![](media/image235.png){width="5.985416666666667in"
height="2.0993055555555555in"}

FIG. 13.5.2-(1) Estimations vs. illumination levels of bullet-type
sample stimuli

\[Keys, FIG. 13.5.2-(1)\]

(Across top, left to right) Glare Discomfort Pleasantness

FIG. 13.5.2-(2) shows estimations of the parameters depending on
illumination levels per LED in case of surface mounted type sample
stimuli. In addition, FIG. 13.5.2-(3) shows the dependence of the
estimations of "glare" and "discomfort" on the level of illumination of
sample stimuli. Although the above data confirmed differences in
estimation values for various parameters depending on illumination
levels per 1 LED between sample stimuli with different LED intervals, in
terms of the dependence of the estimation value on the illumination
level of the sample stimuli taken as a whole, no differences in the
estimation values depending on the LED interval were observed, and
"glare and "discomfort" were conjectured to exhibit a tendency to change
logarithmically with respect to the illumination level. Thus, we
attempted to corroborate the logarithmic change tendency by means of
regression analysis. In addition, as a result of conducting variance
analysis of the measured estimation values, it was found that the main
effects of integration density originating from the LED intervals and
the illumination level per LED were statistically significant for all
the parameters. On the other hand, for any of the parameters, no
statistically significant differences were revealed in estimation values
depending on the illumination level of the sample stimuli taken as a
whole.

![](media/image236.png){width="5.996527777777778in"
height="1.7180555555555554in"}

FIG. 13.5.2-(2) Estimation values of surface mounted type sample stimuli
(depending on illumination level per 1 LED)

\[Keys, FIG. 13.5.2-(2)\]

(Across top, left to right) Glare Discomfort Pleasantness

![](media/image237.png){width="5.985416666666667in"
height="2.6729166666666666in"}

FIG. 13.5.2-(3) Dependence of estimation value on illumination level for
surface mounted type sample stimuli

\[Keys, FIG. 13.5.2-(3)\]

(Across top, left to right) Glare Discomfort

FIG. 13.5.2-(4) shows the correlation between "glare" and "discomfort."
Regression analysis results showed that for "glare" the correlation
coefficient was 0.972 and the explanatory ratio was 94.5%, whereas for
"discomfort" the correlation ratio was 0.934 and the explanatory ratio
was 87.2%, and there was a very strong linear relationship in both. In
other words, it was confirmed that the estimation values of "glare" and
"discomfort" were not related to the integration density of LEDs and
were characterized by exhibiting a tendency towards logarithmic change
with respect to the illumination level of the entire sample stimuli.
Furthermore, as a result of subjecting the relationship between the two
to correlation analysis, it was confirmed that there was a very strong
positive correlation between the two, with a correlation coefficient of
0.944 and an explanatory ratio of 89.2%. In other words, it was
confirmed that the estimation of the psychological effects of "glare"
and that of "discomfort" strongly influenced each other, and, as a
result, the estimation results for "glare" and "discomfort" were
similar.

On the other hand, regarding the parameter of "pleasantness," we used
correlation analysis to probe the relationship between "pleasantness"
and "glare" and between "pleasantness" and "discomfort." As a result, it
was found that in the former case the correlation coefficient was −0.727
and the explanatory ratio was 52.8% and in the latter the correlation
coefficient was −0.802 and the explanatory ratio was 64.4%, with
"pleasantness" having a negative correlation with respect to both
"glare" and "discomfort." FIG. 15.5.2-(5) shows the correlation between
"pleasantness" and "discomfort," but the correlation observed here was
not as strong as in the case of "glare" and "discomfort." For this
reason, it is believed that "pleasantness" and "discomfort" are not in a
directly opposed relationship. This indicates that the relationships of
"pleasantness" to "glare" and that of "discomfort" to "glare" do not
yield completely opposed results. Therefore, it is necessary to proceed
with further clarification of the parameter of "pleasantness."

![](media/image238.png){width="5.986805555555556in"
height="2.4652777777777777in"}

Left: FIG. 13.5.2-(4) Correlation between "glare" and "discomfort"

\[Keys, FIG. 13.5.2-(4)\]

(X-axis) Discomfort

(Y-axis) Glare

Right: FIG. 13.5.2-(5) Correlation between "pleasantness" and
"discomfort"

\[Keys, FIG. 13.5.2-(5)\]

(X-axis) Discomfort

(Y-axis) Pleasantness

As described above, as a result of estimation of visual stress
experienced by test subjects using various integrated light sources such
as bullet type LEDs and surface-mounted type LEDs, it was found that
with the increase in the illumination level of the sample stimuli,
"glare" and "discomfort" increased and there was a strong correlation
between the two. However, influence due to the LED intervals could not
be confirmed, and it was not possible to establish its relationship to
the visual stress generated by two-dimensional matrix-like arrangements
of LED point light sources. Results of previous research into visual
stress, in which healthy individuals were used as test subjects,
indicate that, as far as geometric patterns are concerned, the
discomfort increases when the spatial frequency is in the range of about
1\~5 cpd. By contrast, as far as the LED arrangement patterns of the
surface mounted type sample stimuli used in the present study are
concerned, in terms of the spatial frequencies obtained when scanned in
the horizontal direction, in case of the 5-mm interval, 10-mm interval,
and 20-mm interval, we had frequencies of 4 cpd, 2 cpd, and 1 cpd,
respectively, among which no differences in the estimations of
discomfort were observed. Considering that the duty ratios of the sample
stimuli were also changing, the fact that no differences between the
estimations were observed is extremely interesting. In connection with
this result, we should consider factors such as differences in the
properties of the stimuli themselves (the ratio of illuminance at the
edge of the pattern) and in the future it is necessary to conduct
further investigations based on this point of view. In addition,
although last year we conducted investigations using a theoretically
derived estimation formula, but this study has shown that corroboration
by means of psychophysical testing using actual test subjects is
extremely important in applied studies such as the environmental
compatibility evaluation of illumination.

## Summary

In this year's research, we determined the optical characteristics of
YAG phosphors based on actual measurements and conducted an
investigation into reflective LED light source devices combining blue
LEDs with YAG phosphors using optical simulation. It was confirmed that
the results of the simulation were close to the optical characteristic
values of real-life prototype models fabricated using commercially
available LEDs, which proved that the use of the present simulation
technique was efficient in the optical control and design optimization
of light source devices. In addition, based on the results of the
simulation, it was found that light source devices of the reflective
architecture investigated in the current study were characterized by
optical energy losses of 70% or more, which makes improvements aimed at
increasing the efficiency of the devices by suppressing the optical
energy losses an important task for the future.

On the other hand, by combining currently available white LEDs with red
and green LEDs, we conducted an investigation into a correlated color
temperature alteration technique and also examined color rendering
properties obtained in the process. As a result of studying the spectral
distributions of three kinds of LEDs based on the method of additive
mixture of color stimuli, we obtained correlated color temperature
conditions for incandescent light-bulb (L: Tcp=2800 K), warm white (WW:
Tcp=3370 K), white (W: Tcp=4246 K), daylight white (N: Tcp=5026 K), and
daylight (D: Tcp=6186 K) by setting the emission intensity ratio of the
green LEDs to approximately 20% and controlling the emission intensity
ratios of the white LEDs and red LEDs. In addition, it was confirmed
that using daylight conditions resulted in high color rendering
properties of Ra=95, which exceeded the Ra=85 of currently available
white LEDs.

In our research into environmental compatibility evaluation, we
conducted psychophysical testing using test subjects to determine the
psychological influence exerted by matrix-like two-dimensionally
arranged integrated LED light sources. As a result, we were unable to
determine how visual stress depended on the spatial factors of the LED
point light sources. However, in the experiments, it was found that
there was a strong correlation between the "glare" and "discomfort," and
their estimations were proportionate to a logarithm of the illumination
of the sample stimuli.

## Future plans

This year's results have shown that optical energy losses of 70% or more
occur in LED light source devices utilizing a reflective architecture;
thus, in the future, we plan to increase the efficiency of LED light
source devices by making the most of the optical simulation technique
that has been developed. In addition, we will try and elucidate the
heat-emitting structure of LED light source devices in search of
possibilities of further efficiency improvements.

On the other hand, in the area of environmental compatibility evaluation
research, we will conduct more detailed psychophysical testing to
evaluate the "glare," "discomfort," and "pleasantness" of
LED-illuminated environment while trying to clarify the characteristics
of the influence exerted by discomfort glare and the influence of visual
stress originating from factors related to the spatial arrangement of
point light sources.

## References

\[1\] M. Miyazaki, Journal of Illuminating Engineering Institute of
Japan, 81, 558 (1997).

\[2\] T. Mukai, Journal of Illuminating Engineering Institute of Japan,
83, 447 (1999).

\[3\] T. Taguchi, Display (monthly), No. 3, 1 (2000).

\[4\] Y. Hirota, Materials of 2000 All-Japan Conference of Illuminating
Engineering Institute of Japan (2000), pp. 277-278.

\[5\] K. Bando, S. Koda, M. Kohashi, T. Nishioka, A. Sakano, K.
Nagamine, K. Sakai, K. Arai, Y. Kiyomizu, Preprints of Joint Symposium
on Fluorescent Materials No. 286 (2001), pp. 17\~24.

\[6\] N. Simazaki "Lighting Design Encyclopedia, Planning and Design
Techniques for Lighting," Sangyo Chosakai (1986), p.70.

\[7\] Illuminating Engineering Institute of Japan, "Lighting Handbook,"
Ohmusha (1987) pp. 64-65.

\[8\] N. Ohta, "Color Engineering," Tokyo Denki University Press (1993).

\[9\] K. Naya, "Industrial Color Science," Asakura Shoten (1980).

\[10\] S. Kaneya, "Optics," 13, 331 (1984).

\[11\] A. J. Wilkins, "Visual Stress," Oxford University Press (1995)

\[12\] S. S. Stevens, American Psychology, 17, 29 (1962)

\[13\] S. S. Stevens, "Psychophysics: Introduction to Its Perceptual,
Neural, and Social Prospects," John Wiley (1975).

\[14\] M. Ohyama, "Experimental Psychology," Tokyo University Press
(1984).

\[15\] N. Ichikawa, "Introduction to Measurement Methods in Psychology,"
Science (1991).

\[16\] H. Hibino, Materials of Symposium of P&P Research Branch of
Japanese Cognitive Science Society, 26, 19 (1998).

\[17\] H. Hibino and Y. Sano, Perception, 28, Supplement (1999) p. 94.

\[18\] H. Hibino, A. Itoh, Published Articles of 63^rd^ Conference of
Japanese Psychological Association (1999) p. 372.

\[19\] H. Hibino, Y. Sano, "Animation Research," 2, 5 (2000)

\[20\] H. Hibino, K. Tsubota, Published Articles of 64^th^ Conference of
Japanese Psychological Association (2000) p. 372.

# Research and development of light-emitting-region conversion illumination systems

## Achievements in 1998\~1999

Illumination systems utilizing LEDs are facing two mutually exclusive
requirements:  integrated arrangement of LEDs for low cost and
improvement maintenance, and  providing two-dimensional light-emitting
regions for improved visibility and pleasantness.

In the course of the present study, we conducted research and
development in the area of optimization of light-emitting-region
conversion optical systems as a first step towards making
light-emitting-region-conversion illumination systems a reality.

In 1998\~1999, as a first step towards making light-emitting region
conversion-type illumination systems a reality, we compared three
representative systems and identified problems associated with each one
of them. The results are summarized in Table 14.1.1.

Based on the comparative results shown in Table 14.1.1 as well as the
cost reduction and improved maintenance due to the fact that the light
sources can be concentrated in one point, it is believed that the light
guide panel system is more advantageous.

The uniformity of luminance of an illumination system is not commonly
used as a criterion for the evaluation of illumination systems. However,
in order to better illustrate the advantages of LED illumination as a
new type of illumination system, or to try to develop a unique niche in
the illumination market for LED illumination that would be different
from general illumination, it is important to continue development of
illumination systems capable of utilizing this unique effect of LED
illumination.

Furthermore, in the present study, we developed an original vector
radiation coupling theory \[1, 2\], built a prototype of a 4-inch light
guide panel, and confirmed its effects. Based on these results, we
decided to proceed with research and development aimed at practical
realization of light guide panel systems.

+---------+------------------+---+------------------+---+-------------+
|         | **Light guide    |   | **Direct         |   | **Indirect  |
|         | panel system**   |   | downlight        |   | system**    |
|         |                  |   | system**         |   |             |
+---------+------------------+---+------------------+---+-------------+
|         | Light guide      |   | LED              |   | Reflector   |
|         | panel            |   |                  |   | plate       |
|         |                  |   | Reflector plate  |   |             |
|         | LED module       |   |                  |   |             |
+---------+------------------+---+------------------+---+-------------+
|         | ![]              |   |                  |   |             |
|         | (media/image239. |   |                  |   |             |
|         | png){width="5.02 |   |                  |   |             |
|         | 7083333333334in" |   |                  |   |             |
|         | height="1.5416   |   |                  |   |             |
|         | 666666666667in"} |   |                  |   |             |
+---------+------------------+---+------------------+---+-------------+
| **Light | 92%              | 9 |                  | 9 |             |
| use     |                  | 5 |                  | 6 |             |
| effic   |                  | % |                  | % |             |
| iency** |                  |   |                  |   |             |
+---------+------------------+---+------------------+---+-------------+
| **Unifo | 100% (in         | 1 |                  | − |             |
| rmity** | theory)\*1       | 0 |                  |   |             |
|         |                  | 0 |                  |   |             |
|         | 70% (sample)     | % |                  |   |             |
|         |                  | ( |                  |   |             |
|         |                  | i |                  |   |             |
|         |                  | n |                  |   |             |
|         |                  | t |                  |   |             |
|         |                  | h |                  |   |             |
|         |                  | e |                  |   |             |
|         |                  | o |                  |   |             |
|         |                  | r |                  |   |             |
|         |                  | y |                  |   |             |
|         |                  | ) |                  |   |             |
|         |                  | \ |                  |   |             |
|         |                  | * |                  |   |             |
|         |                  | 2 |                  |   |             |
+---------+------------------+---+------------------+---+-------------+
| **Thic  | 10 mm            | 5 |                  | 1 |             |
| kness** |                  | 0 |                  | 0 |             |
|         |                  | m |                  | 0 |             |
|         |                  | m |                  | m |             |
|         |                  |   |                  | m |             |
+---------+------------------+---+------------------+---+-------------+
| **Advan | Higher degree of | S |                  | − |             |
| tages** | freedom in       | l |                  |   |             |
|         | design of        | i |                  |   |             |
|         | integrated       | g |                  |   |             |
|         | structures made  | h |                  |   |             |
|         | up of light      | t |                  |   |             |
|         | sources          | l |                  |   |             |
|         |                  | y |                  |   |             |
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+---------+------------------+---+------------------+---+-------------+
| **Pro   | \*1 Pattern      | \ |                  | − |             |
| blems** | visibility       | * |                  |   |             |
|         |                  | 2 |                  |   |             |
|         |                  | N |                  |   |             |
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+---------+------------------+---+------------------+---+-------------+
| **      | −                | − |                  | C |             |
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+---------+------------------+---+------------------+---+-------------+

Table 14.1.1. light-emitting region conversion-type illumination
systems.

## Summary of research and development in 2000

This year, to conduct our design work with account taken of possible
mass production of light guide panels in the future, we conducted
verification of improvements in terms of light use efficiency from the
standpoint of tolerances used in the fabrication process. Specifically,
using a computer simulation model, we calculated the correlation between
parameters, for which unpredictable fluctuations are expected in the
process of fabrication (sagging during formation of pattern shapes, LED
installation position, etc.), and the light use efficiency, etc., in
order to obtain the tolerances of the respective parameters.
Furthermore, we proposed robust systems that can tolerate variation
during fabrication, such as imparting a trapezoidal shape to the
pattern, positioning of LEDs in the corner, etc., and confirmed their
effects using computer simulation models.

In addition, in order to investigate high directivity applications such
as footlights and spotlights, we studied the effects of directivity
conversion systems utilizing prism sheets. This year, we used the
prototype of the 4-inch light guide panel to confirm that the
directivity of the emanating light is improved by the installation of a
prism sheet in the light guide panel. Furthermore, it was confirmed that
the uniformity of the luminance of the light guide panel did not
decrease after the installation of the prism sheet.

## Verification of improved efficiency of coupling elements

The basic configuration of a light guide panel system, based on the
results of investigations conducted prior to last year, is shown in Fig.
14.3.1 (a), (b).

This system is made up of a reflector plate/diffuser plate, a light
guide panel, and an LED module, which consists of an LED chip and a
white frame. In addition, light guide panel coupling elements, which
direct light from the LED module into the light guide panel, and
radiation coupling elements (pattern), which cause the light guided
through the light guide panel to be released in the direction of light
emanation, are provided, respectively, in the portion, where the light
emitting module and the light guide panel are coupled together, and on
the surface of the reflector plate side of the light guide panel.

![](media/image240.png){width="3.875in" height="2.3055555555555554in"}

FIG. 14.3.1 Basic configuration

\(a\) Oblique view

\(b\) Cross sectional view.

\[Keys, FIG. 14.3.1\]

\(a\) LED module; Reflector plate; 0º; Light guide panel; Diffuser plate

\(b\) LED module Reflector plate Radiation coupling elements (pattern);
Light guide plate; Diffuser plate

The light guide panel coupling elements and radiation coupling elements
are suggested as the most important factors determining the light use
efficiency of the light guide panel system. If we designate the
luminance distribution of the light coming out of the light guide panel
as P(r,Θ), the light emanating from the light guide panel coupling
elements into the light guide panel as Φ(Θ), and the distribution of the
radiation coupling elements as H(r,Θ), then the respective correlation
can be expressed in the following manner:

P(r,Θ) = F\[H(r,Θ), Φ(Θ)\] (1)

In case of an ordinary light guide panel system, light propagates while
undergoing diffusion, and the function F, which determines the coupling
of H and Φ, cannot be given analytically. However, in case of the vector
radiation coupling system proposed in this study, light from the
coupling section propagates through the light guide panel in a linear
fashion. For this reason, light propagating in direction Θ is influenced
only by the radiation coupling pattern in direction Θ, and F \[*sic*; *−
trans*.\] is expressed by equation (2).

P(r,Θ) = H(r,Θ)⋅Φ(Θ) (2)

Equation (2) means that the influence exerted by the light guide panel
coupling elements and influence exerted by the radiation coupling
elements can be considered independently. For this reason, in this study
we conducted investigations into light guide panel coupling elements and
radiation coupling elements separately.

### Methods and means

#### Simulation model for radiation coupling elements

The simulation of the radiation coupling elements essentially consisted
in using simulation tools to reproduce the portion emanating from the
light guide panel coupling elements in the basic configuration shown in
Fig. 14.3.1 and investigating its characteristics using a ray tracing
simulation program. However, in case of ray tracing models that express
even shapes that take into account fabrication tolerances, reproduction
of the shapes requires building complicated shapes on a computer, and
this, in turn, requires a huge amount of calculation time because of the
huge number of points in the model.

Thus, this year, in order to carry out the simulation within realistic
calculation time limits, we used only part of a model obtained by
selecting only a direction in the vicinity of Θ=0º in the basic
configuration shown in Fig. 14.3.1. By doing so the computation load was
reduced approximately 100 times. The simulation model used for radiation
coupling elements is shown in Fig. 14.3.2

![](media/image241.png){width="4.365972222222222in"
height="2.5125in"}Fig. 14.3.2 Simulation model

\[Keys, Fig. 14.3.2\]

0 extraction

Extraction

As was described above, in the vector radiation coupling system, light
from the light guide panel coupling portion propagates in a linear
fashion to other edge surfaces, with the respective angular directions
being independent and the behavior of light being practically identical.
Thus, because the correlation between fine shape changes of the
radiation coupling pattern and the extraction efficiency, which is the
purpose of the present simulation, is believed to be independent of the
direction, in which light emanates from the light guide panel coupling
elements, we decided to use the results obtained for direction Θ=0º as
representative values.

The input parameter used in the present simulation was the pattern shape
of the radiation coupling elements and the output was the amount of
light emanating upwards. Other parameters were as follows.

  -------------------- --------------------------------------------------
  Incident light       measured light intensity distribution of incident
                       light

  Pattern density      density, such that the luminance is constant in
                       any location of the light guide panel (low density
                       in the vicinity of the light source, high density
                       at edge on the opposite side)

  Light guide panel    10 mm
  thickness            
  -------------------- --------------------------------------------------

#### Simulation model for light guide panel coupling elements

In models of light guide panels used prior to last year, the
light-emitting module was located in the center at one of the edges of
the light guide panel. However, we are aware of the fact that the
efficiency of the light guide panel changes if the position of the
light-emitting module is changed. Typically, the following positions are
considered as positions at which the light-emitting module is installed:

  ----------------- -----------------------------------------------------
   At the edge     (Fig. 14.3.3 (a))

   In the corner   (Fig. 14.3.3 (b))
  ----------------- -----------------------------------------------------

![](media/image242.png){width="4.353472222222222in"
height="2.8291666666666666in"}Fig. 14.3.3 Position of light-emitting
module

\[Keys, Fig. 14.3.3\]

\(a\) At the edge

**Spread** of incident light: about ±90º

\(b\) In the corner

Spread of incident light: about ±45º

In case of , in order to convert the incident light from the light
source to cover the entire surface of the light guide panel, a spread of
about ±90º is necessary in the light guide panel coupling elements. On
the other hand, in case of , a spread of about ±45º in the light guide
panel coupling elements is enough. In these two models  and , we used
ray tracing simulation to determine the efficiency of light use obtained
when the incident light is spread over the entire surface of the light
guide panel and determined the optimum installation position for the
light-emitting module.

### Results and discussion

#### Radiation coupling elements

[a. Effects of pattern shape fabrication tolerance]{.underline}

Fig. 14.3.4 shows a pattern in the shape of a cylindrical lens, which
was proposed in the past. The parameters used in this simulation were as
shown in Fig. 14.3.4. Defects due to fabrication irregularities in the
pattern of Fig. 14.3.4 included edge R, which was caused by deformation
of the edge section during injection molding, deformation of the width
and height of the pattern due to the shrinkage and expansion of the
resin, and variations in the refraction coefficient caused by variations
in the characteristics of the resin and by differences in the process of
curing.

![](media/image243.png){width="5.9875in" height="1.7194444444444446in"}

Fig. 14.3.4 Cylindrical lens-shaped pattern

\[Keys, Fig. 14.3.4\]

Edge R

Light guide panel

Height

Bottom angle: about 50º

Air

Width: 11 μm

First of all, we verified the correlation between changes in the edge R
and the extraction efficiency.

The correlation between the edge R and extraction efficiency for the
cylindrical lens-shaped pattern shown in Fig. 14.3.4 is shown in Fig.
14.3.5. The cylindrical lens height H was selected as the parameter.

According to the results of a simplified injection molding test, it was
found that in case of a fine pattern of the specified size, the edge R
could reach approximately 2 *μ*m. In Fig. 14.3.5, the value of R=2 *μ*m
gives us a decrease, as compared with the ideal state, of about 22% in
case of H=2.8 *μ*m and about 38% in case of H=3.5 *μ*m. This edge
irregularity does not occur uniformly and is subject to considerable
variation depending on the fabrication conditions, etc. Thus, based on
these results, it was found that in case of a cylindrical lens-shaped
pattern, it was not resistant to edge R variation and could bring about
a decrease in the extraction efficiency and considerable luminance
non-uniformity.

![](media/image244.png){width="4.524305555555555in"
height="3.0243055555555554in"}Fig. 14.3.5 Effects of cylindrical lens
edge R on extraction efficiency

\[Keys, Fig. 14.3.5\]

(X-axis) Edge R due to injection molding (μm)

(Y-axis) Decrease in luminance (relative value): Edge R=0 as 1

(Box) ♦ Cylindrical H=2.8

 Cylindrical H=3.5

For this reason, we proposed a trapezoidal pattern shown in Fig. 14.3.6
as a pattern resistant to edge R variation. Details related to the
selection of the trapezoidal pattern are described below. In the same
manner as with the cylindrical lens pattern, occurrence of fabrication
irregularities is possible in case of the trapezoidal pattern.

![](media/image245.png){width="5.986805555555556in"
height="1.9652777777777777in"}

Fig. 14.3.6 Trapezoidal lens-shaped pattern

\[Keys, Fig. 14.3.6\]

Edge R

Height H

Bottom angle α (about 50º)

Width: 11 μm

![](media/image246.png){width="4.280555555555556in"
height="3.158333333333333in"}

Fig. 14.3.7 Effects of trapezoidal lens edge R on efficiency

\[Keys, Fig. 14.3.7\]

(X-axis) Edge R due to injection molding (μm)

(Y-axis) Decrease in luminance (relative value): Edge R=0 as 1

(Box) ♦ Trapezoidal H=2.8

 Trapezoidal H=3.5

The results of the simulation allowed us to determine the correlation
between the edge R of the trapezoidal pattern and extraction efficiency,
which is shown in Fig. 14.3.7. As a result, we can see that while there
was a decrease of not less than 15% in case of the cylindrical
lens-shaped pattern, in case of the trapezoidal pattern, the
corresponding decrease was approximately 10%. In addition, it was found
that in case of a trapezoidal pattern with an H=3.5 *μ*m, the decrease
in the extraction efficiency was maintained within 16%. These results
indicate that the trapezoidal pattern exhibits a smaller decrease in the
luminance at the same variation of the edge R and is a more robust
pattern.

Thus, in connection with the conducted verification, we focused our
further efforts on the study of the trapezoidal pattern.

[b. Theoretical verification]{.underline}

Next, we will consider the theoretical basis of improvements due to the
above-described trapezoidal pattern.

First of all, we will discuss the mechanism of light extraction
depending on the pattern by using the simplified model shown in Fig.
14.3.8. In order for light propagating throughout the light guide panel
to be extracted from the light guide panel by the pattern, during
reflection from the pattern it has to be subject to changes in the
propagation direction of 45º or more (up to an angle exceeding total
reflection angle on the top face of the light guide panel). As a result
of refraction, in some cases light is extracted towards the reflector
plate side as well, but here, in order to facilitate possible changes,
we will only consider extraction based on reflection.

![](media/image247.png){width="5.988888888888889in" height="0.925in"}

Fig. 14.3.8 Differences between efficient regions depending on pattern
shape

\[Keys, Fig. 14.3.8\]

(Left) Height H=3.5

2.8 μm

(α=22.5º or larger)

(Right) Height H=3.5

4.5 μm

(α=22.5º or larger)

In order to bring about these changes of 45º or more, the bottom angle α
on the light source side of the pattern has to be 22.5º or more. Here,
calculation of the length of the inclined surface with an angle
exceeding α=22.5º among the inclined surfaces on the light source side
of the cylindrical lens-shaped pattern and trapezoidal pattern gives 2.8
*μ*m for the cylindrical lens-shaped pattern and 4.5 *μ*m for the
trapezoidal pattern (on the assumption that H=3.5 *μ*m in both cases).

Changes in the edge R take place to the same extent for the respective
patterns. Thus, if we consider a case, in which due to the sagging of
the edge R the above mentioned inclined surface effective in terms of
extraction is decreased 1 *μ*m, in case of the cylindrical shape pattern
the decrease in the extraction efficiency is 1/2.8=35%, and in the case
of a trapezoidal pattern the decrease is 1/4.5=22%, showing that the
trapezoidal pattern, which has more effective inclined surface, is more
robust with respect to edge R variation.

![](media/image248.png){width="3.6222222222222222in"
height="2.6708333333333334in"}Also, Fig. 14.3.9 shows results obtained
by studying the correlation between the bottom angle α of the
trapezoidal pattern and the extraction efficiency. From the results, it
is evident that changes in the inclined surface of the trapezoidal
pattern exert considerable influence on the extraction efficiency.

Fig. 14.3.9 Relationship between bottom angle α and efficiency

\[Keys, Fig. 14.3.9\]

(X-axis) Bottom angle α (deg)

(Y-axis) Relative extraction efficiency (%)

Summing up the above, it is evident that factors that exert strong
influence on the extraction efficiency include the bottom angle on the
light source side of the pattern and the length of the effective
inclined surface. Thus, when designing a pattern of high robustness, a
pattern should be chosen, in which the bottom angle is not likely to
deviate from the design value and the effective inclined surface is
long. However, it is also necessary to pay attention to other
constraints concerning the size of the pattern, such as pattern
visibility, etc.

Next, a correlation between variation of the refractive coefficient
believed to be caused by resin irregularities or curing condition
irregularities and the extraction efficiency is shown in Fig. 14.3.10.
As can be seen from the figure, the actual variation of the coefficient
of refraction is about 0.005 at the most, and, for this reason, it has
almost no effect on characteristics.

![](media/image249.png){width="3.158333333333333in"
height="2.7680555555555557in"}

Fig. 14.3.10 Relationship between bottom angle α \[sic; − trans.\] and
efficiency

\[Keys, Fig. 14.3.10\]

(X-axis) Coefficient of refraction of light guide panel

(Y-axis) Decrease in luminance (relative value)

Results for other parameters are shown in Table 14.3.1.

Because there are no other parameters that would exert adverse
influence, no particular attention should be paid to these parameters in
the design stage. For this reason, here their discussion is omitted.

+----------------------+-----------------------+-----------------------+
| **Parameter**        | **Verification        | **Calculation         |
|                      | region**              | results:**            |
|                      |                       |                       |
|                      |                       | **Amount of deviation |
|                      |                       | from design value     |
|                      |                       | (%)**                 |
+----------------------+-----------------------+-----------------------+
| **Rear bottom angle  | 50+7                  | −0.6                  |
| (deg)**              |                       |                       |
+----------------------+-----------------------+-----------------------+
|                      | 50+7                  | 0                     |
+----------------------+-----------------------+-----------------------+
| **Pattern width      | 10+0.5                | −2                    |
| (*μ*m)**             |                       |                       |
+----------------------+-----------------------+-----------------------+
|                      | 10−0.5                | 7                     |
+----------------------+-----------------------+-----------------------+

Table 14.3.1 Relationship between other parameters and efficiency

\(c\) Light guide panel coupling elements

Results of ray tracing for different positions of the LED module are
shown in Fig. 14.3.11. In this figure, we used the symbol "" to
designate the LED, which is positioned at one of the edges or in a
corner of the light guide panel. Light emitted by the LED is converted
by the light guide panel coupling elements into the desired angular
distribution. The distance from the light guide panel coupling elements
corresponds to the luminous energy for each angle.

In Fig. 14.3.11, the luminous energy E2, which is shown with a thick
black line, is the ideal luminous energy necessary for each angle. If we
want to make constant the luminous energy emitted from the
light-emitting face in any given position of the illumination system,
the larger the distance from the LED to the edge on the opposite side,
the more light will be necessary. In the figure, the distance from the
light guide panel coupling elements to the corner on the opposite side
is the longest, and the ideal luminous energy for the corresponding
angle is greater.

However, it is difficult to bring the luminous energy emitted from the
light guide panel coupling elements in perfect correspondence with the
ideal luminous energy. More realistically, it is easier to ensure proper
control if we create an elliptical distribution of the emanating light.
It is plotted in the figure as E1.

In the luminous energy E1 emanating at a certain angle from the light
guide panel coupling elements, the E1−E2 portion of the luminous energy
becomes unnecessary. On the other hand, the amount of emanating light
can be increased by increasing the density of the pattern if we want to
emphasize the efficiency of light use, but in this case the uniformity
of luminance will deteriorate.

Results regarding light use efficiency E2/E1 in case of uniform
luminance obtained by computer simulation are shown in Table 13.3.2.

  ----------------------- ----------------------- -----------------------
                           Lateral position       Corner position

  Efficiency E2/E1        83%                     95%
  ----------------------- ----------------------- -----------------------

Table 13.3.2 LED module position and light use efficiency

As can be seen from the above results, the efficiency of light use is
higher when the LED module is positioned in the corner (if we want to
emphasize the efficiency of light use, positioning the module in the
corner will decrease the non-uniformity of luminance). Thus, in the
future, when designing prototypes of illumination systems, positioning
the LED in the corner will be our first choice.

![](media/image250.png){width="4.282638888888889in"
height="6.4006944444444445in"}

Fig. 14.3.11 Simulation results for different positions of light
emitting module

\(a\)  Lateral position

\(b\)  Corner position

\[Keys, Fig. 14.3.11 (a)\]

(Left) Light that is not used.

(Right, top to bottom) Design distribution E1

Ideal distribution E2

## Conceptual design of directionality conversion system

### Methods and means (conceptual design of directionality conversion system)

When illumination systems are considered from the standpoint of energy
savings, it becomes important to consider conversion efficiency, which
means how efficiently light from the light source is used to illuminate
the desired illumination area at a uniform lighting level. In other
words, the light that is emitted outside the desired illumination area
is unnecessary, in other words, it is a waste of energy.

For example, in case of a footlight, from the standpoint of the purpose
of the illumination system, the area that has to be illuminated is
limited to the floor surface below the footlight. Specifically, if light
is emitted with a directivity of approximately ±15º, this will be enough
for footlight functionality. However, when you look at actual
illumination, you will find that light sometimes illuminates areas
greater than ±15º. Suppose, if inefficient lighting is used to
illuminate, for example, a ±60º area, then in terms of the magnitude of
the solid angle, such an area will be about 15 times larger than ±15º.
In other words, although the ratio of the luminance of light emitted in
the inefficient area to that of light emitted in the efficient area is
1/10, in terms of energy, more than half is emitted in the inefficient
area, which cannot be simply ignored.

Reduction of this value requires a certain technology for converting the
directivity of the light incident from the light source to the desired
directivity. In case of typical conventional illumination, one of the
most obvious approaches would be to improve directivity by installing
reflectors. Furthermore, especially in case of illumination systems
requiring high directivity, the shape of the reflectors is often
curvilinear (for example, parabolic). However, these systems, especially
curvilinear reflectors and such, are elements supposed to be used with
point light sources and cannot be applied to a light guide panel, which
is a surface light source, and, in addition, their use presents
difficulties because it cancels out one of the advantages of the light
guide panel system, i.e. the fact that it can be made very thin.

![](media/image251.png){width="3.8472222222222223in"
height="1.875in"}Fig. 14.4.1 Prism sheet

\[Keys, Fig. 14.4.1\]

(Top to bottom) Surface light source (light guide panel)

Prism sheet

The above-described reflectors are elements that convert directivity by
making use of light reflection. There is, however, a type of element
that converts directivity by making use of light refraction. Namely, as
shown in Fig. 14.4.1, a prism sheet is a sheet, on which tiny prisms are
molded in array form. Prism-like triangular shapes are formed on one
side of the sheet, and the coefficient of refraction of the prism sheet
is made higher than the coefficient of refraction of air. When light
from a surface light source incident on the top face of the sheet passes
through the sheet and emanates from the bottom face, the light is
subjected to refraction as shown in the figure by the prisms formed on
the bottom face, which improves its directivity. Optimizing the shape of
the prisms makes it possible to convert the directivity of light to the
desired directivity. In addition, the design can be made thinner by
reducing the size (height, pitch) of the prisms, which is why one can
expect prism sheets to find application as a directivity conversion
element in illumination systems of the light guide panel type.

In this study, we conducted simplified tests for the purpose of
confirming the applicability of the light guide panel/prism sheet
combination. As shown in Fig. 14.4.2, two prism sheets (two sheets were
necessary in order to improve directivity in the X direction and Y
direction) were installed on the light-emitting side of a 4-inch light
guide panel prototype, which we had previously fabricated, and a
luminance meter was installed below the light guide panel to measure
luminance in the center of the light guide panel. In addition, the light
guide panel could be rotated about the X axis. Using this setup, we
measured the distribution of the emanating light in the center of the
light guide panel.

![](media/image252.png){width="4.30625in"
height="2.926388888888889in"}Fig. 14.4.2 Directivity measurement system

\[Keys, Fig. 14.4.2\]

(Left) \*Measurements of luminance were conducted by rotating the light
guide panel about the X-axis

(Right, top to bottom) Surface light source (light guide panel)

Diffuser sheet

Prism sheet (for Y-axis)

Prism sheet (for X-axis)

Furthermore, in order to confirm whether the installation of the prism
sheets caused a deterioration in the uniformity of the luminance across
the light guide panel's light-emitting surface, we also measured frontal
luminance in five points: in the center and in the four corners of the
light guide panel. The points, in which the luminance was measured, are
shown in Fig. 14.4.3.

![](media/image253.png){width="3.1in" height="1.5666666666666667in"}Fig.
14.4.3 Uniformity measurement points

### Results and discussion (conceptual design of directionality conversion system)

Fig. 14.4.4 shows an emanating light distribution obtained when no prism
sheets are installed, and Fig. 14.4.5 shows an emanating light
distribution obtained with the prism sheets installed. As can be
understood from the figures, when the prism sheets were installed, the
directivity of the emanating light was improved up to ±17º. In addition,
Table 14.4.1 shows data obtained by subtracting frontal luminance from
Fig. 14.4.4 and Fig. 14.4.5. From the table, it can be seen that the use
of the prism sheets results in an improvement in the directivity and, at
the same time, a doubling of the luminance, so that energy can be
efficiently emitted across the desired area.

![](media/image254.png){width="5.988194444444445in"
height="2.522222222222222in"}

Left: Fig. 14.4.4 Directivity characteristic (without prism sheets)

\[Keys, Fig. 14.4.4\]

(X-axis) Degree (deg)

(Y-axis) Relative luminance (a.u.)

Right: Fig. 14.4.5 Directivity characteristic (with prism sheets
installed)

\[Keys, Fig. 14.4.5\]

(X-axis) Degree (deg)

(Y-axis) Relative luminance (a.u.)

  -------------------------- --------------------- -----------------------
                             Prism sheets          

                             None                  Installed

  Relative luminance         1.00                  2.05
  -------------------------- --------------------- -----------------------

Table 14.4.1 Luminance comparison

Furthermore, results of uniformity evaluation are shown in Table 14.4.2.
As the table shows, even though the uniformity slightly decreased from
13% to 12% when the prism sheets were installed, it was within the
tolerance of measurement. Thus, it was found that the prism sheets
caused practically no decrease in the uniformity of the luminance of the
light-emitting surface.

+-------------------------+---------------------+----------------------+
|                         | Relative luminance  |                      |
+-------------------------+---------------------+----------------------+
|                         | Prism sheets        | Prism sheets not     |
|                         | installed           | installed            |
+-------------------------+---------------------+----------------------+
|                        | 1.00                | 2.05                 |
|                         |                     |                      |
|                        | 1.05                | 2.08                 |
|                         |                     |                      |
|                        | 0.96                | 1.95                 |
|                         |                     |                      |
|                        | 1.08                | 2.05                 |
|                         |                     |                      |
|                        | 1.01                | 1.99                 |
|                         |                     |                      |
| Uniformity (%)          | 12                  | 13                   |
+-------------------------+---------------------+----------------------+

Table 14.4.2 Results of uniformity evaluation

It was found that, as shown above, if prism sheets were used in a light
guide panel system, it was possible to improve the directivity while
maintaining useful characteristics such as thin dimensions.

However, when the present directivity conversion system is applied to
actual illumination systems, it is necessary to decide whether the
present system needs to be used with account taken of the intended use
of the illumination system. The present system is useful in illumination
systems requiring high directivity, such as, for example, footlights,
table lamps, road illumination, etc. However, because in the case of
illumination systems such as signaling devices, signs, etc. the purpose
of the system is to convey information to people in the surrounding
vicinity, increasing directivity would actually be counterproductive,
and the use of the present system would not be appropriate in many
cases. Furthermore, it is believed that in case of indoor illumination,
the distribution of light emitted by an illumination system is
determined with account taken of the ergonomic properties, pleasantness,
and intended use (direct illumination, indirect illumination, etc.) of
the illumination, and corresponding decisions have to be made regarding
the appropriateness of the use of the present system.

## Summary

This year, in order to create a prototype of a light guide panel system
intended for actual illumination, we built and practically used optical
simulation tools using the ray tracing technique for design with account
taken of fabrication errors etc. believed to occur during mass
production. As a result, by elucidating the correlation between the
variability of the shape of the light guide panel coupling elements and
arrangement of radiation coupling elements in case of fabrication errors
etc. and the efficiency of energy conversion, we reached the following
conclusions.

1\. With respect to the pattern formed in the light guide panel, it was
found that a trapezoidal lens-shaped lens pattern exhibited a smaller
decrease in the extraction efficiency due to molding errors than the
cylindrical lens-shaped pattern, which had been proposed in the past.

2\. With respect to the position of the module, it was found that
positioning the module in a corner of the light guide panel provided for
a higher light use efficiency than positioning it at one of the edges of
the light guide panel.

3\. With respect to other parameters, it was found that they exhibited
high resistance to fabrication errors and required no special
countermeasures.

In addition, we determined and decided that using prism sheets as a
directivity conversion system for achieving further energy saving
effects in illumination systems requiring high directivity was the best
strategy in case of a light guide panel system and, after installing
such sheets in a prototype of a 4-inch light guide panel and measuring
the corresponding optical characteristics, reached the following
conclusions:

4\. Installing prism sheets improves the directivity of light emanating
from a light guide panel up to a directivity of ±17º in terms of the
half-value angle.

5\. Inserting a prism sheet does not reduce the uniformity, which is an
advantage of the light guide panel system.

## Plans for the future

Using the shape precision/energy conversion efficiency correlation
determined as a result of this year's investigations, next year, we will
focus on determining the problems expected to arise in the production
process and on their precision analysis with a view to develop actual
illumination systems. In the verification process, we will concentrate
mostly on the light guide panel coupling elements, which exert
considerable influence on the optical characteristics.

In addition, we will combine it with studies related to the basic
specification of the light guide panel system illumination device we
plan to fabricate during the final year.

[(1) Robust design]{.underline}

Previously, with a view to determine the general principles of the light
guide panel system, we have fabricated a prototype of a light guide
panel using the 2P technique (Photo-Polymerization), which is superior
in terms of simplicity and shape transfer properties. However, this
molding technique, albeit superior in transferability of fine shapes,
has numerous problems in terms of productivity indicators, such as tact,
material costs, etc., as well as in terms of application to large
surface areas. Therefore, injection molding, which will be superior in
productivity as a production system for light guide panels, is believed
to be a more advantageous technique. However, the problem with injection
molding is its inferior shape transfer properties as compared with the
2P technique. Thus, next year we will try and elucidate the
transferability of fine patterns under the injection molding method and
will use the obtained characteristics as a basis for robust design of
fine pattern shape and arrangement. Specifically, using transferred
shape evaluation samples fabricated under the injection molding method,
we will obtain new results related to the precision of shape transfer
and its variation characteristics. As criteria for transfer
characteristics evaluation, we are planning to use the linearity of the
transferred shapes and the edge R. Using the obtained shape transfer
characteristics, we will carry out optimal design using optical
characteristics as evaluation criteria, and will verify its validity
using computer simulation techniques.

In addition, in the vector radiation coupling system, it is essential to
substantially localize the LED, and it is expected that the extent to
which we will be able to improve the degree of LED integration will
determine whether it will be possible to realize the system or not. It
is believed that at 100 lm/W, i.e. the target of efficiency of the
present project, the amount of released heat will be smaller than in a
fluorescent lamp and even with integration, the optimization of the
heat-removal structure of the module will permit suppression of
temperature rise. However, the low reliability of heat-related
simulations is a fact, and we will have to keep in mind the possibility
that as a result of prototyping studies, we will not be able to raise
the degree of integration. In addition, considering that we have reached
the middle of the project with a lower LED efficiency than the one we
planned to achieve, we will have to consider scenarios, in which the
amount of released heat will be greater than expected and the degree of
integration will be impossible to increase. Should such concerns prove
realistic, we will study the possibility of integrate LEDs not in one
location of the light guide panel, but rather in several locations of
it. In addition, it is believed that in this case, for the purpose of
maintaining uniformity, it will be better to divide the light guide
panel into several portions, and this idea will be investigated at the
same time.

[(2) Preparation of basic specification]{.underline}

We will define several different shapes of the light guide panel system
LED illumination proto-sample, which will be fabricated in 2002, the
final year of the project. We will further zero in on the intended use,
shape and robust design criteria to prepare a general outline of the
specification.

## References

\[1\] Masayuki Shinohara, Masataka Tei, Shigeru Aoyama, and Masashi
Takeuchi, "Vector Radiation Coupling Method for High Efficiency and High
Uniformity," Diffractive Optics and Micro Optics, 1998 OSA Technical
Digest Series, Vol. 10, 189-191 (1998).

\[2\] Masayuki Shinohara, Shigeru Aoyama, and Masashi Takeuchi, "Vector
Radiation Coupling Type LED Backlight," Optical Alliance, Japan
Industrial Publishing Co., Ltd., Vol. 9, No. 5, 13-16 (1998).

# Lighting Fixture Development Based on Novel LED Concepts

## Achievements in 1998\~1999

We attempted to verify whether it was possible to use LEDs available on
the market as light sources for illumination, and, assuming that it was
possible, to determine what kind of fixtures could be employed.
Furthermore, along with analyzing the characteristics and performance of
LEDs and trying to determine ways in which they could be used as light
sources, we created research prototypes of lighting fixtures based on
novel concepts applicable to LEDs.

### Achievements in 1998

-   Commercially available LEDs were used to fabricate a test piece. As
    a result, we determined that the test piece input efficiency was
    7.18 lm/W and the lamp efficiency was 9.65 lm/W.

-   By measuring the light distribution characteristics of the
    above-mentioned test piece, we found that it could be utilized as a
    spot light for local lighting. In addition, we found that in such a
    case the light level was 131 lx at a distance of 100 cm, 524 lx at a
    distance of 50 cm, and 1450 lx at a distance of 30 cm, which met the
    requirements for reading (300\~750 lx), handicrafts and sewing
    (750\~2000 lx), specified as light level standards in JIS Z9110, and
    fell within a range permitting use in practical applications.

-   Based on the above-described results, we created prototypes of
    various lighting fixtures that took advantage of LED
    characteristics.

### Achievements in 1999

-   Rectangular or round-shaped LED integrated light sources were
    fabricated to investigate the relationship between the size of the
    light sources and the temperature rise. As a result, we determined
    that in case of configurations consisting of closely packaged LEDs,
    their temperature increased to levels at which they could not
    operate.

-   With regard to configurations of LED light sources used for lighting
    fixtures, we found that a "centerless" array obtained by removing
    the central portion of an array ensured the necessary luminous
    outputs and could suppress the temperature rise.

-   We also found that the use of heat sinks was effective as a means
    for suppressing the temperature rise in LEDs. As a result, we
    implemented prototypes of lighting fixtures capable of achieving the
    current target of an illuminance of 1000 lx or more and a
    temperature rise of 30ºC or less.

### Discussion of Achievements in 1998\~1999

Based on the above results, we summarized the relationship between the
characteristics and market applications of LEDs as shown in Table 15.1.

+---+----------------------+----------------------+---------------------+
|   | LED Characteristics  | Lighting Fixture     | Market Applications |
|   |                      | Concepts             |                     |
+---+----------------------+----------------------+---------------------+
| 1 | Directionality of    | Reflectors are       | Fixtures for local  |
|   | light                | unnecessary, high    | lighting            |
|   |                      | fixture efficiency   |                     |
+---+----------------------+----------------------+---------------------+
| 2 | The freedom to       | The freedom to       | Fixtures providing  |
|   | combine circuits and | combine lamps,       | different light     |
|   | lamps                | colors, and beam     | distribution        |
|   |                      | angles               | depending on the    |
|   |                      |                      | intended use        |
|   |                      |                      |                     |
|   |                      |                      | Appropriate         |
|   |                      |                      | illuminance at      |
|   |                      |                      | minimum power       |
+---+----------------------+----------------------+---------------------+
| 3 | Low heat generation  | Thin lighting        | Producing thinner   |
|   |                      | fixtures             | downlights          |
+---+----------------------+----------------------+---------------------+
| 4 | Long service life    | Maintenance-free     | Makes maintenance   |
|   |                      |                      | easier and simpler  |
|   |                      |                      | due to reducing the |
|   |                      |                      | amount of           |
|   |                      |                      | maintenance         |
+---+----------------------+----------------------+---------------------+
| 5 | Lightweight          | Mounting on          | Mounting on         |
|   |                      | nonstationary        | automobiles,        |
|   |                      | equipment            | Shinkansen          |
|   |                      |                      | (Japanese bullet)   |
|   |                      |                      | trains, and other   |
|   |                      |                      | means of            |
|   |                      |                      | transportation and  |
|   |                      |                      | equipment           |
+---+----------------------+----------------------+---------------------+
| 6 | Low temperature      | Fixtures used at low | Possible use in     |
|   | resistant            | temperatures         | cold regions, cold  |
|   |                      |                      | storage facilities, |
|   |                      |                      | etc. is being       |
|   |                      |                      | investigated        |
+---+----------------------+----------------------+---------------------+
| 7 | Impact resistant     | High reliability     | Mounting on mobile  |
|   |                      | under impact and     | equipment           |
|   |                      | vibration conditions |                     |
+---+----------------------+----------------------+---------------------+

Table 15.1 Characteristics and Market Applications of LEDs.

With their current characteristics, modern LEDs produce insufficient
luminous output for lighting applications and the field of their
possible uses is limited to local lighting. In the future, the range of
their uses in lighting applications is expected to expand as a result of
a considerable increase in luminous output per LED.

## Summary of Research and Development in 2000

On the basis of research results obtained up till this year, we
investigated the characteristics of LEDs and their potential as light
sources for illumination; this year, we focused our research on
clarifying the benefits of LED light sources as revealed through
comparisons with conventional light sources. We created prototypes of
lighting fixtures using LEDs having the highest level of luminous
efficiency (15 lm/W) among commercially available white LEDs. In
particular, this year, in search of possibilities in the field of LED
light sources and LED lighting fixtures, we investigated applications of
chip-type LEDs and conducted research concerning fixture miniaturization
and lighting characteristics.

Guided by the new LED concepts, we conducted an investigation into
various topics, such as how to miniaturize the dimensions of
commercially available lighting fixtures, and, in this connection, what
had to be done about the temperature of their various components, how to
handle lighting characteristics, how to mount fixtures, etc.

In addition to providing evidence that miniaturization could be
realized, and that design freedom in design could be increased while
achieving a reduction in power consumption difficult to attain with
conventional lighting fixtures, fabrication and evaluation of prototypes
of LED table lamps and LED ceiling downlights also confirmed their
advantages in terms of ease of installation.

## Prototyping and Evaluation of LED Table Lamp

### Methods and Means

As shown in FIG. 15.3.1-(1), an LED light source substrate was
fabricated by mounting 24 chip-type LEDs on an aluminum substrate. A
schematic diagram of its circuit is displayed in FIG. 15.3.1-(2). In the
circuit, an AC power supply is full-wave rectified, and, though the
medium of resistors, 24 LEDs are connected in series. The
voltage-current, temperature and lighting characteristics of an LED
table lamp prototype fabricated by combining the LED light source
substrate and the circuit were evaluated, and its characteristics were
further verified by comparing its dimensions and shape with table lamps
utilizing conventional light sources.

![](media/image255.png){width="5.998611111111111in"
height="1.4395833333333334in"}Left: FIG. 15.3.1-(1) LED light source
substrate.

Right: FIG. 15.3.1-(2) Schematic circuit diagram. Labels: (Across top)
LED × 24 × 1 circuit.

### Results and Discussion

A photograph of the LED table lamp prototype is shown in FIG.
15.3.2-(1). Evaluation results for the characteristics of the LED table
lamp prototype are shown in FIG. 15.3.2-(2). We found that after
applying AC 100V, the temperature
![](media/image256.png){width="3.4430555555555555in"
height="2.533333333333333in"}rise in the LED junction portion (called
the "LED portion" below) of the table lamp was 26.2ºC, which satisfied
the target value of 30.0ºC or less. In addition, the temperature rise in
the outer shell of the fixture reached 11.9ºC; assuming that the ambient
temperature was 30.0ºC, the temperature of the outer shell portion was
41.9ºC, which was sufficient to meet the requirement of JIS C8109
(General Requirements for Luminaires) of 55ºC as the temperature limit
for parts that come in contact with human skin during operation.

FIG. 15.3.2-(1) LED Table Lamp Prototype

Also, we found that after applying AC 100V, the illuminance was 490 lx,
which was within the range 300\~750 lx, i.e. the JIS standard for
reading; therefore, the required illuminance conditions were satisfied
as well. In addition, the input power at such time was extremely low,
approximately 2.2W, which confirmed that the use of LEDs offered
possibilities for power consumption reduction on a scale that was
difficult to achieve with conventional lighting fixtures.

![](media/image257.png){width="5.986111111111111in"
height="1.7770833333333333in"}

FIG. 15-3-2-(2) LED Table Lamp Prototype Evaluation Results

//Keys, FIG. 15.3.2-(2), left to right//

(Left, across top) Table Lamp (Voltage-Current)

(Middle, across top) Table Lamp (Voltage-Temperature Rise)

(Middle, box) LED Outer Shell

(Right, across top) Table Lamp (Voltage-Illuminance)

(Right, right-hand side) Illuminance 150 mm directly below, standard
posture

A comparison between the dimensions and shapes of the LED table lamp
prototype and table lamps utilizing conventional light sources is shown
in FIG. 15.3.2-(3). The volume ratio of the lamp bodies was about 1:27
in case of LED : incandescent lamp and about 1:80 in case of LED :
fluorescent lamp, which was indicative of unprecedented miniaturization
potential. Accordingly, the lamp could be considered a highly adaptable
next-generation table lamp that, in addition to a higher degree of
freedom in terms of design versatility, offered the advantage of being
easily transportable.

  --------------- ----------------- ------------------ ------------------
                  LED Table Lamp    Incandescent Table Fluorescent Table
                                    Lamp               Lamp

  Shape           Lamp body portion Lamp Body Portion  Lamp Body Portion

  Method of       Placed directly   Placed directly on Attached using
  attachment      on tabletop       tabletop           special fittings

  Lamp Body       104×20×12 mm      Diameter 92×100 mm 163×248×50 mm
  Portion Size                                         
  --------------- ----------------- ------------------ ------------------

FIG. 15.3.2-(3) Comparison of table lamp dimensions and shapes

## Prototyping and Evaluation of LED Ceiling Downlight

### Methods and Means

As shown in FIG. 15.4.1-(1), an LED light source substrate was
fabricated by mounting 44 chip-type LEDs on a glass-epoxy substrate. A
schematic diagram of its circuit is shown in FIG. 15.4.1-(2). In the
circuit, an AC power supply is full-wave rectified and two series
circuits made up of 22 LEDs with resistors interposed between are
connected in parallel. The voltage-current, temperature and lighting
characteristics of an LED ceiling downlight prototype fabricated by
combining the LED light source substrate and the circuit were evaluated,
and its characteristics were verified by comparing its dimension and
shape with those of downlights utilizing conventional light sources.

![](media/image258.png){width="5.995138888888889in"
height="1.5979166666666667in"}

Left: FIG. 15.4.1-(1) LED light source substrate. //Keys, FIG.
15.4.1-(1)// (Right-hand side) Diameter 48 mm.

Right: FIG. 15.4.1-(2) Schematic circuit diagram. //Keys, FIG.
15.4.1-(2)// (Across top) LED × 22 × 2 circuits.

### Results and Discussion

![](media/image259.png){width="3.6930555555555555in"
height="2.652083333333333in"}A photograph of the LED ceiling downlight
prototype is shown in FIG. 15.4.2-(1). An evaluation of characteristics
for the LED ceiling downlight prototype is shown in FIG. 15.4.2-(2). We
found that after applying AC 100V, the value of the temperature rise in
the LED portion of the ceiling downlight was 40.9ºC, which did not
satisfy the target value of 30.0ºC or less. We assumed that this was
attributable to the use of a glass-epoxy substrate for the LED light
source substrate and that the temperature conditions could be corrected
by using a heat-conductive aluminum substrate in the same manner as in
the case of the above-described LED table lamp.

FIG. 15.4.2-(1) LED ceiling downlight prototype.

In addition, we found that temperature conditions that were close to the
target value could be obtained and the temperature rise could be reduced
by lowering the input voltage to AC 90V. In particular, the value of the
temperature rise in the outer shell portion of the fixture was 13.1ºC,
and, assuming that the ambient temperature was 30.0ºC, the temperature
of the outer shell portion was 43.1ºC, which was indicative of
operational safety in the same manner as in the case of the
above-described LED table lamp. Also, the outer shell portion
temperature value of 43.1ºC was acceptable even when working with vinyl
materials (temperature limit: 60ºC), which are said to be the least
temperature resistant of all inner packaging materials. On the other
hand, examination of illuminance after applying AC 90V showed that the
light level at a distance of 300 mm directly below the fixture was 180
lx; and, when the ceiling downlight was mounted in a location that had a
ceiling height of 2.4 m, an illuminance of 2.8 lx was obtained at the
floor level, i.e. the fixture met requirements for night-time safety
lighting. It is believed that this illuminance level provides a basis
for possible development of under-shelf lighting, etc. In addition, the
input power at such time was approximately 2.6W, which showed a
possibility for a large-scale reduction in power consumption in the same
manner as in the case of the above-described LED table lamp.

![](media/image260.png){width="5.993055555555555in"
height="1.8493055555555555in"}

Figure15.4.2-(2) LED ceiling downlight prototype evaluation results.

//Keys, FIG. 15.4.2-(3)//

(Left, across top) Ceiling Downlight (Voltage-Current)

(Middle, across top) Ceiling Downlight (Voltage-Temperature Rise)

(Middle, box) LED Outer shell

(Right, across top) Ceiling Downlight (Voltage-Illuminance)

(Right, right-hand side) Illuminance 300 mm directly below, standard
posture

A comparison between the dimensions and shape of the LED ceiling
downlight prototype and downlights utilizing conventional light sources
is shown in FIG. 15.2.2-(3). The volume ratio of the fixtures was about
1:21 both in case of LED : incandescent lamp and in case of LED :
fluorescent lamp, which indicated that the use of LEDs permitted a high
degree of miniaturization and could contribute to a reduction in labor
because of eliminating the need for drilling holes in the ceiling.

![](media/image261.png){width="6.161111111111111in"
height="2.7222222222222223in"}

  ------------- ------------------ ------------------- -------------------
                Ceiling-mounted    Ceiling-mounted     Ceiling-mounted
                LED fixture        incandescent light  fluorescent fixture
                                   fixture             

  Shape         Ceiling surface    Ceiling surface     Ceiling surface

  Method of     Attached directly  Drilling of holes   Drilling of holes
  attachment    to ceiling         in ceiling was      in ceiling was
                                   necessary           necessary

  Dimensions of                    181 mm              185 mm
  recess in                                            
  ceiling                                              

  Protrusion    19 mm              5 mm                5 mm
  from ceiling                                         
  surface                                              

  Outside       Dia. 95 mm         Dia. 142 mm         Dia. 142 mm
  diameter                                             
  ------------- ------------------ ------------------- -------------------

FIG. 15.4.2-(3) Comparison of downlight dimensions and shapes.

## Conclusions

As a result of fabricating and evaluating the characteristics of
lighting fixture prototypes comprising as their light sources chip-type
LEDs possessing the highest level of commercially available luminous
efficiency (15 lm/W), we arrived at the following conclusions:

-   It is possible to realize ultra-miniature fixtures completely
    different from lighting fixtures used in the past..

-   It is possible to realize fixtures generating energy savings on a
    scale difficult to achieve with conventional light sources.

-   At present, practical application of such fixtures is possible for
    local lighting purposes.

-   The degree of freedom in the selection of installation locations has
    increased because thermal constraints related to the mounting of
    fixtures have been reduced.

## Agenda for the Future

So far, prototypes of light source substrates utilizing bullet LEDs and
chip-type LEDs as light sources for illumination, as well as prototypes
of lighting fixtures that take advantage of their characteristics, have
been created and have confirmed the possibility, as well as the
benefits, of putting them to practical use. A clear-cut development
orientation addressing the needs of the market will be indispensable
from now on. Table 15.6 illustrates how the LED lighting fixture
concept, which was formulated when the research was in its initial
phase, has evolved towards the Next Generation.

  --- ----------------- ---------------------- ---------------------------
      LED               Initial Concept        Next Generation Concept
      Characteristics                          

  1   Directionality of Reflectors are         Development of lighting
      light             unnecessary, high      fixtures that save
                        fixture efficiency     resources

  2   The freedom to    The freedom to combine Light distribution
      combine circuits  lamps, colors, and     characteristics optimized
      and lamps         beam angles            for specific applications

  3   Low heat          Thin lighting fixtures Free choice of installation
      generation                               locations

  4   Long service life Maintenance-free       Applicability to an ageing
                                               society

  5   Lightweight       Mounting on            Fixture holders
                        nonstationary          unnecessary, labor savings
                        equipment              during installation

  6   Low temperature   Fixtures used at low   Fixtures used at low
      resistant         temperatures           temperatures (continuing
                                               effort)

  7   Impact resistant  High reliability under High reliability under
                        impact and vibration   impact and vibration
                        conditions             conditions (continuing
                                               effort)
  --- ----------------- ---------------------- ---------------------------

Table 15.6 LED Characteristics and Next Generation Concept.

FIG. 15.6-(1) and FIG. 15.6-(2) show an image of architectural lighting
and specific lighting techniques used for comfortable space management
that takes advantage of their functional superiority. In comparison with
ordinary lighting fixtures, lighting fixtures used for architectural
lighting have numerous mounting constraints (size, heat, etc.), and thus
they are worth exploring as a research topic and in the future we will
try to keep an eye on possibilities of expansion into the area of
architectural lighting.

In addition, as far as problems related to light source levels are
concerned, it should be noted that because modern LEDs were developed on
the basis of a semiconductor component-oriented philosophy, they are not
necessarily suitable for light sources used for illumination. Therefore,
the downside of their benefits, such as the freedom of arrangement and
combination of sources, etc., is the need to integrate multiple LEDs
because their luminous output is low in comparison with conventional
light sources. As a result, variations in light color, color rendition,
etc. at the level of the integrated light source caused by variations in
individual LEDs may present a problem. In addition, another problem to
consider is the heat problem, which becomes aggravated as the number of
LEDs grows. Therefore, now it is time to investigate LEDs as light
sources for illumination at the structural level and to implement
specific solutions based on efforts by all the groups involved in the
project. In the future, we want to use LEDs of novel structures to
create prototypes of lighting fixtures based on new concepts and to
continue our support for the present project.

![](media/image262.png){width="5.991666666666666in"
height="2.3319444444444444in"}

Left: Figure 15.6-(1) Lighting Techniques

//Keys, FIG. 15.6-(1)//

(Left, across top) Hotel corridor lighting technique using LED-based
architectural lighting

(Right, above 2 circles) Functionality providing maximum effects can be
realized at minimum power if very small architectural lighting fixtures
taking advantage of LED size are employed

(Left circle, inside, top) Door

(Left circle, inside, left) Diffuser

(Right circle, inside) Wall

(Below & between 2 circles) Enlarged view

(Below left circle) Door

(Below right circle) Wall

Right: Figure 15.6-(2) Architectural lighting image.

As a specific course of action, we will be creating the following
prototypes for the purpose of investigating and evaluating LED
structures that can be used as new light sources for illumination.

1\) Using commercially available LEDs, we will be creating prototypes of
lighting fixtures designed for a luminous efficiency of 80 lm/W.

2\) We will be creating prototypes of lighting fixtures utilizing LEDs
of novel structures fabricated in the course of this project.

3\) We will be creating prototypes of lighting fixtures incorporating
new concepts (such as architectural lighting, etc.).

We will also strive to raise the efficiency of the lamp circuits we have
implemented in this year's prototypes while continuing to investigate
the following topics in order to ensure better reliability:

1\) Selection and verification of reliability testing parameters for
LED-based lighting devices.

2\) Guidelines for ensuring safety in connection with the influence that
may be exerted on the human body by increased LED output.

# Conclusions and Future Course

## Planning and Investigations

We have participated in symposia and academic meetings both domestically
and overseas, researched technology trends, and conducted patent
searches on GaN-based LEDs. In the future we shall continue our research
to keep this project active.

## Basic Research on Light Emission Mechanisms

\(1\) With regard to the two components of light emission from
In~0.08~Ga~0.92~N thin films, PL measurements were performed at
excitation energies both higher and lower than the GaN band gap. No
large variations in the emission peak position of the two components
were found. Nonetheless, when GaN is not excited the PL intensity is
reduced, indicating a significant role for GaN as a carrier supply
source, a role which grows in importance with increasing temperature.

\(2\) Excitation spectra of In~0.08~Ga~0.92~N thin films were measured
and a clear absorption peak was observed in the temperature range from
4K to room temperature. It also became clear that at 4K the free A, B,
and C excitons within GaN contribute to the photoluminescence of
In~0.08~Ga~0.92~N. This suggests the possibility that the GaN belongs
primarily to the buffer layer. The absorption positions of the two
emission components are the same and it is surmised that the excited
carriers first relax from the excitation energy position to the higher
energy level and then transition to a lower energy level. Upon measuring
the temperature dependence of the excitation spectrum there was no large
variation in the peak position relative to temperature, so it is thought
that the band gap of In~0.08~Ga~0.92~N shows no startling temperature
dependence.

\(3\) Among the two light emission components, the emission peak
position of the high energy component and the peak position of the
excitation spectrum shifted in a parallel manner with temperature,
indicating no temperature dependence of the Stoke shift of the high
energy component and a value of about 50 meV. In the strain contributing
to localized excitons, it is known that as the temperature rises,
excitons delocalize and the energy difference (Stokes shift) between the
absorption PL spectra becomes smaller. Considering this, the large
Stokes shift observed in In~0.08~Ga~0.92~N is not likely due to
localized excitons. With regard to the photoluminescence source of the
high energy component, the Stokes shift is large and yet it is possible
to maintain it to room temperature.

When thinking about the emission mechanism model into which a polaron
concept has been introduced, as we proposed last year, the Stoke shift
expected from the electron-polaron interaction is α~e~ηω~l~ = 44.2 meV
(α~e~ : Frohlich interaction constant. ηω~l~: LO phonon energy) and
closely matches the results from this experiment, so we found that this
could be explained without contradiction.

\(4\) From the fact that the peak positions in the absorption spectrum
and excitation spectrum are at nearly the same energy position, we
conclude that light absorbed by the In~0.08~Ga~0.92~N specimen excites
carriers within the crystals and that these carriers contribute to the
radiative recombination of In~0.08~Ga~0.92~N in the same manner as in
typical semiconductors.

\(5\) Excitation spectroscopy was performed on InGaN with In
compositions ranging from 4 to 14%. The lower the In composition the
clearer the absorption peak was observed in the excitation spectra.
Also, higher compositions of In showed a larger Stokes shift.

\(6\) Homoepitaxial growth by RF-MBE was conducted on a GaN bulk single
crystal substrate fabricated by pressure controlled solution growth. As
a result, epitaxial thin films that represented a continuation of the
polarity of the GaN bulk single crystals used for the substrates were
obtained. Also, it was confirmed that when a GaN epitaxial thin film is
grown on the N-face of the GaN bulk single crystal, it grows three
dimensionally but when grown on the Ga-face, it grows two dimensionally.
PL spectroscopy on these homoepitaxial thin films showed predominant
emission lines produced by extremely sharp, strong neutral-donor-bound
excitons with hardly any deep level emission observed. In comparison to
heteroepitaxial growth on a sapphire substrate, huge improvements are
seen, such as the emission peak width at half height narrows to less
than 1/2, the PL intensity strengthens by 2 orders of magnitude, and the
band-edge emission / deep level PL intensity ratio increases by 1 to 2
orders of magnitude.

\(7\) The effects of nitriding sapphire substrates with NH~3~ gas and of
AlN buffer layers on crystal polarity have been clarified with regard to
control of initial stages of growth by NH~3~ GS-MBE. N-face growth of
GaN epitaxial thin film was found to be predominant regardless of the
presence or absence of nitriding of the sapphire surface. In contrast to
this, by introducing an AlN buffer layer prior to growth of the
epitaxial thin film, it was found that Ga-face growth could be attained
irrespective of the presence or absence of nitriding. The ability to
control single polarity faces is valuable to high quality growth of
crystals by NH~3~ GS-MBE. Moreover, the results of RBS analysis of
crystallinity of GaN epitaxial thin films grown by NH~3~ GS-MBE using an
AlN buffer layer show that the crystallinity of the attained epitaxial
thin films was extremely high.

Future challenges and policies are noted below.

\(1\) There are insufficient experimental results to make clear
conclusions regarding the low energy component of the two emission
components. Still, based on numerous test results it is expected that
the low-energy component contributes an important effect on the
photoluminescence characteristics of In~x~Ga~1-x~N. At this time, it is
unknown whether the low energy component observed at low temperature
exists at high temperature. Also, from the fact that the strength of LO
phonon replicas changes with different In compositions, it is expected
that the low energy component is highly dependent on the In composition.
In the future, the origin of the low energy component will be uncovered
through more detailed measurements of specimens having different In
compositions.

\(2\) The line width of the low energy component is wider than that of
the high energy component and does not show dependences on external
forces such as excitation strength or temperature. Additionally, the
localization effect within the In~x~Ga~1-x~N among the specimens with
wide emission line widths may shroud the emission characteristics of the
In~x~Ga~1-x~N itself. Therefore, high quality crystals fabricated by
joint R&D organizations will be used.

\(3\) When the photoluminescence of In~x~Ga~1-x~N was researched, there
was no mistake that GaN contributes to the PL intensity. In particular,
carriers from the GaN become crucial the higher the temperature, but
more detailed examination of this mechanism will be conducted. This is
extremely interesting from the perspective of device fabrication.

\(4\) Improvements will be made in the crystallinity of GaN thin films
by homoepitaxial growth, especially with regard to surface flatness,
which was inadequate this time. For the Ga-polar surface, which has
stepped irregularities in the substrate surface, it is necessary to find
techniques to fill in the substrate irregularities, such as lengthening
the growth time or lengthening the source gas dispersion. With regard to
N-polar surface, we will examine the introduction of shutter control
techniques to reduce step-like three dimensional structures that form
during homoepitaxial growth. We will also look at homoepitaxial growth
by NH~3~ GS-MBE, which easily attains films with good photoluminescence
characteristics. High quality InGaN mixed crystals will be grown on good
homoepitaxial GaN buffer layers.

\(5\) Structural and optical characteristics of homoepitaxial thin films
will be researched. To uncover the relationship between crystallinity
and photoluminescence characteristics, RBS and X-ray diffraction
measurement will be conducted on GaN epitaxial thin films and GaN bulk
single crystals. With RBS measurement in particular, more detailed
measurements of the channeling axes other than the \<0001\> axis
evaluated this time will be performed.

## Properties of GaN-based Semiconductors and Discovery of Light Emission Mechanisms

\(1\) Many research organizations have examined photoluminescence
properties of GaInN and it is believed that the key to the light
emitting mechanisms lies in the spatial localization effect of carriers
in GaInN. It is thought that the photoluminescence characteristics
(especially carrier localization) of GaInN are largely dependent on the
In composition and evaluation while systematically varying the In
composition is important. Therefore we systematically examined the
photoluminescence characteristics over a broad range of temperatures (7K
to 540K) using GaInN/GaN quantum well LEDs of varying In compositions
(emission wavelengths). The results indicate an emission life
temperature dependence split among three temperature regions and can be
explained by changes in the carrier localization state. Additionally,
simulation using a simple localization level model shows that an
increase in the In composition ratio produces an increase in
localization energy of the carriers and an increase in point defect
density. The correlation between an LED's quantum efficiency and the
differences in emission life temperature dependence according to In
composition leads us to believe that localization energy and point
defect density are predominant parameters affecting LED efficiency.

\(2\) To research the relationship between carrier localization
characteristics and quantum efficiency, PL spectroscopy and
time-resolved PL spectroscopy was conducted on two LED specimens having
different quantum efficiencies but equivalent emission wavelengths.
Results from the high quantum efficiency specimen show a large shift to
higher energy in the PL peak accompanying excitation intensity, a large
energy dependency with regard to the emission life, and a long emission
life at the emission peak. From the results of these experiments, it is
thought that the deep localization states of the carriers inhibit
transition toward non-radiative recombination centers in the high
quantum efficiency specimen. We believe the deep localization states of
the carriers are a primary cause indicating high quantum efficiency.

\(3\) This year we introduced a near-probe PL system to measure at a
high spatial resolution the in-plane distribution of light emission.
First, basic data was collected from a typical multiple quantum well LED
sample having an emission wavelength of 460 nm. A distribution of PL
intensities ranging between 0.3 and 0.5 µm was observed within the 4 µm
x 4 µm area measured and the PL intensity at the low position was found
to be 25% of the maximum intensity. The data is preliminary and requires
accumulation of further data, including confirming reproducibility, to
correctly interpret the data.

\(4\) The effects of Si doping of the quantum well activation layer were
evaluated by reverse bias PL and time-resolved PL spectroscopy from the
perspective of the quantum well band structure including piezoelectric
effects. Changes in the spectrum of an applied electrical field were
measured by measuring the depletion layer capacitance when a reverse
bias was applied and estimating from the calculated value of the
depletion layer width the external electrical field actually applied to
the quantum well. The results from the low Si-doped specimens (4 x
10^17^cm^-3^) showed a shift of the PL peak to higher energy when an
external electrical field is applied, a long life (52 ns) at zero bias,
and a reduction in the life when an external electrical field is
applied. It is estimated from the peak shift results and calculation of
the transition energy that the piezoelectric field acting upon the
quantum well is 1.48 MV/cm with In composition of x = 0.23. In contrast,
a very slight shift of the PL peak to lower energy was observed with the
high Si-doped specimen (4 x 10^18^cm^-3^). Also, since the life remained
at 10 to 14 ns regardless of bias, it is believed that the electric
field induced by the piezoelectric effect is neutralized by carriers
from donor impurities. While Si doping of the activation layer is
important to neutralize internal electrical fields since it influences
spreading of the depletion layer in the pn connection, optimum design is
important for efficient current injection.

Future plans are described here. First, as an extension of research
conducted up to last year, we will explore the origin of carrier
localization. We will study the two dimensional spatial distribution
within a quantum well and investigate Stokes shift through selective
excitation PL and the depth of localization levels. Additionally, LED
specimen structures and the relationship between crystal growth
conditions and carrier localization characteristics will be studied for
the purpose of raising efficiencies still further.

The latter half of this project will focus on studying the knowledge
obtained thus far and investigating the relationship between crystal
growth parameters and photoluminescence characteristics of near
ultraviolet light that is the goal of this project. To this end,
information and samples will be closely shared between the epitaxial
group and Yamaguchi University. Between the crystals grown within this
project and the UV LEDs of other companies the conditions contributing
to highly efficient emission of ultraviolet light will be uncovered. To
be more concrete, the following main topics will be researched:

-   effects from differences between heteroepitaxy and homoepitaxy,
    effects of crystal dislocation and skewing on photoluminescence
    characteristics, and differences in In doping effects

-   effects of doping the UV emission activation layer with impurities

-   optimization of doping profiles through simulation

This research will yield invaluable knowledge on optimal UV LED
structures.

## Basic Research on Bulk GaN Single Crystals by Solution Growth Method

The effects on the growth of bulk GaN single crystals of supersaturating
a Ga melt with nitrogen using the PC-SG method were uncovered. With the
intent of understanding the conditions needed to grow large, high
quality bulk GaN single crystals, an ultra high pressure HIP device and
an ultra high pressure crystal growth device introduced in 1999 were
used to explore the effects of seed crystals, equilibrium pressure,
acceleration speed of nitrogen pressure, and crystal growth time. Also,
the characteristics of the GaN bulk single crystals attained were
evaluated and the relationship between crystal quality and the surface
condition of GaN bulk single crystals was examined. The details of the
results are noted below.

\(1\) The equilibrium pressure of GaN at a heater temperature of 1475ºC
was 590 to 730 MPa. At a crystal growth pressure of 980 MPa,
overpressure was about 300 MPa.

\(2\) By reducing the acceleration speed of nitrogen pressure in the
ultra high pressure HIP device, a 120 mm^2^ GaN single crystal with a
good surface condition was attained. This is larger than the 104 mm^2^
size GaN single crystal attained by HP-SG.

\(3\) A 334 mm^2^ GaN single crystal (equivalent to diameter 20.6 mm)
was attained using a ultra high pressure crystal growth device with the
nitrogen pressure acceleration speed at 69 MPa/h. This is the best data
regarding size of a GaN single crystal grown via a solution growth
method. A GaN single crystal with a good surface condition was attained
at a pressure acceleration speed of 9 MPa/h, but the crystal size was
about 18 mm and did not enlarge the GaN single crystal by reducing the
pressure acceleration speed.

\(4\) We found in (2) and (3) above that reducing the acceleration speed
of nitrogen pressure is an effective means of attaining GaN single
crystals with good surface condition.

\(5\) From observing the relationship between temperature fluctuations
and cell nuclei occurrence density in (2) and (3) above, we've learned
that reducing temperature fluctuations and lessening the nuclei
occurrence density in GaN crystals is necessary to achieve large bulk
GaN single crystals.

\(6\) Large GaN single crystals are more easily attained on a C-face
just sapphire substrate than on a graphite plate, due to the
relationship between seed crystals arranged in the crucible and the size
of GaN single crystals that form near the surface of the Ga melt due to
natural nuclei occurrence.

\(7\) Results suggest that prolonging the hold time of final crystal
growth pressure has no effect on enlargement of the crystal size and
that the size of the crystal is determined by the pressure rising
process to final pressure.

\(8\) Results from the evaluation of GaN bulk single crystal
characteristics show that differences in the GaN single crystal surface
condition reflect differences in the crystal growth state. Single
crystals having good surface condition are the same as those exhibiting
good crystal characteristics.

-   Tilt angle of 43 arcsec and twist angle of 18 arcsec attained with a
    four crystal x-ray rocking curve profile are values of a level
    equivalent to those of undoped GaN single crystals raised by HP-SG
    and show superior crystallinity.

-   AFM observation showed flatness at a single atom layer level.

-   PL spectroscopy at 4.2 K and room temperature showed good PL
    characteristics with band edge PL intensity greater than the yellow
    band. Additionally, with regard to PL intensity temperature
    dependency of band edge emissions, temperature variations in the PL
    intensity were small in the bulk GaN single crystals raised by
    PC-SG.

Predictions on the achievement of the goal (GaN single crystal size of
25 mm diameter or greater; defect density below 10^5^ cm^-2^) and future
plans are noted below.

In bulk GaN single crystals formed by PC-SG crystal defect density
estimated by TEM observation is 10^5^ cm^-2^ and it is predicted that
the goal can be reached. Still, it is nearly impossible to correctly
evaluate defect densities via TEM observation when the crystal defect
density drops below 10^6^ cm^-2^, so it will be necessary to conduct
evaluation by etch pitting. It was also found that bulk GaN single
crystals formed by PC-SG show good crystal characteristics, especially
superior PL characteristics. However, we were unable to confirm whether
these crystals were superior as epitaxial substrates. Consequently,
topics which should be confirmed or resolved include (1) suitability as
a substrate for homoepitaxy, and (2) attainment of larger bulk GaN
single crystals.

With regard to (1), evaluation of homoepitaxial growth on the bulk GaN
single crystals formed by this research began at the end of 2000 with
the cooperation of Yamaguchi University's Taguchi Research Lab and other
organizations participating in this project. In order to evaluate
characteristics of the bulk crystals it is necessary to have a fixed
size and quantity so it is important to be able to consistently
fabricate single crystals of at least 10 to 15 mm diameters. Moreover,
the handling of crystals having a thickness of 20 to 30 µm is difficult
so we plan to examine the crystal growth process and crystal thickness
inhibiting factors. Study of polarity control is also needed and we plan
to verify effects of Mg doping. These crystals will be supplied to
organizations performing research on LEDs and epitaxial growth
techniques. By feeding back the results to growth technologies it is
hoped that it will be possible to use them as epitaxial substrates.

With regard to (2), GaN single crystals with an area of 334 mm2
(equivalent to a diameter of 20.6 mm) were attained by PC-SG using a
multi-level crucible in an ultra high pressure crystal growth device.
This gives us confidence that meeting the final year goal of 25 mm
diameter crystals is not impossible. Nevertheless, technological
improvements or breakthroughs are required to make this a reality. On
this point, it is thought that reduction of temperature fluctuations is
most effective, thus modifications to the heating device and the ultra
high pressure crystal growth device are planned for this year. Improving
temperature fluctuations will contribute not only to larger single
crystals through reduction in crystal nuclei density, but will also
improve reproducibility through a more stable temperature environment.

It is expected that the achievements of this year's research, through
the technology studies noted above, will result in consistent
fabrication of bulk GaN single crystals that exhibit good crystal
surfaces, good crystal characteristics, and exceed 25 mm diameter in
size.

## Research into Crystalline Substrate Surfaces

With regard to techniques for precision lapping of the surface of a
sapphire substrate, we were able to completely remove surface scratches
by employing MCP in the latter part of normal lapping and achieved a
surface roughness of 0.3 nm. Measurement was accomplished using AFM.

By using custom microscopes, scratches and contaminants can be detected,
making evaluation at the substrate stage possible. Cleaning was changed
from a multiple chamber ultrasound cleaning system to a single wafer
cleaning system, reducing metallic element contaminants to less than
1/10^th^ previous amounts. Analysis was conducted via total reflection
fluorescent X-ray.

Additionally, precision of the C-face azimuth was controlled to less
than ±0.05 degrees relative to the specified axis (with m-axis at 0.15
degrees off as standard). Variation in the epitaxial effect due to off
angle variation of the user's substrate has been reduced.

Lapping and polishing of the GaN substrate was performed experimentally
with a high press lapping machine introduced to this research. Lapping
is possible, but with the coarse surface of the GaN crystals this time,
the target roughness could not be attained.

Future research plans with Yamaguchi University call for evaluation of
GaN heteroepitaxial growth using MBE in pursuit of better substrates. In
concrete terms, we intend to reduce GaN epitaxial film defects by
varying the surface properties through stepping and the like while
maintaining a high standard for surface roughness. We shall also
research methods to greatly reduce GaN surface defects, aiming for a
surface roughness of 1 nm or less.

## Research and Development of GaNAsP-based Multicolored Light Emitting Materials by MOCVD

Two GaNP growth methods to achieve GaNP with a high P composition were
studied: laser-assisted MOCVD and MOCVD using DMHy and PH~3~ source
gases.

The experiment with MOCVD using DMHy and PH~3~ source gases was
conducted keeping the ratio of V/III source gases constant and varying
the growth temperature. With growth temperature set low, a P composition
of 0.5% was confirmed using SIMS analysis. Among photoluminescence
characteristics, a peak shift in the band edge caused by adding P to GaN
via PH~3~ was observed. Newly studied was growth of GaNP thin film using
a laser-assisted MOCVD method. Results showed the source gas readily
decomposed at low temperature due to the light energy and a P mix of
about 10% was confirmed using SIMS analysis. TBP, which easily
decomposes, was used as the P source. As a result, crystals with a high
P composition were attained at a growth temperature of 850ºC. PL
measurements showed the band edge peak shift 0.2 eV toward the long
wavelength compared to GaN. What's more, the degree of red shift was
greater than that reported in previous announcements, and we believe
that we've confirmed band bowing in the GaNP although not
quantitatively. Thus the red shift of the PL spectrum of GaNP using
laser-assisted MOCVD has been shown to be two to three times that of
conventional MOVPE, making laser-assisted MOCVD a promising crystal
growth technology.

It is necessary to move forward with more emphasis on quantitative
research in 2001. With the creation of a prototype GaNP SQW LED, we were
the first to confirm blue white light emission by current injection.
This shows that, at a minimum, GaNP LEDs hold promise as a blue LED.

Future plans are noted below. In 2001, we intend to work with Professor
Onabe of Tokyo University in growing GaNP (As) crystals and finding the
optimum growth conditions under laser-assisted MOCVD by raising the
intensity of the laser and improving the equipment itself. Tokyo
University will examine crystal growth of GaNAs or GaNP by gas source
MBE which has a stronger non-equilibrium state, large band gap bowing,
and a high expectation for wave length shifting. We will also work on
optimizing the design of equipment used for laser-assisted MOCVD. In
other words, we will optimize the growth conditions to achieve GaNP with
a high P composition by making modifications to the substrate susceptor,
heater construction, and gas inlets. We will raise the photoluminescence
intensity of the LED by further studying GaNP multiple quantum well
structures and epitaxial structures. To understand the light emission
mechanism of GaNP we will continue to research in detail photonic
characteristics through photoluminescence measurements. We will use
selective epitaxial growth to increase high brightness of the LED and
strive to raise the crystal quality. Stated differently, selective
epitaxial growth, where epitaxial growth is performed locally and at
specified locations of the substrate surface instead of across the
entire surface, is thought to be more effective in attaining high
quality. Evaluation of photonic characteristics and the light emission
mechanism will be researched together with Professor Taguchi of
Yamaguchi University. Lastly, we will work on raising the P composition
and discovering the optimum growth structures and conditions for high
brightness luminescence.

## Development of Low Defect Density GaN Thin Film Epitaxial Substrates through MOCVD

Using a new FACELO (Facet Controlled Epitaxial Layer Overgrowth) method,
dislocation density was reduced to an order between 10^6^ cm^-2^ and
10^4^ cm^-2^. Controlling initial nuclei density, FACELO growth on a GaN
substrate crystal (10 mm square) exhibiting low dislocation density was
conducted by double FACELO -- {11-22} FACELO, then {11-20} FACELO -- to
gradually reduce dislocation density.

To the best of our knowledge, we attained the world's lowest dislocation
density in GaN on a sapphire substrate. We hypothesize that the
dislocation reduction lies in dislocation bending horizontally along the
c-axis through {11-22} triangular facet, resulting in propagation being
terminated by voids formed at mask center.

Dislocation induced from the upper edge of the voids and arriving to
persist as lines at the surface was found to be effectively blocked by
{11-20} rectangular facet. We began study of 2 inch diameter specimens
but it ended in incomplete embedding and inadequate surface morphology.

Results from TEM analysis of dislocation lines in FACELO-GaN make it
clear that combined dislocations bend toward the mask region and are
terminated by voids, and knife-shaped dislocations bend parallel to the
mask stripe. Through this mechanism, dislocation lines occurring in the
boundary between the GaN and sapphire substrate do not reach the
surface, greatly reducing dislocation density.

It was found that a PL spectrum distribution in relation to the growth
mechanism could be obtained through a micro PL method. It is believed
that a large number of defects and impurities are contained in the
regrowth boundary of the facet. It is thought that the trend to collect
impurities by the facet is lowest in the c-face, followed by the {11-22}
face, and then the {11-20} face.

Results from time-resolved spectroscopy confirm that the life of free
excitons is longer in ELOG-GaN using a W mask than ELOG-GaN using an
SiO~2~ mask and that non-radiative recombination processes are
inhibited.

Future plans are described here. We have accumulated many fundamental
technologies, such as new FACELO methods and the use of W masks, that
reduce dislocation defects on 10 mm square substrates. Dislocation
density is on the order of 10^6^ -- 10^4^ cm^-2^, nearly reaching
targeted levels. However, study of 2-inch diameters with a modified
MOCVD growth device was unsuccessful in reproducing these FACELO
results, ending in an inadequate surface morphology after embedding.

Further efforts at studying 2-inch diameters is needed to realize a
practical 2-inch diameter GaN on sapphire substrate with dislocation
density of less than 10^4^ cm^-2^ targeted by this project. To this
effect, we shall conduct additional basic research on reducing
dislocation density and will combine elemental technologies discovered
thus far. To efficiently combine technologies, it is necessary to fully
understand through TEM observation the propagation and elimination
mechanisms of dislocation. The peculiar mechanisms of MOCVD will be
better understood through examination and comparison with HPVE.
Additionally, now that we understand differences in GaN crystal quality
at the mask and at the window with ELOG-GaN, fine region crystal
evaluation will be conducted through microphotoluminescence,
time-resolved photoluminescence, and cathode luminescence.

By applying the above knowledge to 2-inch diameter GaN, timely
development of practical, high quality GaN on sapphire substrate will
occur. The fabricated substrate will be made available to the LED
epitaxial group for use as a homoepitaxial substrate or to the substrate
group for high-grade crystals to confirm the effects of low dislocation
substrates.

## Development of GaN Epitaxial Substrates through Low Pressure Vapor Phase Method

We examined the synthesis condition of GaN by adjusting the microwave
output to optimize temperature and pressure in the region where plasma
exists in high density, taking source gas temperature, substrate
temperature, relative position of the Ga crucible to the substrate, and
N2 pressure as parameters. Results show that what was heretofore a
powder now shows growth of GaN crystals having a compact film arranged
on the c-axis on sapphire substrates in the range of 10 x 4 mm^2^. X-ray
diffraction spectroscopy also confirms a strong single peak in the
sapphire (0002). Moreover, surface dilapidation was absent on 5 mm
square GaN substrates, and synthesis of c-axis oriented crystals having
surface morphology with good symmetry was achieved.

Additionally, with regard to synthesis speed of max. 100 µm/h, we
experimentally uncovered correlations among various parameters,
attaining the degree of freedom needed to improve crystallization. By
changing growth parameters, such as the partial pressure of the reactive
radicals and the activation method, we were also able to confirm that it
is possible to raise synthesis speed to the highest level of 1 mm/h with
vapor phase growth of compound semiconductors.

Future plans are described here. Among recent research surrounding GaN
substrates, one sees progress in development of thick film GaN single
crystals using HVPE and bulk single crystals by high pressure solution
method, or increasing use of SiC substrates to attain higher quality or
larger size. However, all of these methods involve hexagonal crystals
with small inclination grain boundaries or thread dislocation and at
present problems are piling up. There are also many challenges in
establishing low cost supply as LEDs for general lighting. The synthesis
speed of GaN via low pressure vapor phase has been estimated to
logically be 1 to 6 mm/h. If manufacturing techniques can be established
for direct synthesis of GaN at 1 mm/h or higher, the cost of epitaxial
substrates will decrease dramatically to a level equivalent to present
practical GaAs substrates.

In the future, we will further study a broad range of conditions
together with the knowledge gained thus far and examine the potential as
a GaN single crystal LED substrate, with the aim of fabricating large
GaN bulk single crystals of 10 mm diameter or greater. How well the
stoichometry of Ga and N~2~ can be controlled is crucial. To this end,
single crystal growth conditions such as temperature and pressure must
be optimized through logical observations and systematic experiments.
Also, addition of reactive gases like H~2~ or HCl will be tried as a
promising means to raise growth speed. Moreover, we aim for growth of
low dislocation density crystals by using SiC or GaN crystals as the
substrate, or through substrate surface treatments, or by conducting low
super saturation growth during initial growth processes.

## Development of LED Epitaxial Process Technologies

\(1\) Fabrication of AlGaN/GaInN UV MQW Structures

The AlGaN/GaInN MQW luminescent layer was examined as a luminescent
layer for UV LEDs. By growing an AlGaN barrier layer at a high
temperature of 1100ºC, a MQW structure exhibiting good surface
morphology and interface sharpness was attained. PL measurement showed a
peak wavelength of 385 nm and PL intensity on par with the
photoluminescence of a blue MQW. Investigation of the PL intensity ratio
(I~300K~/I~10K~) showed 33% with the UV MQW using an AlGaN barrier layer
and was high compared with 23% exhibited by a blue MQW using a GaN
barrier layer.

Reduction of the PL intensity ratio in the UV region of a GaN barrier
layer MQW was suppressed by using a AlGaN barrier layer.

\(2\) Study of High AlN Mole Fraction AlGaN Clad Layers

Growth of high AlN mole fraction AlGaN clad layers for UV LEDs was
examined. Growth pressure and the total gas flow was found to be
effective parameters for high AlN mole fraction AlGaN growth. An AlGaN
layer was attained with a AlN mole fraction exceeding 20%.

\(3\) Fabrication of UV LED Element Structures

An LED structure using AlGaN/GaInN MQW as the luminescence layer was
fabricated. Current injection UV emission at an emission wavelength of
390 nm was confirmed. Operating voltage is 3.7 V when energized at 20
mA. Luminescence output was weaker as compared to blue LEDs with a
lamp-translated equivalent of 1 mW. Optimization of growth conditions
must be studied to make progress toward high efficiency.

Future plans include further optimization of growth conditions and
establishment of highly efficient UV LED epitaxial technologies.
Specific challenges are noted below.

\(1\) Reduction of Non-Radiative Paths

Further optimization of growth conditions is required to reduce
non-radiative paths that decrease the output of UV LEDs and it is also
necessary to raise the quality of MQW in particular. We wish to
investigate the effects on LED brightness and improvements in the
crystallinity of the MQW layer by introducing a super lattice clad
layer.

Additionally, it is estimated from I-V characteristics that the cause
lies in the boundary between the MQW layer and the p layer (pn
connection) and examination of a p-AlGaN clad layer is thought to be
important. It is also important to study high carrier concentration
p-AlGaN growth as a means for good interface sharpness of the
pn-connection and improvements in electrical characteristics.

\(2\) AlGaInN Light Emitting Layers (High InN Mole Fraction)

InN mole fraction in the GaInN luminescence layer becomes lower in the
ultraviolet region. Consequently the effect of In weakens, reducing the
PL intensity. With an AlGaInN luminescence layer, a high InN mole
fraction is achieved so we have high expectations for In effect. UV
emission at 360 nm was observed from an LED structure using a
four-dimensional mixed crystal AlGaInN MQW structure by current
injection. We previously studied AlGaInN luminescence layers but the
strength decreased by about a factor of 1 compared with blue MQW. The
introduction of Al into the luminescence layer worsens surface flatness
and is a cause of reduced PL intensity. We shall aim for highly
efficient UV luminescence by raising the quality of the AlGaInN layer
through optimization of the growth conditions.

\(3\) ELO Method

With regard to ELO, further reduction of dislocation and optimization of
regrowth conditions will be carried out. A UV LED structure on an
ELO-GaN substrate shall be fabricated and the effect on low dislocation
will be confirmed.

\(4\) Large MOCVD Equipment

We wish to uncover the effects of substrates on LED luminescence
efficiency, making use of multiple charge characteristics. Dislocation
density, luminescence layer skew, and control of the substrate surface
condition are thought to effect luminescence efficiency. Using large
MOCVD equipment, comparison is possible under fixed growth conditions
with substrates with varying dislocation densities, differences in
lattice inconsistencies with the luminescence layer, and different
surface treatments. We wish to investigate how the different dislocation
densities of GaN substrates and ELO-GaN substrates, the different
lattice inconsistencies of SiC substrates and sapphire substrates, and
the pre-processing conditions effect LED luminescence efficiency.
Moreover, it is expected that observation systems using visible light
surface reflectivity will be an effective tool for control of substrate
surface conditions before growth.

\(5\) Applications with SiC Substrates

Compared to sapphire substrates, SiC substrates have characteristics
advantageous to device fabrication such as substrate conductivity and a
smaller lattice inconsistency with the GaN.

## Development of High Output Ultraviolet Emitting Elements

\(1\) The amoung of Si doping of the barrier layer in a UV LED were
examined and an output of 1.2 mW (at 20 mA) was attained with Si doping
of about 2 x 10^17^ cm^-3^. It is believed that Si doping has the effect
of suppressing diffusion of Mg to the luminescence layer and at the same
time works to degrade crystal quality. Upon making this chip into a
flipchip LED, the output was improved by a factor of 2.4. Moreover, by
fabricating a prototype blue LED lamp using RGB phosphors, we were able
to attain a luminescence efficiency of 1.7 lm/W.

\(2\) By treating substrate irregularities and, by using a method which
promotes lateral growth from protrusions (LEPS method) we attempted to
reduce dislocation density of GaN with a single, maskless growth. The
result was a dramatic reduction in the dislocation density of the
laterally grown portion. Also, by applying LEPS method to UV LEDs, an
output of 2.5 mW was attained, which is 3 times that on a conventional
substrate. In this way, we found that LEPS was an effective method for
reducing dislocation density and improving luminescence output.

\(3\) For the purpose of comparison to UV LEDs, we fabricated a blue LED
lamp and a white LED lamp. The blue LED lamp exhibited an output of 9.1
mW with an external quantum efficiency of 17.1%. The white LED's
luminescence efficiency was 12 lm/W.

Future plans are described here. This research has proven that LEPS can
be used to reduce dislocation density and improve ultraviolet
luminescence output. However, dislocation density observed in CL was
still on the order of 10^7^ parts/cm^2^, even if within the low
dislocation region, and on average was 6 x 10^8^ parts/cm^2^. In the
future further advances in the study of LEPS method will result in
greater reductions in dislocation density. By using the LEPS method we
improved the external quantum efficiency of a UV LED to 3.7% while
energized at 20 mA. By improving light extraction efficiency through
flipchip mounting technology and resin sealants, and with further
optimization of device structures and growth conditions, it is thought
that achievement of a UV LED with an external quantum efficiency
exceeding 20% is possible. From here, we plan on working together with
the light source device group to advance research on white lamps using
RGB phosphors and high output UV LED lamps.

Additionally, we plan to study UV LED applications for new substrates,
such as low dislocation GaN substrates fabricated by firms from the
substrate group that will replace conventional sapphire substrates. We
also plan to further reduce dislocations and attain higher output UV
LEDs. We shall also investigate the relationship between higher output
through low dislocation density and wavelength and shall achieve an LED
with the highest output in the ultraviolet region.

## Development of Highly Efficient Phosphor Substances

A method of measuring the quantum efficiency of phosphors was
determined. The measurement involves a process of separating three
characteristics -- absorption ratio, quantum efficiency, and
luminescence efficiency -- and the results are fed back to an experiment
to improve composition of red phospors. The results showed improvement
in luminescence efficiency of 150 to 200%. Even with blue and green, a
quantum efficiencies higher than 1-dimensional model were seen. However,
even among phosphors showing the best quantum efficiency, red, green,
and blue were all in the 60% range. This leaves a large gap between
today and the goal of 90%.

Future plans are described below.

### Improving Quantum Efficiency

Study shall proceed according to the cycle shown below. In concrete
terms, improvements in quantum and luminescence efficiency will be
targeted by searching for a sensitizer capable of communicating energy
to the luminescence center Eu3 of red phosphor Ln2O2S:Eu. Achievements
here will then be expanded to green and blue colors.

Figure 11.6-(1): Means to Improve Quantum Efficiency

Left Column, top to bottom: Phosphor development; Collect data on
existing phosphors \[quantum efficiency of each phosphor, absorption
efficiency, excitation wavelength, intensity dependence\]; Analysis
\[categorized according to host matrix, activating agents, luminescence
type\]; Improvement of existing materials and design of new materials.

Right Column, top to bottom: Establish method to measure powder quantum
efficiency \[standardization, reliability\]; Study of quantum efficiency
measurement in package environment \[includes evaluation of coating film
structure\].

### Synthesis Method

We shall expand ambient atmospheric baking by choosing parameters
different than in the past. Since the gap between current and target
quantum efficiency is large, new concepts are needed. Ultra fine grains
and thin film are said to improve quantum efficiency and the potential
of these will be considered.

### Other

Problems in making technologies practical and solutions thereof will be
pursued, including:

-   Ramp up, measurement, and evaluation of temperature characteristics.

-   Design of equipment to experimentally accelerate degradation.

-   Simulation of white color mixtures

-   Full understanding of light emission mechanisms (joint research with
    Yamaguchi University)

## Development of Lead Bonding Methods, Packaging Technologies, and White LEDs

### Electrode Formation Technology

In addition to achieving high reflectivity in conventional Ti/Al
electrodes, high reflectivity n-type electrode research suggests that
electrodes based on new ZnO materials hold promise.

The optimum electrode is a Pt/Ag electrode. Pt/Ag electrodes have been
found to achieve contact resistance of 10^-4^ Ωcm^2^ and reflectivity of
71.5%.

### Improvement in Light Extraction Efficiency

Etching facet inclination techniques were examined. GaN dry etching is
thought to be effective in not only raising efficiency of light
extraction, but also for luminescence efficiency of phosphors and
UV-resistant sealants. The method involved preparation of two
photoresist masks, each with a different viscosity, and then study of
dependencies among various photoresist conditions on the GaN facet
angle.

The GaN facet angle showed a tendency to become smaller with an increase
in the thickness of the photoresist film. It was also observed that the
GaN facet angle tends to become smaller when the post bake temperature
is raised, even though the resulting photoresist film thickness remains
nearly the same. Examination of the post bake temperature dependence
showed no perceptible change in the GaN facet angle at post bake
temperatures up to 130ºC on Type A photoresist. By post baking at 140ºC,
the GaN facet inclination reached the target 45º slant. With Type B
photoresist a tendency toward reduction of the GaN facet angle with a
rise in post bake temperature was observed. At a post bake temperature
of 140ºC a GaN inclined facet of 45º was attained.

A dependence on the orientation of the GaN facet angle was examined and
none was observed either vertically or horizontally to the c-face
sapphire substrate\[1-100\] direction. Evaluation of reproducibility of
the GaN facet angle under differing photoresist conditions suggests that
the GaN facet angle can be reproduced consistently when the photoresist
mask is thick and the post bake temperature is high.

### Development of White LEDs

A UV LED chip was used to fabricate a white LED and this was evaluated.

Ultraviolet light was shown on the phosphor layer mounted on a UV LED
chip to form a photonic thin film filter transmitting visible light,
then by coating the UV LED chip edge with phosphors we were able to
verify an improvement of 30% in the white LED lamp's total luminous flux
and transmitted ultraviolet light was suppressed from 31% to 13%
compared to an unprocessed one. It was also realized that study of
structures to draw the radiative light from the UV LED to the phosphor
layer is needed to get the most out of photonic thin film filter
characteristics.

White light color rendering of various phosphors was evaluated from the
perspective of the luminescence spectrum shape. It was confirmed that
color rending can be improved by adjusting the phosphors containing high
quantities of components having a broad spectrum distribution. White LED
lamps combining phosphors with broad spectrum distribution, such as
those G and B model phosphors evaluated here, hold promise as a
practical light source for general illumination.

### Development of Resin Sealants for UV LED

In studying hardening accelerators when using hydrogenised bisphenol A
glycidyl ether as an epoxy monomer, we found that the optimum hardening
accelerators were phosphorus-based ones which do not have double bonds.
Since hydrogenised bisphenol A glycidyl ether has a low resistance to
heat, it was found that by doping with a heat tolerant alicyclic epoxy,
heat discoloration could be suppressed.

Future plans are described here. Among development of electrode
formation technologies, we plan to investigate electrode materials and
the effects of different electrode structures on the potential
efficiency and luminescence efficiency of UV LEDs and then identify
electrodes with optimal device characteristics. We shall also evaluate
electrode reliability from the LED characteristics and narrow down final
electrode materials and structures.

We will apply facet inclination techniques established this year via RIE
to LED fabrication and verify the effects of light extraction efficiency
on improvements in luminescence efficiency. Additionally, we shall begin
development of fine processing techniques for chips with the initial aim
of achieving a light extraction efficiency of 50% or greater.

Within development of UV resistant resin technologies, we will not only
continue the light experiments currently underway, but will also
investigate oxidant inhibitors and ultraviolet absorbents to suppress
discoloration. Also, we plan to study reliability and various
characteristics (such as viscosity, heat resistance, water resistance,
and insulation) needed for LED sealant resins.

On the white LED development front, we shall verify the behavior of
visible light conversion efficiency using a chip to which facet
inclination techniques have been applied. Also, we shall examine the
structure of ultraviolet light reflective film that matches the
wavelength of a UV LED and consider the benefit of using excitation
photoluminescence.

## Development of Light Emitting Diode Light Source Devices for Illumination

In this year's research, photonic characteristics of a YAG phosphor were
constructed using actual measurements and a reflective structured LED
light source device combining a blue LED and a YAG phosphor were studied
through optical simulation. The results of this simulation approximate
the photonic characteristics of a real device model built from a blue
LED sold in the marketplace. It was proven that this simulation method
is effective in the design optimization and optical control of light
source devices. Additionally, it was found that light source device
using the reflection model structure examined this time resulted in an
optical energy loss of greater than 70% and so improvement in
efficiencies while suppressing optical energy loss is an important
future research topic.

In contrast, correlated color temperature conversion techniques were
studied by combining red and green LEDs with current blue LEDs and color
rendering was explored. Examination of the relative spectral energy
distribution of three types of LEDs via additive color mixture by
setting the photoluminescence intensity ratio of the green LED to about
20% and then controlling the photoluminescence intensity ratio of the
white LED and the red LED shows achievement of each correlated color
temperature condition with bulb color (N: Tcp = 5026 K), white white
color (WW: Tcp = 3370 K), white color (W: Tcp = 4246 K), neutral white
(N: Tcp = 5026 K), dayglow color (D: Tcp = 6186 K). What's more, high
color rendering of Ra = 95 was attained under dayglow conditions,
thereby exceeding the Ra = 85 of current white LEDs.

With regard to research into environmental suitability, a psychophysics
experiment was conducted to verify the psychological effects on subjects
that an integrated light source in which LEDs are arranged in a
two-dimensional matrix may have. We were unable to understand the
relationship of vision stress induced by the spatial factors of the LED
points. Nevertheless, in this experiment we were able to establish that
a strong correlation exists between glare and discomfort and that it is
logarithmically proportional to the illumination intensity of the sample
stimulus.

Future plans are described here. This year's research showed an optical
energy loss of greater than 70% in light source devices using a
reflection model structure. Using the established optical simulation
methods we will raise efficiency of LED light source devices. By
introducing heat flow analysis and simulation we will come to understand
heat-dissipating structures for LED light source devices and pursue even
greater efficiencies.

With regard to environmental suitability, further detailed psychophysics
experiments will be conducted to evaluate glare, discomfort, and comfort
under an LED illumination environment. The effects of uncomfortable
glare will be made clear and the effects of vision stress induced by the
spatial factors of LED point light sources will be studied.

## Research and Development of Zone Conversion Illumination Systems

This year an optical simulation tool using ray tracking was constructed
and used to design practical illumination prototypes while considering
fabrication errors expected to occur during mass production. As a
result, the following came to be understood:

-   A trapezoidal shape lens pattern, rather than the cylindrical-shaped
    lens pattern proposed before, results in less degradation in light
    extraction efficiency caused by fabrication errors.

-   It was found that locating the LED module at the corner of the photo
    plate rather than along its side resulted in a higher light
    utilization factor.

-   Other parameters are strongly resistant to fabrication errors and do
    not require any special methods.

Since illumination systems demand a high degree of directivity we found
that using a prism sheet as a directional conversion means to achieve
even greater energy saving effect was optimal for photonic plates. By
creating a prototype with a prism mounted to a 4 inch photonic plate,
optical characteristics were measured and the following was discovered.

-   By attaching a prism sheet, the directivity of the light radiating
    from the photonic plate was half perpendicular, with directivity
    improvement to ±17º.

-   The uniformity of the photonic plate is not reduced with the
    introduction of the prism sheet.

Future plans are described here. Using the correlations between shape
precision and energy conversion efficiency discovered in this year's
research as guidelines, next year we shall conceptualize illumination
systems, enumerate challenges thought to exist in the fabrication
process, and analyze fabrication process precision. Our efforts will
focus on photonic plate coupled elements that are greatly impacted by
optical characteristics.

Additionally, we will begin assembling specifications for the
illumination system planned in the final year.

### Robust Design

To this point the objective has been to confirm the principles of
photonic plates and to create prototypes using a simple 2P
(photo-polymerization) method which has superior shape transfer.
However, although this molding method is superior in transferring fine
shapes, it has many problems with regard to manufacturing speed,
materials cost, and large areas. Therefore, it is thought that injection
molding is a better method for manufacturing, but it is not as good at
transferring shapes as the 2P method. This year we shall investigate the
ability of injection molding to transfer fine patterns and then use the
results to create a robust design of fine pattern shapes and
arrangements. To be specific, using transfer shape evaluation samples
created by injection molding, knowledge will be acquired regarding
precision and variability of shape transfer. Taking the transfer
characteristics as an index, it will then be used for shape transfer of
edge R and line formations. Using the resulting transfer
characteristics, the photonic characteristics will be taken as an
evaluation index to optimize design, verifying suitability through
simulation.

With a vector radiative coupling method, it is critical that the LED be
fundamentally localized and the degree to which LED integration can be
pushed is thought to be the determining factor in whether it will be
realized or not. With regard to heat problems, heat dissipation of 100
lm/W targeted by this project is low compared to fluorescent lamps and
it is believed that, even in integrated modules temperature rises can be
suppressed through optimization of non-thermal structures. Still,
reliability of heat simulation is low at present and it is conceivable
that the prototype is unable to raise the level of integration.

It can also be thought that the LED efficiency at the halfway point of
the project is lower than targeted and that the heat radiated will be
greater than expected, preventing a rise in the level of integration. If
this turns into a reality, breaking the LEDs among multiple photonic
plates will be studied. Some think that this is better for the purposes
of maintaining uniformity and this will be studied at the same time.

### Fundamental Specifications

In 2002 -- the final year of this project -- several types of photonic
plate LED illumination prototype samples will be extracted. From these,
we will narrow it down from the perspective of application, shape, and
design robustness to decide the summary specifications.

## Development of Illumination Devices Based on New LED Concepts

We fabricated lighting fixtures using chip-type LEDs having the highest
level luminescence efficiency (15 ml/W) in commercially available
devices. Evaluation of their characteristics made the following clear.

\(1\) It is possible to achieve extremely compact fixtures that are
completely different from conventional lighting fixtures.

\(2\) It is possible to achieve power-saving fixtures through a range
difficult for conventional light sources.

\(3\) It is possible to presently make practical LED illuminated devices
for local light.

\(4\) Heat restrictions for fixture attachment can be relaxed, creating
greater freedom in where a fixture is affixed.

Future plans are described here. To this point we've created prototype
light source substrates using shell-type LEDs and chip-type LEDs as the
illumination source and made the most of these features in the formation
of prototype light fixtures. We confirmed the practicality and merit of
such approaches. From here, it is important to direct development toward
meeting the needs of the marketplace. Thus, LED lighting fixture
concepts held during the initial stages of this research are expanded to
next generation concepts in the table below.

  ---- ----------------- ----------------------- -------------------------
       LED               Initial Concept         Next Generation Concept
       Characteristic                            

  1    Light having      Reflective plate        Resource saving lighting
       directivity       unnecessary. High       fixtures
                         fixture efficiency.     

  2    Free arrangement  Free to arrange color,  Arrange optimally
       of circuits and   number of lamps, and    according to application
       lamps             beam angle              

  3    Heat radiation    Thin lighting fixtures  Freedom in fixture
       low                                       attachment

  4    Long life         Maintenance-free        Perfect for aging society

  5    Lightweight       Portable fixtures       Fixture holding materials
                                                 unneeded. Less effort to
                                                 install.

  6    Withstand low     Low temperature use     Low temperature use
       temperatures      fixtures                fixtures (carried over)

  7    Withstand shock   High reliability under  High reliability under
                         shock and vibration     shock and vibration
                                                 (carried over)
  ---- ----------------- ----------------------- -------------------------

Table 16.15-(1): LED Characteristics and Next Generation Concepts

Compared to general lighting fixtures, architectural lighting fixtures
are subject to greater restrictions (such as size and heat). This
research will expand its view to include these architectural lighting
fixtures since there is adequate value in studying this as well.

With regard to light source level problems, present LEDs are thought up
as semiconductor devices and are not necessarily suitable as an
illumination light source. Consequently, while there are merits in
freedom of arrangement and combination, the flip side to this is full of
problems with variances in color rendering and the color of light
emitted at the integration level due to differences among each LED.
Moreove, when the LED integration density is increased it is conceivable
that temperature-related troubles will occur with the increase in LED
counts. This makes it important to examine at a structural level the
form that LEDs as an illumination light source should take and this
requires cooperation horizontally among the many groups involved in this
project. In the future, we wish to support this project by creating
prototype lighting fixtures based on new concepts using new LED
structures.

The following prototypes will be created for the purpose of
investigating and evaluating LED structures as a new illumination light
source.

1\) Using a commercially available LED, a prototype lighting fixture
with a target luminescence efficiency of 80 lm/W will be created.

2\) A prototype lighting fixture using a new LED structure devised by
this project will be created.

3\) A prototype lighting fixture introducing new concepts (architectural
lighting, for example) will be created.

Additionally, we shall also advance high efficiency with a prototype
circuit created this year and undertake the following studies with the
aim of assuring reliability.

1\) Examination of LED lighting fixture reliability

2\) Establish safety after understanding the effects of LED high output
on people.

# List of Research Publications and Articles

+----------------------+---------+---------------+-------------+----+
| Publication Subject  | Authors | Where         | When        | Se |
|                      |         | publis        | publishe    | ri |
|                      |         | hed/presented | d/presented | al |
|                      |         |               |             | N  |
|                      |         |               |             | o. |
+----------------------+---------+---------------+-------------+----+
| Radiative            | H.      | Physica       | 2000        |    |
| Recombination        | Kudo,   | Status Solidi |             |    |
| Dynamics of Carriers |         | (a) 180, pp.  |             |    |
| in In~x~Ga~1−x~N     | T.      | 27-21.        |             |    |
| Epitaxial Layers     | Tanabe, |               |             |    |
| Revealed by          |         |               |             |    |
| Temperature          | H.      |               |             |    |
| Dependence of        | Ish     |               |             |    |
| Time-resolved        | ibashi, |               |             |    |
| Photoluminescence    |         |               |             |    |
| Spectra              | R.      |               |             |    |
|                      | Zheng,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Optical properties   | Y.      | IEICE         | 2000        |    |
| of bound excitons    | Yamada, | Transactions  |             |    |
| and biexcitons in    |         | on            |             |    |
| GaN                  | C.      | Electronics   |             |    |
|                      | Sasaki, | E83-C(4), pp. |             |    |
|                      |         | 605-611.      |             |    |
|                      | Y.      |               |             |    |
|                      | Y       |               |             |    |
|                      | oshida, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Su      |               |             |    |
|                      | gahara, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | N       |               |             |    |
|                      | ishino, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Sakai   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Effects of electric  | H.      | Physica E7,   | 2000        |    |
| field on             | Kudo,   | pp. 949-952.  |             |    |
| photoluminescence    |         |               |             |    |
| spectra in InGaN     | Y.      |               |             |    |
| ultraviolet          | Yamada, |               |             |    |
| light-emitting       | and     |               |             |    |
| diodes               |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Homoepitaxial growth | S.      | Vacuum, 59,   | 2000        |    |
| of GaN thin layer by | Kubo,   | pp. 277-283.  |             |    |
| molecular beam       |         |               |             |    |
| epitaxy with an RF   | S.      |               |             |    |
| nitrogen plasma      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | O       |               |             |    |
|                      | kazaki, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Nanba,  |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Manabe, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Fundamental          | T.      | Transactions  | 2000        |    |
| characteristics of   | Tamura, | of The        |             |    |
| light sources        |         | Institute of  |             |    |
| utilizing InGaN      | T.      | Electrical    |             |    |
| semiconductor-based  | Se      | Engineers of  |             |    |
| white LEDs (in       | tomoto, | Japan, A 120. |             |    |
| Japanese)            | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Radiative            | H.      | Proceedings   | July, 2000  |    |
| recombination        | Kudo,   | of SPIE,      |             |    |
| mechanisms in        |         | "O            |             |    |
| InGaN/AlGaN          | T.      | ptoelectronic |             |    |
| single-quantum-well  | Tanabe, | Materials and |             |    |
| LED revealed by      |         | Devices II,"  |             |    |
| time-resolved        | H.      | 4078, pp.     |             |    |
| photoluminescence    | Ish     | 8-15.         |             |    |
| spectra under        | ibashi, |               |             |    |
| external electric    |         |               |             |    |
| fields               | R.      |               |             |    |
|                      | Zheng,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Effect of            | S.      | Proceedings   | July, 2000  |    |
| pre-treatment of GaN | Kubo,   | of SPIE,      |             |    |
| substrate for        |         | "O            |             |    |
| homoepitaxial growth | T.      | ptoelectronic |             |    |
| by rf MBE            | O       | Materials and |             |    |
|                      | kazaki, | Devices II,"  |             |    |
|                      |         | 4078, pp.     |             |    |
|                      | S.      | 58-65.        |             |    |
|                      | Manabe, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Characteristics of   | Y.      | Proc. of      | July, 2000  |    |
| high-efficiency      | Uchida, | SPIE,         |             |    |
| InGaN-based white    |         | "Display      |             |    |
| LED Lighting         | T.      | Technologies  |             |    |
|                      | Se      | IV," 4079,    |             |    |
|                      | tomoto, | pp. 120-126.  |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| White illumination   | K.      | Proc. of      | July, 2000  |    |
| characteristics of   | Mu      | SPIE,         |             |    |
| ZnS-based phosphor   | rakami, | "Display      |             |    |
| materials excited by |         | Technologies  |             |    |
| InGaN-based          | T.      | III," 4079,   |             |    |
| ultraviolet          | T       | pp. 112-118.  |             |    |
| Light-emitting diode | aguchi, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Yoshino |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Recombination        | H.      | Proceedings   | Sept., 2000 |    |
| dynamics due to      | Kudo,   | of 25^th^     |             |    |
| polaron state of     |         | International |             |    |
| electrons in         | T.      | Conference on |             |    |
| In~0.08~Ga~0.92~N    | Tanabe, | the Physics   |             |    |
| epitaxial layers     |         | of            |             |    |
|                      | H.      | Se            |             |    |
|                      | Ish     | miconductors, |             |    |
|                      | ibashi, | in press.     |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Zheng,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth and           | S.      | Proceedings   | Sept., 2000 |    |
| Characterization of  | Kurai,  | of            |             |    |
| GaN Epilayer on      |         | International |             |    |
| Sapphire Substrate   | S.      | Workshop on   |             |    |
| by Ammonia Gas       | Kubo,   | Nitride       |             |    |
| Source MBE           |         | S             |             |    |
|                      | T.      | emiconductors |             |    |
|                      | O       | 2000, pp.     |             |    |
|                      | kazaki, | 213-216       |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Ro      |               |             |    |
|                      | samond, |               |             |    |
|                      |         |               |             |    |
|                      | J. Van  |               |             |    |
|                      | Hove,   |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | P. Chow |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Role of              | R.      | Proceedings   | Sept., 2000 |    |
| Nanostructures in    | Zheng,  | of            |             |    |
| the Radiative        | and     | International |             |    |
| Recombination        |         | Workshop on   |             |    |
| Process in           | T.      | Nitride       |             |    |
| InGaN-based          | Taguchi | S             |             |    |
| Light-Emitting       |         | emiconductors |             |    |
| Diodes               |         | 2000, pp.     |             |    |
|                      |         | 860-863.      |             |    |
+----------------------+---------+---------------+-------------+----+
| Effects of AlN       | S.      | Proceedings   |             |    |
| buffer layer on GaN  | Kurai,  | of 8^th^      |             |    |
| thin film growth by  |         | Symposium of  |             |    |
| RF-MBE (in Japanese) | T.      | Society for   |             |    |
|                      | O       | Applied       |             |    |
|                      | kazaki, | Plasma        |             |    |
|                      |         | Research,     |             |    |
|                      | Y.      | 2000 "Plasma  |             |    |
|                      | Nanba,  | applications  |             |    |
|                      |         | and composite |             |    |
|                      | T.      | functional    |             |    |
|                      | Tanabe, | materials,"   |             |    |
|                      |         | Vol. 10, pp.  |             |    |
|                      | S.      | 83-84         |             |    |
|                      | Kubo,   |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Manabe, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Semiconductor        | T.      | Japan Society | May 25,     |    |
| Lighting − Light for | Taguchi | for the       | 2000        |    |
| the 21^st^ Century   |         | Promotion of  |             |    |
| based on LEDs (in    |         | Science,      |             |    |
| Japanese)            |         | Committee No. |             |    |
|                      |         | 125 Symposium |             |    |
|                      |         | (invited)     |             |    |
+----------------------+---------+---------------+-------------+----+
| Behavior of excited  | T.      | The Japan     | May 26,     |    |
| electrons and        | Taguchi | Society of    | 2000        |    |
| defects in           |         | Applied       |             |    |
| InGaN-based liquid   |         | Physics,      |             |    |
| crystal              |         | Chugoku       |             |    |
| semiconductors (in   |         | Branch        |             |    |
| Japanese)            |         | Symposium     |             |    |
|                      |         | "Crystal      |             |    |
|                      |         | Defects       |             |    |
|                      |         | Control using |             |    |
|                      |         | Electron      |             |    |
|                      |         | Excitation"   |             |    |
|                      |         | (invited)     |             |    |
+----------------------+---------+---------------+-------------+----+
| GaN growth by        | S.      | 31^st^        | July        |    |
| ammonia source MBE   | Kurai,  | Conference on | 26\~28,     |    |
| and evaluation of    |         | Crystal       | 2000        |    |
| crystal properties   | S.      | Growth        |             |    |
| (in Japanese)        | Kubo,   |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | O       |               |             |    |
|                      | kazaki, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | P. Chow |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Radiative            | H.      | Photonics,    | July 26-28, |    |
| recombination        | Kudo,   |               | 2000        |    |
| mechanisms in        |         | Taiwan        |             |    |
| InGaN/AlGaN          | T.      |               |             |    |
| single-quantum-well  | Tanabe, |               |             |    |
| LED revealed by      |         |               |             |    |
| time-resolved        | H.      |               |             |    |
| photoluminescence    | Ish     |               |             |    |
| spectra under        | ibashi, |               |             |    |
| external electric    |         |               |             |    |
| fields               | R.      |               |             |    |
|                      | Zheng,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Effect of            | S.      | Photonics,    | July 26-28, |    |
| pre-treatment of GaN | Kubo,   |               | 2000        |    |
| substrate for        |         | Taiwan        |             |    |
| homoepitaxial growth | T.      |               |             |    |
| by rf MBE            | O       |               |             |    |
|                      | kazaki, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Manabe, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Characteristics of   | Y.      | Photonics,    | July 26-28, |    |
| high-efficiency      | Uchida, |               | 2000        |    |
| InGaN-based white    |         | Taiwan        |             |    |
| LED Lighting         | T.      |               |             |    |
|                      | Se      |               |             |    |
|                      | tomoto, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| White illumination   | K.      | Photonics,    | July 26-28, |    |
| characteristics of   | Mu      |               | 2000        |    |
| ZnS-based phosphor   | rakami, | Taiwan        |             |    |
| materials excited by |         |               |             |    |
| InGaN-based          | T.      |               |             |    |
| ultraviolet          | T       |               |             |    |
| Light-emitting diode | aguchi, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Yoshino |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Homoepitaxial GaN    | Y.      | Preprints of  | July 29,    |    |
| thin film growth by  | Nanba,  | Regular       | 2000        |    |
| RF-MBE with shutter  |         | Meeting of    |             |    |
| control (in          | T.      | The Physical  |             |    |
| Japanese)            | O       | Society of    |             |    |
|                      | kazaki, | Japan,        |             |    |
|                      |         | Ch            |             |    |
|                      | S.      | ugoku/Shikoku |             |    |
|                      | Manabe, | Branch and    |             |    |
|                      |         | The Japan     |             |    |
|                      | S.      | Society of    |             |    |
|                      | Kubo,   | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | S.      | Chugoku       |             |    |
|                      | Kurai,  | Branch        |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Exciton-based        | Y.      | Preprints of  | July 29,    |    |
| emission             | Y       | Regular       | 2000        |    |
| characteristics of   | oshida, | Conference of |             |    |
| ELO GaN with W-mask  |         | The Physical  |             |    |
| (in Japanese)        | Y.      | Society of    |             |    |
|                      | Yamada, | Japan,        |             |    |
|                      |         | Ch            |             |    |
|                      | T.      | ugoku/Shikoku |             |    |
|                      | T       | Branch and    |             |    |
|                      | aguchi, | The Japan     |             |    |
|                      |         | Society of    |             |    |
|                      | K.      | Applied       |             |    |
|                      | Hi      | Physics,      |             |    |
|                      | ramatsu | Chugoku       |             |    |
|                      |         | Branch        |             |    |
+----------------------+---------+---------------+-------------+----+
| In doping effects on | T.      | Preprints of  | July 29,    |    |
| GaN thin film growth | O       | Regular       | 2000        |    |
| by RF MBE (in        | kazaki, | Conference of |             |    |
| Japanese)            |         | The Physical  |             |    |
|                      | Y.      | Society of    |             |    |
|                      | Nanba,  | Japan,        |             |    |
|                      |         | Ch            |             |    |
|                      | S.      | ugoku/Shikoku |             |    |
|                      | Manabe, | Branch and    |             |    |
|                      |         | The Japan     |             |    |
|                      | S.      | Society of    |             |    |
|                      | Kubo,   | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | S.      | Chugoku       |             |    |
|                      | Kurai,  | Branch        |             |    |
|                      | and T.  |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Temperature          | H.      | Preprints of  | July 29,    |    |
| dependence of PL     | Ish     | Regular       | 2000        |    |
| spectra of           | ibashi, | Conference of |             |    |
| In~0.08~Ga~0.92~N    |         | The Physical  |             |    |
| mixed crystal thin   | T.      | Society of    |             |    |
| films (in Japanese)  | Tanabe, | Japan,        |             |    |
|                      |         | Ch            |             |    |
|                      | H.      | ugoku/Shikoku |             |    |
|                      | Kudo,   | Branch and    |             |    |
|                      |         | The Japan     |             |    |
|                      | Y.      | Society of    |             |    |
|                      | Yamada, | Applied       |             |    |
|                      | and T.  | Physics,      |             |    |
|                      | Taguchi | Chugoku       |             |    |
|                      |         | Branch        |             |    |
+----------------------+---------+---------------+-------------+----+
| Reverse bias         | T.      | Preprints of  | July 29,    |    |
| dependence of PL     | Tanabe, | Regular       | 2000        |    |
| spectra of           |         | Conference of |             |    |
| In~0.2~Ga~0.8~N SQW  | H.      | The Physical  |             |    |
| Blue LED (in         | Ish     | Society of    |             |    |
| Japanese)            | ibashi, | Japan,        |             |    |
|                      |         | Ch            |             |    |
|                      | H.      | ugoku/Shikoku |             |    |
|                      | Kudo,   | Branch and    |             |    |
|                      |         | The Japan     |             |    |
|                      | Y.      | Society of    |             |    |
|                      | Yamada, | Applied       |             |    |
|                      | and T.  | Physics,      |             |    |
|                      | Taguchi | Chugoku       |             |    |
|                      |         | Branch        |             |    |
+----------------------+---------+---------------+-------------+----+
| Excitation           | C.      | Preprints of  | July 29,    |    |
| wavelength           | Sasaki, | Regular       | 2000        |    |
| dependence of        |         | Conference of |             |    |
| emission spectra of  | M.      | The Physical  |             |    |
| In~x~Ga~1−x~N/GaN    | Iwata,  | Society of    |             |    |
| multiple quantum     |         | Japan,        |             |    |
| well structures (in  | Y.      | Ch            |             |    |
| Japanese)            | Yamada, | ugoku/Shikoku |             |    |
|                      |         | Branch and    |             |    |
|                      | T.      | The Japan     |             |    |
|                      | T       | Society of    |             |    |
|                      | aguchi, | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | S.      | Chugoku       |             |    |
|                      | Wa      | Branch        |             |    |
|                      | tanabe, |               |             |    |
|                      | M. S.   |               |             |    |
|                      | Minsky, |               |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Sc      |               |             |    |
|                      | hneider |               |             |    |
+----------------------+---------+---------------+-------------+----+
| In composition       | M.      | Preprints of  | July 29,    |    |
| dependence of room   | Iwata,  | Regular       | 2000        |    |
| temperature emission |         | Conference of |             |    |
| characteristics in   | C.      | The Physical  |             |    |
| In~x~Ga~1−x~N/GaN    | Sasaki, | Society of    |             |    |
| multiple quantum     |         | Japan,        |             |    |
| well structures (in  | Y.      | Ch            |             |    |
| Japanese)            | Yamada, | ugoku/Shikoku |             |    |
|                      |         | Branch and    |             |    |
|                      | T.      | The Japan     |             |    |
|                      | T       | Society of    |             |    |
|                      | aguchi, | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | S.      | Chugoku       |             |    |
|                      | Wa      | Branch        |             |    |
|                      | tanabe, |               |             |    |
|                      | M. S.   |               |             |    |
|                      | Minsky, |               |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Sc      |               |             |    |
|                      | hneider |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Emission mechanism   | T.      | 33^rd^        | August      |    |
| and emission         | Taguchi | All-Japan     | 24-25, 2000 |    |
| efficiency           |         | Conference of |             |    |
| improvement strategy |         | The Institute |             |    |
| for GaN-based blue   |         | of            |             |    |
| and ultraviolet LEDs |         | Illuminating  |             |    |
| (in Japanese)        |         | Engineers of  |             |    |
|                      |         | Japan         |             |    |
+----------------------+---------+---------------+-------------+----+
| Design of white LED  | Y.      | 33^rd^        | August      |    |
| illumination devices | Uchida, | All-Japan     | 24-25, 2000 |    |
| using multiple point | T.      | Conference of |             |    |
| light source         | Taguchi | The Institute |             |    |
| illuminance          |         | of            |             |    |
| calculation          |         | Illuminating  |             |    |
| technique (in        |         | Engineers of  |             |    |
| Japanese)            |         | Japan         |             |    |
+----------------------+---------+---------------+-------------+----+
| Emission             | K.      | 33^rd^        | August      |    |
| characteristics of   | Mu      | All-Japan     | 24-25, 2000 |    |
| Y, Sr, ZnS-based     | rakami, | Conference of |             |    |
| white fluorescent    | T.      | The Institute |             |    |
| materials excited by | T       | of            |             |    |
| In~x~Ga~1−x~N        | aguchi, | Illuminating  |             |    |
| ultraviolet LEDs (in |         | Engineers of  |             |    |
| Japanese)            | Y.      | Japan         |             |    |
|                      | Yoshida |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Development of solar | T.      | 33^rd^        | August      |    |
| power-based          | Se      | All-Japan     | 24-25, 2000 |    |
| illumination systems | tomoto, | Conference of |             |    |
| utilizing white LEDs | T.      | The Institute |             |    |
| (in Japanese)        | Taguchi | of            |             |    |
|                      |         | Illuminating  |             |    |
|                      |         | Engineers of  |             |    |
|                      |         | Japan         |             |    |
+----------------------+---------+---------------+-------------+----+
| Homoepitaxial GaN    | S.      | Preprints of  | Sept. 3-7,  |    |
| thin film growth by  | Kubo,   | 61^st^        | 2000        |    |
| ammonia gas source   | T.      | Scientific    |             |    |
| MBE (in Japanese)    | O       | Symposium of  |             |    |
|                      | kazaki, | The Japan     |             |    |
|                      | S.      | Society of    |             |    |
|                      | Manabe, | Applied       |             |    |
|                      | Y.      | Physics, Vol. |             |    |
|                      | Nanba,  | No. 1         |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      | and T.  |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| External electric    | H.      | Preprints of  | Sept. 3-7,  |    |
| field dependence of  | Kudo,   | 61^st^        | 2000        |    |
| PL spectra in        | T.      | Scientific    |             |    |
| In~0.2~Ga~0.8~N SWQ  | Tanabe, | Symposium of  |             |    |
| blue LEDs (in        | H.      | The Japan     |             |    |
| Japanese)            | Ish     | Society of    |             |    |
|                      | ibashi, | Applied       |             |    |
|                      | Y.      | Physics, Vol. |             |    |
|                      | Yamada, | No. 1         |             |    |
|                      | and T.  |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Excitation           | C.      | Preprints of  | Sept. 3-7,  |    |
| wavelength           | Sasaki, | 61^st^        | 2000        |    |
| dependence of        |         | Scientific    |             |    |
| emission             | M.      | Symposium of  |             |    |
| characteristics of   | Iwata,  | The Japan     |             |    |
| In~x~Ga~1−x~N/GaN    |         | Society of    |             |    |
| multiple quantum     | Y.      | Applied       |             |    |
| well structures (in  | Yamada, | Physics, Vol. |             |    |
| Japanese)            |         | No. 3         |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Wa      |               |             |    |
|                      | tanabe, |               |             |    |
|                      | M. S.   |               |             |    |
|                      | Minsky, |               |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Sc      |               |             |    |
|                      | hneider |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Fundamental physical | R.      | Preprints of  | Sept. 3-7,  |    |
| properties of InN    | Zheng,  | 61^st^        | 2000        |    |
| quantum lines in     |         | Scientific    |             |    |
| In~x~Ga~1−x~N        | T.      | Symposium of  |             |    |
| epitaxial thin films | Taguchi | The Japan     |             |    |
| (in Japanese)        |         | Society of    |             |    |
|                      |         | Applied       |             |    |
|                      |         | Physics, Vol. |             |    |
|                      |         | No. 1         |             |    |
+----------------------+---------+---------------+-------------+----+
| Evaluation of        | Y.      | 2000 Combined | Sept. 7-8,  |    |
| characteristics of   | Uchida, | Conference of | 2000        |    |
| white LED            | T.      | Fundamen      |             |    |
| illumination light   | Taguchi | tal/Materials |             |    |
| sources using        |         | Sections of   |             |    |
| multiple point light |         | The Institute |             |    |
| source illuminance   |         | of Electrical |             |    |
| calculation          |         | Engineers of  |             |    |
| technique (in        |         | Japan         |             |    |
| Japanese)            |         |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Recombination        | H.      | 25^th^        | Sept.       |    |
| dynamics due to      | Kudo,   | International | 17-22, 2000 |    |
| polaron state of     |         | Conference on |             |    |
| electrons in         | T.      | the Physics   |             |    |
| In~0.08~Ga~0.92~N    | Tanabe, | of            |             |    |
|                      |         | S             |             |    |
|                      | H.      | emiconductors |             |    |
|                      | Ish     |               |             |    |
|                      | ibashi, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Zheng,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and T.  |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth and           | S.      | International | Sept.       |    |
| Characterization of  | Kurai,  | Workshop on   | 24-27, 2000 |    |
| GaN Epilayer on      |         | Nitride       |             |    |
| Sapphire Substrate   | S.      | Se            |             |    |
| by Ammonia Gas       | Kubo,   | miconductors, |             |    |
| Source MBE           |         | 2000          |             |    |
|                      | T.      |               |             |    |
|                      | O       |               |             |    |
|                      | kazaki, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Ro      |               |             |    |
|                      | samond, |               |             |    |
|                      |         |               |             |    |
|                      | J. Van  |               |             |    |
|                      | Hove,   |               |             |    |
|                      | and P.  |               |             |    |
|                      | Chow    |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Role of              | R.      | International | Sept.       |    |
| Nanostructures in    | Zheng,  | Workshop on   | 24-27, 2000 |    |
| the Radiative        | and T.  | Nitride       |             |    |
| Recombination        | Taguchi | Se            |             |    |
| Process in           |         | miconductors, |             |    |
| InGaN-based          |         | 2000          |             |    |
| Light-Emitting       |         |               |             |    |
| Diodes               |         |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Strategy of the      | T.      | Ministry of   | October 6,  |    |
| Compound             | Taguchi | Educations,   | 2000        |    |
| Semiconductor-based  |         | Sports,       |             |    |
| "Lighting for the    |         | Science, and  |             |    |
| 21^st^ Century"      |         | Technology    |             |    |
| National Project and |         | Special Area  |             |    |
| Future Prospects (in |         | Development   |             |    |
| Japanese)            |         | Symposium     |             |    |
|                      |         | (special      |             |    |
|                      |         | invitation)   |             |    |
+----------------------+---------+---------------+-------------+----+
| Compound             | T.      | 27^th^        | October     |    |
| Semiconductor        | T       | International | 2-6, 2000   |    |
| Lighting Based on    | aguchi, | Symposium on  |             |    |
| InGaN Ultraviolet    | and K.  | Compound      |             |    |
| LED and Phosphor     | M       | S             |             |    |
| System               | urakami | emiconductors |             |    |
+----------------------+---------+---------------+-------------+----+
| Effects of AlN       | S.      | 8^th^         | March       |    |
| buffer layer on GaN  | Kurai,  | Symposium of  | 16-17, 2001 |    |
| thin film growth by  |         | Society for   |             |    |
| RF-MBE (in Japanese) | T.      | Applied       |             |    |
|                      | O       | Plasma        |             |    |
|                      | kazaki, | Research      |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Nanba,  |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Tanabe, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kubo,   |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Manabe, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| White LEDs using     | K.      | Preprints of  | March       |    |
| ZnS-, Y-, and        | Mu      | The 48^th^    | 28-31, 2001 |    |
| Sr-based phosphor    | rakami, | Combined      |             |    |
| materials and        |         | Conference on |             |    |
| In~x~Ga~1−x~N        | H.      | Applied       |             |    |
| ultraviolet LEDs (in | Kudo,   | Physics, Vol. |             |    |
| Japanese)            |         | No. 1         |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      | and M.  |               |             |    |
|                      | Yoshino |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Implementation of    | T.      | Preprints of  | March       |    |
| energy-saving street | Taguchi | The 48^th^    | 28-31, 2001 |    |
| lighting systems     |         | Combined      |             |    |
| based on special     |         | Conference on |             |    |
| emission mechanisms  |         | Applied       |             |    |
| of InGaN-based LEDs  |         | Physics, Vol. |             |    |
| (in Japanese)        |         | No. 1         |             |    |
+----------------------+---------+---------------+-------------+----+
| GaN homoepitaxial    | T.      | Preprints of  | March       |    |
| thin film growth on  | Tanabe, | The 48^th^    | 28-31, 2001 |    |
| GaN single crystal   |         | Combined      |             |    |
| substrates by RF-MBE | Y.      | Conference on |             |    |
| (in Japanese)        | Nanba,  | Applied       |             |    |
|                      |         | Physics, Vol. |             |    |
|                      | S.      | No. 1         |             |    |
|                      | Manabe, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kubo,   |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Inoue,  |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Kurita  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Excitation levels of | H.      | Preprints of  | March       |    |
| In~0.08~Ga~0.92~N    | Kudo,   | The 48^th^    | 28-31, 2001 |    |
| mixed crystal thin   |         | Combined      |             |    |
| films (in Japanese)  | K.      | Conference on |             |    |
|                      | Mu      | Applied       |             |    |
|                      | rakami, | Physics, Vol. |             |    |
|                      |         | No. 1         |             |    |
|                      | S.      |               |             |    |
|                      | Kubo,   |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Inoue,  |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Kurita  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Time-resolved        | Y.      | Preprints of  | March       |    |
| non-linear emission  | Y       | The 48^th^    | 28-31, 2001 |    |
| in exciton-based     | oshida, | Combined      |             |    |
| optical transition   |         | Conference on |             |    |
| process in ELO-GaN   | H.      | Applied       |             |    |
| (in Japanese)        | Ku      | Physics, Vol. |             |    |
|                      | gimiya, | No. 1         |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Well layer thickness | H.      | Preprints of  | March       |    |
| dependence of        | Kudo,   | The 48^th^    | 28-31, 2001 |    |
| optical              |         | Combined      |             |    |
| characteristics in   | K.      | Conference on |             |    |
| InGaN multiple       | Mu      | Applied       |             |    |
| quantum well         | rakami, | Physics, Vol. |             |    |
| structures (in       |         | No. 1         |             |    |
| Japanese)            | H.      |               |             |    |
|                      | Ish     |               |             |    |
|                      | ibashi, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Tsu     |               |             |    |
|                      | nekawa, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Ouchi,  |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | O       |               |             |    |
|                      | kagawa, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | datomo, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Imada   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Room temperature     | M.      | Preprints of  | March       |    |
| emission             | Iwata,  | The 48^th^    | 28-31, 2001 |    |
| characteristics of   |         | Combined      |             |    |
| In~x~Ga~1−x~N/GaN    | C.      | Conference on |             |    |
| multiple quantum     | Sasaki, | Applied       |             |    |
| well structures (in  |         | Physics, Vol. |             |    |
| Japanese)            | Y.      | No. 1         |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Wa      |               |             |    |
|                      | tanabe, |               |             |    |
|                      |         |               |             |    |
|                      | M. S.   |               |             |    |
|                      | Minsky, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | S. Y.   |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Sc      |               |             |    |
|                      | hneider |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Excitation spectra   | C.      | Preprints of  | March       |    |
| of In~x~Ga~1−x~N/GaN | Sasaki, | The 48^th^    | 28-31, 2001 |    |
| multiple quantum     |         | Combined      |             |    |
| well structures (in  | M.      | Conference on |             |    |
| Japanese)            | Iwata,  | Applied       |             |    |
|                      |         | Physics, Vol. |             |    |
|                      | Y.      | No. 1         |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Wa      |               |             |    |
|                      | tanabe, |               |             |    |
|                      |         |               |             |    |
|                      | M. S.   |               |             |    |
|                      | Minsky, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | S. Y.   |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Sc      |               |             |    |
|                      | hneider |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Study of initial     | S.      | Preprints of  | March       |    |
| stage processes in   | Kubo,   | The 48^th^    | 28-31, 2001 |    |
| GaN thin film growth |         | Combined      |             |    |
| by ammonia source    | S.      | Conference on |             |    |
| MBE (in Japanese)    | Onishi, | Applied       |             |    |
|                      |         | Physics, Vol. |             |    |
|                      | T.      | No. 1         |             |    |
|                      | O       |               |             |    |
|                      | kazaki, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Temperature          | S.      | International | 2000        |    |
| Dependent Carrier    | Wa      | Workshop on   |             |    |
| Dynamics in          | tanabe, | Nitride       |             |    |
| GaInN/GaN Multiple   |         | Se            |             |    |
| Quantum Well with    | M. S.   | miconductors, |             |    |
| Varying In           | Minsky, | 2000          |             |    |
| Composition          |         |               |             |    |
|                      | N. Y.   |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | R.      |               |             |    |
|                      | Sch     |               |             |    |
|                      | neider, |               |             |    |
|                      |         |               |             |    |
|                      | C.      |               |             |    |
|                      | Sasaki, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Iwata,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| High Temperature     | M. S.   | 61^st^        | Sept. 6,    |    |
| Time Resolved        | Minsky, | Scientific    | 2000        |    |
| Photoluminescence of |         | Symposium of  |             |    |
| InGaN/GaN Multi      | S.      | The Japan     |             |    |
| Quantum Well LEDs    | Wa      | Society of    |             |    |
|                      | tanabe, | Applied       |             |    |
|                      |         | Physics       |             |    |
|                      | R.      |               |             |    |
|                      | Sch     |               |             |    |
|                      | neider, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | C.      |               |             |    |
|                      | Sasaki, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Iwata,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Emission efficiency  | S.      | 62^nd^        | 2001        |    |
| and emission         | Wa      | Scientific    |             |    |
| lifetime             | tanabe, | Symposium of  |             |    |
| characteristics of   |         | The Japan     |             |    |
| GaInN/GaN-based      | Milan   | Society of    |             |    |
| multiple quantum     | Minsky, | Applied       |             |    |
| well LEDs (in        |         | Physics       |             |    |
| Japanese)            | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | N. Y.   |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | C.      |               |             |    |
|                      | Sasaki, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Iwata,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Photoluminescence    | M. S.   | 61^nd^        | 2001        |    |
| peak shift in        | Minsky, | Scientific    |             |    |
| GaInN/GaN            |         | Symposium of  |             |    |
| multi-quantum wells  | S.      | The Japan     |             |    |
| with Si doped        | Wa      | Society of    |             |    |
| barriers             | tanabe, | Applied       |             |    |
|                      |         | Physics       |             |    |
|                      | T.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | keuchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      |         |               |             |    |
|                      | W.      |               |             |    |
|                      | Goetz,  |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | G       |               |             |    |
|                      | ardner, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | St      |               |             |    |
|                      | ockman, |               |             |    |
|                      | and M.  |               |             |    |
|                      | Krames  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Carrier              | S.      | Submitted to  | 2001        |    |
| Recombination        | Wa      | PIERS2001     |             |    |
| Processes in         | tanabe, |               |             |    |
| GaInN/GaN Multiple   |         |               |             |    |
| Quantum Well LED     | M. S.   |               |             |    |
| with varying In      | Minsky, |               |             |    |
| Composition          |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | T       |               |             |    |
|                      | akeuchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| GaN bulk wafer and   | O. Oda  | SEMI          | June 1,     |    |
| its applications (in |         | Ja            | 2000        |    |
| Japanese)            |         | pan-sponsored |             |    |
|                      |         | Workshop on   |             |    |
|                      |         | Compound      |             |    |
|                      |         | S             |             |    |
|                      |         | emiconductors |             |    |
|                      |         | "             |             |    |
|                      |         | Technological |             |    |
|                      |         | Strategy for  |             |    |
|                      |         | Compound      |             |    |
|                      |         | Semiconductor |             |    |
|                      |         | Materials and |             |    |
|                      |         | Devices"      |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of Bulk GaN   | T.      | The 1^st^     | August 31,  |    |
| Single Crystals by   | Inoue,  | Asian         | 2000        |    |
| the                  |         | Conference on |             |    |
| Pressure-Controlled  | Y.      | Crystal       |             |    |
| Solution Growth      | Seki,   | Growth and    |             |    |
| Method               |         | Crystal       |             |    |
|                      | O. Oda, | Technology    |             |    |
|                      |         | (CGCT-1),     |             |    |
|                      | S.      | T-B-12        |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Pressure-Controlled  | T.      | 9^th^         | Sept. 25,   |    |
| Solution Growth of   | Inoue,  | Conference on | 2000        |    |
| Bulk GaN Crystals    |         | High Pressure |             |    |
| under High Pressure  | Y.      | Semiconductor |             |    |
| (invited)            | Seki,   | Physics       |             |    |
|                      |         | (HPSP9), 25C1 |             |    |
|                      | O. Oda, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of Bulk GaN   | T.      | 48^th^        | March 30,   |    |
| Single Crystals By   | Inoue,  | Combined      | 2001        |    |
| Pressure-Controlled  |         | Conference on |             |    |
| Solution Growth      | H.      | Applied       |             |    |
| Method (II) (in      | Kurita, | Physics       |             |    |
| Japanese)            |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Evaluation of Bulk   | T.      | 48^th^        | March 30,   |    |
| GaN Single Crystals  | Inoue,  | Combined      | 2001        |    |
| Grown By             |         | Conference on |             |    |
| Pressure-Controlled  | H.      | Applied       |             |    |
| Solution Growth      | Kurita, | Physics       |             |    |
| Method (in Japanese) |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of Bulk GaN   | T.      | Jpn. J. Appl. | 2000        |    |
| Single Crystals by   | Inoue,  | Phys. 39,     |             |    |
| the                  |         | 2394-2398.    |             |    |
| Pressure-Controlled  | Y.      |               |             |    |
| Solution Growth      | Seki,   |               |             |    |
| Method               |         |               |             |    |
|                      | O. Oda, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Development of GaN   | O. Oda, | IEICE Trans.  | 2000        |    |
| Bulk Substrates for  |         | Electron      |             |    |
| GaN Based LEDs and   | T.      | E83-C,        |             |    |
| LDs                  | Inoue,  | 639-646       |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Seki,   |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Wa      |               |             |    |
|                      | kahara, |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Y       |               |             |    |
|                      | oshida, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| GaN Bulk Substrates  | O. Oda, | Phys. Stat.   | 2000        |    |
| for GaN Based LEDs   |         | Sol. 180,     |             |    |
| and LDs              | T.      | 51-58         |             |    |
|                      | Inoue,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Seki,   |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Ka      |               |             |    |
|                      | inosho, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Ya      |               |             |    |
|                      | egashi, |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Wa      |               |             |    |
|                      | kahara, |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Y       |               |             |    |
|                      | oshida, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of Bulk GaN   | T.      | J. of Jpn.    | 2000        |    |
| Single Crystals by   | Inoue,  | Association   |             |    |
| Pressure-Controlled  |         | of Crystal    |             |    |
| Solution Growth      | Y.      | Growth 27,    |             |    |
| Method (in Japanese) | Seki,   | 54-59         |             |    |
|                      |         |               |             |    |
|                      | O. Oda, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Pressure-Controlled  | T.      | Phys. Stat.   | 2001        |    |
| Solution Growth of   | Inoue,  | Sol. (b) 223, |             |    |
| Bulk GaN Crystals    |         | 15-27.        |             |    |
| under High Pressure  | Y.      |               |             |    |
| (Editor's Choice)    | Seki,   |               |             |    |
|                      |         |               |             |    |
|                      | O. Oda, |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Kurai,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Yamada, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Taguchi |               |             |    |
+----------------------+---------+---------------+-------------+----+
| GaNP growth by       | Y.      | Japanese      | July 26,    |    |
| Photo-assisted MOCVD | Y       | Association   | 2000        |    |
| (in Japanese)        | oshida, | of Crystal    |             |    |
|                      |         | Growth        |             |    |
|                      | Y.      |               |             |    |
|                      | Itoh,   |               |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Nitride rich         | S.      | Workshop on   | Sept. 27,   |    |
| GaN~1−x~P~x~ growth  | Y       | Nitride       | 2000        |    |
| by photo-assisted    | oshida, | Semicond.     |             |    |
| MOCVD and Int'l      |         | IWN2000       |             |    |
| Workshop on Nitride  | J. J.   |               |             |    |
| Semicond.            | Kikawa, |               |             |    |
|                      |         |               |             |    |
|                      | Y. Itoh |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Evaluation of GaNP   | Y.      | 61^st^        | Sept. 5,    |    |
| grown by MOCVD (in   | Itoh,   | Scientific    | 2000        |    |
| Japanese)            |         | Symposium of  |             |    |
|                      | S.      | The Japan     |             |    |
|                      | Y       | Society of    |             |    |
|                      | oshida, | Applied       |             |    |
|                      |         | Physics       |             |    |
|                      | J. J.   |               |             |    |
|                      | Kikawa. |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Hexagonal GaNP       | S.      | The 1^st^     | August 2000 |    |
| growth by            | Y       | Asian Conf.   |             |    |
| laser-assisted MOCVD | oshida, | On Crystal    |             |    |
|                      |         | Growth and    |             |    |
|                      | J.      | Crystal       |             |    |
|                      | Kikawa, | Tech., Sendai |             |    |
|                      |         |               |             |    |
|                      | Y. Itoh |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Organometallic       | T.      | 61^st^        | Sept. 5,    |    |
| growth of N rich     | Kimura, | Scientific    | 2000        |    |
| hexagonal and cubic  |         | Symposium of  |             |    |
| GaNP mixed crystals  | S.      | The Japan     |             |    |
| (in Japanese)        | Y       | Society of    |             |    |
|                      | oshida, | Applied       |             |    |
|                      |         | Physics       |             |    |
|                      | Y.      |               |             |    |
|                      | Itoh,   |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Chudan, |               |             |    |
|                      |         |               |             |    |
|                      | Wu Jun, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Onabe,  |               |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Shiroki |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Nitride rich GaNP    | S.      | 2000 MRS Fall | Nov. 27,    |    |
| hexagonal growth     | Y       | Meeting,      | 2000        |    |
| using MOCVD          | oshida, | Boston        |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa, |               |             |    |
|                      |         |               |             |    |
|                      | Y. Itoh |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Nitride rich         | J. J.   | Proc. Int.'l  | Dec. 2000   |    |
| GaN~1−x~P~x~ growth  | Kikawa, | Workshop on   |             |    |
| by photo-assisted    |         | Nitride       |             |    |
| MOCVD and its LED    | S.      | Semicon. IPAP |             |    |
| property             | Y       | Conf. Ser. 1, |             |    |
|                      | oshida, | 429           |             |    |
|                      |         |               |             |    |
|                      | Y. Itoh |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Hexagonal GaNP       | J. J.   | J. Crystal    | May 2001    |    |
| growth by            | Kikawa, | Growth, in    |             |    |
| laser-assisted MOCVD |         | press         |             |    |
|                      | S.      |               |             |    |
|                      | Y       |               |             |    |
|                      | oshida, |               |             |    |
|                      |         |               |             |    |
|                      | Y. Itoh |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of GaNP by    | Y.      | 9^th^         | December    |    |
| photo-assisted MOCVD | Itoh,   | Symposium on  | 13-14, 2000 |    |
| and its evaluation   |         | SiC and       |             |    |
| (in Japanese)        | S.      | related wide  |             |    |
|                      | Y       | bandgap       |             |    |
|                      | oshida, | s             |             |    |
|                      |         | emiconductors |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Nitride rich GaNP    | J. J.   | To be         | Apr., 2001  |    |
| hexagonal GaNP       | Kikawa, | published in  |             |    |
| growth using MOCVD   |         | MRS symp.     |             |    |
|                      | S.      | Proc., 2001   |             |    |
|                      | Y       | (MRS Web      |             |    |
|                      | oshida, | site)         |             |    |
|                      |         |               |             |    |
|                      | Y. Itoh |               |             |    |
+----------------------+---------+---------------+-------------+----+
| GaNP LED prototype   | S.      | 48^th^        | March 31,   |    |
| (in Japanese)        | Y       | Conference of | 2001        |    |
|                      | oshida, | The Japan     |             |    |
|                      |         | Society of    |             |    |
|                      | Y.      | Applied       |             |    |
|                      | Itoh,   | Physics       |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Onabe,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Shiroki |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Evaluation of        | Y.      | 48^th^        | March 30,   |    |
| MOCVD-grown GaNP (2) | Itoh,   | Conference of | 2001        |    |
| (in Japanese)        |         | The Japan     |             |    |
|                      | S.      | Society of    |             |    |
|                      | Y       | Applied       |             |    |
|                      | oshida, | Physics       |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Onabe,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Shiroki |               |             |    |
+----------------------+---------+---------------+-------------+----+
| GaNP growth by MOCVD | Y.      | 48^th^        | March 31,   |    |
| (in Japanese)        | Itoh,   | Conference of | 2001        |    |
|                      |         | The Japan     |             |    |
|                      | N.      | Society of    |             |    |
|                      | Chudan, | Applied       |             |    |
|                      |         | Physics       |             |    |
|                      | S.      |               |             |    |
|                      | Y       |               |             |    |
|                      | oshida, |               |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Onabe,  |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Shiroki |               |             |    |
+----------------------+---------+---------------+-------------+----+
| MOVPE growth of      | N.      | 48^th^        | March 30,   |    |
| lattice-matched      | Chudan, | Conference of | 2001        |    |
| quaternary           |         | The Japan     |             |    |
| Ga~1-x~In~x~P~1-y~N  | T.      | Society of    |             |    |
| mixed crystals on    | Kimura, | Applied       |             |    |
| GaP substrate (in    |         | Physics       |             |    |
| Japanese)            | Wu Jun, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Itoh,   |               |             |    |
|                      |         |               |             |    |
|                      | S.      |               |             |    |
|                      | Y       |               |             |    |
|                      | oshida, |               |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Kikawa, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Onabe,  |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Shiroki |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Recent progress in   | K.      | The 3^rd^     | July 5,     |    |
| selective area       | Hi      | Int. Conf. On | 1999        |    |
| growth and ELO of    | ramatsu | Nitride       |             |    |
| III-nitrides         |         | S             |             |    |
|                      | K.      | emiconductors |             |    |
|                      | Nis     |               |             |    |
|                      | hiyama, |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Mo      |               |             |    |
|                      | togaito |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Recent progress in   | K.      | Phys. Stat.   | July 4,     |    |
| selective area       | Hir     | Sol.          | 1999        |    |
| growth and Epitaxial | amatsu, |               |             |    |
| Lateral Overgrowth   |         | \(a\) 176,    |             |    |
| of III-Nitrides:     | K.      | 535           |             |    |
| Effects of Reactor   | Nis     |               |             |    |
| Pressure in MOVPE    | hiyama, |               |             |    |
| Growth               |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Mot     |               |             |    |
|                      | ogaito, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Optical and          | H.      | Jpn. J. Appl. | 1999        |    |
| crystalline          | Sone,   | Phys. 38,     |             |    |
| properties of        |         | L356          |             |    |
| epitaxial lateral    | S.      |               |             |    |
| overgrown GaN using  | Nanbu,  |               |             |    |
| W-mask by HVPE       |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Kaw     |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Yam     |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Sawaki  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Crystal orientation  | Y.      | Jpn. J. Appl. |             |    |
| fluctuation of       | Honda,  | Phys. 38,     |             |    |
| epitaxial lateral    |         | L1299         |             |    |
| overgrown GaN with W | Y.      |               |             |    |
| and SiO~2~ mask      | Iy      |               |             |    |
| observed by TEM and  | echika, |               |             |    |
| X-ray rocking curves |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Sone,   |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | N. N.   |               |             |    |
|                      | Sawaki  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Effects of heat      | S.      | The 46^th^    | March 1999  |    |
| treatment on W/n-GaN | Nanbu,  | Combined      |             |    |
| junction (in         |         | Conference on |             |    |
| Japanese)            | H.      | Applied       |             |    |
|                      | Sone,   | Physics       |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Kaw     |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Sawaki, |               |             |    |
|                      |         |               |             |    |
|                      | U.      |               |             |    |
|                      | Mot     |               |             |    |
|                      | otsune, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| W buried structure   | K.      | 60^th^        | September   |    |
| in selective lateral | Hir     | Scientific    | 1999        |    |
| growth and its       | amatsu, | Conference of |             |    |
| evaluation (in       |         | The Japan     |             |    |
| Japanese)            | S.      | Society of    |             |    |
|                      | Nanbu,  | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | Y.      | 2p-W-6        |             |    |
|                      | Kaw     |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Sawaki, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Haino,  |               |             |    |
|                      |         |               |             |    |
|                      | U.      |               |             |    |
|                      | Mot     |               |             |    |
|                      | otsune, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Effects of pressure  | H.      | 60^th^        | September   |    |
| on GaN grown by      | Mizuya, | Scientific    | 1999        |    |
| selective lateral    |         | Conference of |             |    |
| growth using         | M.      | The Japan     |             |    |
| reduced-pressure     | Haino,  | Society of    |             |    |
| MOVPE (in Japanese)  |         | Applied       |             |    |
|                      | K. K.   | Physics,      |             |    |
|                      | Nis     | 2p-W-3        |             |    |
|                      | hiyama, |               |             |    |
|                      |         |               |             |    |
|                      | U.      |               |             |    |
|                      | Mot     |               |             |    |
|                      | otsune, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hi      |               |             |    |
|                      | ramatsu |               |             |    |
+----------------------+---------+---------------+-------------+----+
| MOVPE-GaN selective  | S.      | 60^th^        | September   |    |
| growth-based         | Nanbu,  | Scientific    | 1999        |    |
| fabrication of W     |         | Conference of |             |    |
| buried structures    | Y.      | The Japan     |             |    |
| (in Japanese)        | Kaw     | Society of    |             |    |
|                      | aguchi, | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | N.      | 2p-W-6        |             |    |
|                      | Sawaki, |               |             |    |
|                      |         |               |             |    |
|                      | U.      |               |             |    |
|                      | Mot     |               |             |    |
|                      | otsune, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Evaluation of ELO    | Y.      | 60^th^        | September   |    |
| GaN with W mask by   | Honda,  | Scientific    | 1999        |    |
| TEM (in Japanese)    |         | Conference of |             |    |
|                      | Y.      | The Japan     |             |    |
|                      | Iy      | Society of    |             |    |
|                      | echika, | Applied       |             |    |
|                      |         | Physics,      |             |    |
|                      | T.      | 2p-W-6        |             |    |
|                      | Maeda,  |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Sone,   |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Sawaki  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Crystalline and      | K.      | Special Issue | 2000        |    |
| optical properties   | Hir     | on Blue Laser |             |    |
| of ELO GaN by HVPE   | amatsu, | Diodes and    |             |    |
| using tungsten mask  |         | related       |             |    |
|                      | A.      | Devices,      |             |    |
|                      | Mot     | IEICE Trans.  |             |    |
|                      | ogaito, | Electron.,    |             |    |
|                      |         | Vol. E83-C,   |             |    |
|                      | H.      | p. 620.       |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Honda,  |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      |         |               |             |    |
|                      | F.      |               |             |    |
|                      | B       |               |             |    |
|                      | ertram, |               |             |    |
|                      |         |               |             |    |
|                      | J.      |               |             |    |
|                      | Ch      |               |             |    |
|                      | risten, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Hoffman |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Buried tungsten      | M.      | Jpn. J. Appl. | 2000        |    |
| metal structure      | Haino,  | Phys., 39,    |             |    |
| fabricated by        |         | L449          |             |    |
| epitaxia             | M.      |               |             |    |
| l-lateral-overgrowth | Yam     |               |             |    |
| GaN via low-pressure | aguchi, |               |             |    |
| MOVPE                |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | A.      |               |             |    |
|                      | Mot     |               |             |    |
|                      | ogaito, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Kaw     |               |             |    |
|                      | aguchi, |               |             |    |
|                      |         |               |             |    |
|                      | N.      |               |             |    |
|                      | Sawaki, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Selective lateral    | H.      | The 47^th^    | March 2000  |    |
| overgrowth (ELO) of  | Miyake, | Combined      |             |    |
| GaN using WN~x~ as a |         | Conference on |             |    |
| mask and WN~x~/GaN   | K.      | Applied       |             |    |
| electrode            | Hir     | Physics       |             |    |
| characteristics (in  | amatsu, |               |             |    |
| Japanese)            |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| GaN layer structures | K.      | European-MRS  | May 30,     |    |
| with buried Tungsten | Hir     |               | 2000        |    |
| Nitride (WN) using   | amatsu, |               |             |    |
| epitaxial lateral    |         |               |             |    |
| overgrowth (ELO) via | H.      |               |             |    |
| MOVPE                | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Fabrication and      | H.      | The Institute | May 2000    |    |
| evaluation of GaN    | Miyake, | of            |             |    |
| structures with      |         | Electronics,  |             |    |
| buried WN~x~         | K.      | Information,  |             |    |
| electrode by ELO (in | Hir     | and           |             |    |
| Japanese)            | amatsu, | Communication |             |    |
|                      |         | Engineers     |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Reduction of         | H.      | The Institute | May 2000    |    |
| dislocations in GaN  | Miyake, | of            |             |    |
| by FACELO            |         | Electronics,  |             |    |
| (facet-controlled    | K.      | Information,  |             |    |
| ELO) (in Japanese)   | Hir     | and           |             |    |
|                      | amatsu, | Communication |             |    |
|                      |         | Engineers     |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Fabrication and      | K.      | Int. Conf. On | June 5,     |    |
| characterization of  | Hir     | Metal Organic | 2000        |    |
| low defect density   | amatsu, | Vapor Phase   | (invited)   |    |
| GaN using facet      |         | Epitaxy       |             |    |
| controlled epitaxial | H.      |               |             |    |
| lateral overgrowth   | Miyake, |               |             |    |
| (FACELO)             |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Evaluation of FACELO | H.      | All-Japan     | July 7,     |    |
| (facet-controlled    | Miyake, | Conference on | 2000        |    |
| selective lateral    |         | Crystal       |             |    |
| overgrowth)-grown    | K.      | Growth        |             |    |
| GaN crystals (in     | Hir     |               |             |    |
| Japanese)            | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Reduction of         | H.      | IWN 2000      | September   |    |
| dislocation density  | Miyake, |               | 24, 2000    |    |
| in GaN by Facet      |         |               |             |    |
| Controlled ELO       | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Epitaxial lateral    | H.      | IWN 2000      | September   |    |
| overgrowth of GaN    | Miyake, |               | 24, 2000    |    |
| using Tungsten       |         |               |             |    |
| Nitride (WN~x~) mask | K.      |               |             |    |
| via MOVPE and        | Hir     |               |             |    |
| electrical           | amatsu, |               |             |    |
| properties of        |         |               |             |    |
| WN~x~/GaN contacts   | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
|                      | et al.  |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Carrier gas          | S.      | Preprints of  | March       |    |
| dependence of GaN    | Mac     | The 48^th^    | 28-31, 2001 |    |
| grown using the ELO  | himoto, | Combined      |             |    |
| technique by HVPE    |         | Conference on |             |    |
| (in Japanese)        | M.      | Applied       |             |    |
|                      | Fukui,  | Physics, Vol. |             |    |
|                      |         | No. 1         |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hir     |               |             |    |
|                      | amatsu, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Iy      |               |             |    |
|                      | echika, |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Maeda   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Transmission         | Y.      | Jpn. J. Appl. | 2001        |    |
| Electron Microscopy  | Honda,  | Phys., 40,    |             |    |
| Investigation of     |         | L309          |             |    |
| Dislocations in GaN  | Y.      |               |             |    |
| Layer Grown by       | Iy      |               |             |    |
| Facet-Controlled     | echika, |               |             |    |
| Epitaxial Lateral    |         |               |             |    |
| Overgrowth           | T.      |               |             |    |
|                      | Maeda,  |               |             |    |
|                      |         |               |             |    |
|                      | H.      |               |             |    |
|                      | Miyake, |               |             |    |
|                      | and     |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Hi      |               |             |    |
|                      | ramatsu |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of GaN Single | M.      | The 31^st^    | July 2000   |    |
| Crystals from Vapor  | T       | All-Japan     |             |    |
| Phase (in Japanese)  | atsumi, | Conference on |             |    |
|                      |         | Crystal       |             |    |
|                      | T.      | Growth        |             |    |
|                      | Hirota, |               |             |    |
|                      |         |               |             |    |
|                      | M.      |               |             |    |
|                      | Kusao   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of GaN        | T.      | The 1^st^     | August 31,  |    |
| Crystals from Vapor  | Hirota  | Asian         | 2000        |    |
| Phase                | and     | Conference on |             |    |
|                      |         | Crystal       |             |    |
|                      | M.      | Growth and    |             |    |
|                      | Tatsumi | Crystal       |             |    |
|                      |         | Technology    |             |    |
+----------------------+---------+---------------+-------------+----+
| Growth of low        | H.      | Preprints of  | March       |    |
| dislocation density  | O       | The 48^th^    | 28-31, 2001 |    |
| GaN by lateral       | kagawa, | Combined      |             |    |
| overgrowth on        |         | Conference on |             |    |
| processed substrates | T.      | Applied       |             |    |
| (2) (in Japanese)    | Shi     | Physics, Vol. |             |    |
|                      | roichi, | No. 1         |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Ouchi,  |               |             |    |
|                      |         |               |             |    |
|                      | T.      |               |             |    |
|                      | Tsu     |               |             |    |
|                      | nekawa, |               |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Ta      |               |             |    |
|                      | datomo, |               |             |    |
|                      |         |               |             |    |
|                      | Y.      |               |             |    |
|                      | Imada   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Study of light       | K.      | Proc. of      | August 25,  |    |
| source materials     | Yamada, | All-Japan     | 2000        |    |
| combining blue LED   |         | Conference of |             |    |
| with YAG phosphors   | S.      | The Institute |             |    |
| (in Japanese)        | Murai,  | of            |             |    |
|                      |         | Illuminating  |             |    |
|                      | Y.      | Engineers of  |             |    |
|                      | Imai,   | Japan, 2000   |             |    |
|                      |         |               |             |    |
|                      | K.      |               |             |    |
|                      | Ishii   |               |             |    |
+----------------------+---------+---------------+-------------+----+
| Study of structure   | K.      | Proc. of      | November    |    |
| and emission         | Yamada, | Kansai Branch | 26, 2000    |    |
| characteristics of   |         | Conference of |             |    |
| phosphor conversion  | Y.      | The Institute |             |    |
| type LED light       | Imai,   | of            |             |    |
| source devices (in   |         | Illuminating  |             |    |
| Japanese)            | K.      | Engineers of  |             |    |
|                      | Ishii   | Japan, 2000   |             |    |
+----------------------+---------+---------------+-------------+----+

PATENTS

+-----------------+------------------+---------------+----------------+
| Title of        | Inventor(s)      | Application   | Application    |
| Invention       |                  | Date          | No.            |
+-----------------+------------------+---------------+----------------+
| GaN Based Light | Kiyoteru YOSHIDA | May 11, 2000  | Japanese       |
| Emitting        |                  |               | Patent         |
| Element and     |                  |               | Application    |
| Method of Its   |                  |               | No.            |
| Manufacture     |                  |               | 2000-138978    |
+-----------------+------------------+---------------+----------------+
| Lighting        | Osamu UEDA,      | May 16, 2000  | Japanese       |
| Fixture         |                  |               | Patent         |
|                 | Akihito YASUHARA |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2000-142690    |
+-----------------+------------------+---------------+----------------+
| Lighting Lamp   | Osamu UEDA,\     | May 16, 2000  | Japanese       |
|                 | Masaki KOYAMA    |               | Patent         |
|                 |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2000-142691    |
+-----------------+------------------+---------------+----------------+
| Lighting        | Kunio SATO,\     | May 16, 2000  | Japanese       |
| Fixture         | Kin'ichi SUGINO  |               | Patent         |
|                 |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2000-142692    |
+-----------------+------------------+---------------+----------------+
| Method for      | Tatsu Hirota,\   | May 26, 2000  | Japanese       |
| Growing III-V   | Masami TATSUMI   |               | Patent         |
| Nitride         |                  |               | Application    |
| Semiconductor   |                  |               | No.            |
| and Vapor       |                  |               | 2000-157036    |
| Growth Device   |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | May 26, 2000  | Japanese       |
| Nitride         | Masami TATSUMI   |               | Patent         |
| Semiconductor   |                  |               | Application    |
| Growth Method   |                  |               | No.            |
| and Growth\     |                  |               | 2000-157041    |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | June 29, 2000 | 00113814.8     |
| Nitride         | Masami TATSUMI   |               | (Europe)       |
| Semiconductor   |                  |               |                |
| Growth Method   |                  |               |                |
| and Vapor Phase |                  |               |                |
| Growth          |                  |               |                |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | June 29, 2000 | 09/605845      |
| Nitride         | Masami TATSUMI   |               |                |
| Semiconductor   |                  |               | (United        |
| Growth Method   |                  |               | States)        |
| and Vapor Phase |                  |               |                |
| Growth          |                  |               |                |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | June 29, 2000 | 2313155        |
| Nitride         | Masami TATSUMI   |               |                |
| Semiconductor   |                  |               | (Canada)       |
| Growth Method   |                  |               |                |
| and Vapor Phase |                  |               |                |
| Growth          |                  |               |                |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | June 29, 2000 | 36354/00       |
| Nitride         | Masami TATSUMI   |               |                |
| Semiconductor   |                  |               | (South Korea)  |
| Growth Method   |                  |               |                |
| and Vapor Phase |                  |               |                |
| Growth          |                  |               |                |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | June 30, 2000 | 89112997       |
| Nitride         | Masami TATSUMI   |               |                |
| Semiconductor   |                  |               | (Taiwan)       |
| Growth Method   |                  |               |                |
| and Vapor Phase |                  |               |                |
| Growth          |                  |               |                |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Tatsu Hirota,\   | June 30, 2000 | 001200164\.6   |
| Nitride         | Masami TATSUMI   |               |                |
| Semiconductor   |                  |               | (People's      |
| Growth Method   |                  |               | Republic of    |
| and Vapor Phase |                  |               | China)         |
| Growth          |                  |               |                |
| Apparatus       |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Lighting        | Osamu UEDA,      | July 17, 2000 | Japanese       |
| fixture         |                  |               | Patent         |
|                 | Masaki KOYAMA,   |               | Application    |
|                 |                  |               | No.            |
|                 | Akihito YASUHARA |               | 2000-215410    |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | September 5,  | Japanese       |
| Light Emitting  | TADATOMO,        | 2000          | Patent         |
| Element         |                  |               | Application    |
|                 | Hiroaki OKAGAWA, |               | No.            |
|                 |                  |               | 2000-268274    |
|                 | Tatsuo           |               |                |
|                 | SHIROICHI,       |               |                |
|                 |                  |               |                |
|                 | Yoichiro OUCHI,  |               |                |
|                 |                  |               |                |
|                 | Takashi          |               |                |
|                 | TSUNEKAWA        |               |                |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | September 18, | Japanese       |
| Substrate       | TADATOMO,        | 2000          | Patent         |
| Material        |                  |               | Application    |
|                 | Hiroaki OKAGAWA, |               | No.            |
|                 | Yoichiro OUCHI,  |               | 2000-282047    |
|                 |                  |               |                |
|                 | Takashi          |               |                |
|                 | TSUNEKAWA        |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | September 22, | Japanese       |
| compound        | HIRAMATSU,       | 2000          | Patent         |
| semiconductor   |                  |               | Application    |
| and method of   | Takayoshi MAEDA, |               | No.            |
| producing the   |                  |               | 2000-288259    |
| same            | Yasushi IYECHIKA |               |                |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | November 24,  | Japanese       |
| Light Emitting  | TADATOMO,        | 2000          | Patent         |
| Element         |                  |               | Application    |
|                 | Hiroaki OKAGAWA, |               | No.            |
|                 |                  |               | 2000-357026    |
|                 | Tatsuo           |               |                |
|                 | SHIROICHI,       |               |                |
|                 |                  |               |                |
|                 | Yoichiro OUCHI,  |               |                |
|                 |                  |               |                |
|                 | Takashi          |               |                |
|                 | TSUNEKAWA        |               |                |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | December 25,  | Japanese       |
| Element         | TADATOMO,        | 2000          | Patent         |
|                 |                  |               | Application    |
|                 | Hiroaki OKAGAWA, |               | No.            |
|                 | Yoichiro OUCHI,  |               | 2000-393229    |
|                 |                  |               |                |
|                 | Takashi          |               |                |
|                 | TSUNEKAWA        |               |                |
+-----------------+------------------+---------------+----------------+
| Emission        | Satoshi          | January 19,   | Japanese       |
| Spectrum        | WATANABE,        | 2001          | Patent         |
| Measurement     |                  |               | Application    |
| Apparatus and   | M. S. MINSKY     |               | No.            |
| Measurement     |                  |               | 2001-011214    |
| Method          |                  |               |                |
+-----------------+------------------+---------------+----------------+
| Color           | Yasuo IMAI,      | March 14,     | Japanese       |
| Temperature     |                  | 2001          | Patent         |
| Variable LED    | Ken'ichi ISHII,  |               | Application    |
| Light Source    |                  |               | No.            |
| Module          | Ken'ichi Y.      |               | 2001-071940    |
|                 | Yamada           |               |                |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kiyoteru Y.      | March 21,     | Japanese       |
| Light Emitting  | Yoshida          | 2001          | Patent         |
| Element         |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2001-080739    |
+-----------------+------------------+---------------+----------------+
| Nitride         | Kiyoteru Y.      | March 21,     | Japanese       |
| Semiconductor   | Yoshida          | 2001          | Patent         |
| Light Emitting  |                  |               | Application    |
| Element         |                  |               | No.            |
|                 |                  |               | 2001-080740    |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | March 22,     | Japanese       |
| compound        | HIRAMATSU,       | 2001          | Patent         |
| semiconductor   |                  |               | Application    |
| and method of   | Hideto MIYAKE,   |               | No.            |
| producing the   |                  |               | 2001-082240    |
| same            | Takayoshi MAEDA, |               |                |
|                 |                  |               |                |
|                 | Yasushi IYECHIKA |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | March 20,     | 90106463       |
| compound        | HIRAMATSU,       | 2001          |                |
| semiconductor   |                  |               | (Taiwan)       |
| and method of   | Hideto MIYAKE,   |               |                |
| producing the   |                  |               |                |
| same            | Takayoshi MAEDA, |               |                |
|                 |                  |               |                |
|                 | Yasushi Y.       |               |                |
|                 | Iyechika         |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | March 21,     | 200101805-0    |
| compound        | HIRAMATSU,       | 2001          |                |
| semiconductor   |                  |               | (Singapore)    |
| and method of   | Hideto MIYAKE,   |               |                |
| producing the   |                  |               |                |
| same            | Takayoshi MAEDA, |               |                |
|                 |                  |               |                |
|                 | Yasushi Y.       |               |                |
|                 | Iyechika         |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | March 22,     | 101 14 029.0   |
| compound        | HIRAMATSU,       | 2001          |                |
| semiconductor   |                  |               | (Germany)      |
| and method of   | Hideto MIYAKE,   |               |                |
| producing the   |                  |               |                |
| same            | Takayoshi MAEDA, |               |                |
|                 |                  |               |                |
|                 | Yasushi Y.       |               |                |
|                 | Iyechika         |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | March 23,     | 09/814,875     |
| compound        | HIRAMATSU,       | 2001          |                |
| semiconductor   |                  |               | (United        |
| and method of   | Hideto MIYAKE,   |               | States)        |
| producing the   |                  |               |                |
| same            | Takayoshi MAEDA, |               |                |
|                 |                  |               |                |
|                 | Yasushi Y.       |               |                |
|                 | Iyechika         |               |                |
+-----------------+------------------+---------------+----------------+
| Group III-V     | Kazumasa         | March 23,     | 1              |
| compound        | HIRAMATSU,       | 2001          | 0-2001-0015251 |
| semiconductor   |                  |               |                |
| and method of   | Hideto MIYAKE,   |               | (South Korea)  |
| producing the   |                  |               |                |
| same            | Takayoshi MAEDA, |               |                |
|                 |                  |               |                |
|                 | Yasushi IYECHIKA |               |                |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | March 27,     | Japanese       |
| Element         | TADATOMO         | 2001          | Patent         |
|                 |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2001-091129    |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | March 27,     | Japanese       |
| Light Emitting  | TADATOMO         | 2001          | Patent         |
| Element         |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2001-091141    |
+-----------------+------------------+---------------+----------------+
| LED Light       | Ken'ichi YAMADA, | March 29,     | Japanese       |
| Source Device   |                  | 2001          | Patent         |
| Used for        | Ken'ichi ISHII,  |               | Application    |
| Illumination    |                  |               | No.            |
| and Lighting    | Yasuo IMAI       |               | 2001-096197    |
| Fixture         |                  |               |                |
+-----------------+------------------+---------------+----------------+
| LED Light       | Ken'ichi ISHII,  | March 29,     | Japanese       |
| Source Fixture  |                  | 2001          | Patent         |
|                 | Yasuo IMAI,      |               | Application    |
|                 |                  |               | No.            |
|                 | Ken'ichi YAMADA  |               | 2001-096198    |
+-----------------+------------------+---------------+----------------+
| Reflection Type | Ken'ichi YAMADA, | March 29,     | Japanese       |
| LED Light       |                  | 2001          | Patent         |
| Source          | Ken'ichi ISHII,  |               | Application    |
|                 |                  |               | No.            |
|                 | Yasuo IMAI       |               | 2001-096541    |
+-----------------+------------------+---------------+----------------+
| LED Light       | Ken'ichi ISHII,  | March 29,     | Japanese       |
| Source Device   |                  | 2001          | Patent         |
| and Lighting    | Yasuo IMAI,      |               | Application    |
| Fixture         |                  |               | No.            |
|                 | Ken'ichi YAMADA, |               | 2001-097222    |
|                 |                  |               |                |
|                 | Chiyoharu        |               |                |
|                 | MAKIHARA         |               |                |
+-----------------+------------------+---------------+----------------+
| Semiconductor   | Kazuyuki         | April 20,     | Japanese       |
| Light Emitting  | TADATOMO         | 2001          | Patent         |
| Element         |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2001-123345    |
+-----------------+------------------+---------------+----------------+
| GaN-based Light | Kiyoteru YOSHIDA | April 26,     | Japanese       |
| Emitting        |                  | 2001          | Patent         |
| Element         |                  |               | Application    |
|                 |                  |               | No.            |
|                 |                  |               | 2001-129492    |
+-----------------+------------------+---------------+----------------+

[^1]: Translation from Japanese to English sponsored by the
    Optoelectronics Industry Development Association and Sandia National
    Laboratories, and completed March 29, 2002 by Kirill V Sereda
    (kvsereda@worldnet.att.net) and Jeff Y Tsao (Sandia National
    Laboratories \-- jytsao@sandia.gov).