JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 26, Number 6, June 1997 This Month Featuring: Proceedings of the 1996 U.S. Workshop on the Physics and Chemistry of II-VI Materials. View June 1997 Table of Contents.

SPECIAL ISSUE PAPERS

Subject areas discussed at the 1996 workshop include epitaxial material growth, defects and impurities, doping, characterization, device properties, x-ray and [gamma]-ray detectors, blue-light-emitters and photo-refractivity. The meeting was attended by more than 160 participants from 16 countries. Of the 61 invited and contributed presentations, 50 papers appear in this publication. The credit for this successful endeavor goes to the authors and the referees for their careful and prompt review of the manuscripts.

The organization and arrangements of a workshop of this magnitude would have been impossible without the dedication of the chairmen, Peter Norton and Ishwara Bhat, the active participation of the program committee, and the workshop coordinators. The program committee members were P.M. Amirtharaj, NIST; J.M. Arias, Rockwell International; R.S. Balcerak, DARPA; I. Bhat, Rensselaer Polytechnic Institute; L.E. Brown, U.S. Air Force Wright Laboratories (US-AFWL); T.N. Casselman, Lockheed Missiles and Space Co.; J.P. Faurie, University of Illinois at Chicago; C.R. Helms, Stanford University; S.M. Johnson, Santa Barbara Research Center; M. Martinka, U.S. Army Night Vision and Electro-Optics Systems Directorate (US-NVESD), T.C. McGill, California Institute of Technology; P.W. Norton, Lockheed Martin IR Imaging Systems; H.F. Schaake, Texas Instruments; A. Sher, SRI International; R. Singer, Institute for Defense Analyses; C.J. Summers, Georgia Tech Research Institute; H.R. Vydyanath, Avyd Devices; and J.R. Waterman, U.S. Naval Research Laboratory (US-NRL). The workshop arrangements were coordinated by Jay Morreale and William Klein, Palisades Institute for Research Services, Inc. Special thanks for the continued sponsorship by the US-NVESD, US-NRL, US-AFWL, the American Physical Society, and The Minerals, Metals and Materials Society.

The proceedings of the first ten workshops (from 1981-91) have been published in the Journal of Vacuum Science and Technology. Papers from the workshop from 1992 onward appear in special issues of Journal of Electronic Materials. On to Santa Barbara!

Paul M. Amirtharaj, NIST, Gaithersburg, MD
Ishwara B. Bhat, Rensselaer Polytechnic Institute, Troy, NY
Proceedings and Special Issues Editors

GROWTH OF MATERIALS

KEY WORDS Infrared detectors, mid-wave detectors, molecular beam epitaxy (MBE), two-color detectors

The first report of molecular beam epitaxial growth and performance of HgCdTe two-color detectors for the simultaneous detection of radiation at 4.1 and 4.5 µm is presented. In-situ doped devices with the n-p-n architecture were grown by molecular beam epitaxy on (211)B CdZnTe substrates. Representative structures exhibited x-ray rocking curves with full width at half-maxima of 40-60 arc-s. The typical near surface etch pit density in these structures were 4-7 x 10⁶ cm^-2. The devices were processed as mesa diodes and electrical contacts were made to the two n-type layers and the p-type layer to facilitate simultaneous operation of the two p-n junctions. The spectral response characteristics of the devices were characterized by sharp turn-on and turn-off for both bands, with R₀A >5 x 10⁵ cm² at 77K. The detectors exhibited quantum efficiencies >70% in both bands.

MOCVD of Bandgap-Engineered HgCdTe p-n-N-P Dual-Band Infrared Detector Arrays
P. MITRA¹, S.L. BARNES,¹ F.C. CASE,¹ M.B. REINE,² P. O'DETTE,² R. STARR,³ A. HAIRSTON,² K. KUHLER,² M.H. WEILER,² and B.L. MUSICANT²
1--Lockheed Martin Vought Systems, Dallas, TX 75265-0003. 2--Lockheed Martin IR Imaging Systems, Lexington, MA 02173-7393. 3--Lockheed Martin IR Imaging Systems, Lexington, MA 02173-7393. Present address: Intel Corporation, Chandler, AZ.

KEY WORDS Detector arrays, dual-band, focal plane array (FPA), HgCdTe, long wavelength infrared (LWIR), metalorganic chemical vapor deposition (MOCVD), mid wavelength infrared (MWIR), p-n photodiodes

We report the implementation of recent advances in metalorganic chemical vapor deposition (MOCVD) for in situ growth of four-layer HgCdTe mid wave/long wave (MW/LW) simultaneous dual-band 64 x 64 infrared detector detector arrays. This independently accessed, simultaneous, double-heterojunction p-n-N-P dual-band detector has two back-to-back stacked photodiodes grown on CdZnTe (100) substrates. The LW photodiode is a p-on-n heterojunction grown on top of an MW N-on-P heterojunction photodiode. Secondary ion mass spectrometry depth profiles of these 28 µm thick p-n-N-P dual-band films show four well-defined regions of alloy composition and doping, and agree well with the device design. 64 x 64 arrays of dual-band detectors were fabricated from these films using electron cyclotron resonance dry etching and CdTe passivation, and hybridized to a dual-band readout chip. Two bump inter-connects in each unit cell provide independent electrical access to the back-to-back MW and LW photodiodes, and allow the MW and LW photocurrents to be separate and independent. The dual-band infrared focal plane arrays (IRFPAs) spectral response data at 78K are well-behaved and are fully consistent with that observed in individual single-band LW p-on-n and MW N-on-P heterojunction devices of the same design. The hybridized 64 x 64 dual-band FPAs have MW and LW average in-band quantum efficiencies of 79 and 67%, and median D^* values of 4.8 x 10¹¹ and 7.1 x 10¹⁰ cm-Hz/W, in the respective spectral bands at 78K. The data demonstrate that MOCVD has progressed significantly toward being a practical and viable vapor phase in situ growth technology for advanced bandgap-engineered HgCdTe detector arrays.

MBE-Grown HgCdTe Multi-Layer Heterojunction Structures for High Speed Low-Noise 1.3-1.6 µm Avalanche Photodetectors
OWEN K. WU,¹ RAJESH D. RAJAVEL,¹ TERRY J. DE LYON,¹ JOHN E. JENSEN,¹ MIKE D. JACK,² KEN KOSAI,² GEORGE R. CHAPMAN,² SANGHAMITRA SEN,² BONNIE A. BAUMGRATZ,² BOBBY WALKER,³ and BILL JOHNSON³
1--Hughes Research Laboratories, Malibu, CA. 2--Santa Barbara Research Center, Goleta, CA. 3--Laboratory for Physical Sciences, University of Maryland, Colleage Park, MD.

KEY WORDS Avalanche photodetectors, HgCdTe, molecular beam epitaxy (MBE)

HgCdTe is an attractive material for room-temperature avalanche photodetectors (APDs) operated at 1.3-1.6 µm wavelengths for fiber optical communication applications because of its bandgap tunability and the resonant enhancement of hole impact ionization for CdTe fractions near 0.73. The HgCdTe based separate absorption and multiplication avalanche photodetector is designed and fabricated for backside illumination through a CdZnTe substrate. The multi-layer device structure is comprised of seven layers including 1). n⁺ contact 2). n^- diffusion buffer 3). n^- absorber 4). n charge sheet 5). n^- avalanche gain 6). p to form junction and, 7). p⁺ contact. Several wafers were processed into 45 µm x 45 µm and 100 µm x 100 µm devices. The mean value of avalanche voltage is 63.7 V measured at room temperature. At 1 GHz, the device shows a gain of about 7 for a gain-bandwidth product of 7 GHz. This first demonstration of an all molecular beam epitaxially grown HgCdTe multi-layer heterojunction structure on CdZnTe substrates represents a significant advance toward the goal of producing reliable room temperature HgCdTe high speed, low noise avalanche photodetectors.

Improving Material Characteristics and Reproducibility of MBE HgCdTe
D.D. EDWALL, M. ZANDIAN, A.C. CHEN, and J.M. ARIAS
Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360.

KEY WORDS HgCdTe, molecular beam epitaxy (MBE)

This paper describes our progress to improve the material quality, reproducibility, and flexibility of molecular beam epitaxial (MBE) growth of HgCdTe. Data, statistics, and yields according to defined screen criteria are presented for n-type layer carrier concentration and mobility, void defect density, and dislocation density for more than 100 layers. Minority carrier lifetime data are also presented. Continued improvements in impurity reduction have allowed us to achieve, for the first time, reproducible, low n-type carrier concentration in the mid-10¹⁴ cm^-3 range with high electron mobility. Data are presented that show that low dislocation density films are obtained for growth on CdZnTe substrates with a wide range of Zn concentration. Results are presented from a nine-growth run first pass success demonstration run to further assess material quality reproducibility and flexibility of wavelength band tuning. These results demonstrate the promising potential of MBE growth for flexible manufacturing of HgCdTe for infrared focal plane arrays.

Spectroscopic Ellipsometry for Monitoring and Control of Molecular Beam Epitaxially Grown HgCdTe Heterostructures
M.J. BEVAN,¹ L.A. ALMEIDA,² W.M. DUNCAN,¹ and H.D. SHIH¹
1--Texas Instruments Incorporated, Corporate Research & Development, P.O. Box 655936, MS 150, Dallas, TX 75265. 2--Texas Instruments Incorporated, Corporate Research & Development, P.O. Box 655936, MS 150, Dallas, TX 75265. Permanent address: Microphysics Laboratory, Department of Phjysics, M/C 273, University of Illinois at Chicago, 845 W. Taylor, Chicago, IL 60607.

KEY WORDS HgCdTe, in-situ control, infrared (IR) detectors, molecular beam epitaxy (MBE), spectroscopic ellipsometry

A systematic study of the effect of measurement perturbation on in situ monitoring of the composition of molecular beam epitaxially (MBE) grown Hg_1-xCd_xTe using spectroscopic ellipsometry was carried out. Of the five variables investigated, which included angle of incidence, wavelength of the light beam, modulator rotation, analyzer rotation, and modulator amplitude, the angle of incidence and the modulator rotation had the strongest effect on the in situ Hg_1-xCd_xTe composition monitoring process. A wobble-free sample manipulator was installed to reduce the impact of these two variables. With these improvements, the spectroscopic ellipsometer is now routinely used to monitor Hg_1-xCd_xTe compositions during MBE growth of heterostructures and is a useful tool in diagnosing growth-related problems. Examples are included for both application areas, that include the control of the interface between Hg_1-xCd_xTe layers of different compositions, i.e. device engineering.

Strain Effects in CdTe (111) Epitaxial Layers Grown on GaAs (100) Substrates by Molecular Beam Epitaxy
M.S. HAN,¹ T.W. KANG,² J.H. LEEM,¹ B.K. SONG,¹ Y.B. HOU,¹ W.H. BAEK¹ M.H. LEE,³ J.H. BAHNG,³ K.J. KIM,³ J.M. KIM,⁴ H.K. KIM,⁴ and T.W. KIM⁵
1--Department of Physics, Dongguk University, Seoul 100-715, Korea. 2--Department of Physics, Dongguk University, Seoul 100-715, Korea. Author to whom all correspondence should be addressed. 3--Department of Physics, Kun-Kuk University, Seoul 133-701, Korea. 4--Agency for Defence Development 4-5-1, Daejeon 305-600, Korea. 5--Department of Physics, Kwangwoon University, Seoul 139-701, Korea.

KEY WORDS CdTe, GaAs, molecular beam epitaxy, strain effects

Spectroscopic ellipsometry and photoreflectance measurements on CdTe/GaAs strained heterostructures grown by moleculcer beam epitaxy were carried out to investigate the effect of the strain and the dependence of the lattice parameter on the CdTe epitaxial layer thicknesses. Compressive strains exist in CdTe layers thinner than 2 µm. As the strain increases, the value of the critical-point energy shift increases linearly. These results indicate that the strains in the CdTe layers grown on GaAs substrates are strongly dependent on the CdTe layer thickness.

Selective Area Epitaxy of CdTe
Y.Y. LUO, A. CAVUS, and M.C. TAMARGO
Center for Advanced Technology on Ultrafast Photonic Materials and Applications, Center for Analysis of Structures and Interfaces and Department of Chemistry, City College of CUNY, New York, NY 10031.

KEY WORDS CdTe, in-situ patterning, molecular beam epitaxy, patterned growth, shadow mask

We have performed selective area epitaxy (SAE) of CdTe layers grown by molecular beam epitaxy using a shadow mask technique. This technique was chosen over other SAE techniques due to its simplicity and its compatibility with multiple SAE patterning steps. Features as small as 50 microns x 50 microns were obtained with sharp, abrupt side walls and flat mesa tops. Separations between mesas as small as 20 microns were also obtained. Shadowing effects due to the finite thickness of the mask were reduced by placing the CdTe source in a near normal incidence position. Intimate contact between the mask and the substrate was essential in order to achieve good pattern definition.

Lattice Mismatch Induced Morphological Features and Strain in HgCdTe Epilayers on CdZnTe Substrates
DAVID R. RHIGER,¹ SANGHAMITRA SEN,¹ JEFFREY M. PETERSON¹ HUA CHUNG,² and MICHAEL DUDLEY²
1--Santa Barbara Research Center, 75 Coromar Dr., Goleta, CA 93117. 2--Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794.

KEY WORDS CdZnTe substrates, HgCdTe, lattice mismatch

HgCdTe epilayers on CdZnTe substrates can exhibit a cross-hatch pattern of periodically varying strain and surface undulations as revealed by x-ray topography, and in some cases by Nomarski optical microscopy. On {111} oriented material, the pattern appears as three sets of parallel lines in the <110> slip directions (60° apart). To investigate this phenomenon and its impact on photovoltaic device performance, we have characterized several liquid phase epitaxy (LPE)-grown HgCdTe epilayer samples by means of Lang x-ray reflection topography, synchrotron white beam x-ray topography (SWBXT), etch pit density, and other techniques. The cross hatching generally shows a correlation with the ZnTe mole fraction of the substrate. In particular, the pattern is likely to appear when the natural lattice parameter of the layer at room temperature is slightly larger or smaller than that of the substrate in the same region. We also find the corresponding pattern in {211} oriented layers grown by MBE. Although substantial compositional interdiffusion occurs at the layer/substrate interface during LPE growth at around 500°C, this is not a necessary condition for the cross-hatch pattern, as demonstrated by the occurrence of the pattern in MBE material grown at less than 200°C. In terms of device performance, the pattern is manifested as lines of diodes in an array having greater leakage than their neighbors. In addition to these results, we have investigated other anomalies, by means of SWBXT applied to large-area diodes that have been electrically tested. A novel technique called absorption edge contour mapping, using synchrotron white beam x-rays with a molybdenum filter, was applied to reveal the longer range lattice strain.

Optimization of the Structural Properties of Hg_1-x Cd_xTe (x = 0.18-0.30) Alloys: Growth and Modeling
A. PARIKH,¹ S.D. PEARSON,¹ R.N. BICKNELL-TASSIUS,¹ L.H. ZHANG,¹ R. BENZ,¹ and C.J. SUMMERS²
1--Advanced Materials Technology Division, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332-0850. 2--Advanced Materials Technology Division, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332-0850. e-mail: chris.summers@gtri.gatech.edu.

KEY WORDS HgCdTe, metalorganic molecular beam epitaxy (MOMBE), neural networks, thermodynamic model, void defects

The conditions for the metalorganic molecular beam epitaxial growth of Hg_1-xCd_xTe ( x = 0.18-0.32) alloys at very low growth temperatures (T 150°C) have been optimized by correlating the surface properties and crystalline perfection with the incident Hg flux. A window for growth has been defined for x = 0.18, 0.23, and 0.32. A thermodynamic model has been developed to account for void formation. A neural net model has been used for the first time to model the dependence of void density on the Hg flux and the x-ray rocking curve widths on growth parameters. The combination of these two complementary modeling techniques allows for a flexible process optimization to be carried out with a minimum effort spent in calibration runs.

SURFACES AND INTERFACES

KEY WORDS Alpha particle mapping, cadmium zinc telluride, etching, high resolution x-ray diffraction, polishing, radiation detectors

We examined the effects of polishing and etching on the structural and electrical properties of various high pressure Bridgman Cd_1-xZn_xTe (CZT) crystals using high resolution x-ray diffraction (HRXRD) and alpha particle mapping. Two etching solutions investigated are: (1) standard bromine-methanol solution, and (2) standard solution mixed with lactic acid. HRXRD, and in particular, triple axis x-ray diffraction (TAXRD) showed the effectiveness of the bromine-methanol etch in removing residual strain and damage from the CZT crystal. TAXRD mapping of a ~5 cm² CZT crystal after etching resulted in a reduction of the average rocking curve full width at half maximum to 15 arc-sec (compared to 23 arc-sec for the "as-received"). Alpha particle mapping of the electron risetime and the pulse height spectrum, along with leakage current measurements, showed varying effects of the different etching solutions on the surface properties (and hence their influence on the electrical and detector properties). These preliminary results show the importance and the sensitivity of the overall detector properties on the surface preparation conditions of CZT crystals used as radiation detectors.

The Use of Atomic Hydrogen for Substrate Cleaning for Subsequent Growth of II-VI Semiconductors
L.S. HIRSCH,¹ ZHONGHAI YU,¹ S.L. BUCZKOWSKI,¹ T.H. MYERS,² and M.R. RICHARDS-BABB³
1--Department of Physics, West Virginia University Morgantown, WV 26506-6045. 2--Department of Physics, West Virginia University Morgantown, WV 26506-6045. email: myers@wvu.edu. 3--Department of Chemistry, West Virginia University Morgantown, WV 26506-6045.

KEY WORDS II-VI semiconductors, atomic hydrogen, CdTe, HgCdTe, molecular beam epitaxy, surface cleaning, ZnSe

Atomic hydrogen is shown to be particularly efficacious for the preparation of substrates for subsequent growth of II-VI compounds by molecular beam epitaxy. A commercial thermal cracker was used to produce atomic hydrogen in the molecular beam epitaxy growth chamber for in-situ cleaning. This paper discusses the use of atomic hydrogen for both oxide removal from GaAs prior to ZnSe and CdTe growth, and for low-temperature oxide removal from CdTe and HgCdTe. Reflection high energy electron diffraction, ultra violet fluorescence microscopy, Nomarski interference contrast microscopy, and atomic force microscopy were used to characterize the growths.

Surface Cleaning and Etching of CdZnTe and CdTe in H₂/Ar, CH₄/H₂/Ar, and CH₄/H₂/N₂/Ar Electron Cyclotron Resonance Plasmas
ROBERT C. KELLER,² H. ZIMMERMANN,² M. SEELMANN-EGGEBERT,² and H.J. RICHTER²
1--Fraunhofer-Institut für Angewandte Festkörperphysik, 79108 Freiburg, Germany. Presently at Texas Instruments, Inc., Sensors and Infrared Laboratory, Dallas, TX 75265. 2--Fraunhofer-Institut für Angewandte Festkörperphysik, 79108 Freiburg, Germany.

KEY WORDS CdZnTe substrate cleaning, electron cyclotron resonance (ECR) plasma etching, HgCdTe epitaxy, long wavelength infrared devices

This paper compares H₂/Ar, CH₄/H₂/Ar, and CH₄/H₂/N₂/Ar plasma etch processes for CdZnTe and CdTe substrates in view of their potential to provide high-quality substrate surfaces for subsequent HgCdTe epitaxy. An electron cyclotron resonance source was used as plasma generator, and ellipsometry, angle-resolved x-ray photoelectron spectroscopy and low energy electron diffraction were applied to characterize roughness, composition, and order of the resulting substrate surfaces. It was found that CdZnTe is much more susceptible to evolving surface roughness under H₂/Ar plasma exposure than CdTe. The severe roughening observed at 100°C sample temperature was found to be correlated with a build-up of ZnTe at the surface, which suggests that the roughness formation may result from a preferential etching of the CdTe component. This surface degradation could be reduced by the addition of CH₄ to the process gases. However, only a further addition of nitrogen gas balanced and substantially improved the plasma process so that atomically clean, very smooth, and stoichiometrically composed CdZnTe surfaces of long-range order were eventually obtained.

Surface Passivation of HgCdTe by CdZnTe and Its Characteristics
T.S. LEE,¹ K.K. CHOI,¹ Y.T. JEOUNG,¹ H.K. KIM,¹ J.M. KIM,¹ Y.H. KIM,¹ J.M. CHANG,² W.S. SONG,² S.U. KIM,² M.J. PARK,² and S.D. LEE³
1--Agency for Defense Development, Youseong P.O. Box 35, Taejeon, Korea. 2--Korea University, Anamdong 5-1, Seongbugku, Seoul, Korea. 3--Wonju National Jr. College, Heungup-meun, Wonju, Korea.

KEY WORDS CdZnTe, HgCdTe, metal-insulator-semiconductor (MIS) capacitors, surface passivation

In this paper, we report the results of capacitance-voltage measurements conducted on several metal-insulator semiconductor (MIS) capacitors in which HgCdTe surfaces are treated with various surface etching and oxidation processes. CdZnTe passivation layers were deposited on HgCdTe surfaces by thermal evaporation after the surfaces were etched with 0.5-2.0% bromine in methanol solution, or thin oxide layers (t_ox ~ few ten Å) were grown on the surfaces, in order to investigate effects of the surface treatments on the electrical properties of the surfaces, as determined from capacitance-voltage (C-V) measurements at 80K and 1 MHz. A negative flat band voltage has been observed for MIS capacitors fabricated after etching of HgCdTe surfaces with bromine in methanol solutions, which is reported to make the surface Te-rich. It is inferred that residual Te on the surface is a positive charge, Te⁴⁺. C-V characteristics for MIS capacitors fabricated on oxide surfaces grown by air-exposure and electrolytic process have shown large hysteresis effects, from which it is inferred that imperfect and electrically active oxide compounds and HgTe particles near the surface become slow interface states.

New Surface Treatment Method for Improving the Interface Characteristics of CdTe/Hg_1-xCd_xTe Heterostructure
SEONG HOON LEE,¹ HYUNGCHEOL SHIN, ² HEE CHUL LEE,² and CHOONG KI KIM²
1--Department of Electrical Engineering and Center for Electro-Optics, Korea Advanced Institute of Science and Technology, 373-1 Kusong-Dong, Yusong-Gu, Taejon, 305-701 Korea. Also with Hyundai Electronics Industry; e-mail: sshlee@eekaist.kaist.ac.kr. 2--Department of Electrical Engineering and Center for Electro-Optics, Korea Advanced Institute of Science and Technology, 373-1 Kusong-Dong, Yusong-Gu, Taejon, 305-701 Korea.

KEY WORDS AFM, CdTe/HgCdTe, chemical oxidation of HgCdTe, fixed charge density, hysteresis capacitance-voltage (C-V) curve, MIS capacitors, slow surface state density

This paper presents a new simple method of HgCdTe surface treatment which consists of chemical oxidation of HgCdTe with nitric acid and removal of the oxide with ammonium hydroxide. The electrical properties of the electron-beam deposition CdTe passivation of Hg_0.7Cd_0.3Te are investigated with regard to the effects of HgCdTe surface etching, exposure to nitric acid, and the new surface treatment method. As the HgCdTe surface is progressively etched with bromine in methanol (Br-MeOH), the surface becomes rougher and a higher density of fixed charge is induced at the interface between CdTe and HgCdTe. Exposure to HNO₃ results in a very high density of fixed charge and performance degradation in metal insulator semiconductor (MIS) capacitors, which is due to the chemical oxide grown by HNO₃. The oxide growth rate is enhanced as the concentration of HNO₃ increases or as more H₂O is added. This oxide can be removed with NH₄OH. After the new surface treatment, MIS capacitors of Hg_0.7Cd_0.3Te show substantial improvement in electrical properties, such as low density of fixed charge and reduced hysteresis width, regardless of previous surface etching.

CHARACTERIZATION

KEY WORDS Critical point parameters, Hg1-xCdxTe, optical properties, spectroscopic ellipsometry

The alloy composition of Hg_1-xCd_xTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg_1-xCd_xTe by spectroscopic ellipsometry (SE). The spectral data of Hg_1-xCd_xTe with x = 1, 0.235, and 0.344 were obtained from room temperature to 800K in the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E₀, E₀+₀, E₁, E₁+₁, E₂(X), and E₂(). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.

Application of Urbach Rule Optical Absorption to Composition Measurement of Cd_1-yZn_yTe
A.J. SYLLAIOS, P.-K. LIAO, B.J. GREENE, H.F. SCHAAKE, H.-Y. LIU, and G. WESTPHAL
Texas Instruments Incorporated, Corporate Research & Development, P.O. Box 655936, MS 150, Dallas, TX 75265.

KEY WORDS Absorption coefficient, CdZnTe, Urbach rule, Vegard's law

The optical absorption coefficient of Cd_1-yZn_yTe near the fundamental band edge was measured at room temperature using transmission spectroscopy. Like in other II-VI semiconductors, it was found that the absorption coefficient exhibits an exponential dependence on incident photon energy according to Urbach's rule. It was also found that the exponential parameters depend on composition, y, of Cd_1-yZn_yTe. A technique is described for determining the composition of Cd_1-yZn_yTe from optical transmission spectroscopy. This technique has been implemented in the manufacturing of Cd_1-yZn_yTe substrates for lattice matched epitaxial growth of HgCdTe.

Electronic Structure, Absorption Coefficient, and Auger Rate in HgCdTe and Thallium-Based Alloys
SRINIVASAN KRISHNAMURTHY,¹ A.-B. CHEN,² and A. SHER¹
1--SRI International, 333 Ravenswood Ave., Bldg. R, Menlo Park, CA 94025-3493. 2--Physics Department, Auburn University, Auburn, AL 36349.

KEY WORDS Absorption coefficient, Auger rate, band structure, HgCdTe, infrared applications, minority carrier lifetime, thallium alloys

The band structures, absorption coefficients, and Auger recombination rates in narrow-gap alloys HgCdTe, InTlP, InTlAs, and InTlSb in the zinc blende structure, along with those of GaAs, are calculated using a hybrid pseudopotential and tight-binding method. The composition-dependent band gaps of the thallium-based alloys are reported along with those of several other semiconductor alloys. Within 50 meV from the absorption edge, the absorption coefficient of In_xTl_1-xP is found to have about the same magnitude as that of Hg_xCd_1-xTe and GaAs, while that of In_xTl_1-xAs and In_xTl_1-xSb is much smaller. In agreement with previous theories, the calculated Auger lifetimes in Hg_0.78Cd_0.22Te with unit or k · p overlap agree very well with experiments. Among the thallium alloys studied, the Auger lifetimes are longest in In_0.33Tl_0.67P, but still shorter than those in Hg_0.78Cd_0.22Te by an order of magnitude. In addition, realistic overlaps produce lifetimes one to two orders of magnitude larger than those observed.

Evaluation of Low-Temperature Interdiffusion Coefficients in Hg-Based Superlattices by Monitoring the E₁ Reflectance Peak
M.A. MATTSON¹ T.H. MYERS,² M.R. RICHARDS-BABB,³and J.R. MEYER⁴
1--Department of Physics, West Virginia University, Morgantown, WV 26506-6315. 2--Department of Physics, West Virginia University, Morgantown, WV 26506-6315. email: tmyers@WVU.EDU. 3--Department of Chemistry, West Virginia University, Morgantown, WV 26506-6045. 4--Code 5613, Naval Research Lab, Washington, DC 20375.

KEY WORDS HgTe-CdTe superlattice, k · p calculation, low temperature interdiffusion, molecular beam epitaxy

We show that variations of the E₁ reflectance peak in Hg-based superlattices can be used to probe low-temperature interdiffusion by monitoring the shift of the E₁ peak with time over extended periods. Little evidence of interdiffusion was detected for a number of HgTe/CdTe and HgCdTe/CdTe superlattices stored at room temperature for approximately two years. Two HgTe/CdTe superlattices and one HgCdTe/CdTe superlattice were subsequently annealed in a dry nitrogen atmosphere at 100°C for approximately six months, and then at 150°C for 24 days. During these intervals, the superlattices were periodically removed from the anneal for reflectance measurements to assess the extent of the interdiffusion. Comparison of these results with calculations of superlattice bandgaps and interdiffusion profiles has led to an evaluation of the low temperature interdiffusion coefficients. These extend previous results to lower temperatures and confirm that the degradation of Hg-based superlattices devices due to thermal interdiffusion under normal processing, storage, and operating conditions should not be an issue of concern.

Inter-Layer Subband Mixing in MBE-Grown HgTe/CdTe Superlattices
J.W. PARK,¹ JAESUN LEE,¹ Y.M. MOON,¹ J.B. CHOI,² M.S. HAHN,³ B.K. SONG,³ Y.B. HOU,³ T.W. KANG,³ KYUNG-HWA YOO,⁴C.A. HOFFMAN,⁵ J.R. MEYER,⁵ Y.T. JEOUNG,⁶ H.K. KIM,⁶ and J.M. KIM⁶
1--Department of Physics, Chungbuk National University, Cheongju 360-763, Korea. Department of Physics, Chungbuk National University, Cheongju 360-763, Korea. e-mail: jungchoi@cbucc.chungbuk.ac.kr. 3--Department of Physics, Dongguk University, Seoul 100-715, Korea. 4--Korea Research Institute of Standards and Science, Taejon 305-763, Korea. 5--Naval Research Laboratory, Washngton, DC. 20475. 6--Agency for Defense Development, Taejon 300-600, Korea.

KEY WORDS Effective 2D channels, HgTe/CdTe superlattice, inter-layer subband mixing, magnetotransport, quasi-3D growth-direction energy dispersion

We report transport results for molecular beam epitaxially grown HgTe/CdTe superlattices (SLs) with CdTe barrier thickness of L_b systematically varied from 100Å down to 20Å, and demonstrate the increasing intersubband mixing between adjacent layers with decreasing L_b. Magnetotransport data measured at 1.5K and for B up to 10T show monotonously varying features with L_b, implying a dimensional change from 2D to quasi-3D. As L_b decreases, the quantum Hall plateaus become weaker and the number of effective 2D channels are decreased, which indicates increasing growth-direction energy dispersion. This enhanced interlayer subband coupling with decreasing L_b is further confirmed by the angle-dependence of SdH oscillations. For L_b = 20Å, all of 2D-related transport behavior are washed out. A quantitative analysis with calculated subband energy dispersion relations demonstrates the close interplay between superlattice barrier thickness and the strength of the intersubband mixing. These transport results are directly related to successful control of the growth-axis carrier effective mass and will contribute to the development of high-performance HgTe/CdTe SL-based IR photo-voltaic devices and lasers, in which tunneling noise due to diffusion currents can be considerably reduced.

Improved Determination of Matrix Compostion of Hg_1-xCd_xTe by SIMS
JACK SHENG, LARRY WANG, GAYLE E. LUX, and YUMIN GAO
Charles Evans & Associates, 301 Chesapeake Dr., Redwood City, CA 94063.

KEY WORDS Composition analysis, heterostructure layer thickness calibration, HgCdTe, secondary ion mass spectrometry (SIMS)

Results are presented to show an improved method for composition characterization of HgCdTe heterostructure using secondary ion mass spectroscopy. This method utilizes the molecular ions CsM⁺ rather than M^+/- ions. The advantage is that the molecular CsM⁺ ion yield, unlike the atomic M^+/- ions, is quite insensitive to the matrix material from which they are emitted. Composition of multilayer HgCdTe structure can be determined with excellent accuracy and depth resolution. Layer thickness of HgCdTe heterostructure can also be calibrated.

An Optical Alternative to the Hall Test
FREDERICK W. CLARKE
Weapons Sciences Directorate, AMSMI-RD-WS-CM, Research, Development, and Engineering Center, U.S. Army Missile Command, Redstone Arsenal, AL 35898-5248. Present address: 8253 Old Madison Pike, Madison, AL 35758.

KEY WORDS Electron effective mass, electron mobility, Faraday rotation, free carrier absorption, IR detector materials, magneto-optical, nondestructive testing

Effective mass ratios, m*, of electrons in n-type InSb, GaAs, and near intrinsic and n-type Hg_1-xCd_xTe for 0.20 < x < 0.30 over the temperature range 77K < T < 296K were measured using Faraday rotation spectroscopy. m* ranged from 0.0186 to 0.0357 for InSb with carrier concentrations, N, in the range 1.76 < N <; 110 x 10¹⁶ cm^-3 at 296K, in good agreement with available values in the literature. Effective masses of HgCdTe were found to be about twice as large at room temperature as band edge effective mass, m*_be calculations. These calculations can be corrected for thermal excitation by adding a factor, m**, to the band edge calculation: m* = m** m*_be, where m** was found empirically to be m** = 4.52 x 10^-3 T + 0.78. The electron's mobility is proportional to the ratio of the electron's Faraday rotation to its absorption; that is, the absorption due to the intraband transitions of the electron itself, not the sample's total absorption, which may include holes, interband transitions, and the like. The constant of proportionality, or the "mobility constant", was measured in n-type GaAs and InSb doped above 18 x 10¹⁶ cm^-3 using absorption directly. Both HgCdTe and InSb have large intrinsic carrier concentrations, on the order of 10¹⁶ cm^-3. Hole absorption is the majority component of the sample's absorption at lower n-type dopant concentrations. In these cases, the mobility constant was determined using an absorption cross section.

X-Ray Rocking Curve Analysis of Ion Implanted Mercury Cadmium Telluride
B.L. WILLIAMS,¹ H.G. ROBINSON,² C.R. HELMS,² and N. ZHU³
1--Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. 2--Department of Electrical Engineering, Stanford University, Stanford, CA 94305. 3--Phillips Semiconductors, Sunnyvale, CA 94088.

KEY WORDS Defects, dislocations, HgCdTe, ion implantation, mercury cadmium telluride, transmission electron microscopy (TEM), x-ray diffraction

Junction formation by ion implantation is a critical step in producing high quality infrared focal plane arrays in mercury cadmium telluride (MCT). We have analyzed the structural properties of MCT implanted with B at doses of 10¹⁴ and 10¹⁵ cm^-2 using double and triple crystal x-ray diffraction (DCD and TCD) to monitor the disorder and strain of the implanted region as a function of processing conditions. TCD (333) reflections show that a distinct tensile peak is produced by the high dose implant while the low dose implant shows only a low angle shoulder on the substrate peak. A preliminary association of the low angle shoulder with point defects has been made since no extended defects have been observed in the low dose range. For the high dose implant, extended defect formation has been reported and may be responsible for the tensile peak. After annealing, the low angle shoulder on the low dose implant has disappeared, while the high dose implant exhibits an increase in the tensile strain from 6.5 x 10^-4 to 9.3 x 10^-4 after 24 h of annealing and then decreases in tensile strain to 7.3 x 10^-4 after 48 h of annealing. It is believed the changes in strain are associated with the Oswald ripening and dissolution of extended defects, which has been observed during annealing of ion implanted Si.

Characterization of CdTe/Hg_1-xCd_xTe Heterostructures by High-Resolution X-Ray Diffraction
N. MAINZER,¹ D. SHILO,² E. ZOLOTOYABKO,² G. BAHIR,³ and A. SHER⁴
1--Faculty of Materials Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel or SCD-Semiconductor Devices, P.O. Box 2250/99, Haifa 31021, Israel. 2--Faculty of Materials Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel. 3--Faculty of Electrical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel. 4--Department of Solid State Physics, Soreq NRC, Yavne 81800, Israel.

KEY WORDS CdTe/Hg1-xCdxTe, heterostructure, implantation, x-ray diffraction

CdTe/Hg_1-xCd_xTe heterostructures variously heat-treated and implanted with boron were studied by means of high-resolution x-ray diffraction. A novel procedure for simulating diffraction spectra was developed, which is based on direct summation of scattered waves across a heterostructure. In that routine, short-range variations of structural parameters, including concentration of the components, can be taken into account. The new approach allows precise characterization of II-VI heterostructures, because it correctly treats atomic diffusion effects in diffraction spectra. As a result, subtle spectral modifications induced by boron implantation could be detected and were attributed to the dynamics of post implantation point defects.

DEFECTS

KEY WORDS CdTe, diffusion, structural defects, zinc, ZnxCd1-xTe

CdTe slices have been diffused in sealed silica capsules under conditions of saturated vapor pressure due to zinc in the temperature range 390-950°C. All slices annealed with zinc at temperatures above 450°C displayed extensive surface cracking both by slip along specific crystallographic planes and by brittle fracture, whereas below this temperature no such effects were detected. In addition, some slices diffused at 800°C with small quantities of zinc, which was insufficient to maintain saturated vapor pressure throughout the diffusion experiment, did not show any cracks. The interpretation of this behavior can be divided into three temperature ranges and can be explained in terms of the critical resolved shear stress and the misfit ratio. This is discussed extensively in this paper.

Numerical Simulation of Clustering Phenomena for Point-Defects in HgCdTe
IWAO SUGIYAMA, NOBUYUKI KAJIHARA, and YOSHIHIRO MIYAMOTO
Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan.

KEY WORDS Defects, diffusion, HgCdTe, random telegraph noise (RTN), simulation

We investigated the formation of cluster defects in HgCdTe materials by numerical simulation. The equations used for the simulation include change in the activation energy for Hg vacancy diffusion dependent on the local cadmium composition. This study demonstrated how defects grow during thermal treatment. We simulated the annealing process for initial composition fluctuations in uniform n-type Hg_0.78Cd_0.22Te. When the local composition fluctuations increase, we found that the concentration of vacancies around the initial core increases because of the composition difference between the initial core and bulk region. Concurrently, if the fluctuation range is narrow, the resulting large constant for interstitial diffusion accelerates this process. We determined that the critical fluctuation range is in the order of 50 nm for annealing at 150°C. The concentration of clustering vacancies around the initial core reached 10¹⁶ cm^-3, meaning that the conductivity of the cluster region changed to the p-type. We conclude that such cluster defects have a structure consisting of a core surrounded by a p-type shell.

Mode of Arsenic Incorporation in HgCdTe Grown by MBE
S. SIVANANTHAN,¹ P.S. WIJEWARNASURIYA,¹ F. AQARIDEN,¹ H.R. VYDYANATH,² M. ZANDIAN,³ D.D. EDWALL,³ and J.M. ARIAS³
1--Microphysics Laboratory, Department of Physics (M/C 273), University of Illinois at Chicago, 845 W. Taylor St., Room 2236, Chicago, IL 60680. 2--Microphysics Laboratory, Department of Physics (M/C 273), University of Illinois at Chicago, 845 W. Taylor St., Room 2236, Chicago, IL 60680 or Avyd Devices, P.O. Box 7942, Huntington Beach, CA 92615-7942. 3--Science Center, Rockwell International Corporation, 1049 Camino Dos Rios, Thousand Oaks, CA 91358.

KEY WORDS Arsenic doping, HgCdTe, molecular beam epitaxy (MBE), p-type doping, secondary ion mass spectrometry (SIMS)

The results of arsenic incorporation in HgCdTe layers grown by molecular beam epitaxy (MBE) are reported. Obtained results indicate that arsenic was successfully incorporated as acceptors in MBE-HgCdTe layers after a low temperature anneal. Secondary ion mass spectrometry and Hall effect measurements confirm that arsenic is incorporated with an activation yield of up to 100%. This work confirms that arsenic can be used as an effective dopant of MBE-HgCdTe after a low temperature annealing under Hg-saturated conditions.

Behavior of p-Type Dopants in HgCdTe
M.A. BERDING, A. SHER, and M. VAN SCHILFGAARDE
SRI International, Menlo Park, CA 94025.

KEY WORDS Amphoteric dopants, doping, HgCdTe

Obtaining high concentrations of active p-type dopants in HgCdTe is an issue of much current interest. We discuss the results of our calculations on column IB and VA dopants. The full-potential linear muffin-tin orbital method, based on the local density approximation is used to calculate electronic total energies and localized levels in the band gap. Free energies are predicted and incorporated into a thermodynamical model to calculate impurity and native defect concentrations as a function of temperature, stoichiometry, and total impurity density. Copper, silver, and gold are found to be incorporated nearly exclusively on the metal sublattice and to be 100% active for all near-equilibrium growth and processing conditions. The density of interstitial copper is high enough to impact copper diffusion. In contrast, significant concentrations of phosphorus, arsenic, and antimony are found on the metal sublattice where they behave as n-type dopants, accounting for highly compensated, or even n-type material, depending on the equilibration temperature and equivalent mercury partial pressure.

Modeling of Junction Formation and Drive-In in Ion Implanted HgCdTe
S. HOLANDER-GLEIXNER,¹ B.L. WILLIAMS,¹ H.G. ROBINSON,² and C.R. HELMS²
1--Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. 2--Department of Electrical Engineering, Stanford University, Stanford, CA 94305.

KEY WORDS Diffusion limited, flexible manufacturing, HgCdTe, infrared focal plane arrays, ion implantation, n-on-p, process modeling

N-on-p junction formation and drive-in in ion implanted Hg_0.8Cd_0.2Te photodiodes have been studied. A model of the junction formation and drive-in processes has been developed that accounts for the variations in injected Hg interstitial concentration, background point defect and extrinsic doping levels, sample geometry, and annealing conditions. The limiting mechanisms controlling junction drive-in were investigated using the model. Experimental data showed the junction drive-in rate was proportional to the square root of time, indicating a diffusion limited process. The diffusion limited process is the result of a solubility limit for the Hg interstitial concentration. This limit is approximately the same value as that obtained for Hg interstitials in Hg saturated Hg_0.8Cd_0.2Te in type conversion and self-diffusion experiments (D_IC_I = 1.43 x 10¹³exp(-.457 eV/ kT)*P_Hg).

DEVICES

KEY WORDS Defects, diode, HgCdTe, lateral collection, surface inversion channel, surface recombination

Variable-area diodes are routinely used to separate surface from bulk effects in the characterization of HgCdTe photodiodes. In this work, the traditional models are reviewed and clarified by comparison to the results of numerical models of diffusion and other effects. It is shown that the characteristic lengths associated with lateral collection of both thermal and photo-generated carriers are not necessarily equal to each other and also are not equal to the minority carrier diffusion length. The effective surface recombination velocity in the traditional model for surface generation-recombination current is shown to have an alternative explanation as the effect of a surface inversion channel. Finally, the relation of the traditional models to the effects of point defects is examined. The average effects of defects with bulk or surface distributions are shown to contribute either zero or positive slope, respectively, in a plot of the inverse of the zero-bias resistance-area product vs the perimeter to area ratio. A negative slope arises from defects whose conductance increases faster than linearly with the diode radius. Distributions of diode dynamic resistance data from over 100 HgCdTe test arrays were analyzed to determine how the probability of a diode being defective varies with diode size. Plots of the diode conductance data ordered by increasing conductance indicate that the onset of defects occurs for a percentage of diodes of a given size which increases approximately linearly with the diode perimeter. This suggests that these defects have a perimeter rather than bulk origin.

1/f Noise Studies in Uncooled Narrow Gap Hg_1-xCd_xTe Non-Equilibrium Diodes
C.T. ELLIOTT,¹ N.T. GORDON,¹ R.S. HALL,¹ T.J. PHILLIPS,¹ C.L. JONES,² and A. BEST²
1--DRA Malvern, St. Andrews Road, Malvern, WR14 3PS, UK. 2--GEC Marconi Infrared Ltd., Millbrook Industrial Estate, Southampton, UK.

KEY WORDS 1/f noise, HgCdTe, narrow-gap, non-equilibrium diodes, uncooled

We have studied the 1/f noise current in narrow gap semiconductor heterostructure diodes fabricated in mercury cadmium telluride (HgCdTe) and designed to operate in a non-equilibrium mode at room temperature. HgCdTe heterostructure diodes exhibit Auger suppression giving current-voltage characteristics with high peak-to-valley ratios (up to 35), and low extracted saturation current densities (e.g., 20 Acm^-2 at 10 µm at 295K) but high 1/f knee frequencies (e.g., 100 MHz at 10 µm at 295K). A comparison is made with the noise levels found in room temperature non-equilibrium mode heterostructure InAlSb/InSb diodes. The devices are being used at high frequencies for CO₂ laser heterodyne detector demonstrators. For the devices to be useful in low frame-rate imaging arrays, the 1/f noise level must be sufficiently low that the signal is not swamped. Ideally, the knee frequency should be below the frame rate. The relationship between the noise current and reverse bias voltage, current density, and temperature will be examined in order to attempt to identify the principal 1/f generation mechanisms.

High Performance SWIR HgCdTe Detector Arrays
L.O. BUBULAC,¹ W.E. TENNANT,¹ J.G. PASKO,¹ L.J. KOZLOWSKI,¹ M. ZANDIAN,¹ M.E. MOTAMEDI,¹ R.E. DeWAMES,¹ J. BAJAJ,¹ N. NAYAR,¹ W.V. McLEVIGE,¹ N.S. GLUCK,¹ R. MELENDES,¹ D.E. COOPER,¹ D.D. EDWALL,¹ J.M. ARIAS,¹ and R. HALL,¹ A.I. D'SOUZA²
1--Rockwell Science Center, Inc., 1049 Camino Dos Rios, Thousand Oaks, CA 91360. 2--Rockwell Electro-Optical Center, 3370 Miraloma Ave., Anaheim, CA 92803.

KEY WORDS HgCdTe, IR detectors, molecular beam epitaxy (MBE)

Short wave infrared (SWIR) devices have been fabricated using Rockwell's double layer planar heterostructure (DLPH) architecture with arsenic-ion implanted junctions. Molecular beam epitaxially grown HgCdTe/CdZnTe multilayer structures allowed the thin, tailored device geometries (typical active layer thickness was ~3.5 µm and cap layer thickness was ~0.4 µm) to be grown. A planar-mesa geometry that preserved the passivation advantages of the DLPH structure with enhanced optical collection improved the performance. Test detectors showed Band 7 detectors performing near the radiative limit (~3-5X below theory). Band 5 detector performance was ~4-50X lower than radiative limited performance, apparently due to Shockley-Hall-Read recombination. We have fabricated SWIR HgCdTe 256 x 12 x 2 arrays of 45 µm x 45 µm detector on 45 µm x 60 µm centers and with cutoff wavelength which allows coverage of the Landsat Band 5 (1.5-1.75 µm) and Landsat Band 7 (2.08-2.35 µm) spectral regions. The hybridizable arrays have four subarrays, each having a different detector architecture. One of the Band 7 hybrids has demonstrated performance approaching the radiative theoretical limit for temperatures from 250 to 295K, consistent with test results. D* performance at 250K of the best subarray was high, with an operability of ~99% at 10¹² cm Hz^1/2/W at a few mV bias. We have observed 1/f noise below 8E-17 AHz^-1/2 at 1 Hz. Also for Band 7 test structures, Ge thin film diffractive microlenses fabricated directly on the back side of the CdZnTe substrate showed the ability to increase the effective collection area of small (nominally <20 µm µm) planar-mesa diodes to the microlens size of 48 µm. Using microlenses allows array performance to exceed 1-D theory up to a factor of 5.

VSWIR to VLWIR MBE Grown HgCdTe Material and Detectors for Remote Sensing Applications
A.I. D'SOUZA,¹ L.C. DAWSON,¹ E.J. ANDERSON,¹ and A.D. MARKUM¹ W.E. TENNANT,² L.O. BUBULAC,² M. ZANDIAN,² J.G. PASKO,² W.V. McLEVIGE,² D.D. EDWALL,² J.W. DERR,³ and J.E. JANDIK³
1--Boeing North American Electro-Optical Center, Anaheim, CA 92803. 2--Rockwell Science Center, Thousand Oaks, CA 91360. 3--ITT Defense & Electronics, Fort Wayne, IN 46801.

KEY WORDS HgCdTe, molecular beam epitaxy (MBE), remote sensing

The molecular beam epitaxy (MBE) growth technology is inherently flexible in its ability to change the Hg_1-xCd_xTe material's bandgap within a growth run and from growth run to growth run. This bandgap engineering flexibility permits tailoring the device architecture to the various specific requirements. Material with active layer x values ranging from ~0.198 to 0.570 have been grown and processed into detectors. This wide range in x values is perfectly suited for remote sensing applications, specifically the National Polar Orbiting Environmental Satellite System (NPOESS) program that requires imaging in a multitude of infrared spectral bands, ranging from the 1.58 to 1.64 µm VSWIR (very short wave infrared) band to the 11.5 to 12.5 µm LWIR (long wave infrared) band and beyond. These diverse spectral bands require high performance detectors, operating at two temperatures; detectors for the VSWIR band operate near room temperature while the SWIR, MWIR (mid wave infra red), LWIR and VLWIR (very long wave infrared) detectors operate near 100K, because of constraints imposed by the cooler for the NPOESS program. This paper uses material parameters to calculate theoretical detector performance for a range of x values. This theoretical detector performance is compared with median measured detector optical and electrical data. Measured detector optical and electrical data, combined with noise model estimates of ROIC performance are used to calculate signal to noise ratio (SNR), for each spectral band. The SNR are compared with respect to the meteorological NPOESS system derived focal plane. The derived system focal plane requirements for NPOESS are met in all the spectral bands.

Photocurrent Effect on the Zero-Bias Dynamic Resistance of HgCdTe Photodiode
KWAN KIM, HAN JUNG, HYUNGCHEOL SHIN, HEE CHUL LEE, and CHOONG-KI KIM
Department of Electrical Engineering and Center for Electro-Optics, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Taejon 305-701, Korea.

KEY WORDS Depletion region, HgCdTe, photocurrent, RoA

A new physical model is presented for the illumination-dependence of the zero-bias resistance-area product (R_oA) of HgCdTe photodiode. The model is based on three independent mechanisms. They are the depletion region volume change with the applied bias, the diffusion distance change with the moving depletion region edge, and the minority carrier accumulation in the depletion region which affects the minority carrier diffusion. Analytic equations are derived for the photodiode current-voltage characteristics and R_oA products. The results of the model have been compared with experimental data obtained from several Hg_0.7Cd_0.3Te diodes with an identical diode structure having different absorbing amount of light. The model showed good agreement with the experimental data.

Resonant-Cavity Infrared Optoelectronic Devices
J.L. PAUTRAT,¹ E. HADJI,² J. BLEUSE,² and N. MAGNEA²
1--Departement de Recherche Fondamentale sur la Matière Condensée, SP2M/PSC, CEA-Grenoble. 17 rue des Martyrs- F 38054 Grenoble Cedex 9, France. e-mail: jpautrat@cea.fr. 2--Departement de Recherche Fondamentale sur la Matière Condensée, SP2M/PSC, CEA-Grenoble. 17 rue des Martyrs- F 38054 Grenoble Cedex 9, France.

KEY WORDS HgCdTe, infrared optoelectronic devices, molecular beam epitaxy, resonant cavity

The Cd_xHg_1-xTe compounds are well suited to the design of resonant microcavity devices. Indeed these compounds display a wide variation of bandgap and refractive index with composition, while the lattice parameter remains practically unchanged. Microcavities resonating in the 3-5 µm range have been prepared by molecular beam epitaxy. Light emitting diodes (LEDs) are obtained by stacking a lower Bragg mirror (10.5 periods) which is doped n-type and a nominally undoped cavity medium containing a 50 nm active layer (CdTe-HgTe pseudo-alloy). The upper mirror is a gold layer deposited on the cavity, which is partly p-type doped. The diode emission is observed under direct bias, up to room temperature, in coincidence with the cavity resonance mode (linewidth 8 meV). It is much narrower than the inhomogeneous linewidth of the active layer (60 meV at 300K). The directivity is also much better. The diode properties are only very slightly dependent on temperature. A similar device can also be designed to make an infrared detector whose active layer thickness is reduced with respect to conventional detectors. The detector efficiency at the resonance wavelength may be increased by a factor close to the cavity finesse. With 16.5 periods in the lower mirror and a dielectric mirror as upper mirror (seven periods of ZnS/YF₃), it has been possible to make a cavity resonating at 3.06 µm whose quality factor reaches 350. By photopumping the cavity across the dielectric mirror with a YAG microlaser, a laser emission occurred at the cavity resonance. At 10K, the laser threshold is 45 kW/cm² and the linewidth is only 1.7 meV. These results demonstrate the usefulness of the microcavity concept for designing new devices such as LED or lasers which could be the basis for new applications of CdHgTe compounds.

Modeling of Heterojunction HgCdTe Photodiodes Using Approximate k · p Approach
V. ARIEL¹ and G. BAHIR²
1--IBM Israel - Science and Technology, MATAM, Haifa 31905, Israel. 2--Department of Electrical Engineering, Technion, Haifa 32000, Israel.

KEY WORDS Heterojunction, HgCdTe, k · p approach, modeling

In this paper, we describe a new approach to simulation of HgCdTe devices based on an approximation for the k · p model. We use an approximate expression for the carrier density which includes effects of carrier degeneracy and conduction band nonparabolicity. These effects are essential for accurate modeling of HgCdTe devices. The simplicity of this approximation allows us to use well-established methods of semiconductor device modeling. On the other hand, the new approach demonstrates similar accuracy to more complex models based on numerical evaluation of the nonparabolic Fermi-Dirac integral. The new model is applied to energy band calculation of HgCdTe heterojunctions and dark current calculation of HgCdTe photodiodes.

Device Modeling of HgCdTe Vertically Integrated Photodiodes
D.H. MAO,¹ H.G. ROBINSON,² D.U. BARTHOLOMEW,³ and C.R. HELMS²
1--Department of Electrical Engineering, Stanford University, Stanford, CA 94305. 2--Applied Physics Dept., Stanford University, Stanford, CA 94305. 3--Central Research Laboratories, Texas Instruments, Dallas, TX 75265.

KEY WORDS Burstein-Moss shift, HgCdTe, ion implant, nonparabolic, photodiode, surface charge, tunneling

Simulations of current-voltage characteristics of ion-implanted n-on-p photodiodes have been performed using SemiCad Device. In order to accurately simulate this device structure, several modifications to the simulator were implemented. These include the modified carrier statistics to account for the nonparabolic band structure of HgCdTe, the correct physics parameters for Shockley-Read-Hall, optical, and Auger recombination, and the Burstein-Moss shift for optical absorption important for heavily doped n-type HgCdTe. With these and other improvements, SemiCad Device is calibrated with the measured ideal dark current of an ion implanted diode and is used to simulate a source of non-ideal dark current from surface-charge induced band-to-band tunneling.>

MODELING

KEY WORDS IR detectors, phase diagrams, thallium

Thermodynamical factors that affect growth of the thallium-bearing zinc-blende alloys InTlP and InTlAs are examined within the local density approximation (LDA), using the linear muffin-tin orbital method. The LDA predicts TlP and TlAs to be unstable with respect to decomposition into the elemental constituent solids, or marginally stable if conservative estimates of the LDA errors are made. Several thallium-rich and anion-rich compounds have also been examined; some are found to have excess energies per atom comparable to the zinc-blende phase. The equilibrium partial pressures over InTlP and InTlAs have also been calculated as a function of composition. Even with conservative error estimates, we predict that only low concentrations of thallium can be achieved in InTlP (<5% at 350°C) using gas source molecular beam epitaxy (GSMBE), far less than the 67% needed for long-wave infrared (LWIR) applications. Although much less than 1% thallium is predicted to be soluble in InTlAs for GSMBE growth at 350°C, the addition of error estimates into the calculation indicates that obtaining the 15% thallium needed for LWIR applications may be possible. Native defect populations have been calculated for alloy compositions corresponding to band gaps in the LWIR, and the anion antisite densities are predicted to be quite high, especially in InTlP, reflecting the comparable stability of the TIP and TlP₃ phases.

Applications of Thermodynamical Modeling in Molecular Beam Epitaxy of Cd_xHg_1-xTe
T. COLIN¹ and T. SKAULI²
1--Norwegian Defence Research Establishment, Division for Electronics, P.O. Box 25, N-2007 Kjeller, Norway. e-mail: Thierry.Colin@ffi.no. 2--Norwegian Defence Research Establishment, Division for Electronics, P.O. Box 25, N-2007 Kjeller, Norway.

KEY WORDS Condensation and re-evaporation coefficients, HgCdTe, molecular beam epitaxy (MBE), Te-rich phase boundary, thermodynamic prediction of optimal growth temperature

It is well known that the crystalline quality of Cd_xHg_1-xTe grown by molecular beam epitaxy is critically dependent on the substrate temperature. The optimal growth temperature has been identified immediately below the crossing of the Te-rich phase boundary, that is just below the temperature range where Te precipitation occurs in the layer. It is potentially very useful to be able to predict the optimal temperature and its variation with other growth parameters, but no general guidelines for this can be found in the literature. We have studied experimentally the variation of the optimal growth temperature with Hg flux, Cd mole fraction and growth rate. These results are compared with a thermodynamical model published previously by Gailliard. We find that the modeled position of the phase boundary coincides well with the observed variations in optimal growth temperature for growth on Te-terminated surfaces, within the uncertainties of available thermodynamical constants. We show that the optimal substrate temperature depends mainly on the Hg flux and Cd mole fraction, while the dependence on growth rate can be neglected in practical molecular beam epitaxy conditions. The experimental observation of optimal layer quality at the phase boundary could suggest the existence of an adsorbed layer of Te, acting as a reservoir for Te atoms and reducing the supersaturation of the growth reaction. Simultaneous growth on the (211)B and (100) orientations reveals a clear, although not very large, difference in optimal growth temperature and Cd incorporation, indicating a difference in growth kinetics. This can be accounted for in the thermodynamical model by condensation and evaporation coefficients.

WIDE-GAP MATERIALS

KEY WORDS Blue-green laser diodes, metalorganic vapor phase epitaxy (MOVPE), ZnSSe

ZnS_ySe_1-y/ZnSe/GaAs (001) heterostructures have been grown by photoassisted metalorganic vapor phase epitaxy, using the sources dimethylzinc, dimethylselenium, diethylsulfur, and irradiation by a Hg arc lamp. The solid phase composition vs gas phase composition characteristics have been determined for ZnS_ySe_1-y grown with different mole fractions of dimethylselenium and different temperatures. Although the growth is not mass-transport controlled with respect to the column VI precursors, the solid phase composition vs gas phase composition characteristics are sufficiently gradual so that good compositional control and lattice matching to GaAs substrates can be readily achieved by photoassisted growth in the temperature range 360°C T 400°C. ZnSe/GaAs (001) single heterostructures were grown by a two-step process with ZnSe thicknesses in the range from 54 nm to 776 nm. Based on 004 x-ray rocking curve full width at half maximums (FWHMs), we have determined that the critical layer thickness is h_c 200 nm. Using the classical method involving strain, lattice relaxation is undetectable in layers thinner than 270 nm for the growth conditions used here. Therefore, the rocking curve FWHM is a more sensitive indicator of lattice relaxation than the residual strain. For ZnS_ySe_1-y layers grown on ZnSe buffers at 400°C, the measured dislocation density-thickness product Dh increases monotonically with the room temperature mismatch. Lower values of the Dh product are obtained for epitaxy on 135 nm buffers compared to the case of 270 nm buffers. This difference is due to the fact that the 135 nm ZnSe buffers are pseudomorphic as deposited. For ZnS_ySe_1-y layers grown on 135 nm ZnSe buffers at 360°C, the minimum dislocation density corresponds approximately to room-temperature lattice matching (y ~ 5.9%), rather than growth temperature lattice matching (y ~ 7.6%). Epitaxial layers with lower dislocation densities demonstrated superior optical quality, as judged by the near-band edge/deep level emission peak intensity ratio and the near band edge absolute peak intensity from 300K photoluminescence measurements.

Molecular Beam Epitaxial Growth of P-ZnSe:N Using a Novel Plasma Source
K. KIMURA,^1,2,3 S. MIWA,^1,2,4 T. YASUDA,^1,5 L.H. KUO,^1,2 A. OHTAKE,^1,2 C.G. JIN,^1,6 K. TANAKA,^1,5,6 and T. YAO^1,5,7
1--Joint Research Center for Atom Technology, 1-1-4 Higashi, Tsukuba 305, Japan. 2--Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba 305, Japan. 3--On leave from Sumitomo Electric Industry Ltd. 4--On leave from Sony Corp. 5--National Institute for Advanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba 305, Japan. 6--University of Tsukuba, 1-1 Tennohdai, Tsukuba 305, Japan. 7--Japan or Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan.

KEY WORDS Molecular beam epitaxy (MBE), nitrogen doping, plasma source, ZnSe

We have studied the p-type doping in ZnSe molecular beam epitaxial growth using a novel high-power (5 kW) radio frequency (rf) plasma source. The effect of growth conditions such as the rf power, the Se/Zn flux ratio and the growth temperature on p-ZnSe:N was investigated. The net acceptor concentration (N_A - N_D) of around 1 x 10¹⁸ cm^-3 was reproducibly achieved. The activation ratio ((N_A - N_D)/[N]) of p-ZnSe:N with N_A - N_D of 1.2 x 10¹⁸ cm^-3 was found to be as high as 60%, which is the highest value so far obtained for N_A - N_D ~ 10¹⁸ cm^-3. The 4.2K photoluminescence spectra of p-ZnSe:N grown under the optimized growth condition showed well-resolved deep donor-acceptor pair emissions even with high N_A - N_D.

A Comparison of Ethyl Iodide and Hydrogen Chloride for Doping ZnSe Grown by Photoassisted MOVPE
D.W. PARENT,¹ S. KALISETTY,² X. G. ZHANG,² G. ZHAO,² W. ZAPPONE,² J. ROBINSON,² E. HELLER,² J.E. AYERS,² and F.C. JAIN²
1--260 Glenbrook Road, Storrs, CT 06269-2157. e-mail:dave@newton.ee.uconn.edu. 2--Electrical and Systems Engineering Department, The University of Connecticut, 06269-2157.

KEY WORDS Blue-green lasers, metalorganic vapor phase epitaxy (MOVPE), n-type doping, photoassisted MOVPE, ZnSe

We have conducted a study of the electrical and photoluminescence properties of ZnSe films grown by photoassisted metalorganic vapor phase epitaxy (MOVPE) (250 Torr, 400°C) with ethyl iodide and hydrogen chloride as n-type dopant sources. A higher peak electron concentration and a lower minimum resistivity were observed using hydrogen chloride (5.4 x 10¹⁸ cm^-3, and .0070 ohm-cm, respectively), as opposed to ethyl iodide (1.55 x 10¹⁷ cm^-3, and 0.067 ohm-cm, respectively). We show that the higher electron concentrations observed in the chlorine doped layers are due to a higher incorporation of chlorine atoms than that of iodine atoms, and that this may be a result of the different tetrahedral misfit factors for these atoms. Our photoluminescence and 77K Hall effect data support this conclusion. Growth rate depression was observed to be more severe for iodine doped layers than for chlorine doped layers. Thus, it appears that hydrogen chloride is a superior dopant source for low-temperature photoassisted MOVPE ZnSe growth of n-type layers for blue-green laser diodes in the pressure-temperature regime investigated.

Degradation of ZnSe/ZnTe Multiquantum Well Contacts to p-ZnSe
JOHN J. FIJOL¹ and PAUL H. HOLLOWAY²
1--University of Florida, Gainesville, FL 32611. Currently at Lockheed-Martin IR Systems, Lexington. MA. 2--University of Florida, Gainesville, FL 32611. Currently at Lockheed-Martin IR Systems, Lexington.

KEY WORDS Degradation, multiquantum well (MQW), ohmic contacts, ZnSe

The work presented in this paper studied the degradation of ZnTe/ZnSe multiquantum well contacts to p-ZnSe under high current loading (1000 to 1500 A/cm²). During degradation, localized heating (up to 200°C > the bulk substrate and heat sink) was observed to occur at the point were electrical power was supplied. Auger data from degraded samples indicated that due to the localized heating, Zn and Te from the ZnTe layers and Zn from the ZnSe layer diffused through the Au metallization to the samples surface. In addition, thermal stress from the localized heating generated micro-cracks in the ZnSe which acted as high diffusivity paths for impurities. Rectangular defects were also found to form in the degraded region. These defects were oriented to the micro-cracks and had similar geometries as dislocation patches (dark line defects) which have been reported to form in the quantum well region of degraded ZnSe based laser devices. The similarities between the rectangular defects and dark line defects suggest the formation of similar dislocation patches in the quantum well region of the multiquantum well contacts.

New Mechanisms in Photo-Assisted MOVPE of II-VI Semiconductors
S.J.C. IRVINE,¹ A. STAFFORD,¹ M.U. AHMED,¹A. BROWN,² and H. KHEYRANDISH²
1--Opto-electronic Materials Research Laboratory, North East Wales Institute, Wrexham, LL11 2AW, UK. 2--MATS UK Ltd., Wavertree Technology Park, Liverpool L7 9PG, UK.

KEY WORDS II-VI semiconductors, metalorganic vapor phase epitaxy (MOVPE), photo-assisted growth

The first detailed comparison has been made of the metalorganic vapor phase epitaxy growth rates of CdTe, ZnTe, and ZnSe, measured in situ with laser reflectometry. The comparison also includes the photo-assisted growth with visible radiation from an argon ion laser. Using a standard Group II precursor (DMCd or DMZn.TEN) partial pressure of 1.5 x 10^-4 atm, VI/II ratio of 1 and DIPM (M = Te, Se) the maximum growth rates are in the region of 10 to 15 AU/s. Decrease in growth rates of ZnTe at higher temperatures or higher laser powers have been attributed to the desorption from the substrate of unreacted Te precursor. The behavior of DTBSe is quite different from DIPSe for both pyrolytic and photo-assisted growth. The maximum growth rate is around 1 AU/s with very little photo-enhancement, except at 300°C. Secondary ion mass spectroscopy analysis of hydrogen concentration in the ZnSe layers shows high concentrations, up to 5.9 x 10¹⁹ atoms cm^-3 for DTBSe grown ZnSe under pyrolytic conditions. These results show that the growth kinetics play an important part in the incorporation of hydrogen and passivation of acceptor doped material.

Gas Source Molecular Beam Epitaxy Growth of SrS:Ce for Flat Panel Displays
W. TONG, T. YANG, W. PARK, M. CHAICHIMANSOUR, S. SCHöN, B.K. WAGNER, and C.J. SUMMERS
Phosphor Technology Center of Excellence, Manufacturing Research Center, Georgia Institute of Technology, Atlanta, GA 30332-0560.

KEY WORDS Electroluminescence, gas source molecular beam epitaxy (GSMBE), SrS:Ce, thin films

SrS:Ce thin films have been grown by gas source molecular beam epitaxy (GSMBE). The growth conditions have been systematically investigated as a function of growth temperature, sulfur to strontium flux ratio, and cerium flux. Single crystal SrS and high quality SrS:Ce were successfully grown on GaAs and glass substrates, respectively, without post-annealing process at temperature as low as 600°C. It was found that the electroluminescence (EL) performance was greatly improved by addition of ZnS during the growth.

Photoluminescence of Nitrogen-Doped Zinc Selenide Epilayers
M. MOLDOVAN, S.D. SETZLER, ZHONGHAI YU, T.H. MYERS, L.E. HALLIBURTON, and N.C. GILES
Department of Physics, West Virginia University, Morgantown, WV 26506-6315.

KEY WORDS Molecular beam epitaxy (MBE), photoluminescence (PL), ZnSe

Photoluminescence (PL) studies of nitrogen doped ZnSe epilayers grown by molecular beam epitaxy have been performed as a function of excitation wavelength, power density, and temperature. The broad emission from heavily doped ZnSe:N is composed of two distinct bands which we label as N_I and N_II. The dominant band N_I appears at 2.54 eV, while the N_II band position is sensitive to excitation power and occurs between 2.55 and 2.61 eV. The N_I emission energy is insensitive to incident power or temperature over the ranges studied. Further, a 69 meV localized phonon of the N_I band is observed. We propose that the N_I band is related to transitions within a (V_Se⁺-Zn-N_Se^-)⁰ close-associate pair. The N_II band displays characteristics consistent with the conventional donor acceptor pair model. A third band N_III at 2.65 eV is observed under high-power pulsed excitation. Previous studies of heavily doped ZnSe:N had suggested that the broad emission band was described by a modified donor-acceptor pair model. Our PL study does not support this previous model. In addition, our data suggests that singly ionized selenium vacancy complexes form in heavily doped ZnSe:N and play a role in compensation.

X-RAY AND GAMMA-RAY

New Developments in CdTe and CdZnTe Detectors for X and -Ray Applications

KEY WORDS CdTe detectors, CdZnTe detectors, pulse height correction method, x- and -ray applications, x-ray imager

There has been considerable recent progress in II-VI semiconductor material and in methods for improving performance of the associated radiation detectors. New high resistivity CdZnTe material, new contact technologies, new detector structures, new electronic correction methods have opened the field of nuclear and x-ray imaging for industrial and medical applications. The purpose of this paper is to review new developments in several of these fields. In addition, we will present some recent results at LETI concerning first the CdTe 2-D imaging system (20 x 30 mm² with 400 x 600 pixels) for dental radiology and second the CdZnTe fast pulse correction method applied to a 5 x 5 x 5 mm³ CdZnTe detector (energy resolution = 5% for detection efficiency of 85% at 122 keV) for medical imaging.

Fabrication and Characterization of CdZnTe Radiation Detectors with a New P-I-N Design
R. SUDHARSANAN,¹ G.D. VAKERLIS,¹ and N.H. KARAM²
1--Spire Corporation, One Patriots Park, Bedford, MA 01730. 2--Spire Corporation, One Patriots Park, Bedford, MA 01730. Currently at Spectrolab, Inc., Sylmar, CA 91342.

KEY WORDS CdZnTe, gamma-ray detector, HPB-CdZnTe, P-I-N

We report on the design, fabrication, and performance of CdZnTe gamma-ray detectors with a new P-I-N structure for spectroscopic applications. High-pressure and conventional vertical-Bridgman CdZnTe crystals were used for detector fabrication. P and n layers were deposited by thermal evaporation, and by optimizing the deposition conditions we achieved low leakage current (approximately 15 nA at 1000 V) and good performance. Spectral response data at high bias voltages showed improved energy resolution and peak-to-valley ratios for ²⁴¹Am and ⁵⁷Co compared to metal-semiconductor-metal detectors.

Linear X-Ray Detector Array Made on Bulk CdZnTe for 30~100 keV Energy
S.S. YOO,¹ G. JENNINGS,² and P.A. MONTANO¹
1--Microphysics Laboratory, Department of Physics, University of Illinois at Chicago, Chicago, IL 60607-7059 and Material Science Division, Argonne National Laboratory, Argonne, IL 60439. 2--Material Science Division, Argonne National Laboratory, Argonne, IL 60439.

KEY WORDS CdTe, CdZnTe, gamma-ray detector, high pressure Bridgman, linear array, radiation detector, x-ray detector

A CdZnTe detector grown by the high pressure Bridgman (HPB) growth technique was tested using high energy x-rays (30~100 keV), and the performance was compared with a commercially available NaI scintillating detector of 5 cm thickness. The charge collection efficiency of a CdZnTe detector is as high as 90% at relatively low electric field, 600 V/cm. At high x-ray photon energies, the detection efficiency is reduced due to the thickness of the CdZnTe. A 32 channel linear array was fabricated on 1.2~1.7 mm thick CdZnTe, of which the detector area was 175 x 800 µm² and the pitch size 250 µm. The measured dark current for the 16 element detector was as low as 0.1 pA at 800 V/cm with an excellent uniformity. Energy spectra were measured using a Co⁵⁷ radiation source. A small pixel effect and charge sharing were observed. The energy resolution was improved and compared with the large area detector. The array detector gave an average 5.8% full-width-half-maximum (FWHM) at 122 keV photopeak. The large area detector of the same material before fabrication exhibited a low energy tail at the photopeak, which limits the photopeak FWHM to 8%.

Study of Contacts to CdZnTe Radiation Detectors
Y. NEMIROVSKY,¹ A. RUZIN,² G. ASA,² Y. GORELIK,² L. LI³
1--Kidron Microelectronics Research Center, Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel. e-mail: nemirov@ee.technion.ac.il. 2--Kidron Microelectronics Research Center, Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel. 3--Cleveland Crystals, Inc., 19306 Redwood Avenue, Cleveland, OH 44110-2738.

KEY WORDS CdZnTe, contacts, gamma-ray, noise, resistive Schottky detectors, x-ray

This study characterizes, for the first time, contacts to CdZnTe radiation detectors by measuring the dark noise spectra as a function of the applied bias. The noise currents are correlated with the dc dark current-voltage characteristics of CdZnTe x-ray and gamma-ray detectors. In order to identify and separate the role of the contacts in the overall performance, the measured noise phenomena is correlated with detector configuration and contact design as well as the growth method of the CdZnTe crystals, contact technology, and passivation. Several contact technologies (electroless gold, and a number of evaporated metallic contacts including gold, indium, zinc, titanium, aluminum, and platinum contacts) are compared. Contacts to CdZnTe crystals grown by high pressure Bridgman are compared with contacts to CdZnTe crystals grown by modified Bridgman. Contacts of resistive detectors as well as of Schottky detectors are reported. Large area symmetric contacts are compared with small area pixelized contacts. The role of the metallization used for contacts, the role of surface effects and passivation, and the role of contact design are discussed.

Development of a 64 x 64 CdZnTe Array and Associated Readout Integrated Circuit for Use in Nuclear Medicine
H.B. BARBER,^1,2 H.H. BARRETT,^1,2 F.L. AUGUSTINE,³ W.J. HAMILTON,⁴ B.A. APOTOVSKY,⁵E.L. DERENIAK,² F.P. DOTY,⁵ J.D. ESKIN,² J.P. GARCIA,² D.G. MARKS,² K.J. MATHERSON,² J.M. WOOLFENDEN,¹ and E.T. YOUNG⁶
1--Division of Nuclear Medicine, Department of Radiology, University of Arizona, Tucson, AZ 85724-5068. 2--Optical Sciences Center, University of Arizona, Tucson, AZ 85721-0094. 3--Augustine Engineering, Encinitas, CA 92024. 4--Santa Barbara Research Center, Hughes Aircraft Company, Goleta, CA 93117. 5--Digirad, San Diego, CA 92121. 6--Steward Observatory, University of Arizona, Tucson, AZ 85721-0065.

KEY WORDS CdTe, CdZnTe, gamma-ray imaging, hybrid gamma-ray detectors, nuclear medicine, readout multiplexer, semiconductor detector arrays

Previous work has shown that hybrid semiconductor detector arrays similar to those used in infrared focal-plane arrays are very attractive for use in nuclear medicine and other gamma-ray imaging applications. In this paper, we describe the development of a 64 x 64 readout multiplexer specifically for use in gamma-ray imaging; we also describe the construction of 64 x 64 CdZnTe hybrid detector arrays using the new readout. The readout and detector array are both about one inch square (2.5 cm x 2.5 cm) and have 380 µm pixel pitch. Some initial assembly problems have been resolved by stabilizing the hybrids with epoxy. Preliminary testing results are presented that verify that the 64 x 64 CdZnTe arrays perform as excellent imaging spectrometers.