Study of new optical blinking phenomena on
InGaN/GaN single quantum well2020
Master's thesis
Yokohama City University
Graduate School of Nanobioscience
Department of Materials System Science
Yusuke Fujii

CONTENTS
1

2

Introduction................................................................................................. 5
1.1

History of InGaN and its application to the present...................................................5

1.2

Physical properties of InGaN material...................................................................6

1.2.1

InxGa(1-x)N................................................................................................ 6

1.2.2

Vegard's law............................................................................................. 7

1.2.3

Structural defects and their effects..................................................................9

1.3

Research Purpose........................................................................................... 11

1.4

Structure of this thesis..................................................................................... 12

Blinking Phenomena on InGaN/GaN SQW........................................................14
2.1

Blinking Phenomena...................................................................................... 14

For the proposed model of the blinking phenomena................................................15

2.3

Previous studies on the blinking phenomena in InGaN/GaN SQW..............................193

2.2

Samples and measuring systems......................................................................28
Samples...................................................................................................... 28

3.2

Selective excitation fluorescence microscope........................................................30

3.3

Video data processing methods (quantification).....................................................35

4

3.1

Temperature dependence of blinking phenomena.................................................37
Introduction................................................................................................. 37

4.2

Experimental method...................................................................................... 37

4.3

Experimental result........................................................................................ 38

4.4

Discussion................................................................................................... 42

5

4.1

Change in blinking phenomena due to liquid adhesion...........................................44
Introduction................................................................................................. 44

5.2

Experimental method...................................................................................... 44

5.3

Experimental result........................................................................................ 45

5.4

Discussion................................................................................................... 47

6

5.1

Statistical analysis of the blinking phenomena.....................................................48
6.1

Consideration of how the blinking point glow using Standard Deviation map.................48

6.2

Correlation between blinking points....................................................................49

6.3

Conclusion and Discussion............................................................................... 58

7

The Electronic Properties of Chlorine in GaN (External Collaboration).....................60

8

Bibliography.............................................................................................. 63

9

Acknowledgement....................................................................................... 67

10

Conferences and Publications.........................................................................69

11

Summary of the research (Japanese).................................................................70

Study of new optical blinking phenomena on
InGaN/GaN single quantum well

Author: Yusuke Fujii
Supervisor: Ruggero Micheletto
Subadvisors: Masanori Tachikawa
Subadvisors: Hirokazu Tanimoto

Study of new optical blinking phenomena on InGaN/GaN single quantum wellDepartment of Materials System Science Yusuke Fujii
Supervisor Ruggero Micheletto[KeyWords]
 Blinking Phenomena: A phenomena in which a luminescent region of a few microns insize blinks unsteadily when a sample is excited. In 2006, The blinking phenomena in
InGaN/GaN Single Quantum Well was first discovered by R. Micheletto et al. [1].
 InGaN/GaN Single Quantum Well (InGaN/GaN SQW): A structure in which an InGaN
layer with a smaller band gap due to the addition of indium is sandwiched between two
GaN layers with a larger band gap.
 Quantum Confined Stark Effect (QCSE): The generation of a piezoelectric field inside
the sample, which causes the separation of the electron and hole wavefunctions and adecrease in the recombination rate.[2] It is thought that blinking is caused by the repeatedincrease and decrease in the overlap of the wave functions.
[Introduction]
Indium gallium nitride (InGaN) is now used as a material for blue light emitting diodes,
and by changing the composition ratio of indium, it has become possible to cover not only
blue but also wavelengths in most visible light regions. It is a device that is attracting a great
deal of attention because it emits light with high brightness and long life. However, manythings have not been clarified about this physical property. Among them, this study deals withthe blinking phenomena.[1] Irradiation of InGaN/GaN Single Quantum Well by ultra violet
with emission wavelength of 365nm as an excitation light results in the formation of localized
domains where the optical emission is unstable. It is considered that lattice mismatch and highdislocation due to the use of sapphire substrate causes blinking phenomena, but it is not clear
why blinking occurs. In this study, I attempted to clarify the physics mechanism of theblinking phenomena by using InGaN/GaN SQW.

[Method]
I used InGaN/GaN SQW with emission wavelengths of 460nm and 510nm provided by
Prof. Yoichi Kawakami of Kyoto University as the measurement samples. This sample wasgrown by the metal-organic vapor deposition method, and has a double heterostructure
consisting of a 4μm GaN layer, a 3nm InGaN layer, and a 5nm GaN layer in sequence on asapphire substrate. As a measurement system, I used a selective excitation fluorescencemicroscope. The blinking was photographed by a 3CCD camera attached to the top of the
microscope, and the data was sent to a computer for analysis. In this study, I explored thetemperature dependence, the change in blinking due to the adhesion of liquid, and thecharacteristics of the blinking points by conducting statistical analysis.

[Results and Discussion](1) Temperature dependence:
The temperature dependence of the blinking phenomena is tested using a standard deviation
map. InGaN/GaN SQW was placed on the heater, the temperature was changed from 300K to
373K, and the change in blinking was investigated. As a result, the emission wavelengths ofboth 460 nm and 510 nm blinked most strongly in the relatively low temperature range, but
the blinking tended to become weaker at higher temperatures. At 373K, the 460nm sample
was barely blinking, but the 510nm sample was still blinking. It is thought that one of thefactors that alleviate the intensity of blinking due to temperature rise is that thermal expansion
increases the defect density, making it easier for carriers to be trapped, and as a result, nonradiative recombination has done that does not emit light. However, since various physicalquantities such as electron mobility change with temperature, it was not possible to identify aclear cause in this survey.
(2) Change in blinking phenomena due to liquid adhesion:
I have found that blinking is induced by the adhesion of liquid. In this study, water andethanol were utilized and measured while adhering to the sample. When water was attached tothe sample, the number of blinking points increased and the blinking was weak, but when
ethanol was attached to the sample, the blinking tended to be incredibly intense. This wascommonly observed for InGaN/GaN SQW with emission wavelengths of 460nm and 510nm.

If the blinking phenomena is caused by QCSE, it is considered that the liquid would have theeffect of rapidly intensifying and weakening the magnitude of the piezoelectric field. The
optical intensity at the blinking points also increased, suggesting that the liquid may becreating a state that promotes electronic excitation.
(3) Correlation among blinking points
By enlarging the blinking points displayed on the standard deviation map, I found several
blinking points where two areas are blinking significantly. When I calculated the correlationof the other to one of these locations, it became clear that the regions blinked with negative
correlation to each other. By expanding this to the entire area, the correlation was also shownwith other blinking points in the surrounding area, and it became clear that the other blinking
points are composed of two regions with negative correlation. The correlation here means thecorrelation of optical intensity in time series. The intensity of each blinking point was alsoplotted in time series by region. It was found that the intensity of the blinking points seemed
to be synchronized even though they are spatially different. If the blinking phenomena iscaused by QCSE, it is considered that the magnitude of application of the piezo electric field
is also synchronized.
Fig1. Time series data of optical intensities in two regions consisting of negatively correlated.
A-point and B-point are spatially distinct blinking points.
[Bibliography][1] R.Micheletto et al. Appl. Phys. Lett. 88,061118 (2006)[2] J.W.Robinson et al. Appl. Phys. Lett. 86,213103 (2005)

【日本語概要】
発光デバイスの１つでもある InGaN/GaN 単一量子井戸型構造に対し、光励起を行うと不安定な点滅を繰り返す輝点が無数に発生する。しかしながら、この点滅現象は未だ原因不明であり、メカニズムの解明が求められている。そこで、本研究では点滅現象の物理学的なメカニズムを解明することを目的とした。温度依存性、液体の付着による点滅現象の変化を追究したところ、これらの要素が点滅へ強い影響を及ぼしている事が分かった。また統計的な分析を進めることで、点滅点同士の相関性も明らかにした。

1 Introduction
1.1

History of InGaN and its application to the present

In modern society, Indium Gallium Nitride (InGaN) is used as a material for blue lightemitting diodes, which are used in home appliances, traffic lights, and Christmas lights. LEDs
have the advantages of long life, high efficiency, and even lower power consumption, and area good alternative to the traditional incandescent bulbs. There is a trend to move to LED bulbs
and the number of users is expected to increase. This feature is expected to be used in the next
generation of medical devices and new displays.
In 1989, blue light-emitting diodes had not yet been developed while yellow, red andyellow-green diodes were being developed. However, the creation of blue light-emitting
diodes was made possible by Isamu Akasaki and Hiroshi Amano, who won the Nobel Prize inPhysics in 2014; the pair succeeded in creating the world's first blue light-emitting diode, but
the light intensity was 10 mcd and the emission wavelength was very dark, close to violet, at
470nm. Four years later, however, in 1993, Shuji Nakamura and his colleagues at
Tokushima's Nichia Corporation succeeded in developing a blue light-emitting diode withhigh luminance and long life, although this was thought to be impossible. They were also able
to create white LEDs from the three primary colors of light. This breakthrough was madepossible by InGaN. Previously, silicon carbide was used as a material for semiconductor
devices, but by switching to InGaN, which has a higher electron mobility than silicon, it can
now be used as a material for high-frequency devices.[1]As the implementation phase of what was adopted in the Paris Agreement began in 2020,
we must focus our efforts on combating global warming. It is about keeping the average
global temperature rise well below 2 degrees Celsius and limiting it to 1.5 degrees Celsius
compared to the pre-industrial period. What we can do to achieve this goal is to reduce carbon
dioxide emissions, and the practical application of LEDs will reduce electricity consumption
and carbon dioxide emissions from thermal power plants. If all the incandescent light bulbs inJapan could be converted to LEDs, the energy equivalent of 4.5 power plants could be saved.
From this point of view, the development of light-emitting devices based on InGaN is
attracting attention for its environmental benefits.

1.2

Physical properties of InGaN material

1.2.1

InxGa(1-x)N

InxGa(1-x)N is a mixed crystal of Indium Nitride (InN) and Gallium Nitride (GaN), and its
bandgap value can be adjusted from infrared to ultraviolet light by changing the Indium
composition ratio. This means that the material covers the wavelengths of visible light,
making it possible to emit light in colors we can see. Figure1.1 shows the relationshipbetween the lattice parameter and bandgap of group Ⅲ nitride semiconductors and othersemiconductor materials.[2] It can be seen that the band gap of InN and GaN traverse visible
light. The materials GaN and InN have many thermodynamically stable hexagonal wurtzite
type structures as shown in Figure1.2 [3]1.2.2

Vegard's law

The InxGa(1-x)N band gap becomes smaller as the Indium composition ratio increases, andthe emission wavelength shifts to longer wavelengths. This relationship follows the Vegard's
law. The Vegard's law is a relationship between the composition ratio and the band gap, asshown in the following equation (1).

E g . InGaN ( x )=x Eg . InN + ( 1−x ) E g ,GaN −bx ( 1−x )(1)E g . InN represents the band gap of InN (0.65eV), and similarly, E g ,GaN represents the bandgap

of GaN (3.4eV). x indicates the amount of Indium. b is called the Bowing parameter.
Figure1.3 is a plot of the relationship between the composition of Indium and the band gap
when the bowing parameter is set to 1. According to this figure, As the indium composition
increases, the band gap decreases exponentially and the emission wavelength shifts to higher
wavelengths.

Figure1.1 Relationship between lattice parameter and band gap of wurtzite structure [2]Figure1.2 GaN crystal with a hexagonal wurtzite structure; InN crystal has a similar structure.
black is nitrogen and gray is gallium or indium. [3]Figure1.3 The relationship between the concentration of Indium and the band gap according
to the Vegard's law, with Bowing set to 1. The band gap decreases exponentially as theconcentration of Indium increases

1.2.3Structural defects and their effects

There is no substrate suitable for the epitaxial growth of GaN material with a perfect lattice
constant. For this reason, sapphire is often used as substrate because their lattice constants areclose to those of other substrates, they are relatively inexpensive, and they can withstand hightemperatures. However, even if sapphires are used, the thermal expansion coefficients andlattice constants are very different, which leads to large structural defects and threading
dislocation.[4] Table 1.1 lists the lattice parameters, thermal conductivity, and coefficient ofthermal expansion for various materials used as a substrate.[5] These defects have a negative
impact on the entire devise. The formation of the threading dislocation traps electrons in deeplevels and converts the light energy caused by the recombination of electrons and holes into
thermal energy. The phenomena is called nonradiative recombination. How an LED using
InGaN as the luminescent layer could shine so brightly despite the formation of many defects
that cause nonradiative recombination had been a mystery. However, Y. Narukawa et al.
concluded that fluctuations in the composition of Indium can cause excitons to move to sites
where indium with lower potential is localized, resulting in luminescent recombination. [6,7]In fact, by using SNOM, they show that InGaN emits blue light where the potential isconcave. [8]The mechanism of high brightness luminescence in spite of many structural defects is being
elucidated. However, there are several phenomena in InGaN that have not yet been resolved,
which are thought to be caused by structural defects. One of them is the key to this thesis:
Blinking Phenomena. [9] I discuss the phenomena in detail in Chapter 2. The Blinking
phenomena is considered to be caused by a structural defect, but the physical mechanism is
not yet understood.

Table1.1 Substrates used in III-V nitride semiconductors and their physical properties. [5]1.3

Research Purpose

The purpose of this study is to elucidate the unexplained optical properties of InGaN
materials for blue LED light emitting devices. In particular, I focus on the "blinking
phenomena" discovered by R. Micheletto et al. in 2006.[9] As mentioned above, there is no
physical model that can explain blinking phenomena. In an ideal structure, the blinking
phenomena would not occur. If the physical mechanism of the blinking phenomena isclarified, it leads to the development of a very innovative new device. A detailed explanationof the blinking phenomena, possible theoretical models, and examples of previous research
discuss in Chapter 2. In this study, I used two types of InGaN/GaN single quantum well
(SQW) structures with emission center wavelengths of 460nm and 510nm. A detailed
description of the samples is given in Chapter 3.

12

In this study, I investigated the characteristics of blinking by analyzing the temperaturedependence of the blinking phenomena, the change caused by the adhesion of liquid, andstatistical analysis, and from these results, I attempted to clarify the physical mechanism of
the blinking occurrence.

1.4

Structure of this thesis

Chapter 1 summarizes the history of the development of InGaN nitride semiconductors andtheir universal physical properties.
Chapter 2 summarizes the specific explanation of the blinking phenomena, the physicalmodel that is considered to be the cause of the blinking phenomena, and examples of previous
research.
Chapter 3 summarizes the experimental samples, the selective excitation fluorescencemicroscope used in this study, and the details of the processing of the obtained video data.
Chapter 4, the temperature dependence of the blinking phenomenon in the range of 300K to
373K is investigated using InGaN/GaN SQW with emission center wavelengths of 460nm
and 510nm.
Chapter 5, I observed the blinking behavior by photoexcitation with a liquid attached. The
liquids which I used were water and ethanol, and a comparison was made with the state
without any liquid attached. As in Chapter 4, two types of samples with emission wavelengthsof 460nm and 510nm were utilized.
Chapter 6, I discuss what was revealed by proceeding with the statistical analysis, such as
standard deviation map and correlation map.
Chapter 7, I summarize our collaborative work with Dr. Giovanni Alfieri at Hitachi Powergrip in Switzerland. It has been suggested that chlorine can be contaminated in reactive ion
13etching, one of the production processes of gallium nitride (GaN), and the physical properties
of GaN when contaminated with chlorine have not been clarified. Therefore, we investigatedthe electronic properties of GaN with impurity chlorine by performing the density functional
theory using SIESTA, one of the first-principles calculation software. Since this is not mymain research, I only briefly describe it.
The following chapter 8 consists of references, chapter 9 consists of acknowledgments,
chapter 10 contains research achievements, and the last chapter 11 contains the abstract of thisthesis in Japanese.

14

2 Blinking Phenomena on InGaN/GaN SQW
2.1

Blinking Phenomena

Irradiating the surface of an InGaN/GaN SQW used in a light emitting device with narrowband ultraviolet light produces a myriad of bright spots that repeat unstable blinking. This isthe blinking phenomenon, and it is roughly a few microns in size. In 2006, R. Micheletto firstobserved the blinking phenomena in InGaN/GaN SQW growing in the [0001] direction on asapphire substrate.[9] The phenomena occurs most often when only the peak of 365nm is
taken out from the wavelengths irradiated from the mercury lamp and irradiated on the sample
as excitation light. The observed blinking phenomena is shown in Figure2.1 and this data isfor InGaN/GaN SQW with an emission wavelength of 510nm. In addition to this, samples
with emission wavelengths of 460nm and 540nm were used and the blinking of the threesamples were observed, there is a data that the longer the emission wavelength is, the more
likely the blinking occurs.[3] As mentioned above, the ideal InGaN/GaN SQW does not cause
blinking phenomena. Many physical models of the blinking phenomena have been proposed,
but there is no physical model that we can use. In addition to InGaN, blinking phenomena
have also been observed in quantum dots such as InP[10] and GaAs [11]. The next section
describes some of the proposed models.

15

Figure2.1 A blinking point is circled. (a) is on-time and (b) is off-time. Sample is 510nm
InGaN/GaN SQW [9]2.2

For the proposed model of the blinking phenomena

In this section, I describe the model of the blinking phenomena that has been proposed.
1.

Quantum Confined Stark Effect (QCSE)
The generation of a piezoelectric field inside the sample, which causes the separation of theelectron and hole wavefunctions and a decrease in the recombination rate, is called Quantum
Confined Stark Effect (QCSE).[12] In nitride semiconductors, two types of electric fields are
generated: spontaneous electric fields and piezoelectric fields. The spontaneous electric fieldis generated due to the lack of symmetry of the crystal in the C-axis direction. The
piezoelectric field is caused by a lattice constant mismatch with the substrate. InGaN/GaN
SQW based on sapphire substrates are considered to have a large lattice mismatch, which maylead to a large piezoelectric field. Repeatedly overlapping and pulling apart of the
wavefunctions could be the result of blinking mechanism. Figure2.2 shows a simple

16

schematic diagram of QCSE. This figure is taken from a master's thesis by K.Oikawa who
graduated from our laboratory and is partially modified.[14]Due to the distortion of the bandgap, the effective bandgap becomes smaller in the order of(a), (b), (c). Therefore, it is theoretically considered that the emission wavelength shifts to thelonger wavelength side as the band gap is distorted. Experimental research is also being
conducted to measure the spectra of (a) when the blinking light intensity is high (on-state) and(c) when the blinking light intensity is low (off-state). According to the Figure2.6 which is
introduced in the next section, the blinking emission wavelengths are mostly distributed in the
low wavelengths in the on-state, but the emission wavelengths are mostly distributed in thehigh wavelengths in the off-state. Therefore, from the perspective of both theory andexperimental results, the hypothesis that the blinking phenomenon is caused by QCSE is very
credible.
Since the piezoelectric field is detrimental to the device, some research has been done to
reduce the piezoelectric field. For example, it has been reported that QCSE is suppressed forsamples with quantum well type structures created on substrates with different surface
orientations.[13]17Figure2.2 A simple model of the causal relationship between QCSE and blinking phenomena
in InGaN/GaN SQW. As the piezoelectric field increases, the overlap of the wave function
decreases and the recombination rate of electrons and holes decreases. As a result, theemission intensity also decreases. The blinking phenomena can be considered as a repetitionof (a), (b) and (c).[14]2.

Quantum Jump Model
A quantum jump model has been proposed by Cook and Kimble.[15] The ground state isconsidered to be 0 and the excited state is set to 1. In addition to these two, there is also ametastable state 2. Radiative recombination occurs as the carriers move back and forth
between the 0⇆1 states. However, there are occasional carrier jumps to state 2 at random.
Carriers transitioning to state 2 are non-radiative recombination and therefore do not
encourage emission. This quantum jump model can be used as a theoretical model of theblinking phenomena by applying it to the rate equation.

18

Figure2.3 Simple Energy-level scheme [15]3.

Auger Ionization
The on/off state of emission in quantum dots is consistent with the Auger ionization

mechanism.[16] Quantum dots usually have a carrier or a single electron-hole pair. However,if there are two electron-hole pairs, when one electron-hole pair recombines, the energy
released from the recombination is given to the other electron, which transitions to the traplevel. The electrons in the trap level do not contribute to the luminescence because of the nonradiative recombination. Therefore, the radiative recombination rate is reduced, which has anegative effect on the luminescence.

2.3 Previous studies on the blinking phenomena in InGaN/GaN SQW
The blinking phenomena of InGaN/GaN SQW was discovered by R. Micheletto et al. in
2006. [9] They used a sample grown in the [0001] direction on a sapphire substrate with anemission wavelength of 510nm. This was made under the same conditions by the same
preparers as the sample used in my research. The details of the samples describe in Chapter 3.
According to their research, in Figure2.4, the blinking phenomena does not occur at 405nm,
where only the quantum well layer is excited, but it occurs at 365nm, where the quantum welland GaN layer are excited, so it was suggested that the carriers flowing from the GaN layer to19the InGaN quantum well layer are not spatially uniform. Blinking can be divided into two
types: those that move back and forth between the two intensities and those that behave
continuously and chaotically. Details are in Figure2.5 They measured the wavelengths of theblinking when the light is on and off. As shown in Figure2.6, the spectra shifted to the low
energy side when the light is on and to the high energy side when the light is off.
The temperature of the blinking phenomena has also been reported. As shown in Figure2.7,
the temperature dependence of the blinking phenomena is measured every 10K in the range of
200 K to 290 K, it is the result of a 70-second exposure to excitation light with emission peak
365nm. The graph shows that the intensity and amplitude of the blinking increased as thetemperature increased. Figure2.8 shows a histogram of the luminescence intensity at eachtemperature. The luminescence intensity shifts to higher intensity as the temperature isincreased, and a wide range of values are taken.
T.Toshiaki et al. who graduated from our laboratory also reported on the temperature andpressure dependence of the total emission of InGaN/GaN SQW.[17] [18] The sample is asimilar one grown in the [0001] direction on a sapphire substrate, with an emission
wavelength of 540nm.
First of all, I discuss the temperature dependence of the experiment: Tsutsumi used atemperature control device (Cryogenic Control System's Model 32) to observe theluminescence change of InGaN/GaN SQW between 290 K and 320 K in atmospheric air. A
sample is placed on a heater connected to the temperature control device and the temperatureof the heater is simply changed. He measured the temperature dependence at 365nm and405nm excitation light, respectively, and I will discuss the 365nm results in this paper.
Figure2.9 shows the changes in temperature and emission intensity while the sample was20

excited with 365nm. Figure2.9 shows the changes in temperature and emission intensity while
exciting the sample with 365nm excitation light for 2000 seconds. The results show that as thetemperature increases, the total emission increases, and conversely, as the temperature
decreases, the total emission tends to decrease. As for the number of blinking points, heconcludes qualitatively that the number does not change. So, Quantitative data have not beenobtained on how the blinking flashes due to temperature changes.
Next, I report on the pressure dependence. The sample was placed in a vacuum chamber(RC102-CFM, CIA, Inc.) and the luminescence change was observed as the pressure was
reduced from 1000 mmbr to 10-5 mmbr. As shown in Figure2.10, the overall luminescenceintensity tended to gradually increase as the pressure is reduced. Conversely, when thepressure is increased rapidly, the total emission decrease. Although the result is qualitative,
there is a tendency for the blinking points to decrease with decreasing pressure. Therefore, it
can be suggested that the blinking may be induced by surface deposits.
Based on these results, they modeled the changes in the amount of adsorption and theoverall luminescence intensity using an equation, assuming that the luminescence intensity
decreases as the amount of physisorption increases and increases as the amount ofphysisorption decreases. Set the luminescence intensity be I(t). Assuming that this is proportional tothe amount of adsorbed material on the surface, n(t), it can be expressed as follows.

I ( t )=−β ×n ( t ) + I max(1)

Where β is the loss of luminosity they set and I max represents the maximum luminescenceintensity. Furthermore, the time evolution of n ( t ) is expressed by the following equation.

dn ( t )=−n ( t ) γ des ( t ) + ( n max−n (t ) ) γ |¿|( t )¿
dt

21

(2)

γ des ( t ) represents the amount of desorption per unit time and γ|¿|( t )¿ indicates the amount ofadsorption per time. nmax represents the maximum value of physical amount attached to thesurface. Since γ des ( t ) and γ|¿|( t )¿ are not constant and depend on the gas pressure, so they madethe following relationship as a model.
γ|¿|=α

|¿|¿¿

¿

(3)

γ des=α des ( g desmn −gmn ) P+ gmx(4)α
where gmx and gdes
mn represent the range of pressure and α |¿|¿ and des are coefficient fitted to|¿|¿experimental data. Using the model equations (1) to (4), the relationship between the
excitation time and the emission intensity is plotted in Figure2.11. It can be seen that themodel has a similar curve to the time variation of the pressure obtained from the experiments.

Figure2.4 Blinking phenomena of an InGaN/GaN SQW with a luminescence wavelength of
510nm. (a) is at an excitation wavelength of 365nm and (b) is at 405nm. Blinking appears
easily in (a), while no blinking occurs in (b) when only the quantum well layer is excited. [9]22Figure2.5 InGaN/GaN SQW has two types of Blinking, one that moves back and forth between thetwo intensities, as in (a), and the other, chaotic blinking via all values, as in (b). [9]Figure2.6 (a) Time-series data of the luminescence intensity of the InGaN/GaN SQW
blinking phenomena: (b) Relationship between peak wavelength and luminescence intensity.
[9]23

Figure2.7 Time-series variation of blinking intensity at different temperatures [9]24Figure2.8 Histogram of the blinking intensity at each temperature [9]25Figure2.9 The temperature dependence of the surface luminescence of InGaN/GaN SQWwith a luminescence wavelength of 540nm irradiated with 365nm excitation light. The blue
and red lines show temperature and intensity changes, respectively. [17]Figure2.10 The pressure dependence of the surface luminescence of InGaN/GaN SQW with aluminescence wavelength of 540nm irradiated with 365nm excitation light [18]26Figure2.11 Time series data of the experimental pressure variation and the modeled
luminescence intensity [18]273Samples and measuring systems

3.1 Samples
Samples I used in this study are shown in Figure3.1. These were provided by Prof. Yoichi
Kawakami of Kyoto University and Nichia Corporation, and are the same as those used by
T.Tsutsumi, who completed his studies in our laboratory five years ago. The double
heterostructure consists of a 4µm GaN layer, a 3nm InGaN layer, and a 5nm GaN layer grownin the [0001] direction on the sapphire substrate in that order. The growth method used was
the metallic organic vapor deposition method, and no doping was applied to any of the layers
in the sample. As noted in Chapter 1, the emission wavelength can be changed by changing
the indium composition ratio x. In this study, I used samples with emission wavelengths of
460nm and 510nm.

Figure3.1 Structure and photographs of the InGaN/GaN SQW used in this study

28

Figure3.2 Blinking phenomena observed at an emission wavelength of 460nm

Figure3.3 Blinking phenomena observed at an emission wavelength of 510nm. The blinking
points tend to be larger than that of the 460nm sample.

29

3.2 Selective excitation fluorescence microscope
A schematic diagram of the selective excitation fluorescence microscope used in this study
are shown in Figure3.4. The microscope is an Olympus BX51WI-33FL-YS2, and the mercury
lamp used as a light source is an Ushio BC2078 5000273 100W USH-102D. The light fromthe mercury lamp passes through an excitation filter, which allows only the excitation
wavelength to be transmitted. Excitation light is irradiated onto the sample and fluorescenceand excitation light are emitted. By passing through the absorption filter, the excitation light isreflected and only the fluorescence wavelengths are transmitted to our eyes. The combinationof the excitation filter and the absorption filter determines the method of excitation. The
combination of filters can be selected from the mirror unit on the microscope. We have fivedifferent mirror units in our laboratory: BFL, NU, NBV, NIBA and WIG. The spectra of
excitation and fluorescence wavelengths that can be transmitted by each unit are shown
below. This figure was provided to us by Olympus. Blinking is generated by taking out the
wavelength of 365nm only in the mercury lamp and using it as an excitation light. In addition,
as mentioned earlier, since the fluorescence wavelengths of 460nm and 510nm are observed,
the necessary mirror unit for observing the blinking is NU, and the blinking does not occur
under the conditions of mirror units other than NU. The blinking is observed with the 3CCD
camera (HDR-SR1, SONY) mounted on the top of the microscope, which has a magnificationof 10x and the objective lens is set to 50x, so the blinking is observed with a total of 500x
magnification. The resulting video data is 1440 pixels by 1088 pixels with a frame rate of
30fps. By processing this recorded data by programming, it is possible to perform statisticalanalysis.

30

Figure3.4 Diagram of the selective excitation fluorescence microscope31

Figure3.5 Spectral distribution of the mercury lamps used in this study (I modified) [19](https://www.ushio.co.jp/documents/products/light_source/USH102D_J.pdf)

32

Figure3.6 The spectra of excitation and fluorescence wavelengths that can be transmitted by
NU. The blue line indicates the excitation filter, the red line indicates the absorption filter, andthe green line indicates the dichroic mirror [20]Figure3.7 The spectra of excitation and fluorescence wavelengths that can be transmitted by
NBV [20]33

Figure3.8 The spectra of excitation and fluorescence wavelengths that can be transmitted by
NBV [20]

Figure3.9 The spectra of excitation and fluorescence wavelengths that can be transmitted by
WIG [20]

3.3 Video data processing methods (quantification)
Directly obtained blinking video data is relatively large in size and contains noise. I used
Avidemux 2.7(Qt 5) to crop and remove noise to reduce the size of the video. The optical
34

intensity of the time series can be obtained by calculating and integrating the optical intensity
at each frame. In addition, since it can be expressed in terms of 8-bit emission intensity, theoptical intensity per pixel is calculated by calculating the brightness (0-255) in red, green, andblue, respectively, as shown in equation (1), and averaging them. It is also possible tocalculate the overall intensity by specifying an area instead of a pixel unit as in equation (2).
I ( t )=x2

R ( t ) G ( t ) B (t )
+
+
3
3
3

(1)

y2

R ( x , y , t )+G ( x , y , t )+ B( x , y ,t)(2)
3 ( x 2−x 1 ) ( y 2− y 1)
x=x 1 y= y 1

I ( t )= ∑ ∑Although I do not directly measure the spectral intensity of the blinking, the image analysis asshown above allows us to analyze the time variation of the optical intensity of the blinking
and other statistical analysis.

35

Figure3.10 The video data processing method is represented as a diagram. By recording thecoordinates ( x 1 , y 1) of the upper left part and ( x 2 , y 2) of the lower right part of the area where
we want to measure the intensity, and applying equation (2) described on the previous page,
we can arrange the intensity for each frame in chronological order, resulting in a time seriesgraph like the one on the right. Also, since this is a video data processing, it is an image
coordinate system.

36

4 Temperature dependence of blinking phenomena
4.1

Introduction

As mentioned in Chapter 2, the temperature dependence of InGaN/GaN SQW has beenstudied in the past. However, there has been no quantitative discussion of how the blinking
points behave in response to large temperature changes. Here, I used InGaN/GaN SQW withemission center wavelengths of 460nm and 510nm to study the behavior of the blinking pointfrom 300K to 373K at 10K intervals.

4.2

Experimental method

I measured the temperature dependence of the blinking phenomena. The temperaturedependence of the blinking phenomena can be measured by placing InGaN/GaN SQW on aheater attached to a temperature controller and adjusting the temperature controller. The
temperature controller used was Cryogenic Control System's Model 32. After the temperature
displayed on the temperature controller was kept constant, the shutter on the microscope was
opened and the excitation light was irradiated for 30 seconds in order to make the comparison
under the same conditions as possible. The temperature was measured in two ways: fromlower to higher and from higher to lower. The blinking phenomena was captured on a 3CCD
camera mounted on top of the microscope. The spatial distribution of the blinking points wascalculated by analyzing the video data. This method of image analysis is also described in
Chapter 3. To quantify the blinking phenomena, I created a standard deviation map. The
standard deviation map allows us to show the blinking area quantitatively. More intenseblinking areas result in higher standard deviation values and weaker blinking areas result in
lower standard deviation values.

37

4.3

Experimental result

The standard deviation map for each temperature is shown in Figure4.1. This is the resultof irradiating InGaN/GaN SQW with an emission wavelength of 460nm with excitation lightof 365nm. From the standard deviation map, it is clear that blinking is most active at low
temperature environment, but it can be seen that the intensity of blinking tends to be relievedby increasing temperature. At the maximum temperature of 373K, the intensity of blinking
was most moderated, and more than 90% of blinking points disappeared. This result is clear
not only on the standard deviation map but also visually.
Figure4.2 is the result of irradiating InGaN/GaN SQW with an emission wavelength of
510nm with excitation light of 365nm. Similar to InGaN/GaN SQW with an emission
wavelength of 460nm, it can be seen that the blinking becomes most intense in a lowtemperature environment, and the intensity of blinking points tend to be relieved as thetemperature rises. However, unlike the 460nm InGaN/GaN SQW, blinking was still present
even at the maximum temperature of 373K. This result is also visible visually as well as thestandard deviation map.
Furthermore, when comparing the standard deviation values of both samples, the result ishigher for InGaN/GaN SQW at 510nm. This standard deviation value is based on thetemperature that the blinking was most intense, and the color bar is created. In other words, itmeans that the InGaN/GaN SQW having an emission wavelength of 460nm blinks more
rapidly than the InGaN/GaN SQW having an emission wavelength of 510nm. As reported by
C. Feldmeier, 460nm InGaN/GaN SQW is less prone to blinking points than 510nm
InGaN/GaN SQW.[3] Same result was obtained in this study, and it was found that the more
likely the sample to blink, the greater the intensity of the blinking.

38

Figure4.1 I created a standard deviation map to quantify the blinking phenomena. This is thestandard deviation map for each temperature, and is the result for an InGaN/GaN SQW with
an emission wavelength of 460nm. The larger the value, the more intense the blinking.

39

Figure4.2 The standard deviation map is the same as in Figure4.1. This is the result of an
InGaN/GaN SQW with an emission wavelength of 510nm.

40

4.4

Discussion

It was found that blinking behavior changed significantly with temperature changes, andthat when excited in a relatively low temperature environment, blinking was most likely to be
active in both samples. According to the Figure4.1 and Figure4.2, it can be seen that theblinking points gradually decrease as the temperature rises, and the blinking disappeared on
the 460nm sample, but the blinking was still present even at the maximum temperature of
373K on the 510nm sample. However, depending on the location, there was an area where theblinking gradually decreased every 10K, and there was an area where the blinking increased
at 10K or 20K and then decreased after that. The cause of this has not been identified.
However, there was a common case that blinking was likely to occur in a relatively low
temperature range, and blinking was unlikely to occur in a high temperature range.
Furthermore, in the 460nm sample, the blinking almost disappeared at the maximum
temperature of 373K, and in the 510nm sample, the blinking was present even at 373K. Since
the only difference between the two is the indium content, it is thought that the cause is that
there are many defects due to the difference in the indium content.
It is considered that the coefficient of thermal expansion increased and the distortion inside
the crystal increased as a factor of the decrease of the blinking point when the temperaturerise. The carriers required for recombination are more likely to be trapped by larger defects
and can result in non-radiative recombination that emits heat rather than light. Another factor
may be reduced electron mobility, but the study could not determine the exact factor because
various physical quantities change with temperature.

41

5 Change in blinking phenomena due to liquid adhesion
5.1

Introduction

As mentioned in Chapter 2, the luminescence change phenomena were caused by thevacuum change, suggesting that surface deposits are involved in the luminescence andblinking. Therefore, I studied how the blinking phenomena changes when the liquid isdeposited on the surface.

5.2

Experimental method

I used InGaN/GaN SQW with an emission wavelength of 460nm and InGaN/GaN SQWwith an emission wavelength of 510nm. I deposited a liquid on the surface of the sample
before excitation with light. I covered the surface with a cover glass after depositing it so thatthe liquid is uniformly distributed. Since the transmittance of the cover glass is almost 100%,
I do not expect it to have much effect on the observation of the blinking phenomena. The
liquids used in this study are water and ethanol and I excited the sample for 30 seconds. To
quantify the blinking phenomena, I made a standard deviation map as well as the temperaturedependence.

42

5.3

Experimental result

The results of the experiment showed that the behavior of blinking varied greatly dependingon the liquid deposited on the surface. Figure5.1 shows the standard deviation map for
InGaN/GaN SQW with an emission wavelength of 460nm, and Figure5.2 shows the standard
deviation map for InGaN/GaN SQW with an emission wavelength of 510nm. The same
results were obtained for both samples, indicating that the blinking point is induced by theadhesion of liquid. The blinking is weakest when nothing is attached to the surface, and theintensity of the blinking increases slightly when water is attached. When ethanol adhered tothe surface, the blinking became incredibly intense. I quantified the blink phenomena with astandard deviation map, and the actual visual observation shows that the change in blinking
behavior due to liquid adhesion is remarkable. I also performed the experiment with different
excitation locations and obtained similar results.

43

Figure5.1 A standard deviation map for each liquid adhesion. The sample is an InGaN/GaN
SQW with an emission wavelength of 460nm. It can be seen that the blinking is especiallyintense for ethanol adhesion.

Figure5.2 A standard deviation map for each liquid adhesion. The sample is an InGaN/GaN
SQW with an emission wavelength of 510nm. It can be seen that the blinking is especiallyintense for ethanol adhesion.

5.4

Discussion

It was found that the blinking phenomena was induced by the adhesion of the liquid andthat the behavior of the blinking points differed depending on the liquid type. This suggeststhat the adsorbed material on the surface plays an important role in the blinking phenomena.
The blinking is thought to be caused by the QCSE as described in Chapter 2. If the overlap of44

the wavefunctions increases and decreases at high speed, the blinking is expected to be more
intense. If the magnitude of the piezoelectric field is constantly changing over time, the above
phenomena is likely to occur. The optical intensity of the blinking also increased, so it is alsosuggested that the adsorbent increases the density of states near the valence band, creating anenvironment in which many electrons can be excited at once, and increasing therecombination rate of carriers.

45

6 Statistical analysis of the blinking phenomena
6.1

Consideration of how the blinking point glow using Standard
Deviation map

The blinking phenomenon can be quantified by expressing it as a standard deviation map.
By enlarging the blinking points displayed on the standard deviation map, we can see the
distribution of the variation in optical intensity over the measurement time of a single blinking
point. When we took out the blinking points and examined them one by one, we found that
there are several types of blinking points, and their blinking patterns are different. Figure6
shows the blinking patterns that were particularly conspicuous. Figure6.1(a) shows a patternof two blinking points blinking significantly, Figure6.1(b) shows a donut-shaped patternwhere the center of the blinking point hardly blinks but the surrounding area blinks
significantly, Figure6.1(c) shows a pattern where the center of the blinking point blinks
significantly, and finally Figure6.1(d) shows that the intensity of blinking is a uniform pattern.
Since the maximum standard deviation differs for each blinking point, the standard deviation
is normalized.

46

Figure6.1 Enlarging the standard deviation map, the above blinking pattern is conspicuous

6.2

Correlation between blinking points

As shown in Figure6.1(a), I report a very interesting feature of the blinking point where theintensity varies significantly between two locations. In particular, I cut out one pixel in aregion with a high standard deviation and calculated its correlation with another region with ahigh standard deviation. The correlation here refers to the correlation of optical intensities inthe time series. As shown in Figure6.2, the correlation was relatively strong and negative.
There were some blinking points of weak correlation, but the result was definitely negative.
This means that when the optical intensity of one become higher, the intensity of the otherbecome lower, which occurs every one frame (0.03 seconds). Although it is not possible toinclude all the blinking data in this paper, Figure6.3 shows a plot of the correlation between
the two regions where the optical intensities show a negative correlation. It is also clear fromthis correlation that the two regions are negatively correlated with each other.

47

Figure6.2 Cut out a single pixel with a high standard deviation value and transformed it into
a correlation distribution of intensities for that location. This data is just one example among
many blinking points.

48

Figure6.3 Correlation diagram of two regions where optical intensities are negatively
correlated. It is not possible to put all the blinking data, so the data of six blinking points areshown. Correlation coefficients are also shown.

49

The data on the previous page shows the correlation for a single blinking point. Thus, I havealso performed extensive correlation mapping for a single pixel. Similarly, I picked out theblinking points in Figure6.1(a) that had a particularly high standard deviation, and calculated
the overall correlation for those points. The result is shown in Figure6.4 and Figure6.5. Fromthese results, we can see that there is a correlation not only between one of the blinking
points, but also between other blinking points that exist in the surrounding. It is also apparent
that there are multiple points consisting of two regions with negative correlations. As shownin Figure6.1, there are various ways to blink, but as a result of the above approach, I foundthat blinking points other than (a) also tend to consist of two blink regions with negative
correlation. Even for such blinking points, the correlation diagram was plotted, and a negative
correlation with a downward slope as shown in Figure6.3 could be obtained.
Figure6.4 and Figure6.5 show that almost all blinking points consist of two negatively
correlated regions. I selected other blinking points that were negatively correlated with eachother and measured their optical intensity in the same time series. The results are shown in
Figure6.6 and Figure6.7. These data show that the blinking points behave as if they were insync with each other. Two seconds after the start of measurement, the blinking opticalintensity of A (G), B (H), and C (I) increases, and processing takes about 23 seconds. In
contrast, blinking D (J), E (K), and F (L) work in contrasting ways. That is, not only the
optical intensity shows a negative correlation for each frame (0.03 seconds), but also theoptical intensity shows a negative correlation even when considered on a long-time scale. At
the same time, the optical intensity in the surrounding non-blinking area was also measured,
but there was no characteristic increase or decrease in light intensity as shown in the graph,and there was no sign of synchronization, Thus, I think this is not due to noise or lightsources.
50

51

Figure6.4 The correlations shown in Figure6.2 have been transformed into an extensive
correlation mapping, showing that there is correlation not only in one blinking point, but also
in others present in the surroundings. There are also many points that consist of two blinking
regions with negative correlation. The data is based on (378,320) pixels.

52

Figure6.5 Correlation mapping as in Figure6.4. The results are measured in a different region.
This map also shows that the blinking consists of two regions with negative correlation. The
data is based on (163,406) pixels.

53

54

Figure6.6 Time series optical intensity of two regions composed of negatively correlated. These data are spatially different blinking
points.

55

56

Figure6.7 Time series optical intensity of two regions composed of negatively correlated. These data are spatially different blinking
points.

57

6.3 Conclusion and Discussion
As a result of statistical analysis, it was found that the blinking point is composedof two regions composed of negative correlations. Negative correlation means thatas the optical intensity of one increase, the optical intensity of the other decreases. I
also found that these two areas are related to the other two areas of the surrounding
blinking points and behave as if the blinking intensities were synchronized.
However, there were some blinks that behaved in the opposite way when viewed ona long-time scale.
Since the two regions can show a relatively high negative correlation, the numberof carriers reaching one blinking point is considered constant, and many carriers are
involved in recombination in one region. On the other hand, the number of carriers
involved in recoupling may be small in other regions. This means that the QCSE
wavefunction overlap increases in one region and decreases in the other. As
mentioned in Chapter 2, QCSE is due to the piezo electric field. In addition, since
the behavior of the other two blinking regions existing in the surroundings and the
behavior of the two regions are synchronized, the application of the piezo electricfield is synchronized in the portion where the indium corresponding to the blinking
portion is localized. On the contrary, regarding blinking that behaves in the opposite

58

direction without synchronization, it is considered that the application of the piezo
electric field also behaves in the opposite direction. As a future prospect, it isnecessary to directly show by experiment whether the magnitude of the piezoelectricfield is constantly changing in the blinking part and whether the application of thepiezoelectric field is synchronized in the blinking part.

59

7 The Electronic Properties of Chlorine in
GaN(External Collaboration)

Although the main focus of my research is the blinking phenomena, I also
elucidated the effects of internal impurities in GaN using ab initio calculations and I
published a paper.[21] This work was done in collaboration with Dr. Giovanni
Alfieri, who works at ABB Power Grid Research in Switzerland. It is well known
that internal impurities can significantly alter the overall functionality of a device,and the presence of this defect cannot be ignored in the development of GaN
optoelectronics. For example, it has been reported that nitrogen vacancies in GaN
can prevent the formation of p-type GaN. Carbon unintentionally trapped in GaN
has also been studied. Carbon is easily replaced by nitrogen in GaN and is known tocause yellow luminescence. Thus, the presence of point defects has been reported tohave a negative effect on devices.
We focused on chlorine, which is a chlorine-based reactive ion etching process
for the fabrication of GaN semiconductor devices. In this process, chlorine can beunintentionally mixed into GaN. Chlorine-based etching has been reported to reducethe carrier concentration in p-type GaN by two orders of magnitude; in n-type GaN,
the ohmic contact resistivity is reported to be reduced or even increased. Although60the above experimental facts have been presented, the overall interpretation is
ambiguous. To answer this question, we calculated the defect formation energy by
performing SIESTA-based density functional theory [22] for three structures: one
with substituted chlorine, one with both vacancies and substituted chlorine, and one
with substituted chlorine and magnesium. By calculating the defect formationenergy, it is possible to quantitatively evaluate in which charged state the impurity
exists stably in the band gap, and whether it behaves as a donor that emits electrons
or as an acceptor that receives electrons.
In using SIESTA, Troullier-Martines norm-conserving pseudopotentials [23] wasused to take into account the influence of the core electrons, and the Ceparley-Alderform of the local density approximation (LDA) [24] was used to approximate theexchange correlation potential. The double-ζ plus polarization is used as the basis
for the atomic orbitals, and the d-orbitals are treated as valence electrons in the Ga
atom. The equilibrium configuration of GaN, which contains a total of 96 atoms,
was determined using the conjugate gradient method until the maximum nuclear
energy was below 0.025 eV and the stress tensor was 0.5 Gpa. The charge density
was projected onto a real space grid with a cutoff of 375 Ry. Each grid constant was
1% larger than the experimental value and the direct band gap was 2eV.

61

The density functional theory calculations show that the chlorine substitution at the
N site behaves as a double donor. In the other two, the impure chlorine also behaves
as a donor and is responsible for releasing electrons. This could explain theexperimental increase in electron concentration in dry-etched n-type GaN.

62

8 Bibliography
1. THEMIS editorial department Japan. Blue light-emitting diode (2004)
2. D.S.Arteev et al Investigation of Statistical Broading in InGaN Alloys 2018
J. Phys.: Conf. Ser. 1135 012050
3. Christian Feldmeier, Blinking Phenomena in InGaN Quantum Wells,
University of Regensburg Master's thesis (2009)
4. Josh Abell and T.D. Moustakas, The role of dislocations as nonradiative
recombination centers in InGaN quantum wells, Applied Physics
Letters,92,091901 (2008)
5. H.Morkoc, S.Strite, G.B.Gao, M.E.Lin, B.Sverdlov, and M.Burns, Largeband-gap SIC, Ⅲ - Ⅴnitride, and Ⅱ- Ⅵ ZnSe-based semiconductor devise
technologies, Journal of Applied Physics, Volume76, Issue3 (1994)
6. Y.Narukawa, Y.kawakami, M.Futano, Sz.Fujita. and Sg.Fujita, Role of selfformed InGaN quantum dots for exciton localization in the purple laser diodeemitting at 420 nm, Applied Physics Letters, 70,981 (1997)

63

7. Y.Narukawa,Y.Kawakami,S.Fujita and S.Nakamura, Recobination dynamics
of localized excitons in oo multiple quantum wells, PHYSICAL REVIEW
B, 55,R1938 (1997)
8. A.Kaneta,M.Futano,and Y.Kawakami, Nanoscopic recombination processes
in InGaN/GaN quantum wells emitting violet,blue,and green spectra,
PHYSICAL REVIEW B, 78, 125317 (2008)
9. Ruggero Micheletto, Masayoshi Abiko, Akio Kaneta, Yoichi Kawakami,
Yukio Nakamura, and Takashi Mukai, Observation of optical instabilities in
the photoluminescence of InGaN single quantum well, Applied Physics
Letters, 88,061118 (2006)
10. M-E.pistol, P.Castrillo, D.Hessman,J.A.Prieto, and L.Samuelson, Random
telegraph noise in photoluminescence from individual self-assembled
quantum dots, Physical Review B, VOLUME59,NUMBER16 (1998)
11. D.bertram, M.C.Hanna, and A.J.Nozik, Two color blinking of single straininduced GaAs quantum dots, Applied Physics Letters 74,2666, (1999)

12. J W.Robinson , J H Rice, Kwan H Lee, Jong H. Na, and R A.Taylor D
G.Hasko R A.Oliver, M J.Kappers and C J.Humphreys, G.Andrew D.Briggs,

64

Quantum-confined Stark effect in a single InGaN quantum dot under alateral electric field, Applied Physics Letters, 86,213103 (2005)
13. S H.Park and S L.Chuang, Crystal-orientation effects on the piezoelectricfield and electronic properties of strained wurtzite semiconductors, PhysicalReview B, VOLUME59,NUMBER7 (1999)
14. K.Oikawa, Study of optical blinking phenomena on nitride semiconductors,
Yokohama City University Graduate School of nanoscience Master thesis(2011)
15. R.J.Cook, and H.J.Kimbie, Possibility of Direct Observation of Quantum
Jumps, PHYSICAL REVIEW LETTERS, VOLUME54, NUMBER 10
16. M.Nirmal, B.O.Dabbousi, M.G.Bawendi, J.J.Macklin, J.K.Trautman,
T.D.Harris and L.E.Brus, Fluorescence intermittency in single cadnium
selenide nanocrystals, Nature 383,802-804 (1996)
17. T.Tsutsumi, The influence of surface conditions in InGaN quantum well
photoluminescence dynamics, Yokohama City University Graduate Schoolof nanobioscience Master's thesis (2015)65

18. T.Tsutsumi, G.Alfieri, Y.Kawakami, R.Micheletto, The relation between
photoluminescence properties and gas pressure with [0001] InGaN single
quantum well systems, Applied Surface Science 392, 256-259 (2017)19. USHIO Specification Sheet, USH-102D, USHIO Super High Pressure UV
Lamp(https://www.ushio.co.jp/documents/products/light_source/USH102D_J.pdf)
20. Modified version provided by Olympus Corporation (I received 2020 7/30)
21. Y.Fujii, R.Micheletto, and G.Alfieri, The Electronic Properties of Chlorine in
GaN: An Ab Initio Study, Physical Status Solidi B, 2000303 (2020)
22. J.M.Soler, J.D.Gale, A.Garcia, J.Junquera, P.Ordejon, and D.Sanchez-Portal,
The SIESTA method for ab initio order-N materials simulation, Journal of
Physics, Condens.Matter 14 2745 (2002)
23. N.Troullier and J.L. Martins, Efficient pseudopotentials for plane-wave
calculations, Physical Review B, 43,1993 (1991)
24. D.M.Ceparly and B.J.Alder, Ground State of the Electron Gas by a
Stochastic Method, Physical Review letters, 45,566 (1980)

66

9 Acknowledgement
I would like to thank Professor Ruggero Micheletto for his very enthusiastic
guidance in conducting this research on myself. He also taught me aboutprogramming and helped me to develop skills that I can use in the future as amember of society. I would like to express my deepest gratitude to him.
Professor Yoichi Kawakami of Kyoto University also gave me extensive guidanceon knowledge. Thanks to Dr. Kawakami's advice, I was able to conduct meaningfulresearch. I would like to express my deepest gratitude to him.
Dr. Giovanni Alfieri, who belongs to Hitachi Power Grid, Lenzburg, Switzerland,gave me knowledge about first-principles calculations. I asked a lot of questions by
e-mail every day, but I received accurate and easy-to-understand answers from him.
I would like to express my deepest gratitude to him.
I would also like to thank Mr.K.Sano of Katsumichi Corporation for providing mewith useful instruments for measurements. I would like to express our deepest
gratitude to him.
I would like to thank Mr. Rei Maekawa and Mr. Kazuki Konno, who graduatedfrom our laboratory last year, for their precise advice mainly on seminar activities.

67

We had a very meaningful time together outside of research activities, such as
discussions over drinks. I would like to express my deepest gratitude to them.
I would like to thank Makoto Ono and Akira Hirai, who are my classmates in thelaboratory, we were able to work hard and polish each other's research for two
years. Although they sometimes gave me harsh advice, it was because of them that I
was able to conduct good research. I am deeply grateful to them.
I have fond memories of not only discussing my research with Yoshinori Tsuchiya
and Kenya Tsukada, who are one year behind me, but also discussing about myfuture and job hunting activities. It was a great opportunity for me to learnsomething that I did not know before. I would like to express my deepest gratitude
to them.
I was able to discuss with Masaya Tsukida and Yuya Okajima, who are two years
behind me, from morning till night in the laboratory. We were able to discuss not
only our own research, but also how each other's research could be useful in the
future. They taught me a lot about statistics and increased my curiosity about
statistics. I would like to express my deepest gratitude to them.

68For a short time, I was able to have an intense discussion with Shunra Yoshida,
who is three years my junior, about the blinking phenomena. I was amazed at theamount of knowledge he had. I would like to express my deepest gratitude to him.
I would like to thank my family for supporting me for the past 24 years in good
health.

69

10 Conferences and Publications[Conference Oral Presentation]
1. International Workshop on Nitride Semiconductors 2020, August 23-28, 2020 inBerlin, Germany. (Oral session)
Title: The effect of absorbed liquids on the optical blinking of InGaN/GaN
quantum Wells

2. Nano-Optics Research Group 27th Research Discussion Meeting (Japan). January
22 2021 (Oral session held online)
Title: Study on unstable blinking phenomenon on InGaN/GaN single quantum well

ナノオ プティクス 研究グループ第２ ７回研究討論会 オン ライン開催
発表題目

2021年

1月22日

InGaN/GaN 単一量子井戸型構造上の不安定点滅現象に関す る研究[Publication]
Y.Fujii, R.Micheletto, and G.Alfieri, The Electronic Properties of Chlorine in GaN:
An Ab Initio Study, Physical Status Solidi B, 2000303 (2020)70

11 Summary of the research (Japanese)【日本語要旨】
発光デバイスに使われるInGaN/GaN単一量子井戸型構造上(InGaN/GaN
SQW)に励起波長365nmの紫外線を照射すると、表面に不安定な点滅を繰り返す
無数の輝点が発生する。これはInGaN以外の量子ドットでも観測されるが、原因不明な現象の一つでありメカニズムの解明が求められている。そこで、本研究ではこの不安定点滅現象の物理学的なメカニズムを解明することを目的とし、点滅現象の温度依存性、液体の付着による点滅現象の変化、さらに、統計的な分析を行うことで点滅
点の相関関係などを調査した。
第１章ではInGaN窒化物半導体の開発の歴史と基礎的な物性についてまとめた
第２章ではミケレット・ルジェロらによって発見された InGaN/GaN SQW上の点滅現象についての具体的な説明、点滅現象の要因とされている物理学的モデルや過去の先行研究事例についてまとめた。
第３章では実験に利用したInGaN/GaN SQWの内部構造、本研究で利用したオ
リンパス社製の選択励起蛍光顕微鏡のメカニズム、得られたビデオデータの処理方法についてまとめた。
第４章にて点滅現象の温度依存性について、発光波長が 460nmと510nmのInGaN/GaN SQWを利用することで検討を行った。両サンプルに共通して、相対的に低い温度環境で光励起を行うと点滅が発生しやすい結果が得られた。また、温度上昇により点滅の激しさが和らぎ、点滅点も消滅していく傾向が見られ、発光波長が460nmのInGaN/GaN SQWでは最高温度の373Kにて、90％以上の点滅が消滅した。510nmの場合も同様に温度上昇によって点滅の激しさは和らいでいくが、最高温度の373Kでも点滅し続ける輝点が存在している結果となった。温度上昇による熱膨張が数多くの欠陥を生成し、キャリアが欠陥にトラップされやすくなったが故に
非輻射再結合を行うようになった結果、点滅の激しさが和らいだと 私は考えている。
他の要因として電子移動度の低下などが挙げられるが、温度上昇によりあらゆる物
理量が変化するため、明確な原因については本研究で突き止めることができなかった。
第５章では液体を付着させた状態で光励起を行い、点滅の 挙動を観察した。利用した液体は水とエタノールであり、何も付着させていない状態との比較を行った。試料は、第4章と同様に発光中心波長が460nmと510nmの2種類を利用している。こ71

の結果、液体の付着が点滅現象を 誘発していることが両者共通して明らかとなった水を付着させた場合、輝点が増え、点滅の激しさが若干増すが、エタノールの場合、点滅が非常に激しくなる現象が顕著に見られた。液体の付着がInGaN/GaN SQW
内部の電子状態を変え、点滅を引き起こしやすい環境を作り出していることが示唆される。仮説としては、価電子帯近傍の状態密度が大きくなり、励起に必要な電子の席が増えた。または、点滅現象が量子閉じ込めシュタルク効果により発生しているのであれば、点滅箇所におけるピエゾ電界の印加の大きさが高速で増減を繰り返していると考えられる。
第6章では統計的分析により、点滅は負の相関である2つの領域で構成されていることが明らかになった。負の相関とは、2つの領域のうち片方の光学強度が上昇す
れば、もう片方が下降していく現象が1フレーム毎（約0.03秒）に発生していることを意味する。また、1つの点滅点だけでなく、周囲に存在している他の点滅とも相関性が見られ、点滅内の2つの領域の光学強度が同期しているような振る舞いをしていることが分かった。すなわち、約0.03秒毎に光学強度が相関性を示すだけでなく、長い時
間スケールで考えても、光学強度が相関性を示していることになる。仮に点滅現象が量子閉じ込めシュタルク効果により引き起こされているならば、点滅箇所に印加されるピエゾ電界の大きさも時系列的に同期していると考えられ、再結合率も同期すると考えることができる。点滅が負の相関を示すことについては、片方の領域ではピエゾ
電界の大きさが大きくなりキャリアの再結合率が下がっていくが、もう片方では対照的な現象が発生しているからと考える。
第7章ではスイスの日立パワーグリップ社に所属する、ジョバンニ・アルフィエリ博士との共同研究について 簡単にまとめた。窒化ガリウム（GaN）の製造プロセスの1つである反応性イオンエッチングにおいて、塩素が混入されてしまうことが示唆されており、塩素が混入された場合のGaNの物性について明らかにされていない。そこで第一原理計算ソフトの一つであるSIESTAを利用した密度汎関数法を実行し、欠陥
生成エネルギーを算出することで不純物塩素が含まれたGaNの電子物性について定量的に評価した。結果としてGaN内部の塩素は電子を放出することで安定な構造をとるドナーとしてGaN内部で振る舞うことが明らかになった。
続く第8章は参考文献、第9章は謝辞から構成され、第10章に研究実績をまとめた。

72