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PUBLfCATIONS

NIST SPECIAL PUBLICATION 400-94

U.S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

or Measurement Technolo.

Improved Characterization and Evaluation Measurements

for HgCdTe Detector Materials, Processes, and Devices

Used on the GOES and TIROS Satellites

U57

NO. ^00-91

199^1
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Semiconductor Measurement Technology:

Improved Characterization and Evaluation Measurements

for HgCdTe Detector Materials, Processes, and Devices

Used on the GOES and TIROS Satellites

David G. Seller, Jeremiah R. Lowney, W. Robert Thurber,

Joseph J. Kopanski, and George G. Harman

Semiconductor Electronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

April 1994

U.S. DEPARTMENT OF COMMERCE, Ronald H. Brown, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar, Director
National Institute of Standards and Technology Special Publication 400-94

Natl. Inst. Stand. Technol. Spec. Publ. 400-94, 184 pages (April 1994)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1994

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
TABLE OF CONTENTS

Page

Abstract 1 Key Words 1 Executive Summary 2

1. Introduction 5

A. Background 5

(1) Infrared Detectors Used on the GOES and TIROS Satellites 5

(2) Previously Published Report on HgCdTe Detector Reliability

Study for the GOES Program 8

(3) Fabrication of Photoconductive HgCdTe Infrared Detectors 8

B. Importance of Work Presented Here 10

C. Outline and Organization of Report 11 PART I - HIGH-FIELD MAGNETOTRANSPORTSTUDIES

2. Characterization of GOES and TIROS HgCdTe IR Detectors by Quantum

Magnetotransport Measurements 13

A. Review of Shubnikov-de Haas (Oscillatory Magnetoresistance) Effect 13 B. Physical Modeling and Fundamental Theory 13

(1) Theory of Shubnikov-de Haas Effect 13

(2) Fourier Transform Analysis 15

(3) Determination of Effective Masses 15

(4) Model for Subbands 17

(5) Determination of Dingle Temperatures 27

(6) Dependence of SdH Signal on Angle of B-field 27

C. Results of Measurements on Specific Detectors 28

(1) Typical Data 28

(2) Results on GOES Detectors 28

(a) Supplier 1 28

(b) Supplier 2 37

(c) Supplier 3 49

(3) Results on TIROS Detectors 62

(4) Intercomparisons 71

D. Summary of Quantum Magnetotransport Characterization Measurements 81

(1) Comparison between Theory and Experiment for Two Different

Detector Types 81

(2) Comparison with Detector Performance and Identification of Trends 84

3 . DC Magnetoresistance Characterization of Detectors 85

A. Background 85

B. Theoretical Analysis 85

C. Experimental Work and Results 87

D. Conclusions 96

iii
TABLE OF CONTENTS (Continued)

Page

PART II -OTHER CHARACTERIZATION STUDIES

4. Bonding, Metallization, and Packaging for GOES and TIROS Infrared Detectors . . . 97

A. Overview and Rationale 97

B. Accomplishments 98

C. Recommended Practice for Wire Bonding and Metallization Used in

Radiation Detectors Prepared for Use in GOES, TIROS, and Other Satellites 99

D. Glossary 101

E. Typical Bonding Characteristics and Appearance of Plated Gold Films 102

5. Semiconductor Electronic Test Structures: Appiications to Infrared Detector

Materials and Processes 103

A. Introduction 103

B. Conclusions 105

6. Scanning Capacitance Microscopy: A Nondestructive Characterization Tool 106

A. Background 106

B. Applications of SCM to GOES and Related Infrared Detectors 106

C. Establishment of Facility 107

D. Preliminary Results 109

E. Summary 113

7. NIST Review of the GOES Calibration Program 114

A. Purpose 114

B. Summary of Visits to Facilities 114

C. Recommendations 115

8. Summary and Conclusions 117

A. Magnetotransport Measurements 117

B. Other Characterization Studies 118

9. References 120

APPENDICES

A. Reprint of Published Paper "Heavily Accumulated Surfaces of Mercury Cadmium

Telluride Detectors: Theory and Experiment"

B. Reprint of Published Paper "Review of Semiconductor Microelectronic Test Structures with Applications to Infrared Detector Materials and Processes"

C. Reprint of Published Paper "Hgi.^Cd^Te Characterization Measurements: Current

Practice and Future Needs"

IV
LIST OF FIGURES

Page

1 . 1 Principal components of a HgCdTe GOES detector element 9

2.1a Built-in potential for accumulation layer with total electron density of 8.9 x 10^^ cm'^

and alloy fraction x = 0.191 21

2.1b "E versus k" dispersion relations for the subbands for this potential, showing

spin-splitting 22

2.2a Electron density computed by solving Poisson's equation for a charge continuimi

and from full quantum-mechanical calculations for the case of figure 2.1b 23

2.2b Calculated subband densities as a function of total density 24

2.3a Subband Fermi energies as a function of total density, measured from the bottom

of each subband 25

2.3b Ratio of subband cyclotron effective masses to the free electron mass at the Fermi

energy as a function of total density 26

2.4 The ac and dc signals of the magnetoresistance of a typical detector element, 31 IB 29

2.5 The ac signal for detector element 311 IB showing the oscillations imposed on a background which initially rises abruptly and then falls slowly 30

2.6 The ac signal which has been centered by doing a spline fit to the average value of

regions of the original signal and then replotting the signal relative to the fit 31

2.7 The ac signal as a function of inverse magnetic field 32

2.8 Fourier transform of the signal in the previous figure 33

2.9 Fourier transforms from elements of two different Supplier 1 detectors, 1 and 2 34

2.10 Subband carrier density versus total density for the Supplier 1 detector elements in figure 2.9 38

2.11 Shubnikov-de Haas traces of four elements of detector 2III1 from Supplier 2 39

2.12 Temperature dependence of the SdH oscillations of element 2III IB 40

2.13 Traces of four elements of detector 2III2 41

2.14 Fourier transforms for the elements of detector 2III1 42

V
LIST OF FIGURES (Continued)

Page

2.15 Fourier transforms for the elements of detector 2III2 44

2.16 Results of rotating element 2III1B in the magnetic field 45

2.17 Shubnikov-de Haas oscillations from the three Supplier 2 detectors fabricated for

diagnostic purposes 46

2.18 Fourier transforms of the SdH response in figure 2.17 for the experimental

Supplier 2 detectors 47

2.19 Response curves for the two types of Supplier 3 detectors 50

2.20 Temperature dependence of the SdH response of a Supplier 3 type I element, 3I2A .... 51

2.21 Fourier transforms of the temperature data in figure 2.20 52

2.22 Fourier transforms from five type I detector elements from Supplier 3 53

2.23 Temperature dependence of the response of element 311 IC from a Supplier 3 type II detector 57

2.24 Fourier transforms of four elements of a Supplier 3 type II detector 58

2.25 Subband carrier density as a function of total density for the Supplier 3 type I detector elements listed in table 2.3 61

2.26 Subband carrier density as a function of total density for the Supplier 2 and

Supplier 3 type II detector elements listed in tables 2.2 and 2.4, respectively 63

2.27 Response of element 3I2A when rotated in the magnetic field 64

2.28 Fourier transforms of the rotation traces in the previous figure 65

2.29 Normalized frequency dependence of the Fourier transform peaks for the major

subbands of element 3I2A as a function of the angle of rotation in the magnetic

field 66

2.30 Comparison of synthesized and original data for element 3I2A 67

2.31 Reconstruction of the complicated SdH response for element 311 IB 68

2.32 Shubnikov-de Haas response of six TIROS detectors 69

2.33 Fourier transforms of the six TIROS detectors in the previous figure 70

vi
LIST OF FIGURES (Continued)

Page

2.34 Shubnikov-de Haas traces for detector T56238 at five temperatures 74

2.35 Fourier transforms of the SdH traces in the previous figure 75

2.36 Subband density as a function of total density for the six TIROS detectors discussed

in this section 76

2.37 Fit to determine the effective mass for subband 0' 77

2.38 Illustration of the determination of the Dingle temperature Tj) 78

2.39 Representative SdH traces from each supplier and commercial detector type 79

2.40 Fourier transforms of the SdH traces in figure 2.39 80

2.41a Fourier transform of SdH data for detector element 3I2A; the label "H" stands for

harmonic; peaks are labeled by subband number 82

2.41b Fourier transform of SdH date for detector element 3II1B; the peaks are labeled by

subband number 83

3.1 Relative resistance as a function of ijlB for different length-to-width ratios a, from reference 3.8 86

3.2 Illustration of a typical photoconductive HgCdTe infrared detector 88

3.3 Two-terminal transverse magnetoresistance as a function of B for a type I passivated detector and a type II passivated detector at 6 K 90

3.4 Expanded scale of the low-field behavior of the transverse magnetoresistance as a function of B for type I and type II detectors at 6 K and 77 K 91

3.5 Two-terminal transverse magnetoresistance as a function of B for a type III passivated

detector at 6 K 92

3.6 Expanded scale of the low-field behavior of the transverse magnetoresistance as a function of B for a type III detector at 6 and 77 K 93

3.7 Two-terminal transverse magnetoresistance as a function of B for a multi-element

type III detector at 6 K 94

6.1 Digital Instruments Inc. Nanoscope III atomic force microscope with a large sample

stage similar to the system operational at NIST 108

vii
LIST OF FIGURES (Continued)

Page

6.2 Low-magnification AFM image showing parts of three photoconductive detectors ..... 110

6.3 Cross-sectional analysis of an AFM image of the boundary between the passivated

active layer and the metal pad of a detector element Ill

6.4 Atomic force microscopy image of the edge of a photoconductive detector revealing fme

structure in the topography clustered at the edge of the active area 112

LIST OF TABLES

Page

1 . 1 Imager Channel Functions 6

1.2 Partial Listing of Spaceborne Infrared Sensor Programs Using Mercury-CadmiumTelluride Detectors 7

2.1 Results of Shubnikov-de Haas Analysis for Supplier 1 Detectors 35

2.2a Results of Shubnikov-de Haas Analysis for Supplier 2 Detector 43

2.2b Results of Shubnikov-de Haas Analysis for Supplier 2 Experimental Detectors 48

2.3 Results of Shubnikov-de Haas Analysis for Supplier 3 Type I Detectors 54

2.4 Results of Shubnikov-de Haas Analysis for Supplier 3 Type II Detectors 59

2.5 Results of Shubnikov-de Haas Analysis for TIROS Detectors 72

3.1 Parameter Values Extracted by the Transverse Magnetoresistance Method 95

viii
Semiconductor Measurement Technology :

Improved Characterization and Evaluation Measurements

for HgCdTe Detector Materials, Processes, and Devices

Used on the GOES and TIROS Satellites

D. G. Seller, J. R. Lowney, W. R. Thurber,

J. J. Kopanski, and G. G. Harman

Semiconductor Electronics Division

National Institute of Standards and Technology

Gaithersburg, MD 20899

ABSTRACT

An extensive study was carried out to improve the characterization and evaluation

methods used for HgCdTe (mercury-cadmium-telluride) photoconductive infrared

detectors used in GOES and TIROS satellites. High-field magnetotransport techniques were used to determine the electrical properties of the detector accumulation layers,

which partially control their detectivities. Assessments were made of the quality of the

bonding and packaging used in detector fabrication, and a list of recommended practices was produced. The applicability of scanning capacitance microscopy and test structures

to detector-array evaluation is discussed, and, finally, recommendations are made for

standardized detector calibration. The results of this work have provided new and more

refined measurement methods that can be adopted by the detector manufacturers to improve performance and yield.

KEY WORDS: bonding; geostationary environment satellite; infrared detector; IR

detector calibration; magnetoresistance; mercury cadmium telluride; packaging;

scanning probe microscopy; Shubnikov-de Haas; test structure

Disclaimer: Certain commercial equipment, instruments, or materials are identified in

this report in order to specify the experimental procedure adequately. Such

identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment

identified are necessarily the best available for the purpose.

1
EXECUTIVE SUMMARY

This report summarizes results of extensive studies carried out by the National Institute of Standards

and Technology (NIST) for the National Oceanic and Atmospheric Administration (NOAA) on

improving characterization and evaluation measurements of HgCdTe infrared detector materials,

processes, and devices used for the Geostationary Operational Environmental Satellite (GOES) and the

Television and Infrared Operational Satellite (TIROS) systems.

NIST has provided services to NOAA, the National Aeronautics and Space Administration (NASA),

ITT Aerospace Communications Division in Fort Wayne, Indiana, and several other detector

fabrication companies in areas of detector packaging, bonding, and metallization. Numerous detector

committee meetings and briefings were attended by NIST personnel.

The techniques developed by NIST and reported here have the advantage that they can be applied to actual, small-area, commercial detectors being manufactured for the GOES and TIROS Programs.

These measurements provide high-quality data which are demonstrated to provide a unique

characterization signature for an infrared detector. A physical model of the detector surface layers

has been developed relating detector parameters to performance, thus permitting a better

understanding and engineering of current detectors as well as future generations. In addition, the

techniques developed here provide a diagnostic tool to characterize effects of processing on detector

performance, as well as the ability to characterize detector stability and reliability. New processing

fabrication procedures being developed can now be much better understood and monitored.

NIST has carried out state-of-the-art applied and fundamental research on two magnetic-field-based

characterization measurements needed for the HgCdTe-based infrared photoconductive detectors of the

GOES and TIROS Programs. The oscillatory variation in resistance with magnetic field, i.e., the

Shubnikov-de Haas (SdH) effect, and the behavior of the dc magnetoresistance are both shown to provide crucial understanding and characterization of the properties (electron concentrations and

mobilities) of the two-dimensional electron gas (2DEG) in the accumulation layers produced by the

passivation process. The detector performance depends to a great extent upon the type and quality of

the passivation process. Ten samples were prepared for low-temperature Shubnikov-de Haas and

other measurements for the NIST HgCdTe detector studies.

Shubnikov-de Haas oscillations in the transverse magnetoresistance have been used to characterize

accumulation layers of the infrared detectors used in GOES and TIROS weather satellites. Electron

densities, cyclotron effective masses, and Dingle temperatures can be obtained from the data for each

subband in the 2D electron gas formed by the accumulation layer. A first-principles calculation of the

subband energy dispersion relations has been performed in order to compare theory and experiment.

The model is needed to extract the electron density from the data because the energy bands are very

nonparabolic in narrow-gap HgCdTe. The agreement between predicted and measured masses and

Fermi energies was excellent for anodically oxidized layers. Effective masses could not be obtained

for other processes because signals were weak and complex. A large number of detectors from each

of three suppliers were measured by the SdH effect, and the data were analyzed. Results obtained for

devices with type I passivation (anodic oxidation) gave Fourier transforms with large, well-defined

peaks from which the carrier density of the accumulation layer was obtained. Detector elements with

different passivations, type II and type III, had a weak SdH response. The carrier density of

accumulation layers of type II and III detectors were much greater than those for type I detectors.

The generally lower mobilities and higher densities of accumulation layers in type II and type III
detectors led to their improved performance because of reduced leakage and decreased surface

recombination. Angular rotation studies were done on devices from two suppliers to verify that the

SdH signal was coming from the two-dimensional accumulation layers. Effective masses and Dingle

temperatures were calculated for one or more elements with type I passivation. The values were in good agreement with theoretical calculations.

A new and simpler method to characterize infrared detectors has been developed based on dc

magnetoresistance upon which the small-amplitude SdH oscillations are superimposed. Electron

density and mobility in the top accumulation layer can be determined from the magnetic-field

dependence of the transverse magnetoresistance at high fields. Agreement between densities and

mobilities in accumulation layers of type I and type II detectors with Hall measurement data supplied

by the manufacturers showed the method was accurate. Measurements were made on a large number

of detectors. The results showed variability of the accumulation-layer density by 20% among three

elements of a multi-element detector. This method can be applied directly to the fabricated detectors

because it requires only two terminals.

A total of six visits were made to three GOES and TIROS infrared detector manufacturers during

fiscal years 1992 and 1993. The first visit to each site served to evaluate the production lines and the

processes. Later visits were made to help them improve the detector packaging. This included one

2.5-h seminar on wire bonding and reliability of metallurgical systems used in packaging HgCdTe

detectors. Over 30 people attended that seminar. For another company, a more informal hour-long

presentation was made to 6 or 7 engineers and management personnel. Extra time was later spent in visiting their packaging laboratories. A scanning electron microscope study was made of detectors

from one manufacturer that showed several defects (the results of this study were presented at a GOES project review at NASA Goddard, July 10, 1992). This information, with proposed solutions, was also fed back to the manufacturer to help them improve their product. Studies were carried out

at NIST and at two detector manufacturing sites to establish the best molecular-cleaning methods that are compatible with normal HgCdTe detector packaging methods. For this work, ultraviolet cleaning

equipment was handcarried to detector manufacturers so that tests could be performed there.

Because of the substantial impact of test structures on other semiconductor circuits, the current

state-of-the-art applications of test structures to HgCdTe-based IR detectors were comprehensively

reviewed. To place these applications in context, the general principles of applying test structures,

determined through experience with silicon integrated circuits (ICs) and GaAs monolithic microwave

integrated circuits (MMICs), were also reviewed. From these two reviews, principles and ideas were

extracted for test-structure applications that could be used to further enhance the manufacturability,

yield, and performance of IR detectors. To communicate and encourage application of test structures,

the results of the study were presented at the Measurement Techniques for Characterization of MCT

Materials, Processes, and Detectors Workshop held in Boston, Massachusetts, during October 1992

and published in Semiconductor Science and Technology. A reprint of this paper is included in this report as an appendix.

Scanning capacitance microscopy (SCM) is a new, nondestructive metrology tool that merges a highsensitivity capacitance sensor with an atomic force microscope (AFM). SCM applications that could

be expected to have a large impact on the quality, yield, and manufacturability of IR detectors

include: nondestructive diagnosis of material variations within the active regions of detectors,

nondestructive prefabrication materials evaluation, and depth profiling of dopants in nanostructures. AFM images were made of some photoconductive detector elements to illustrate the feasibility,

3
potential resolution, and image quality of SCM applied to IR detectors.

NIST staff from the Radiometric Physics Division visited and examined the radiometric calibration

programs of the detector suppliers and the system integration contractor where the final radiometric

calibrations are performed. NIST recommends that a fundamental calibration program be established

that is coordinated between the different manufacturers and assemblers. NIST also recommends that the GOES detectors be calibrated several times before launch to establish a calibration history and

base line.

4
1. INTRODUCTION

A. Background

(1) Infrared Detectors Used on the GOES and TIROS Satellites

The National Oceanic and Atmospheric Administration (NOAA) has the responsibility for producing,

launching, and operating a multiple Geostationary Operational Environmental Satellite (GOES)

system. The primary purpose of the GOES Program is the continuous and reliable collection of

environmental data in support of weather forecasting and related services. The data obtained by the

GOES satellites provide information needed for severe storm detection, monitoring, and tracking;

wind measurements from cloud motion; sea surface thermal feamres; precipitation estimates; frost monitoring; rescue operations; and research. The geostationary orbit of these satellites allows

continuous observation of a portion of the earth and its atmosphere. Since 1974, these GOES

satellites have been used to collect and disseminate environmental data for the United States National

Weather Service. At present, there is only one aging satellite, GOES H or (GOES-7), in orbit. The

United States National Weather Service now relies heavily on this aging satellite GOES-7 for crucial

weather information.

New weather satellites are being produced by a program known as GOES-NEXT, for the next generation of Geostationary Operational Environmental Satellites. A series of five satellites,

designated by the letters I-M, are scheduled to be produced. There are significant differences

between the GOES I-M series satellites and the earlier series. The GOES D-H satellites had a

passive, spin-stabilized, attitude control system. The GOES I-M series of satellites uses a three-axis

attitude control system. Unlike the GOES D-H series, the GOES I-M satellites support separate

imager and sounder instruments that operate independently and simultaneously perform imaging and

sounding operations. These satellites perform a number of functions including visible and infrared

imaging (Imager) and atmospheric sounding (i.e., depth profiling of the atmosphere) (Sounder) by

using various types of detectors.

The GOES sensors provide two-dimensional cloud and temperature imagery in both visible and

infrared spectra, radiometric data that provide the capability to determine the three-dimensional

structure of atmospheric temperature and water-vapor distribution, and solar and near-space

environmental data. Three different types of detectors are used in each of the Imager and Sounder

systems: silicon (Si) photovoltaic detectors for visible radiation, indium-antimonide (InSb)

photovoltaic detectors for infrared radiation, and mercury-cadmium-telluride (HgCdTe)

photoconductive detectors for various infrared-radiation spectral regions. There are five channels for

the Imager. Table 1.1 shows their specifications for detector type, wavelength range, and their

purpose. Spectral separation in the Imager is done by fixed dichroic beam splitters, permitting

simultaneous sampling of all five spectral channels.

The Sounder instrument has 19 channels. There are four Sounder bands containing Si detectors for

the visible, InSb detectors for the shortwave infrared, and HgCdTe detectors for both the midwave

and longwave infrared regions. These bands provide information on atmospheric temperature

profiling. The visible spectrum and the three infrared bands are separated by dichroic beam splitters.

The three infrared bands then pass through three concentric rings of a filter wheel where channel

filters provide sequential sampling of the seven longwave, five midwave, and six shortwave channels.

5
1

Table 1.1. Imager Charaiel Fmictions

Spectral Channels

1 2 3 4 5

Detector Type Si InSb HgCdTe HgCdTe HgCdTe

Wavelength (/xm) 0.55 to 0.75 3.80 to 4.00 6.50 to 7.00 10.20 to 11.20 11.50 to 12.50

Function Cloud

Cover

Nighttime

Clouds

Water Vapor

Surface Temperature

Sea Surface Temperature &

Water Vapor

The ternary intermetallic compound Hgi.,CdJe is one of the most important materials used in

infrared detectors. These infrared detectors are widely used for military applications and civilian

purposes such as in satellites that need spaceborne infrared sensors for remote temperature sensing.

Interest also exists in using these detectors for evaluating home and industrial energy loss, medical

thermography (i.e., breast cancer detection), astronomical research, spectrophotometers, laser light

detection, remote controls for TV sets and VCRs, etc.

The Television and Infrared Observation Satellite (TIROS) also performs meteorology functions using

HgCdTe infrared (IR) detectors incorporated into two instruments: the Advanced Very High

Resolution Radiometer (AVHRR) and the High Resolution Infrared Radiation Sounder (HIRS). In

fact, there are a large number of commercial and defense satellites that incorporate HgCdTe IR

detectors in their instruments as illustrated in Table 1.2.

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(2) Previously Published Report on HgCdTe Detector Reliability Study for the GOES Program

The results of a special assessment carried out by NIST, at the request of NOAA, of the reliability of

certain infrared detectors for the Geostationary Operational Environmental Satellite system were summarized in a special report, NISTIR 4687, by D. G. Seller, G. G. Harman, J. R. Lowney, S. Mayo, and W. S. Liggett, Jr. [1.1]. The data made available by ITT on detector resistances and

signals supported the conclusion that degradation of some detector responses had occurred, even when

the estimated measurement uncertainty was included. Statistical analysis of the 11-jwm detectors

confirmed that one detector decreased in signal with time. The existing data available to NIST are not sufficient to identify uniquely the cause of degradation or unstable behavior present in a number

of detectors. NIST's physical examination of several detectors by optical and SEM microscopy

methods and an examination and analysis of the Detector Measurement Database has yielded several

plausible possible mechanisms for the observed degradation. These possible mechanisms are related

to the detector fabrication or processing steps and include: incomplete or poor passivation

procedures, excess mercury diffusion resulting from the ion-beam milling fabrication step, poor

indium electrical contacts produced by the indium-plating fabrication step, and delamination of the

ZnS anti-reflection optical coating. Other observed problems were poor wire bonding, use of tin-lead

solder to couple the fine gold wire (bonded to the detector) to the package terminal, and use of

silicone RTV to stake the bond wires to the edge of the ZnS substrate.

One of the recommendations given at the end of this assessment report suggested that magnetic-fieldbased measurements such as Hall-effect and Shubnikov-de Haas effect measurements be performed to determine the properties of the accumulation layers produced by the passivation process. The work

presented here addresses this recommendation.

(3) Fabrication of Photoconductive HgCdTe Infrared Detectors

Infrared photoconductive detectors are devices that convert electromagnetic radiation to electric

signals by direct conversion of incident photons into conducting electrons or holes. The signals can

then be processed to obtain information from the intensity and wavelength distribution of the incident

radiation. Figure 1.1 shows the principal elements of the HgCdTe GOES detectors [1.2].

There are a number of reasons why Hgj.^Cd^Te alloys are used. By varying the mole fraction x, the

energy gap can be continuously adjusted from below 0.04 to above 1.3 eV, covering the 1- to 25-fim

infrared region. Tailor-made materials can thus be grown to respond to preselected wavelengths,

providing one the opportunity to make a range of temperature measurements from orbit. Quantum

efficiencies approaching 100% for 12- to 16-/xm-thick devices are possible. Material having a long

carrier lifetime can be produced even at relatively high processing temperatures. The material can

also be made quite pure (approaching electrical levels of approximately 1 x 10^'* cm'^ carriers). In

addition, the surfaces can be passivated by any of a large number of approaches, including using ZnS,

native (anodic) oxides, sulphides, fluorides, etc. It is important to note that the performance of the

detector depends to a great extent upon the type of passivation process.

Important factors that influence the responsivity, impedance, and noise of the photoconductive

detectors are the energy gap, doping concentration, electron and hole mobilities, carrier lifetimes,

passivation properties, the effects of ion millings, and the contacts. Effects associated with the device

contacts and surfaces can cause gross distortions of the detector operating characteristics. The

processing details for fabricating contacts to HgCdTe are based largely upon empiricism. A

8
ZnS

antireflective

coating

Gold bond

wires

Passivation

r \

HgCdTe

Glue

ZnS

substrate

Figure 1.1 Principal components of a HgCdTe GOES detector element [1.2]. For the Imager,

the 7-/im-wavelength detectors have two elements; the 11-^m- and 12-/Lim-wavelength detectors, four.

9
fundamental understanding is lacking. Formation of Schottky barriers causes voltage instabilities and

problems with reproducibility and reliability.

Passivation is used to produce high carrier densities at the detector surfaces to reduce recombination

and noise by repelling minority carriers and filling surface traps. Passivation also serves to stabilize

the surfaces. An adequate passivation layer must (1) be a good insulator, (2) adhere sufficiently well

to the HgCdTe, (3) be time stable, (4) be stable against the atmosphere (unless a hermetic seal is used), (5) not be attacked by chemicals necessary for making the device, (6) be sufficiently nonporous

that atmospheric gases cannot move through it and attack the HgCdTe, and (7) produce an interface

which is sufficiently inert electrically so that it does not degrade the operation of the detector.

Inhomogeneity or nonuniformity in the relative concentrations of mercury and cadmium, doping

concentration level, or defects throughout the wafers can cause problems. Production yields for the

high-performance detectors are typically low, on the order of 5 to 10% or even lower.

B. Importance of Work Presented Here

The HgCdTe infrared detectors used in the GOES and TIROS Satellite Programs are essential

components of the satellite. The success or failure of the many functions of these satellites depends

upon the proper and reliable operation of these detectors. The detectors planned for use must have

high reponsivities or detectivities along with low 1/f noise characteristics in order to meet

specifications required by NOAA.

These state-of-the-art detectors are not easy to manufacture, and production yields are correspondingly

very low. In addition, it may be that "hot" detectors, i.e., those that meet the stringent specifications, are more susceptible to degradation or reliability problems because of the use of newer technologies.

It is, thus, imperative that a physically based understanding, rather than just empirically based

knowledge, be acquired for the selection, operation, and determination of the limitations and

reliability of the detectors.

The GOES detector degradation task force team "struggled" with the question of how to characterize

and understand the stability or reliability of the photoconductive detectors. A significant diagnostic

technique was found to be lacking. The development of the magnetotransport methods presented here

now provides a diagnostic tool to be used.

The HgCdTe industry is very concerned about the following issues that were directly raised by

several companies:

• "Why do individual detectors fail? Local characterization techniques are needed. There is no

well-understood body of knowledge available for HgCdTe. In wide-gap HgCdTe, one obtains

good signatures by photoluminescence methods, but in narrow-gap material, understanding of the

issues and characterization methods is lacking."

• "Many issues exist: nonuniform impurity distributions, defects and impurities, complexes, ... We don't know how to analyze the structures we are growing. THE ABILITY TO GROW

STRUCTURES HAS OUTGROWN THE ABILITY TO CHARACTERIZE THE MATERIAL.

There is a need to develop a good signature by using a particular characterization technique."

• Producing high-performance detectors is still a "trial and error" process using "black magic" and

10
"lots of sweat." Industry has to "build a learning curve" first.

NIST has thus investigated, developed, and performed state-of-the-art applied and fundamental

research related to new and improved characterization measurements for HgCdTe-based infrared

detectors needed by the GOES and TIROS Programs. NIST has developed a number of magneticfield-based techniques that have the proven capabilities of providing the necessary understanding and

characterization of HgCdTe detectors. These techniques include: (1) variable magnetic field and

variable-temperature magnetotransport measurements, and (2) oscillatory magnetoresistance (periodic

variation of resistance with magnetic field), i.e., Shubnikov-de Haas measurements. These techniques

are capable of determining the necessary information about the properties of the two-dimensional

electron gas (2DEG) in the accumulation layers produced by the passivation process and of the bulk

HgCdTe. This information includes the concentrations and mobilities of electrons in the 2DEG and

bulk, and related properties such as energy gaps and impurity- and defect-level information. These

m.agnetic-field-based measurement results can then be compared with the device parameters so that a

direct correlation of materials, processing, and device properties is achieved. This allows the

establishment of a database linking the detector parameters to specific aspects of the material

properties and the effects of the processing.

NIST is also developing and establishing scanning capacitance microscopy (SCM) as a new tool for

contactless, nondestructive characterization of HgCdTe wafers and processing technologies.

Nondestructive evaluation techniques are urgently needed for enhancing the yields of the HgCdTe

infrared detectors. SCM combines two established NIST technologies: capacitance-voltage

characterization of semiconductors and atomic-force microscopy. SCM will provide high spatial

resolution mapping of native and process-induced lateral variations in the electrical properties of

HgCdTe, including the bulk and accumulation layer regions.

C. Outline and Organization of Report

The following items summarize the content of this report, which is divided into two parts. Part I relates to high-field magnetotransport characterization of detector accumulation layers, while Part II relates to a number of other characterization techniques and issues.

• Section 2 gives extensive, systematic results on the use of quantum magnetotransport

measurements (Shubnikov-de Haas oscillations) to characterize GOES and TIROS HgCdTe IR

photoconductive detectors. The fundamental theory is presented, as well as the proper physical

modeling for the electric subband energies and densities, accumulation-potential wells at the

HgCdTe interface, and the subband effective masses and Dingle temperatures.

• Section 3 presents the results of a new technique developed to extract the electron density and

mobility in the top accumulation layers. It is based on measurements of the dc magnetoresistance

(the background signal on which the Shubnikov-de Haas oscillations are superimposed). This

technique is easy to apply and should be easily adopted by the detector manufacturers to improve

their quality control of existing detectors as well as to help engineer new, improved detectors.

• Section 4 presents the bonding, metallization, and packaging consulting work done for the GOES

and TIROS infrared detectors. An overview, rationale, and accomplishments of the work are

given.

11
Section 5 summarizes NIST work done on reviewing the use of test structures in the

semiconductor manufacturing industry, with a particular emphasis on HgCdTe-based IR

detectors. This section, along with the detailed results presented in Appendix B, gives a complete review of test structures applied to IR detectors and contains suggestions on how to improve IR detector process control, yield, performance, and reliability through the intelligent

application of test structures.

Section 6 gives a report on the establishment at NIST of scanning capacitance microscopy. SCM

has important capabilities to image nanoscale variations in dopant concentration, composition,

defects, mobility, and charge distributions within the detector elements.

Section 7 discusses recommendations for detector calibration.

Section 8 summarizes and concludes the work presented.

Section 9 lists the references.

Appendix A is a reprint of an article, "Heavily Accumulated Surfaces of Mercury Cadmium

Telluride Detectors: Theory and Experiment" which reports NIST work on characterizing the

GOES detectors [1.3]. These initial results were disseminated to the HgCdTe detector

community through a talk and this paper given at the 1992 U.S. Workshop on the Physics and

Chemistry of HgCdTe and Other IR Materials. The paper is published in the Journal of

Electronic Materials in August 1993.

Appendix B is a reprint of an invited article "Review of Semiconductor Microelectronic Test

Structures with Applications to Infrared Detector Materials and Processes," published in June

1993. This paper was given at the 1992 Workshop on Measurement Techniques for

Characterization of HgCdTe Materials, Processing, and Detectors and was published in

Semiconductor Science and Technology in 1993.

Appendix C is a reprint of an invited article "Hg^.^Cd^Te Characterization Measurements:

Current Practice and Future Needs," was published in Semiconductor Science and Technology in

1993.

12
PART I - HIGH-FIELD MAGNETOTRANSPORT STUDIES

2. CHARACTERIZATION OF GOES AND TIROS HgCdTe IR DETECTORS BY

QUANTUM MAGNETOTRANSPORT MEASUREMENTS

A. Review of Shubnikov-de Haas (Oscillatory Magnetoresistance) Effect

The two-terminal resistance of a semiconductor generally rises in a transverse magnetic field, B, and

this effect is referred to as transverse magnetoresistance. It can be caused either by macroscopic

current bending by the magnetic field or microscopic effects resulting from the admixture of carriers

with different mobilities. This nonoscillatory or dc magnetoresistance is discussed in section 3. The

magnetoresistance effect discussed here is due to the small Shubnikov-de Haas (SdH) oscillations in resistance that are superimposed on the large dc magnetoresistance background. These oscillations are quantum-mechanical in nature and result from the successive crossing of Landau levels by the

Fermi energy. Analysis of these oscillations leads to values for the densities and effective masses of

the contributing carriers, and thus this technique can be very useful in characterizing important

properties of semiconductors.

Shubnikov-de Haas analyses have been performed on HgCdTe infrared detectors to determine the

carrier densities in their accumulation layers [2.1]. The accumulation layers form a two-dimensional

(2D) electron gas with several allowed subbands. The bulk, which is three-dimensional (3D) in behavior, does not contribute significantly to these oscillations because of its low carrier density

(~3 X 10^^ cm'^). This is illustrated for the GOES detectors by the angular dependence of the

oscillations in section 2.B.6. Each subband produces its own oscillation, and Fourier transform

techniques are then used to separate and identify the components. This technique has been used and

extended in this work to characterize the properties of accumulation layers, which have a controlling

influence on long-wave infrared detector performance.

The detector sample is mounted in a liquid-helium, variable-temperature cryostat in a superconducting

magnet capable of 9 T. The plane of the sample is perpendicular to the field so that the field is perpendicular to both the current and the accumulation layers. The oscillations are very small and

must be enhanced by lock-in amplifier techniques. A small ac magnetic field is superimposed on the

dc magnetic field, and the ac signal is then measured by a lock-in amplifier that uses a reference

signal with twice the frequency of the ac signal. The resultant signal is greatly magnified, and is comparable to a second derivative of the initial signal. This second-derivative-like signal is then

Fourier analyzed to obtain subband densities and effective masses. A review of these techniques is given by Seller et al. [2.2] and Yamada et al. [2.3].

B. Physical Modeling and Fundamental Theory

(1) Theory of Shubnikov-de Haas Effect

The Shubnikov-de Haas effect is a small oscillation in the magnetoresistance of a solid at high

magnetic fields. It is due to the redistribution of carriers among the Landau levels, which are the

allowed energy levels in the presence of a magnetic field, when one of the Landau levels crosses the

Fermi energy. This oscillatory behavior is characteristic of the properties of the conducting electrons,

e.g., carrier density, effective mass, and mobility, and can be used as an important characterization

13
tool. It is a much simpler measurement to make on a fabricated detector than the more common Halleffect measurement because it can be made with only two terminals.

The fundamental theory for the effect has been developed by Ando [2.4], who derived the following

equation for the oscillatory magnetoresistance signal in two dimensions:

where

n 00

Po 5=0 r=0

X Z),(co^,T/,r^)(-l) ^ cos

(1)

2

' (2)

27r2(r, + l)^5r

(3)

= cos ((r,+l)7rm,^,/2) , (4)

Z), K,r/,r,) = exp (-(r,+l)7r/co,T,') , (5)

2^[%A (6)

and where is the resistivity at zero magnetic field, Ap is the oscillatory part of the

magnetoresistance at a magnetic field B, s is the subband index, is the harmonic index, oj^ is the

cyclotron frequency, is the scattering lifetime, t^' is the Landau-level broadening lifetime, is the

cyclotron effective mass, is the effective g-factor, Ep ^ is the Fermi energy, is Boltzmaim's

constant, T is the equilibrium lattice temperature, J2 is the second-order Bessel function, and is the

amplitude of the modulating B-field.

14
The term (co^, r^, r^) is from Ando's self-consistent solution of magneto-transport and is left out by some authors to simplify eq (1) since is slowly varying with magnetic field,

phenomenological form is also sometimes used:

A simplified

-1 (7)

The term (co^, T, r^) involving temperature accounts for the broadening of the Fermi-Dirac

distribution function, which greatly reduces the oscillation amplimde at high temperatures because of

the gradual change in Landau-level occupancy as a function of level energy. The effect of spin

splitting of the Landau levels is accounted for by the term (m^, g^, r^), which averages Ap over

both spin components. The term (co^, r^', r^) accounts for the broadening of the Landau levels due

to collisions between the electrons and scattering centers. The values are related to the scattering

times Tj for mobility, but differ in value from them because low-angle scattering contributes much

more to the Landau-level broadening than to the reduction in mobility. The term 1) (^j) occurs because the signal is measured with a lock-in amplifier that uses a signal with twice the

frequency of the modulating B field to detect the signal.

(2) Fourier Transform Analysis

The first step in extracting useful data from the signal is to obtain its Fourier transform. The

cyclotron frequency, oj^, equals eB/m^ in SI units, and thus the oscillatory term in eq (1) becomes

cos JfieB). It is periodic in 1/5, and its Fourier transform has peaks corresponding to the

m^p ^ products for each subband. If the subband is parabolic, then A^^ = m^pjlir, where is the

subband carrier density, and the peak positions yield the densities directly. However, HgCdTe is very nonparabolic, especially for long-wavelength detectors, which have small energy gaps, and a model is needed to determine A^^ from the peak positions.

All the terms involving B other than the oscillatory one should be removed prior to the taking of the

transform. However, it is not possible to do so generally because w^,r^, and t^' are not known a

priori. It mms out that this is not necessary because the oscillations are much more rapid than the

variations associated with these terms. Thus, the entire signal is used in the transform.

The Fourier transforms can exhibit quite complicated spectra. Peaks may appear for one or two

harmonics (r= 1, 2) and may be identified because they are multiples of the fundamentals. Generally,

the harmonic peaks are much smaller than the fundamental peaks and thus may be ignored. Peaks

may be split due to spin-splitting by the high electric field in the accumulation layer, or there may be

multiple peaks for each subband because of the two separately passivated surfaces of the detector or

variations in density within one surface. Skill is therefore needed in identifying peaks, and the model

discussed below helps in interpreting the Fourier transform.

(3) Determination of Effective Masses

The Fourier transform peaks provide the m^p ^ products for each subband; to proceed further, the

values of must be determined. The most direct way to find is to first decompose the signal into

its individual oscillatory components by inverse transforming each peak in the Fourier transform.

15
Then one obtains each value from a best fit to the temperature dependence of the amplitude of

each elemental oscillation at a fixed B value:

where S^(T) is the experimental amplitude of the fundamental signal for subband s, and co^ is chosen

at a field that corresponds to a strong oscillation peak at all temperatures. A nonlinear least- squares fit is made to extract the value of from the data for several temperatures according to the

dependence given by eq (8). The total signal is decomposed by taking the inverse Fourier transform

of its Fourier transform only between frequencies where the given subband peak is above the

background. The remainder of the Fourier transform is set to zero except for the corresponding peak

at very high frequency, which is not shown in the figures. This corresponding peak occurs in

discrete Fourier transforms because of the equivalence between the evaluation at points k and n - k,

where n is the number of points. However, if the peaks are not well separated, the decomposition

becomes difficult, and several peaks must be included in one to obtain an "average" peak. A small

error is also incurred by abruptly terminating a peak when the background is noticeably above zero. This is the main method used in this work.

An alternative method is to use the temperature dependence of the amplitude of the Fourier transform

peaks directly. This approach requires the determination of an average 5-field to use in the analysis

because the Fourier transform involves an integral over l/B. Mathematically, the weighted average

1/B is given by:

<l/B >

0 - .

1 ± Ap{B) cos _li d{l/B)

B

I Ap{B) cos d(l/B)

(9)

where is the peak frequency. In practice, the entire complex transform is used to avoid having to determine the phase of the signal.

As long as the region in l/B over which the integrals in eq (9) vary from zero to their final values is sufficiently small and symmetrical about the determined average 5-field, this method should be

adequate. The average 5-field also must not vary significantly with temperature because only one Bfield value can be used in the analysis for a given subband. This method is somewhat easier to implement than the decomposition approach and can be used for closely spaced peaks. However,

occasional checks with the decomposition approach should be made to make sure the averaging is working.

After the masses have been found, the Fermi energies are then found from the values of the Fourier

transform peaks. Comparison with the model discussed below yields the surface electron density that

16
gives the best agreement between experimental and theoretical values. This analysis depends on the

assimiption that bulk and surface electron mobilities do not depend on temperature in the measurement

range, so that the only temperature dependence occurs in the term fi/w^, T, r^) in eq (1). This is usually a good assumption for temperatures between 4 K and 30 K, the temperature range most often

used in measurements to extract effective masses.

(4) Model for Subbands

First-principles calculations have been used to determine the accumulation-layer potentials, electron

densities, cyclotron effective masses, and Fermi energies of the 2D electron gases produced by the

passivation process. For parabolic subbands, the electron densities can be obtained directly from the

frequencies of the peaks in the Fourier transform. However, HgCdTe is very nonparabolic because

of its small energy gap, and therefore a model is needed to deduce the densities, as stated earlier. The subband Fermi energies can be obtained from the fundamental frequencies once the cyclotron

effective masses are determined from the measured temperature dependence of the amplitude of the

oscillations. The model then relates these measured quantities to the electron densities.

The work of Nachev [2.5], who has performed the most rigorous analysis of the subband dispersion

relations, has been extended to the entire set of allowed subbands for a wide range of electron

densities. Initial results of theoretical calculations and a brief comparison to some data have been

published, the reprint of which is in Appendix A [1.3]. Nachev has derived the 8 x 8 matrix

Hamiltonian for the conduction band and the heavy-hole, light-hole, and split-off valence bands for

both spin directions (±). He then reduced the Hamiltonian to a second-order differential equation for

the wave function </> , in the direction z-perpendicular to the accumulated surface:

- a

d<^. (10)

dz

where

a = 2a^ + p^ dV(z)

2a + p dz

(11)

b = 3y

(12)

2a + p dz

(13)

and

17
P

=

1 E(z)-A-E^

Y

- E(z) - E-

(14)

(15)

(16)

In the above equations, k is the wave number in the plane of the surface and refers to the 2D free electron gas; V(z) is the built-in potential in the accumulation layer due to the oxide charge; EJz) and

EJz) are the conduction and valence band energies, respectively, including the effect of the potential;

A is the split-off band energy separation from the valence band edge; Pq is a number proportional to the momentum matrix element as given in Kane's band model for HgCdTe [2.7]; and are the

eigenvalues for the two spin directions.

The wave functions (t)^(k,z) are real, and the actual subband wave functions, \p, can be computed

from them by solving for the eight envelope functions /„ because

8 = E ' ^^^^

where u^(r) denotes the periodic part of the Bloch function at k=0. The envelope wave functions of

the conduction hand, fj(z) and f^(z), are found directly in terms of (t)^(k,z):

^(k,z) = i = + iky (18)

The other envelope functions are found from the matrix Hamiltonian in terms of these two envelope

functions by direct substitution. The equations for them, which are somewhat lengthy, involve the

derivatives of fj(z) and f^(z) as well because of the momentum operator in the Hamiltonian. The

volumetric electron density can then be computed directly from ^ and the areal density of the 2D

electron gas.

The initial potential is found by solving Poisson's equation for a nonquantized 3D free electron gas. The standard Kane k • p band model [2.7], which treats the coupling of the light-hole and split-off

valence bands with the conduction band, is used. Poisson's equation is solved by a nonlinear

two-point boundary value method based on finite differences with deferred correction and Newton

18
iteration. Equation (10) is then solved by integrating the equation from an initial value and slope at the boundary opposite to the accimiulation layer, and eigenvalues are found by selecting those

solutions that vanish at the surface. As in reference [2.5], the interior boundary, where the

conduction-band wave functions go to zero, has been chosen to be at the middle of the energy gap for a given eigenvalue or at a maximum distance of 0.5 ^m if the eigenvalue always lies above midgap.

Thus, only those states that are effectively bound by the conduction band and constitute a 2D electron

gas are accepted. This approximation is based on the assumption that the wave functions decay

sufficiently in the energy gap so that those associated with the conduction band can be separated from

those associated with the valence band. As the gap becomes very small, this approximation breaks

down, and a continuum background of states that traverse the entire thickness of the detector becomes

allowed. This is a limitation for gaps smaller than 40 meV and densities greater than 10^^ cm"^.

Equation (10) is solved for at most 50 k values to construct the dispersion relations for the allowed

subbands. A new potential is computed from the calculated wave functions between the surface and

0.1 ^m; beyond this point the original bulk potential is used. Were the calculated wave functions

beyond 0.1 ^m to be used, there would be a difficulty because of the artificial boundary condition at 0.5 /xm where the wave functions are forced to be zero. The process is iterated until the input and

output potentials agree to within 1 % . In order to prevent the potentials from gradually diverging

from their original values, they are scaled each time by the ratio of the initially computed areal electron density to that just computed [2.8]. When this factor is between 0.99 and 1.01, convergence

is obtained. It was discovered that convergence could be obtained more rapidly, and often only, if the initial potential were modified slightly between the surface and 0.025 pim to take into account the

strong differences between the electron density computed initially and quantum-mechanically near the

surface in the accumulation layer. The initial potential was thus subsequently scaled by a quadratic

function to make it agree better with the shape of the first calculated potential over this range.

Once the self-consistent subband dispersion relations were found, the subband carrier densities, Fermi

energies, and cyclotron effective masses at the Fermi energy were computed by performing either a parabolic spline interpolation or linear extrapolation of the computed eigenvalues to the Fermi energy.

The spin-averaged cyclotron effective mass, m , is obtained from the expression

2

^ U

(19)

evaluated at the Fermi energy, Ej. These quantities now allow one to compute the value of the

subband densities from the peaks in the Fourier transform of the SdH data. The frequencies

corresponding to the peaks equal m E^/he for each subband [2.3]. The value of m is determined

from the measured temperature dependence of the amplitude of the SdH oscillations [2.2]. Thus, one

can find the subband density for which the theoretically computed product of m Ef has the measured

value for each subband. For the case of parabolic subbands, m Ej = h ttN, where is the electron

density, and the peak frequencies provide directly. This relation is referred to as the parabolic

approximation, which is used throughout this work except in section 2.D.

Calculations of the subband dispersion relations and related quantities have been made here for the

range of areal electron densities between 0.1 and 5.0 x 10^^ cm"^. The j:-value of the detector was

taken to be 0.191, with a corresponding energy gap of 41.1 meV [2.9], which is representative of

19
long-wavelength detectors and equal to that of the detectors reported on in detail below. The bulk

electron doping density was assumed to be 3.9 x lO^'^ cm'^, which was reported for these detectors,

along with a bulk mobility of 2.5 x 10^ cm^/Vs at 77 K. Bulk SdH oscillations were not observed

because of their low frequency, which implies that they would have been observed only at low

magnetic fields where broadening effects greatly reduce their signal strength. The temperature is taken to be 6 K, at which the material is degenerate. As an example, the calculated built-in field and

subband dispersion relations for an areal density of 8.9 x 10^^ cm'^ are shown in figures 2.1a and

2.1b, respectively. The surface potential in figure 2.1a is about three times greater than the energy of

the lowest subband edge and over five times the energy gap. The spin splitting is evident in figure

2.1b, and is greatest for the lowest subband. The density of electrons in the spin-up subband is about

15% greater than in the spin-down subband for the lowest four subbands. At the lower densities this percentage decreases somewhat, especially for the higher subbands, while at higher densities it remains nearly the same for all subbands. Note also that the deviation from a parabolic to a nonparabolic, nearly linear dependence of £ on A: is clear for energies only about 10 meV above the

subband edges. The small oscillations in these curves are due to 1% numerical uncertainty in the

solutions.

The corresponding electron density in the accumulation layer is shown in figure 2.2a for both the

semi-classical result from the initial solution of Poisson's equation and the final quantum-mechanical

result from the subbands. The width of the accumulation layer is seen to be about 0.1 ixm. The

latter density is greatly reduced at the surface because of the boundary condition on the wave

functions. It goes to zero discontinuously across the boundary because of the dependence of the wave

function on the derivatives offj and /j, which undergo a discontinuous change from a finite to zero value at the boundary. Therefore, the shape of the potential near the interface is different in the two

cases, and the value of electron density obtained quantum-mechanically is less than that of the initial semi-classical solution. The electron densities of the first four subbands are plotted as a function of

total density in figure 2.2b. The total density is computed from a sum over only the first four

subbands, for which accurate computations can be performed. The error incurred by this approximation is estimated to be less than 1 % . The relations are nearly linear with average slopes of

0.673, 0.223, 0.077, and 0.027 for the first (n = 0) through fourth (n = 3) subband, respectively. The

deviations from linearity are less than 1 % . This near linearity shows that the shape of the potential

distribution is relatively insensitive to the magnitude of the surface potential. This linearity has been

observed before experimentally as well, and the values of the experimental slopes are nearly the same

as those calculated [2.1].

The subband Fermi energies and cyclotron effective masses are shown in figures 2.3a and 2.3b,

respectively, as a function of total density. The Fermi energy in the bulk is computed to be 5.44

meV for an assumed bulk density of 3.9 x 10^^ cm"^. The scatter in the mass values is due to the

derivative in eq (19). Although the calculated eigenvalues appear relatively smooth in figure 2.1b,

they are only accurate to about 1 % , and this uncertainty as well as that due to the discreteness of the

A:-values causes the theoretical masses to have errors of about 5% occasionally. A more refined

calculation would lead to better accuracy. The strong variations of the masses with density attest to the nonparabolicity of the dispersion relations, which have an effect on the optimization of device

performance. The serpentine shape of the curves is due to the strong curvature of the built-in

potential.

In conclusion, the dispersion relations for the 2D subbands in the accumulation layers of HgCdTe

detectors have been computed by solving the 8 x 8 matrix Hamiltonian for a large range of electron

20

C\2

O

C\2

O

C\2

O

CO

O

I o o

CO

I o

I

o

LTD O

O

CO

Pi

a;

>

Ml

CO

g

O

CO I

^ 2 ^ S

O ^

o

^

•Jo -3

"-I S 1)CO X)

= <u

C«

3

.1^

{a9^) ADH3Na QNvaaas

22
23
CO

a>

CO

I '<-> cH

c4

a>

i-i

<Uc

XS' ON c o

•a II 3 X

4—* a

o

c

-aC

3

U

3

-a

(z.xno^jOT) AiisNaa aNvaans

24
o o o o O Irt O iO

C\2

25
o o

COo

CV2 o oo

C/5 So

bo a; c §

X)

<u 3 s ^ D (L) (U ^

<i2 o

(U ^

52

Sen

11C .5 X .

.2 -So I I

II

tl-l T-l 2 M-T

l-c -^S 3 O o £ <S o

26
densities (0.1 to 5 x 10^^ cm"^). The subband densities, Fermi energies, and cyclotron effective masses have been computed as a function of the total electron density. The results show the effects of

strong nonparabolicity. The near linear dependence of the subband densities on total density, which

has been observed experimentally, has been confirmed theoretically.

(5) Determination of Dingle Temperatiu-es

The oscillations decay with increasing 1/5 (decreasing B) because of the scattering of the electrons by

the lattice, defects, and impurities, which disrupt the cyclotron orbits and broaden the Landau levels. The term D^(u}^, t^', r^) in eq (1) contains this effect, which is described by the scattering time t/.

This ' differs in value from that of the electron mobility r, because low-angle scattering has a much

smaller effect on t [2.10,2.11]. However, the scattering mechanisms are the same in both cases, and

the Landau-level broadening can at least be correlated to the mobility.

The values for r^' for each subband are obtained from the decomposed signals S^{B,T^) by dividing

out all the terms in eq (1) except D^{o}^, t^', 0), and plotting the absolute result semilogarithmically,

as demonstrated later. The slope of the line that is tangent to the peaks of the absolute value of the

oscillations gives r^' directly. Another parameter, the Dingle temperature T^,^, is commonly used

instead of t^'. It is defined by:

and is an equivalent measure of the scattering, with high Dingle temperatures implying low

mobilities. The Dingle temperature can only be determined if nearly all the current goes through the

top accumulation layer, which occurs at sufficiently high B-fields, so that the signal amplitude does

not vary because of current division between the accumulation layers and the bulk.

(6) Dependence of SdH Signal on Angle of B-Field

The equations derived in this section are for the 5-field perpendicular to the passivated interface.

This configuration gives cyclotron orbits in the plane of the accumulation layer, which maximizes the

effect of the field on the 2D electron gas. If the angle 6 of the field with respect to the perpendicular

is not zero, then only its component in the perpendicular direction relates to cyclotron orbits. Thus,

all the terms involving B should be replaced with B cos 6, and the peak frequencies/^ all vary as 1/cos d to compensate.

This dependence of the peak frequencies on 6 is a definitive test of whether or not the electrons are behaving as a 2D electron gas. An example of this is shown later for a GOES detector 2D electron

gas system. In a 3D system, there is no angular dependence because the cyclotron orbits can occur in any direction. Thus, one can separate 2D and 3D behavior by measuring the angular dependence of

the SdH oscillations.

27
C. Results of Measurements on Specific Detectors

(1) Typical Data

Shubnikov-de Haas oscillations can be recorded with either ac or dc techniques. However, for the

weak oscillatory structure present in most GOES detectors, the much more sensitive ac method which

involves using ac magnetic-field modulation and phase-sensitive detection techniques is chosen for

use. Figure 2.4 shows the dc magnetoresistance compared with the signal from the lock-in amplifier

obtained by the ac technique. As can be seen in the figure, the dc signal has a large monotonic

background which is essentially eliminated by the ac method. Sometimes the ac trace is essentially

flat, and no further data manipulation is needed prior to obtaining the Fourier transform. However,

frequently the ac response has some gradual background variation, as seen in figure 2.5, which is not

related to the SdH oscillations. A varying background gives rise to structure at low frequencies in the

Fourier transform which can mask or distort peaks in the region. Background effects are removed

from the ac trace by a procedure called centering. For this procedure, the trace is divided into 5 to 20 regions, depending on the nature of the background curvature and oscillation frequency, and the

average value of each region is found. Then the average values are connected by a spline fit, and the

trace is replotted relative to the fitted background, as seen in figure 2.6. The next step is to spline fit the data to obtain a file, typically 2000 points, equally spaced in inverse magnetic field. The inverse

field plot is shown in figure 2.7. The region of oscillations associated with each subband is labeled.

This complex spectrum results from several surfaces, each with its own set of subbands, such that the

regions are a mixture of oscillations of several frequencies. The final step in the analysis is to do the

Fourier transform, as plotted in figure 2.8. From the multitude of peaks, it is apparent that several

surfaces of slightly different carrier concentration are present. For a sample this complex, there is some uncertainty in assigning peaks to a particular surface and subband. However, peaks in figure

2.8 were assumed to be associated with subbands 0, 1,2, and 3 from three surfaces: unprimed,

primed, and double-primed.

(2) Results on GOES Detectors

(a) Supplier 1 A typical commercial GOES imager detector consists of four individual detector elements, each with a length of 50 to 100 /xm, a width of about 50 fxm, and a thickness of 6 to 8 /xm. Elements of two

detectors from Supplier 1, passivated by the anodic oxidation process (type I), were measured at several temperatures. The Fourier transforms of five elements at 6 K are shown in figure 2.9. In the

element code, the supplier number is first, then the passivation type, next the detector number, and

finally the letter of the particular element. The results of the three elements from detector 1 (IIIB,

IIIC, HID) are very similar and can be interpreted as two surfaces, probably front and back, with

slightly different carrier densities. The subband 0 (first surface) and 0' (second surface) peaks are

close together but completely resolved. The peaks for subbands 1 and 1' overlap because the spacing

decreases by about a factor of three in going to higher subbands. The 1 peak is seen as a shoulder on

the right side of the 1' peak. The two elements of detector 2 differ from each other and from the

elements of detector 1. The assignment of peaks to subbands is given in table 2.1, but there is some

uncertainty for the elements of detector 2. Carrier densities were calculated by using the parabolic

approximation. Effective masses and Dingle temperatures were determined for the dominant

subbands in elements HID and 112A. (The latter values, given in table 2.1, are considered to have

an uncertainty of about 20% from estimates of the residual errors obtained from the goodness of the

28
(ALU) 3SN0dS3a oa

S ° 2i moo

o

^ *->

^ <L> n\ ° ^ ^ aj on *-> o

C > (/3

S g, ^

§3 2 o S ^ S

-s -s g

o ^ <u i2 o §

g *o S

^ ^ "S

O (73

fU -t—• -t-j H .S ^

^ s -§

CN on "

1) D y

> D

29

31
32
33
aaniiNovH rhojsnvhi namnoi

34
Table 2.1. Results of Shubnikov-de Haas Analysis for Supplier 1 Detectors

Detector Electric

OUL/UailU

Frequency m* Ep

(meV)

Carrier

(xl0"cm-2)

m /nig Cp^inc Y )

IIIB 0 17.70 2.050 8.55

1 5.33 0.617 2.57

2 1.30+

3

- 0.41 + SUM 12.83

0' 14.24 1.649 6.88

1' 4.86 0.563 2.35

2' 1.83 0.212 0.88

3' - 0.33 + SUM 10.44

IIIC 0 18.22 2.110 8.80

1 5.66 0.655 2.73

2

- 1.34 +

3

- 0.43 + SUM 13.30

0' 14.50 1.679 7.00 r 4.94 0.572 2.39

2' 1.85 0.214 0.90

3' 0.34 + SUM 10.63

HID 0 17.86 2.068 8.62 0.017 120 48

1 5.50 0.637 2.66

2 1.43 +

3

_ 0.45 + SUM 14.15

0' 14.32 1.658 6.92 0.016 104 40

1' 4.86 0.563 2.35 0.011 53 31

2' 1.83 0.212 0.89 0.0063 34 20

3' 0.34 + SUM 10.50

35
Table 2,1 (Continued)

Detector

Plpni£*nt

Electric U UL/fl 1 H 1

Frequency

V * / m Ep

(meV)

Carrier Density

(xl0"cm-2)

in*/niQ Er^CmeVI

112A 0 19.86 2.300 9.59 0.021 109 37

1 7.90 0.915 3.81

2 3.18 0.368 1.53

3

- 0.49+ SUM 15.42

0' 13.66 1.582 6.60 0.022 73 34

1' 4.6 0.533 2.22 0.014 38 17

2' 1.72 0.199 0.83 0.0093 21 13

3' - 0.32+ SUM 9.97

1I2D 0 26.40 3.057 12.75

1 10.69 1.238 5.16

2 2.09 +

3 0.66 + SUM 20.66

0' 16.94 1.962 8.18 r 6.0 0.695 2.90

2' 2.77 0.321 1.34

3' 0.41 + SUM 12.83

Calculated estimate based on densities of other subbands

36
fit.) The subband densities are plotted as a function of total density Nj in figure 2.10. The results of

the linear fits are: Nq = 0.638 N^, = 0.225 N^, and N2 = 0.077 Nj. The subband 3 values

v/ere all calculated by using the relationship N3 = 0.032 Nj Note that the theoretically calculated

values given earlier were 0.673, 0.223, and 0.077, respectively.

(b) Supplier 2

Two detectors of four elements each from Supplier 2 were measured at several temperatures. The

back and front surfaces were passivated by different processes (the combination designated type III), with the result that the SdH traces are complex and very difficult to analyze. Oscillations associated

with the second process are closely spaced, even at high fields, and continue with the same amplitude

to at least 9 T. Traces at 6 K of the four elements of the first detector are shown in figure 2.11. The

temperature dependence of element 2III1B is plotted in figure 2.12. The figure shows that the

amplitude of the oscillations decreases with increasing temperature as expected. Traces of the four

elements of detector 2III2 are shown in figure 2.13. The Fourier transforms for detector 2III1 are

plotted in figure 2.14. The identification of peaks associated with particular subbands is difficult. A

possible assignment for element 2III1D is shown on the figure and is given in table 2.2a. The

Fourier transforms for detector 21112 are shown in figure 2.15 and are similar to those for detector

2ini. This type of detector is better characterized using the new dc magnetoresistance method

presented in section 4.

To determine if the oscillations were indeed coming from the two-dimensional surfaces, a limited

rotational study was undertaken, as shown in figure 2.16. The magnetic field is perpendicular to the

surface layers at 0° and parallel to them at 90°. The decrease in the response with rotation indicates

that the oscillations are associated with the surfaces rather than the bulk. The residual amplitude at 90° is considered to be the noise level.

In order to better understand the detectors of Supplier 2 with their two different passivations, a series

of three experimental detectors was prepared by Supplier 2 for SdH studies. Detector 2IV5 had the

back surface prepared with the first process passivation, and the front surface was unfinished

(designated by type IV). Detector 216 had both surfaces passivated by the first process, whereas

detector 2III7 had the back surface passivated with the first process and the front surface done with

the second process. The SdH response of these three detectors is shown in figure 2.17. The addition

of the second process is seen to strongly interfere with the oscillations from the first process at low

magnetic fields. The Fourier transforms of these spectra are shown in figure 2.18. The first process

peaks are identified and are similar to those seen in detectors from other suppliers with this type of

passivation. The n=0 subband has three peaks, indicating that the surface has at least three regions

with slightly different carrier densities. The addition of a second surface of the same type gives a response which is similar to that of one surface with only slight changes in peak frequency and

relative height. The detector with two surfaces passivated, each of a different passivation, has quite a

different behavior. The first process peaks are difficult to identify since they are greatly reduced and

appear to have shifted slightly. The only definite front surface peak (second process) is the one

labeled Is. The Os response is fairly weak and the 2s peak occurs in the region of structure from the

other surface.

Effective masses were determined for four of the subbands of detector 2IV5. The values are in good

agreement with those found for the Supplier 1 detectors which had similar passivation. The

information on carrier density, effective mass, and Dingle temperature is listed in table 2.2b.

37
o 1—4 CV2 CO

II II II II

\ cp

Cv2

O

Cv2 M

I o

o

Et—

<

m

o

E-O

— lO

in

s .s -s

•S o -a ^ ^ S

C 3 (U 5 =1 I- w c«' ci

fc y O P l-c o >

0) 0) ^

J3 --^^

!u c 6 J,

;3 1) •

S .1

O I (U ill »- 5

go

0) CO

CO J3

(^-^^iiOt) AXiSNaa aNvaans

(N CO

>

'5b

CO

13 . O fi

><

:^ o

' ^o

n g

ca o

0) a N

38
oo

IS

CO 13

1)O oo

Si

M

a- S

00

X)

4-1 (L> O

-H 3 — O

a. -

<U CO

-4-^ I—-I (U T3 C

.2

O

11

Vh CO

lis o

M-iO

O -Z!

CO o K Si 73 O

;-i CO I—I

(NO

<U CO

1/3 tL, O

39
asNOdsan Hoioaxaa ov aAixvian

40
41
42
Table 2.2a. Results of Shubnikov-de Haas Analysis for Supplier 2 Detector

Detector Element

Electric Subband

Frequency

(T)

m Ep

(meV)

Carrier

Density

(xlOiicm-2)

2111 ID 0 42.0 4.86 20.29

1 16.05 1.86 7.75

2 3.27 +

3 1.03 + SUM 32.34

0' 30.8 3.57 14.88

r 12.81 1.48 6.19

2' 2.45 +

3' 0.78 + SUM 24.30

Calculated estimate based on densities of other subbands

43
o

44
45
46
47
Table 2.2b. Results of Shubnikov-de Haas Analysis for Supplier 2 Experimental Detectors

Detector ElementElectric SubbandFrequency

(T) m_Ep

^0

(meV)

Carrier Density

(xl0**cm-2)

m*/nip Ep(meV) (K)

2IV5 0 14.93 1.729 7.21 0.021 83 34

1 5,70 0.660 2.75

2 - 1.16+

3

- 0.37+ SUM 11.49

0' 11.60 1.343 5.60 0.018 75 35

1' 4.47 0.518 2.16 0.013 40 16

2' 1.80 0.208 0.87 0.0076 27 13

3' 0.66 0.32 SUM 8.95

2III7 Os 49.8 5.767 24.05

Is 18.70 2.165 9.03

2s 6.37 0.738 3.08

3s 2.36 0.273 1.14 SUM 37.30

+ Calculated estimate based on densities of other subbands

48
(c) Supplier 3

Figure 2.19 shows the responses of the two types of Supplier 3 detectors. One element has type I (anodic oxidation) passivation, whereas the other element has a different, proprietary, passivation

designated here as type II. It is readily apparent that these give very different SdH signatures. The

magnitude of the oscillations is much greater for the type I detectors when recorded at the same

current and instrument settings. The type I element shows the normal SdH behavior in that a series

of oscillations is periodic in inverse magnetic field. For the type II element, the spacing of the

oscillations does not show any obvious pattern due to the complicated, high frequency signal,

although the Fourier transform does show a set of peaks.

The temperature dependence of a type I device, 312A, is shown in figure 2.20. Note that the

oscillations at low magnetic field damp out first with increasing temperature. Figure 2.21 shows the

Fourier transforms of the temperature traces. The transform peaks broaden somewhat with

temperature as evidenced by the decreased resolution of the subband 0 and 0' peaks at 18 K. The

temperature dependence of the peak amplitude can be used to obtain the effective mass as discussed

elsewhere.

Figure 2.22 shows the Fourier transforms from five Supplier 3 type I detector elements. Elements

3I2A through 3I2D were from one detector and 3I3C was from another detector. Elements 3I2A and

3I2B are seen to be essentially identical, 3I2C and 3I2D have a stronger n=0 and n = 1 surface region

with apparent reduction in the other surfaces (denoted by the primes and double primes), and 3I3C

shows a new surface region at the expense of the dominant surface (double-primed subbands) of 3I2A

through 3I2D. The most probable explanation of these Fourier spectra is related to the geometry of

the detector elements. All of the elements have the same active area but the device length, and

consequently the resistance, varies from element to element. Elements 312A and 3I2B have the same

length, 0.0071 cm, and the same Fourier spectra. Element 3I2C is longer, 0.0107 cm, and its spectrum shows a significant amount of the unprimed surface (subbands 0, 1, and 2). Element 3I2D

is even longer, 0.0132 cm, and has more of the unprimed surface in its spectrum. It appears that the

unprimed surface is the region of the device outside the active area. A coating, or the lack of it, on

the non-active regions produces a surface layer with a different carrier density, which in turn gives

rise to a different Fourier spectrum. The decrease in the peaks of the long elements, compared to the

peaks seen in the short elements, may be due to the effect of series and parallel resistances of the

various layers. Element 3I3C from another detector is equivalent in length to 3I2C. It basically has

the same number of surfaces, but with the strongest one (see for example, the 0" peak) in elements

3I2A through 3I2D essentially absent and one new surface region present. All of the major peaks in

the transforms are listed in table 2.3.

Figure 2.23 shows SdH traces of a Supplier 3 type II detector element at a series of temperatures.

With the many high-frequency oscillations, it is difficult to track a specific oscillation as a function of

temperature. The Fourier transforms for the elements of a type II detector are shown in figure 2.24.

The assignment of peaks to specific subbands is shown for element 3II1B, and given in table 2.4 for

all elements, but the identification is somewhat arbitrary. The unlabeled peaks are associated with

other regions of the surfaces.

Experimental subband carrier densities are plotted versus total density in figure 2.25 for the Supplier

3 type I detector elements listed in table 2.3. The linear fits gave: Nq = 0.656 Nj, Nj = 0.225 N^,

and N2 — 0.087 N^. These compare favorably to the theoretically calculated values given earlier.

49
Q

OI—

H

o

o o 1

c «3 2

° «3 o « ?^ t2

C/3 C o

DO

1/3 Kl U o 0) 2 ffi (U T3

_ 1/3 ^

3

4^ CIhJS

<u

(/3

C/2 CI, C

O

.> >b &( (u

G

O —

2 ^ >p

0)

o o

Si ^

asNOdsan Hoxoaiaa ov aAUVian

50
51
52
Q

I—

enO oo

(U (U - c

fs -S OQ

IZ) 3 OC ^ 5 :t5 -a (L)

(Uooc 6 g « i2 ^ - g ^ S J33

-H :S ' a. o o o -a >e^ m (/I I—

o(+5 Oo

tin

O

C/2

l-l

l-i Co

bO (uB i3

<uocoo

53
Table 2.3. Results of Shubnikov-de Haas Analysis for Supplier 3 Type I Detectors

Detector ElementElectric Subband

Frequency

(T) \ * /

m* Ep

0 (meV)

Carrier Density

(xlO'lcm-2)

ni*/nig

3I1B 0 11.70 1.355 5.65

1 4.17 0.483 2.01

2 1.89 0.219 0.91

3

- 0.28+ SUM 8.85

0' 10.0 1.158 4.83

1' 3.33 0.386 1.61

2' 1.61 0.186 0.78

3' - 0.24+ SUM 7.46

3I1C 0 11.00 1.274 5.31

1 4.08 0.472 1.97

2 1.54 0.178 0.74

3

- 0.27+ SUM 8.29

3I1D 0 10.70 1.239 5.17

1 3.94 0.456 1.90

2 1.48 0.171 0.72

3 - 0.26+ SUM 8.05

0' 8.55 0.990 4.13

1' 3.03 0.351 1.46

2' 1.48 0.171 0.72

3' - 0.21 + SUM 6.52

-

3I2A 0' 16.75 1.940 8.09 0.022 88 29

1' 5.83 0.675 2.82

2' 1.27+

3' 0.40+ SUM 12.58 54
Table 2.3 (Continued)

Detector Electric Frequency m Ep Carrier Element Subband (T) Density EpCmeV) Td(K)

(meV) (xl0"cm-2)

0" 13.60 1.575 6.57 0.022 72 28 1" 5.05 0.585 2.44 0.014 42 23 2" 2.29 0.265 1.11 3" 0.33 + SUM 10.45

3I2B 0' 16.60 1.922 8.02

1' 5.5 0.637 2.66

2' 2.20 0.255 1.06

3' 0.39+ SUM 12.13 0" 13.74 1.591 6.64

1" 5.5 0.637 2.66 2" 1.08 +

3" 0.34 + SUM 10.73

3I2C 0 33.24 3.849 16.05

1 11.35 1.314 5.48

2 3.7 0.428 1.79

3 0.77 + SUM 24.09

0' 16.23 1.879 7.84 r 5.47 0.633 2.64

2' 2.3 0.26 1.11

3' 0.38+ SUM 11.97 0" 13.54 1.568 6.54

1" 4.58 0.530 2.21 2" 2.1 0.24 1.01 3" 0.32+ SUM 10.08

55
Table 2.3 (Continued)

Detector ElementElectric SubbandFrequency

(T) m Ep mo

(meV)

Carrier Density

Will t

ni*/m(j EF(meV) Td(K)

u jz.z J. /zy 1 J.JJ

i iU. /J 17/1^ < 1 Q

z i.jy n /1 1

A

U.'f 10 1 71

J

f\ 7/1 +

U. /4 oUM 7*2 7 1

(J lo.j 1 nil1 .yl 1 7 n7 7.97

1 j.oU n /;77 U.O/Z z.oO

Z

1 7C +

l.ZD

3' 0.40+ SUM 12.42 0" 12.81 1.483 6.19

1

4.85 0.562 2.34 2" 2.15 0.249 1.04 3" 0.32 + SUM 9.89

3I3C 0 33.27 3.853 16.07

1 10.75 1 .245 5.19

2 3.85 0.446 1.86

3 0.76+ SUM 23.88

0' 25.75 2.982 12.44 r 8.87 1.027 4.28

2' 3.55 0.411 1.71

J U.Ol SUM 19.04 0" 16.4 1.899 7.92 1" 5.88 0.681 2.84 2" 2.52 0.292 1.22 3" 0.40+ SUM 12.38

Calculated estimate based on densities of other subbands

56
1 I

I I I I I U I I 0 12 3 4

MAGNETIC FIELD, B (Tesia)

Figure 2.23 Temperature dependence of the response of element 311 IC from a Supplier 3 type II detector. The quiet trace at 78 K suggests that noise is low for these spectra.

57
58
Table 2.4. Results of Shubnikov-de Haas Analysis for Supplier 3 Type II Detectors

Detector

HiClUCIlL

Electric OUUUdXlU

Frequency m Ep

(meV)

Carrier Density

(xlO^'cm-^) m /hIq n.p(,me V j

3111/^ 0 192.6 22.30 93.03

1 65.1 7.54 31.44

2 22.0 2.55 10.63

3 7.0 0.81 3.38 SUM 138.48

0' 128.9 14.93 62.26 r 43.0 4.98 20.77

2' 13.4 1.55 6.47

3' 5.2 0.60 2.51 SUM 92.01 0" 95.4 11.05 46.08

1" 32.0 3.71 15.46 2" 9.8 1.13 4.73 3" 3.5 0.41 1.69 SUM 67.96

3II1B 0 144.0 16.68 69.55

1 53.9 6.24 26.03 0.015 416

2 20.4 2.36 9.85

3 7.2 0.83 3.48 0.0081 102 SUM 108.91

0' 115.6 13.39 55.83 r 44.2 5.12 21.35

2' 16.0 1.85 7.73

3' 5.4 0.63 2.61 SUM 87.52 0" 84.8 9.82 40.96 0.023 427

1" 28.4 3.29 13.72 2" 11.0 1.27 5.31 3" 4.1 0.47 1.98 SUM 61.97

59
Table 2.4 (Continued)

Detector ElementElectric buDband

Frequency

(1) m Ep

nig (meV)

Carrier Density

(xl0"cm-2)

bp(meV)

3II1C 0 123.3 14.28 59.55

1 39.7 4.60 19.18

2 15.0 1.74 7.25

3 6.1 0.71 2.95 SUM 88.93

0' 75.6 8.75 36.51

1' 30.0 3.47 14.49

2' 11.8 1.37 5.70

3' 3.5 0.41 1.69 SUM 58.39

3II1D 0 115.3 13.35 55.69

1 36.3 4.20 17.53

2 15.5 1.79 7.49

3 5.8 0.61 2.80 SUM 83.51

0' 84.9 9.83 41.01

1' 28.6 3.31 13.81

2' 11.1 1.29 5.36

3' 3.9 0.45 1.88 SUM 62.06

60
61
0.673, 0.223, and 0.077, respectively. All of the subband 3 values were calculated using N3 =

0.032 Nj. Figure 2.26 is a subband carrier density plot for the Supplier 2 type III and Supplier 3

type II detector elements given in tables 2.2 and 2.4, respectively. The subbands identified for the

Supplier 2 element are considered to be associated with the second passivation process, and the

Supplier 3 type II elements had the same alternate passivation on both sides. The results of the linear

fits are: Nq = 0.660 N^, Nj = 0.229 N^, N2 = 0.083 N^, and N3 = 0.029 N^. Once again, these

are in excellent agreement with the theoretically calculated values.

Figure 2.27 shows SdH data for the rotation of element 312A from a type I detector with respect to the magnetic field direction. At 0° the field is perpendicular to the accumulation layers, whereas at 90° it is in the plane of the layers. The disappearance of the oscillations with rotation shows that the

SdH effect is coming from the two-dimensional layers rather than the bulk. Particular peaks in the

waveform are seen to be at higher fields at 30° than at 0°. The quantization depends on the

component of the field in the direction of the layer thickness. Thus, the peak position varies as the

cosine of the angle. This can be seen best in figure 2.28 which shows the Fourier transforms of the

data at the four angles. The identified peaks are seen to shift to higher frequency with increasing

angle as predicted. The rotation is seen to be useful in resolving double peaks. - The 1' peak is not

clearly resolved in the 0° transform, but the 60° transform shows distinct 1 and 1' components. The

rotation also confirms that the subband 0 peak has two components. Figure 2.29 is a plot of the peak

positions, normalized to the 0° value, as a function of the rotation angle. The expected cos 6

dependence is shown, and the agreement is considered reasonable in that the rotation was

accomplished by wedges rather than a precision mechanism.

It is instructive to see how well the major peaks in the Fourier transform describe the original SdH

data. To do this, an inverse transform of each peak is obtained and the results summed. Some

distortion is introduced because the peaks are truncated on each side. Inverse transforms were done

for the peaks identified as subbands 0', 0", 1", 2", and harmonic 1" for element 312A in figure

2.21. The waveforms are shown in figure 2.30 along with their sum and the original data. The

synthesized spectrum is seen to be a good reproduction of the original. This same reconstruction was

done for the more complicated data of element 3II1B from figure 2.24. As shown in figure 2.31,

inverse transforms of 12 Fourier peaks were combined and compared with the data. The sum is a reasonable representation of the data, especially at higher values of inverse magnetic field because a

greater percentage of the Fourier peak area was identified and transformed at lower frequencies. It is useful to sum the identified components to see if the signal is well represented by them.

(3) Results on TIROS Detectors

Figure 2.32 shows the SdH traces for six different TIROS detectors. These are large, single-element

devices, about 170 ixm by 170 ixm. The TIROS detectors of figure 2.32 varied in resistance by a

factor of five and the detector current for the measurements was adjusted accordingly. Although

similar type SdH oscillations are observed for all the devices, there are reproducible differences which

demonstrates that each SdH trace is a unique signature for an individual detector.

The Fourier transforms of the six detectors in figure 2.32 are shown in figure 2.33. They can be

categorized into two groups of three detectors each, based on the position of the 1 and 1' subbands.

The groups were received at different times which suggests that they were chosen from lots with

somewhat different material characteristics or surface passivation. The subband 0 response for

detector T60367 is composed of three or four peaks, which suggests a corresponding number of

62
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partial surfaces with slightly different electron densities. The composite nature gives rise to shoulders

in the subband 1 peak. Detectors T46044 and T46004 were fabricated from the same wafer and have

the most similar Fourier transforms, but reproducible differences are still seen. The parameters

calculated from the subband peak positions are given in table 2.5.

Figure 2.34 shows the SdH traces for detector T56238 at five temperatures. Evidence of the largest

oscillations is seen even at 28 K. The Fourier transforms of these traces are plotted in figure 2.35.

The major subband peaks are identified and the frequency positions and calculated parameters are

listed in table 2.5. Subband density is plotted versus total density for the TIROS detectors in figure

2.36. Linear fits gave the following relationships good to better than 1%: Nq = 0.667 Nj, Nj =

0.220 N^, and N2 = 0.083 N^, in excellent agreement with the theoretically calculated values. All of

the subband 3 densities, except one, were calculated using N3 = 0.032 Nj.

Effective masses for the various subbands were obtained from fits of the inverse transform amplitude

as a function of temperamre as discussed in an earlier section. Specifically, the logarithms of the

amplitudes are fit to the theoretical variation ln{Tsinh {^Tjn /m^)/T^inh (^Tm /m^)] by choosing

m to give the best nonlinear least-squares fit. T^ is the reference temperature, usually the lowest

temperature, and (3 = lirkgmjefi = 14.68 T/K. The use of the logarithmic dependence produces a more accurate fit because the small signal values at higher temperature contribute more than they

would in a linear plot. The fit for the mass of subband 0' for detector T56238 is shown in figure

2.37. The masses obtained for the other subbands of this detector are given in table 2.5.

Dingle temperatures were calculated for the subbands of detector T56238; the values are listed in table 2.5. The procedure is to obtain the inverse transform of the Fourier transform peak for the

subband of interest. The inverse signal is divided by all terms except that containing the Dingle

temperature. The log of the absolute value of this quantity is plotted as a function of 1/B and the

Dingle temperature is calculated from the slope of the line that is tangent to the peaks in the region

where the inverse transform is strongest. The plot for subband 0' is shown in figure 2.38.

(4) Intercomparisons

An SdH trace from each group of detectors is shown in figure 2.39. The responses from devices

fabricated with alternate passivation, 2III1D and 311 IB, have a noisy look, and are much lower in magnitude than the responses from the devices with the standard anodic oxide. When calculating the

scale factors, no allowance was made for differences in magnetic field modulation, current, or goes

here resistivity. The Fourier transforms of these traces are shown in figure 2.40. Again, the

amplitude of the devices with alternate passivation is much less than that of the other devices. Even

though the anodic oxide devices all have basically the same set of subband peaks, there are still significant differences. The devices with alternate passivation have a carrier density about 3 to 10

times higher than those with anodic oxidation passivation. The responsivity of the alternate devices

reported by the manufacturers is also much higher.

Dingle temperatures could not be reliably extracted for the nonanodically oxidized surfaces.

However, the Dingle temperatures are in general agreement from device to device for the anodically

oxidized surfaces except that element 3I2A has lower temperatures for the 0 subbands than the rest of

the devices. The theory predicts, and NIST results confirm, a systematic decrease in the Dingle

temperature in going from subband 0 to subband 3. As discussed in a previous section, this is because the electrons in the lower subbands are closer to the surface, with its lower mobility, than the

71
Table 2.5. Results of Shubnikov-de Haas Analysis for TIROS Detectors

Detector Electric Frequency m Ep Carrier Element Subband (T) Density EF(meV) Td(K)

(meV) (xl0"cm-2)

T56238 0 19.3 2.235 9.32 0.018 122 47

1 - 3.95+

2 - 1.55 +

3

- 0.49+ SUM 15.30

0' 15.95 1.847 7.70 0.020 92 43

1' 5.30 0.614 2.56 0.012 50 30

2' 2.10 0.243 1.01

3' - 0.37 + SUM 11.64

T60367 0 17.35 2.009 8.38

1 5.74 0.665 2.77

2 2.23 0.258 1.08

3

- 0.40+ SUM 12.63

T56247 0 15.79 1.828 7.63

1 5.42 0.628 2.62

2 2.23 0.258 1.08

3

- 0.37+ SUM 11.70

T54428 0 29.05 3.364 14.03

1 9.10 1.054 4.40

2 3.32 0.384 1.60

3

- 0.66+ SUM 20.69

0' 23.12 2.677 11.17

1' 7.70 0.892 3.72

2' 2.80 0.324 1.35

3' 0.54+ SUM 16.78 72
Table 2.5 (Continued)

Detector ElementElectric SubbandFrequency

(T) m Ep

nig (meV)

Carrier Density

(xlO"cm"2)

Ep(meV) Td(K)

T46044 0 22.59 2.616 10.91

1 7.92 0.917 3.83

2 2.93 0.339 1.42

3 0.93 0.108 0.45 SUM 16.61

T46004 0 24.70 2.860 11.93

1 7.92 0.917 3.83

2 3.03 0.351 1.46

3 0.57 + SUM 17.79

Calculated estimate based on densities of other subbands

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electrons in the higher subbands.

The values of the Dingle temperature were quite large compared with the values around 3 K which

were computed on the basis of the measured mobility. Theoretical calculations [2.11] have shown

that the Dingle temperatures can be much larger than those derived from the conductivity mobility

because the relaxation time affecting the Dingle temperature is much more sensitive to forward

scattering. For Coulomb repulsion due to surface-charge scattering, it is expected that the Dingle

temperatures should be as large as measured. Thus, it is concluded that the behavior of the

anodic-oxide passivation is well understood.

D. Summary of Quantmn Magnetotransport Characterization Measwements

(1) Comparison between Theory and Experiment for Two Different Detector Types

Shubnikov-de Haas measurements have been made on a number of long-wavelength, commercial,

n-type PC detectors with total densities between 5 x 10^^ and 1 x 10 cm"^. The SdH Fourier

transforms of two different representative detectors are shown in figures 2.41a and 2.41b. The

notations ' and " refer to different surfaces or to different regions on the same surface, since both

surfaces of the detector were passivated. The detector in figure 2.41a was passivated with the usual

anodic oxidation process [2.12,2.13], while that in figure 2.41b was passivated by a different method.

The separations of peaks due to spin-splitting as predicted by theory are about half of that observed,

and, thus, it is expected that these multiple peaks are due to different surface densities. The splittings

vary somewhat from detector to detector as well. Thus, the effect of spin-splitting is just to broaden

each peak. The transform corresponding to the detector with lower total density, element 312A in

figure 2.41a, has clear, strong peaks, while that for the one with higher density, element 3II1B in

figure 2.41b, has peaks that rise less above the background. One reason is that the penetration of the

energy gap by the wave functions is much greater for the heavily accumulated case, especially for the

lowest subband. There is a mixture of subband states bound to the conduction band and continuum

states that traverse the entire detector. Thus, the layer of electron accumulation is not well described

by a simple 2D electron gas at high densities and small energy gaps. The Fourier transform for a heavily accumulated, narrow-gap detector can have a broad background of contributions from the

range of 3D states corresponding to the graded 3D electron density in the accumulation layer. The

2D states, which are bound to the conduction band, appear as well in the Fourier transform and rise above the background because they contribute at only one frequency for each subband. The

complexity of the background signature in figure 2.41b may indicate that there is nonuniformity in the

surface density as well because of the two surfaces and multiple regions within a surface. Even

though the relative amplitude of the oscillations of element 311 IB was almost ten times smaller than

that of element 3I2A, the Fourier transform successfully resolved the unique signature of the detector.

The subband electron densities have been computed from the measured cyclotron effective masses and

Fermi energies for the first detector. The results from the two surfaces were averaged because the

splitting could not be resolved at higher temperatures. For element 3I2A, the measured data and their

uncertainties obtained from the fitting are: rriQ* = 0.022 ± 0.001, E^^ = 79 + 4 meV, nij* = 0.014

± 0.001, and Ej =42 + 3 meV. The corresponding theoretical values (and their uncertainties due

to calculational imprecisions), based on the calculations shown in section 2.B.4, were obtained by

finding the total density for which the m Ey values for the subbands best agreed with experiment:

niQ* = 0.023 + 0.001, Ef = 75 + 2 meV, nij* = 0.015 + 0.001, and e/ = 38+1 meV. The

predicted total density is found to be 7.9 x 10 cm'^, which is considerably less than the value of

81
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82
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1.2 X 10^^ cm that is obtained from the parabolic approximation for the density. The agreement

between theory and experiment is within the assigned uncertainty, but the theoretical values for the

effective masses are slightly larger than measured. Thus, it is expected that the energy gap may be

smaller than the one used. Band-gap narrowing which occurs at high densities because of the manybody interactions between electrons has been included [2.14]. The band-gap reduction is at most 10%

at this density. The new theoretical values are: itiq = 0.022 + 0.001, £y =75 + 2 meV, mj* =

0.014 + 0.001, and E^^ = 39+1 meV, which are somewhat closer to the experimental values. The

remaining differences in Fermi energy may be due either to a slightly smaller value of x or slight

differences in the built-in potential between theory and experiment.

No attempt was made to analyze the second detector because its electron density was above the range

of the calculations and the peaks were not clean enough to determine the effective masses. Results

for the subband densities for the peaks are given in table 2.4, based on the parabolic approximation.

These values for densities are probably on the order of 30 to 50% larger than they would be if the

effects of nonparabolicity were included.

The agreement of calculations with experiment is good for anodically oxidized detectors and is even improved somewhat by inclusion of the effect of band-gap narrowing. Models such as these provide

a basis to characterize the electron states in the accumulation layers of these detectors, which have a

large effect on device performance.

(2) Comparison with Detector Performance and Identification of Trends

It is not possible to make detailed comparisons between the responsivities and detectivities of different

detectors and the results of the SdH analyses because: 1) the measurement practices differ from

supplier to supplier; 2) the substrate wafers are different; and 3) the side-wall delineation, bonding,

and packaging are different. However, it should be possible to compare the different elements of a

detector because variations due to effects other than passivation should be small. Overall, the

element-to-element variations were not very large, both in SdH signature and in responsivity. In

section 4, element-to-element variations are shown for multielement detectors by the dc

magnetoresistance technique with correlations to detector responsivities.

It is possible, however, to correlate the results of the SdH analyses with expected device performance.

The goal of the passivation treatment is to reproducibly create an accumulation layer at the top and

bottom surfaces that repels minority carriers so that surface recombination is suppressed. This greatly

increases the minority-carrier lifetime in the detectors. However, it is also necessary that the

conduction through the accumulation layer be minimized so that the signal is not shunted. Therefore,

relatively high electron density and low mobility are desired. The improved responsivities and

detectivities of the detectors that use treatments other than anodic oxidation processes are achieved by

the higher electron densities and lower mobilities resulting from these processes. Unfortunately, these treatments also lead to a wide variation of detector parameters from device to device. There are two

reasons why the corresponding mobilities in these heavily accumulated layers are lower: 1) the

conductivity effective mass is larger for the higher electron densities and 2) the surfaces are probably

damaged somewhat by the processing. The SdH measurements have shown why these alternative

passivation treatments are better and have also indicated the need for further study to understand their

chemical and physical properties better. Hall measurements should be performed on similarly

processed wafers as well as chemical analyses of the constituents. The stability of these surfaces over

84
time can be studied by continually measuring the SdH signatures of several detectors. Thus, the SdH

method is a very useful tool in determining the nature of the passivation layers of HgCdTe detectors.

3. DC Magnetoresistance Characterization of Detectors

A. Background

The SdH technique just described furnishes detailed information on the electrical properties of the

subbands in the accumulation layer. However, it is a somewhat sophisticated technique which

requires a high level of analysis. Therefore, a new technique has been considered which is based on

the dc magnetoresistance that corresponds to the background signal on which the SdH oscillations are superimposed. This technique is easier to apply and furnishes the total electron density and average

mobility of the carriers in the top accumulation layer of the detectors. This new method should be

easily adopted by detector manufacturers to improve their quality control.

The magnetic-field dependence of the two-terminal magnetoresistance has been used as a tool to measure both the free-carrier density and the Hall mobility of the top accumulation layers of the

detectors situated in a magnetic field, B, which is perpendicular to the plane of the sample. The

magnetoresistance normalized to B=0, which increases as the length-to-width ratio, a, decreases, has

usually been considered an unwanted effect in standard magnetotransport measurements such as the

Hall effect, but it was shown to be a valuable characterization technique. It is due to the shorting

effect of the Hall voltage by the contacts and is not to be confused with the bulk or "long-bar"

transverse magnetoresistance, which is usually much smaller because it only relates to effects due to scattering-lifetime distribution functions and band-structure [3.1]. The cases treated here are very

degenerate, and the bulk magnetoresistance is negligible. The two-terminal transverse magnetoresistance effect (or "geometric effect") described here has been used in the past to characterize GaAs Gunn diodes [3.2-3.5] and FET contacts [3.6], but the limiting case of a -» 0 was

used because it was considered to be adequate for these cases. This limit is not appropriate for the

infrared detectors. Thus, an approximate, general formula for the magnetoresistance as a function of

both B and a has been derived here to allow this method to be applied to a wider range of

semiconductor structures currently being manufactured.

B. Theoretical Analysis

A conformal-mapping calculation was performed by Lippmann and Kuhrt [3.7,3.8] to obtain the

normalized magnetoresistance, R{B,a), of a semiconductor layer as a function of B and a, as shown

in figure 3.1. A nonlinear least-squares fitting routine [3.9] was used to derive an approximate

analytical equation from these calculated curves:

R(B,a) = a

(1 + 11 (3.1)

ne\it

where

r = (0.98a + 1.01)

-1 (3.2)

85
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86
and where e is the electronic charge, n is the carrier density, \l is the Hall mobility, and t is the layer

thickness. SI units are used throughout. Note that the curves gradually progress from a nearly linear

to parabolic dependence on B with decreasing a. The fit is good to about 1 % over the fitted range

0.2 < a. < 5 and jiB < 5. Analytic results exist for a = 0 and a — 1, for which eq (3.1) holds

with r = 1.0 and 0.5, respectively. These values of r are close to those obtained from the fit, 0.990

and 0.503, respectively. It is expected that the fit should not degrade substantially outside of the

fitted range because of the agreement with these two cases for all values of B. Because of the

functional form for these two cases, the form of eq (1) was chosen. The case a = 0 corresponds to a Corbino disk, for which this method has been applied in the past [3.1]. For a multilayer structure, eq

(1) can be used for each layer with its own set of parameters, but only if the layers are electrically

isolated so that circulating currents associated with different Hall voltages in each layer can be

neglected. It is important that the value of a be known accurately, which should be possible from the

mask dimensions used to fabricate the devices. To employ the technique, one requires high enough

fields such that fiB at least approaches unity. Thus, for layers with [x = 10 m^/Vs, a field of only

about 0.1 T would suffice, whereas for layers with n = OA m^/Vs, a superconducting magnet

producing fields up to 10 T would be needed.

C. Experimental Work and Results

Four detectors have been characterized, some with multi-elements, to demonstrate the usefulness of

this technique and determine the properties of the accumulation layers. An illustration of a typical

detector element is shown in figure 3.2. Typical dimensions are a length of 50 to 100 ^m, a width of

about 50 /xm, and a thickness of 6 to 8 ixm. The detectors were placed in a cryostat with the

temperature varied between 6 and 80 K. In this temperature range intrinsic carriers can be neglected.

A superconducting magnet was used to obtain a magnetic field of up to 7 T. The voltage was

measured across the detector at a fixed current of 100 ^A as a function of B to obtain the resistance.

The sample geometries were supplied by the manufacturer and checked with an optical microscope.

Data were obtained on a digital oscilloscope and transferred to a computer for analysis.

The contacts are made to the top of the detectors with the current spreading down into the multilayer

structure. At B = 0, much of the current flows through the bulk, which has a nominal electron

density of 3 x 10^^ cm"-^ and a mobility of 25 m^/Vs at 77 K. Even higher mobilities occur at 6 K.

As the magnetic field increases, the current eventually flows primarily through the top accumulation

layer because the bulk magnetoresistance rises rapidly with field as a result of its high mobility. This

prevents current from flowing in the bulk and bottom accumulation layers. It is not possible to fit the

bulk or bottom accumulation layer because of the current spreading and the circulatory currents

associated with the different Hall voltages of each layer. Therefore, values have been obtained from

the fits only for high enough fields that current is limited primarily to the top accumulation layer.

Some leakage current could travel along the passivated sides of the sample, but it is estimated to be

small. The equation to be solved for the total resistance of the top accumulation layer at sufficiently

high B is Rj{B,oc) = + R{B,a) where R^ is the contact resistance. A nonlinear-least-squares fit is used to extract the parameters R^, N, and fi, where = nt. Contact resistance, an unavoidable

aspect of two-terminal measurements, can in principle be found from these fits. However, to reduce

the number of unknowns, it may be possible to estimate it from other measurements.

Three different types of passivation were studied with this technique. The first, type I, is a typical

anodically oxidized detector, while the second and third, types II and III, respectively, employ

87
1
proprietary passivation techniques that have a smaller surface conductance and are used when high

responsivity is required. The dependences of the detector resistance on magnetic field for type I and

type II detectors at 6 K are shown in figure 3.3 along with the fits of the high-magnetic-field portions.

Note how different the curves are for the two detector types. An expansion of the data at low field is shown at both 6 and 77 K in figure 3.4. It is interesting that the temperature dependence of the

mobilities of the two types is opposite in size between 6 and 77 K. For the type I detector, which has

a = 1.21, the resistance has a nearly linear dependence once the bulk-related contribution to the total conduction is greatly reduced above about 0.1 T. The type II detector, on the other hand, which has

a — 2.04, shows bulk-related curvature up to nearly 2 T. The bulk remains relatively more

conducting to higher fields in this detector because of its larger value of a and the higher zero-field

resistance of its accimiulation layer. Thus, the resistance-vs. -field data above 0.1 T for type I detectors and above 2 T for type II detectors have been fit because conduction is then mainly through

the top accumulation layer.

The magnetoresistance of the third type of detector, type III, is shown in figure 3.5 at 6 K along with

the fit. It has ol = 1.21 and resembles the dependence of the type II detectors. The fit was made

above 4 T so that the current flows primarily through the top layer. An expansion of the

magnetoresistance at low field is shown in figure 3.6 at 6 and 77 K. The magnetoresistance of a

different multi-element type III detector is shown in figure 3.7. It has a - 1.33 and is similar to the

other type III detectors. The elements are all different, and this shows that variations in accumulation-layer properties can occur across the elements of a detector. Specific numerical

comparisons are made below. Reduction in these variations can lead to improved pixel performance

and better signal-to-noise ratios.

The results of the nonlinear-least-squares fits to the data at 6 and 77 K are shown in table 3.1. The

average deviation of the fits from the fitted data was less than 1 % . A constant value of 10 fl was

used for the contact resistance. This value was found by comparing two type II detectors with

different lengths at 6 K at 5 = 0 and agrees with the manufacturers' estimates. The value of

could not be determined by the fit because it is small compared with the fitted resistance values.

Manufacturers' values at 77 K for the product of the carrier density and mobility independently

determined by multilayer Hall measurements on fully processed wafers formed by both processes are given in table 3.1 [3.10,3.11]. The piN product was found to be nearly constant over their ranges of

measured densities for each type of detector. Better agreement was found for the type II detector than

for the type I detector. The reason is that the value of a is near unity for the type I detector, and the

layer resistance becomes nearly independent of [x for ixB > 1. The value of the carrier density,

however, is easily extracted and agrees well at both temperatures. The values of the carrier densities

for both detectors were also within the range of the manufacturers' estimates. Thus, it has been

shown that this technique can determine N and fi for the top layers of these infrared detectors.

The type III detectors have top accumulation layers with as low mobility as the type II detectors.

However, their electron densities are between a third and a half of the type II detectors. Thus, the

leakage through the accumulation layers of the type III detectors as evident by the Nfjt, product is less than for the type II detector, which have in turn less leakage than the type I detectors. The

effectiveness of the accumulation layer in repelling minority carriers, as evident by the value of A^, is best for the type II detectors, next best for the type III detectors, and worst for the type I detectors.

Thus, both proprietary processes provide accumulation layers with improved performance over the

anodic oxidation process. This technique is seen as providing the analysis necessary for

understanding why these processes are better.

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A comparison of elements A, B, and C of detector 2III2, shown in figure 3.7, with the measured

responsivities, shows how the accumulation-layer properties correlate with performance. At 77 K the

inverse ratios of the densities and /xA^ products of the elements B and C to element A are 0.86 + 0.03

and 0.82 + 0.03, respectively, while the corresponding ratios of the responsivities are 0.91 ± 0.01

and 0.82 + 0.01, respectively. The uncertainties given here are from the residuals resulting from the

DATAPLOT fits. The agreement is good, which shows the controlling effect of the top accumulation

layer.

In order to obtain information about the bulk, bottom accumulation layer, and contact resistance, the

top layer conductance determined from the fit can be subtracted from the total conductance at B = 0. Variations in this conductance difference as well as in the top-layer conductance from detector to detector can be used to determine when the process is not in control. The reasons for the variations can then be diagnosed. For example, the conductances of elements A, B, and C of detector 2III2 at B

= 0 and at 77 K are 8.33 x 10"^, 9.01 x 10"^, and 9.80 x 10"^ S, respectively. Subtracting the

respective top-layer conductances leads to the remaining conductances of elements A, B, and C which

are 6.77 x 10'^, 7.21 x 10"^, and 7.88 x 10"^ S, respectively. If the bulk conductance is the same

for all three elements, these variations would be due to the bottom accumulation layer.

D. Conclusions "

In conclusion, a practical magnetoresistance method has been demonstrated for measuring the carrier

density and Hall mobility of a semiconductor layer by using only a two-terminal configuration, a

situation applicable to a number of industrially relevant devices. An equation has been fit to published calculations of this phenomenon so that it can be applied to a wide range of rectangular

geometries. Application of this technique to four GOES HgCdTe PC infrared detectors gives results which are in excellent agreement with previous Hall data, showing the validity and usefulness of this method. The variation in the electron density of the top accumulation layer of three elements of a multi-element detector was shown to correlate with the responsivity. The reduced leakage and

increased electron density of the type II and type III processes are shown to be the reasons for their

improved performance characteristics. This technique can be used in the future by the detector

manufacturers to characterize their processes and thereby improve detector performance.

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PART 11 - OTHER CHARACTERIZATION STUDIES

4. BONDING, METALLIZATION, AND PACKAGING FOR GOES AND TIROS INFRARED

DETECTORS

A. Overview and Rationale^

Evaluation studies of GOES HgCdTe detector packaging were made at NIST during FY 1991 and

1992. Problems that can reduce the yield (increasing the cost) and/or reliability of the devices were

found. Several such problems were docimiented in earlier reports (see NISTIR 4687, September

1991) and in NCAA program reviews. Others are retained in files of NIST's scanning electron

microscope (SEM) studies of these devices. Therefore, work was done in FY 1992 in the area of

detector packaging to improve the above situation and, hopefully, to prevent surprise problems from

arising that can delay launch dates and necessitate retrofits or otherwise increase NOAA satellite costs. The objectives and rationale of the NIST packaging work during FY 1992 are given below.

1) Develop a set of Recommended Practices (guidelines) for packaging of NOAA space sensors

(detectors). This will include recommendations for reliable wire bonding (interconnections),

package electroplating, device metallization, and molecular cleaning methods.

Circulate the Recommended Practice to appropriate detector contractors and other interested

organizations for their comments. Their comments will be evaluated, negotiated, and

incorporated in the final document. This document will be given out to appropriate contractors

for implementation on future NOAA infrared sensors.

2) Request the detector manufacturers to submit examples of detectors (electrical or optical rejects)

mounted in packages designed for incorporation in NOAA satellites. NIST will then examine

(with SEM and/or other appropriate analytical methods) for compliance with the above guidelines

in (1), with emphasis on potential reliability and yield enhancement. If problems are found,

NIST will then:

a) Assist sensor (detector) contractors with necessary information, visits, and training, so they can package devices in conformance with these guidelines.

b) Serve as packaging and materials science consultants for any problems that may arise in NOAA infrared detector manufacturing, and present training seminars on reliable packaging.

3) Many wire bond yield and reliability problems are traceable to the lack of molecularly cleaned

bonding surfaces. Studies will be carried out to establish the best cleaning methods that are compatible with the normal HgCdTe detector packaging methods.

Large integrated circuit (IC) packaging houses make hundreds of thousands of devices and

millions of wire bonds per week. They can therefore employ a number of engineers who are experts on bonding. However, infrared sensor companies make so few bonds that no one

engineer can specialize in this area; thus such companies may have more bond yield and/or

reliability problems than the IC companies.

97
4) Cooperate with other NIST HgCdTe detector studies, such as preparing samples for low

temperature Shubnikov-de-Haas and other measurements.

6. Accomplishments

A first version of the

" RECOMMENDED PRACTICE FOR THE WIRE BONDING AND

METALLIZATION USED IN RADIATION DETECTORS PREPARED FOR USE IN GOES.

TIROS. AND OTHER SPACECRAFT" was written and circulated to eight organizations and, in some cases, to several individuals within an organization. After receiving responses (several did not respond) when questions remained, those individuals were contacted for resolution. The final document includes references to ASTM as well as to various Military Class "S" (space) specifications

and standards. The "Recommended Practice" document is in Section C. Because of some late responses there was not time to recirculate the new document for final comments. Readers are urged

to make any such comments as soon as possible after receiving this report. If significant changes are suggested and made, then the "Recommended Practice" will be revised and republished later as

final. The document was also presented to a standards organization, ASTM Subcommittee F-1.07

(the wire bonding committee) at its meeting on January 28, 1993 for additional comments from a

different but very knowledgeable audience.

A total of six visits were made to three GOES and TIROS infrared detector manufacturers during FY

1992. The first visit to each site was made to evaluate their production lines and processes. Later

visits were made to help them with the packaging of their detectors including, in one case, a 2.5-hour

seminar on wire bonding and the reliability of the metallurgical systems used in packaging HgCdTe

detectors. Over 30 people attended that seminar. A more informal, hour-long presentation was made

at another company where six/seven engineers and management personnel attended. Extra time was

spent in their packaging laboratories discussing equipment, etc. A study was made of detectors from

one manufacturer and several defects were found (the results were presented at a GOES project

review at NASA Goddard on July 10, 1992). This information, with proposed solutions, was also fed

back to the manufacturer to help them improve their product.

Many wire-bond yield and reliability problems are traceable to molecular-level contamination of the

bonding surfaces resulting from handling, epoxy outgasing residues, etc. The high-reliability

microelectronics hybrid industry routinely uses oxygen and argon plasma or ultraviolet-ozone (UVozone) cleaning procedures to greatly enhance their wire-bond yield and reliability. The HgCdTe

detector industry has been afraid to use such cleaning methods before bonding because of possible

damage to the detectors or to their surface coatings. These concerns have never been documented,

but there is some possibility that oxygen plasma cleaning could attack the antireflection coatings.

Therefore, NIST proposed to study the effects of UV-ozone cleaning on HgCdTe detectors. This

work was planned to determine whether the cleaning had any effect on the sensitivity and noise of the

detectors, as well as any effect on the various optical absorptive coatings that may be used inside the

housing (but not on the device).

Preliminary UV-ozone cleaning experiments were carried out at NIST on available detectors and

absorptive coatings. No physical damage was observed. However, there was no equipment available

at NIST to evaluate any possible optical or electrical damage to the detectors. Thus, for these

additional measurements, it was necessary to visit two cooperating detector manufacturers and

perform the experiments there. For this, the UV-ozone cleaning equipment was hand-carried [on

airplanes] to the manufacturers' locations. Previously tested detectors were irradiated in the NIST

98
equipment at their plants, but retesting for electrical and optical properties had to be scheduled for a

later time. One organization has reported its first set of measurements made within one week of

irradiation. The data indicate that the detector noise measurements show no change due to UV-ozone

treatment. However, the infrared sensitivity was found to decrease in all samples by an average of

11%. The organization plans to remeasure the detectors in 3 months to see if this decrease is temporary or permanent. If temporary it should be removed by minimal heat treatment (perhaps an hour at 80 °C). However, if the sensitivity loss is permanent, then another method of cleaning will

have to be investigated. Cured black light-absorbing paint was irradiated at NIST, and no damage to

its light-absorbing properties was detected.

In addition to the possible detector damage studies, other experiments were performed to show the

effects of cleaning on wire bondability at the manufacturers' plants. Problems immediately arose at one manufacturer because the intended bonder was in use and another had to be substituted. This

bonder was not in good condition and did not make reproducible bonds (the work-holder was not flat, the bonding tool moved laterally during bonding, etc.) Also, some of experiments that were

performed appeared to show that cleaning actually decreased bondability (something that has not been

observed before and is presumed to result from the bonder problems). The organization planned to obtain their own UV-ozone cleaner and rerun the experiments later. Other problems relating to peeling metallization were found at another manufacturer. Thus, in order to reach any conclusion,

both of these experiments must be continued.

Ten special HgCdTe infrared detectors received from manufacturers were prepared and mounted in low-temperature carriers for Shubnikov-de-Haas and other liquid-helium temperature measurements.

New removable adhesives that would tolerate these low temperatures had to be found, and special

carriers (packages) were made. Several repairs to soldered or wirebonded interconnections were

affected. In addition, various other techniques such as microsoldering had to be developed.

C. Recommended Practice for Wire Bonding and Metallization Used in Radiation Detectors

Prepared for Use in GOES, TIROS, and Other Satellites

1) The bonding wire should be made of the same metal as the bond pads on both the detector and

the package. This is normally gold. If there is a difference between the metallization on the

chipside and the packageside, then the wire material will match that of the chip-side bond pads,

a) Electroplated (or vapor deposited, etc.) gold [metal] should be pure, soft and uniform in optical appearance and thickness. It should contain < 1 ppm of thallium and have <50 ppm

total impurity content. The hardness should be < 80 Knoop. Any unusual surface

appearance should be investigated with appropriate magnification (lOOx optical microscopy

or up to 400x for a scanning electron microscope). If blistered, cracked, or peeling, it should be further investigated (see Mil-Std-883, Method 2010.10, 1 3.1.1.4). It is clearly

rejectable if a metal-deposition-defect is in the bond pad area and is equal to or greater than one quarter the area of the bond nugget and positioned so that the bond must be placed on

top of it. The same condition should be applied to an external soldering area (substituting

solder in place of bond nugget). It is also of concern if the metal defect is elsewhere, since

this indicates that the metallization process is out of control and may therefore produce

defects in critical areas on other packages in the same lot that may not undergo inspection.

2) Bond pads should not be placed on brittle, crater-prone material such as HgCdTe. Special

designs involving (>0.5^m) under-layers of hard metals such as titanium and/or tungsten on

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brittle materials may be acceptable if demonstrated to prevent bond cratering. (Such layers have

long been used to prevent cratering in silicon and gallium arsenide devices. For HgCdTe

devices, a hard underlayer thickness of 0.5 to 1 ixm is a good starting point, followed by > Ijixm

of gold.)

3) The bond pad metal must be clean and uniform to be wire bondable with a high yield.

a) Cleaning of the open, packaged, detector should be done (with plasma (O2 or Ar) or UV/Ozone) within 2 hours of bonding, if such cleaning processes do not damage the device.

Vapor degreasing has been used, but it is not as effective as one of the molecular cleaning

methods above. (The recommended method of cleaning HgCdTe detectors before bonding is currently being investigated by NIST in cooperation with two detector manufacturers.)

b) Any evidence of scratching the detector or package bond pad metallization, either during

handling or to mechanically clean the surface intentionally in order to enhance bondability,

should be questioned and will be cause for rejection if more than 25% of the bond pad area

is disturbed (see Mil-Std-883, Method 2010. 10, t 3. 1 . 1 . 1 .h). Probe marks and carefuUy

removed test bonds, are exempt unless their remains are so large as to interfere with final bonding.

c) Any evidence of rebonding, either lifts or residual pieces of earlier bond attempts (other than

necessary test bonds), should also be questioned. Any of these evidences may be cause for

rejection or performing a nondestructive wirebond pull test to verify the bond integrity if they interfere with the final chip-to-package bonding. (Note: Use of the nondestructive pull

test will raise the loop height, and may result in the wire interfering with a closely placed

lens or some other package height constraint.)

4) Bonding requirements and methods for gold-gold interfaces

a) Although gold-gold bonds can be made reliably at room temperature, the bonding window is narrow and any surface impurities will reduce the bond yield significantly. The highest bond

yield and the broadest bond parameter window occurs when the interface is heated.

Therefore, when using ultrasonics, it is also desirable to apply work-stage heat or use a heated bonding tool. The latter must be 200 - 300 °C to be helpful. In general, use the

highest bond interface temperature possible (up to 300 °C) considering the thermal

limitations of the device, epoxies, and other packaging materials. Even a 75 °C heated

substrate improves the bondability and reliability of a gold-gold bond.

b) The wedge bonding of gold wire to gold pads normally requires the use of a special gold

bonding tool, which will have a cross-groove and/or a roughened surface.

c) Wire bonding takes place at a higher bond yield if the surfaces are flat (perpendicular to the

axis of the bonding tool). Round or angled surfaces reduce the bond yield and should be

avoided. If they cannot be flattened, then special care should be taken, such as tipping the

substrate, bonding straight along the highest portion of a cylindrical surface, etc., in order to maximize the bond yield.

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5) The bond appearance should be normal.

a) Ultrasonic wedge bonds should not be over- or under-deformed (the range for reliable gold

wedge bonds is between approx. 1.5 to 2.5 wire diameters and 1.5 to 2.0 wire diameters for

aluminum wire bonds). There should be no evidence of smearing, either on the bond itself or on the adjacent bond pad area. (When gold wire is wedge bonded to gold at low

temperature (< 100 °C), the bonding window is narrow and bonds with low deformation

(< 1.5 wire diameters) may not be reliable.)

b) The bond heels should not be cracked, and particularly, if a cross-grooved bonding tool was

used for gold bonding, both the front and rear of the bond should be normal. (For example,

the front portion, in front of the groove, should be neither separated from the body, over- nor imder-deformed, nor be missing.)

6) Wedge bonds should be tested using an appropriate statistical sampling method (e.g., MIL-M385 lOJ, Appendix B) using the pull test for machine setup and evaluation. Electrical reject

samples may be used for setup purposes, and several bonds may be placed on one pad if there is room. Under some conditions, the nondestructive pull test may be used on completed devices,

where critical verification is necessary. Use class "S" requirements in the test methods below (if

applicable).

a) Use Mil-Std-883, Method 2011 for the destructive pull test.

b) Use Mil-Std-883, Method 2023 for the nondestructive pull test.

c) Use ASTM F 1269-89 (Ball Shear Test) if ball bonds are used.

7) Fine gold wires (<75 ^m diameter) must not be soldered, but must be welded by a standard

method used in microelectronics (thermosonic, ultrasonic, split-electrode electrical discharge,

etc.).

8) This Recommended Practice may also be applied to bonding in non-HgCdTe types of detectors

such as bolometers, etc.; in that case the same metal (gold in 1) may be omitted, and either the

pad and/or the wire may be aluminum. (If the wire is aluminum, then no bonding advantage is obtained from using special bonding tools.)

a) On occasion, conductive polymers (e.g., gold- or silver-filled epoxy) are used instead of

metallurgical welding to obtain electrical conductivity. This presents many complex material

choices and cannot be addressed by this Recommended Practice. (For example, silver-filled epoxy should not be used to electrically connect wires to aluminum surfaces, but under some

circumstances may be acceptable for use at cryogenic temperatures.)

D. Glossary

HgCdTe is mercury cadmium telluride, a semiconductor material used for far-infrared detectors.

This material is very brittle and has a low fracture toughness. It craters easily. It is typically used at cryogenic temperatures < 100 K.

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Cratering, crater-prone. Cratering refers to crystallographic damage in a semiconductor underneath

a bond. It can range from microscopic damage to large divots of the semiconductor that are pulled

up during bonding or in a pull test. Brittle materials with low fracture toughness crater easily during

bonding, and HgCdTe is such a material.

Military Standards: Mil-Std-883. This is the "Test Methods and Procedures for Microelectronics."

The specific version referred to in this Recommended Practice is 883D, 15 November 1991.

Although it was not specifically designed for spacecraft detectors, its class "S" provisions as well as

its bond pull test requirements are used almost universally for other parts of spacecraft electronics.

Only those parts applicable to this Recommended Practice are cited. MIL-M-38510. This is "Military Specification, Microcircuits, General Specification For" This is cited for its statistical sampling methods.

Yield is used in this document to mean the fraction of acceptable wire bonds produced in the

manufacturing process.

E. Typical Bonding Characteristics and Appearance of Plated Gold Films

(Not a part of the Recommended Practice~for information only)

(1) Gold films that bond well and are reliable:

a) Are pure yellow in color.

b) Are uniform, matte, and lusterless in appearance.

c) Are smooth and free from pits, blisters, or other blemishes.

d) Are soft (<80 Knoop), ductile, and dense.

e) Have <50 ppm of total impurity content.

f) Have < 1 ppm thallium and low hydrogen content for Al wire bonding.

g) Have a wide bonding parameter window.

(2) Gold films bond poorly and/or are not reliable if they:

a) Appear dark, have large nodules, or are very bright.

b) Shed minute particles on handling.

c) Have a lenticular or columnar structure and/or high porosity.

d) Have a high hydrogen or thallium content and/or >50 ppm total impurities.

e) Are hard, nonductile, and/or have high internal stress.

f) Give poor corrosion protection.

g) Have a narrow bonding parameter window.

(3) A pragmatic test to evaluate gold films for aluminum ultrasonic bonding: make aluminum bonds

to the film, bake 300 °C for 1 h, and reject if bond lifts in pull test. (This is based on Mil-Std-883,

Method 5008, f 3.5.3.3). This is a destructive test and is often used to screen packages or films

before acceptance from a vendor. It would not be used on a completed detector.

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5. SEMICONDUCTOR ELECTRONIC TEST STRUCTURES: APPLICATIONS TO

INFRARED DETECTOR MATERIALS AND PROCESSES

A. Introduction

Test structures are microelectronic devices which are fabricated by the same processes used to manufacture semiconductor electronic products, and that are used to measure selected material,

process, or tool parameters by means of electrical tests. Properly designed test structures can be used

to evaluate semiconductor materials, to determine and monitor process-centering parameters, to measure critical device and circuit parameters (which can be input to device models), to identify and

quantify yield limiting defects, and to measure processing tool performance.

Test structures are essential tools for fabrication process development and process maintenance of

silicon integrated circuits (ICs) and gallium arsenide monolithic microwave integrated circuits

(MMICs). However, test structures have not been as widely applied in the process development and

manufacture of infrared detectors, nor have the ways in which they are used been well documented in the open literature. Considering the substantial impact of test structures on other semiconductor

products, the current state-of-the-art applications of test structures to HgCdTe-based IR detectors have

been reviewed. The smdy had several goals:

1) To communicate and encourage application of the substantial experience gained with test structures in Si and GaAs to HgCdTe IR detectors;

2) To document test structures, appropriate to IR detector technology, so that they could be more

readily assembled into test chips;

3) To determine new ways in which test structures could be applied to improve HgCdTe uniformity,

to increase detector yield, and to enhance performance of detectors;

4) To identify test structures and measurements to complement and correlate with the Shubnikov-de

Haas and other magnetotransport measurements.

To meet the goal of communicating to the IR detector community, the results of this study were

presented at the Measurement Techniques for Characterization of MCT Materials, Processes, and

Detectors Workshop in Boston, Massachusetts, on October 16, 1992. A publication entitled: "Review

of Semiconductor Electronic Test Structures with Applications to Infrared Detector Materials and

Processes" has appeared in the workshop proceedings published in Semiconductor Science and

Technology in 1993 [5.1]. This publication is the most complete review of test structures applied to IR detectors and contains suggestions on how to improve IR detector process control, yield,

performance, and reliability through the intelligent application of test structures. The complete paper

is reproduced in Appendix B. A major element of this study was a comprehensive review of specific test structures used by the IR

detector industry. A thorough review of the literature concerning applications of test structures to IR

detectors was conducted. Details of device design and measurement methodologies were also

considered. In addition, experts in the electrical characterization of IR detector materials and devices were consulted. A site visit to Loral Infrared and Imaging Systems was conducted; other

consultations took place via telephone. A variety of reasons were suggested for the underutilization

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of test structures with IR detectors:

1) The relatively small numbers of devices required and the nature of the market have not demanded

large yields to satisfy product demand or to assure profitability. However, even if production

goals can still be satisfied with a low-yield process, the long-term reliability of devices fabricated

with a low-yield process could be compromised.

2) The need to test devices at low temperatures and as a function of irradiation has often been most

easily satisfied by packaging devices before any testing. Consequently, development and use of

low-temperature automated probe stations to acquire statistically significant amounts of data at the

wafer level and before packaging (a major application of Si and GaAs test structures) were not vigorously pursued.

3) The perceived need to package devices before test resulted in a conclusion that completed

detectors were their own best test structure. A larger emphasis was placed on the quality of

detector performance rather than the direct measurement of specific material and process

parameters with test structures. Instead of test structure measurements, the electrical

characteristics and figures-of-merit of detector elements have been used to deduce some material

parameters, and as a general indicator of device and process quality.

4) Early on, the poor quality of HgCdTe and resultant low yield of fabricated devices has,

paradoxically, encouraged a reluctance to devote valuable HgCdTe surface area to test structures. Without test structures to verify process and material parameters, poor detector performance or unexplained failure of elements in an array is often attributed, without evidence, to low-quality

starting material.

The experience of both the silicon IC and GaAs MMIC industries has shown that a comprehensive

test structure program is essential for effective process development and maintenance.

Communication of this previous experience would encourage wider applications of test structures in

the manufacture of IR detectors. Thus, a second critical aspect of this study was to review the

general principles of applying test structures determined through experience with silicon ICs and

GaAs MMICs. Much of this experience is in contrast to the practices described above. For example,

it is accepted that for silicon ICs, an intelligently selected and applied set of test structures is more

informative than finished product performance in the diagnosis of which specific process steps may be

limiting detector performance or yield. Test vehicle (chip) design methodology facilitates judgment as

to the appropriate number and kind of test structures, as well as a percentage of substrate area to devote to test structures, even for processes with very low yields.

The third aspect of this study was to extract principles and ideas for test structure applications from

the Si and GaAs experience that could be used to further enhance the manufacturability of IR

detectors. The central lesson is that yield improvement requires improved process control by means

of the intelligent application of test structures . An increased emphasis on applications of test structures is indicated for IR detectors. While the methodologies developed from previous experience can serve as a model for implementing test structures as tools for IR detector process development

and control, substantial effort will be required to:

• Determine what information is needed from test structures by correlating detector performance

and yield with material and process parameters.

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• Determine how that information will be obtained by correlating the output from test structure measurements with relevant HgCdTe material and process parameters.

• Develop and document standard, well-characterized test structure designs as cells in a CAD

library.

•

" Develop and document a detailed test plan.

• Develop and document detailed data analysis procedures.

Data acquisition may be simplified and accelerated by development of automated probe stations which

are compatible with low-temperature measurements and which use lightly contacting or noncontact

(i.e., low-damage) probes.

B. Conclusions

The conclusions of this study can be summarized as follows: Greater application of test structures to IR detector manufacture could lead to real improvements in the yield, reliability, and performance of

the detectors. The previous experience with test structures in the Si and GaAs industries provides a

useful guide and methodology for expanding the applications of test structures to IR detectors.

However, further applied research will be necessary to optimize these applications of test structures.

The experience of researchers at the National Institute of Standards and Technology is a resource for

extending the current applications of test structures to IR detectors. Potential areas of impact will be

to augment the design and applications of the existing test structures, to develop new test structures aimed at specific process or material problems, and to develop comprehensive test structure implementation plans. Cooperation among the users can further increase the impact of test structures on IR detector manufacturability. NIST and industry should work together to develop common test structures and implementation approaches.

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6. SCANNING CAPACITANCE MICROSCOPY: A NONDESTRUCTIVE

CHARACTERIZATION TOOL

A. Background

Scanning Capacitance Microscopy (SCM) is being established at NIST as an internally funded

initiative. SCM is a type of scanning probe microscope (SPM), like the scanning tunneling or atomic

force microscopes (STM or AFM). SPMs produce images by recording the strength of some

interaction between a mechanical probe tip in close proximity to a surface as that probe tip is scanned

in two-dimensions above the surface. Nanometer resolution is readily achievable.

Scanning Capacitance Microscopy is expected to be sensitive to nanoscale variations in dopant

density, composition, defects, mobility, and charge within a semiconductor. Because of these

capabilities, SCM will have some very important applications to IR detector material, process, and

device characterization. For example, the spatial resolution of the SCM will be such that performance difference between two photoconductive detector elements, which appear to be identical

by visual inspection, can be investigated by imaging the electrical properties of the active region

between the contacts. SCM has the potential to provide nondestructive prefabrication material

screening, so that detector fabrication is undertaken by using only the highest quality material.

Examination of cross sections of epitaxial layers or diffused regions will allow the dopant and

compositional variations to be determined with unprecedented resolution and accuracy. In addition,

topography, such as epitaxial island thickness and uniformity, can be measured with an AFM.

One possible implementation of SCM uses the Metal tip of an STM and the Insulating air gap

between the tip and a Semiconductor to form an M-I-S capacitor. Capacitors of this sort have long

been a basic test structure to determine the properties of semiconductors. The SCM provides an MIS

capacitor that is nondestructive, highly localized, and mobile. Thus, many of the measurements made

with capacitive test structures on semiconductors could now be implemented as a microscopy; i.e., the

spatial variations of the measured parameter could be mapped into an image. The concept of a scanning capacitance microscope was demonstrated by Matey in 1985 [6.1], and a capacitance probe was first integrated with an STM by Williams in 1989 [6.2]. SCM has also been demonstrated by

using an AFM by Barrett in 1991 [6.3].

B. Applications of SCM to GOES and Related Infrared Detectors

Scanning Probe Microscopes are expected to have an enormous impact on the semiconductor

electronics industry, perhaps surpassing that of electron microscopy. Significant applications are envisioned not only in the silicon integrated circuit industry, but throughout the wide range of

industries based on semiconductors, including infrared detectors based on HgCdTe and other

materials. SPMs will be important because of these unique capabilities:

1) High Resolution — Atomic resolution is "easily" achievable with STM and other SPMs. As

device dimensions are reduced below a micrometer, SPMs allow features to be seen that are beyond the resolution of optical or electron microscopes.

2) Nondestructive — In general, SPMs are noncontact and nonintrusive. Microscope vendors

have begun building stages compatible with in-line examination of 200-mm-diameter wafers.

Thus, most nominally flat surfaces are accessible, as well as some types of packaged devices.

106
3) Sensitivity to material properties — SPMs exist that are sensitive to various physical

parameters of importance to semiconductor electronics. These include the AFM which is sensitive to surface topography, SCM which is sensitive to the parameters that determine

capacitance (such as dopant density), as well as other microscopes such as a Kelvin probe

microscope which is sensitive to work function differences.

The possible applications of SCM to metrology problems of importance to GOES and related infrared

detectors include:

Materials Characterization

Nanoscale microscopy of bulk electrical properties

Dopant density

Composition

Defects

Interface properties

Mobility

Pre-fabrication wafer screening

Characterization of nanostructured materials, such as structures grown by molecular beam epitaxy

Process/Device Characterization

Junction profiling

Insulator/passivation charge mapping and manipulation

Topography and surface roughness

Fine-scale C-V measurements (and associated analysis)

Insulating layer integrity/defects/damage/uniformity

Metallization integrity/grain structure

Failure Analysis

Process-induced damage

Electrostatic discharge damage

Process fault and material diagnosis

C. Establishment of Facility

The significant challenges in bringing SCM from the lab to a practical (potentially in-line) analytical

technique are to 1) develop measurement procedures, 2) improve signal-to-noise and sensitivity, and

3) develop the theoretical interpretation and models to relate measured signals to relevant material

parameters.

A Digital Instruments Inc. Nanoscope III^ with a large sample stage was procured as the basis for

the NIST SCM. A schematic of the interface of the SCM with the AFM is shown as figure 6.1. The

system can perform both STM and AFM and has a fully automated stage with 200-mm wafer

Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by NIST, nor does it imply that the materials or equipment used are necessarily the best

available for the purpose.

107
Co - ir:

108
capability. Scanning capacitance microscopy will be initially implemented in the atomic-forcemicroscopy mode. SCM in the scanning-tunneling-microscopy mode will also be investigated in the

coming year. The SCM will be an important addition to existing efforts to characterize and screen

infrared detector materials and devices.

Capacitance-sensitive circuit

After Matey [6.1] and Williams [6.2], the capacitance-sensitive circuit from an RCA video disc player

will initially be used. This provides an inexpensive, high-sensitivity, and miniaturized capacitance

sensor. Other means of capacitance measurement will be investigated with the goals of increased

sensitivity and dynamic range of the capacitance measurement.

Modeling effort

An effort to model the measurement of capacitance with the SCM probe is ongoing. The SCM

capacitor is in a hemispherical geometry rather than the more usual and more highly studied parallel

plates. The initial thrust will be to numerically simulate the electromagnetic field between the probe

tip and semiconductor by using currently available software. While several groups have demonstrated

the concept of SCM, the measured signal has not yet been quantitatively related to the semiconductor

dopant variations.

It is worth noting that applications of the interactions of a scaiming probe tip and a semiconductor,

especially when subjected to transient electrical and optical signals, are still a new area of

investigation. It is likely that many semiconductor parameters will ultimately be accessible with some

type of SPM.

D. Preliminary Results

At the writing of this report, the AFM/STM which will form the basis of the SCM facility has been

operational only a short time. AFM images were made of some photoconductive detector elements to

illustrate the potential resolution and image quality of an SCM based on the AFM system. Images were made at room temperature and in air on a packaged device. No sample preparation other than

mounting the package to the microscope stage was needed.

Figure 6.2 is a low-magnification AFM image showing parts of three photoconductive detectors.

AFM provides a precise means of determining layer heights and device topography. In the initial implementation of SCM, AFM will be used to measure material topography simultaneously with SCM

measurements. An independent measurement of topography greatly simplifies the interpretation of the

scanning capacitance image.

Since an SPM acquires images as arrays of digital data, rapid image manipulation and analysis is possible. Figure 6.3 shows a cross-sectional analysis of an AFM image of the boundary between the

passivated active layer and the metal pad of a detector element from figure 6.2. The AFM image

reveals a 150-nm dip in the passivation layer at the probe-pad boundary and a step in the probe-pad

wall which could not have been as easily imaged with an optical microscope or SEM. Statistical analysis of topography or surface roughness is also readily available.

109
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112
Figure 6.4 shows a higher magnification AFM image of the edge of the active area of a detector

element. Considerable structure to the topography is seen clustered along the edge of the active area with resolution of less than 1 ^m. Resolution of AFM is less than 10 nm and would permit the

surface roughness of materials and device layers to be quantified. SCM will certainly be capable of

similar resolution which would be sufficient for planar materials characterization. Resolution of

100 nm has been demonstrated with SCMs similar to the one being constructed at NIST, enabling the

dopant variations across p-n junctions or heterostructures to be quantified.

It is difficult to identify the origin of the fine structure seen in the topography at the detector edge

without more extensive measurements. It may be due to particulate contamination concentrated by a wet process step before or after passivation deposition. Indeed, development of SCM will permit the

electrical effects of such topography disturbances to be probed and their likely effects on detector

performance to be evaluated.

E. Summary

Scanning capacitance microscopy is a new, nondestructive metrology tool expected to have significant

applications to infrared detector materials, processes, and devices. SCM will produce images that can be related to material electrical properties with spatial resolution comparable to the dimensions of

detector elements. SCM applications expected to have a large impact on the quality, yield, and

manufacturabilty of detectors include: nondestructive diagnosis of material variations within the

active regions of detectors, nondestructive prefabrication materials evaluation, and dopant depth

profiling of nanostructured materials.

113
7. NIST REVIEW OF THE GOES CALIBRATION PROGRAM

Contributed by

B. Saunders, C. Johnson, and C. Chromer

Radiometric Physics Division, NIST

A. Purpose

The goals of this task were to investigate the radiometric calibration procedures of the GOES program

and to report NIST evaluation of the calibration capabilities of the prime vendors. Out of this evaluation NIST was to suggest recommendations for an improved calibration program for the

infrared sensor system on the GOES platform. NIST staff would visit the three main contractors

associated with developing the instruments for the infrared radiometric measurements. Although

LORAL is the main contractor for the GOES program, they were not directly involved in the

radiometric calibrations, and as a consequence, NIST staff did not visit or discuss radiometric

calibrations with LORAL staff.

NIST staff first visited and examined the radiometric calibration programs of the detector vendors and

followed with a trip to the system integration contractor (ITT Fort Wayne) where the final radiometric

calibrations of the GOES satellite are performed.

B. Summary of Visits to Facilities

1) Supplier 2

Supplier 2 supplied the indium antimonide detectors and the associated collecting lens system used for

the short wave infrared channels of the GOES sensor system. The only radiometric calibration

specification that the detector was contractually required to meet was a specified blackbody D* for

bare detectors. These test were performed using a 500-K blackbody as a standard for the blackbody

D* determination. This is a straightforward measurement and is routinely made at a precision of few

percent. NIST observation at Supplier 2 confirmed the measurements were being done according to accepted practice. Because of the detector sensitivity requirements of the GOES sensors, only one

detector in a hundred met the requirements and could be selected for further processing. At this point, the calibration program at the detector manufacturer ceases, and the detectors are not further

characterized until they arrive at ITT for integration. The next step at the detector manufacturer is the fitting of the detector optics onto the detector. The only requirement for the detector

manufacturer is that the optics meets certain fabrication specifications and that the optical elements be

tested for transmittance. The detector manufacturer is not informed about the criteria for the final selection process for the integrated detector package. The detector manufacturer is not responsible for

the technical operation of the assembled detector after the lens is integrated on the detector. No

check or test of responsivity or noise of the lens-detector systems is performed by the primary

manufacturer.

2) Supplier 3

Supplier 3 is the manufacturer of HgCdTe detectors for the long wave channels. During the visit,

NIST staff was shown the manufacturing process for the detector but was not able to review any

calibration program of the detector package. This apparently was due to the fact that the calibration

114
program for the detectors was not operational at the time of the visit. Suppher 3 performs some

general radiometric characterizations but does not provide detailed radiometric characterizations.

Limited performance checking is the basis for selection of detectors to be used for the GOES satellite.

C. Recommendations

a. The calibration process must be studied from the fundamental optical physics viewpoint, the sources of error identified, and their effects on the final measurement determined. A

comprehensive approach to this problem involves writing a measurement equation that incorporates all the relevant parameters and their relationships.

b. Determine the critical parameters that are needed to characterize and develop a calibration

plan before manufacturing the components. This calibration plan can be done in several

steps. In GOES, there should be a calibration and characterization program for each step of

the manufacturing process. The different components, such as the detectors should be

characterized before proceeding to the next step of reassembly. These detector packages are

the heart of the GOES sensor system and are required to meet specifications that are at the

theoretical limits of the detectors. Presently, the bare detectors are only being tested for

blackbody D*. Although this is a critical parameter, the detectors should be tested for

parameters other than blackbody D*. Parameters that affect detector performance are

internal shunt resistance, spatial radiometric uniformity, stability, spectral responsivity,

spectral D*, and linearity; if any of these parameters is unacceptable, then it is pointless to proceed to the integration of the lens onto the detector. The parameters such as spatial

radiometric uniformity and geometric radiometric response function (response as a function

of angle) should be determined after the lens is mounted on the detector.

c. Detailed records showing the history of the contributing test and changes should be

maintained. These records should follow the component though its incorporation into the

satellite. These records could be used to determine the stability and performance of the

detectors [aspect of the sensor system.]

d. There should be a coordinated effort to maintain calibration assurance between the different

manufacturers and the assemblers. This will help each manufacture plan a suitable and

appropriate calibration program. In NIST's investigation, it was found that the different

manufacturers of the detectors involved did not know how the detectors were going to be

tested in the next stage. Not only is this costly but a lot of valuable expertise is not passed

down the manufacturing chain. Also, the manufacturer is not held responsible for the

performance and ability of his product to integrate it to successive stages of the assembly.

Each manufacturer should be held responsible for meeting the specification of the next stage.

e. To achieve a reliable sphere source, considerably more attention to the calibration and care of the sphere will be necessary. The sphere should be calibrated more frequently and a history of the calibration maintained. Departures or significant change from the baseline

need immediate attention. The calibration schedule of the sphere should closely bracket the

calibration of the GOES sensors.

f. Since the final product is a radiometric calibration problem, staff trained in radiometric

measurements are essential in the GOES program. Although the GOES program includes

115
many talented people whose expertise is needed, NIST did not find evidence of radiometric

training or expertise on the calibration staff.

g. The GOES should be calibrated as close to the time of its use as possible. This will help ensure that the GOES has the optimal calibration at the time of launch. However, this procedure will not ensure that the GOES can make measurements of the same accuracy later due to changing environmental circumstance that can effect the optics and detection in space

and during launch.

h. It is recommended that the GOES be calibrated several times before launch. This will

establish a calibration history and base line. It will also set a limit as to the accuracy that can be achieved in the use of the instrument once launched. While it is a recognized

departure from present strategy and perhaps contains difficulties, it is recommended that NOAA/NASA consider developing a strategy for calibration checks or calibration while the

instruments are in space orbit. This could include onboard calibration devices or the use of

celestial sources that could be viewed for calibration and stability checks. If a scenario for

the present GOES cannot be developed that addresses this problem, the insights gained in

such a study can be used for future design considerations in future observational satellites.

116
8. SUMMARY AND CONCLUSIONS

A. Magnetotransport Measurements

Shubnikov-de Haas (SdH) oscillations in the transverse magnetoresistance have been used to characterize the accumulation layers of the infrared detectors used in the GOES and TIROS weather

satellites. The electron densities, cyclotron effective masses, and Dingle temperatures can be obtained

from the data for each subband in the 2D electron gas formed by the accumulation layer. A

first-principles calculation of the subband energy dispersion relations has been performed in order to compare theory and experiment. The model is needed in order to extract the density from the data,

because the energy bands are very nonparabolic in narrow-gap HgCdTe. The agreement between

predicted and measured masses and Fermi energies was excellent for anodically oxidized layers. The

masses could not be obtained for the other processes because the signals were weak and complex.

A large number of detectors from each of three suppliers were measured by the SdH effect and

analyzed to obtain the densities, effective masses, and Dingle temperatures of their accumulation

layers. Results were obtained for devices (five elements from two detectors) from Supplier 1 with

type I (anodic oxidation) passivation. The Fourier transforms have large, well-defined peaks from

which the carrier density of the accumulation layer was obtained. Two detectors with four elements

each from Supplier 2 were measured. The front and back surfaces have different passivations

(combination designated type III), which make it difficult to interpret the results. Consequently, three

experimental detectors were fabricated and measured to help understand the individual surfaces. Two

classes of detectors were received from Supplier 3. The eight elements with type I passivation were

similar to the type I units from Supplier 1 . The four elements with a different passivation, type II, had weak SdH response which resembled that of the type III devices from Supplier 2. The carrier

densities of the accumulation layers of type II and III detectors were much greater than those for the

type I detectors. Six TIROS detectors were also measured. These are larger devices with type I passivation. The carrier densities were similar to those on the other type I detectors. Angular

rotation smdies were performed on devices from Suppliers 2 and 3 to verify that the SdH signal was

coming from the two-dimensional accumulation layer. Effective masses and Dingle temperatures were calculated for one or more elements with type I passivation from each supplier. The values were in good agreement with the theoretical calculations. More work is needed to obtain meaningful

values of effective mass and Dingle temperature on other types of accumulation layers.

A new and simpler method that is based on the dc magnetoresistance upon which the SdH oscillations occur has been investigated. The electron density and mobility of the top accumulation layer can be

determined from the magnetic-field dependence of the magnetoresistance at high magnetic field. Agreement between the densities and mobilities of the accumulation layers of type I and type II detectors with Hall measurements by the manufacturers showed that the method was accurate. Measurements were made on a large number of detectors from Suppliers 2 and 3. The results showed variability of the accumulation-layer density by 20% among three elements of a multi-element

detector. The generally lower mobilities and higher densities of accumulation layers on type II and

type III detectors led to their improved performance because of reduced leakage and decreased surface

recombination.

117
B. Other Characterization Studies

Bonding, Metallization, and Packaging

A total of six visits was made to three GOES and TIROS infrared detector manufacturers during FY

1992. The first visit to each site was made to evaluate their production lines and processes. Later

visits were made to help them with the packaging of their detectors. This included one 2.5-hour

seminar on wire bonding and the reliability of the metallurgical systems used in packaging HgCdTe

detectors. Over 30 people attended that seminar. For another company, a more informal hour-long

presentation was made to six/seven engineers and management personnel. Extra time was later spent

in their packaging laboratories. An SEM study was made of detectors from one manufacturer and

several defects were found (the resuUs were presented at a GOES project review at NASA Goddard

July 10, 1992). This information, with proposed solutions, was also fed back to the manufacturer to help them improve their product. Studies were carried out at NIST and at two detector manufacturing

sites to establish the best molecular-cleaning methods that are compatible with the normal HgCdTe

detector packaging methods. For this work, ultraviolet cleaning equipment was hand carried to detector manufacturers so that tests could be performed there. A set of recommended practices was

developed and provided to the manufacturers to help them improve their yields.

Test Structures . - -

^

Because of the substantial impact of test structures on other semiconductor circuits, the current

state-of-the-art applications of test structures to HgCdTe-based IR detectors were comprehensively

reviewed. To place these applications in context, the general principles of applying test structures,

determined through experience with silicon ICs and GaAs MMICs, were also reviewed. From these

two reviews, principles and ideas were extracted for test structure applications that could be used to further enhance the manufacturability, yield, and performance of IR detectors. To communicate and

encourage application of test structures, the results of the study were presented at the Measurement

Techniques for Characterization of MCT Materials, Processes, and Detectors Workshop held in

Boston, MA during October 1992 and published in Semiconductor Science and Technology. A

reprint of this paper is attached in Appendix B.

Scanning Capacitance Microscopy

Scanning capacitance microscopy is a new, nondestructive metrology tool that merges a highsensitivity capacitance sensor with an atomic force microscope (AFM). SCM applications that could

be expected to have a large impact on the quality, yield, and manufacturability of detectors include:

nondestructive diagnosis of material variations within the active regions of detectors, nondestructive

prefabrication materials evaluation, and dopant depth profiling of nanostructures. AFM images were made of some photoconductive detector elements to illustrate the feasibility, potential resolution, and

image quality of SCM applied to IR detectors.

GOES Calibration Program

NIST staff visited and examined the radiometric calibration programs of the detector suppliers as well

as the system integration contractor where the final radiometric calibrations are performed. NIST

recommends that a fundamental calibration program be established that is coordinated between the

118
different manufacturers and assemblers. NIST also recommends that the GOES detectors be

calibrated several times before launch to establish a calibration history and base line.

Industrial Survey of Characterization Measurements for HgCdTe Materials, Processes, and Devices

Appendix C contains a reprint of this work. An extensive industrial survey of the importance and use

of characterization measurements for HgCdTe materials, processes and devices has been completed.

Seventy-two characterization/measurement techniques were considered, and thirty-five responses were

received. This information was sought for a study on materials characterization and measurement

techniques of parameters and properties necessary to improve the manufacturing capabilities of

HgCdTe infrared detectors. The nature of materials characterization is defined, and an overview is given of how it is related to improving IR detector manufacturing. Finally, we present a description

of the characterization survey and a summary of the survey results. Major aspects of the results include: (1) ranking the 72 techniques by their importance and frequency of use, (2) listing the

parameters or properties determined by each technique, (3) enumerating the most important properties

that need to be measured, (4) indicating the key measurement techniques that most need to be

developed, enhanced or improved, and (5) giving key overall comments.

119
9. REFERENCES

[1.1] D. G. Seiler, G. G. Harman, J. R. Lowney, S. Mayo, and W. S. Liggett, Jr., HgCdTe

Detector Reliability Study for the GOES Program, National Institute of Standards and

Technology, NISTIR 4687 (September 1991).

[1.2] HgCdTe Detector Degradation Task Force Meeting Booklet, May 2, 1991, p. 76.

[1.3] J. R. Lowney, D. G. Seiler, W. R. Thurber, Z. Yu, X. N. Song, and C. L. Littler,

Heavily Accumulated Surfaces of Mercury Cadmium Telluride: Theory and Experiment, J. Electronic Materials 22, 985 (1993).

[2.1] R. J. Nicholas, F. Nasir, and J. Singleton, High Magnetic Field Characterization of

(Hg,Cd)Te Surface Layers, J. Crystal Growth 86, 656 (1988).

[2.2] D. G. Seiler, G. B. Ward, R. J. Justice, R. J. Koestner, M. W. Goodwin, M. A. Kinch,

and J. R. Meyer, Shubnikov-de Haas Measurements on n- and p-type HgTe-CdTe

Superlattices, J. Appl. Phys. 66, 303 (1989).

[2.3] S. Yamada, H. Asai, and Y. Kawamura, Subband Effective Mass and Mobility of

Two-Dimensional Electrons in Uniformly Si-Doped GalnAs/AlInAs Multiple Quantimi

Wells, J. Appl. Phys. 72, 569 (1992).

[2.4] T. Ando, Theory of Quantum Transport in a Two-Dimensional Electron System under

Magnetic Fields. IV. Oscillatory Conductivity, J. Phys. Soc. Japan 37, 1233 (1974).

[2.5] I. Nachev, Bound States in Inversion layers on p-Hgj.^Cd^Te: Self-Consistent Results,

Semicond. Sci. Technol. 3, 29 (1988).

[2.6] See Ref. 1.3.

[2.7] E. Kane, Band Structure of Indium Antimonide, J. Phys. Chem. Solids 1, 249 (1957).

[2.8] J. Sune, P. Olivo, and B. Ricco, Self-Consistent Solution of the Poisson and Schrodinger

Equations in Accimiulated Semiconductor-Insulator Interfaces, J. Appl. Phys. 70, 337

(1991).

[2.9] D. G. Seiler, J. R. Lowney, C. L. Littler, and M. R. Loloee, Temperature and

Composition Dependence of the Energy Gap of Hgi.xCd^Te by Two-Photon

Magnetoabsorption Techniques, J. Vac. Sci. Technol. A8, 1237 (1990).

[2.10] J. P. Harrang, R. J. Higgins, R. K. Goodall, P. R. Jay, M. Laviron, and P. Delescluse,

Quantum and Classical Mobility Determination of the Dominant Scattering Mechanism in

the Two-Dimensional Electron Gas of an AlGaAs/GaAs Heterojunction, Phys. Rev. B 32,

8126 (1985).

[2.11] S. Das Sarma and F. Stem, Single-Particle Relaxation Time Versus Scattering Time in an Impure Electron Gas, Phys. Rev. B 12, 8442 (1985).

120
[2.12] Y. Nemirovsky and G. Bahir, Passivation of Mercury Cadmium Telluride Surfaces, J. Vac.

Sci. Technol. A7, 450 (1989).

[2.13] Y. Nemirovsky and I. Kidron, The Interface between Hgj.xCd^Te and Its Native Oxide,

Solid-state Electronics 22, 831 (1979).

[2.14] A. J. Rimberg and R. M. Westervelt, Electron Energy Levels for a Dense Electron Gas in Parabolic GaAs/AlGaAs Quantum Wells, Phys. Rev. B 40, 3970 (1989).

[3.1] K. Seeger, Semiconductor Physics , Springer-Verlag, New York, 1985, p. 63.

[3.2] R. D. Larrabee, W. A. Hicinbothem, and M. C. Steele, A Rapid Evaluation Technique for

Functional Gunn Diodes, IEEE Trans. Electron Devices ED- 17, 271 (1970).

[3.3] T. R. Jervis and E. F. Johnson, Geometrical Magnetoresistance and Hall Mobility in Gunn

Effect Devices, Solid-State Electronics 13, 181 (1970).

[3.4] W. S. C. Gumey and J. W. Orton, New Techniques for the Study of Gunn Diode Contacts,

Solid-State Electronics 17, 743 (1974).

[3.5] M. J. Howes, D. V. Morgan, and W. J. Devlin, Applications of Magnetoresistance

Measurements in the Evaluation of Transferred Electron Device Performance, Phys. Stat. Sol. (a) 41, 117 (1977).

[3.6] D. C. Look, A Two-Layer Magneto-TLM Contact Resistance Model: Application to Modulation-Doped FET Strucmres, IEEE Trans. Electron Devices ED-35, 133 (1988).

[3.7] H. J. Lippmann and F. Kuhrt, Geometry Influence on the Transverse Magnetic Resistance

Effect in Rectangular Semiconductor Plates [Der Geometrieeinflusz auf den transversalen

magnetischen Widerstandseffekt bei rechteckformigen Halbleiterplatten], Z. Naturforsch.

13a, 462 (1958) (in German).

[3.8] H. Weiss, Magnetoresistance, in Semiconductors and Semimetals, Vol. 1, ed. by R. K.

Willardson and A. C. Beer, Academic Press, New York, 1966, p. 315.

[3.9] DATAPLOT is a software package developed at NIST for analyzing and displaying data; see DATAPLOT- Introduction and Overview, NBS Special Publication 667 (U.S. GPO,

Washington, DC, 1984).

[3.10] M. B. Reine, K. R. Maschhoff, S. P. Tobin, P. W. Norton, J. A. Mroczkowski, and E. E. Krueger, The Impact of Characterization Techniques on HgCdTe Infrared Detector

Technology, Semicond. Sci. and Technol. 8, 788 (1993).

[3.11] M. B. Reine, private communication.

[5.1] J. J. Kopanski and C. E. Schuster, Review of Semiconductor Microelectronic Test

Structures with Applications to Infrared Detector Materials and Processes, Semicond. Sci. Technol. 8, 888-910 (1993).

121
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(1985).

[6.2] C. C. Williams, W. P. Hough, and S. A. Rishton, Scanning Capacitance Microscopy on a 25 nm Scale, Appl. Phys. Lett. 55, 203, (1989).

[6.3] R. C. Barrett and C. F. Quate, Charge Storage in a Nitride-Oxide-Silicon Medium by

Scanning Capacitance Microscopy, J. Appl. Phys. 70, 2725 (1991).

122
Journal of Electronic Materials. Vol. 22, No. 8, 1993 APPENDIX A

Special Issue Paper

Heavily Accumulated Surfaces of Mercury Cadmium

Telluride Detectors: Theory and Experiment

J.R. LOWNEY, D.G. SEILER, and W.R. THURBER

Semiconductor Electronics Division, National Institute of Standards and

Technology, Gaithersburg, MD 20899

Z. YU, X.N. SONG, and C.L. LITTLER

Department of Physics, University of North Texas, Denton, TX 76203

Some processes used to passivate n-type mercury cadmium telluride photoconductive infrared detectors produce electron accumulation layers at the surfaces,

which result in 2D electron gases. The dispersion relations for the electric subbands that occur in these layers have been calculated from first principles.

Poisson's equation for the built-in potential and Schroedinger's equation for the

eigenstates have been solved self-consistently. The cyclotron effective masses and Fermi energies have been computed for each subband density for 12 total densities between 0. 1 to 5.0 x 10^- cm"^. The agreement with Shubnikov-de Haas

measurements is very good at lower densities with possible improvement if band-gap narrowing effects were to be included. At higher densities, larger

differences occur. The simple 2D description is shown to break down as the

density increases because the wave functions of the conduction and valence

bands cannot be well separated by the narrow band gap of long-wavelength

detectors. These results provide a basis for characterizing the passivation

processes, which greatly affect device performance.

Key words: HgCdTe photoconductive detectors, Shubnikov-de Haas oscilla- tions, surface passivation, 2D electron gas

INTRODUCTION

The II-VI compound semiconductor Hgj^Cd^Te is today's most widely used infrared detector material.

It provides, for example, high-performance photoconductive (PC) detectors in the 4 to 6 and 8 to 12 |j.m

spectral ranges. The properties of these detectors are very sensitive to their surface passivation, which is often a dominant factor in limiting device perfor- mance. The complexity of the surface passivation

process arises from the different chemical properties

of the constituents and the formation of electrically active defects in the interface region. ^ Often HgCdTe

detectors are passivated by processes (e.g. anodic

oxidation) that produce accumulated surfaces, which

resiilt in 2D electron gases with areed electron densities on the order of 10^^ to 10^ cm-^. The resulting

surface potential can be greater than the band-gap

(Received October 12, 1992; revised January 4, 1993)

energy in long-wavelength detectors, and the sur- faces can thus greatly influence the characteristics of the detector. Consequently, the development of theories and models that predict the conduction properties of the electrons in these accumulation layers is very important. In this paper, first-principles calculations have

been used to determine the accumulation-layer potentials, electron densities, cyclotron effective masses, and Fermi energies ofthe resulting 2D electron gases. Shubnikov-de Haas (SdH) measurements were also carried out on a wide variety ofn-type PC detectors to compare them with the calculated results. The elec- tron densities and mobihties of such layers were

obtained in the past by Nemirovsky and Kidron^ by

using Hall-effect and capacitance-voltage measure- ments for a wide range of densities. Nicholas et al.^ have used SdH measurements to characterize these

surface layers, and they have found that the layers can be described by a 2D electron gas with a number

985
986 Lowney, Seller, Thurber, Yu, Song, and Littler

of 2D subbands. SdH oscillations occxir in the magnetoresistance at high magnetic field and result from

the redistribution ofcarriers caused by the crossing of

the Fermi energy by a Landau level. This technique

has the advantage that measurements can be made

directly on commercial photoconductive detectors,

which have only two terminals. The oscillations are

periodic when plotted as a function of the inverse of

the magnetic field, B, and a Fourier transform of the

signal as a function of LIB shows peaks at fundamental frequencies corresponding to the densities of each

subband. For parabolic subbands, the electron densities can be obtained directly from these frequencies.

However, HgCdTe is very nonparabolic because ofits small energy gap, and therefore a model is needed to deduce the densities. The subband Fermi energies can be obtained from the fundamental frequencies once the cyclotron effective masses are determined

from the measured temperature dependence of the

amplitude of the oscillations. The model then relates

these measured quantities to the electron densities.

We have calculated the dispersion relations of the

subbcinds self-consistently from the accumulationlayer potentials to obtain the model needed to relate the electron densities to the measured parameters.

There have been various approaches to the solution of

the subband dispersion relations, most ofwhich have

used a WKB approximation.'*-^ We have followed the

work ofNachev,' who solved the matrix Hamiltonian

for the conduction band axid heavy-hole, light-hole,

and split-off valence bands. The built-in potential in

the electron accumulation layer is included directly in

the matrix equation, and a second order differential

equation is obtained for a wave function that contains

terms that depend both on the potential and the

electric field in the accimiulation layer. The field terms cause a spin-splitting of the eigenvalues. The

actual subband wave functions can then be determined from the solutions ofthis differential equation.

First we solved Poisson's equation for a continuum

model ofthe electron density to obtain an initial built- in potential. Then we solved Schroedinger's equation

for the eigenfunctions and eigenvalues for the electric subbands that are allowed in the 2D electron gas that

describes the accumulation layer. From the wave

functions obtained, we recomputed the potential and

iterated the potential imtil self-consistency was ob- tained. We then computed the Fermi energies and

cyclotron masses from the calculated subband dis- persion relations for twelve electron densities between 0. 1 to 5.0 X 10^2 cm-2. Agreement ofour theoretical results with our measurements for a detector with an electron density of 7.9 x 10^^ cm-^ is very good. In

contrast, the theory predicts values for cyclotron

effective masses that are much smaller and Fermi

energies that £ire much larger than the measured

values for a detector with 3.4 x 10^^ cm^^. We expect

that if we were to include the effect of band-gap

narrowing,* which occurs at these fairly large carrier

densities, we would obtain better agreement. There is also the possibility that the surface could be imder

some strain, which could affect the energy gap. There is a gradual breakdown in the 2D electron

gas model for the accumulation layers as the density

increases because the energy gaps are so small in

long-wavelength detectors. It becomes difficult for the

electrons to remain confined solely to the conduction

band because their wave functions cam cross the

energy gap into the valence band when the potentials

become large. For the 2D model to be completely valid,

the conduction-band wave functions must decay sufficiently in the energy gap that they can be considered

isolated. For the low-order subbands in very heavily

accumulated surface layers, this isolation weakens,

and a large number of states are then allowed that

traverse the entire thickness of the detector. Thus,

the model approaches the limiting case ofa graded 3D

layer with a SdH Fourier transform that has a broad

featureless background for the range of 3D electron

densities in the accumulation layer.

THEORY

We have extended the work of Nachev,^ who has

performed the most rigorous analysis of the subband

dispersion relations, to the entire set of allowed

subbands for a wide range of electron densities. He

has derived the 8x8 matrix Hamiltonian for the

conduction band and the heavy-hole, light-hole, and

split-offvalence bands for both spin directions (±). He

then reduced the Hamiltonian to a second-order dif- ferential equation for the wave function <t)^. in the

direction z perpendicular to the accumulated surface:

where

a =

2a^ + dV(z)

2a + p dz

b =

3y

c =

Po^(2a + P)

3(a^-p^)dV(z)

2a-i-p dz

and

a = E,(z)-E*

1 Ejz)-A-E*

y = E,(z)-E*

(1)

(2)

(3)

(4)

(5)

(6)

(7)

In the above equations, k is the wave number in the

plane ofthe surface and refers to the 2D free electron
Heavily Accumulated Surfaces of Mercury Cadmium

TeUuride Detectors: Theory and Experiment 987

gas, V(z) is the built-in potential in the accumulation

layer due to the oxide charge, E^(z) and E^(z) are the

conduction and valence band energies, respectively,

including the effect of the potential, A is the spht-off

band separation from the valence band edge, is a number proportional to the momentimi matrix ele- ment as given in Kane's band model for HgCdTe,^ and

E- are the eigenvalues for the two spin directions.

The wave functions <t)^(k,z) are real and the actual

subband wave functions, can be computed from

them by solving for the eight envelope functions f^^ because

^(r) = if.(zK(r) (8)

n=l where u_^(r) denotes the periodic part of the Bloch

function at k = 0. The envelope wave functions of the

conduction band, /Jzj and j-iz), are foimd directly in terms of ({) . (k,z):

(!)^ (k,z) = i (k /k)/,(z) ± /^(z), k^ = k + ik„. (9) The other envelope functions are found from the

matrix Hamiltonian in terms of these two envelope

functions by direct substitution. The equations for them, which are somewhat lengthy, involve the de- rivatives of /j(zj and /.(z) as well because of the

momentimi operator in the Hamiltonian. The volumetric electron densit>' can then be computed directly

from ^ and the areal density of the 2D electron gas. The initial potential is found by solving Poisson's

equation for a non-quantized 3D free electron gas. The standard Kane k-p band model,^ which treats the

coupUng of the light-hole and split-off valence bands

with the conduction band, is used. Poisson's equation

is solved by a nonlinear two-point boimdary value

method based on finite differences with deferred correc- tion and Newton iteration. Equation 1 is then solved

by integrating the equation from an initial value and

slope at the boimdary opposite to the accumvdation

layer, and eigenvalues are found by selecting those

solutions that vanish at the surface. As in Ref 7, the interior boundary, where the

conduction-band wave functions go to zero, has been

chosen to be at the middle ofthe energy gap for a given

eigenvalue or at a maximum distance of 0.5 |im if the

eigenvalue always hes above midgap. Thus, we accept

only those states that are effectively bound by the

conduction band and constitute a 2D electron gas. This approximation is based on the assumption that

the wave functions decay sufficiently in the energy

gap that those associated with the conduction band

can be separated from those associated with the

valence band. As the gap becomes very small, this approximation breaks down and a continuimi background of states that traverse the entire thickness of

the detector becomes allowed. This effect is discussed

further below.

Equation 1 is solved for at most 50 k values to construct the dispersion relations for the allowed

subbands. A new potential is computed from the

calculated wave functions between the surface and

0.1 |a.m; beyond this point, the original bulk potential

is used. Were we to use the calculated wave functions

beyond 0.1 |j.m, there would be a difficulty because of

the artificial boundary condition at 0.5 |im where the wave functions are forced to be zero. The process is iterated vmtil the input and output potentials agree to within 1%. In order to prevent the potentials from

gradually diverging from their original values, they

are scaled each time by the ratio of the initially

computed areal electron density to that just computed.^" When this factor is between 0.99 and 1.01,

convergence is obtained. It was discovered that con- vergence could be obtained more rapidly, and often

only, if the initial potential were modified slightly

between the surface and 0.025 p.m to take into account

the strong differences between the electron density

computed initially and quantiun-mechanically near

the surface in the accumulation layer. The initial potential was thus subsequently scaled by a quadratic function to make it agree better with the shape

of the first calculated potential over this range. Once the self-consistent subband dispersion re- lations were foimd, we computed the subband densities, Fermi energies, and cyclotron effective masses

at the Fermi energy by performing either a parabolic

spline interpolation or linear extrapolation of our computed eigenvalues to the Fermi energy. The spinaveraged cyclotron effective mass, m*, is obtained

from the expression

(10)

evaluated at the Fermi energy, E^ These quantities now allow one to compute the value of the subband

densities from the peaks in the Fourier transform of

the SdH data. The frequencies corresponding to the

peaks equal m*E//2 e for each subband. The value of

m* is determined from the measured temperature

dependence ofthe ampUtude ofthe SdH oscillations.^^ Thus, one can find the subband density for which the

theoretically computed product of m*Ef has the measured value for each subband. For the case of parabolic subbands, m*Ef = ^^nN, where N is the electron

density, and the peak frequencies provide N directly.

We refer to this relation as the parabolic approximation. EXPERIMENT

Shubnikov-de Haas measurements have been made

on a series of long-wavelength n-t3T3e HgCdTe PC

infrared detectors with accumulation-layer electron

densities between mid lO^^ and mid 10^^ cm-^. They

were square with active areas between 2 and 4 x 10"* cm^. Their top and bottom surfaces were passivated

by various methods that produced electron accumulation layers. Indium contacts, sometimes extended, were used to achieve excellent ohmic behavior.

Some of these detectors were measured as a function of temperature between 1.5 and 30K to obtain

effective mass data. Here we report on two detectors
988 Lowney, Seller, Thurber, Yu, Song, and Littler

ENERGY BAND DIAGRAM CONTINUUM AND QUANTUM SOLUTIONS

-250 -

-300

INTERFACE

0.05 0.1 POSITION ifim)0.15

15

a2 10

CONTINUUM QUANTUM

0.025 0.05 0.075 POSITION (^m)

0.1

SUBBAND DISPERSION RELATIONS SUBBAND POPULATIONS

-80

-- SPIN DOWN — SPIN UP

I

' I

' 0.05 0.1 WAVENUMBER (nm"')

0.15 0.2

Fig. 1 .(a) Built-in potential for accumulation layer with total electron density of 8.9 x 10" cm-^ and alloy fraction x = 0.191. The horizontal

lines are the subband-edge energies for the first three subbands (n =

0,1,2). (b) Dispersion relations for the subbands for this potential, showing spin-splitting. The curves are labeled by subband number

from 0 to 6.

TOTAL DENSITY (lO'^cm"')

Fig. 2. (a) Electron density computed by solving Poisson's equation for a charge continuum (dashed) and from full quantum-mechanical

calculation (solid) for case of Fig. 1 . (b) Suband densities as a function

of total density. Lines are linear least-squares fit. The curves are

labeled by subband number from 0 to 3, and x = 0.1 91

.

that have had extensive measurements made on

them. Ac magnetic-field modulation and lock-in amplifier techniques that use the second harmonic detec- tion were used to obtain the signal for magnetic fields up to 12 The SdH oscillations are a small percentage ofthe total magnetoresistance, and ac techniques

significantly enhance the measured SdH oscillations

in these detectors. Excellent signal-to-noise ratios were obtained in

most detectors even at temperatures as high as 30K.

Measurements made at 30, 60, and 90 degree rota- tions of the sample about the perpendicular to the

surface showed that the peak frequencies varied as 1/ cos(9), where 9 is the rotation angle, as expected for a 2D electron gas.

RESULTS AND DISCUSSION

Calculations of the subband dispersion relations

and related quantities have been made for the range

of areal electron densities between 0.1 and 5.0 x 10^^ cm"^. The x-value of the detector was taken to be

0. 191, with a corresponding energy gap of41. 1 meV,^^

which is representative of long-wavelength detectors

and equal to that ofthe detectors we report on below.

The background electron doping density was assumed

to be 3.9 X 10^* cm-^, which was reported for these

detectors, along with a bulk mobility of 2.5 x 10^ cmV

Vs at 77K. Bulk SdH oscillations were not observed

because of their low frequency, which implies that

they would have been observed only at low magnetic
Heavily Accunmlated Surfaces of Mercury Cadmium

Telluride Detectors: Theory and Experiment 989

fields where broadening effects greatly reduce their

signal strength. The temperature is taken to be 6K, at which the material is degenerate. As an example, the

built-in field and subband dispersion relations for an

areal density of 8.9 x 10^^ cm*- are shown in Figs, la

and lb, respectively. The siirface potential in Fig. la

is about three times greater than the energy of the

lowest subband edge and over five times the energy

gap. The spin splitting is evident in Fig. lb, and is greatest for the lowest subband. The density of elec- trons in the spin-up subband is about 15% greater

than in the spin-down subband for the lowest four

subbands. At the lower densities, this percentage

decreases somewhat, especially for the higher subbands, while at higher densities it remains nearly the same for all subbands. Note also that the deviation

fi-om a parabolic to a nearly linear dependence ofE on k IS clear for energies only about 10 meV above the

subband edges. The small oscillations in these curves

are due to 1% numerical uncertainty in the solutions.

The corresponding electron density in the ac- cumulation layer is shown in Fig. 2a for both the semiclassical result from the initial solution of Poisson's

equation and the final quantum-mechanical result

fi"om the subbands. The width of the accumulation

layer is seen to be about 0.1 jam. The latter density is greatly reduced at the surface because of the boundary condition on the wave functions. It goes to zero discontinously across the boiindary because of the

dependence ofthe wave function on the derivatives of

/j and /j, which undergo a discontinuous change from

a finite to zero value at the boundary. Therefore the

shape of the potential near the interface is different in

the two cases, and the value of electron density

obtEiined quantum-mechanically is less than that of

the initial semiclassical solution. The electron densities ofthe first four subbands are plotted as a function

of total density in Fig. 2b. We compute the total density from a sum over only the first four subbands,

for which we can perform accurate computations. We

estimate the error incurred by this approximation to be less than 1%. The relations are nearly Hnear with

average slopes of0.673, 0.223, 0.077, and 0.027 for the

first (n = 0) through fourth (n = 3) subband, respectively. The deviations from linearity are less than 1%.

These values differ somewhat from our corresponding

experimental values of 0.609, 0.258, 0. 101, and 0.032,

which have deviations from linearity of only a few

percent, although the experimental values may not be

completely accxirate because they are based on the

parabolic approximation for the density in the SdH

analysis. This near linearity shows that the shape of

the potential distribution is relatively insensititve to the magnitude ofthe surface potential. The difference

between theoretical and experimental slopes may

indicate that the potential distribution is somewhat

distorted, which can be caused by strain or band gap

narrowing.

The subband Fermi energies and cyclotron effective masses are shown in Figs. 3a and 3b, respectively, as a function of total density. The Fermi energy in the

bulk is computed to be 5.44 meV for an assumed bulk

density of 3.9 x 10" cm-^. The scatter in the mass

values is due to the derivative in Eq. 10. Although the

calculated eigenvalues appear relatively smooth in

Fig. lb they are only accurate to about 1%, and this

uncertainty as well as that due to the discreteness of

the k-values causes the theoretical masses to have

errors ofabout 5% occasionally. A more refined calculation would lead to better accuracy. The strong

variations of the masses with density attest to the

nonparabolicity of the dispersion relations, which

have an effect on the optimization of device perfor- mance. The serpentine shape of the curves is due to the strong curvature of the built-in potential.

We have made SdH measurements on a number of

FERMI ENERGIES

200

12 3 4 TOTAL DENSITY {iC'cm"')

CYCLOTRON EFFECTIVE MASSES

0.05

0.04 -

0.03

0.02 -

0.01

y0 0 ^ 1

B 2

B 3

b

TOTAL DENSITY (10"cm'')

Fig. 3. (a) Subband Fermi energies as a function of total density, measured from the bottom of each subband. (b) Ratio of subband

cyclotron effective masses to the electron mass at the Fermi energy as

a function of total density. Lines are cubic least-squares fit. The curves

are labeled by subband number from 0 to 3, and x = 0.191

.
990 Lowney, Seller, Thurber, Yu, Song, and Littler

FOURIER TRANSFORM OF SHUBNIKOV-DE HAAS DATA

0 10 20 30 40 FREQUENCY (T) a

FOURIER TRANSFORM OF SHUBNIKOV-DE HAAS DATA

, FREQUENCY (T) b

Fig. 4. (a) Fourier transform of SdH data at 5K for Sample 1 ; the label

"h" stands for harmonic; peaks are labeled by subband number, (b) Same for Sample 2, except at 2K. Actual SdH data for the signals as a function of magnetic field are given in insets and x = 0.191.

long-wavelength, commercial, n-t3rpe PC detectors

with total densities between 5 x 10" and 5 x 10^^ cm-^. The SdH Fourier transforms of two different repre- sentative detectors are shown in Figs. 4a and 4b along

with the actual SdH data, which show the oscillations vs magnetic field. The notations ' and " refer to different surfaces or to different regions on the same

surface since both surfaces ofthe detector were passivated. The detector in Fig. 4a was passivated with the

usual anodic oxidation process,^ while that in Fig. 4b was passivated by a different method. The separations of peaks due to spin-splitting are predicted by

our theory to be about half of that we observed, and

thus we expect that these multiple peaks are due to different surface densities. The splittings vary some- what from detector to detector as well. Thus, the effect of spin spUtting is just to broaden each peak. The

transform corresponding to the detector with lower

total density, Sample 1, in Fig. 4a, has clear, strong

peaks, while that for the one with higher density,

Sample 2, in Fig. 4b, has peaks that rise less above the

background. One reason is that the penetration ofthe

energy gap by the wave functions is much greater for the heavily accumulated case, especially for the lowest subband. There is a mixture of subband states bound to the conduction bcind and continuum states that traverse the entire detector. Thus, the layer of

electron accumulation is not well described by a simple 2D electron gas at high densities and small

energy gaps. The Fourier transform for a heavily

accumulated, narrow gap detector can have a broad

background of contributions from the range of 3D

states corresponding to the graded 3D electron density in the accumulation layer. The 2D states, which

are bound to the conduction band, appear as well in

the Fourier transform and rise above the background

because they contribute at only one frequency for each

subband. The complexity ofthe background signature

in Fig. 4b may indicate that there is nonuniformity in

the surface density as well because ofthe two surfaces

and multiple regions within a surface. Even though

the relative ampUtude of the oscillations of Sample 2 was almost ten times smaller than that of Sample 1, the Fourier transform successfully resolved the signature of the detector. We have computed the subband electron densities

from the measured cyclotron effective masses and

Fermi energies for these two detectors. The results

from the two surfaces were averaged because we

could not resolve the sphtting at higher temperatures. For sample 1, the measured data are: m^ =

0.022 ± 0.001, E° = 79 ± 4 meV, m; = 0.014 ± 0.001, and

Eji = 42 ± 3 meV. The corresponding theoretical values were obtained by finding the total density for which

the m*Ef values for the subbands best agreed with

experiment: m; = 0.023 ± 0.001, E? = 75 ± 2 meV, m;

= 0.015 ± 0.001, and EJ = 38 ± meV. The predicted total density is found to be 7.9 x 10" cm-^, which is considerably less than the value of 1.2 x 10^^ cm-^ that is obtained from the parabolic approximation for the

density. The agreement between theory and ex- periment is vdthin the assigned uncertainty vidth the

theoretical values for the effective masses slightly

larger than measured. Thus, we expect that the en- ergy gap may be smaller than the one we are using.

This may occur because ofband-gap narrowing, which

occurs at high densities because of the many-body

interactions between electrons. We estimate the bandgap reduction to be at most 10% at this density.^ For

Sample 2, we report the results for the peaks labeled

with " because the unprimed peaks were above our

calculated densities, and some of the single-primed

peaks were not as well separated from the background. We do not report the n = 2 and n = 3 peaks

because they are also not very well separated. The

measured data for the peaks labeled with " are: m* =

0.020, E? = 312 meV, m; = 0.010, and EJ= 277 meV.

The uncertainties for these nimibers are about 20%.
Heavily Acciomulated Surfaces of Mercury Cadmium

Telluride Detectors: Theory and Experiment 991

We obtain from the total density that gives best

agreement between our theory and the measured

subband m*Ef values: m; = 0.039 ± 0.002, E? = 160 ±

3 meV, m; = 0.029 ± 0.001, and E} = 80 ±2 meV. The

predicted total density is then found to be 3.4 x lO^^ cm-2, which is also considerably less than the value of

4.3 X 10^2 cm-2 from the parabolic approximation.

However, Sample 2, which was made by a different

process from the traditional anodic oxidation method^

that was used for Sample 1, disagrees with the predictions, because the measured masses are so much

smaller and the Fermi energies so much larger than

the predicted values. Band-gap narrowing would be

larger for these samples, probably in excess of 10% of

the energy gap, and the gap could become very small.

However, much more work is needed to understand

the cause of this disagreement, which may involve

strain as well. The relatively large Fermi energies

and small masses could imply that the built-in potential is more like a square well in these detectors with

heavy accumulation.

CONCLUSIONS

The dispersion relations for all the 2D subbands in the accimiulation layers of HgCdTe detectors have

been computed by solving the 8x8 matrix Hamiltonian for a large range of electron densities (0.1 to 5 x

10^- cm-2). The subband densities, Fermi energies,

and cyclotron effective masses have been computed as a function of the total electron density. The results show strong nonparabolicity and a breakdown in the

simple 2D model of the electron gas at high densities

and small energy gaps. The near linear dependence of

the subband densities on total density, which has

been observed experimentally, has been confirmed

theoretically. The agreement of our calciilations with

experiment is good at the lower densities and may

possibly be improved by inclusion ofthe effect ofbandgap narrowing. At the liigher densities differences are

large, and further work including the effects of bandgap narrowing and strain is needed to understand the

measurements. Models such as these provide a basis

to characterize the electron states in the accumulation layers of these detectors, which have a large

effect on device performance.

The direct application of these results to the characterization of detectors is not simple, however. The

properties of the accumulation layers constitute only one ofthe contro.lling factors in detector performance.

Others include the bulk electrical and optical properties, the type and quality of the contacts and packaging, and the effects from the method ofdelineating the

detectors, such as ion milling. The contribution ofthis

work has been to present a direct, two-terminal measurement that can be used to determine the electron

density of the accumulation layers of a detector. Failure to have sufficient accumulation can lead to increased surface recombination and degradation of

detectivity. Excessive accumulation, on the otherhand, can lead to a large shimt conductance, which also can

degrade detectivity. Thus, one can use this method to study the dependence of detectivity on the method of

passivation and the resiilting level of accumulation.

The optimum process can be determined, and subsequently this method C£in be used to monitor the

accvmiulation layers during manvifacturing and to monitor their stabiHty as a function of time.

REFERENCES

1. Y. Nemirovsky and G. Bahir, J. Vac. Sci. Technol. A7, 450

(1989).

2. Y. Nemirovsky and I. Kidron, Solid-State Electron. 22, 831

(1979).

3. R.J. Nicholas, F. Nasir and J. Singleton, J. Cryst. Growth 86, 656(1988).

4. W. Zawadzki. J. Phys. C 16, 229 (1983).

5. Y. Takada, J. Phys. Soc. Jpn. 50, 1998 (1981).

6. G. Nachtwei, P. Schulze, G. Gobsch, G. Paasch, W. Kraak, H. Kruger, and R. Herrmann, Phys. Status Solidi b 148, 349

(1988). 7. I. Nachev, Semicond. Sci. Technol. 3, 29 (1988).

8. G. Trankle, E. Lach, A. Forchel, C. Ell, H. Haug, G. Weimann,

G. Griffiths, H. Kroemer, and S. Subbanna, J. de Phys. Coll. €5,48,385(1987).

9. E. Kane, J. Phys. Chem. Solids 1, 249 (1957).

10. J. Sune, P. Olivo and B. Ricco, J. Appl. Phys. 70, 337 (1991).

11. S. Yamada, H. Asai and Y. Kawamura, J. Appl. Phys. 72, 569

(1992).

12. D.G. Seller, G.B. Ward, R.J. Justice, R. J. Koestner, M. W.

Goodwin, M. A. Kinch and J. R. Meyer, J. Appl. Phys. 66, 303

(1989). 13. D.G. Seller, J.R. Lowney, C.L. Littler and M. R. Loloee, J. Vac.

Sci. Technol. A8, 1237 (1990).
APPENDIX B

S€micond. Sci. Technol. 8 (1993) 888-910. Printed in the UK

INVITED PAPER

Review of semiconductor microelectronic

test structures with applications to

infrared detector materials and processes'^

J J Kopanski and C E Schuster

Semiconductor Electronics Division, National Institute of Standards and

Technology, Gaithersburg, MD 20899, USA

Abstract. The impact of microelectronic test structures, as they have been applied

to silicon integrated circuits (ics) and gallium arsenide monolithic microwave

integrated circuits (mmics), is reviewed. General principles for the use of test structures with possible applications to infrared (iR) detector technology based on HgCdTe and other materials are emphasized. The uses of test structures for Si and GaAs, test chip design methodology and some examples of how test structures have been applied for process control and to increase yield are

discussed. Specific test structures and techniques that have been applied to ir detectors are also reviewed The basic design considerations and measurements

possible with each class of test structure are discussed. The important experience

of the Si and GaAs industries, applicable to ir detectors, is that significant yield improvement is possible with improved process control using test structures. Increased research efforts to expand the applications of test structures to ir detector manufacture are indicated.

1. Introduction

Since the mid-1970s, the National Institute of Standards

and Technology has engaged in a programme to develop

well characterized test structures, test methods and data

analysis techniques for use by the semiconductor

manufacturing industry. Test structures are microelectronic devices that are fabricated by the same

processes used to manufacture semiconductor integrated

circuit products, and that are used to measure selected material, process or tool parameters by means of

electrical tests. Properly designed test structures can be

used to evaluate semiconductor materials, to determine

and monitor process centring parameters, to measure

critical device and circuit parameters (which can be input

to device models), to identify and quantify yield-limiting

defects and to assess processing tool performance. As they are used today, test structures are indispensable to the successful process development and manufacture of Si and GaAs devices and circuits. The use of

test chips speeds up process development, product

fabrication and test, and enhances the reliability of the product—all at decreased cost. The use of test structures

as part of a comprehensive test programme can ensure

that performance and reliability have been built into

circuits. The prominence of test structures is the result

* Contribution of the National Institute of Standards and

Technology; not subject to copyright.

of many years of experience with their use in the manufacture of silicon ics and, more recently, GaAs

MMICS. The next section contains a tutorial describing how

test structures have been applied to silicon ics and GaAs

MMICS. It is the intention of this tutorial to encourage

greater acceptance and use of test structures in ir detector manufacture by developing an understanding of the ways

test structures have had an impact on the larger silicon and GaAs industries. Elements of a hierarchical test chip

design methodology, which maximizes the impact of using test structures, are described. This bottom-up

approach to test chip design provides a blueprint for applying test structures to any semiconductor device

fabrication process. On the basis of experience with

silicon and GaAs test structures, a guide to selecting the appropriate kind and number of test structures, as well as principles for effective application of these devices, is provided. Three examples of how test structures have

been applied for process control and to increase yield are

included. The first two examples are from Si technology

and illustrate process/tool control using the analysis of

results of measurements on test structures. The third example describes how test structures suggested an improvement of an etch process for GaAs mmic fabrica- tion and illustrates the importance of using a high area

density of test structures early in process development.

In the third section, the current use of test structures and specialized measurements most commonly applied

0268-1242/93/060888 + 23 $07.50 © 1993 lOP Publishing Ltd
Review of test structures for infrared detectors

to IR detector devices, processes and materials are

discussed. The characterization concerns of ir detectors

are compared and contrasted with Si and GaAs circuits. Design considerations and the uses of each test structure

are discussed. The section is meant to be a comprehensive

listing of devices that are well described in the literature and that are thus candidates for inclusion in ir detector

test chips.

The final section contains a discussion of specific test structures and principles for the expanded application of

test structures which have the potential for enhancing ir detector technology. Areas where further study is needed

are outlined. Relevant principles from the current body

of test structure experience are used to suggest changes

in practices that could ultimately affect the manufacturability of ir detectors.

2. Using test structures effectively

This section summanzes test structure utilization from a

Si and GaAs perspective, but with the goal of featuring

ideas which should also be applicable to the ir detector

industry. First, some selected applications and implementations of test structures are presented to illustrate why and how test structures can be used. Then, a discussion of test structure methodology explains how

to develop successful test structure implementations.

FinaUy, case studies from the Si and GaAs industries are used to show how three types of problem, potentially

applicable to ir detector manufacturing, were discovered and solved using test structures.

2.1. Why test structures are used

Test structures provide a means for obtaining parameters

that can be used to diagnose, monitor and predict the performance of the fabrication process, the products

manufactured and the manufacturing equipment. Test

structure data can be used to estabhsh, improve or ensure

process control and product yield. The results of test structure data analysis may be used as feedback to the manufacturing process or as the basis for future development work.

Test structure data can be collected rapidly using a computer-controlled probe station. This enables many

samples of many types of parameters to be collected during processing without a significant impact on

manufacturing time. Test structure measurements also enable users to derive parameters that cannot be obtained

at all, or as economically, from other forms of testing. If the collected data are efficiently reduced and evaluated,

processing and yield-limiting problems can be detected and solved promptly, improving the economics of the manufacturing process. Test structures can be applied to processes, pro- ducts and process tools throughout the manufacturing

lifecycle—in development, production, technology transfer and buying and selling. The following discussion

describes some typical uses of test structures during these

times.During developmental phases, process control and

adequate yields must be established for successful production of products. Collected test structure data are evaluated to identify critical factors in the various process

steps that must be controlled and to detect problems that Hmit product yield. When diagnosing control and

yield-limiting problems, the user may also correlate

collected data with historical or simulated data. Collected data can also be used as input to modelling studies used

in extended developmental evaluations, such as new

process development, device and circuit modelling and

logic and fault simulation. During development and production, after process

control and adequate yields have been achieved, test structures can be used to monitor the critical parameters.

Collected test structure data are compared with data from previously verified lots to ensure that process

control is maintained. For a well controlled and well characterized process, test structure data can also be evaluated to determine if existing correlations can be used

to reduce the set of test structures needed to monitor the

process. In efl"orts involving technology transfer, comparison

of test structure data between participants is useful for verifying that the desired performance criteria are

successfully met on all manufacturing lines. This is important in developing multiple sources for materials, processes and products and in demonstrating the applicabihty of new processing tools. When buying and selling, performance criteria derived from test structures can be used as manufacturing

sp>ecifications. Such specifications help the seller to market products and services and enable buyers to make

comparisons when selecting materials, processes, devices,

circuits and processing tools. Test structures to support these applications can be

categorized into several classes according to their general purpose: to extract material, process, device or circuit parameters; to detect or quantify random faults and

processing defects; to establish product reliability and

yield; to formulate layout design rules; and to assess processing tool performance. The remainder of section

2.1 discusses typical user goals that can be achieved, and

the test structures, parameters and types of analysis

required. It also provides references for further reading.

The reader should note that, in many cases, the references (and their references) reveal assumptions, limitations or requirements which are beyond the scope of this discussion.

2.1.1. Test structures to extract material, process, device and circuit parameters. Material, process, device and

circuit parameters are either directly measured or

indirectly extracted from test structures. These characteri- zation parameters are then compared and correlated to diagnose and monitor processes or products. Characteri- zation parameters can also be used to build models for software simulation of processes, devices or circuits

889
J J Kopanski and C E Schuster

[1, 2]. Such simulations provide results which can be

used in evaluations to diagnose existing process problems, develop process enhancements or future processes, determine target and tolerance values or layout rules, or

design devices and circuits. The discussion below

identifies the major characterization parameters and test structures used to characterize materials, processes, devices and circuits.

Materials. Test structures can be used to determine

properties of starting materials, surface and interfacial regions, ion-implanted layers and deposited films. Here,

test structures provide an efficient and economical means

of measuring parameters that are otherwise impossible

or difficult to obtain, such as those requiring advanced

physical analysis techniques. Evaluation and diagnosis

of materials parameters are used to ensure the uniformities needed to support manufacture of functioning

devices, as they may correlate with performance factors

[3] or device quality [4, 5]. Since materials parameters

are obtained early in the processing, correlations with

other test structure parameters are timely indicators for monitoring and predicting product performance.

To determine starting material parameters associated with deep-level defects, an integrated gated-diode electro- meter is used [6, 7]. This test structure consists of a

gated-diode with a preamplifier and is used to measure

leakage current, from which both carrier lifetime and

surface recombination velocity can be extracted by using an equivalent circuit model. Information related to impurity concentration in the material is determined

using a Hall structure for the Hall mobility and a long

gate-length transistor for the drift mobihty [8]. For ion-implanted layers, test structures are used to determine the doping profile and information about

residual substrate impurities [4]. The dopant density and

depletion width can be determined using transistors, capacitors or diodes [9]. These structures also allow C-V

profihng to be done, which indicates further information

about drift mobility, the presence of traps, activation energy, carrier concentration, ideality factor and barrier height [5, 10]. Parameters such as conductivity and contact resist- ance are sometimes considered as measures of material

quality [8, II], but are more accurately measures of process quahty [12]. Regardless of this, these parameters

are critical to making good Hall measurements [II] and

must be evaluated when making Hall measurements. Test

structures and references for conductivity and contact

resistance measurements are found in the following

section.

Processes. Process parameter test structures provide

information about layer uniformity, layer interface conditions and feature definition resulting from the various process steps. These parameters are important

for making correlations related to monitoring, diagnosing

and predicting process and process tool performance. The

test structures most frequently used to provide process parameters include van der Pauw sheet resistors.

cross-bridge sheet resistors, interfacial contact resistors, potentiometric alignment structures, transistors, capacitors and diodes. The van der Pauw sheet resistor, cross-bridge sheet

resistor, interfacial contact resistor and potentiometric

ahgnment structure are implemented in each of the process layers where uniformity or particular target values are critical. The van der Pauw sheet resistor measures conductivity [4, 13-15]. The interfacial contact

resistor provides the interfacial contact resistance, which

is used to evaluate the quality of the contact formation

process [16-18]. A cross-bridge sheet resistor [14, 15, 19-21], which provides sheet resistance and allows

extraction of Hnewidth, is used to evaluate the quahty of

film deposition processes. The potentiometric alignment

structure [22, 23] provides a measure of the misregistration of photomask features between two conducting

layers. Alignment structures are also used to indicate process tool performance (see section 2.1.5) and to formulate or evaluate geometric layout rules (see section

2.1.4). Transistor, capacitor and diode structures provide

information about processing, as well as materials. For

example, threshold voltage data from transistor-based

test structures have been correlated with sheet resistance data to diagnose contamination problems [24]. Threshold voltage data have also been used to infer parameters,

such as dopant concentration, for which no direct measurement method exists, as described in section 2.4.1. Other process parameters commonly measured are oxide

thickness using capacitors, and leakage currents using

diodes.

Devices. Test structures for characterizing device para- meters are useful in process control and simulation. Like

process parameters, device parameters can be indicators of process quahty or product yield, as demonstrated in section 2.4.2. Device parameters also frequently correlate with other test structure data. Test structures for characterizing devices include transistors, diodes, resis- tors and capacitors that have the same layouts, operating

conditions and measurement conditions as the devices used to construct the ic product. This allows meaningful

comparison and correlation for monitoring, diagnosing

or predicting product performance. Such test structures can also be used to develop specifications for designers,

buyers and sellers. Parameters typically measured from field effect transistors include threshold or pinch-off voltage, saturation voltage, breakdown voltage, leakage current, trans- conductance, parasitics associated with device terminals and any other parameters needed for equivalent circuit models [12, 25]. Many of these parameters can be

extracted from diodes as well. Information needed by a

circuit designer to interconnect devices and assess loading

conditions can be determined using resistance, capacit- ance, delay and gain values measured from resistors, capacitors and transistors.

Circuit elements. Circuit characterization test structures

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Review of test structures for infrared detectors

are used to determine circuit performance and to verify or predict that functional circuits can be fabricated by a

given process [26]. They provide data that can often be

correlated with fundamental processing parameters, as

in the example of ring oscillator frequency and linewidth

[27]. They also provide parameters that can be used as product specifications. Basic circuits often used include

inverters, counters, amphfiers and oscillators. Typical

circuit parameters that are measured or extracted include frequency response, delays [28], gains, noise, drive

capability and power dissipation.

2.1.2. Test structures to detect or quantify random faults

(yield). Random fault test structures help users monitor

materials-based process faults, their failure rates and their

effects on product yield [24]. They provide information

about critical process steps such as etching, metallization and contact formation. Random fault test structures are

useful in that process problems unknown to a manufacturer, which also reduce yield, can be identified and

corrected, the performance of multiple manufacturers can

be compared and product acceptance criteria can be

specified. These structures are also used to determine

layout design rule constraints such as minimum spacings

for successful isolation or contacts (see section 2.1.4). The random faults to be detected are due to either a physical fault that causes a circuit failure or a parametric

fault that may or may not cause a circuit failure. Examples

of random physical faults are a pinhole in oxide, a break

in a metal line, or a gate-to-source short in a transistor. Random fault test stuctures are generally implemented as arrays of identical transistors, capacitors or

resistors. The test structure arrays are sized to permit

fault detection and to provide statistically significant data

for obtaining failure rates for the process. Based on the

results of continuity tests, these arrays indicate what

percentage of the total elements are successfully fabri- cated. Examples of random fault test structures include a capacitor array for detecting oxide pinholes, a

serpentine array of resistors for detecting breaks in metal

step coverage [29], a transistor array for dielectric integrity or transistor defect type and rate, and a comb

configuration of resistors for isolation integrity.

The arrays can be addressable, to allow location of

the sites of failures. As the array sizes needed for statistics are large, using addressable arrays to locate the faults makes visible mspection following fault location tract- able. Addressable arrays also enable possible clustering of faults to be observed. For example, in a mosfet array

[24], clustering of excessive leakage current and low

breakdown voltage were found. Random fault data have further utility related to test vector generation and logic simulation [1]. Logic

simulation performed with fault models constructed from

test structure data can indicate design flaws which could

be actuated by random faults. This permits faults to be found before completing product fabrication. Random

fault data also provide the basis for models used in fault

simulations that describe more than the classical stuckat-one, stuck-at-zero or open-circuit faults.

2.1.3. Test structures to establish product reliability. Reliability test structures [26] are used to identify faults due to environmental stresses such as temperature,

voltage, humidity and radiation. These stress-related

faults are most frequently manifested as problems in oxide

or metal that prevent the final product from functioning, or lead to its early failure. Like random fault test structures, reliability test structures are often implemented as arrays and provide failure-rate data. Capacitor

and transistor arrays can detect oxide problems by

determining dielectric breakdown, leakage current and

oxide charge density. Resistor-based structures, implemented as serpentines and combs, can detect opens and

shorts in metallization due to electromigration [30] and

corrosion.

2.1.4. Test structures to formulate layout design rules. These test structures are used to formulate the design

rules for a process. Test structure data relevant to width

and spacing of process features are evaluated to produce

the geometric layout rule set. Optimum widths and

spacings of the process features can be determined on

the basis of yield for each of the test structure types, which are implemented in various widths and spacings

[31]. The test structures used include cross-bridge

resistors for feature linewidth, alignment resistors for feature-to-feature ahgnment, isolation resistors for feature isolation [26] and contact arrays for contact window

opening sizes and spacing [7].

2.1.5. Test structures to assess processing tool performance. This class of test structures is used mainly to evaluate lithographic systems for performance or accept- ance of equipment, for operator performance, and for separation of equipment-related problems from other processing or materials effects. One strength of the test structures in this class is that they provide accurate and

rapid measurements needed for successful feedback

related to tool performance.

The cross-bridge can be applied in a variety of ways

related to its capability to rapidly provide accurate Hnewidths. It has been used to evaluate the linewidth uniformity of a wafer stepper system [32], a step-andrepeat camera [33], a mask ahgner [34], and to evaluate

the performance of a plasma descumming step intended

to improve linewidth uniformity achievable by an

electron-beam system [35]. An extension of the cross- bridge can be used to estimate proximity exposure effects in an electron-beam system [35]. A cross-bridge used in conjunction with serpentine and comb interconnect

resistors can be used to evaluate and improve the software and procedures for exposing electron-beam resist in the submicrometre regime [36].

Electrical alignment test structure measurements can

help to determine origins of alignment errors. Whether

errors are caused by operator rotation or translation, wafer warpage, photomask problems or the actual

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J J Kopanski and C E Schuster

alignment characteristic of the aHgnment equipment itself can be resolved through analytical methods and vector maps of measurements across a wafer [34].

2.2. How test structures are implemented

Test structures are commonly implemented on a semi- conductor wafer in one of four arrangements. First, discrete structures are placed adjacent to particular devices or products at selected locations on the wafer. Second, a collection of structures small enough to be

located on-chip or in the kerf (between products) area is used. These test strip-type implementations have various names, including test strip, test coupon, plug-bar and

pellet. Third, a test chip is formed from a collection of

structures and used in place of an entire product chip at selected chip sites on the wafer. These are usually referred to as drop-ms. Fourth, a high density of the chips on the wafer are test chips. Particular applications and details

for each of these implementations are described below.

Discrete test structures monitor specific parameters

of high interest in production environments. Known

correlations between the test structure and device or product must be well established for this implementation

to be useful. Uniformity of the correlated parameters must exist if discrete structures are used at only a few

locations on the wafer. Test strip-type and drop-in implementations are used

in production environments to assure process control. They are sometimes called a process control monitor

(pcm) or process validation monitor (pvm). Since they

consist of various types of test structures, they provide more information than a discrete structure and can be

used to evaluate wafer-to-wafer and lot-to-lot performance. Their advantage is that they collect a variety of parameters without using a lot of wafer area, so their use has a minimal effect on production economics. Because

these implementations monitor only specific wafer areas, process uniformity is a prerequisite for their use, as shown

in section 2.4.2. Test strip-type structures may be placed either at multiple locations on the wafer, near particular circuits that they are designed to monitor or in one location on

the wafer. A popular use of test strips is in Si foundries

to verify that the products have been correctly fabricated. Some GaAs manufacturers who need an on-wafer plating bar accompany it at the wafer flat with a plug-bar

implementation of test structures to ensure process

control. Drop-ins are usually geometrically placed on the

wafer. Some frequent placements include one chip per quadrant and one in the centre, or all the chips along

the diagonals in a cross pattern. High-density implementations are needed if evaluating intrawafer uniformity is critical. They are used in both production and development environments. Since

high-density implementations replace significant amounts of product area with test chips, they are suitable

for production environments only if they are applied as a process validation wafer (pvw). In a pvw, test chips cover an entire wafer, but only one or two wafers per lot

are pvws. Applied in this manner, pvws can be used for vendor qualification and circuit acceptance, where

lot-to-lot process and device uniformities are critical [37,

38]. In development environments, full-wafer or other

high-density implementations are used to evaluate

uniformities. Other high-density implementations

arrange test chips and product chips in checkerboard-like

patterns. This allows spatially sensitive correlation of process and product characteristics. Also, the process and

products can be diagnosed using the same mask and

possibly the same wafers. Several lots of several wafers each may be needed to obtain significant statistics when

establishing process control, building software models or applying built-in reliabiUty concepts [39].

23. Developing successful test structure implementations

The test vehicles described in the previous section are most successfully implemented when the user makes a

concerted effort to develop and document a test structure methodology. Development of a test structure methodology is a hierarchical process. The user needs to address

the elements of the methodology in this order:

(i) user specific application;

(ii) test structure designs;

(iii) measurement methods;

(iv) data analysis techniques; and

(v) test vehicle implantation.

When developing a methodology, users must consider many factors in their own environment in addition to the principles of the methodology discussed below.

Trade-offs should not be made that are counter to the most basic goals of test structure use. Test structures are intended to provide accurate and rapid diagnostic

information about manufacturing processes. Sufficient data need to be collected and reduced in forms that meet

user goals. Test structures must be compatible with the user environment, use a minimum of space, be easily fabricated and, most importantly, must address user

goals. These concepts have been effective for nearly 20

years and are expressed in more detail in one of the first documents on test structure methodology [40] and in a more recent one [12]. The test structure methodology has been developed

in the context of test structure data collection using a computer-controlled parametric test system with wafer probes that make electrical contact with the probe pads

of the test structure. Parameters are obtained by direct measurement from the test structure or by extraction from test structure measurements. The collected measure- ments are reduced to the desired parameters using

physics-based computations, statistical analysis, graphical techniques and modelling efforts. Further analysis

is performed using computer-based techniques to achieve

the user-specific application goals. For HgCdTe detectors, the same series of steps needs to be addressed while taking into account the so-far limited applications of automated probe stations with this technology.

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Review of test structures for infrared detectors

23.1. User-specific application. Until it can be clearly stated why test structures are needed, the data that must

be collected to meet goals cannot be specified, and test structure and test vehicle designs caimot be selected. The

initial consideration involves whether test structures are

to monitor well controlled production environments or

to support development environments. This influences

the test vehicle implementation. Knowledge of the

materials, process, devices and circuits helps to define the parameters that need to be evaluated and the precision

and accuracy required. This influences the test structure designs to be implemented on the test vehicle.

23.2. Test structure designs. Test structure designs should be modular and contained in a computer-aided

design (cad) test structure library. This allows efficient placement of the same designs anywhere on the test vehicle and ensures that designs on future test vehicles provide data consistent with established process history.

The designs should be integrated with modular prooe

pads [41] to ensure rapid collection of data and ease of

test code development. Kelvin probe pads should be used

to eliminate probe-to-probe pad parasitic resistance, thereby improving measurement accuracy. Test structures should be designed to minimize the

effects of process faults or fabrication limitations; otherwise, the test structure may not be functional. Only

well characterized test structures producing accurate, vahdated and consistent results should be implemented

unless the purpose is to assess new test structure designs.

Designs should generally use the same lay-out rules as the process, be tolerant of misalignments and use

conservatively sized features unless smaller sizes are needed to induce the random faults the user wants to quantify. Users should consider designing structures with

several sizes of linewidths, contacts and spacings,

including those smaller than current design rules, to

assess design rule limits and to enable the same mask to be used on future processes having smaller feature sizes. The designs and design constraints must be documented

so that the cad for the masks can be generated using the document, rather than relying on a collection of experts.

233. Measurement methods. Test equipment must be

available to provide the precision required for the test structures selected to meet user goals. For example, a

test structure such as the potentiometric alignment

structure requires that enough current be forced to measure only a small difference in voltage; this measure- ment requires microvolt resolution. If an instrument with

this precision is not available, making this type of measurement is likely to be useless. The hardware and software of the tester system need

to be validated to ensure quality measurements. Instru- ments should be regularly calibrated. The switching matrix and probe card responsible for connections used

in probing should be verified using tester system

diagnostics. Benchmarks and known artifacts should be

used to check that, under software control, the correct

currents and voltages are forced or measured at the right times and that sufficient settling times are allowed. Once measurements are made, their reproducibility should be ensured. This is done after the desired set of measurements is made, by retesting selected sites on the

wafer. For a pvw (process validation wafer) implementation, this usually means making complete test chip measurements at one chip site in each quadrant and one

in the centre. Similarly to the documentation for test structure

designs, measurement method documentation should

enable test code to be written and the measurements

made.

2.3.4. Data analysis techniques. Data management was

cited as a key problem when the first test structures were

being measured [40] and remains so today. Large

numbers of data can be easily collected, but presenting

them for analysis is not trivial. Before analysis related to the user goals is begun, two evaluations should be made.

First, retest data should be examined to ensure that the measurements are reproducible. If they are not, a deter- mination of the cause and remeasurement are needed. A second pre-analysis evaluation is then made to exclude

outliers [42] that will skew further statistical analysis. In performing further analysis, experts in materials,

processing and statistical engineering are needed. A

variety of graphical representations of data [40] is useful for analysis. Some of these, such as box plots and

wafer maps to show lot-to-lot and intrawafer variations and correlations, are illustrated in the case studies. These

analytical techniques are successfully used to address user

goals, but they are time-consuming and the results are somewhat analyst-dependent.

Advanced data analysis techniques based on machine

learning [43], neural nets [44] and expert systems [45]

are being investigated. They can address user goals in a more direct and consistent manner with varying degrees

of success. These methods are not intended to replace an analyst but to aid in improving the scope and

consistency of analysis.

2.3.5. Test vehicle implementation. The test vehicle implementation depends on the user goals identified in the application description. For process diagnosis and

modelling tasks, a full-wafer or high-density implementation is needed. For comparing or monitoring a well controlled and well characterized process, reduced implementations are adequate. If valid comparisons must

be ensured between manufacturers, a standard test vehicle

is needed. To lay out the selected test vehicle, the user must

decide what test structures should be placed near each

other to allow correlation and wafer mapping of the

material, process, device and circuit parameters of

interest. Designers should also prioritize the selected set of test structures so that if space limitations are encountered, the most important structures are not

omitted. If space and the nature of a structure permit,

893
J J Kopanski and C E Schuster

validation test structures should be included to cor- roborate test structure performance. For example, a validation alignment structure with a built-in offset, placed adjacent to the actual measurement structure, provides verification of the actual structure and is more

efficient than optical verification. Once a conceptual layout is specified, the use of a CAD cell library expedites layouts for the mask layers. During layout, the documented layout constraints should

be observed if library designs are modified, because failure to do so can defeat the intended purpose of the test structure.

2.4. How test chips are applied to solve problems

This section presents three case studies examining how

test structure data analysis techniques have been used to solve different types of process control problems. The

first and second examples relate to a Si-based process and the third to a GaAs process. In the first example, a

test structure serves as a direct process monitor for layer

uniformity. The second example shows how a test structure can be used as an indicator to monitor a process parameter for which no measurement method is known.

The third example illustrates using a test structure as a

yield monitor. These examples show the use of box plots, wafer maps and basic physics. They also demonstrate the

significance of intrawafer variations, lot-to-lot variations,

and the correlation and lack of correlation between

parameters. All the examples support the need for a process expert to be involved in the data analysis.

2.4.1. Solving process control problems in a Si-.gate process. Both of the examples in this case study are based on data from the same test vehicle [24]. For every lot of wafers manufactured in a radiation-hardened Si-gate

CMOS/SOS process, one process validation wafer (pvw) was

included. The purpose of the pvw was to identify critical parameters that could be used for process monitoring

and control. The pvw contained process-related para- metric test structures and random fault test structures. Data sets collected over about two years were evaluated

using statistical parameters, box plots and correlation of wafer map patterns. The first example application shows how a nonuniformity problem in a phosphorus implant layer that was unknown to the manufacturer was discovered and

found to be yield-hmiting. The next example shows how

a critical process parameter, the n-island doping concentration, for which no measurement method existed, was found to have a control and variability indicator in the p-channel threshold voltage.

Identifying a non-uniform layer problem. This example

illustrates the use of test structures as process monitors

for layer uniformity [24]. Contact resistance data for nearly 100 metal-to-n"^ contacts per lot showed wide

lot-to-lot variation, as shown in the upper part of figure 1. As the data for the metal-to-p"^ contacts, as shown in

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years, utilizing NBS-16 process validation wafers fabricated with a radiation-hardened Si-gate cmos process. The

average metal-to-n* contact resistance (top) shows large

lot-to-lot variations; however, the average metal-to-p* contact resistance (bottom) remains under control over the same time period.

the lower part of figure 1, showed httle lot-to-lot variation and both types of contact were defined with the same

process steps, lithography problems were not a likely cause of the variations. Further analysis involved looking

for a correlation of the metal-to-n"^ contact resistance with some other process parameter. Once a correlation was found with the n sheet resistance, as shown in figure

2, it appeared that a process step related to sheet

resistance was a cause. The rest of the diagnosis was to determine which process steps and measured parameters were related to sheet resistance. The sheet resistance was controlled by phosphorus

implants through a gate oxide. When wafer maps of gate

oxide thickness from a capacitor test structure were examined, a correlation with the sheet resistance was

found. This led to the conclusion that non-uniform gate

oxide caused variations in the concentration of the phosphorus implant, which caused the variations in the sheet resistance.

Identifying an indicator for a non-measurable parameter.

This example traces the development of a means to control a process parameter for which no measurement method was available [24]. Data collected from nearly 100 MOSFETs per pvw showed a distinct intrawafer

894
Review of test structures for infrared detectors

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Rgure 2. Similar wafer map patterns indicate a correlation between two process parameters from the NBS-16 process

validation wafer in lot A10 of a radiation-fiardened Si-gate CMOS process. Test results from sites with an 'X' symbol were used to calculate mean, median and standard

deviation and to produce the wafer maps of the metal-to-n* contact resistance (top) and the n* sheet

resistance (bottom).

Figure 3. The similar patterns in wafer maps from NBS-16

process validation wafers in lot A16 (top) and lot A11

(bottom) indicate minimal lot-to-lot variations in a process parameter (p-channel threshold voltage). Distinct patterns

in wafer maps indicate consistent intrawafer variation, indicating a process control problem related to wafer

orientation.

variation and minimal lot-to-lot variations in the p-channel threshold voltage, as depicted in figure 3. As

the intrawafer variation is notable with respect to the wafer flat, the cause is deduced to be related to some

processing step where the wafer orientation remains fixed. For this process, the lithography and epitaxial growth

steps must be examined further.

If lithography problems are the cause, the threshold

voltage should correlate to Unewidth data for polysilicon,

epitaxial and metal. Since none of these Unewidth wafer maps correlated with the threshold voltage wafer map,

the conclusion was that lithography problems were not

the cause. Evidence concerning the epitaxial growth process was

then investigated. From basic physics, dependence of the p-channel threshold on other parameters serves as a clue. The threshold voltage depends partly on the n-island dopant concentration, which is dependent only on the epi- taxial growth process. The threshold voltage also depends

partly on the gate oxide capacitance, and the surface state charge, both of which are obtainable from test structure

data. As no correlation existed between threshold voltage

and either gate oxide capacitance or surface state charge,

the answer appears to be in the process controlling the

n-island doping. Here, a logical explanation is found for the threshold voltage variation: the wafer is always placed

in the epitaxial reactor with the same orientation of the

flat, and the wafer map pattern observed is consistent with the expected variation due to depletion of the dopant

across the wafer during epitaxial processing.

Once the critical processing steps which affect the variabihty of the threshold voltage have been identified, control of the threshold voltage is then possible. Although

the n-island concentration could not be directly measured, the p-channel threshold voltage provides an

accurate indicator for its control. In a similar manner,

predictive indicators can be developed for important

parameters in any process.

895
J J Kopanski and C E Schuster

2.4.2. Solving a yield and yield-monitoring problem in a GaAs process. The utility of a test structure methodology

using high-density test structures and a comprehensive,

standard test chip was demonstrated during the DARPA/

Tri-Service Microwave/Millimeter Wave Monolithic

Integrated Circuit (mimic) Program [46]. As in the Si example, yield-hmiting problems that were unknown to a manufacturer were found because of the high-density

implementation of a comprehensive test chip. Because a standard test chip was used, lessons learned by one

vendor could be more easily learned by another. Although a cooperative effort such as this may at first seem to be compromising competitive advantages, the end result was that vendors had more to gain than lose and freely participated without proprietary worries. One

example that demonstrates these benefits follows. During the three-year Phase 1 effort, four contractor teams produced sets of test wafers in addition to product

development wafers. The vendors' processes were similar but not the same and all were in the later stages of development. With minor exceptions for processing

differences, all the vendors used a standard, comprehensive test chip in a full-wafer implementation with about

200 test chips per wafer. Each test chip included fets, parametric test structures and manufacturer-specific MMics. Each wafer also included manufacturer-specific

PCMs at five drop-in sites, at the middle and in each quadrant of the wafer. This implementation allowed

significant statistical analysis, correlation and comparison of material, process and device performance among

lots and between vendors. Including the vendor-specific

PCM enabled correlation to be made between the Phase

1 data and historical data obtained from test structures

that were designed differently but which measured the same parameters as the Phase 1 test structures. In the following example, the drain-to-source current,

for a 200 nm fet was examined. The data distribution from the full-wafer data was compared with that from the drop-in areas. Only 48.3% of the full-wafer values for I^^ were within the parameter target range, as indicated by

the black areas of the wafer map in figure 4. Also, the percent standard deviation (%o-) is an unacceptable

28.1%. Note that all five drop-ins are within the black

areas, indicating acceptable I^^ values. These data also have a much lower %(t of 10.7%. Had only the limited drop-in area data been considered, yield could have been

considered adequate, rather than poor. The variability

in the two data sets is further contrasted by the figure 4 box plots. The range of the two quartiles of data (the bounds of the box) around the median (line inside the box) is much larger for the full-wafer data than the limited

data.These high-density data raised a process control concern not evident with limited data. Investigation revealed that during the manual dip process for etching

the channel, I^^ data from the drop-ins were used as a

process control monitor. If the drop-in data did not meet

the target values, additional partial dipping was done to

'correct' the etch depth. The process of manually

correcting five areas of the wafer introduced the

Ids Yield

550 r- 500 i- 450 \r

250 ^ ^ 200 E- i

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Figure 4. Drain-source current, /^^. data from a transistor

test structure used In a GaAs mimic process. The map of

full wafer data (top) reveals poor yield, and the variability

in the full wafer data (box plot, bottom left) reveals poor

uniformity. The low variability in 1^^ from the limited area

drop-in site data (box plot, bottom right) illustrates how

insufficient test structure sites can be misleading.

variability and non-uniformity observable in the box

plots and wafer maps. This analysis prompted the manufacturer to replace the manual dip etch process with an automated spray etch process. This resulted in the improvements shown in figure 5. The full-wafer data yield increased to 83.1%, its %cr decreased to 8.0%, and the distribution and variability of the full-wafer and limited data sets became similar. This indicates good process

control for I^^. Now, data from the drop-in areas alone could be used to successfully predict yield. When this example was presented to the other manufacturers, some

pursued their own analysis in this area and also adopted

the automated spray etch process to improve yield. Thus, for processes where control has not been

demonstrated, high-density data can help manufacturers

to detect and solve yield-hmiting problems. In developmental or uncontrolled processes, relying on limited drop-in data could cause manufacturers to continue to expend resources on non-productive processing.

3. Current uses of test structures for ir detectors

This section describes the current use of test structures and specialized measurements most commonly applied

896;;
Review of test structures for infrared detectors

Ids Yield

Yield: 893 (1074) = 83.1%

83.1% 250 < Ids < 350

Percent std = 8.0%

550 -

500 f- 450 ^ _ t I 400 r 1 350 j- § 300 [ 250 f- 200

I

Figure 5. Following process modifications indicated from

test structure results, wafer maps of /^^ show improved

and controlled yield (top). Once the yield-limiting problems were controlled, the variabilities of the data from the full wafer (bottom left) and the limited area drop-in sites (bottom right) became comparable.

to IR detector devices, processes and materials. Section

3.5 presents a comprehensive listing of devices that are

well described in the literature and that are thus candidates for inclusion in m detector test chips.

IR detectors are fabricated as a wide range of devices based on a variety of materials, with a number of basic device structures, and encompassing everything from

discrete detectors to large infrared focal plane arrays

(iRFPAs). Most of the following discussion focuses on

photovoltaic (f>-n junction) diodes fabricated in HgCdTe,

the technology with currently the most pressing and

challenging applications for test structures. However,

other significant devices, such as photoconductive and

MIS detectors, and materials such as CdTe and silicon, are included as appropriate.

3.1. Similarities and differences with Si and GaAs ics

iR detectors share some concepts in common with silicon

ICS and GaAs mmics: they are produced with much the same equipment and processes, and they have similar types of device structures. Some irfpas are nearing the same complexity and dimensions as large-scale integra- tion (LSI) Si memory chips. Like memories, detector arrays contain many similar, regularly spaced devices.

However, there are pronounced differences between

IR detectors and ics. Infrared detectors are photosensitive

devices which interact with electromagnetic radiation,

similarly to Si photovoltaic solar cells. Many ir detectors require a narrow-bandgap semiconductor, such as HgCdTe, and must be operated at low temperatures. The

need to characterize the material and devices as a function

of incoming Hght and at low temperature has often necessitated that test structures be packaged and cooled

in a Dewar before measurement. Hgi_j.Cd^Te (HgCdTe) is a compound semiconductor whose electrical and optical properties are a

function of composition. Detector performance is thus dependent upon composition, and arrays of detectors with uniform responses require material highly uniform

in composition. Many detector apphcations also require

thin layers of active material, requiring a precisely controlled thinning process. HgCdTe materials are grown

by a variety of methods, including bulk growth,

liquid-phase epitaxy (lpe), metal-organic chemical vapour deposition (mocvd) and molecular beam epitaxy

(mbe). The evolving crystal growth technology demands

test structures and characterization procedures to eval- uate material quahty, uniformity and process effects rapidly and precisely.

Compared with Si, HgCdTe is easily damaged by

temperature and radiation. Many fabrication processes can change the properties of the material by distributing

composition or introducing electrically active defects. Test structures can be used to monitor such changes, as

in the use of mis capacitors to quantify ion milling damage

[47]. Damage caused by contact with mechanical probes,

especially in the active region of a device, can change the

electrical properties of the material. Compared with many semiconductor products, ir detectors are currently produced in low volume and with

yields that are often very low. The starting substrates have small surface areas and, for some applications, the highest quality material is not plentiful. As a consequence, manufacturers have been reluctant to devote large areas on each substrate or in each fabrication lot, to test structures. In fact, the completed ir detectors themselves

often serve as the primary test structures.

3.2. Characterization needs and concerns

ir detector material and producibility issues were

reviewed in the keynote addresses at the 1991 HgCdTe

Workshop [48, 49]. Here are a few of the key areas

that are of general concern for HgCdTe photovoltaic

materials, processes, devices and packages:

Materials. The uniformity of electrical and optical

properties that depend upon composition, defects and

dopant variations is a pressing concern for Hg, .^Cd^^Te. To achieve a uniform device response over large areas, a low density of bulk defects and rigorous control of

surface effects are required. With any photosensitive

device, the critical parameters which determine device response are those which describe the recovery of the

897
J J Kopanski and C E Schuster

semiconductor to equilibrium: minority carrier lifetime

T, diffusion length L and surface recombination velocity

So.

Processes. Surface effects often dominate ir detector performance. Characterization of the process by which

the passivation layer is formed and the quality of that passivation layer are therefore critical. The choice of

passivation is more complex for HgCdTe than for Si, with no obvious optimum process for all applications from

the wide variety of potentially useful native and deposited

insulating layers examined to date. Increasingly with future generations of detectors, performance will depend on the carefully tailored variations in material properties with depth. A variety of techniques is available to determine dopant profiles with depth (such as secondary-ion mass spectrometry

(siMS), neutron depth profihng and junction capacitancevoltage dopant-depth profiling), but existing nondestructive techniques are limited in their abihty to resolve variations between multiple nanoscale layers.

Devices. Characterization of the individual detector performance and variations among detectors is of obvious concern, and is often addressed with test structures, i.e. detector elements fabricated especially for

this purpose.

Infrared detectors are produced in four major classes of photon detectors (photoconductive, photovoltaic (p-n

junctions), metal-insulator-semiconductor (mis) and

Schottky barrier), each with different device characterization concerns. Photoconductive detectors (see the review by Broudy and Mazarczyck [50]) contain no rectifying

junctions, limiting the complexity of test structures. For

photovoltaic detectors (reviewed by Reine et al [51]), the

electrical characteristics of the p-n junction photodiodes must be determined, typified by the product of the photodiode dynamic resistance at zero-bias voltage and

junction area, RqA. Much effort has been devoted to understanding the origins of and controlling the dark

current and noise. Metal-insulator-semiconductor photocapacitors (reviewed by Kinch [52]) are characterized by their capacitance and conductance versus voltage behaviour. An RqA value is also an important figure of merit for mis detectors, as well as other mis properties

including: the interface charges, tunnelling currents and

the storage time of the mis device. Schottky diodes sue characterized by the Schottky barrier height.

Package. Practical detectors depend on sophisticated interconnect and packaging techniques for interfaces with

additional signal processing electronics, for interfaces with the incident signal and for permanent installation in Dewars. The rehability of the device, interconnect and

package is often characterized by elevated temperature

bake-out and thermal cycling.

33. Current approaches to test structure use For IR detectors, a larger emphasis has been placed on

the quality of the detector performance, rather than the

direct measurement of specific material and process parameters with test structures. Electrical characteristics and figures of merit of detector elements, such as RqA,

have been used as diagnostic tools. Measurements of material parameters have been developed using detector array elements as the test structure, i.e. using the RqA of

a photovoltaic detector as a measure of the lifetime of

the material [53, 54], and estimating diffusion length by

injecting current at one diode and determining the change

in leakage at an adjacent diode. Considerable pre-detector-fabrication characterization also takes place. For example, it is common practice

to make use of a 'fast diode' process. This is a minimum-step, fast turn-around, diode process which can

be fabricated on a comer of a substrate and be used to evaluate the quality of the material from the qualities of

the 'fast diodes'. Prefabrication characterization also commonly includes Hall effect measurements and many

of the non-test-structure characterization techniques

discussed in the next section. As far as test structures are concerned, a common

strategy has been to make use of a series of devices which

differ in one physical aspect and to see how that variation

affects the measured electrical properties. Devices of this type include: (1) variable area diodes, (2) constant area, variable perimeter diodes [55], (3) diodes with varying

contact metal areas [56] and (4) diodes with contact pads,

both on the diode and off the active area. As mentioned above, measurements are often not conducted until the devices have been packaged. Measure- ments on bonded devices from an m detector process are

often more repeatable and reliable than measurements

using a probe station. The background photon flux can

be difficult to control and quantify in measurements with

probe stations. Also, determination of crosstalk and 1// noise is much easier with packaged devices. However,

the increased time and cost of packaging devices has no

doubt hindered test structure apphcations in HgCdTe.

Recent advances in probe station technology incorporate

both low temperature and the use of a non-contact probe

adapted from a scanning tunnelhng microscope [57], and

may facilitate greater pre-packaging test. Despite the emphasis on other characterization procedures, test structures and test chips more along the hnes of those applied in the silicon ic industry have been

developed. Test chips consisting of both drop-ins for inclusion with product wafers and complete wafers of test chips for statistical parameter characterization are

available for use in the m detector industry. A layout of a test chip with a comprehensive set of test structures, courtesy of Rockwell International Inc.f, is shown in figure 6. This test chip, for an epitaxial, photovoltaic

detector process, contains a mini-array of detectors, variable area diodes, mis capacitors, hthographic Hall

t Certain commercial equipment, instruments or materials are

identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by NIST, nor does it imply that the materials or equipment used are necessarily the best available for the purpose.

898
Review of test structures for infrared detectors

Rgure 6. The comprehensive test chip for an epitaxial

layer, HgCdTe photodiode process designed by McLevige

at Rockwell International Corp., Science Center. (Courtesy Rockwell International Inc., with special thanks to M V

McLevige and J Bajaj.)

effect structures and transmission line test structures. Uses of each of these classes of test structures are included

in the discussion that follows.

3.4. Non-test-stnicture characterization Due in part to the deUcate nature of HgCdTe, various

optical and non-contact electrical methods have been

highly developed and applied to HgCdTe. In practice, these non-contact techniques play at least as large a part

as test structures in the characterization of the material and processes. A complete review of such techniques is not possible here. However, any discussion of test structures must take into account the functions of several prevalent non-test-structure characterization techniques. In particular, optical infrared transmission measure- ments are universally available to determine wafer composition (bandgap energy) and epitaxial layer thick- ness [58]. Instruments are available that can rapidly

produce compositional maps of bandgap energy at nine or more points evenly spaced across the HgCdTe surface. Photoreflectance [59] provides a non-contact and

low-temperature method with which to determine

optical transition lineshapes and information on composition and defects. Other commonly applied optical

techniques include: visible and far-infrared reflectivity, ellipsometry, electrolyte electroreflectance and laser- beam-induced current (lbic) images of electrically active

defects [60]. Also prevalent are non-contact methods to measure lifetime [61], such as optically modulated

absorption, microwave transmission or reflection techni- ques, and the frequency response of photoluminescence.

Destructive characterization techniques are also used.

Dopant density as a function of depth has been

determined by ion sputtering in conjunction with siMS, Auger and x-ray photoelectron spectroscopy [62]. A

simple measure of dislocation density and uniformity is the etch pit density (epd) following exposure to an etchant

[63]. Electron-beam-induced current (ebic), usually

measured in a scanning electron microscope (sem), is used

to determine minority carrier difi"usion length [53, 64].

3.5. Specific test structures Of the wide variety of test structures that have been

developed for Si and GaAs, a smaller subset has been

adapted for use with er detector technology. This section contains a comprehensive listing of the test structures and some of the test methods used to characterize ir detector devices, processes and materials. Design considerations and the uses of each test structure are

discussed. Most test structures were originally developed on the basis of Si technology. References in this section are made to applications to HgCdTe, along with a few key

references to the original applications in Si.

33.1. Detector array elements as test structures. Detector

array elements themselves, and mini-arrays (of around

five-by-five elements), are important test structures. Separate elements for test allow the detector response to be characterized without compromising a functional

array. Since the adjacent elements can limit the optical area and otherwise alter the response of a detector

element, mini-arrays are useful to determine device performance while surrounded by other functioning

detectors. For ease of interpretation, it is desirable for characterization to be made on detectors identical to working array elements. Variable-area diodes, discussed

in the next section, can also be used for this sort of measurement and offer the advantage of quantifying the perimeter and area dependence of any measurement.

Array elements have been used to characterize the detectors themselves, the noise and crosstalk, and to estimate minority carrier lifetime.

Detector characterization. Detector elements are quantified and compared by means of several well known figures of merit, which are discussed in detail in previous reviews

[51, 52, 65]. The response of an infrared detector to an

incident signal is characterized by the responsivity, jR, in VW~\ the quantum efficiency, tj, the relative spectral response at the cut-off and peak wavelength, and /i^^^y., in )im, and the time constant of the frequency response,

T in s. The signal-to-noise ratio of the elements is characterized by the noise equivalent power, nep, in W

Hz"^'-^, and the normalized detectivity, D*, in cm Hz''^ W~ ^ Capacitance-voltage dopant-depth profihng of the p-n junction photodiode has also been commonly

implemented [64, 66].

Noise characterization. The sources of current noise in photodiodes include: Johnson-Nyquist noise of the

899
J J Kopanski and C E Schuster

zero-bias impedance, IkT/R^^, shot noise, thermal noise due to thermally generated dark currents, and 1// noise,

which is characterized by a spectrum that varies as 1//", with a close to 1 [51, 67]. Much effort has been devoted to the characterization and reduction of excess 1// noise, a limiting factor in the performance of photodetectors and focal plane arrays [68-71]. Understanding the origins of 1// noise is critical to guide actions to reduce it. Measurement of the noise

versus frequency spectrum is straightforward (see, for example, the experimental set-up in [70]). Noise current

density is usually measured over frequencies in the range

of 1 Hz to 10 kHz, and also as a function of diode reverse

bias and temperature. The 1// noise has been correlated with detector bias, temperature, diode area and surface leakage currents [69, 71]. More detailed noise studies require a set of variable-area diodes, as discussed below,

so that the surface or bulk origin of the noise, the mechanism that generates noise, and the relationship

between noise and dark current can be determined [17].

Crosstalk determination. The influence of adjacent ele- ments potentially affects the spatial and frequency

resolution of detector array elements. The most straightforward method to quantify crosstalk is to measure the response of an element which is blocked to irradiation while operating adjacent (non-blocked) elements. Alternatively, a focused monochrome beam can be used to actuate individual elements while monitoring the res- ponse of adjacent elements.

Minority carrier lifetimefrom zero bias resistance product. For n"^ on p junction photodiodes, an estimate of minority carrier lifetime can be obtained from the measured RqA versus temperature, T, via [53, 54]

RoA = {NJqnf\kTlqy^\zJnyi^ (1)

where TV^ is the base acceptor concentration, q is the electron charge, k is Boltzmann's constant, is the minority electron lifetime and is the electron mobility.

The intrinsic carrier concentration, n„ is calculated from

expressions relating it to x, T and the bandgap, £g [54,

72]; mobility is obtained from published data, and

estimates of from Hall measurements before device

fabrication.

3^.2. Variable-area diodes. The perimeter-to-area ratio,

P/y4j, of a square or circular diode will increase as the length of its side or radius is made smaller. Thus, a

smaller-area diode will be more dependent on perimeter

or surface effects and less dependent on area or bulk

effects than a diode with the same shape but a larger

area. This natural separation of surface and bulk effects makes a series of diodes with varying areas a powerful

test structure for the evaluation of the potential

performance of infrared photovoltaic devices, as well

as the quality of the material, process and surface

passivation. Variable-area diodes refer to a series of three or more

p-n junction diodes, identical except for their area. Usually, the diodes are square or circular with junction

areas varying by about three orders of magnitude, for example a set of five diodes with areas from 9.0 x 10"^

to 2.5 X 10"^ cm^ [70]. Some variable-area-diode test structures have been designed with contacts off the active area (to eliminate damage induced by wire bonding) [70]

and with a biasable guard ring (to better define the

effective optical area of the diode) [73]. Variable-area diodes have been used to evaluate differences in diodes

formed in the mesa or planar structures, as well as the

qualities of different passivation procedures. A variation

of this test structure would be a series of diodes with

constant area, but variable perimeter (i.e. with different shapes) [55]. Parameters measured with variable-area-diode test structures include RqA, dark current density and/or noise current density, often as a function of temperature.

Cahbration of the actual fabricated diode areas versus designed diode areas would also be desirable. Variable- area diodes have been used to determine the P/Aj

dependence of {R^AY^, the area dependence of the noise

current, and the l/T dependence of RqA.

{RqA)~^ versus P/Aj. RqA^ has been modelled as a sum

of bulk-dependent and P/Aj proportional terms [74]: {RoA)-' = (/?o^)b"Jk + (<7"iSo lV,/V^.)(P/Ai) (2) where Sq is the surface recombination velocity in the depletion region where it intersects the HgCdTe surface, Wq is the depletion width at zero bias and F^j the built-in potential of the junction. The slope of the plot of (/?o^)

~

versus P/A^ is proportional to Sq, while the projected

intercept with P/A^ = 0 (corresponding to an infinite diode) gives (i?o^)buik- (^o^)buik is characteristic of the

film and should not be dependent on diode geometry or

process variations [75]. The degree that {RqA)~^ depends on P/Aj is a relative measure of the influence of surface originating currents on device behaviour. The ideal case

is for the lowest possible contribution from surface

currents, a low surface recombination velocity (Sq

approaching 0), and no P/Aj dependence.

A linear trend is only observed when hole diffusion

lengths, Z-h, are short compared with diode radii [75]. When Ly, is long compared with diode radii, a significant

component of the dark current will be due to lateral diffusion. In this case, smaller diodes will have a larger

proportion of diffusion current than larger diodes, and

(^0^)"' versus P/A^ will have a parabohc dependence

[75, 76].

If dark current is proportional to (RqA)'^, it would

be expected to show the same type of P/A^ dependence.

Noise current density versus Ay In an analogous manner,

variable-area diodes have been used to determine whether

noise currents in midwavelength infrared (mwir) diodes

originate at the surface or in the bulk [70, 77]. The

analysis is based on Hooge's empirical equation for the noise power spectral density, Sj, in p-n junctions [70, 78]:

S, = a^PlfN. (3)

900
Review of test structures for infrared detectors

Here, aH is a fitted parameter referred to as the Hooge

parameter, / is the current flowing in the system and A'^ the number of carriers. Noise current is the square root

of S,. If the noise is due to an area (bulk) source, then both / and N should var\' as the diode area, and noise

current density should depend on A^'^. If the noise is due

to a perimeter (surface) source, then both / and A'^ should

vary as the square root of diode area, and noise current

density should depend on /l^'*.

RqA and/or dark current density versus 1/T. The

temperature dependence oi RqA and dark current density can be used to separate the contribution of diffusion currents and generation-recombination (g-r) currents

to the total current [56. 70, 79]. Insight into the origin of the dominant current components in infrared detectors has been useful in optimizing device performance.

Common practice is to measure Rq in a temperature

range where the dominant carrier mechanism is changing.

A plot of Rq versus I T is then compared with hnes approximating diffusion current (l/nf temperaturedependence dommated)

i?o/lD.FF = {kT/q^)iNJnf)(rJD„y" (4)

and generation-recombination current (l/nj temperaturedependence dominated) [79, 80],

RoAc-K = [(Na/cJA) ln(N^No/nf)V'^izJqn,) (5)

where the subscripts diff and g-r refer to the contributions of diffusion and generation-recombination currents. resp>ectively. From the known behaviour of current-generating

processes, diffusion currents are expected to dominate at temperatures higher than around 50 K, while g-r current dominance is expected at lower temperatures. Deviations

from the g-r line at lower temperatures are often assumed

to be from unknown sources of (most likely) surface leakage [56, 79]. However, tunnelling is often the dominant current mechanism in HgCdTe. Measurements

on diodes consisting of n^ on bulk p-type Hg^.^Cdj-Te

(x =r 0.22) have shown that Rq is diffusion-current dominated at high temperatures, while at lower temperatures, trap-assisted and band-to-band tunnelling currents dominate depending on reverse bias and temperature [56,

66].An activation energy can be extracted from the inverse temperature dependence of dark current density and

noise current densities using an Arrhenius-type equation

[70]. The activation energy of the dark current should

relate to the energy bandgap, while the activation energy

of the 1// noise current has been related to the location

of g-r centres at about 0.75£g within the bandgap.

Mini or lithographic Hall effect test structures. Hall

effect measurements are of central importance to the characterization of detector materials as they provide

reliable values for carrier concentration and mobility. Hall measurements can be made on small, lithographically fabricated structures, as well as on the bulk samples.

Most interpretation has been developed under the assumption of sample homogeneity perpendicular to the magnetic field. Lithographic Hall bars provide data from

a more localized area. When fabricated as part of a detector process, lithographic Hall bars can be made from

both the substrate and any epitaxial layer(s). A variety of lithographic Hall bar and van der Pauw geometries

have been used [81, 82]. For a discussion of design

considerations for lithographic Hall test structures, see

[11, 83]. To make the measurement most useful, the key concern is interpretation of the Hall effect measurements and relating the measured electrical quantities to the semiconductor electrical properties. This is particularly

important for the case of two-layer structures [84], such

as epitaxial layers, which are common in detector technology.

More detailed magnetotransport measurements, as a function of magnetic field and temperature, are of great

interest for infrared detector materials because of the coexistence of multiple-carrier species [85]. For example,

for p-type HgCdTe, variable magnetic field Hall effect measurements have yielded the Hg vacancy ionization

energies, as well as carrier concentrations and mobilities

for the majority carriers (heavy holes), minority carriers (electrons) and light holes [86]. The lithographic Hall effect structure can also be used

in a variety of other measurements. Conductivity/

resistivity measurements are commonly made in conjunction with Hall effect. With the addition of a source of

illumination, Hall effect can be extended to the photoHall effect [87, 88] or the light-modulated Hall eff'ect [89], which provides a measure of electron mobihty in p-type material. Hall effect structures are also used for the photoelectromagnetic (pem) effect (which can provide

a measure of surface recombination velocity) [90-92],

Shubnikov-de Haas effect (oscillatory magnetoresistance) [93], and photoconductivity (lifetime) measurements. The Hall effect measurement has been used as a monitor of processing effects on materials, for example,

ion beam milling effects on the surface of HgCdTe [47].

3.5.4. Methods to determine lifetime. Carrier hfetime is an essential physical parameter for a complete description

of the response of any ir detector material. A variety of contact methods, or methods using simple test structures, have been employed in the measurement of carrier

lifetime in m detector materials. For a recent review of the issues and techniques for the measurement of lifetimes in HgCdTe, see the paper by Lopes et al in this issue [61]. An important issue in the measurement of lifetime is the distinction between

lifetime measured by transient methods versus steadystate methods [94]. Significant differences between

transient and steady-state lifetime have been detected and

are a function of minority carrier trapping [94]. The

steady-state hfetime appears to be more significant for device performance and a more appropriate measure of material quahty [94].

901
J J Kopanski and C E Schuster

Lifetime measurements are often conducted as a

function of temperature, carrier concentration and

composition. The dependence of the lifetime on these variables has been used to identify the dominant

generation-recombination process within the test struc- ture [64]. Minority carrier lifetime has been related to bandgap and carrier concentration [61]. The difference

in lifetime between impurity-doped and vacancy-doped

HgCdTe has been studied [94]. The temperature

dependence of lifetime has been used to determine the trap energy and density of bulk Shockley-Read centres

[64, 95]. The most common methods of determining

carrier hfetimes in HgCdTe include the following.

Photoconductive decay (pcd). In the photoconductive

decay technique, lifetime is determined from the slope or

1/e point of the current or voltage decay following

irradiation with a short laser pulse. The theory and measurement instrumentation is described in some detail

in [81, 82, 95-97]. The pair of (ohmic) contacts required

for measurement of lifetime by pcd has been obtained

from simple circular dots. Hall devices [94], a multiprobe

Hall bar with six sets of contacts spaced 5 x 10~-^cm

apart down the length of the sample [81], and using a

pair of plated electrodes, each covering about one-third

of the wafer perimeter, allowing mapping of lifetime over

a circular wafer [95]. The measurement can also be made

on an mis capacitor and ohmic contact, providing a measure of surface recombination velocity [98]. The

measurement is dependent on the spacing and size of the

contacts, which must be chosen to avoid minority carrier sweep-out [99]. Lifetimes measured by pcd and by

optically modulated absorption have been compared

[96], as well as lifetimes by the pcd, photoconductivity

and PEM techniques [91].

Diode reverse-bias pulsed recovery technique (prt). The

pulsed recovery technique determines lifetime by the recovery of the current following an abrupt reversal of a junction from forward to reverse bias. The theory and

instrumentation for HgCdTe photodiodes is summarized

in Polla et al [64]. A simple interpretation of prt

measurements, developed for Si diodes without necessarily abrupt junctions [100], is applicable to HgCdTe

[64]. Both p-n junction [64] and Schottky barrier [101]

diodes have been used in the measurement.

Steady-state photoconductivity. For the steady-state

photoconductivity measurement, the sample is illumi- nated with a laser or blackbody through a low-frequency

chopper and the PC signal measured by a lock-in [47, 91, 94]. The measured photoconductive signal is proportional

to fi„t„. The technique is dependent on a separate measurement of minority carrier mobihty. The light- modulated Hall effect has been used for this mobility measurement [90]. The steady-state photoconductivity measurement was then made with the same Hall devices.

Zero-bias resistance-area product. The determination of

lifetime from the RqA product of a p-n junction

photodiode [53, 54, 79] is described in section 3.5.1.

Photoconductivefrequency response {frequency domain or frequency roll-off). The lifetime of a semiconductor can

be obtained from the short-circuit ac photocurrent as a

function of light intensity modulation frequency response

of a diode [102, 103]. Photocurrent (or responsivity of a photodiode) is plotted versus chopping frequency. The

hfetime and fall-off frequency are related. The test structure is a photodiode or Schottky contact.

Photoelectromagnetic effect (pem). A discussion of the technique and considerations for its application to HgCdTe is given in [90]. In the photoelectromagnetic

effect, a magnetic field is applied perpendicular to a

diffusion current of optically generated excess carriers. Analysis of the pem current, /pem. versus magnetic field gives surface recombination velocity, carrier mobility and

bulk lifetime. The test structure used is the Hall bar. By

combining photoconductivity and pem measurements,

both bulk minority carrier Hfetime and mobility can be

obtained [91].

3.5.5. Metal-insuiator-semiconductor (mis) capacitors.

Metal-insulator-semiconductor capacitors can be fabri- cated along with photovoltaic detectors, as well as mis photocapacitor detectors. The capacitors can be formed on all layers of an epitaxial process. Information about

the semiconductor, the insulator, and the semiconductorinsulator interface can be obtained. The mis test structure should incorporate a guard-ring gate, surrounding the primary mis capacitor, to isolate the test structure from

the surrounding surface [104]. The various insulators and passivation schemes used

in infrared detectors have recently been reviewed [105].

Insulating layers for mis capacitors on HgCdTe consist of either deposited dielectric films or thin native films with an overcoat of a deposited dielectric. Popular

passivation materials for HgCdTe include: evaporated or

sputter-deposited ZnS [90, 106-109], anodic (native)

oxide (usually with an overcoating of ZnS) [104, 110, 111], anodic (native) sulphide (usually with an overcoating of ZnS) [90-92, 1 12-114], SiOj (deposited by several techniques) [115-117], evaporated Znj.Pbi_j.S [118],

SiN^ [105], anodic (native) fluoride [119] and other II-VI compounds such as CdTe [120]. The mis capacitor

is the essential test structure for the evaluation and

comparison of passivation and insulating layers of HgCdTe.

Like photovoltaic diodes, mis photocapacitors have

been their own most important test structure. Key

parameters are the RqA product and the storage time,

Tj,. Storage time refers to the time required for minority

carrier dark current to fill the potential well resulting

from a voltage pulse of the mis capacitor into deep

depletion [52]:

Ts, = Q.K/J, (6)

where C„„ is the insulator capacitance, Kthe voltage pulse,

C„jF is the well capacity and is the total minority

carrier dark current density.

902
Review of test structures for infrared detectors

Beyond detector characterization, the mis capacitor can be used for the determination and study of other

material, interface and process parameters:

Capacitance, conductance, and current versus gate voltage.

A large variety of standardized measurements developed

for Si Mos capacitors, as detailed in [121], has been

applied to mis capacitors on HgCdTe. Primarily, these involve the measurement of the capacitance, conductance

and DC current of the mis capacitor as a function of gate

bias, AC measurement frequency, temperature, time and

illumination. Properties of the insulator that can be

obtained include thickness, resistivity, breakdown electric

field, insulator fixed charge and traps within the insulator. The doping level of the bulk semiconductor is obtainable from the change in capacitance from accumulation to inversion, but more importantly, the formation and

dynamics of an inversion layer induced by the applied

electric field can be studied [112]. This can provide

insight into the minority carrier generation and recombination rates and mechanisms. At the interface, the

properties of the fast and slow surface states can

be obtained, as well as an indication of the metalsemiconductor work function [122] and the semi- conductor-insulator barrier height. Interface states in HgCdTe .MIS capacitors have often been quantified via the conductance method [1 12, 116, 123], or the high/low

C-K technique [117]. The C-V of MIS capacitors has also been used to

profile the carrier concentration of ion-implanted junctions in HgCdTe by fabricating an array of mis capacitors,

separated by 500 ^m, on a wafer whose surface has been

slant etched, resulting in a gentle grade in depth [91]. The properties of mis structures fabricated on doublelayer heterostructures, consisting of a 1 iim layer of wide-bandgap HgCdTe on top of a 5 ^m layer of narrow-bandgap HgCdTe, have also been reported

[124].

Unique concerns of mis capacitors from HgCdTe. Inter- pretation of electrical measurements of HgCdTe mis

structures is not as straightforward as for an Si mos

[104, 112, 125, 126]. To adequately explain the observed behaviour of mis capacitors on narrow-bandgap semi- conductors, the following factors have to be considered:

(i) the non-parabolic conduction band [104, 126];

(ii) degeneracy in the occupancy of the free carriers [104, 126];

(iii) comp)ensation and partial ionization of impurities and defects [126];

(iv) inversion layer quantization [104]; and

(v) interband or Zener tunnelling [104, 125].

As a consequence of conduction band degeneracy, the most obvious effect on the measured C-K characteristics

is that the capacitance does not saturate in accumulation

for n-type capacitors, or in the strong inversion for p-type

capacitors in the low-frequency regime [104, 126]. Oscillations in G-K response of mis capacitors on p-type

HgCdTe are explained as arising from indirect (trapassisted) tunnelling and inversion layer quantization

effects [104, 127]. With the application of a magnetic

field, the formation of Landau levels in the two- dimensional gas at the oxide/HgCdTe mis interface has been observed at temperatures around 10 K [128]. The

oscillations in the C- V and G- V response are shifted by an apphed magnetic field in a manner consistent with

tunnelling into the lowest Landau level associated with each electric subband [128]. Spicer has pointed out that due to the 'weak' Hg-Te bond, the bulk and surface properties of HgCdTe intercommunicate and are more

closely related in HgCdTe than in III-Vs and Si [129].

Defect and damage characterization. An important subclass of measurements concerns the determination of the

effects of radiation damage on the mis capacitor. Even

low levels of short-wavelength visible light can create

lateral non-uniformities in the fixed charge density of the HgCdTe-insulator interface [104]. Visual light exposure can result in a multiple flatband effect, though the effect can be removed by annealing. The mis capacitor has also been used as a monitor of ion-milling damage [47, 130]. Dislocations and hillocks have been observed to affect the storage time and tunnelling currents of mis capacitors on MBE-grown HgCdTe [110, 111].

3^.6. Gated p-n junction diodes and misfets. Gated

diodes and metal-insulator-semiconductor field-effect transistors (misfets) are the most complex test structures

in common usage. They require formation of p-n junctions, high-quality insulating layers and ohmic contacts.

Gated p-n junction diode. A gated p-n junction diode (gd)

consists of a p-n junction with a concentric mis gate

surrounding and slightly overlapping the junction at the

surface. A second, outer concentric gate is often employed

to isolate the gd from the surface beyond the inner gate

[115]. A gated diode is also formed by the gate and one

source/drain region of a misfet [131], though it is preferred that the gate completely surround the junction.

The gate(s) of the gd offer the abihty to precisely control

the surface potential around the p-n junction. As a test structure applied to ir detectors, gated diodes have been

used to localize and identify surface leakage effects and

currents, as well as to provide an indication of surface recombination velocity and lifetime. Typical characterization measurements include the reverse junction current (at a series of fixed diode reverse

bias) and the RqA product as a function of gate voltage.

The outer guard gate (if it exists) would be held at a constant voltage, usually to accumulate the surface as determined from C- F measurements. The R^A product

is also measured as a function of gate voltage and l/T. The gd has also been employed for C-F dopant profiling of the region beneath the junction [66]. The well established interpretation of gated-diode

reverse current versus gate voltage characteristics for Si gds is described in [132]. As with the mis capacitor, gds

in HgCdTe differ from those in Si; extensions of the

903
J J Kopanski and C E Schuster

existing theory to cover gds in HgCdTe are discussed in [51, 113, 115]. In narrow-bandgap material (x ss 0.22) and temperatures of 77 K and below, gated diode

behaviour is dominated by tunnelhng currents [51, 66, 113, 1 14, 133]. The surface potential determines whether

surface or bulk tunnelling prevails [113]. When the gate

is biased to cause accumulation of the surface adjacent

to the junction, surface tunnelling across the surface junction dominates. When the area adjacent to the junction has been inverted, tunnelling currents in the

field-induced junction dominate. For HgCdTe with a wider bandgap (x x 0.45 to 0.3) and at higher temperatures (145 K), gated diode characteristics are closer to those of Si gds [115]. When the gate is biased to cause accumulation of the surface adjacent to the junction, the gated diode current is dominated by trap and band-to-band tunnelling. When

the area adjacent to the junction is biased in depletion,

generation current from interface states will contribute

to the reverse-bias junction current. A measure of surface recombination velocity, Sq, at the intersection of

the depletion region and the gate, can be obtained from

the magnitude of the current in this region [115, 133]. The

calculation is complicated due to uncertainties in the

effective area of the induced junction [133]. When

the area adjacent to the junction has been inverted, the effects of surface generation are suppressed and the

effective area of the junction is increased by the

field-induced junction. The magnitude of the current in

this region is dependent on the minority carrier lifetime

in the metallurgical and field-induced junction.

Gated diode studies ofsurface passivation. The gated diode,

like the mis capacitor, is a useful test structure with which

to study surface effects. Measurements of noise and dark

currents in gated diodes as a function of temperature,

junction area and gate bias have been used to separate bulk and surface mechanisms contributing to l/f noise

[113, 134]. Gated diode characteristics have been used

as a monitor of fixed charge and interface trapped charge

density in long-wavelength infrared (lwir) HgCdTe as a function of gamma radiation total dose [133].

MiSFETs. Enhancement- and depletion-mode misfets in HgCdTe [107, 114, 131, 135-137] and a metal-semiconductor FET (mesfet) in CdTe [135], as well as simple

digital circuits [106, 136, 138] have been reported, misfets

in Hgi_^Cdj.Te offer some prospect for monolithic

integration of signal processing functions with detectors

[131]. The suitability of fets for use in electronics circuits

is commonly assessed from their DC characteristics (drain-to-source current as a function of drain-to-source

voltage and gate voltage, threshold voltage, surface mobihty, etc) and small-signal ac characteristics (drain- source resistance, amplification factor, transconductance,

etc) [106, 136]. Simple misfet digital invertor circuits on HgCdTe have been characterized at speeds to 1 MHz

[106]. misfets in HgCdTe have been used as test structures

to study quantum phenomena in reduced dimensions.

Subband spectroscopy, the observation of subbands due

to inversion layer quantization effects, has been reported

using gated diodes and misfets [107, 108]. misfets have

been used as a test structure for the quantum Hall effect [139] and magnetotransport measurements [107]. By

changing the gate voltage on an HgCdTe misfet, the magnetotransport of the two-dimensional electron gas has been modulated from classical free electron behaviour

to weak-localization behaviour [107].

3.5.7. Ohmic and Schottky contacts. Simple metal-to- semiconductor contacts are critical components of

detectors, and their behaviour must be characterized. Ohmic and Schottky contacts are also employed as test structures in a number of different types of measurement.

Theoretical models and experimental experience indicate that for X < 0.4, metal contacts are expected to be ohmic

contacts on n-type and Schottky barriers on p-type

Hgi_^Cd^Te [140]. For x > 0.4, metal contacts are expected to be Schottky barriers on both n- and p-type

material [140, 141].

Ohmic contacts. All detector structures require ohmic

contacts. High-quality ohmic contacts are essential for photoconductive detectors. Ohmic contacts are characterized primarily by their resistance, which can be divided

into components due to the semiconductor and to the contact resistance. Methods to determine the specific contact resistivity, the geometry-independent parameter

which describes the ohmic metal-to-semiconductor contact, are described in the following sections. Stable ohmic contacts have been used as test structures to measure minority carrier hfetime via the photoconductive decay technique, as well as for spreading

resistance [62], resistivity and contact resistivity. Resistivity can be related to doping concentration and, at room temperature, to the x value of Hgi_jCdjjTe [72, 142].The defect structure and surface chemistry play an

important part in determining the quality of an ohmic

contact to HgCdTe [143, 144]. Ohmic contact test structures have been used to evaluate the effects of the contact formation process, surface preparation, intentional interfacial layer formation and annealing [144].

Schottky contacts. Schottky barriers to HgCdTe are themselves another class of photodiodes [101] and a

critical component necessary before a mesfet can be fab- ricated [141]. Schottky contacts are commonly characterized by their current-voltage and capacitance-voltage

responses. From the /-K characteristic, the diode ideality

factor, n, reverse leakage current and breakdown electric

field can be determined. The Schottky metal-to-semi- conductor contact is characterized by the barrier height,

(/)b, which can be determined from I-V or C-V

measurements. The barrier height has also been obtained

from the activation energy of the variation of the saturation current with inverse temperature [101]. Dopant density profiles have also been obtained from

C- K measurements [145].

904:
Review of test structures for infrared detectors

1000 iim 't^gj.L^

BflHIIIHIin iHri<lllflrllllMlinillllllllhllHIIIIIMIIIHIMMHIinii HliniHIMliirH1liihM

3 oaw.

0.2330

0.2320

Rgure 7. Magnified image of x-value variations in solid state recrystallized Hg,_j,Cd^Te

produced with a high spatial density of identical test structures and automated data

collection. The region mapped is 0.5 by 1.0 cm. Each gray scale level represents a

variation in x of 0.001.

Schottky contacts have been employed to study the process of their formation and to evaluate the effects of metal/HgCdTe chemical reactivity on contact properties

[141]. In addition, Schottky barriers have been employed

in DLTS studies of CdTe [145], for determination of minority carrier diffusion lengths by ebic [101] and

determination of minority carrier lifetimes by the reverse recovery technique [64].

Spreading resistance. Information about dopant varia- tions and heterojunctions in HgCdTe has been obtained

by the spreading resistance technique, implemented with

mechanical point-pressure contacts along a wafer-level slant-etched surface [62]. Fine-scale variations in dopant

as a function of depth is of growing concern for next-generation detector structures grown by mbe. Spreading resistance measurements of HgCdTe can

also be conducted using an array of lithographic ohmic

contacts [142]. In concept, when combined with a

wafer-level slant-etched technique, like that used by

Fraenkel et al [91] with mis capacitors, a contact array may also be used for spreading resistance dopant depth

profiling.

Transmission line. The transmission line model (tlm) has been commonly applied for contact resistivity measure- ments of metal-to-HgCdTe contacts. The tlm can be implemented with the transmission line tap resistor, though simpler versions, like that described in [146], are

in use. In essence, the transmission line consists of a series of metal contacts with varying spacing, often four metal

rectangles with three different spacings are employed.

Resistance between the contacts is plotted as a function

of contact separation, with the contact resistance estimated as the resistance projected to zero contact

separation. A measure of semiconductor sheet resistance can also be obtained. The advantage of test structures based on tlm is their ease of fabrication: they can be realized by one

photolithographic step and etch of a deposited metal

layer. A disadvantage of transmission line-type test structures is that the total resistance is very sensitive to contact area variations and contains parasitic resistances due to current crowding effects. For low-resistance contacts on silicon ics, test structures incorporating a

single contact, such as a four-terminal contact resistor, have come to be preferred over tlm [16]. Contact

resistivity and test structures for its determination have

recently been reviewed [147].

Variable-area ohmic contacts. A set of variable area ohmic

contacts has been used to determine the contact resistivity via the method of Cox and Strack [148], and to quantify

the 1// noise of the contact [143]. The range of areas of the ohmic contacts was used to determine a dependence

of power spectral density of resistance fluctuations on

contact diameter of Sf^a:d~'", which can be related to the source of the 1// noise [143].

Contact array. The structure consists of an array of

contacts, precisely defined by photolithography through an insulating layer, and metal probe pads [142]. A

contact diameter of 5 |i.m and a contact spacing of 40 |im

have been used, though contact dimensions are limited only by photolithography. Two-probe spreading resist- ance, four-point probe resistivity and contact resistivity measurements can be made. Maps of HgCdTe resistivity variations with 120 |im spatial resolution have been

produced by rapid measurement of many contact sites with an automatic probe station [142].

Resistivity variations are then related to x-value

variations. The technique is illustrated in figure 7, in which the typically axial variations in x value, from a

cross section of an HgCdTe crystal grown by solid state recrystallization, have been mapped.

Figure 7 represents another class of test structure

applications, a refinement of the low-resolution waferscale maps shown in figures 2 to 5. Here, a high spatial

905
J J Kopanski and C E Schuster

density of identical test structures and automated data

collection are used to map variations in material parameters (resistivity and x value). The technique results

in magnified images, such as would be obtained from a microscope sensitive to resistivity and x-value variations.

3.5.8. Miscellaneous test structures.

Variably spaced diodes. Variably spaced diodes are used

to determine the minimum spacing at which adjacent

diodes short, thus providing an estimate of junction

spread. The same devices can be used to estimate diffusion length by injecting current at one diode and determining

the change in leakage at an adjacent diode.

Deep-level characterization techniques. A variety of techniques, which make use of the depletion region of a p-n

junction, Schottky barrier or mis capacitor, has been

employed to characterize deep levels in HgCdTe. These

include deep-level transient spectroscopy (dlts) [145,

149, 150], admittance spectroscopy (as) [150] and

thermally stimulated current (tsc) [150].

4. An expanded role for test structures?

This section contains suggestions for test structures with

potential applications to IR detectors. Section 4.2 contains

a discussion of how the silicon methodology can be

applied to HgCdTe ir detectors, as well as some practices

that the experience with silicon and GaAs test structures suggests would be beneficial if applied to ir detectors.

4.1. Test structures with potential applications to m

detectors The devices discussed in section 3 form a relatively

complete set of test structures for materials characterization, including some applications which are not usual for

Si, such as magnetotransport measurements. One test structure that has been developed and useful in Si technology, is the contact resistor [16, 18]. The contact

resistor is a single-contact, Kelvin-probe-type test struc- ture used for the determination of contact resistance and

contact resistivity. Contact resistors avoid the problems

with some parasitics and sensitivity to material nonuniformity that plague tlms.

In addition, some classes of test structures that are important in Si technology are scarcely mentioned in connection with HgCdTe. These are primarily test structures concerned with evaluation of photohthography and fabrication defect density, which can be

important for yield prediction.

Linewidth. Numerous variations of bridge and van der Pauw structures for the measurement of sheet resistance and electrical linewidth have been developed [14, 23]. These are useful for evaluation of both the etching and

photolithography processes used to define the different

layers of the process. Linewidth test structures are very

similar to photoHthographic Hall bars.

Alignment. Variations of sheet resistors can also be

applied as potentiometric electrical alignment test struc- tures [21, 23]. The residual misahgnment over a wafer,

possibly due to wafer warpage and non-planarity, or

misregistration, can be detected and quantified.

Meander contact chains. Contact chains are a series of

contacts with tabs of metal connecting every other pair of contacts. These can be short chains of contacts, taking

up a very small area, with probe pads every few contacts,

to determine if the process to open and form contacts is functioning as desired. Larger, addressable arrays of

contacts are also used to determine the yield of the contact process.

Step coverage. Step-coverage structures are simply

two-terminal resistors designed to evaluate metaloverlayer electrical continuity [24, 29]. A typical

step-coverage structure consists of a serpentine metallization line crossing a parallel array of epitaxial island lines at hundreds or thousands of steps. With the growing

complexity and reduced dimensions projected in next- generation IRFPAS, some of these test structures may find application for evaluation of step coverage.

Gate dielectric integrity array. The fault density of gate

insulators can be determined from an array of capacitors

[151]. The leakage current, caused by photolithographyinduced defects or dielectric breakdown of gate and

passivation insulators, is determined as a function of capacitor array size. Along similar lines is the random

access fault test structure, consisting of an array of

individually addressable misfets.

4.2. Strategies for extending applications of test structures The literature contains references to an abundance of test structures that have been applied to materials and process

characterization for ir detectors. The techniques to

fabricate the test structures and an understanding of the unique aspects of applying the test structures to HgCdTe

have been developed. These test structures have been

applied to a wide range of specific problems, most notably

to the isolation and optimization of sources of dark

current and noise in photodetectors. Major manufacturers of ir detectors have a range of test chips for their processes, which include some or many of the devices described above. However, information about specific instances of application of test structures for process

control is harder to come by.

The central lesson from the experience of the Si and

GaAs industries is that yield improvement requires

improved process control by means of the intelligent

application of test structures. The optimum test structure design and implementation for each appUcation can only be achieved through an extensive effort as part of process

906
Review of test structures for infrared detectors

development, guided by the methodology that we have

presented. The key elements of that methodology

translate into the following specific areas demanding

further study:

(i) Determine what information is needed from test structures by correlating detector performance and yield with material and process parameters.

(ii) Determine how that information will be obtained. Correlate the output from test structure measurements

with relevant HgCdTe material and process parameters.

Develop and document standard, well-characterized test structure designs as cells in a cad hbrary. Develop and

document a detailed test plan.

(iii) The information from test structures should get

to someone who can act upon it in a short period of

time. Develop and document detailed data analysis

procedures.

The experience of applying test structures to Si ics and GaAs mmics also indicates that changes in some

current practices and trends in IR detector manufacture

could produce positive results. Low required product volume cannot justify a low-yield

process. The relatively small numbers of devices required

and the nature of the market have not demanded large

yields to satisfy product demand or to ensure profitability. Even if production goals can be satisfied with

a low-yield process, the quaUty and reliabihty of

devices fabricated with a low-yield process could be

compromised . The substantial advantages of automated testing prior

to packaging should motivate development and applications of cryogenic probe stations with low pressure or noncontact (i.e. non-damaging) probes. The need to test devices at low temperatures and as a function of

illumination has often been most easily satisfied by

packaging devices before any testing. Consequently,

development and use of cryogenic automated probe

stations to acquire statistically significant numbers of data at the wafer level and before packaging (a major

apphcation of Si and GaAs test structures) have not been

vigorously pursued. Automated testing prior to packaging

allows a statistically significant number of data to be

rapidly acquired, and determination of the product

functionahty prior to the expensive packaging steps. Designated test chips, made to be compatible with

automated probing, could be included on production

substrates even if the final product were unsuited to prepackage probing. Probe measurements on these designated test chips will not compromise products.

An intelligently selected and applied set of test structures is more informative than finished product

performance in the diagnosis of which specific process steps may be limiting detector performance or yield. A larger

emphasis has been placed on the quality of detector performance rather than the direct measurement of

specific material and process parameters with test structures. Instead of optimized test structure measurements, the electrical characteristics and figures-of-merit

of detector elements have been used to deduce some

material parameters, and as a general indicator of device and process quality.

Low yield of a process justifies larger wafer area devoted to test structures. Early on, the poor quality of HgCdTe and resultant low yield of fabricated devices

has, paradoxically, encouraged a reluctance to devote

valuable HgCdTe surface area to test structures. Without

test structures to verify process and material parameters, poor detector performance or unexplained failure of elements in an array is often attributed, without evidence,

to low-quality starting material. Test vehicle (chip) design

methodology facilitates judgment as to the appropriate

number and kind of test structures, even for processes with very low yields. Cooperation between users, and standardization of test structure designs arui measurement methods, can result in gains in productivity beneficial to all. The experience of researchers at the National Institute of Standards and

Technology (NIST) can play a part in extending the current applications of test structures to ir detectors. Potential areas of impact will be to augment the design

and applications of the existing test structures, to develop new test structures aimed at specific process or material problems, and to develop comprehensive test structure implementation plans. Cooperation among the users can

further increase the impact of test structures on ir detector manufacturability. NIST and industry should work together to develop common test structures and

implementation approaches.

5. Summary

The experience of the sihcon ic and GaAs mmic industries has proved that the inteUigent application of test structures will result in significant yield improvement and

yield maintenance through improved process control. In

these industries, test structures are commonly employed

throughout the product cycle to determine and monitor

process centring parameters, to measure critical device and circuit parameters, to identify and quantify yield

limiting defects and to measure processing tool per- formance. An abundance of test structures has also been applied

to materials and process characterization for ir detectors. These test structures have been applied to a wide range

of specific problems, most notably to the isolation and

optimization of sources of dark current and noise in photodetectors. However, the ir detector industry does

not appear to employ test structures for process

control with the same discipline as the Si and GaAs

industries. We have described the uses of test structures, as

well as a methodology for implementing user-specific

applications of test structures. If these recommendations

are observed in the manufacture of ir detectors, real gains in yield, in reliability and in performance can be

expected.

907
J J Kopanski and C E Schuster

Acknowledgments

Thanks to all who shared their knowledge in developing

the compilation of test structures applied to IR detectors,

especially: Marion Reine, Loral Infrared and Imaging

Systems; Jagmohan Bajaj, Rockwell International; Scott Johnson, Hughes, Santa Barbara Research Center; Art Simmons, Texas Instruments; and Ted Wong, EG&G

Judson. Test structure data in figures 4 and 5 were

provided by the US Air Force, Wright Laboratory from

a DARPA/Tri-Service mimic Program contractor.

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910.
APPENDIX C

Semicond. Sci. Technol. 8 (1993) 753-776. Printed In the UK

INVITED PAPER

Hg^.j^Cdj^Te characterization

measurements: current practice and

future needs*

D G Seller, S Mayo and J R Lowney

Materials Technology Group, Senniconductor Electronics Division, National

Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Abstract. An extensive industrial survey of the importance and use of characterization measurements for HgCdTe materials, processes and devices has been completed. Seventy-two characterization, measurement techniques were

considered and thirty-five responses were received. This information was sought

for a study on materials characterization and measurement techniques of parameters and properties necessary to improve the manufacturing capabilities of HgCdTe infrared detectors. The nature of materials characterization is defined, and an overview is given of how it is related to improving m detector manufacturing. Finally, we present a description of the characterization survey and

a summary of the survey results. Major aspects of the results include: (1) rantcing the 72 techniques by their importance and frequency of use. (2) listing the parameters or properties determined by each technique, (3) enumerating the most

important properties that need to be measured. (4) indicating the key measurement techniques that most need to be developed, enhanced or improved, and (5) giving key overall comments

1. Introduction

Mercury cadmium telluride, Hgj _ ^Cd^Te, where x is the Cd mole fraction of the semiconducting alloy, is the

essential matenal for fabrication of intrinsic infrared detectors for a wide variety of military and space

applications. The focus of the materials technology has evolved from small, bulk-grown wafer material to

relatively large epitaxial wafers grown in special reactors with multiple capabilities. However. Hgj.^Cd^Te is a complex material, and because of the large number of array elements used for imaging and the wide range of

its uses, it may be ten times more complex than GaAs,

and even more when compared with Si [1]. In addition, the cost and afTordability of IR focal plane arrays are now

of critical concern and must be addressed along with the quality and performance required [2]. It is our belief that, in order to attain high quahty and top performance of

IR detectors at affordable costs, enhanced understanding

and use of key materials/process/'device characterization methods are critical. The development of new or improved diagnostic and screening techniques will have

an impact on device yields and lead to the establishment

of empirical and physical models necessary for quantitative prediction of detector behaviour. Finally, we stress

•Contribution of the National Institute of Standards and

Technology, not subject to copyright

that revolutionary new techniques might spring from the combination or revitalization of older ones. Improved ir detector manufacturing may also come

if US companies link their own R&D efforts involving

characterization measurements more closely to the production side. As pointed out by Reich, most American

technological firms draw a sharper distinction between R&D on the one side and production and marketing on

the other than Japanese firms [3]. In Japan, research, product development and the design of manufacturing

processes are carried out such that they are not isolated from each other. As a result, simultaneously, knowledge

from one area can readily influence decisions made in other areas. This paper attempts to describe the current import- ance and use of materials characterization methods now

being utilized for Hgi-^Cd^Te materials, processes and

devices as well as to present future needs. The paper is based upon an extensive industrial survey (72 characteri- zation/measurement techniques listed and 35 responses

received) carried out over the past two years. First, in section 2, we describe the nature of materials characteri- zation and related measurements by defining it in terms

of its activities. Next, in section 3 and in three appendices, we present a description of the survey and its results. Major aspects of the survey results presented here include:

(1) ranking the 72 techniques by their importance and

frequency of use, (2) listing the parameters or properties

0268-1242/93/060753-1-24 $07.50 © 1993 lOP Publishing Ltd 753
D G Seiler et al

determined by each technique, (3) enumerating the most important properties that need to be measured,

(4) indicating the key measurement techniques that most need to be developed, enhanced or improved, and

(5) giving key overall comments.

2. Materials characterization—nature of, activities and definition

Materials characterization is an important subject. Many

published papers, reports, activities and even organizational structures within industrial laboratories use these words readily. Unfortunately, materials characterization

is a rather complex area, and many types of definitions and common usage, often assumed, have evolved. It is the purpose of this section to review the activities and

nature of materials characterization, as well as to come

up with the best overall definition that encompasses all aspects of this topic.

Over the years, many people have adopted the

definition developed in 1967 by the Committee on

Characterization of Materials, Materials Advisory Board,

National Research Council: 'Characterization describes those features of composition and structure (including

defects) of a material that are significant for a particular

preparation, study of properties or use, and suffice for reproduction of the material' [4]. Hannay also seems to have adopted this approach in 1967—'In simple terms our definition of characterization means what atoms are present and where they are. This is all that is required

in principle, for characterization. . . . Eventually, however, one hopes to understand properties in terms of composition and structure, and to eliminate the need for a description of properties, as well as the method of preparation, in characterization' [5]. Unfortunately,

these definitions hmit the characterization methods to those that provide information about composition,

structure and defects, and exclude those methods that

yield information primarily related to materials properties, such as thermal, electrical, optical, mechanical, etc. Also, some workers use on a less frequent basis the terminology 'analysis of materials' in an interrelated way. For example, Meieran et al (1987) in defining this subject

say, 'The intent of analysis of materials is to understand

material properties in order to modify them to make

more usable, useful, economical objects' [6]. We give here what we feel is the broadest (and perhaps

best) possible definition for materials characterization

that includes its incorporation as an integral part of the manufacturing process; Activities that 'determine the structure and composition, properties, and performance of

materials, and the interrelationships among these elements'

Specific subcomponents relate to: '(7) structure and

composition, including the development of instrumentation and its application to determine chemical and geometric

descriptions of materials, from atomic to macro scales;

(2) properties, involving the measurement of properties and

their relationship to structure and composition; and

(5) performance, involving analysis of the behaviour of

materials in simulated or actual use' [7]. In most industrial laboratories, materials characteri- zation measurements reflect an exceptionally diverse, multi- and interdisciplinary set of activities. The back- ground and skills required to achieve enhanced yields of Hgi_^Cd;,Te iR focal plane arrays at low costs do not

coincide with those of the ordinary disciplines. The scope

of materials characterization in device production covers

a wide range of activities—quality assurance of incoming

materials, wafer screening methods, proper control and

monitoring of manufacturing processes, diagnostic and

failure analyses, and in playing the essential role of determining how a manufactured device differs from its intended design and function. Consequently, its import- ance is well established in the semiconductor industry;

e.g. 'a key to the continued progress in vlsi technology

is the refinement and development of new materials characterization tools. . . . The coupling of characterization methods to the fabrication process permits proper

control of the resultant product and the tailoring of material properties to specific physical requirements, for practical apphcations or for fundamental studies' [8]. 'Materials characterization and device development have

evolved in a synergistic partnership which has been

fundamental to the semiconductor industry' [9]. The materials characterization/measurement techni- ques for HgCdTe materials, processes and devices listed in the survey involve three major areas of characterization: chemical and physical (or structural), electrical and

optical. Figure 1 shows a simple overview of these areas, showing some representative examples, what the techni- ques determine and some general comments. In most

cases there exists a strong interrelationship between these areas or techniques. For example, the composition of Hgi_^Cdj,Te greatly affects the electrical and optical

properties, as well as the chemical and physical.

Consequently, relevant measurements in any of the three categories of techniques shown in figure 1 can provide a determination of the composition. Obviously, inter- compansons of techniques must be carried out by

correlating all the results.

3. Description of characterization survey

The survey was designed to measure the importance and

use of various characterization/measurement techniques

for Hgi_^Cd^Te materials, processes and tr detector

devices by industrial laboratories. An extensive list of 72

techniques that cover the vast majority of methods used

to characterize semiconductors was assembled. Note that

the authors realized that this list would not be exhaustive and so asked a general question in the survey as to what

measurement techniques have been inadvertently omitted

from the hst. In addition, for each technique, the respondent was asked to list the key parameters or

properties determined. Finally, four questions were asked

at the end of the survey as shown below. The format of

754
Hg,_^Cd^Te characterization measurements

the survey given out was thus as follows;

Technique Importance Use of Technique

Characterization/ Measurement Technique Dont Know

Not

Imp.

Imp. Very

Imp.

Daily Weekly Monthly Seldom Never Should Use More

Key Parameters

or Properties Measured By

1. Admittance

Spectroscopy

2. Atomic Absorption

Spe«roscc5py

3. Atomic Emission

Spectroscopy

4. Auger Electron Spectroscopy

72. Surface Topography

CHEMICAL AND PHYSICAL

Examples: TEM, SIMS, neutron diEfraction/scattering, X-ray

topography, phoioemission, SEM, STM, AES, EBIC, ESCA WDX EDX XRF. XRD. EXAFS, mass spearometiy

Comments: High spatial resolution (atomic)

Ability to identify elements and compounds

Rather complex equipment used

Interpretation of data frequently difficult Generally lower sensitivity Usually performed by specialists

Determines: Atomic coordination, composition, chemical

bonding, elements, stoichiomeiry. surface topography

ELECTRICAL

Examples: Hall, resistivity, mobility,

lifetime, C-V, DLTS, spreading

resistance, admittance spectroscopy, quantum magnetotransport

Comments: Very prevalent Both contacting and non-contacting methods

Determines: Carrier concentrations,

lifetime, mobility, resistivity (for pro61es and maps), carrier type, contaa resistance

OPTICAL

Examples: Optical microscopy,

ellipsometry, FTIR, PL, Raman, PR,

reflectance, modulation spectroscopy,

photoconductivity, LBIC, OMA

Faraday rotation, PEM,

magnetoabsorption, photothermal

spectroscopy

Comments: Contactless, high

sensitivity Some used routinely by nonspecialists

Determines: Optical constants;

epitaxial layer and insulator thicloiess; optical images of surfaces; impurity and defect type and size; composition;

stress/strain; damage/struaural

imperfections; wafer cleanliness; carrier

lifetimes

Figure 1. Some simple examples of materials characterization techniques representing chemical and physical, electrical and optical measurements. Also shown are some of the properties they determine along with some general comments. The acronyms are as follows: tem (transmission electron microscopy), sims (secondary ion mass spectroscopy), sem (scanning

electron microscopy), stm (scanning tunnelling microscopy), aes (Auger electron spectroscopy), ebic (electron beam

induced current), esca (electron spectroscopy for chemical analysis), wdx (wavelength dispersive x-ray analysis), edx (energy dispersive x-ray analysis), xrf (x-ray fluorescence), xrd (x-ray diffraction), exafs (extended x-ray absorption fine structure), c-v (capacitance-voltage), dlts (deep level transient spectroscopy), ftir (Fourier transform infrared spectroscopy), pl (photoluminescence spectroscopy), pr (photoreflectance spectroscopy), lbic (laser beam induced current), OMA (optical modulation absorption), rem (photoelectromagnetic effect).

755
D G Seller et al

Table 1. Top twenty ctiaracterizatlon measurements tor Hg^.^^Cdj^Te and their scores ranked in order of their importance and their use.

Imp. Characterization Imp. Use Characterization Use

rank measurement score rank measurement score

1 29. Fourier transform Infrared spectroscopy 60 1 26. Etching for defects 544

2 30. Hall pffprt 60 2 537

3 •) 1 1 1 tri il—viji lay

c

53 3 ijU. Mall nail ckffor^t eneci 510

4 26. Etching for defects 51 4 45. Optical microscopy 483

5 45. Optical microscopy 51 5 11. Current-voltage 452

6 7 49 6 •JO. n^dioU VI ly 368

7 62. ^oponHarv/ inn mace cnoflmmetrv 48 7 PdlipoUillt^li 1 1 i nc o mot r\/y 296

8 16. 45 8 7 278

9 Dhntnf*nnHi i^ti\/it\/ r 1 IfJlULrUI IUU<->II VI ly 40 9 lO. Double crystal x-ray rockinQ curve 250

10 58. Resistivity 40 10 6. Breakdown voltage 241

11 60. wV^Ol llllll^ dC7^^ll\^ll IIIIV^I \JO\^\J\Jj 39 11 43 193

12 24. Elllpsometry 38 12 49. Photoconductivity 191 13 6. Breakdown voltage 34 13 72. Surface topography: optical interferometry, stylus, scanning

tunnelling microscopy

174

14 43. Mos capacitance 32 14 60. Scanning electron microscopy 128

15 72. Surface topography; optical interferometry, stylus, scanning

tunnelling microscopy

32 15 31. Laser beam induced current 88

16 4. Auger electron spectroscopy 31 16 41. Microwave impedance 80

17 15. Double crystal x-ray topography 31 17 25. Energy dispersive x-ray analysis 74

18 19. Electron beam induced current 30 18 46. Optical modulation absorption 72

19 20. Electron spectroscopy for chemical

analysis 28 19 62. Secondary ion mass spectrometry 66

20 53. Photoiuminescence 28 20 71. Electron diffraction 65

Questions asked:

1. What are the most important (2-4) properties or parameters to measure or determine? Why?

2. What measurement techniques most need to be

developed, enhanced or improved? Why?

3. Any additional helpful, constructive comments?

4. What measurement techniques have been inadvertently omitted from this listing?

4. Survey results

The survey was distributed to numerous representatives

from industry and to several key personnel at the US

Army Night Vision and Electro-Optics Directorate. Two

anonymous responses were also received. Among the 35

responses received, eight companies were represented. Of

these company responses, the vast majority came from

four major HgCdTe focal plane array producers and

research laboratories. Multiple responses received from

the four companies accounted for about two-thirds of

the total responses received. No one company was 'over

represented' in the sense that the number of respondents

from each of these companies was 8, 6, 6 and 5. Consequently, the authors believe that the survey results are representative of the state-of-the-art characteri- zation practices used in manufacturing HgCdTe infrared

detectors.

The details of the survey results are presented in three appendices. Appendix 1 reports the ranking of each

chracterization measurement by its importance. It was

decided to weight a response that was checked 'very important' with a '2', 'important' with a

'1' and 'not important' with a '0.' The top 20 techniques and their scores are listed in order of importance in table 1. In compihng the use ranking, the weight for techniques used

daily was 20, weekly 4, and monthly 1. The top 20

techniques and their use score are also listed in table 1, with all the results tabulated in appendix 1. We note the high correlation between daily or weekly

usage and the importance of the technique. In some cases such as secondary ion mass spectrometry, the complexity

of the technique precludes daily usage, and thus the importance rank of 7 th is much greater than its use rank

of 19.Figure 2(a) shows a histogram of the number of techniques for the response for importance. If eight or more respondents said a technique was in a given

category, then it was counted in that category. We note

that the respondents were not familiar with a sizable portion of techniques. More strikingly, of al! the rest of

the techniques they were familiar with, very few (fewer than 10%) were marked 'not important'. Figure 2(6) shows a histogram of the number of techniques for the response for usage. Note that almost half are never used by more than eight of the respondents and that about

20% are used daily. In appendix 2 are listed the respondents' comments

to the key parameters or properties measured for each

756:
Hgi_j,Cd;,Te characterization measurements

Number ol Techniques (a) Number ol Techniques (b)

30

Category Category

Rgure 2. (a) Importance histogram for characterization techniques, {b) Usage histogram for characterization techniques. In each case a minimum of eight respondents is needed to include the technique in the category.

technique. Each respondent has been identified by a

separate letter of the alphabet. Since there were 35

respondents, double letters had to be used. As can be

seen from these results, the responses for each technique

ranged from 0 to 25, a greater number of responses

usually indicating an important or frequently used

technique. Answers to the four questions in the survey from each

respondent are presented in Appendix 3. We give

summaries of these answers in figures 3-6.

5. Summary and conclusions

To our knowledge, this is the first time such a

characterization survey has been done for HgCdTe. We

believe that results of this survey of the importance and

prevalence of characterization techniques in the HgCdTe

IR detector industry are important. We hope that they will stimulate and challenge existing concepts and practices

and even lead to the development or application of new

characterization techniques for the HgCdTe industry.

Properties/Parameters

alloy composition (Eg,Ag) carrier concentration

mobility

lifetime (minority carrier) impurities (trace, etc.) breakdown voltage

quantum efficiency

tellurium (second phase,

precipitates)

crystalline quality individual layer properties

interface/surface structure and

chemistry

dark currents

1/f noise

impurity content in starting materials defects (dislocations, etc.)

Needs for Improvement

NEED TO MEASURE PROPERTIES

ON SURFACE AND PROFILE IN TO DEPTH OF SAMPLE.

NEED TO KNOW

NONUNIFORMITY AND SPATIAL VARIATIONS.

NEED SUFFICIENT KNOWLEDGE

TO ACCURATELY PREDICT

PERFORMANCE OF REAL

DETECTORS.

Figure 3. Summary of most important properties or parameters to measure for HgCdTe.

757
D G Seller et al

anything related to device

performance (defects, impurities)

starting material analysis

processing characterization device parameter measurements

nondestructive techniques noncontact techniques

techniques that cover large areas

spatially resolved methods

scanning methods

mapping (of defects, concentration,

composition of thickness, lifetime)

in situ monitoring of grow'th processes

characterization of multilayer

structures material screening after growth and

during processing

defect mapping, by DLTS or scanning photoluminescence Te cluster detection and

identification defect detection

detection of p-type micro islands in n-,type matrix dark-current detection and

identification dopant nonuniformrties (sensitivity lO^'' cm-^) surface analysis techniques - surface passivation still limits performance

both qualitative and quantitative

techniques to determine Impurities or control trace amounts very early

in overall process

differential Hall effect Faraday rotation for nondestructive mapping of carrier concentration

laser scanning mass spectroscopy - a good impurity survey technique?

optical probes (PL, OMA, LBIC,

etc.) junction location, quality, and

profiling techniques

Figure 4. Summary of measurement techniques that most need to be developed, enhanced or improved.

HqCdTe analysis is a very drfficult problem. Most important question:

Why do I see what I see, and what

does this mean physically? Answer by carefully designing

experiments combining several

analytical techniques and proper

controls. Focus combined efforts of materials growth, test device (or structure fab), and materials and

device analysis.

Multiorganizational effort is highly

desirable.

Often, materials characterization techniques and device

characterization tools are developed separately. Closed loop

needed between materials parameters and device- performance parameters.

Process-control test features used

in Si and GaAs industries could be

transferred and/or modified for use

in HgCdTe processing.

Develop techniques for better analyses of defect states in MCI,

use as more routine

characterization tools.

Figure 5. Summary of general comments.

Establish clearinghouse of data

correlated to absolute (or, if secrecy

requires, relative) performance

parameters.

Establish agreed measurement

standards and cross-lab correlation

(as was done by NATO composition

exercise in 1980).

Standardization of surface

preparation and ambient during measurements generally lacking.

Any technique requiring >$100K

investment likely to be confined to occasional research and

corroboration of cheaper, less accurate techniques needed in production.

Need to improve detection limits of various analytical techniques such

as SIMS.

Needed a column entitled, "Relative Knowledge of Technique."

758 -
Hgi_^Cdj,Te characterization measurements

• Laser reflectance

• Laser microscopy

• Residual stress measurements in processed devices

• Positron annihilation

• Atomic force microscopy or scanning capacitance microscopy

Quantum efficiency measurements

• Contact resistance (especially for p-type samples)

• Contact and surface recombination

velocity

Open-circuit photovoltage decay

Transmission line measurements for contact, sheet resistance Variable-area-diode measurements RHEED analysis Electrochemical analysis

IR microscopy to image precipates

X-ray Laue back-reflection to determine single crystal orientation X-ray topography techniques other than double crystal

Imaging SIMS

Figure 6. Summary of measurement techniques omitted from list.

CHARACTERIZATION OF HgCdTe MATERIAL

INGREDIENTS AND

PROCESSING RAW

MATERIALS

SINGLE MATERIAL MATERIAL PROPERTIES

DEVICE USES

CORRELATION CORREWTION

STRUCTURE

DEFECTS

OUTPUTS OF ABOVE CYCLE BECOME INPUTS FOR CYCLE BELOW

DEVICE

MATERIAL STRUCTURE CHARACTH8ZH3

MATERIALS PROCESSES DEVICE PROPERTIES SYSTEM

USES

CORRELATE CORRELATE

STRUCTURE

DEFECTS CHARACTERIZATION OF HgCdTe MATERIAL SYSTEM

Figure 7. Diagram of the characterization activities for a single material and a material system (see [10]). Note the strong role of characterization measurements and the need for correlation activites.

The appendices contain all the detailed results from

the survey. Thus each person can individually interpret

and arrive at his/her own conclusions. However, the key

to success may be the adequate use and proper combination

of many of the techniques on the hst. It is worthwhile to remember that relationships exist among the structure, properties, synthesis, processing and performance of HgCdTe materials and devices as shown in figure 7 [10]. Once the material properties are characterized satis- factorily in the first part of the cycle, they become inputs

for the device cycle. Important questions need to be raised and answered: how are the HgCdTe composition,

structure, defects and electrical/optical properties

determined by the fabrication processes, and how, in turn, does this characterization determine the useful properties of the electronic devices? Both scientific and technological

issues and practices need to be thoroughly understood. Only then: (1) will HgCdTe processes and devices be capable of being controlled and continually improved,

(2) can we hope to understand the physical mechanisms

that affect all aspects of producing HgCdTe IR focal plane

arrays, and (3) can we hope to meet the cost and reliability requirements for HgCdTe systems. The top five measurements in order of importance

are (1) Fourier transform infrared spectroscopy, (2) Hall

effect, (3) current-voltage, (4) etching for defects and

759
D G Seller et al

(5) optical microscopy. The top five measurements in order of usage are exactly the same as for the importance

ranking, but with a redistribution: (1) etching for defects,

(2) Fourier transform infrared spectroscopy, (3) Hall

effect. (4) optical microscopy and (5) current-voltage.

The reasons for their high ranking can be ascertained from the key parameters or properties they measure, as tabulated in appendix 2. We summarize the information

obtainable from them and the reasons for their high

ranking. For Hgi_;,CdjjTe materials, the x-value is a very important parameter to determine because, in addition

to the temperature, it affects the energy band structure. The energy band structure, in turn, directly affects the

cut-off wavelength, intrinsic carrier concentration, carrier

mobilities, etc. The technique used more than any other

is infrared transmission, most often called ftir for Fourier transform infrared spectroscopy. The ftir measurement

has numerous advantages: it has mapping capability, it

is contactless and non-destructive, and it accurately

predicts cut-off wavelengths, determines layer thicknesses, and is thus capable of determining the x-value and other compositional information of epitaxial layers

[11].The Hall effect is one of the most important

characterization methods because of its wide application

in the determination of semiconductor resistivity, carrier concentration and mobility. Discussions of the Hall effect can be found in many solid-state and semiconductor

textbooks. It has become an excellent process monitor

for crystal growth with the purity, doping and homogeneity able to be correlated with growth conditions. In addition to bulk electron and hole densities and

mobilities, one can determine inversion and accumulation

layer properties, acceptor binding energies and compensation ratios from the low-temperature freeze-out of free holes, and energy gaps from the temperature dependence

of the intrinsic carrier concentration [12]. Current-voltage measurements to determine the behaviour of diodes are very important for characterizing

diode parameters. Two types of diodes may be distin- guished: p-n junction diodes and metal-insulator-semiconductor (mis) diodes. For modern-day infrared detection the p-n junction photodiode is the more important.

Surface and geometrical effects become increasingly

important as the detector size shrinks. Bulk defects can

have impact on the operation of large arrays. Variable- area diode data analysis is an essential tool in the characterization of HgCdTe infrared detectors [13]. Measurement of the zero-bias resistance-area product

{RqA) and its perimeter/area dependence provides critical information useful in separating surface and bulk effects and evaluating the quality of the surface passivation. The

junction quality can be ascertained and the diffusion.

generation-recombination and tunnelling mechanisms

found. Leakage or dark currents, as well as breakdown

voltages, are also important parameters that can be measured by this technique.

The physical characterization of HgCdTe crystals by

etching for defects is one of the major techniques for measuring the deviation from perfection of the crystal

lattice. The rate of reaction of a solution with a solid surface depends distinctly on the crystallographic orientation. The rate is also significantly affected by local stress caused by defects. Chemical etching proceeds more

rapidly in regions near dislocations or any other physical

defect than in perfect regions. As a result, etch pits are

often formed on the surface. Etch hillocks may also form

and can often be confused with pits in an optical

microscope. Use of a Normarski microscope in conjunction with preferential chemical etching and optical

microscopy is thus one of the easiest and least expensive

techniques for the determination of crystal defects in HgCdTe. Extended crystallographic defects such as dislocations, stacking faults, precipitates, voids and

subgrain boundaries can have a major impact on the performance of focal plane arrays.

Finally, we note that this paper provides a reference

for the importance and use of characterization techniques

in the HgCdTe community. It should be a 'handy'

document for scientists or engineers who want to know

what techniques are available, what they are used for, how important they are and how often they are used.

Acknowledgments

The authors acknowledge the partial support of this survey by the Defense Advanced Research Projects

Agency and the US Army Night Vision and Electro- Optics Directorate. The authors gratefully acknowledge the invaluable help provided by Nancy Seiler and Jane Walters in compiling the results of this survey and the expert

assistance of Tammy Clark. Finally, many thanks are due to the 35 respondents who took the time to fill out

the survey form.

Appendix 1. Importance and usage ranking of characterization measurements

In this appendix the 72 characterization measurements

are ranked by their importance (table A 1) and their usage

(table A2). Note that there is generally (but not always)

a correlation between the importance rank and the usage

rank.

760
Hgi_^Cd^Te characterization measurements

Table A1. Ranking of characterization measurements by their importance.

Technique importance Use of technique

Imp. mp. Characterization Imp. V. Imp. score Daily Weekly Monthly Use score

rank measurement I VI I + 2VI D W M 20D + 4W + M

1 29. Fourier transform infrared spectroscopy 6

2 30. Hall effect 4

3 11. Current-voltage 3 4 26. Etching for defects 9

5 45. Optical microscopy 3 6 7. Capacitance-voltage 11 7 62. Secondary ion mass spectrometry 10 8 16. Double crystal x-ray rocking curve 11 9 49. Photoconductivity 14

10 58. Resistivity 10

11 60. Scanning electron microscopy 15

12 24. Ellipsometry 12

13 6. Breakdown voltage 10

14 43. Mos capacitance 6

15 72. Surface topography: optical 14

interferometry, stylus, scanning

tunnelling microscopy

16 4. Auger electron spectroscopy 13 17 15. Double crystal x-ray topography 15

18 19. Electron beam induced current 12 19 20. Electron spectroscopy for chemical 10 analysis 20 53. Photoluminescence 12

21 67. Transmission electron microscopy 12 22 2. Atomic absorption spectroscopy 15 23 25. Energy dispersive x-ray analysis 14 24 31. Laser beam induced current 16 25 14. Deep level transient spectroscopy 15 26 61. Scanning transmission electron 11 microscopy

27 46. Optical modulation absorption 11 28 40. Mass spectrometry 12 29 12. Diode reverse recovery 11 30 13. Deep level optical spectroscopy 10

31 47. Photo Hall effect 10 32 64. Spreading resistance 7 33 8. Cathodoluminescence 14 34 71. Electron diffraction 6

35 54. Photoreflectivity 4

36 44. Neutron activation analysis 5 37 33. Low energy electron diffraction 4 38 69. X-ray photoelectron microscopy 6

39 70. Electron channelling 8 40 42. Microwave reflection 5

41 56. Raman scattering spectroscopy 7 42 57. Reflectometry 1 43 59. Rutherford backscattering spectroscopy 9 44 1. Admittance spectroscopy 4 45 3. Atomic emission spectroscopy 8 46 5. Beta-ray absorption/backscatter 4

47 21. Electroreflectance 4

48 41. Microwave impedance 2 49 17. Drift mobility 1 50 18. Eddy current 5

51 48. Photocapacitance 3

52 10. Current transient spectroscopy 6 53 65. Surface photovoltage 4

54 22. Electron energy loss spectroscopy 5 55 28. Faraday effect 5 56 32. Laser ionization mass analysis 4

57 37. Magneto-optics 2

58 68. X-ray fluorescence analysis 4

59 23. Electroluminescnce 3

60 34. Magnetoabsorption 2

27 60 26 4 1 537 28 60 25 2 2 510

25 53 22 3 452

21 51 26 4 8 544 24 51 24 3 483

19 49 12 9 2 278

19 48 1 9 10 66

17 45 11 6 6 250

13 40 8 7 3 191 15 40 18 2 368

12 39 4 10 8 128

13 38 14 3 4 296

12 34 11 5 1 241 13 32 8 8 1 193

9 32 8 3 2 174

9 31 2 2 1 49

8 31 2 4 44

9 30 1 3 3 35

9 28 2 1 5 49

8 28 5 5 25

8 28 1 1 8 32

6 27 1 5 9

6 26 2 7 6 74

5 26 3 6 4 88 4 23 3 3 6 23 1 2 22

5 21 3 3 72 4 20 1 1 7 31 4 19 2 2 48

3 16 2 1 9 3 16 3 3 4 15 1 1 3 27

14 3 3

4 14 3 1 1 65

4 12 2 1 41 3 11 1 20

3 10 1 1 24

2 10 1 1 5

1 10 2 3 11 2 9 2 1 1 45

1 9 0 4 9 3 1 64

9 2 2

2 8 2 40

8 1 1 2 8 2 0 1 41 2 8 1 1 5

3 8 4 80

3 7 1 1 1 25

1 7 2 1 1 45

2 7 1 20

6 2 1 9

1 6 0

5 1 1 5 1 1 24 4 0

1 4 1 1 5 4 1 1 3 0 2 0{Continued)

761
D G Seller et al

Table A1. (Continued)

Imp. rankCharacterization measurement

Technique importance

Imp. V. Imp.

1 VI

Imp. score

1 + 2VI

Use of technique

Daily Weekly (Vlonthly D W M

Use score 20D + 4W + M

61 38. Magnetoresistance 2 2 0 62 50. Photoelectromagnetic effect 2 2 1 1 63 51. Photoemission spectroscopy 2 2 0

134 27. Extended x-ray absorption fine structure 0 65 35. Magnetoconductivity 0

66 39. Magnetophonon spectroscopy 0

67 52. Photoinduced transient spectroscopy 0 68 63. Shubnikov-de Haas effect 0 69 66. Thermal wave microscopy 0 70 9. Charged particle activation analysis 0 0

71 36. Magnetoreflectivity 0 0 72 55. Photothermal spectroscopy 0 0

Table A2. Ranking of characterization measurements by their usage.

Technique importance Use of technique Use

Use Characterization . Imp. V. Imp. score Daily Weekly Monthly Use score

rank measurement 1 VI 1 -1- 2VI D W M 20D -1- 4W -1- M

1 26. Etching for defects 9 21 51 26 4 8 544

2 29. Fourier transform infrared spectroscopy 6 27 60 26 4 1 537

3 30. Hall effect 4 28 60 25 2 2 510

4 45. Optical microscopy 3 24 51 24 3 483

5 11. Current-voltage 3 25 53 22 3 452

6 58. Resistivity 10 15 40 18 2 368

7 24. Ellipsometry 12 13 38 14 3 4 296

8 7. Capacitance-voltage 11 19 49 12 9 2 278

9 16. Double crystal x-ray rocking curve 11 17 45 11 6 6 250

10 6. Breakdown voltage 10 12 34 11 5 1 241

11 43. Mos capacitance 6 13 32 8 8 1 193

12 49. Photoconductivity 14 13 40 8 7 3 191 13 72. Surface topography: optical Interferometry, stylus, scanning

tunnelling microscopy

14 9 32 8 3 2 174

14 60. Scanning electron microscopy 15 12 39 4 10 8 128

15 31. Laser beam induced current 16 5 26 3 6 4 88

16 41. Microwave impedance 2 3 8 4 80

17 25. Energy dispersive x-ray analysis 14 6 26 2 7 6 74

18 46. Optical modulation absorption 11 5 21 3 3 72

19 62. Secondary ion mass spectrometry 10 19 48 1 9 10 66

20 71. Electron diffraction 6 4 14 3 1 1 65

21 57. Reflectometry 1 4 9 3 1 64

22 20. Electron spectroscopy for chemical

analysis

10 9 28 2 1 5 49

23 4. Auger electron spectroscopy 13 9 31 2 2 1 49

24 12. Diode reverse recovery 11 4 19 2 2 48

25 42. Microwave reflection 5 2 9 2 1 1 45

26 18. Eddy current 5 1 7 2 1 1 45

27 15. Double crystal x-ray topography 15 8 31 2 4 44

28 5. Beta-ray absorptlon/backscatter 4 2 8 2 1 41 29 54. Photoreflectivity 4 4 12 2 1 41 30 1. Admittance spectroscopy 4 2 8 2 40

31 19. Electron beam induced current 12 9 30 3 3 35

32 67. Transmission electron microscopy 12 8 28 1 8 32

33 40. Mass spectrometry 12 4 20 1 7 31 34 64. Spreading resistance 7 4 15 1 3 27

35 53. Photoluminescence 12 8 28 5 5 25

36 17. Drift mobility 1 3 7 1 1 25

37 33. Low energy electron diffraction 4 3 10 1 24

38 28. Faraday effect 5 5 1 24

39 61. Scanning transmission electron 11 6 23 2 22 microscopy

762
Hgi_;,Cd;,Te characterization measurements

Table A2. {continued)

Technique Importance Use of technique Use Use Characterization Imp. V. Imp. score Daily Weekly Monthly Use score

rank measurement 1 VI 1 -1- 2VI D W M 2.\j\J -T 4Vv + M

40 44. Neutron activation analysis 5 3 1

1

1 20

41 48. Photocapacitance 3 2 7 1 20 42 70. Electron channelling 8 1 10 2 3 11 43 2. Atomic absorption spectroscopy 15 6 27 1 5 9 44 13. Deep level optical spectroscopy 10 3 16 2 1 y 45 lU. Current transient spectroscopy 6 6 y 46 69. X-ray photoelectron microscopy D lU 1 ( c0 47 21. Electroreflectance A4

o0 1 1 0 48 37. Magneto-optics 2 1 4 1 1 c0 49 14. Deep level transient spectroscopy 15 4 23 o O 50 4/. rnoto nail enect lU oO ID Oo

oo

51 8. Cathodol u m i nescence 14 14 oo o 52 59. Rutherford backscattering spectroscopy 9 9 d. o

£. 53 i. Atomic emission spectroscopy QO

QO

i

1 1 54 22. Electron energy loss spectroscopy 5 5

4

1

4

1 55 50 Photoelectromagnetic effect o o£. 1 i 56 68. X-ray fluorescence analysis 4

A4A

\

•A1 57 56. Raman scattering spectroscopy 7 1 y

ftU 58 65. Surface photovoltage 4 1 t3

ftU

59 32. Laser ionization mass analysis 4 4

ftu 60 23. Electroluminescence 3 o

ftu

61 34. Magnetoabsorption li o ftu

62 38. Magnetoresistance i. d.

ftu

63 51. Photoemission spectroscopy 2

ftu 64 27. Extended x-ray absorption fine structure 0 65 35. Magnetoconductivity 0 66 39. Magnetophonon spectroscopy 0 67 52. Photoinduced transient spectroscopy 0 68 63. Shubnikov-<Je Haas effect 0

69 66. Thermal wave microscopy 0

70 9. Charged particle activation analysis 0 0

71 36. Magnetoreflectivity 0 0

72 55. Photothermal spectroscopy 0 0

Appendix 2. Key parameters or properties measured

In this appendix the key parameters or properties

measured by each characterization/measurement techni- que are given. Each letter of the alphabet refers to a

specific respondent throughout. The more important and

highly used the technique, the more the responses

received.

1. Admittance spectroscopy

t. (On MIS) A/js, flatband voltage

w. Conductance

dd. Deep level analysis

ff. Interface state density

hh. Lifetimes, activation energies, transition rates

ii. Dark current mechanisms, activation energy

2. Atomic absorption spectroscopy

d. Impurities

f. Cu concentration in Te matrix, etc. Other impurites

g. Used to measure impurities in raw materials and

substrates. Buying instrument

j. Fe, Cu impurity concentrations in bulk CdTe

k. Dopant concentrations—typically In, masked by

"^Cd in mass spectrometry

n. Characterization of starting materials

o. Trace level impurities

p. Impurities, material and source materials & gases

(MOCVD)

r. Trace element analysis, we have a contract with VHG to improve technology

s. Impurity survey in starting materials, including

organometallics using icp

t. Low levels of impurities, good for impurities hard to detect by sims

u. Cu detection in substrates; other impurities: Si, Na, Li, etc.

V. Zeeman corrected. Impurities in films and

substrates

w. II-VI material purity

y. Composition

aa. Composition, impurity concentration

cc. Trace impurities in starting materials, substrates and epilayers. Use zcgfaas technique

dd. Trace element analysis of raw materials and

semiconductor materials

ee. Impurity levels. Need more sensitive technique

Continued

763
D G Seller et al

Appendix 2 (Continued)

3. Atomic emission spectroscopy

d. Impurities

f. Survey elemental Impurities

y. Composition

aa. Impurity concentration

dd. Trace element analysis of raw materials and

semiconductor materials

ff. Impurity concentrations

4. Auger electron spectroscopy

d. Composition profiles

f. Surface analysis and passivation profiles and

compositions

g. Occasionally used during array fabrication process development

j. Profiles of host elements

I. HgCdTe material composition and quality m. Surface analysis

n. Surface impurities

o. Surface composition, Interface chemistry

p. Spatial resolution high, useful In device surface

analysis, passivation, metallization, interface

r. Surface element analysis and bonding

s. Surface contaminants, test of substrate cleaning

t. Composition, Te precipitates, high levels of impurities

u. Surface composition

w. Surface composition

y. Composition, especially surfaces

z. Surface chemical composition

aa. Surface composition, depth profile

cc. Surface deposits, precipitates. Use scanning aes technique

dd. Surface analysis with good spatial sensitivity

surface contaminants

ee. Surface contamination

ff. Concentrations of elements on surfaces, in films or in bulk

5. Beta-ray absorption/backscatter

j. Layer thickness (production line)

s. Thickness measurements when surface morphology rough

t. Nondestructive thickness determination

w. Heterojunctlon profiles dd. Contactless layer thickness, metallization thickness

ee. Thickness of films on reference surface

6. Breakdown voltage

d. Empirical QC factor

f. Breakdown voltage in mis

g. Measure of device/junction quality

h. Used mostly for mis HgCdTe detectors

j. Some Indication of diode quality—part of /l/analysis

s. Indicates presence of defects that will enhance

tunnelling currents; also test passivation

t. (On diodes) carrier concentration

u. Breakdown field

V. Routine characterization for mis devices

w. Breakdown field dd. p-n junction quality ee. Breakdown voltage usually higher than

requirement. I-V measurements more useful

ff. Insulator breakdown strength

hh. Material quality (tunnelling or bandgap states)

ii. Dark current mecanlsms, material defects

7. Capacitance-voltage

d. Process control/doping

f. MIS properties, capacitance

g. Determine surface parameters

mis. Determine

junction profile—photodlodes

h. Junction doping for p-n junctions

I. Diode carrier concentration

j. Near junction concentration, interface states,

VpB. other mis properties (versus temperature)

I. Material doping estimate m. Fixed charge, Interface state density, doping profile

n. Well capacity, carrier concentration

p. MIS device characteristics, passivation

s. Good technique for measuring carrier concentration

in wide bandgap material

t. (On MIS, diodes) carrier concentration near the

junction, flatband voltage, N^^

u. Fixed charge density, carrier concentration

V. Routine characterization for mis devices

w. Threshold voltage; carrier concentration

aa. Carrier concentration

cc. Electrical conductivity, carrier concentration

dd. Carrier concentration, quality of passivation layer

(flatband, hysteresis, etc) ee. Base layer doping extrapolate from C-V,

capacitance Itself

ff. Carrier concentration, flatband voltage, hysteresis,

high/low frequency characteristics

hh. Material quality, surface passivation quality

ii. Carrier concentration

8. Cathodoluminescence

f. Defects in substrates

g. We're going to try some measurements

j. Some indication of CdTe or wide band mct

dislocations and impurity properties m. Structural quality of CdTe films

0. Subsurface defect density, carrier concentration

variations

r. We have used in past, may again in future

s. Indicates presence of electrically active defects

but limited to mwir or wide bandgaps

u. Electrical activity of defects such as dislocations and p/n junctions w. CdZnTe substrate uniformity

aa. Structural defects in substrate material (CdTe,

CdZnTe, etc)

cc. There is no commercial source of cl service in CdTe or CdZnTe so far as we know. HELP

dd. Imaging of defects of CdZnTe, have not yet used it for HgCdTe

ee. Lifetime too long to precisely locate junction

(diffusion length too long)

9. Charged particle activation analysis

10. Current transient spectroscopy

d. Lifetime and junction (R^) product

dd. Deep level analysis

ii. Dark current mechanisms, activation energy

11. Current-voltage

d. RqA

f. mis properties, G-V, dark current versus bias

h. p-n junction characterization

1. Many diode parameters

Continued

764
Hg,_,Cd^Te characterization measurements

Appendix 2 (Continued)

j- Provides key insignt botn into diode performance ano incundiiioiiio \vcr5us lemperaiure^

1 m

.

L-caKayc ouiiciii, udiK curreni in pnoiouioues

P- Key device performance parameters, RqA etc for pnOlOVOIlalC OIOQc

r. Diode IV characteristics

s. nyH, uicai\uuwii vuiidyc, piiuiuoui r cm

t (On diodes) origin of lealoQe currents

u. RcA

w.

aa. Diode properties

dd. Junction quality ee. /?(,, Ra(V), /?series Model diffusion, G-R. and

tunnelling mechanisms

ff. RqA. leakage current

hh. Dark current mechanisms, breakdown properties

11. Dark current mechanisms, activation energy

12. Diode reverse recovery

d. Lifetime after diode processing

g- OMA and pc-rolloff to determine lifetime

h. Carrier lifetime

i. Minority carrier lifetime

j- Some idea of minority carrier lifetime near junction (versus temperature)

1. Diode lifetime m. Minority carrier lifetime

t. Lifetime in depletion region

aa. Diode properties

dd. Recombination lifetime in diode; difficult for thin- base diodes

ee. Lifetime

ii. Lifetime, activation energy

Deep level optical spectroscopy

d. Check on process, deep levels, not routine (simpler than dlts) m. Defect density, energy level and capture cross

section

r. Optical determination of impurity levels

s. Midgap states

t. Deep levels

cc. Nature of traps

dd. Deep level analysis

ee. Ej

ff. Energy of trap levels

ii. Activation energy

14. Deep level transient spectroscopy

d. Check on process, deep levels, not routine

f. Au in Si and GaAs

m. Defect density, energy level and capture cross

section

n. Impurity levels

r. Looks at impurity, vacancy levels. Seems

problematic for hct

s Midgap states, levels and concentrations

t. Deep levels

V. Desirable to correlate with device performance

w. Traps in wide bandgap ll-VI materials

dd. Deep level analysis

ee. Trap levels, cross sections (expensive to use)

ff. Energy of trap levels gg Defect levels

ii. Activation energy

15. Double crystal x-ray topography

d. Defects—much too time-consuming

i. Defects

j. Some insight into defects and strain

k. Cd(Zn)Te(Se) substrate quality (substructure,

inclusions, precipitates) m. Structural quality over large areas

n. Crystalline perfection

p. Crystal quality bulk substrates

r. Surface topograph

s. Spatial variations in crystal quality

t. Spatial structural uniformity, defect structure

u. Crystalline perfection, precipitates, dislocation types and arrangement

v. Crystal perfection—films and substrates

w. Structural perfection of ll-VI materials

y. Surface roughness

cc. Crystal perfection, defect topography

dd. High resolution imaging of defect strain field ee. Surface crystal lattice quality

16. Double crystal x-ray rocking curve

d. FWHM of epilayers and substrates

g. Determine quality of layer, crystallinity and lattice matching with substrates

i. Crystal quality

j. Basic crystal quality of epi

I. Substrate quality; substructure, dislocations m. Structural quality of CdTe and HgCdTe layers

n. Subject to interpretation

p. X-tal quality of epi

r. Wo have a Brimrose X-ray mapper. It produces

both topographs and rwhm rocking curve maps

s. Crystalline quality of epilayers, tilt, strain, composition, dislocation density

t. Crystallinity

u. Microstructures/grain boundaries, etc.

V. Crystal perfection—films and substrates

w. Structural perfection of ll-VI materials

y. Structural perfection

z. Crystal lattice perfection

aa. Structural properties/crystal quality

cc. Gives number related to crystal perfection. Must

be in conjunction with x-ray topography or etch

pitting dd. Crystal quality, fwhm of substrates and epitaxial layers, lattice mismatch

ee. Crystal quality

ff. Linewidth, crystalline perfection

gg. Crystalline defects

hh. Epilayer quality, substrate quality

17. Drift mobility

j. Basic to assessing materials electrical properties (versus temperature and field) m. Minority carrier mobility for photoconductors

ee. Mobility

18. Eddy current

f. Electrical conductivity

V. Non-destructive measurement of conductivity in films w. Measure lpe liquidus

cc. Electrical resistivity ee. Resistivity (no instrumentation readily available)

ii. Carrier concentration Continued

765
D G Seller et al

Appendix 2 {Continued)

19. Electron beam induced current

d. Investigation of junction location; check on BAD

devices on fpas in certain configurations

f. Diffusion lengths, electrically active defects, mapping, junction depths

g. Location of electrical versus metallurgical junction

i. Junction position

]. Junction depth, hidden junctions m. Diode junction location, diffusion length

o. Subsurface defect structure, junction depth

s. Indicates junction location and electrically active defects

t. Junction depth in diodes

u. p/n junction depth

w. Type variations in HgCdTe

dd. p-n junction location on cleaved diodes

ee. Electrically active defect mapping, junction

location, x-value, limited spatial resolution

hh. Diode contours, junction profiles

li. Electrically active defect

20. Electron spectroscopy for chemical analysis

f. Passivation compositions

g. Used during process development to obtain chemical information (residue after etching,

etc.)

j. Helps assess surface contamination and processes

I. HgCdTe material

n. Material analysis

o. Surface and interface chemistry

p. Chemical surface and interface information. Compound formation during epi growth,

metallization, etc, non-destructive depth profile MBE interface, electronic parameters

r. Use along with Auger

t. Surface chemistry

w. Surface analysis—cleanups, passivations

z. Surface chemical constituents

cc. Surface chemistry, surface deposits

dd. Surface analysis of insulators; HgCdTe composition

depth profile, interface between insulator/ semiconductor

ee. Surface contamination

ff. Chemical composition of surfaces and films

21. Electroreflectance

j. Tells something about surface composition and

properties, but not too clear how useful

I. HgCdTe composition

n. Results questionable

' s. X profile with depth

t. Composition, E^, Aq, crystalllnity aa. Composition, bandgap

dd. Composition, carrier concentration if calibrated. Requires ohmic contact to sample

ee. MCT x-value

ff. Bulk composition from E^, material quality from

linewith F

II. Composition, maybe carrier concentration

22. Electron energy loss spectroscopy

o. Low Z element surface analysis, electronic

states and chemical bonding

s. Bonding of surface atoms

dd. Used in tem analysis to determine composition

ee. Surface crystal diffraction patterns

ff. Vibrational energies of phonons and adsorbates

(high resolution mode)

23. Electroluminescence

d. Check on junction quality

t. (p-n junctions) radiative quantum efficiency ee. More useful in other material systems

24. Ellipsometry

g. Thin film thicknesses and index, mct surface state

h. Surface preparation monitor for various parts of processes

j. Tells surface condition, composition, film structures m. Film quality in detector. Passivation

n. Film thickness and uniformity

p. Non-destructive mbe growth control

r. Optical constants, film thickness

s. X profile with depth, measure dielectric layers

t. Surface conditions, bandgap

u. Index of refraction for insulators. (Not useful for MOT?)

w. Surface cleanliness, film thickness

y. >p and A, n, film thickness

z. Substrate surface contamination

dd. Excellent measure of surface cleanliness

ee. Surface cleanliness, film thickness

ff. Film thickness and refractive Index

hh. Surface quality, surface contamination

25. Energy dispersive x-ray analysis

f. LPE compositional profiles g Occasionally used during process development.

Would use more if had a good in-house one

j. Gives gross confirmation of compositional profiles m. Determination of X-value in HgCdTe layers

o. X-value, compositional uniformity

p. Film composition average, e.g. X-value

r. Maps of wafer X-value, impurity maps

s. Composition analysis in thick films

t. Compositions and their depth profile w. Chemical mapping on devices

y. Composition

z. Layer composition

aa. Composition

cc. Chemical nature of precipitates, Zn % czt, chemistry of deposits

dd. Destructive measure of composition; routine for failure analysis of processing problems

ee. 'Bulk' composition and impurities

ff. Bulk composition of Hg,_^Cd;,Te

26. Etching for defects

d. Defects

f. Dislocation and Te precipitate density

g. Qualify substrates and check growth process

i. Defects

j. Tells dislocation density of layer—essential to materials improvement

k. Te precipitates, dislocations, substructure

(orientation A/B) m. Indirect assessment of crystalline quality

o. Etch pit density

p. Defect measure, substrate film

r. We don't do this, but others use a lot for material

dislocation density

s. Dislocation density

t. Defect structure, epd

u. Dislocations, Te precipitates

Continued

766
Hgi_j,Cdj,Te characterization measurements

Appendix 2 (Continued)

V. Dislocations—pits—inclusions in films and

substrates

w. Dislocation density In HgCdTe and CdZnTe. Damage

y- Etch pit density, A-B face (111)

z. Crystal lattice perfection

aa. Defects (structural)

cc. Defect densities, dislocation nature and

distribution dd. Best technique available for determining

dislocation density

ee. Correlation of etch defects with device performance

ff. Dislocation density, substructure

hh. Material quality, dislocations and other defects

ii. Dislocations, microstructure

27. Extended x-ray absorption fine structure

28. Faraday effect

f. Local carrier concentration

u. Is this rotation? w. Carrier concentration In HgCdTe

ff. Carrier concentration

gg. Carrier concentration (under development)

hh. Material uniformity, Nq — N/^

ii. Carrier concentration

29. Fourier transform infrared spectroscopy

d. Cutoff, thickness

f. Cutoff measurements

g. Measure transmittance of substrates, films, determine X-value, device spectral response

h. Film cut-off and thickness from ir transmission spectrum. Device cut-off wavelengths and

spectral response

i. Composition

j. Gives composition of film, spectral response

(temperature dependence important)

k. Composition determination, ir transmission

I. Diode spectral response

m. X-value, thickness. We use double beam ir grating

spectrophotometer

n. Composition, uniformity

p. Key performance parameters, absorption a

s. Composition and thickness mapping

t. Composition, gradients, thickness, far ir for transport

u. Composition (from bandgap), thickness, scattering

v. Cut-oH wavelength and thickness in mct films

w. Epilayer thickness, cut-off wavelength

y. Molecular (group) vibrational modes

aa. Cut-on k. thickness, free-carrier absorption

cc. IR transmission, Impurity effects or stoichiometric deviations

dd. Accurate, nondestructive mapping of mct

composition

ee. X-value, transmission

ff. Cutoff wavelength/composition uniformity gg Composition

hh. Energy gap for grown material

ii. Composition, carrier type? (below bandgap

absorption)

30. Hall effect

d. Transport properties

f. Material type and carrier concentration, mobility

g. Process monitor for crystal growth. Carrier density and mobility

h. Versus magnetic field is ESSENTIAL!! Versus

temperature is desirable. With resistivity, gives

carrier concentration and mobility. Multiple

carrier effects must be taken into account in many cases

1. Carrier concentration/mobilities

j- Gives essential layer electrical properties

(profile and versus B and T)

k. Electrical parameters—purity, doping, homogeneity

1. Material doping

m. H, n, /?n, resistivity

n. Carrier concentration, mobility

P- Type determination, doping, carrier behaviour

r. Carrier type, concentration mobility. Use depends

on facilities and demand

s. Carrier concentration and mobility

t. n, n, p

u. /Vjj, mobility, type

V. Electrical properties in mct films w. Carrier concentration, mobility, type

aa. Carrier concentration, mobility, resistivity

cc. Bulk chemistry effects, impurities, process

variations

dd. Destructive measurement of carrier concentration,

mobility, resistivity ee. IJL, Nf^, Nq (versus 7" and versus H)

ff. Carrier concentration, mobility

gg- Carrier concentration (mobility) hh. Material type, carrier concentration, mobility; homogeneity Indicator

ii. Carrier concentration

31. Laser beam induced current

d. Use on FPC diodes restricted to certain configurations. Can only evaluate drastic failure

f. Variations in electrically active defects g Identify active areas and location of 'breakdown'

i. Electrical uniformity

j- Helps assess material quality, junction location, optical area, etc m. Diode junction location, diffusion length

0. Defect features

s. Junction location, electrically active defects

t. Electrically active defects, qe of diodes

u. p-n junctions in uniform mct

w. Uniformity of HgCdTe, junction profiles dd. Difficult to interpret results on n-type material

requires contacts to sample

ee. Defect mapping

ff. Implant uniformity, carrier inhomogeneity, diode

junction position

gg- Electrically active (charged) defects. inhomogeneities

hh. Homogeneity of type of material; diode

contours

ii. Electrically active defect, minority carrier diffusion length

32. Laser ionization mass analysis

f. Impurity analysis

i- Possibly better than sims for certain elements

s. Microanalysis of precipitates

z. Impurity concentrations

aa. Impurity

cc. Specialized impurity analysis. Might offer some

lower detection limits dd. Quantitative trace-level analysis

ee. Use SIMS instead

{Continued)

767
D G Seller et al

Appendix 2 (Continued)

33. Low energy electron diffraction

i. In situ monitoring of mbe growth

s. Surface crystalline structure

t. Surface chemistry, lattice spacing

y. Surface atomic order or disorder

dd. Used as a measure of surface cleanliness and

crystallinity primarily in mbe

ee. Surface crystal quality and lattice parameters, good

for MBE

34. Magnetoabsorption

h. Research technique to obtain fundamental material properties

r. Good technique for looking at impurity levels, carrier lifetime. We have no facilities for this now

s. Fine structure w. Defect and impurity states in HgCdTe

dd. NIST accurate determination of mct bandgap

versus composition and temperature

35. Magnetoconductivity

h. Research technique to obtain fundamental material properties

r. Good technique for looking at impurity levels, carrier lifetime. We have no facilities for this now

s. Fine structure

36. Magnetorefiectivity

h. Research technique to obtain fundamental material properties

r. Good technique for looking at impurity levels, carrier lifetime. We have no facilities for this now

s. Fine structure

37. Magneto- optics

h. Research technique to obtain fundamental

material properties

r. Good technique for looking at impurity levels, carrier lifetime. We have no facilities for this now

hh. Bandgap states investigations. Spectroscopy,

transition rates

ii. Activation energy

38. n/lagnetoresistance

h. Research technique to obtain fundamental material properties

r. Good technique for looking at impurity levels, carrier lifetime. We have no facilities for this now

ii. Carrier type

39. Magnetophonon spectroscopy

h. Research technique to obtain fundamental

material properties

r. Good technique for looking at impurity levels, carrier lifetime. We have no facilities for this now

s. Coordination and short-range ordering in alloy

40. l\Aass spectrometry

f. Impurities, dopant profiles

g. Monitoring high vacuum processes

j. Gives basic purity information on solid source

material

k. Low discharge mass spectrometry is routine purity control

n. Chemical analysis

o. Major constituent and impurity analysis

p. Impurities of source materials

r. Regularly used to look at film junctions,

interdiffusion, impurities

s. GCMS of starting materials can analyse for organic impurities

t. Very important with sims

w. II-VI material purity

y. Trace elements

z. Impurity concentrations

aa. Trace impurity

cc. Glow discharge MS and spark source MS are good

impurity screening techniques but do not give low enough dls. icpms has proved very poor in CT and czT matrices

dd. Trace impurity analysis

ee. Impurity levels

ff. Background gas composition, primary gas

composition

41. Microwave impedance

f. Conductivity

u. Minority carrier lifetime w. Lifetime

dd. Contactless resistivity measurement

ee. Lifetime

hh. Carrier lifetime

42. Microwave reflection

d. Lifetime

f. Lifetime measurements

g. Tried some measurements with inconclusive

results

t. Minority carrier lifetime

V. Minority carrier lifetime in mct films

dd. Contactless measurement of carrier lifetime ee. Lifetime

ff. Minority carrier lifetime hh. Carrier lifetime

43. MOS capacitance

f. Carrier concentration, cut-off, storage time, breakdown voltage, etc

g. Surface passivation quality; fixed charge and

surface states

h. Monitor fixed charge at interface; measure dopant

concentration

i. Surface

j. Basic information on flat band—important to understanding surface properties

I. ZnS/HgCdTe passivation quality and material doping

m. Insulator thickness

s. Storage capacity of mis devices or persistent currents in diodes

t. Carrier concentration, flatband voltage, N^^

w. Carrier concentration

dd. Surface passivation quality, carrier concentration

ee. Surface states, surface charge: process monitor for passivation

ff. Insulator thickness and uniformity

hh. Material quality, surface quality

ii. Carrier concentration—activation energy (dlts)

44. Neutron activaton analysis

f. Residual impurity analysis

k. Special (im)purity analyses

Continued

768
Hgi^Cd^Te characterization measurements

Appendix 2 [Continued)

m. Impurity determination

n. For tellurium and cadmium analysis

w. Impurities in ll-VI materials

cc. Good for a very few impurities in ct/czt materials,

but it has excellent dls when applicable

dd. Trace impurity analysis

ee. Impurity determination (level)

ff. Elemental composition

45. Optical microscopy

d. Need automated scanning of morphology with macro-micro resolution (sampling micro)

f. Dislocation and precipitate densities, microstructure of bulk MCT, defects, morphology

i. Morphology

j. Essential to observation of defects, layer thickness,

crystal morphology and growth features

k. Surface quality

I. Fabrication and material defects m. Surface morphology, defect determination,

thickness

p. General structure of materials and devices

r. Inspection of devices and materials, ir microscope

is also useful

s. Surface morphology, macro-defect density

t. Surface roughness, macro-defects

u. Surface morphology

y Surface features

z. Surface morphology

cc. Manyfeatures of surfaces, cleaved cross sections, damage features

dd. Surface inspection/characterization

ff. Defect density

46. Optical modulation absorption

d. Accurate low temperature cut-off; contactless

lifetime measurements; measure of junction

RqA in LWiR

g. Lifetime measurements, cut-off measurements,

wafer level screening

h. Lifetime; device cut-off wavelength; film quality

j. For T

I Heterojunction material cut-off, response time

m. X-value

o Carrier lifetime

t. Nilinority carrier lifetime, defect levels w. Lifetime

dd. Contactless carrier lifetime measurement and

mapping

ee. Bandgap, lifetime

ff. Carrier concentration

hh. Carrier lifetimes

ii. Lifetime

47. Photo Hall effect

i. Properties of minority carriers

j. For mmority t, /j m. Influence of compensation on mobility

s. Surface states

t. N^.N^

u. Type, donor and acceptor concentration, mobility

hh. Lifetime measurements, transport data versus

temperature, etc

ii. Activation energy, trap levels

48. Photocapacitance

s. Surface states

t. Defect levels

w. Detectors

ee. Used in the past with mis structure to screen wafers

<or

ii. Quantum efficiency, responsivity

49. Ptiotoconductivity

f. Lifetime

g. Photoconductive devices

h. Carrier lifetime

i. Transients for minority carrier lifetime

j. Lifetime and essential to materials/device

understanding

k. Predictor of pc response—used on bulk cmt and epi EMT

m. Lifetime

p. Lifetime measurements

r. Pulse decay lifetime measurements for material

lifetime and surface recombination

s. t\/1inority carrier lifetime

t. Minority carrier lifetime, PC decay on diodes gives

w. Detectors, lifetime

cc. Might correlate to purity and stoichiometry

dd. Carrier lifetime measurement (transient) ee. Lifetime pc device

ff. PC lifetime gg. Lifetime

hh. Material quality, spectral response, lifetimes

ii. Lifetime, surface recombination velocity

50. Photoelectromagnetic effect

ee. Device parameters

hh. Minority carrier versus majority carrier effects

51 . Photoemission spectroscopy

s. Bonding of dopants and impurities

w. Surface analysis

ff. UPS—measures valence electron energies; xps—

see ESCA

52. Photoinduced transient spectroscopy

cc. Traps and defects, impurities, and the way they are tied up in materials

dd. Needs development work

53. Photoluminescence

d. Compositions/lifetimes (77 K)—this technique Is definitely underutilized in epilayer. Also

substrate evaluation of low temperature 4 K

g. Compositional measurement

h. Carrier lifetime; surface quality

j. Provides some useful information on CdTe

substrate quality m. Measure crystalline quality of CdTe epi

n. Characterization of substrates

p. Non-destructive electronic characteristics measurements

s. Important for wide bandgap (defects, impurities, etc)

t. Non-radiative centres (distribution), shallow levels, Te precipitates

u. Very useful for substrates (Cd, Zn)Te. Acceptor

levels/donor levels/£g/[Zn] concentration

w. Wide-gap ll-VI materials purity and defects

z. Impurities

aa. Impurities, composition, crystal quality

cc. Optically active impurity and defect states; effect of various annealing treatments Continued

769
D G Seiler et al

Appendix 2 {Continued)

dd. Primarily used to determine composition of CdZnTe (77 K meas. temp.)

ff. CdTe perfection

hh. Material quality, defect bands, particularly

epilayers

54. Photoreflectivity

d. MoreaccuratesurfaceX-values. Alsosurfacestate

evaluation

p. Film composition, X-value, non-destructive mbe

growth control

r. HCT X-value at the surface; formerly used frequently, equipment currently down

s. Alloy composition

t. Composition, E^, £2, Aq, crystallinity aa. Surface composition

dd. Contactless compared with electroreflectance

ff. Composition (X-value)

ii. Composition—carrier concentration?

55. Photothermal spectroscopy

56. Raman scattering spectroscopy

\. Tried to get surface information (not sensitive). Bulk information not clear as to importance

p. Crystal lattice quality and imperfections

t. Te precipitates, composition

w. Clustering in ll-VI materials

y. Molecular species, group vibrations

dd. Surface ordering, not much work done for hot

ff. Vibrational modes of compounds

Ii. Phonon energies, defects? (anti-site)

57. Reflectometry

g. uv-vis reflectance used to determine X-value

h. Reflectance spectrum near the and -1- A,

transitions. Gives x at the surface

s. Alloy composition

dd. uv reflectance (£, transition) used to map surface composition of mct (300 K meas.)

ee. Surface X-value

58. Resistivity

d. Part of Hall process

f. Hall measurements, determination of p-type

material

g. Part of Hall measurement

i. With Hall for cc and n

j. With Hall, gives mobility—essential to materials, understanding and device modelling

k. Electrical parameters (with Hall)—purity, doping, etc m. Carrier concentration, contact resistance

s. Deduce mobility in conjunction with Hall measurements, contact resistance

t. Product of carrier concentration and mobility w. HgCdTe for photoconductors

cc. Impurities, defects, stoichiometry, annealing effects and effectiveness

dd. Integral part of Hall effect measurements: not separately measured

ee. Done in conjunction with Hall effect

ff. Resistivity of metal film or semiconductor

gg. Mobility

hh. Material type, carrier concentration mobility; homogeneity indicator

59. Ruttterford backscattering spectrometry

f. Passivation characterization and implant profiling

j. Provides information on near-surface composition and damage

m. Chemical analysis, crystal perfection

s. Composition depth profiles, crystallinity and strain

y. Atom %

dd. Crystallinity of epitaxial layers, ion implantation

damage, etc

ee. Passivation layer purity (hard to use for mct)

ff. Film composition, density

60. Scanning electron microscopy

d. Morphology

g. Important for process development

j. Key to detailed morphology examination

I. Fabrication/material analysis, especially defects m. Surface imaging, defects

n. Defects

o. Morphology, crystalline quality, chemical

composition, etc

p. Surface features

r. Used for materials and device inspection

s. Morphology, layer thickness

t. Morphology

u. Surface morphology, composition by wdx, edax

V. Surface defects and other features w. Device examination

y. Microstructure

z. Surface morphology, crystallinity, thickness

cc. Surface damage, surface features, surface deposits

dd. Routine characterization of materials and devices

ee. Structural micro-features

ff. Step coverage, surface morphology

61. Scanning transmission electron microscopy

f. TEM of defects

g- Locating inclusion, dislocations, etc m. Dislocation density

n. Defects

P- Interface structure

t. Defect structure

y- Defect structure

dd. Presently use tem only

ee. Defects. Difficult to use

ff. Presence of dislocations, twinning

hh. Defect quality of material, interfaces

62. Secondary ion mass spectrometry

d. Impurities

f. Chemical analysis of dopant profiles and

impurities

g. Determine impurity locations, dopant versus

X-value profiles

h. lpe film composition and impurity profiles

i. Impurities

j. Key to materials purity and composition and

interface control

I. HgCdTe heterostructure material composition,

impurities m. Impurity determination

n. Analysis of impurity clusters

p. Impurity doping profiles

r. Regularly used to look at film junctions,

interdiffusion, impurities

s. Impurity analysis and depth profiles

t. Impurity concentration (profiles), composition

Continued

770
Hg,_j,Cd^Te characterization measurements

Appendix 2 {Continued)

u. Measure concentration of various impurities in mot

and (Cd. Zn)Te

w. Impurities in II —VI materials

y. Trace elemental analysis

2. Impurity concentration

CC, Depth profiling of trace impurity and dopant

concentrations, interface or surface pile-ups, segregation effects dd. Trace level analysis, doping profiles ee. Impurity levels

ff. Elemental composition, implant depth gg Profiling of impurities, and composition

63. Shubnikov-de Haas effect

s. Interface states and band bending

64. Spreading resistance

g Important for fpa (staring)

h. Gives contact resistance

j- Spatial information

1. Use trans, line method contact resistance

s. Sheet resistance

t. Transport, n^ product

w. Conductivity uniformity

dd. 77 K spreading resistance technique to profile p-n

junctions needs development!

ee. Use to analyse contact resistance

65. Surface photovoltage

d. Carrier concentration;

r. We have used optical scanning technique in conjunction with electrical contacts to see

photovoltaic and photoconductivity response

dd. Not yet developed for mct, requires contacts on samples

ii. Carrier type changes (inhomogeneity)

66. Thermal wave microscopy

j- Tried, but found nothing after a casual look m. Defect imaging

67. Transmission electron microscopy

f. Defect analysis and microstructural analysis

j- Key insight into materials defects m. Dislocation density determination

n. Defects, dislocations P Interface structure local phases

s. Analysis of defects

t. Defect structure

V. Defect analysis

w. Precipitates, dislocation structure, interface in ll-VIs

y- Defect structure

2. Defect structure

CC. High magnification defect and dislocation analysis,

damage structure

dd. Imaging of structural quality of thin-film interfaces

in cross section

ee. Defects, structure. Difficult to use

ff. Crystalline quality

hh. Defect quality of material interfaces

68. X-ray fluorescence analysis m. Impurity determination

t. Composition

u. Not very sensitive

y. Atom %

dd. Composition measurement of large-area

samples

69. X-ray photoelectron microscopy

f. Surface contamination on thin film profiles

I. HgCdTe material composition, impurities

p. Local device chemistry; information on films and

interfaces

r. SEM in this mode gives greater sensitivity for elemental analysis

s. Analysis of Impurity bonding

y. Composition

dd. Surface composition analysis; combined with

sputtering can be used for comp. depth profiling, interface chemical composition

70. Electron cfiannelling

d. Orientation; surface preparation

0. Orientation, near surface crystalline quality

p. Rough orientation of crystal films

s. Crystallinity of thin layers

y. Structural symmetry

z. Crystal orientation and perfection

dd. Rapid surface sensitive measurement of crystallinity (qualitative)

71. Electron diffraction

f. MBE growth, crystal quality and orientation

j. Essential to good mbe

p. Assume rheed, growth monitor mbe

s. In conjunction with tem for analysis of defects

t. Lattice spacing, crystallinity

u. Crystalline quality, orientation. Burger's vector

y. Order/disorder

dd. Useful in high-vacuum environments such as i^be

ff. Surface perfection in mbe

72. Surface topography: optical interferometry, stylus, scanning tunnelling microscopy

d. Topography; defects

f. Nomarski interference microscopy, to see shallow

etch pit defects

g. Thin film process measurements, grown lpe film surface morphology

j. Helps get film thicknesses, wafer depths

n. Uniformity

p. Topography to calibrate film thickness, roughness,

device structures, depth analysis craters, atomic

resolution structure, electronics of surface and

Interface, epi films, non-destructive device

testing

r. Make interferometric topographs. May use to measure thickness of films by looking at bevelled edge of multilayer structure, and detecting

transitions by phase change

s. Measurement of surface topography, height and

period

w. Wafer flatness

y. Roughness at various levels of scrutiny to atomic

resolution

CC. Surface flatness of substrates

dd. Fizeau—flatness; Stylus—etch depths, surface roughness, etc; stm—not widely used in mct (need

clean surfaces)

ff. Film thickness; stress from bow measurement

771
0 G Seller et al

Appendix 3. Results from questions asked

In this appendix we tabulate the answers given by the respondents to four questions asked. Again each letter of the alphabet refers to a specific respondent throughout

this appendix, as in appendix 2.

Questions

1. What are the most important (2-4) properties or parameters to measure or determine? Why?

a. Bandgap, carrier concentration. Hall

mobility, dislocation density, lifetime

c. Carrier concentration, X^, uniformity, charge

storage

d. Cut-off/composition, carrier concentration,

Hfetime, RqA

e. Diode l-V, Hall, spectral response

f. Voltage past threshold (breakdown voltage),

storage time, cut-off wavelength. Hall

carrier concentration and mobihty,

dislocation density, mis performance

g. Where are the leakage currents? Surface or bulk? Are they locaHzed defect: Substrate

related? Properties of the individual layers of heterostructure material

h. Hgi_^Cd^Te alloy composition, carrier concentration, carrier lifetimes, carrier

mobilities. These directly determine device performance

i. Composition and composition variations on

surface and in depth; same for carrier concentration and mobilities; transport properties

of minority carriers; deep levels

j. Nature of point defects, spatially resolved extended defects, spatially resolved surface/

interface flaws. These are the perceived causes

of non-ideal device behaviour

k. • composition (absolute and variation)—governs AA of arrays

D* uniformity {a) bulk, {b)

epilayers. *NON-DESTRUCTIVE

• non-destructive defect mapping—Te ppts, dislocations, mosaic substructure • quick-check predictor of responsivity and

detectivity on relatively unprocessed substrates

1. Hard to choose m. X-value, n, p, n—screen material for processing;

minority carrier lifetime

n. Mobility—crystal perfection and purity; minority

carrier lifetime—same as above; tellurium second phase

ALL mct materials are plagued

with tellurium as a second-phase problem

o. Composition, carrier concentration, junction

position (diodes), crystalHne quality and

minority carrier lifetime

772

p. Materials—crystal quality, defect concentration,

carrier concentration, profile of impurities and

activity. Interface/surface structure chemistry

and electronic property

Deviceqe, RqA, lifetime, carrier concentration

and profile, D*

r. Depends on the use. Substrates: crystalline

perfection, trace element analysis; HgCdTe

films: A'-value, mobility

s. Composition (cut-off wavelength), carrier concentration, hfetime, dopant profiles. Most

fundamental to operation of fr detector

t. Minority carrier lifetime, N^, Nj^—directly affect device performance. Lifetime good indicator of material quahty

Dislocation density, electrically active defects, Te

precipitates Surface state density (mis): I-V characteristics, quantum efficiency

u. Carrier concentration, mobility, Hfetime,

composition, crystalline defects concentration.

If we know these values accurately, then we can

predict the performance. These parameters

uniquely determine the quality of the material

V. For screening mct material: spectral cut-off, carrier concentration, mobility, lifetime For improving mct material: defect levels (dislocations, etc), dopant uniformity

w. Breakdown field, carrier concentration, lifetime, cut-off wavelength, dark current—these five properties determine detector performance.

Need non-destructive means for mapping

HgCdTe wafers for uniformity of these five properties

X. Electrical transport (Hall), composition

(transmission), crystallinity (x-ray)

y. (1) Stoichiometry, (2) trace impurities, (3) oxidation state, (4) dislocation density,

(5) most important—atomic structural order

at surfaces and at interfaces

z. Defect structure and purity—strongest influence on device performance

bb. Composition, grading, thickness and Hall

coefficient and mobihty

cc. (1) Impurity content in Cd, Zn, Te and Se

starting materials—must control these or have no chance for downstream quality. (2) Impurities in substrates—must control as in (1). (3) EPDs—must control in order to assume

epilayer quality yields. (4) Precipitate type, size and distribution—somehow affects epilayer and

device quality and yield dd. (1) Absolute composition and compositional

uniformity—determines wavelength cut-off; strongly affects device quality. (2) Carrier concentration and mobility—strongly affects device properties; control of carrier concentration is essential. (3) Minority

carrier lifetime—directly affects device quality

ff. Carrier concentration, cut-off wavelength.
Hg,_^Cd^Te characterization measurements

minority carrier lifetime, dark current—they

are the factors which most directly affect device performance

gg. Composition, carrier concentration, lifetime, deep-level defects and their spatial distribution

hh. • Diode/Mis dark current—depends on lifetime, bandgap states, surface passivation • Diode/Mis breakdown voltage—depends on doping concentration, density and

position of bandgap states • Diode MIS 1// noise—depends entirely on

surface passivation

ii. Composition, carrier concentration, material

defects—these material properties determine

the device performance

2. What measurement techniques most need to be

developed, enhanced or improved? Why?

a. Imaging sims. defect etching

c. C-F, c-f, Hall effect, fts

d. • Optical probing (oma, pl, thickness mapping

by absorption—non-destructive, fast and

cheap, one set-up, large area • Detailed morphology mapping/pattern

recognition—present R&D, future process

control

f. Position annihilation—its limits and

usefulness are not understood very well. It could provide a means for measuring defects which we have difficulty in observing (i.e., Te vacancy concentration in HgCdTe, CdZnTe,

etc) Photoluminescence—We do not have

sufficient understanding of what point defects or impurities contribute to which peaks • Non-destructive, non-contact, screening

techniques • Non-contact mis; fast qualification of production materials • Current mis device fabrication takes too long

g. • Ability to determine junction location versus

alloy composition for heterostructure

material—potentially a combination of cold

stage Ebic in a sem • Wafer level screening techniques for junction quality

oma response time (at low

backgrounds) has potential

h. • Non-contact measurement of junction

quality (e.g., RqA, quantum efficiency, . . .) would have enormous impact on array costs • Junction (heterojunction) profiling

techniques would greatly facilitate control and diagnostics of advanced device structures • In situ monitoring of vpe film growth of HgCdTe multilayer devices

i. • Characterization of multilayer structures • Diode reverse recovery for measurement of minority earner lifetimes on actual device

(after processing may have changed the properties of the material)

j. All spatially resolved materials analytical

techniques which provide specific, interpretable

information, nde and in situ materials and

device characterization follow close behind.

Priorities should be set by the relevance of

the technique to solving the most important

problems

k. • above; for layers / (x, y, depth) in scanning

mode

• above; but correlated with device (PC and pv

types) performance to permit reasonable

specification of parameters and screening • above; mobiUty, carriers and lifetime are commonly spec'd to prescreen for performance.

AR/A flux has been tested and found effective, but is httle used

1. We badly need a cheap, reliable optical standard (detector, probably) for the 10-12 nm

region

We also need better in-process characterization

of our HgCdTe diodes, which are too fragile to be probed directly and also need to be measured

at low T{ ~ 80 K) and low optical background

m. Non-destructive techniques would be very

useful. X-ray techniques that correlate substrate defects to HgCdTe epi defects, and

device performance

n. Starting material analysis—no suitable technique

exists for ppb or less analysis; tellurium cluster detection and identification—in my opinion, Te clustering is the dominant cause of low

detector yields

p. Non-destructive growth and device parameter measurement and control

q. Defect etches/EPD counts; defect detection (non-destructive if possible)

r. We need a way to map on pixel size level the

carrier type, concentration and mobility.

N-type HCT seems to have p-type microislands through it. We need to see these and

figure a way to remove them

s. In general, techniques that are capable of mapping critical wafer properties that impact

the variability of diode performance. For

structural imperfections, x-ray topography

and long wavelength cathodoluminescence or photoluminescence could be developed. Basic

properties such as carrier concentration and

lifetime also need to be mapped routinely.

Another area of weakness is compositional

inhomogeneities

t. Spatially scanning techniques lbic; electrically active defects ought to be important for electro-optical devices techniques that are non-destructive and can be applied at different states of processing—this will help

identify processing steps that limit performance

773
D G Seller et al

Surface analysis techniques—surface passivation

still limits performance for many applications

u. Mapping techniques for carrier concentration

need to be developed or improved.

Optical techniques utilized to probe the gap

needs to be enhanced and brought into more

laboratories in industry

v. Techniques to measure dopant non-uniformities with sensitivity of 10'* cm"-'. This may be the

limiting factor in charge storage in mis devices on MCT

Non-destructive characterization of defects

needed to improve hetero-interfaces in films w. Current/field, pulse, decay { JIE curves). Great

potential for studying field dependence of dark current in HgCdTe. Shape variations in JIE curves yield strong clues about the source

of the dark current

X. Low-level impurity identification (<1 x 10'"' cm"^)

y. In situ surface chemistry (properties 1-3, Question 1, y) during wet and dry processing

z. Techniques to determine impurities—both

qualitative and quantitative, lcms—Laser Scanning Mass Spectroscopy—good impurity

survey technique?

bb. Differential Hall effect

cc. • Impurity analysis so that we can control trace impurities very early in the overall process

rather than when the epilayer is characterized or the device is made and tested • II-VI incorporated and the whole industry

will significantly benefit from the development

of ZCGFAAS under the now active US Army sbir contract at VHG Labs

dd. • Techniques for trace level impurity measurement at < 10'^ cm " level. Ultimately

correlate with deep level analysis and device

properties • 77 K p-n junction profiling technique—i.e., measure actual profile as opposed to siMS, S-R

measurements (currently use taper-etch C-V

which is time-consuming)

• 77 K carrier concentration mapping (as being

pursued by NIST)—will not be used routinely, but is helpful for development of materials

ee. • It certainly would be nice to profile alloy

concentration and junction location to better resolution to determine just where the junction occurs in heterostructure devices • Anything related to device performance

(defects, impurities)

ff. Non-contact techniques need to be developed/

enhanced/improved to allow material screening quickly after growth and during

processing. The time required to build, bond

and test electrical devices hampers all aspects

of material, process and device development

gg. Faraday rotation for non-destructive mapping

of carrier concentration; defect mapping.

e.g., by DLTS or scanning photoluminescence;

and scanning double crystal x-ray rocking

curves

hh. • Contactless techniques for determining

material parameters relevant to the above

device properties • Rapid turnaround device data at 77 K for diodes/Mis devices

ii. • Localized mapping techniques need to be developed to characterize the material. (Ideally, these techniques should be non-contact) • Techniques need to be developed to identify

microscopic material defects which may

cause carrier-type inhomogeneity,

compositional non-uniformity, etc.

3. Any additional helpful, constructive comments?

d. I do not think the Consortium approach will work. Best approach to long-range problems is to fund Government Labs (MIT, NIST,

CNVEOL, etc) which can subcontract to industry

f Too much weight is put on x-ray rocking curve

half-width and it is not reflective of the

overall crystal quality. Defect etching is much

better

h. NIST should actively get involved in HgCdTe

infrared detector technology, probably in the area of materials/process characterization

j. MCT analysis is a very tough problem. It will not be solved by the magical application of an

overlooked technique. The most important

questions to answer are: Why do I see what I see and what does this mean physically? These

questions will have to be answered by carefully

designing experiments combining (often) several analytical techniques and proper

controls. The issues cannot adequately be addressed without the focused combined

eff"orts of materials growth, test device (or structure fab) and materials and device

analysis. A multi-organizational effort is highly desirable

k. • A clearing house of data correlated to absolute (or, if secrecy requires, relative) performance parameters. Agreed measurement

standards and cross-lab correlation (as was done by NATO composition exercise in 1980) • Standardization of surface preparation and

ambience during measurements is generally

lacking

1. I have a feeling that there are many process- control test features used in the Si and GaAs

industries which could be transferred and/or

modified for use in HgCdTe processing—and some new features which should be developed

for the special HgCdTe diode case where there are a lot of new things such as In bumps

774
Hgi_j,Cd^Te characterization measurements

s. I am unclear of the applicability of some of these techniques to mct (such as charged particle activation analysis, deep level optical

spectroscopy, etc). There is a need to develop some of these techniques so that better analyses can be made of defect states in MCT and to bring them to the point where

they can be used as more routine

characterization tools by the crystal grower

t. Quite often, materials characterization techniques and device characterization tools are developed separately, and very httle effort

is made to close the loop between materials parameters and device performance

parameters

Any defect-detecting technique will show

defects in HgCdTe. The trick is to isolate 'bad' defects from 'good' defects

y. Should have had a column entitled 'Relative knowledge of the technique' (i.e., knowledge

possessed by person filling out form) for cases where "don't know' is inappropriate. Last column should not have had the word 'by'

in it (thought, at first, that you meant 'how'

property is measured)

z. NIST should estabhsh impurity characterization

capability, including lsms (not currently

available in US—Brian Easton at Philips in UK

has it)

cc. Correlation between epds and device performance are coming along well; now we

have got to learn how to lower epds in substrates and epilayers dd. Keep up the good work to measure fundamental

properties such as £g, N-, versus composition

that NIST has already done

ff. I have been exposed to many of these techniques. I am aware that certain techniques

are in use with which I have no experience, so

I cannot say what key parameters are determined by every technique. There

should be a middle ground of minor

importance for techniques which are highly

specialized and useful, but not used on an

everyday basis. I would have placed magnetoabsorption in this class, but instead I was forced

to select 'not important' because the information it provides is more fundamental

and less practical

gg. Need to improve detection limits of various

analysis techniques such as slms, etc.

4. What measurement techniques have been inadvertently omitted from this listing?

a. Lifetime

d. Laser microscopy

f Residual stress measurements in processed

devices; x-ray diffraction for crystal orientation; positron annihilation

h. Atomic force microscopy, scanning capacitance

microscopy; quantum efficiency; contact

resistance (especially important for p-type

samples); contact and surface recombination

velocity; far infrared transmission; variable-area photodiode test arrays; open-circuit

photovoltage decay

j. Nothing strikes me offhand. The key is adequate use and proper combination of many

of the techniques on the hst. Many techniques should be applied versus temperature and

magnetic field (in some cases). Spectroscopy

should include spectral responses versus T. Depth and spatial profiles are very desirable

for many techniques

k. Any technique requiring >S100 K investment

will hkely be confined to occasional research and corroboration of cheaper, grosser techniques which are needed in production

(my feelings after 25 years in the business)

1. • Transmission fine measurements for contact, sheet resistance (we use for p contacts on our p-n diodes) • Variable-area-diode measurements material

optical response, diffusion (RqA), and lateral optical collection effects, perimeter g-r effects

n. Photon backscattering

s. In situ measurement for mocvd such as laser reflectance w. Current/field, pulse, decay (J/£ curves). Great

potential for studying field dependence of dark current in HgCdTe. Shape variations in J/E curves yield strong clues about the source

of the dark current

y. (1) Atomic force microscopy, (2) reflection high energy electron diffraction, (3) electro- chemical techniques (as done by Arthur

Hubbard, Manuel Soriaga and J Stickney),

(4) ADAM (Angular Distribution Auger

MicroscopyScience, in January 1990, 247

182-8) (by Hubbard and Doug Frank)

Z. LSMS

bb. Microscopic etch pit and dislocation density

cc. None that I can cite dd. ( 1

) uv-NiR surface reflectance to measure surface composition from transition (comments

under reflectometry). (2) X-ray topography

techniques other than double-crystal (not a

serious omission). (3) IR microscopy to image

precipitates in substrates and layers. (4) X-ray

lattice constant measurements. (5) X-ray Laue

back-reflection to determine single crystal orientation; powder diffraction and other

specialized techniques

ee. Imaging siMS

ff. MIS reverse recovery lifetime, mis storage time

775
D G Seller et al

References

[1] Tennant W E, Cockrum C A, Gilpin J B, Kinch M A, Reine M B and Ruth R P 1992 J. Vac. Sci. Technol. B 10 1359

[2] Balcerak R and Brown L 1992 J. Vac. Sci. Technol B 10 1353

[3] Reich R B 1989 Sci. Am. 261 41

[4] The Committee on Characterization of Materials, Materials Advisory Board, National Research

Council, MAB-229-M, March 1967

[5] Hannay N B 1967 Trace Characterization, Chemical and

Physical NBS Monograph 100 ed W W Meinke

and B F Scribner (Washington, DC: US

Government) p 1 [6] Meieran E S, Flinn P A and Carruthers J R 1987 Proc. IEEE 75 908

[7] Advanced Materials and Processing: Fiscal Year 1993 Program 1992 (Office of Science and Technology

Policy) p 10

[8] Materials Research Advisory Committee, in Trends and

Opportunities in Materials Research (Washington,

DC: National Science Foundation, 1984) pp 2, 22

[9] SchafFner T J 1987 Scanning Electron Microscopy

(AMF O'Hare, IL: SEM) p 1

1

[10] Adapted from Materials and Processes for Electron Devices (National Academy of Sciences, 1972) p 7 [11] Price S L and Boyd P R 1993 Semicond. Sci. Technol. 8

[12] Meyer J R, Hoffman C A, Bartoli F J, Arnold D A, Sivananthan S and Faurie J P Semicond. Sci. Technol. 8 805

[13] McLevige W V, Williams G M, DeWarmes R E, Bajaj J, Gergis I S, Vanderwyck A H and

Blazejewski E R Semicond. Sci. Technol. 8 946

776 ti U.S. GOVERNMENT PRINTING OFFICE: 1994 - 376-913 - 814/
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