JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 25, Number 8, August 1996 This Month Featuring: Papers from the 1995 U.S. Workshop on the Physics and Chemistry of Mercury Cadmium Telluride and Other IR Materials, which was held on October 10-12, 1995, in Baltimore, Maryland. View August 1996 Table of Contents.

SPECIAL ISSUE PAPERS

Ishwara Bhat
Electrical, Computer & Systems Engineering Department
Rensselaer Polytechnic Institute
Troy, NY 12180-3590

Special Guest Editor

Application of Eddy Current Tecnique to Vertical Bridgman Growth of CdZnTe
R. SHETTY¹ C.K. ARD,¹ and J.P. WALLACE²
1--II-VI Incorporated, 375 Saxonburg Boulevard, Saxonburg, PA 16056. 2--Casting Analysis Corporation, Route 2, Box 113, Weyers Cave, VA.

KEY WORDS Bridgman, CdZnTe, conductivity, crystal growth, eddy current, interface

The eddy current technique was used to reveal the interface shape during vertical Bridgman growth of CdZnTe and to follow changes in the properties of the solidified ingot as it was cooled to room temperature after growth. Experiments were performed where partially solidified charges were decanted to show the interface shape. Eddy current analysis of the partially solidified charge indicated a concave interface shape in qualitative agreement with the shape of the decanted ingot. However, due to noise, interference, and possibly the inhomogeneous nature of the melt, only some of the eddy current signals could be analyzed empirically for interface shape; absolute values of conductivity could not be calculated from the eddy current data. Eddy current measurements made to follow changes in conductivity during post-growth cooling showed a minimum in the data during an 800°C annealing step indicative of a transition in the electrical properties of the ingot. On further cooling, a dramatic increase in the bulk conductivity of the ingot was noted. Such a transition can probably be described as a Mott transition.

Recent Results on Metalorganic Vapor Phase Epitaxially Grown HgCdTe Heterostructure Devices
C.T. ELLIOTT,¹ N.T. GORDON,¹ R.S. HALL,¹ T.J. PHILLIPS,¹ A.M. WHITE,¹ C.L. JONES,² C.D. MAXEY,² and N.E. METCALFE²
1--Defence Research Agency, Malvern, Worcestershire, WR14 3PS, UK. 2--GEC-Marconi Infra-Red, Southampton, SO15 0EG, UK.

KEY WORDS Infrared LEDs, MCT, metalorganic vapor phase epitaxy (MOVPE), nonequilibrium detectors

The ability to grow complex multilayer structures in Hg_1-xCd_xTe by epitaxial techniques has made it possible to produce a range of new devices such as infrared LEDs, lasers, and two-color infrared detector arrays. The devices described here, however, are designed to operate at temperatures above 145K and include both infrared sources and detectors. Three layer ppn structures, where the underlined symbols mean wider gap, have close to Auger limited R_oAs at temperatures above 145K. Under reverse bias, the devices exhibit Auger suppression leading to useful detectivities at room temperature. The diodes exhibit forward biased electroluminescence at room temperature although the efficiency of this emission is found to fall rapidly as the peak wavelength is increased toward 9 µm due to increased Auger recombination rates. By reverse biasing them, however, the devices show negative luminescence as a result of reducing the electron and hole densities below their thermal equilibrium value. The diode emitters have a higher quantum efficiency when used in this mode due to Auger suppression of the dark current.

Minimally Cooled Heterojunction Laser Heterodyne Detectors in Metalorganic Vapor Phase Epitaxially Grown Hg_1-xCd_xTe
C.T. ELLIOTT,¹ N.T. GORDON,¹ T.J. PHILLIPS,¹ H. STEEN,² A.M. WHITE,¹ D.J. WILSON¹ C.L. JONES,³ C.D. MAXEY,³ and N.E. METCALFE³
1--Defence Research Agency, Malvern, Worcestershire, WR14 3PS, UK. 2--Defence Research Agency, Malvern, Worcestershire, WR14 3PS, UK. Permanent address: Norwegian Defence Research Establishment, Kjeller, Norway. 3--GEC-Marconi Infra-Red Limited, Southampton, SO15 0EG, UK.

KEY WORDS Heterodyne detectors, laser detectors, MCT, metalorganic vapor phase epitaxy (MOVPE), nonequilibrium

By taking advantage of Auger suppression techniques, the leakage currents of room temperature infrared detectors operating in the LWIR band can be greatly reduced. At present, these detectors suffer from large 1/f noise and hence the improvement in the detectivity resulting from the reduced leakage currents can only be realized at high frequencies. However, this is not a problem for heterodyne systems which employ intermediate frequencies above 40MHz. A thermo-electrically cooled Auger suppressed infrared detector operated at 260K has been studied as a heterodyne detector. The device was operated with the application of sufficient local oscillator power to double its dark current (about 0.3 mW) and a NEP of 2X10^-19 W Hz^-1 was deduced from heterodyne measurements. The frequency response is presently limited by a combination of the detector capacitance and the series resistance to about 70MHz and ways to reduce this series resistance are considered.

Negative Diffusion Capacitance in Auger-Suppressed HgCdTe Heterostructure Diodes
T.J. PHILLIPS and N.T. GORDON
Defence Research Agency, St. Andrews Road, Malvern, Worcestershire, WR14 3PS, U.K.

KEY WORDS Cadmium mercury telluride, diffusion capacitance, heterostructure diode

An ac analytical model of the diffusion capacitance properties of a p¼n HgCdTe heterostructure extracting diode is developed. The results show that the diffusion capacitance dominates the impedance, and can be negative, depending on the length of the intrinsic region. Experimental results on typical diodes show a capacitance of the magnitude predicted, but with a rather more complex frequency dependence, with two changes in sign in the frequency range 20Hz-1 MHz. The existence of a negative capacitance region is not found to be dependent on the existence of a negative resistance region. The discrepancy between theory and experiment is ascribed to the existence of charge trapping. The results are also discussed in terms of time domain variation of a small perturbation current. The results for the capacitance at high frequency are used to predict the high frequency performance limit of such devices.

Advanced Magneto-Transport Characterization of LPE-Grown Hg_1-xCd_xTe by Quantitative Mobility Spectrum Analysis
J.R. MEYER,¹ C.A. HOFFMAN,¹ F.J. BARTOLI,¹ J. ANTOSZEWSKI,² L. FARAONE,² S.P. TOBIN,³ P.W. NORTON,³ C.K. ARD,⁴ D.J. REESE,⁴ L. COLOMBO,⁵ and P.K. LIAO⁵
1--Code 5610, Naval Research Laboratory, Washington, DC 20375. 2--The University of Western Australia, Nedlands, WA, 6009. 3--Loral Infrared and Imaging Systems, Lexington, MA 02173. 4--II-VI Incorporated, Saxonburg, PA 16056. 5--Texas Instruments Incorporated, Dallas, TX 75265.

KEY WORDS Hall measurements, HgCdTe, magneto-transport measurement

We report the development, optimization, and testing of an advanced quantitative mobility spectrum analysis (QMSA) technique for determining free electron and hole densities and mobilities from field-dependent Hall and resistivity data. Application to temperature-dependent data for a series of 25 LPE-grown n-type and p-type Hg_1-xCd_xTe samples confirms that the fully automated procedure yields accurate and reliable results for all classes of samples, and also has greater sensitivity to minority carrier concentrations than previous mixed-conduction analysis methods. The QMSA is found to be a suitable standard tool for the routine electrical characterization of semiconductor materials and devices.

SIMS Characterization of HgCdTe and Related II-VI Compounds
JACK SHENG, LARRY WANG, and GAYLE E. LUX
Charles Evans & Associates, 301 Chesapeake Dr., Redwood City, CA 94063.

KEY WORDS HgCdTe, secondary ion mass spectroscopy (SIMS), SIMS detection limits, SIMS imaging

The production of consistent high purity materials is critical for improvement in performance and sensitivity of II-VI photovoltaic and photoconductive devices. Information regarding the energy band structure and impurity or defect levels present in the material is essential to understand and enhance the performance of current detectors along with the development of future novel devices. Secondary ion mass spectrometry (SIMS) is capable of providing information of purity, junction depths, dopant distribution, and stoichiometry in the material. SIMS techniques can achieve high detection sensitivities in very small analytical volumes and for a wide range of elements (almost the entire periodic table). SIMS analysis also provides unique capabilities for localizing atomic distribution in two and three dimensions. Ion images can be obtained by registering the positions of mass selected ions formed in the sputtering process. The combination of excellent detection sensitivity, high mass resolution, depth profiling capability, and high resolution image acquisition on a wide spectrum of elements by a SIMS instrument is not matched by any other instrumentation technique.

Ion Drift in Cd-Rich HgCdTe Crystals
J.F. BARBOT, B.O. WARTLICK, L.F. PINHÈDE, and C. BLANCHARD
Laboratoire de Métallurgie Physique, Université de Poitiers, SP2MI-Bd3, Téléport 2, BP179, 86960 Futuroscope Cedex, France.

KEY WORDS Drift of copper, HgCdTe, impurities, pairing reaction

Ion drift in the electric field of a depleted region has been studied at about room temperature in intentionally Cu-doped p-type Hg_0.3Cd_0.7Te bulk materials. Capacitance-voltage measurements have mainly been used to investigate the drift of interstitial copper. Diffusion data of copper has been obtained using classical theoretical models based on the dissociation-formation of acceptor-donor complexes (A_sD_i). The possibility of junction creation by this ion manipulation under a strong electric field is discussed.

Recent Progress on HgCdTe at the National Laboratory for Infrared Physics in China
JUNHAO CHU and DINGYUAN TANG
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China.

KEY WORDS As impurity, electrical properties, HgCdTe, lifetimes, photoluminescence

The characterization of impurities, defects, uniformity and some fundamental properties for the bulk and epitaxial HgCdTe (MCT) material, undoped and doped with Sb, As, Fe, have been investigated recently in the authors' laboratory by means of photoluminescence, magneto-photoconductivity, quantum capacitance spectroscopy, transport measurements, and other contactless and nondestructive methods such as the infrared, far-infrared, and millimeter wave measurements. This paper reports a portion of these new results.

Trace Copper Measurements and Electrical Effects in LPE HgCdTe
J.P. TOWER,¹ S.P. TOBIN,¹ P.W. NORTON,¹ A.B. BOLLONG,² A. SOCHA,² J.H. TREGILGAS,³ C.K. ARD,⁴ and H.F. ARLINGHAUS⁵
1--Loral Infrared & Imaging Systems, Lexington, MA 02173. 2--Johnson Matthey Electronics, Spokane, WA 99216. 3--Texas Instruments Incorporated, Dallas, TX 75265. 4--II-VI Incorporated, Saxonburg, PA 16056. 5--Atom Sciences, Oak Ridge, TN 37830.

KEY WORDS CdZnTe, HgCdTe, sputter initiated resonance ionization spectroscopy (SIRIS), trace impurities

Recent improvements in sputter initiated resonance ionization spectroscopy (SIRIS) have now made it possible to measure copper in HgCdTe films into the low 10¹³ cm^-3 range. We have used this technique to show that copper is responsible for type conversion in n-type HgCdTe films. Good n-type LPE films were found to have less than 1X10¹⁴ cm^-3 copper, while converted p-type samples were found to have copper concentrations approximately equal to the hole concentrations. Some compensated n-type samples with low mobilities have copper concentrations too low to account for the amount of compensation and the presence of a deep acceptor level is suggested. In order to study diffusion of copper from substrates into LPE layers, a CdTe boule was grown intentionally spiked with copper at approximately 3X10¹⁶ cm^-3. Annealing HgCdTe films at 360°C was found to greatly increase the amount of copper that diffuses out of the substrates and a substrate screening technique was developed based on this phenomenon. SIRIS depth profiles showed much greater copper in HgCdTe films than in the substrates, indicating that copper is preferentially attracted to HgCdTe over Cd(Zn)Te. SIRIS spatial mapping showed that copper is concentrated in substrate tellurium inclusions 5-25 times greater than in the surrounding CdZnTe matrix.

Reduction of CdZnTe Substrate Defects and Relation to Epitaxial HgCdTe Quality
S. SEN,¹ C.S. LIANG,¹ D.R. RHIGER,¹ J.E. STANNARD,¹ and H.F. ARLINGHAUS²
1--Santa Barbara Research Center, Goleta, CA 93117. 2--Atom Sciences, Inc., Oak Ridge, TN 37830.

KEY WORDS Annealing, CdZnTe, defects, HgCdTe, impurities, inclusion, IR absorption, liquid phase epitaxy, second-phase, sputter initiated resonance ionization spectroscopy (SIRIS), stoichiometry

We have conducted annealing experiments on CdZnTe wafers to restore stoichiometry, eliminate or reduce second-phase (Cd or Te) inclusions, and investigate effects on the quality of epitaxial HgCdTe grown on the thermally treated substrates. Two categories of second phase features were revealed in these materials. Category 1 has a star-like shape with sixfold symmetry (as seen by infrared transmission microscopy) and a central core consisting of cadmium. These stars were observed only in the more stoichiometric materials (having good infrared transmission characteristics). Category 2 consists of triangular, hexagonal, and irregular shaped tellurium inclusions which are present in the off-stoichiometry materials (which exhibit strong IR absorption). Substrates were annealed at temperatures ranging from 500 to 700°C for one to seven days, in vapor derived from elemental Cd or Cd_1-xZn_x alloy (x = 0.005). These anneals were able to eliminate the excess IR absorption and decrease the apparent sizes of both categories of second-phase features. It was found that pinhole-like morphological defects on the surface of a HgCdTe layer grown by liquid phase epitaxy can be caused by Cd and Te inclusions located within the CdZnTe substrate near the interface. Additionally, measurement and spatial mapping of copper concentration by sputter initiated resonance ionization spectroscopy showed 10 to 100 times higher Cu concentration in the inclusions than in the surrounding matrix areas.

Magnetophonon Oscillations in the Transverse and Longitudinal Magnetoresistance of Hg_1-xCd_xTe
J. BAARS,¹ C.L. LITTLER,² D. BRINK,¹ and M. BRUDER³
1--Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg i.Br. Germany. 2--Department of Physics, University of North Texas, Denton, TX 76203. 3--AEG-Aktiengesellschaft, Infrarotmodule, D-74072 Heibronn, Germany.

KEY WORDS HgCdTe, InSb, magnetophonon resonance, magnetoresistance, thermoelectric power

The transverse and longitudinal magnetoresistance (MR) as well as the longitudinal magneto-thermoelectric coefficient of n-type Hg_1-xCd_xTe (MCT) (0.20 < x < 0.33) have been measured at various temperatures (40 ¾ T ¾ 140K) as a function of magnetic field (0 ¾ B ¾ 18 kG). Both the transverse and the longitudinal MR clearly exhibit oscillations which are described in terms of magnetophonon (MP) transitions involving the HgTe-like and the CdTe-like longitudinal optical (LO) phonons of MCT. The field positions of the transverse MR maxima agree with the calculated MP resonances taking into account nonparabolic bands (k · p model for narrow-gap zinc-blende-type semiconductors) and the polaron effect. Those of the longitudinal MR minima are found to coincide with the oscillation minima in the longitudinal magneto-thermoelectric coefficient. However, these minima are shifted by ¼/2 to lower fields with respect to the positions of the MP resonances. This phase shift was predicted by Barker for the case of strong Landau level damping but has not been previously observed. In contrast, the MP oscillation minima of the longitudinal MR and the oscillation maxima of the transverse MR of n-type InSb (investigated here for comparison) occur exactly at the fields of the MP resonances. Only the oscillation minima of the longitudinal magneto-thermoelectric coefficient are slightly shifted to the side below the MP resonance fields. With regard to the band parameters and the dominant polar optical mode scattering of charge carriers InSb very much resembles MCT. InSb, however, is a binary compound whereas MCT is a solid solution. Thus, the phase shift by ¼/2 to lower fields observed for the oscillation minima in the longitudinal MR and magneto-thermoelectric coefficient of MCT may be due to alloy scattering. The temperature coefficients of the MP resonance fields of MCT are found to be substantially smaller than those reported by Takita et al. and McClure et al. The larger temperature coefficents are presumably due to unresolved two-phonon structures of the MP oscillations

Magnetoluminescence Properties of Hg_1-xCd_xTe Epitaxial Layers and Superlattice Structures Grown by Metalorganic Molecular Beam Epitaxy
T.K. TRAN,¹ A. PARIKH,¹ S.D. PEARSON,¹ B.K. WAGNER,¹ R.G. BENZ II,¹ R.N. BICKNELL-TASSIUS,¹ C.J. SUMMERS,¹ T. KELZ,² J.W. TOMM,² W. HOERSTEL,² P. SCHÄFER,² and U. MULLER²
1--Quantum Microstructures Laboratory, Georgia Tech Research Institute, Atlanta, GA 30332. 2--Institut für Physik, Humboldt-Universität zu Berlin, D-10099 Berlin, Germany.

KEY WORDS HgCdTe, magnetoluminescence, MOMBE, superlattice structures

We present a study of the electro-optical properties of Hg_1-xCd_xTe epitaxial layers and Hg_1-xCd_xTe/CdTe (0.28 < x < 0.30) superlattice structures by x-ray diffraction, lateral transport and photo- and magneto-luminescence measurements. Systematic studies of the excitation intensity and magnetic field dependence of the photoluminescence revealed direct evidence of an excitonic contribution to the observed luminescence in Hg_1-xCd_xTe epitaxial layers. Similar investigations of the superlattice structures indicated that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm^-1 were obtained for an excitation intensity of 100 kW/cm² indicating suitable electro-optical properties for making efficient mid-infrared laser diodes.

Growth and Characterization of InTlSb for IR-Detectors
N.H. KARAM,¹ R. SUDHARSANAN,¹ T. PARODOS,¹ and M.A. DODD²
1--Spire Corporation, One Patriots Park, Bedford, MA 01730. 2--Wright Laboratory, Wright-Patterson Air Force Base, OH.

KEY WORDS III-V compounds, InTlSb, long wavelength infrared (LWIR) detectors, metalorganic chemical vapor deposition (MOCVD), photoconductors

Epitaxial In_1-xTl_xSb films with compositions up to x=0.1 have been demonstrated using the metalorganic chemical vapor deposition technique on InSb and GaAs substrates. A specially designed high-temperature source delivery system was used for the low vapor pressure cyclopentadienylthallium source. Tl-compositions in the deposited films were measured by Rutherford backscattering spectroscopy which confirmed the incorporation of up to 10% Tl. Room temperature infrared transmission spectra of InTlSb exhibited considerable absorption beyond 7 µm. Photoconductive detectors were fabricated in InTlSb films grown on semi-insulating GaAs. Spectral response measurements showed substantial photoresponse at 8.5 to 14 µm. In spite of the large lattice-mismatch (~14%) between InTlSb and GaAs, photoconductive detectors exhibited blackbody detectivities (D^*_bb) of 5.0X10⁸ cm-Hz^1/2W^-1 at 40K.

Electrical Characterization of Very-Narrow-Gap Bulk HgCdTe Single Crystals by Variable Magnetic Field Hall Measurements
J.S. KIM,¹ D.G. SEILER,¹ R.A. LANCASTER,² and M.B. REINE²
1--Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. 2--Loral Infrared & Imaging Systems, Lexington, MA 02173.

KEY WORDS Bandgap-crossing, electron transport, Hall measurements, HgCdTe, high electron mobility, very-narrow-gap semiconductors

Variable-magnetic-field Hall measurements (0 to 1.5 T) are performed on very-narrow-gap bulk-grown Hg_1-xCd_xTe single crystals (0.165 ¾ x ¾ 0.2) at various temperatures (10 to 300K). The electron densities and mobilities are obtained within the one-carrier (electrons) approximation of the reduced-conductivity- tensor scheme. The present data together with the selected data set reported by other workers exhibit a pronounced peak when the electron mobility is plotted against the alloy composition x-value which has been predicted to be due to the effective-mass minimum at the bandgap-crossing (E_g0). The observed position (x0.165), height (4X10² m²/Vs), and width (0.01 in x) of the mobility-peak can be explained by a simple simulation involving only ionized-impurity scattering. A lower bound of the effective mass is introduced as a fitting parameter to be consistent with the finiteness of the observed electron mobility and is found to be of the order of 10^-4 of the mass of a free electron.

Study of the Charge Collection Efficiency of CdZnTe Radiation Detectors
Y. NEMIROVSKY, A. RUZIN, G. ASA, and J. GORELIK
Kidron Microelectronics Research Center, Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel.

KEY WORDS CdZnTe, radiation detectors, Schottky diode

The charge collection efficiency of CdZnTe radiation detectors with two different configurations: a Schottky diode detector and a resistive detector are compared. The average charge collection efficiencies for three different directions of irradiation (negative electrode, positive electrode and perpendicular to the electric field) are calculated. The mobility-lifetime product of the CdZnTe substrates is evaluated from the dependence of the measured spectra upon detector bias voltage. The measurement of the average charge collection efficiency is based on monitoring the shift of the peak channel with bias voltage in an experimental setup which is well calibrated. Two types of radiation are used: gamma photons from several radioactive sources and alpha particles from an ²⁴¹Am source. The models for the evaluation of mobility-lifetime product from the measured data for the two types of detector configurations as well as for the two types of radiation sources, are compared and discussed. The CdZnTe (Zn=10%) substrates under study are obtained commercially and are grown by the high pressure Bridgeman method. The mobility-lifetime products and specific resistivity of the two types of detectors are evaluated and compared. A lower resistivity material has a narrower depletion region and behaves like a thinner detector thus exhibiting better collection efficiencies. Therefore, medium resistivity material which is completely inadequate for resistive detectors can still yield high performance Schottky detectors. The preferred direction of irradiation, i.e. from the negative electrode, is possible only in the case of n-type material which is reverse biased by negative voltages applied to the Schottky gate. The mobility-lifetime products that are derived on both the resistive detector (with specific resistivity of ~1·10¹⁰ ·cm) and the Schottky diode (with specific resistivity of ~1·10⁶ ·cm) are µ_nn 4·10^-4 cm²V^-1 and µ_pp 8·10^-5 cm²V^-1.

Application of II-VI Materials to Nuclear Medicine
H. BRADFORD BARBER
Division of Nuclear Medicine, University of Arizona, Tucson, AZ 85724.

KEY WORDS CdTe, CdZnTe, gamma-ray imaging, nuclear medicine, semiconductor detectors

Semiconductor gamma ray detector arrays made of II-VI materials such as CdTe or CdZnTe hold great promise for improving the spatial resolution and energy resolution of nuclear medicine imaging systems. This field has benefited greatly from technologies developed in infrared imaging. This report surveys the state of the art for producing high-resolution semiconductor arrays with emphasis on II-VI materials and considers the prospects for producing a semiconductor detector gamma camera. A number of practical designs are reviewed that make use of single-charge-carrier dominance effects to improve useful photopeak fraction and thus efficiency.

Drift Mobility and Photoluminescence Measurements on High Resistivity Cd_1-xZn_xTe Crystals Grown from Te-Rich Solution
K. SUZUKI,¹ S. SETO,² S. DAIRAKU,³ N. TAKOJIMA,¹ T. SAWADA,¹ and K. IMAI¹
1--Hokkaido Institute of Technology, Sapporo 006, Japan. 2--Ishikawa National College of Technology, Ishikawa 929-03, Japan. 3--Sumitomo Metal Mining Co. Ltd., Ohme, Tokyo 198, Japan.

KEY WORDS Alloy scattering, Cd1-xZnxTe, drift mobility, photoluminescence (PL), solution growth, time-of-flight (TOF)

The drift mobilities of chlorine doped high resistivity Cd_0.8Zn_0.2Te have been investigated by using the time-of-flight technique. Electron as well as hole mobility in the as-grown crystals is limited by trap-controlled carrier transport. The energy locations of the defects responsible for carrier trapping are determined to be E_c-0.03 and E_v+0.14 eV for electrons and holes, respectively. After annealing at 400°C for 80 h, no evidence of trap-controlled mobility was recognized for electrons. On the other hand, no significant change before and after the annealing was observed for hole transport. Those results and the change in the photoluminescence spectra before and after the annealing are explained by the complex defect model composed of the Cd vacancy and chlorine donor. Further, the alloy scattering potentials of U_e and U_h were estimated by employing the theoretical calculation method recently reported by D. Chattopadhyay [Solid State Commun. 91, 149 (1994)].

Photon Assisted Growth of Nitrogen-Doped CdTe and the Effects of Hydrogen Incorporation During Growth
ZHONGHAI YU,¹ S.L. BUCZKOWSKI,¹ M.C. PETCU,¹ N.C. GILES,¹ T.H. MYERS,¹ and M. RICHARDS-BABB²
1--Department of Physics, West Virginia University, Morgantown, WV 26506-6315. 2--Department of Chemistry, West Virginia University, Morgantown, WV 26506-6045.

KEY WORDS CdTe, hydrogen incorporation, molecular beam epitaxy, nitrogen doping

Nitrogen doping in CdTe epilayers grown by photo-assisted molecular beam epitaxy was demonstrated using an rf plasma source. The effect of the presence of atomic hydrogen during growth of undoped and nitrogen-doped CdTe was investigated. The layers were characterized using photoluminescence spectroscopy (PL), Hall effect, secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy, and atomic force microscopy. PL confirmed the incorporation of nitrogen as acceptors. While p-type carrier concentrations greater than 10¹⁸ cm^-3 were easily obtained, SIMS measurements indicated that nitrogen was concentrated near the undoped-doped and epilayer-substrate interfaces which complicates interpretation of activation efficiency. Hydrogen incorporation was found to be enhanced by the presence of nitrogen. Infrared absorption measurements strongly suggested the formation of N-H complexes. Hall measurements indicated that complexes are formed which are donor-like in nature. The presence of atomic hydrogen during growth radically changed the low temperature photoluminescence in both undoped and nitrogen-doped layers. Exciton-related luminescence was quenched at low temperature. Nitrogen-related donor-acceptor pair luminescence was also absent from the N-doped hydrogenated layers, consistent with complex formation. Copper (a cation-site acceptor) donor-acceptor pair luminescence appeared to be enhanced by hydrogenation.

Comparison of HgTe Materials Grown in (100), (110), (111), and (211) Orientations
SRINIVASAN KRISHNAMURTHY,¹ A.-B. CHEN,² and A. SHER¹
1--SRI International, Menlo Park, CA 94025. 2--Physics Department, Auburn University, Auburn, AL 36349.

KEY WORDS Critical temperature, defect density, growth modeling, growth rate, HgTe, vacancy formation

We calculated energies required to remove atoms from various configurations on (111), (110), (100), and (211) HgTe surfaces. The excess pair energies for various species are then calculated and are used in a thermodynamic model to study the growth. All energies are obtained using a Green's function method. The pair distributions are calculated from these energies in a generalized quasi-chemical approximation. The calculated critical temperatures for surface roughness transition are found to be considerably higher than the usual growth temperature of 185°C, so the growth on these surfaces is expected to be layer-by-layer with formation of two-dimensional islands. However, among the surfaces studied, only the (211) surfaces have an attractive binding energy for Hg, making those surfaces suited for better growth. The critical temperature for growth on (211)Hg is slightly higher than that for (211)Te, but we also find that Hg sticking coefficient on (211)Hg surface is considerably lower than that on (211)Te surface. These calculations are consistent with the observed higher growth rate of the (211)Te surface. Our calculations suggest that there will be fewer grown-in vacancies and Te antisites, at the expense of growth rate and sticking coefficient, for crystals grown on (211)Hg surface. We further calculated the Hg and Te vacancy formation energies as functions of surface orientations and layer depth. The cation vacancy formation energies from completed surface regions (islands) are higher than bulk values near anion terminated surfaces and smaller than bulk values near cation terminated surfaces.

Comparison of the Diffusion of Hg into CdTe and Hg_0.8Cd_0.2Te
M.U. AHMED,¹ E.D. JONES,¹ J.B. MULLIN,² and N.M. STEWART³
1--Applied Physics Division, Coventry University, Priory Street, Coventry, CV1 5FB, UK. 2--EMC, "The Hoo," Brockhill Road, West Malvern, Worcestershire, WR14 4DL, UK. 3--BT Research Laboratories, Martlesham Heath, Ipswich, IP5 7RE, UK.

KEY WORDS CdTe, diffusion, HgCdTe, radiotracer analysis

In this paper, results published recently on Hg diffusion in the important infrared detector material, Hg_0.8Cd_0.2Te, and its common substrate material, CdTe, are compared and discussed. As is customary with diffusion studies in II-VI semiconductors, two component profiles were obtained in the majority of cases, each profile giving two values of the diffusivity. The values of D for the mercury diffusion in CdTe were much less than corresponding values in Hg_0.8Cd_0.2Te. In Hg_0.8Cd_0.2Te diffusion proceeds by volume diffusion followed by short circuit diffusion, whereas in CdTe diffusion is rate limiting volume diffusion involving a slow stream and a fast stream. From pressure dependency measurements, it is proposed that the slow component occurs by an interstitial mechanism at low P_Hg and a vacancy mechanism at high P_Hg, whereas in Hg_0.8Cd_0.2Te the reverse occurs for the fast diffusion component. In CdTe, the slow component increases systematically with etch pit density, whereas in Hg_0.8Cd_0.2Te, the diffusivity is independent of the quality of the material. From the comparisons, it is seen that there are some common features in the diffusion of Hg in the two materials but there are also some clear and distinct differences.

Enhancement of the Steady State Minority Carrier Lifetime in HgCdTe Photodiode Using ECR Plasma Hydrogenation
HAN JUNG,¹ HEE CHUL LEE,² and CHOONG-KI KIM²
1--Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Kusong-Dong, Yusong-Gu, Taejon, Korea and also with the Hyundai Electronics Industry Co. 2--Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Kusong-Dong, Yusong-Gu, Taejon, Korea.

KEY WORDS Diffusion length, field effect transistor (FET), HgCdTe, hydrogenation, lifetime, mobility, photodiode

This paper reports the effects of electron cyclotron resonance (ECR)H₂ plasma hydrogenation on the characteristics of HgCdTe devices for the first time. We compared the characteristics of photodiodes and n-channel enhancement type field effect transistors (FETs) in the hydrogenated regions with those in the unhydrogenated regions on the same wafer. From the measurement of the photodiodes, it was found that the steady-state minority carrier diffusion length was increased from 190 to 28 µm by the hydrogenation. The surface mobility of the n-channel FET was about 5800 cm²/Vs and was not varied by hydrogenation. From these facts, the steady-state minority carrier lifetime is increased about two times by the ECR H₂ plasma hydrogenation. We believe that the ECR hydrogenation can effectively reduce the surface trap-states which results in increasing the minority carrier lifetime and improving the characteristics of HgCdTe devices.

Dry Etching of Hg_1-xCd_xTe Using CH₄/H₂/Ar/N₂ Electron Cyclotron Resonance Plasmas
ROBERT C. KELLER,¹ M. SEELMANN-EGGEBERT,² and H.J. RICHTER²
1--Fraunhofer Institut für Angewandte Festkörperphysik, 79108 Freiburg, Germany. Presently at Texas Instruments, Corporate Research Division, Dallas, TX 75265. 2--Fraunhofer Institut für Angewandte Festkörperphysik, 79108 Freiburg, Germany.

KEY WORDS Dry etching, electron cyclotron resonance (ECR) etching, HgCdTe

An approach is presented which eliminates the problems caused by hydrocarbon polymer deposition during etching Hg_1-xCd_xTe with CH₄/H₂ based plasmas. We find that the addition of N₂ to the plasma inhibits polymer deposition in the chamber and on the sample. We speculate that atomic nitrogen formed from N₂ in the plasma has several beneficial effects: the elimination of polymer precursors, the reduction of the atomic hydrogen concentration, and a potential increase of methyl radical concentration. Evidence for the reaction between the nitrogen and the polymer precursors is presented. It is also demonstrated that the addition of N₂ to CH₄/H₂ based electron cyclotron resonance (ECR) plasmas used to etch HgCdTe eliminates the roughness normally formed during etching and results in a steadier etch rate.

Growth of Fully Doped Hg_1-xCd_xTe Heterostructures Using a Novel Iodine Doping Source to Achieve Improved Device Performance at Elevated Temperatures
C.D. MAXEY,¹ C.L. JONES,¹ N.E. METCALFE,¹ R. CATCHPOLE,¹ M.R. HOULTON,² A.M. WHITE,² N.T. GORDON,² and C.T. ELLIOTT²
1--GEC-Marconi Infra-Red Limited, Southampton, SO15 0EG, UK. 2--Defence Research Agency, Malvern, Worcestershire, WR14 3PS, UK.

KEY WORDS Diffusion, doping, heterojunctions, HgCdTe, metalorganic vapor phase epitaxy (MOVPE)

Band gap engineered Hg_1-xCd_xTe (MCT) heterostructures should lead to detectors with improved electro-optic and radiometric performance at elevated operating temperatures. Growth of such structures was accomplished using metalorganic vapor phase epitaxy (MOVPE). Acceptor doping with arsenic (As), using phenylarsine (PhAsH₂), demonstrated 100% activation and reproducible control over a wide range of concentrations (1X10¹⁵ to 3.5X10¹⁷ cm^-3). Although vapor from elemental iodine showed the suitability of iodine as a donor in MCT, problems arose while controlling low donor concentrations. Initial studies using ethyliodide (EtI) demonstrated that this source could be used successfully to dope MCT, yielding the properties required for stable heterostructure devices, i.e. 100% activation, no memory problems and low diffusion coefficient. Cryogenic alkyl cooling or very high dilution factors were required to achieve the concentrations needed for donor doping below 10¹⁶ cm^-3 due to the high vapor pressure of the alkyl. A study of an alternative organic iodide source, 2-methylpropyliodide (2MePrI), which has a much lower vapor pressure, improved control of low donor concentrations. 2MePrI demonstrated the same donor source suitability as EtI and was used to control iodine concentrations from 1X10¹⁵ to 5X10¹⁷ cm^-3. The iodine from both sources only incorporated during the CdTe cycles of the interdiffused multilayer process (IMP) in a similar manner to both elemental iodine and As from PhAsH₂. High resolution secondary ion mass spectroscopy analysis showed that IMP scale modulations can still be identified after growth. The magnitude of these oscillations is consistent with a diffusion coefficient of 7X10^-16 cm²s^-1 for iodine in MCT at 365°C. Extrinsically doped device heterostructures, grown using 2 MePrI, have been intended to operate at elevated temperatures either for long wavelength (8-12 µm) equilibrium operation at 145K or nonequilibrium operation at 190 and 295K in both the 3-5 um and 8-12 µm wavelength ranges. Characterization of such device structures will be discussed. Linear arrays of mesa devices have been fabricated in these layers. Medium wave nonequilibrium device structures have demonstrated high quantum efficiencies and R₀A = 37 cm² for _co = 4.9 µm at 190K.

HgCdTe/CdZnTe P-I-N High-Energy Photon Detectors
W.J. HAMILTON,¹ D.R. RHIGER,¹ S. SEN,¹ M.H. KALISHER,¹ G.R. CHAPMAN,¹ and R.E. MILLS²
1--Santa Barbara Research Center, Goleta, CA 93117. 2--Hughes Aircraft Company, Newport Beach, CA 92663.

KEY WORDS CdZnTe, gamma-ray detector, HgCdTe, x-ray detector

Classical solid-state detection of x-ray and gamma-ray radiation consists of a high voltage applied between two metallic contacts sandwiching a high resistivity, high dielectric strength material; high voltage and high resistivity are required to enable complete charge collection while minimizing the resolution-degrading leakage current (dark current). We report here the conception and successful fabrication and test of a new device construct which changes this paradigm. P-type and n-type layers are fabricated by mercury cadmium telluride (HCT) liquid phase epitaxy (LPE) on opposite sides of a high-quality wafer of CdZnTe (CZT) in order to construct a p-i-n diode structure. Wafers up to 9 cm² area have been grown. This diode structure provides an extremely high effective resistivity and barrier to the flow of dark current in the device. Several wafer lots have repeatably yielded p-i-n detectors which exhibit typical diode current-voltage (I-V) curves with very low dark currents at very high bias voltages. Spectra obtained from these detectors produce exceptionally sharp photopeaks which exhibit very little low-energy tailing.

Interface Formation Between Deposited Sn and Hg_0.8Cd_0.2Te
H. ZIMMERMANN,¹ ROBERT C. KELLER,² P. MEISEN,¹ H.J. RICHTER,¹ and M. SEELMANN-EGGEBERT¹
1--Fraunhofer-Institut für Angewandte Festkörperphysik, Freiburg, Germany. 2--Texas Instruments, Incorporated, Dallas, TX 75265.

KEY WORDS Alpha-Sn, ARXPS, depth profiling, ohmic contacts, photoelectron diffraction

The structure of the interface formed by the reaction of deposited Sn on Hg_0.78Cd_0.22Te(111)B was investigated by hemispherically scanned x-ray photoelectron spectroscopy including x-ray photoelectron diffraction (XPD). The interface formation was found to proceed as follows: At the onset of Sn deposition, Hg is expelled and substituted by Sn in the topmost monolayer of the Hg_0.78Cd_0.22Te lattice while the zinc-blende structure of the original surface is maintained. With further Sn deposition (and further loss of Hg), an epitaxial layer of cubic SnTe (with inclusions of CdTe) was found to grow. At room temperature, the SnTe growth stopped after a few monolayers, and the epitaxial growth of cubic a-Sn was observed to start on top of it. At elevated deposition temperatures, the SnTe intermediate layer continued to grow up to several 100Å.

P-Type Doping with Arsenic in (211)B HgCdTe Grown by MBE
P.S. WIJEWARNASURIYA, S.S. YOO, J.P. FAURIE, and S. SIVANANTHAN
Microphysics Laboratory, Department of Physics (M/C 273), University of Illinois at Chicago, 845 W. Taylor St., Room #2236, Chicago, IL 60680.

KEY WORDS Arsenic, arsenic-planar doping, diffusion, HgCdTe, ion-implanted

Arsenic incorporation and doping in HgCdTe layers grown by molecular beam epitaxy (MBE) were examined in this paper. Arsenic incorporation into MBE-HgCdTe was carried out in two different ways: (1) ex-situ arsenic ion-implantation on indium-doped n-type HgCdTe layers, and (2) through a new approach called arsenic planar doping. We report on ex-situ arsenic diffusion on indium-doped MBE-HgCdTe layers at 450°C. In the investigated layers, arsenic redistribution occurs with a multi-component character. We obtained a diffusion coefficient of DAs = (1-3)X10^-13 cm²/s at 450°C. Results of differential Hall and fabricated p-n junctions suggest that during high temperature annealing, arsenic preferentially substitutes into Te sublattices and acts as acceptor impurities. In the second case, arsenic has been successfully incorporated during the MBE growth as an acceptor in the planar doping approach. Without ex-situ annealing, as-grown layers show up to 50% activation of arsenic during the growth. These results are very promising for in-situ fabrication of infrared devices using HgCdTe material.

Synchrotron X-Ray Photoconductor Detector Arrays Made on MBE Grown CdTe
S.S. YOO,^1,3 B. RODRICKS,² S. SIVANANTHAN,³ J.P. FAURIE,³ and P.A. MONTANO^1,3
1--Material Science Division, Argonne National Laboratory, Argonne, IL 60439. 2--Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439. 3--Microphysics Laboratory, Department of Physics, University of Illinois at Chicago, Chicago, IL 60607.

KEY WORDS Linear array photoconductor, molecular beam epitaxy (MBE) CdTe, synchrotron x-ray, x-ray detector

We have been fabricating x-ray photoconductor linear array detectors using molecular beam epitaxially (MBE) grown (111)B undoped CdTe layers on (100) Si substrates. A novel technique was developed to remove the Si and to mount the fragile MBE grown CdTe layers onto insulating ceramic substrates. 256 channel linear photoconductor array devices were fabricated on the resulting CdTe layers. The resistivity of MBE (111)B CdTe was high (>10⁸ cm) enough to utilize the material for low energy (8 ~ 25 keV) x-ray detectors. The stability of the detectors are satisfactory, and they were tested at room temperature routinely for over a year. The performance of the photoconductor was greatly improved when the detector was cooled to 230K. Due to its reduced dark current at low temperatures, the dynamic range of the detector response increased to nearly four decades at 230K.

Characteristics and Uniformity of Group V Implanted and Annealed HgCdTe Heterostructure
L.O. BUBULAC, J. BAJAJ, W.E. TENNANT, M. ZANDIAN, J. PASKO, and W.V. McLEVIGE
Rockwell Science Center, Thousand Oaks, CA 91360.

KEY WORDS Heterostructures, HgCdTe, ion implantation, molecular beam epitaxy (MBE)-HgCdTe

This work presents characterization of implanted and annealed double layer planar heterostructure HgCdTe for p-on-n photovoltaic devices. Our observation is that compositional redistribution in the structure during implantation/annealing process differs from that expected from classical composition gradient driven interdiffusion and impacts the placement of the electrical junction with respect to the metallurgical heterointerface, which in turn affects quantum efficiency and R_oA. The observed anomalous interdiffusion results in much wider cap layers with reduced composition difference between base and cap layer composition. The compositional redistribution can, however, be controlled by varying the material structure parameters and the implant/anneal conditions. Examples are presented for dose and implanted species variation. A model is proposed based on the fast diffusion in the irradiation induced damage region of the ion implantation. In addition, we demonstrate spatial uniformity obtained on molecular beam epitaxy (MBE) material of the compositional and implanted species profile. This reflects spatial uniformity of the ion implantation/annealing Processes and of the MBE material characteristics.

CdZnTe Photodiode Arrays for Medical Imaging
R. SUDHARSANAN,¹ T. PARODOS,¹ A. RUZIN,² Y. NEMIROVSKY,² and N.H. KARAM¹
1--Spire Corporation, One Patriots Park, Bedford, MA 01730-2396. 2--Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa, Israel.

KEY WORDS CdZnTe gamma-ray detectors, CdZnTe Schottky photodiodes, medical imaging, radiation detectors

In this paper, we report on the design, fabrication, and performance of the first CdZnTe Schottky photodiode arrays for radiation detection. High pressure Bridgman-grown CdZnTe substrates with bulk resistivities in the range 10⁸ to 10¹⁰ohm-cm were used. CdZnTe Schottky photodiodes were formed with In and Ti/Au contacts. Diode arrays with pixel sizes from 1000X1000 µm to 100X100 µm were fabricated. The diode's I-V characteristics exhibited low leakage current and high bulk resistivity; leakage current decreased as diode pixel size was reduced. Response of these detector arrays to high energy photons was uniform and their energy resolution improved with smaller pixel size.

Materials Inhomogeneities in Cd_1-xZn_xTe and Their Effects on Large Volume Gamma-Ray Detectors
J.M. VAN SCYOC,¹ J.C. LUND,¹ D.H. MORSE,¹ A.J. ANTOLAK,¹ R.W. OLSEN,¹ R.B. JAMES,¹ M. SCHIEBER,¹ H. YOON,² M.S. GOORSKY,² J. TONEY,³ and T.E. SCHLESINGER³
1--Advanced Electronics Manufacturing Department, Sandia National Laboratories, Livermore, CA 94550. 2--Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90024. 3--Department of Electrical and Computer Engineering, Carnegie Mellon University Pittsburgh, PA 15213.

KEY WORDS CdZnTe, gamma-ray detectors, particle induced photo-emission

Cadmium zinc telluride (Cd_1-xZn_xTe or CZT) has shown great promise as a material for roomtemperature x-ray and gamma-ray detectors. In particular, polycrystalline material grown by the high pressure Bridgman method with nominal Zn fraction (x) from 0.1 to 0.2 has been used to fabricate high resolution gamma-ray spectrometers with resolution approaching that of cooled high-purity Ge. For increased sensitivity, large areas (> 1 cm²) are required, and for good sensitivity to high energy gamma photons, thick detectors (on the order of 1 cm) are required. Thus, there has been a push for the development of CZT detectors with a volume greater than 1 cm³. However, nonuniformities in the material over this scale degrade the performance of the detectors. Variations in the zinc fraction, and thus the bandgap, and changes in the impurity distributions, both of which arise from the selective segregation of elements during crystal growth, result in spectral distortions. In this work, several materials characterization techniques were combined with detector evaluations to determine the materials properties limiting detector performance. Materials measurements were performed on detectors found to have differing performance. Measurements conducted include infrared transmission, particle induced x-ray emission, photoluminescence, and triaxial x-ray diffraction. To varying degrees, these measurements reveal that "poor-performance" detectors exhibit higher nonuniformities than "spectrometer-grade" detectors. This is reasonable, as regions of CZT material with different properties will give different localized spectral responses, which combine to result in a degraded spectrum for the total device.

Improved Arsenic Doping in Metalorganic Chemical Vapor Deposition of HgCdTe and in situ Growth of High Performance Long Wavelength Infrared Photodiodes
P. MITRA,¹ Y.L. TYAN,¹ F.C. CASE,¹ R. STARR,² and M.B. REINE²
1--Loral Vought Systems Corporation, Dallas, TX 75265-0003. 2--Loral Infrared and Imaging Systems, Lexington, MA 02173-7393.

KEY WORDS Arsenic doping, HgCdTe, metalorganic chemical vapor deposition (MOCVD), photodiodes, tertiarybutylarsine, tris-dimethylaminoarsenic

Controlled and effective p-type doping is a key ingredient for in situ growth of high performance HgCdTe photodiode detectors. In this paper, we present a detailed study of p-type doping with two arsenic precursors in metalorganic chemical vapor deposition (MOCVD) of HgCdTe. Doping results from a new precursor tris-dimethylaminoarsenic (DMAAs), are reported and compared to those obtained from tertiarybutylarsine (TBAs). Excellent doping control has been achieved using both precursors in the concentration range of 3X10¹⁵-5X10¹⁷ cm^-3 which is sufficient for a wide variety of devices. Arsenic incorporation efficiency for the same growth temperature and partial pressure is found to be higher with DMAAs than with TBAs. For doping levels up to 1X10¹⁷ cm^-3, the alloy composition is not significantly affected by DMAAs. However, at higher doping levels, an increase in the x-value is observed, possibly as a result of surface adduct formation of DMAAs dissociative products with dimethylcadmium. The activation of the arsenic as acceptors is found to be in the 15-50% range for films grown with DMAAs following a stoichiometric anneal. However, a site transfer anneal increases the acceptor activation to near 100%. Detailed temperature dependent Hall measurements and modeling calculations show that two shallow acceptor levels are involved with ionization energies of 11.9 and 3.2 meV. Overall, the data indicate that DMAAs results in more classically behaved acceptor doping. This is most likely because DMAAs has a more favorable surface dissociation chemistry than TBAs. Long wavelength infrared photodiode arrays were fabricated on P-on-n heterojunctions, grown in situ with iodine doping from ethyl iodide and arsenic from DMAAs on near lattice matched CdZnTe (100) substrates. At 77K, for photodiodes with 10.1 and 11.1 µm cutoff wavelengths, the average (for 100 elements 60X60 µm² in size) zero-bias resistance-area product, R₀A are 434 and 130 ohm-cm², respectively. Quantum efficiencies are 50% at 77K. These are the highest R₀A data reported for MOCVD in situ grown photodiodes and are comparable to state-of-the-art LPE grown photodiodes processed and tested under identical conditions.

Modeling Ion Implantation of HgCdTe
H.G. ROBINSON, D.H. MAO, B.L. WILLIAMS, S. HOLANDER-GLEIXNER, J.E. YU, and C.R. HELMS
Department of Electrical Engineering, Stanford University, Stanford, CA 94305.

KEY WORDS Defect modeling, HgCdTe, ion-implantation, process modeling

Ion implantation of boron is used to create n on p photodiodes in vacancy-doped mercury cadmium telluride (MCT). The junction is formed by Hg interstitials from the implant damage region diffusing into the MCT and annihilating Hg vacancies. The resultant doping profile is n⁺/n^-/p, where the n⁺ region is near the surface and roughly coincides with the implant damage, the n^- region is where Hg vacancies have been annihilated revealing a residual grown-in donor, and the p region remains doped by Hg vacancy double acceptors. We have recently developed a new process modeling tool for simulating junction formation in MCT by ion implantation. The interstitial source in the damage region is represented by stored interstitials whose distribution depends on the implant dose. These interstitials are released into the bulk at a constant, user defined rate. Once released, they diffuse away from the damage region and annihilate any Hg vacancies they encounter. In this paper, we present results of simulations using this tool and show how it can be used to quantitatively analyze the effects of variations in processing conditions, including implant dose, annealing temperature, and doping background.

Heteroepitaxy of HgCdTe(112) Infrared Detector Structures on Si(112) Substrates by Molecular-Beam Epitaxy
T.J. DE LYON,¹ R.D. RAJAVEL,¹ J.E. JENSEN,¹ O.K. WU,¹ S.M. JOHNSON,² C.A. COCKRUM,² and G.M. VENZOR²
1--Hughes Research Laboratories, 3011 Malibu Canyon Rd., Malibu, CA 90265. 2--Santa Barbara Research Center, 75 Coromar Drive, Goleta, CA 93117.

KEY WORDS CdTe, CdTe/Si, focal-plane arrays (FPAs), heteroepitaxy, HgCdTe, HgCdTe/Si, hybrid reliability, infrared detectors, molecular-beam epitaxy (MBE)

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2X10⁶ cm^-2 for 8 µm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22X10⁶ cm^-2. These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64X10⁴ -cm² (0 FOV) for devices with 7.8 um cutoff wavelength at 78K to demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.

Orientation Dependence of HgCdTe Epitaxial Layers Grown by MOCVD on Si Substrates
K. SHIGENAKA,¹ K. MATSUSHITA,¹ L. SUGIURA,¹ F. NAKATA,² K. HIRAHARA,¹ M. UCHIKOSHI,³ M. NAGASHIMA,⁴ and H. WADA⁴
1--Materials and Devices Research Laboratories, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan. 2--On assignment at the R amp&; D Center from Komukai Works, Tochib a Corporation, 1, Komukai Tochib-cho, Saiwai-ku, Kawasaki 210, Japan. 3--Second Research Center, TRDI, Japan Defense Agency, 1-2-24, Ikejiri, Setagaya, Tokyo 154, Japan. Present address: Kumamoto Polytechnic College, Kumamoto, Japan. 4--Second Research Center, TRDI, Japan Defense Agency, 1-2-24, Ikejiri, Setagaya, Tokyo 154, Japan.

KEY WORDS HgCdTe, metalorganic chemical vapor deposition (MOCVD), orientation dependence, Si substrates, transmission electron microscopy (TEM)

Orientation dependence of HgCdTe epilayers grown by MOCVD on Si substrates was studied. Substrate orientation is considered to be one of the most sensitive factors to enable hetero-epitaxial growth on silicon substrates, especially in the case of a low temperature growth process. The present work was carried out with characterized features of a low temperature process for HgCdTe growth on Si and using a thin CdTe buffer layer. The (100), (100) misoriented toward [110], (311), (211), (111), and (331) oriented Si substrates were used in the present work. The best results were obtained on (211)Si substrates with an x-ray full width at half maximum of 153 arc sec for a 5 µm thickness HgCdTe layer and 69 arc sec for a 10 µm thickness layer. It was found that the effective lattice mismatch of CdTe/Si heterosystem was reduced to 0.6% (for the 611 lattice spacing of CdTe and 333 spacing of Si) in the case of (133)CdTe/(211)Si.

Growth of (111) HgCdTe on (100) Si by MOVPE Using Metalorganic Tellurium Adsorption and Annealing
K. MARUYAMA,¹ H. NISHINO,¹ T. OKAMOTO,¹ S. MURAKAMI,¹ T. SAITO,¹ Y. NISHIJIMA,¹ M. UCHIKOSHI,² M. NAGASHIMA,³ and H. WADA³
1--Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan. 2--Japan Defense Agency, 1-2-24 Ikejiri, Setagaya, Tokyo 154, Japan. Present address: Kumamoto Polytechnic College, Kumamoto, Japan. 3--Japan Defense Agency, 1-2-24 Ikejiri, Setagaya, Tokyo 154, Japan.

KEY WORDS Adsorption, annealing, antiphase, CdTe, HgCdTe, metalorganic tellurium, metalorganic vapor phase epitaxy (MOVPE), Si, twinning

(111)B CdTe layers free of antiphase domains and twins were directly grown on (100) Si 4°-misoriented toward <011> substrates, using a metalorganic tellurium (Te) adsorption and annealing technique. Direct growth of (111)B CdTe on (100) Si has three major problems: the etching of Si by Te, antiphase domains, and twinning. Te adsorption at low temperature avoids the etching effect and annealing at a high temperature grows single domain CdTe layers. Te atoms on the Si surface are arranged in two stable positions, depending on annealing temperatures. We evaluated the characteristics of (111)B CdTe and (111)B HgCdTe layers. The full width at half maximum (FWHM) of the x-ray double crystal rocking curve (DCRC) showed 146 arc sec at the 8 µm thick CdTe layers. In Hg_1-xCd_xTe (x = 0.22 to 0.24) layers, the FWHMs of the DCRCs were 127 arc sec for a 7 µm thick layer and 119 arc sec for a 17 µm thick layer. The etch pit densities of the HgCdTe were 2.3X10⁶ cm^-2 at 7 µm and 1.5X10⁶ cm^-2 at 17 µm.

Direct Growth of CdTe on (100), (211), and (111) Si by Metalorganic Chemical Vapor Deposition
H. EBE,¹ T. OKAMOTO,¹ H. NISHINO,¹ T. SAITO,¹ Y. NISHIJIMA,¹ M. UCHIKOSHI,² M. NAGASHIMA,³ and H. WADA³
1--Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan. 2--Japan Defense Agency, 1-2-24 Ikejiri, Setagaya, Tokyo 154, Japan. Present address: Kumamoto Polytechnic College. 3--Japan Defense Agency, 1-2-24 Ikejiri, Setagaya, Tokyo 154, Japan.

KEY WORDS CdTe, DETe, metalorganic chemical vapor deposition (MOCVD), Si

CdTe epilayers were grown directly on (100), (211), and (111) silicon substrates by metalorganic chemical vapor deposition (MOCVD). The crystallinity and the growth orientation of the CdTe film were dependent on the surface treatment of the Si substrate. The surface treatment consisted of exposure of the Si surface to diethyltelluride (DETe) at temperatures over 600°C prior to CdTe growth. Direct growth of CdTe on (100) Si produced polycrystalline films whereas (111)B single crystals grew when Si was exposed to DETe prior to CdTe growth. On (211) Si, single crystal films with (133)A orientation was obtained when grown directly; but produced films with (211)A orientation when the Si surface was exposed to DETe. On the other hand, only a (111)A CdTe films were possible on (111) Si, both with and without Te source exposure, although twinning was increased after exposure. The results indicate that the exposure to a Te-source changes the initial growth stage significantly, except for the growth on (111) Si. We propose a model in which a Te atom replaces a Si atom that is bound to two Si atoms.

Metalorganic Vapor Phase Epitaxy of (100) CdZnTe Layers Using Diisopropylzinc Source
K. YASUDA, K. KAWAMOTO, T. MAEJIMA, M. MINAMIDE, K. KAWAGUCHI, and H. MAEBA
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan.

KEY WORDS CdZnTe, CdZnTe/GaAs, growth characteristics, metalorganic vapor phase epitaxy (MOVPE)

Growth characteristics of (100) Cd_1-xZn_xTe (CZT) have been studied using metalorganic vapor phase epitaxy. CZT layers were grown on (100) GaAs substrates using diisopropylzinc (DiPZn), dimethylcadmiun (DMCd), and diethyltelluride (DETe) as precursors. Growths were carried out in the temperature range from 375 to 450°C. Since DiPZn has lower vapor pressure than DMCd, CZT layers with Zn composition below 0.06 were grown with good compositional control. Layers with uniform Zn composition and thickness over an area of 10X15 mm² were grown. Enhancement of CZT growth rate was observed when a small amount of DiPZn is introduced under fixed flows of DMCd and DETe. Zn composition increases abruptly for further increase of DiPZn flow rate, where growth rate decreases. Growth mechanisms for the above growth conditions were also discussed.

Pb_1-xSn_xSe-on-Si LWIR Sensor Arrays and Thermal Imaging with JFET/CMOS Read-Out
H. ZOGG,¹ A. FACH,¹ J. JOHN,¹ J. MASEK,¹ P. MÜLLER,¹ C. PAGLINO,¹ and W. BUTTLER²
1--AFIF at Swiss Federal Institute of Technology, ETH-Teil Technopark, Pfingstweidstrasse 30, CH-8005 Zürich, Switzerland. 2--Ingenieur-Büro, Eschenburg 55, D-45276 Essen, Germany.

KEY WORDS CaF2, IR detectors, PbSnSe IR devices, thermal image

Long wavelength infrared (LWIR) sensor arrays were fabricated in Pb_1-xSn_xSe layers grown epitaxially on Si-substrates by MBE. A CaF₂ intermediate buffer layer 30Å thick was employed for compatibility reasons. The photovoltaic sensors are based on the blocking Pb-contact technique on p-type material. They were fabricated using simple wet-etching process steps only. Cut-off wavelengths were about 10.5 µm, quantum efficiencies >60%, and resistance-aera products above 3-cm² at 90K. A demonstrational LWIR thermal imaging camera was assembled with a 256 element line array with 50 µm pitch. Low-noise signal processing was achieved with sensors with differential resistances in the 10 kOhm range by using JFET/CMOS technology. For each channel, an integrator, correlated multiple sampling and sample/hold amplifier was used before multiplexing to a common output.

In Situ Sensors for Monitoring and Control in Molecular Beam Epitaxial Growth of Hg_1-xCd_xTe
M.J. BEVAN, W.M. DUNCAN, G.H. WESTPHAL, and H.D. SHIH
Texas Instruments Incorporated, Corporate Research & Development, P.O. Box 655936, MS 150, Dallas, TX 75265.

KEY WORDS HgCdTe, in situ sensors, scan spectral spectrometer

The current status of the implementation and refinement of two wafer state sensors for in situ monitoring and control during molecular beam epitaxial (MBE) growth of Hg_1-xCd_xTe will be reported. First a rapid scan spectral ellipsometer has been developed and employed for precisely measuring compositions of Hg_1-xCd_xTe alloys during growth. MBE films in the composition range x = 0.20 to 0.30 have been grown and in situ spectra taken at the growth temperature (180°C) and at room temperature. The MBE films were treated as single layers without the need to invoke any surface film (due to surface roughness, oxide, or of any different composition) as required for ex situ data. The least squares fit over the whole spectral range was used as a measure of the precision. The film composition was also determined ex situ by wavelength dispersive analysis of x-rays and by Fourier transform infrared (FTIR) spectrometry after verifying that there was no lateral variation. A precision of better than +/-0.0015 has so far been demonstrated using in situ spectral ellipsometry for Cd composition or CdTe mole fraction, x, measurements. This compares with +/-0.003 for single wavelength ellipsometry. The composition of Hg_1-xCd_xTe films were also monitored during growth. A spectral pyrometer based on a FTIR spectrometer has also been developed for substrate temperature measurements during growth. The spectral pyrometer measures both the emission and reflectance to give the emissivity of a growing sample over a range of wavelengths spanning the peak of the grey body emission. From the reflectivity measurements, the thickness (in excess of 1 µm) of the growing film is also determined from the interference fringes. The spectral ellipsometer is only capable of measuring thicknesses up to ca. 5000Å (i.e. optically thin). Excellent agreement is obtained between the in situ (at growth temperature) and ex situ (at room temperature) thickness measurements. The small discrepancy can be explained by the refractive index of Hg_1-xCd_xTe being 5% higher at the growth temperature than at room temperature. The combination of in situ sensors now provides a means of continuously monitoring the composition and thickness of the growing Hg_1-xCd_xTe film.

Key Performance-Limiting Defects in P-on-N HgCdTe LPE Hetrojunction Infrared Photodiodes
M.C. CHEN, R.S. LIST, D. CHANDRA, M.J. BEVAN, L. COLOMBO, and H.F. SCHAAKE
Texas Instruments Incorporated, Corporate Research & Development, P.O. Box 655936, MS 150, Dallas, TX 75265.

KEY WORDS Defects, double layer heterojunction (DLHJ), HgCdTe, liquid phase epitaxy (LPE), metallurgical defects, p-on-n diodes

The inoperability of long-wavelength p-on-n double layer heterojunction arrays for 40K low-background application has long been limited by the wide variation in pixel-to-pixel zero bias resistance (R₀) values. Diodes on test structures showing lower performance, with R₀ values below 7X10⁶ ohm at 40K, usually contained gross metallurgical defects such as dislocation clusters and loops, pin holes, striations, Te inclusions, and heavy terracing. However, diodes with R₀ values between 7X10⁶ and 1X10⁹ ohm at 40K contained no visible defects. To study the "invisible" performance limiting defects (i.e. defects that cannot be revealed by etching), a good correlation between the dynamic resistance at 50mV reverse bias (R50) value at 77K and the R₀ value at 40K was first established, and then used as a tool. The correlation allowed measurements of a large number of devices at 77K, rather than relying exclusively on time consuming measurements at 40K. Interesting results regarding R₀ values, such as insensitivity to low density dislocations, mild degradation from Hg vacancies, severe degradation from Hg interstitials, and correlation with junction positioning were obtained from specially designed experiments.

Annealing Studies of Undoped Hg_1-xMn_xTe Bulk Crystals at High Temperatures
SUSAN W. KUTCHER,¹ T.O. POEHLER,¹ SUDHIR TRIVEDI,² ZHENGCHEN YU,² H.R. VYDYANATH,³ and P. BECLA⁴
1--3400 N. Charles St., Johns Hopkins University, Baltimore, MD 21218. 2--Brimrose Corporation of America, 5020 Campbell Blvd., Baltimore, MD 21236. 3--Aerojet Electrosystems Company, 1100 West Hollyvale St., P.O. Box 296, Azusa, CA 91702. 4--Massachusetts Institute of Technology, Cambridge, MA 02139.

KEY WORDS Annealing, defects, HgMnTe, long wavelength infrared (LWIR)/very long wavelength infrared (VLWIR) detectors, pressure-temperature diagram

The effect of high temperature annealing treatments in varying mercury atmospheres on Hg_1-xMn_xTe crystals with long wavelength infrared/very long wavelength infrared cut off wavelengths has been studied. The undoped Hg_1-xMn_xTe crystals were grown using the traveling heater method with a tellurium solvent zone, and composition was verified by infrared transmission measurements. The crystals were subjected to annealing temperatures of 500 and 550°C under mercury pressures varying from Hg-rich conditions to Te-rich conditions. The samples were either air cooled or water cooled to room temperature. Hall effect measurements were carried out at 77K at magnetic fields varying from 500 Gauss to 10 kGauss. The hole concentration in the annealed crystals was found to be roughly inversely proportional to the partial pressure of Hg indicating that the material is essentially intrinsic at the anneal temperature. A defect model and a relationship between the mass action constants for the native acceptor defects of HgMnTe are presented.

Mercury Cadmium Telluride-Based Resonant Cavity Light Emitting Diode
J.L. PAUTRAT, E. HADJI, J. BLEUSE, and N. MAGNEA
Département de Recherche Fondamentale sur la Matière Condensée, SP2M/PSC, CEA-Grenoble, 17 rue des Martyrs- F 38054 Grenoble Cedex 9 France.

KEY WORDS Bragg mirror, infrared emitter, light emitting diode (LED), resonant cavity

A CdHgTe resonant cavity light emitting diode (RCLED) is proposed as a new infrared emitter. The device is prepared by molecular beam epitaxy on a CdZnTe substrate. A 10.5 periods Bragg mirror is first deposited. The cavity material is made of Cd_0.75Hg_0.25Te and contains a wide well (50 nm) designed to emit at 3.2 µm. The last three periods of the mirror are n-type doped while the cavity material is covered by a thin p-type CdZnTe layer. A gold layer closes the cavity, serving as the second mirror of a Fabry-Perot cavity tuned around 3.18 µm. It also provides an ohmic contact to the p-region. Under forward bias, the emission spectrum displays a narrow peak (8 meV full width at half maximum) corresponding to the cavity resonance. The position and linewidth of this line are independent of temperature. The directivity of the diode is also improved with respect to a conventional emitter, in agreement with theoretical expectations. Taking advantage of the spectral properties of the RCLED a new multispectral device has been fabricated.

Uniform Low Defect Density Molecular Beam Epitaxial HgCdTe
J. BAJAJ, J.M. ARIAS, M. ZANDIAN, D.D. EDWALL, J.G. PASKO, L.O. BUBULAC, and L.J. KOZLOWSKI
Rockwell Science Center. Thousand Oaks, CA 91360.

KEY WORDS Double layer planar heterostructure (DLPH), focal plane array (FPA), growth control, HgCdTe, long wavelength infrared (LWIR), molecular beam epitaxy (MBE), p-n photodiodes, uniformity

This paper describes recent advances in MBE HgCdTe technology. A new 3 inch production molecular beam epitaxy (MBE) system, Riber Model 32P, was installed at Rockwell in 1994. The growth technology developed over the years at Rockwell using the Riber 2300 R&D system was transferred to the 32P system in less than six months. This short period of technology transfer attests to our understanding of the MBE HgCdTe growth dynamics and the key growth parameters. Device quality material is being grown routinely in this new system. Further advances have been made to achieve better growth control. One of the biggest challenges in the growth of MBE HgCdTe is the day-to-day control of the substrate surface temperature at nucleation and during growth. This paper describes techniques that have led to growth temperature reproducibility within +/-1°C, and a variation in temperature during substrate rotation within 0.5°C. The rotation of the substrate during growth has improved the uniformity of the grown layers. The measured uniformity data on composition for a typical 3cm X 3cm MBE HgCdTe/CdZnTe shows the average and standard deviation values of 0.229 and 0.0006, respectively. Similarly, the average and standard deviation for the layer thickness are 7.5 and 0.06 µm, respectively. P-on-n LWIR test structure photodiodes fabricated using material grown by the new system and using rotation during growth have resulted in high-performance (R₀A, quantum efficiency) devices at 77 and 40K. In addition, 128X28 focal plane arrays with excellent performance and operability have been demonstrated.

Growth of High Quality CdTe on Si Substrates by Molecular Beam Epitaxy
L.A. ALMEIDA,¹ Y.P. CHEN,¹ J.P. FAURIE, S. SIVANANTHAN,¹ DAVID J. SMITH,² and S.-C.Y. TSEN²
1--University of Illinois at Chicago, Microphysics Laboratory, Physics Department, 845 W. Taylor St., Chicago, IL 60607-7059. 2--Department of Physics and Astronomy and Center for Solid State Science, Arizona State University, Tempe, AZ 85287.

KEY WORDS CdTe, defects, heteroepitaxy, HgCdTe, molecular beam epitaxy, Si substrate

We have systematically studied the growth of CdTe (111)B on Si(001)with different atomic step structures, defined uniquely by miscut tilt angle and direction. X-ray double crystal rocking curve (DCRC) analysis has been used to evaluate the crystalline quality and twin content of the films. High-resolution electron microscopy has been used to examine the CdTe(111)B/Si(001) interface and to follow the microstructural evolution as a function of distance from the interface. Our results show that the formation of double domains and twins is very sensitive to the tilt parameters. When growth conditions are optimized, twins are not observed at distances greater than about 2.5 microns from the substrate surface. The best quality films exhibit a DCRC FWHM of 60 arc sec, for a film thickness of 17 µm, the lowest value ever reported for heteroepitaxial growth of CdTe on Si or GaAs. In efforts to improve the nucleation process, precursors such as Te and As have been used, and we have shown that they improve the stability of the heterointerface.

In-Situ Spectroscopic Ellipsometry of HgCdTe
J.D. BENSON,¹ A.B. CORNFELD,¹ M. MARTINKA,¹ K.M. SINGLEY,¹ Z. DERZKO,¹ P.J. SHORTEN,¹ J.H. DINAN,¹ P.R. BOYD,² F.C. WOLFGRAM,² B. JOHS,³ P. HE,³ and JOHN A. WOOLLAM³
1--Night Vision and Electronics Sensors Directorate, Ft. Belvoir, VA 22060. 2--Army Research Lab, Ft Belvoir, VA 22060. 3--J.A. Woollam Co. Inc., Lincoln, NE 68508.

KEY WORDS Molecular beam epitaxy, spectroscopic ellipsometry

An in-situ spectroscopic ellipsometer has been equipped on a molecular beam epitaxy system to improve control of HgCdTe growth. Using this device, in-situ analysis of composition, growth rate, and surface cleanliness were monitored. A real time model which determined the compositional profile was used. The ellipsometer was employed to give in-situ real time control of the growth process.

Status of MBE Technology for the Flexible Manufacturing of HgCdTe Focal Plane Arrays
R.D. RAJAVEL,¹ D. JAMBA,¹ O.K. WU,¹ J.A. ROTH,¹ P.D. BREWER,¹ J.E. JENSEN,¹ C.A. COCKRUM,² G.M. VENZOR,² and S.M. JOHNSON²
1--Hughes Research Laboratories, Malibu, CA 90265. 2--Santa Barbara Research Center, Goleta, CA 93117.

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

A robust process has been developed for the reproducible growth of in-situ doped Hg_1-xCd_xTe:As alloys by molecular beam epitaxy. Net hole concentrations in excess of 5X10¹⁷ cm^-3, with peak mobilities >200 cm^2/Vs were measured in Hg_0.74Cd_0.26Te:As films. The p-type layers were twin-free and consistently exhibit narrow x-ray rocking curves (<40 arc sec). The reproducible growth of small lots of p-on-n LWIR detector structures has been established. For a typical lot consisting of 13 layers, the average x-value of the n-type base layer was 0.226 with a standard deviation of 0.003. The lateral compositional uniformity across a 2.5 cmX2.5 cm wafer was x = +/- 0.0006. High performance p-on-n LWIR diodes were fabricated that exhibited R_oA_o values (0-fov at 78K) as large as 350 cm² at 10.4 µm.