JOURNAL OF ELECTRONIC M ATERIALS
	ABSTRACTS Volume 24, Number 5, May 1995 This Month Featuring: Proceedings of the 1993 U.S. Workshop on the Physics and Chemistry of HgCdTe and Other IR Materials held in San Antonio, Texas, October 29-31. View May 1995 Table of Contents.

1993 U.S. WORKSHOP ON THE PHYSICS AND CHEMISTRY OF HgCdTe AND OTHER IR MATERIALS

Subject areas discussed in the 1993 workshop include material growth processes, bulk and thin-film substrates, defects, III-V materials, diffusion, devices, and optical characterization. This special issue is arranged by topical area, as presented at the workshop. Forty of the 58 papers presented at the workshop appear in this publication. The editor thanks the referees for their careful and prompt review of the manuscripts and the program committee for having selected such high-quality work for presentation.

The program committee members were S.M. Johnson (Santa Barbara Research Center), C.J. Summers (Georgia Tech Research Institute), R.L. Aggarwal (Massachusetts Institute of Technology), J.M. Arias (Rockwell International), R.S. Balcerak (Advanced Research Projects Agency), I.B. Bhat (Rensselaer Polytechnic Institute), L.E. Brown (Wright Laboratory), T.N. Casselman (Lockheed Missiles and Space Co.), J.P. Faurie (University of Illinois at Chicago), J.K Furdyna (Notre Dame University), C.R. Helms (Stanford University), M. Martinka (U.S. Army CECOM Night Vision and Electronic Sensors Directorate), T.C. McGill (California Institute of Technology), P.W. Norton (Loral IR and Imaging Systems), H.F. Schaake (Texas Instruments), D.G. Seiler (National Institute of Standards and Technology), A. Sher (SRI International), H.R. Vydyanath (Aerojet Electronic Systems), and J.R. Waterman (Naval ResearchLaboratory). Special thanks go to Scott Johnson and Chris Summers, cochairmen of the workshop, and Jay Morreale (Palisades Institute for ResearchServices, Inc.), workshop coordinator.

The workshop was cosponsored by the U.S. Army CECOM Night Vision and Electronic Sensors Directorate, U.S. Air Force Wright Laboratory, and the U.S. Office of Naval Technology.

The editor is grateful to Theodore C. Harman, editor of Journal of Electronic Materials (JEM), Robert Makowski, Director of Publishing Services of The Minerals, Metals, and Materials Society, and members of the JEM Editorial Oversight Committee for their cooperation in publishing this special edition.

James R. Waterman
Naval Research Laboratory
Washington, DC

Special Issue Editor

MATERIALS GROWTH

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

HgCdTe MBE technology is becoming a mature growth technology for flexible manufacturing of short-wave, medium-wave, long-wave, and very long-wave infrared focal plane arrays. The main reason that this technology is getting more mature for device applications is the progress made in controlling the dopants (both n-type and p-type in-situ) and the success in lowering the defect density to less than 2 x 10⁵/cm² in the base layer. In this paper, we will discuss the unique approach that we have developed for growing As-doped HgCdTe alloys with cadmium arsenide compound. Material properties including composition, crystallinity, dopant activation, minority carrier lifetime, and morphology are also discussed. In addition, we have fabricated several infrared focal plane arrays using device quality double layers and the device results are approaching that of the state-of-the-art liquid phase epitaxy technology.

Precise Control of HgCdTe Growth Conditions for Molecular Beam Epitaxy
MASAYA KAWANO, TOKUHITO SASAKI, and NAOKI ODA
Material Development Center, NEC Corporation, Kawasaki, Kanagawa 216, Japan.

KEY WORDS Control of composition, HgCdTe, infrared detectors, molecular beam epitaxy (MBE)

A Hg_1-xCd_xTe growth method is presented for molecular beam epitaxy, which precisely controls growth conditions to routinely obtain device quality epilayers at a certain specific composition. This method corrects the fluctuation in composition x for run-to-run growth by feedback from the x value for the former growth to the fluxes from CdTe and Te cells. We achieved standard deviation (x/ of 3.3% for 13 samples grown consecutively. A substrate temperature drop was found during growth, which considerably degrades the crystal quality of epilayers. In this method, this drop is greatly diminished by covering the holder surface with a heavily doped Si wafer. Finally, etch pit density of 4 x 10⁴ cm^-2 and full width at half-maximum of 12 arc-s for the x-ray double-crystal rocking curve were obtained as the best values.

Metalorganic Vapor Phase Epitaxy In-Situ Growth of p-on-n and n-on-p Hg_1-xCd_xTe Junction Photodiodes Using Tertiarybutylarsine as the Acceptor Source
V. RAO,¹ H. EHSANI,¹ I.B. BHAT,¹ M. KESTIGIAN,² R. STARR,² M.H. WEILER,² and M.B. REINE²
1--Electrical, Computer, and Systems Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180. 2--Loral Infrared Imaging Systems, Lexington, MA 02173.

KEY WORDS As-doping, direct alloy growth, HgCdTe, infrared detectors, metalorganic vapor phase epitaxy (MOVPE)

We report arsenic doping of Hg_1-xCd_xTe (0.2 < x < 0.3) grown using metalorganic vapor phase epitaxy (MOVPE) by the direct alloy growth (DAG) technique. Tertiarybutylarsine (TBAs) was used as a precursor for As doping. Several epilayers were grown at different Hg partial pressures and TBAs bubbler temperatures in order to study the doping characteristics. The amount of As incorporated in the layer as well as the acceptor concentration were found to be a strong function of the Hg pressure. Secondary ion mass spectrometric studies on heterostructures showed that the compositional interdiffusion is less than the diffusion of As during growth. P-N junctions were grown using TBAs for the first time and several of these layers were processed to fabricate photodiodes. A p-on-n grown junction photodiode with a cutoff wavelength of 8.2 µm had an R₀A value of 241 ohm-cm² at 80K and is the highest reported value for p-on-n DAG-MOVPE devices. Methods to improve the device R₀A of the grown junctions are also proposed.

Application of Spectroscopic Ellipsometry for Real-Time Control of CdTe and HgCdTe Growth in an OMCVD System
S. DAKSHINA MURTHY,¹ I.B. BHAT,¹ B. JOHS,² S. PITTAL,² and P. HE²
1--Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY. 2--J.A. Woollam Company, 650 J Street, #39, Lincoln, NE 68508.

KEY WORDS CdTe, GaAs substrates, HgCdTe, in situ monitoring, organometallic vapor phase epitaxy (OMVPE), spectroscopic ellipsometry

A multi-wavelength in-situ spectroscopic ellipsometer system is described. The hardware can acquire accurate ellipsometric data at 44 wavelengths in less than one second, is simple and compact, and is well suited for in-situ monitoring of chemical vapor deposition. The software used for data analysis is capable of determining the growth rate and composition of the growing layer in real time. These tools were used to study the organometallic chemical vapor deposition of CdTe, HgTe, and HgCdTe on GaAs. We could obtain the dielectric constants of these materials at the growth temperature and also the growth rate and composition of the layers in real time. Feedback control of CdTe growth was performed by connecting an analog control voltage line from the data acquisition/analysis computer to the dimethylcadmium mass flow controller. Using dielectric constants of HgCdTe for two different compositions at the growth temperature, composition control of HgCdTe was attempted in a similar manner.

SUBSTRATES

KEY WORDS CdTe/Si, Ge/Si, heteroepitaxy, organometallic vapor phase epitaxy (OMVPE)

Epitaxial (100) CdTe and ZnTe layers with high crystalline quality have been grown on Si substrates by atmospheric pressure organometallic vapor phase epitaxy (OMVPE). A thin Ge interfacial layer grown at low temperature was used as a buffer layer prior to ZnTe and CdTe growth. The layers were characterized by Nomarski optical microscopy and double crystal x-ray diffraction. Double crystal rocking curves with full width at half maximum of about 110 and 250 arc-sec have been obtained for a 7 µm thick ZnTe layer and a 4 µm thick CdTe layer, respectively. The results presented demonstrate a novel method of in-situ Si cleaning step without a high temperature deoxidation process to grow high quality CdTe and ZnTe on Si in a single OMVPE reactor.

Integrated In Situ Wafer and System Monitoring for the Growth of CdTe/ZnTe/GaAs/Si for Mercury Cadmium Telluride Epitaxy
S.J.C. IRVINE,¹ J. BAJAJ,¹ R.V. GIL,¹ and H. GLASS²
1--Rockwell International Science Center, Thousand Oaks, CA 91360. 2--Rockwell International, Electro Optical Center, Anaheim, CA.

KEY WORDS CdTe/ZnTe/GaAs/Si, in situ monitoring, laser reflectrometry, metalorganic vapor phase epitaxy (MOVPE), pyrometer

Reproducible improvements in the metalorganic vapor phase epitaxy (MOVPE) grown CdTe buffer quality have been demonstrated in a horizontal rectangular duct silica reactor by the use of integrated in situ monitoring that includes laser reflectometry, pyrometry, and Epison concentration monitoring. Specular He-Ne laser reflectance was used to in situ monitor the growth rates, layer thickness, and morphology for both ZnTe and CdTe. The substrate surface temperature was monitored using a pyrometer which was sensitive to the 2-2.6 µm waveband and accurate to ±1°C. The group II and group VI precursor concentrations entering the reactor cell were measured simultaneously using two Epison ultrasonic monitors and significant variations were observed with time, in particular for DIPTe. The surface morphology and growth rates were studied as a function of VI/II ratio for temperatures between 380 and 460°C. The background morphology was the smoothest for VI/II ratio in the vicinity of 1.5-1.75 and could be maintained using Epison monitors. Regularly shaped morphological defects were found to be associated with morphological defects in the GaAs/Si substrate. The x-ray rocking curve widths for CuK (531) reflections were in the range of 2.3-3.6 arc-min, with no clear trend with changing VI/II ratio. X-ray topography images of CdTe buffer layers on GaAs/Si showed a mosaic structure that is similar to CdTe/sapphire substrates. The etch pit density in Hg_1-xCd_xTe layers grown onto improved buffer layers was as low as 6 x 10⁶ cm^-2 for low temperature MOVPE growth using the interdiffused multilayer process.

Direct Growth of CdZnTe/Si Substrates for Large-Area HgCdTe Infrared Focal Plane Arrays
S.M. JOHNSON,¹ T.J. de LYON,² C.A. COCKRUM,¹ W.J. HAMILTON,¹ T. TUNG,¹ F.I. GESSWEIN,¹ B.A. BAUMGRATZ,¹ L.M. RUZICKA,¹ O.K. WU,² and J.A. ROTH²
1--Santa Barbara Research Center, Goleta, CA 93117. 2--Hughes Research Laboratory, Malibu, CA 90265.

KEY WORDS CdZnTe, CdZnTe/Si, heteroepitaxy, HgCdTe, infrared focal plane arrays (INFRAs), liquid phase epitaxy (LPE), metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE)

Direct epitaxial growth of high-quality {100}CdZnTe on 3 inch diameter vicinal {l00} Si substrates has been achieved using molecular beam epitaxy (MBE); a ZnTe initial layer was used to maintain the {100}Si substrate orientation. The properties of these substrates and associated HgCdTe layers grown by liquid phase epitaxy (LPE) and subsequently processed long wavelength infrared (LWIR) detectors were compared directly with our related efforts using CdZnTe/GaAs/Si substrates grown by metalorganic chemical vapor deposition (MOCVD). The MBE-grown CdZnTe layers are highly specular and have both excellent thickness and compositional uniformity. The x-ray full-width at half-maximum (FWHM) of the MBE-grown CdZnTe/Si increases with composition, which is a characteristic of CdZnTe grown by vapor phase epitaxy, and is essentially equivalent to our results obtained on CdZnTe/GaAs/Si. As we have previously observed, the x-ray FWHM of LPE-grown HgCdTe decreases, particularly for CdZnTe compositions near the lattice matching condition to HgCdTe; so far the best value we have achieved is 54 arc-s. Using these MBE-grown substrates, we have fabricated the first high-performance LWIR HgCdTe detectors and 256 x 256 arrays using substrates consisting of CdZnTe grown directly on Si without the use of an intermediate GaAs buffer layer. We find first that there is no significant difference between arrays fabricated on either CdZnTe/Si or CdZnTe/GaAs/Si and second that the results on these Si-based substrates are comparable with results on bulk CdZnTe substrates at 78K. Further improvements in detector performance on Si-based substrates require a decrease in the dislocation density.

Suppression of Twin Formation in CdTe(111)B Epilayers Grown by Molecular Beam Epitaxy on Misoriented Si(001)
Y.P. CHEN,¹ J. P. FAURIE,¹ S. SIVANANTHAN,¹ G.C. HUA,² and N. OTSUKA²
1--University of Illinois at Chicago, Microphysics Laboratory, Physics Department (M/C 273), 845 W. Taylor St. Room # 2236, Chicago IL 60607-7059. 2--School of Materials Engineering, Purdue University, West Lafayette, IN. 47907.

KEY WORDS CdTe/Si, heteroepitaxy, molecular beam epitaxy (MBE), twinning

CdTe(111)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(111)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[] along Si[] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].

Large Area Depositon of Cd_1-xZn_xTe on GaAs and Si Substrates by Metalorganic Chemical Vapor Deposition
N.H. KARAM,¹ R. SUDHARSANAN,¹ A. MASTROVITO,¹ M.M. SANFACON,¹ F.T.J. SMITH,² M. LEONARD,³ and N.A. EL-MASRY³
1--Spire Corporation, One Patriots Park, Bedford, MA 01730. 2--Loral Infrared & Imaging Systems, Lexington, MA 02173. 3--North Carolina State University, Raleigh, NC 27695.

KEY WORDS CdZnTe/GaAs, CdZnTe/GaAs/Si, GaAs/Si, CdZnTe, heteroepitaxy, HgCdTe, liquid phase epitaxy (LPE), metalorganic chemical vapor deposition (MOCVD)

Results of large-area (up to 1000 cm²/run) Cd_1-xZn_xTe heteroepitaxy on both GaAs and GaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD) are presented. Cd_1-xZn_xTe (x = 0-0.1) films exhibited specular surface morphology, 1% thickness uniformity (standard deviation), and compositional uniformity (x) of ±0.002 over 100 mm diam substrates. For selected substrate orientations and deposition conditions, the only planar defects exhibited by (111)B Cd_1-xZn_xTe/GaAs/Si films were lamella twins parallel to the CdTe/GaAs interface; these do not propagate through either the Cd_1-xZn_xTe layer or subsequently deposited liquid phase epitaxy (LPE) HgCdTe layer(s). Background Ga and As-impurity levels for Cd_1-xZn_xTe on GaAs/Si substrates were below the secondary ion mass spectroscopy detection limit. Preliminary results of HgCdTe liquid phase epitaxy using a Te-rich melt on Si-based substrates resulted in x-ray rocking curve linewidths as narrow as 72 arc-sec and etch-pit densities in the range 1 to 3 x 10⁶ cm².

Monitoring Vertical Bridgman-Stockbarger Growth of Cadmium Telluride by an Eddy Current Technique
GARY J. ROSEN,¹ FREDERICK M. CARLSON,¹ JEFFREY E. THOMPSON,¹ WILLIAM R. WILCOX,¹ and JOHN P. WALLACE²
1--Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY 13699-5729. 2--Casting Analysis Corporation, Route 2, Box 113, Weyers Cave, VA 24486.

KEY WORDS CdTe, eddy current, vertical Bridgman

The electric conductivity of indium-doped cadmium telluride (CdTe) in the temperature range 600-1080°C was 163 to 1203 ^-1m^-1. In-situ monitoring of vertical Bridgman-Stockbarger growth showed an unexpected step change in the voltage response vs height. Differential thermal analysis of CdTe showed both Cd and Te melting peaks as well as an exothermic reaction above 790°C.

CdZnTe Substrate Impurities and Their Effects on Liquid Phase Epitaxy HgCdTe
J.P. TOWER,¹ S.P. TOBIN,¹ M. KESTIGIAN,¹ P.W. NORTON,¹ A.B. BOLLONG,² H.F. SCHAAKE,³ and C.K. ARD⁴
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.

KEY WORDS CdZnTe, glow discharge mass spectrometry (GDMS), HgCdTe, impurities, infrared detectors, liquid phase epitaxy (LPE), purification

Impurity levels were tracked through the stages of substrate and liquid phase epitaxy (LPE) layer processing to identify sources of elements which degrade infrared photodetector performance. Chemical analysis by glow discharge mass spectrometry and Zeeman corrected graphite furnace atomic absorption effectively showed the levels of impurities introduced into CdZnTe substrate material from the raw materials and the crystal growth processes. A new purification process (in situ distillation zone refining) for raw materials was developed, resulting in improved CdZnTe substrate purity. Substrate copper contamination was found to degrade the LPE layer and device electrical properties, in the case of lightly doped HgCdTe. Anomalous HgCdTe carrier type conversion was correlated to certain CdZnTe and CdTe substrate ingots.

Etch Pit Characterization of CdTe and CdZnTe Substrates for Use in Mercury Cadmium Telluride Epitaxy
W.J. EVERSON,¹ C.K. ARD,¹ J.L. SEPICH,¹ B.E. DEAN,¹ G.T. NEUGEBAUER,¹ and H.F. SCHAAKE²
1--II-VI Incorporated, Saxonburg, PA 16056. 2--Texas Instruments, Dallas, TX 75265.

KEY WORDS CdTe, CdZnTe, defects, etch pit, HgCdTe

A new etch system is described which produces pits on the technologically important B face of (111) and (211) CdTe and CdZnTe which are commonly used in mercury cadmium telluride (MCT) epitaxy. A ratio of approximately 10 wide: 1 deep is achieved with this etch allowing its use without removing excessive material. Examples of the use of this etch are given and a comparison is made with the Nakagawa, A face etch system which is in common use to characterize this family of materials. A screening protocol is discussed which integrates the use of etch pitting into the manufacture of substrates for use in epitaxial MCT applications. Comparisons are made between CdZnTe substrates grown using the horizontal and vertical Bridgman techniques.

Copper Outdiffusion from CdZnTe Substrates and its Effect on the Properties of Metalorganic Chemical Vapor Deposition-Grown HgCdTe
R. KORENSTEIN,¹ R.J. OLSON, Jr.,¹ D. LEE,¹ P.K. LIAO,² and C.A. CASTRO²
1--Raytheon Company, Research Division, Lexington, MA 02173. 2--Texas Instruments, Central Research Laboratories, Dallas, TX 75265.

KEY WORDS CdZnTe, HgCdTe, impurities, metalorganic chemical vapor deposition (MOCVD), Te precipitates

We report that HgCdTe (MCT) epilayers grown by metalorganic chemical vapor deposition can be doped by copper outdiffusing from CdZnTe substrates. The copper content in the substrates was determined by the choice of the purity of the starting raw materials. Copper diffusion could be controlled by adjusting the tellurium precipitate density in the substrates. Growing on substrates with a high concentration of tellurium precipitates resulted in low doped MCT epilayers whereas a high copper concentration was found in MCT grown on substrates with a lower concentration of tellurium precipitates. A mechanism whereby tellurium precipitates getter copper during the post-growth cooldown of CdZnTe boules and trap copper in the substrates is proposed.

Resonance Ionization Spectroscopy for Quantitative and Sensitive Surface and Bulk Measurements of Impurities in II-VI Materials
S. SEN,¹ J.E. STANNARD,¹ S.M. JOHNSON,¹ H.F. ARLINGHAUS,² and G.I. BEKOV²
1--Santa Barbara Research Center, Goleta, CA 93117. 2--Atom Sciences, Inc., Oak Ridge, TN 37830.

KEY WORDS Cd, CdZnTe, HgCdTe, impurities, resonance ionization spectroscopy (RIS), secondary ion mass spectroscopy (SIMS), Te

We have applied resonance ionization spectroscopy for the first time on II-VI materials, Cd, Te, CdZnTe, and HgCdTe for the measurement of trace impurities. It is an analytical technique with extremely high sensitivity, selectivity, dynamic range, and quantitation accuracy. The technique provides virtual freedom from matrix effects and minimizes isobaric and other mass interferences, known to be the shortcomings in secondary ion mass spectroscopy and other mass spectroscopic measurements. Quantitative analysis of Cu in bulk CdZnTe boules has shown Cu concentration in the range low 10¹⁴ to low 10¹⁵ cm^-3 with an average copper content in four different boules near 2 x 10¹⁴ cm^-3. High Cu concentration (1--2 x 10¹⁷ cm^-3) measured in some HgCdTe epitaxial layers correlated with lower Hall mobility in the layer, and in one case the intentionally In-doped, n-type HgCdTe layer turned p-type.

DEFECTS

KEY WORDS Defects, HgCdTe, infrared detectors, molecular beam epitaxy (MBE)

We have carried out a study and identified that MBE HgCdTe growth-induced void defects are detrimental to long wavelength infrared photodiode performance These defects were induced during nucleation by having surface growth conditions deficient in Hg. Precise control and reproducibility of the CdZnTe surface temperature and beam fluxes are required to minimize such defects. Device quality material with void defect concentration values in the low 10² cm^-2 range were demonstrated.

How Dislocations Affect Transport
A.T. PAXTON,¹ A. SHER,² M. BERDING,² M. VAN SCHILFGAARDE,² and M.W. MULLER³
1--Department of Materials, University of Oxford, OXl 3PH, U.K.. 2--SRI International, Menlo Park, CA 94025. 3--Washington University, St. Louis, MO 63130.

KEY WORDS Dislocations, HgCdTe, piezoelectric, transport properties

Dislocations crossing a junction in HgCdTe have little effect on detector responsivity, but are known to reduce the zero bias impedance R₀A and increase the leakage current, especially at low temperatures where R₀A is dominated by tunneling and generation/recombination processes. We have calculated the Coulomb and piezoelectric fields associated with dislocations in an attempt to interpret their effect on the junction's transport properties. Dislocation electric fields can affect transport since they are superimposed on the built-in and applied junction fields which control the currents. The screening of the fields in the neutral region is consistent with the dislocations' small effect on responsivity. Their impact in the space charge region is found to be significant and consistent with the nonlinear dependence of performance on dislocation density. The piezoelectric potential of the typical 60° dislocation in a sphalerite crystal, and the Coulomb potential of a dislocation crossing the junction plane other than normally, are angularly varying in the junction plane. Angular variation of the potentials can be qualitatively interpreted as an angular modulation of the potential barrier. Because of the nonlinear dependence of junction currents on the barrier (or the junction potential), the angular variation of the currents does not vanish upon averaging. We find that the range of the Coulomb potential is too small to account for a major portion of the experimentally reported performance degradation but may be responsible for the reduction of R₀A at cryogenic temperatures and low dislocation density, and that the longer range piezoelectric potential may be important. We also find that superposing the potentials of neighboring dislocations, because of the nonlinear dependence of junction leakage currents onjunction potentials may account for the observed nonlinearity of performance degradation with dislocation density as measured by etch pit density.

Dislocation Profiles in HgCdTe(100) on GaAs(100) Grown by Metalorganic Chemical Vapor Deposition
H. NISHINO, S. MURAKAMI, T. SAITO, Y. NISHIJIMA, and H. TAKIGAWA
Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan.

KEY WORDS CdTe buffer, dislocation, etch pit density (EPD), HgCdTe, incorporation of dislocations, lattice mismatch strain, metalorganic chemical vapor deposition (MOCVD), slip plane

We studied dislocation etch pit density (EPD) profiles in HgCdTe(100) layers grown on GaAs(100) by metalorganic chemical vapor deposition. Dislocation profiles in HgCdTe(111)B and HgCdTe(100) layers differ as follows: Misfit dislocations in HgCdTe(111)B layers are concentrated near the HgCdTe/CdTe interfaces because of slip planes parallel to the interfaces. Away from the HgCdTe/CdTe interface, the HgCdTe(111)B dislocation density remains almost constant. In HgCdTe(100) layers, however, the dislocations propagate monotonically to the surface and the dislocation density decreases gradually as dislocations are incorporated with increasing HgCdTe(100) layer thicknesses. The dislocation reduction was small in HgCdTe(100) layers more than 10 µm from the HgCdTe/CdTe interface. The CdTe(100) buffer thickness and dislocation density were similarly related. Since dislocations glide to accommodate the lattice distortion and this movement increases the probability of dislocation incorporation, incorporation proceeds in limited regions from each interface where the lattice distortion and strain are sufficient. We obtained the minimum EPD in HgCdTe(100) of 1 to 3 x 10⁶ cm^-2 by growing both the epitaxial layers more than 8 µm thick.

The Minority Carrier Lifetime in Doped and Undoped p-Type Hg_0.78Cd_0.22Te Liquid Phase Epitaxy Films
M.C. CHEN, L. COLOMBO, J.A. DODGE, and J.H. TREGILGAS
Texas Instruments Incorporated, Corporate Research and Development, Dallas, TX 75265.

KEY WORDS HgCdTe, impurity doped, minority carrier lifetime, photoconductivity decay

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg_1-xCd_xTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of ⁱ_A7/ⁱ_A1 ratio by lifetime's dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 10¹⁵ to 1.5 x 10¹⁷ cm^-3, and the lifetime varied from 2 µs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 10¹⁵ and 8 x 10¹⁶ cm^-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, ⁱ_A7/ⁱ_A1, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (E_v+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.

Minority Carrier Lifetime in Indium-Doped HgCdTe(211)B Epitaxial Layers Grown by Molecular Beam Epitaxy
P.S. WIJEWARNASURIYA,^1,2 M.D. LANGE,² S. SIVANANTHAN,¹ and J.P. FAURIE¹
1--University of Illinois at Chicago, Microphysics Laboratory, Physics Department, 845 W. Taylor St., Room 2236, Chicago, IL 60607-7059. 2--EPIR Ltd., P.O. Box 803827 - P2E, Chicago, IL 60680-3827.

KEY WORDS HgCdTe, impurity doped, minority carrier lifetime, photoconductive decay

We have studied the minority-carrier lifetime on intentionally indium-doped (211)B molecular beam epitaxially grown Hg_1-xCd_xTe epilayers down to 80K with x 23.0% ± 2.0%. Measured lifetimes were explained by an Auger-limited band-to-band recombination process in this material even in the extrinsic temperature region. Layers show excellent electron mobilities as high as 2 x 10⁵ cm²v^-1s^-1 at low temperatures. When the layers are compensated with Hg vacancies, results show that the Schockley-Read recombination process becomes important in addition to the band-to-band processes. From the values of _n0 and _p0 of one sample, the obtained defect level is acceptor-like and is somewhat related to the Hg vacancies.

III-V MATERIALS

KEY WORDS Band structure, InAs/GaInSb superlattice, magneto-transport, magneto-optical

We have theoretically and experimentally investigated the electronic properties of InAs-Ga_1-xIn_xSb superlattices. It is found that a strong repulsion between the E1 and H1 bands in superlattices with thin Ga_1-xIn_xSb layers leads to dispersion relations that closely resemble those in HgTe-CdTe superlattices. Temperature-dependent magneto-transport and magneto-optical measurements on samples with a range of InAs layer thicknesses confirm several of the theoretically predicted consequences, e.g., the coexistence of two electron species in semimetallic superlattices and a very light electron cyctron mass in narrow-gap semiconducting samples. The electron mobility is found to be dominated by interface roughness scattering under nearly all conditions of interest. Implications for this system as an infrared detector material are discussed.

Photoresponse Study of Normal Incidence Detection in p-Type GaAs/AlGaAs Multiple Quantum Wells
G.J. BROWN,¹ F. SZMULOWICZ,² and S.M. HEGDE²
1--Wright Laboratory, Materials Directorate, WL/MLPO, Wright Patterson AFB, OH 45433-7707. 2--University of Dayton Research Institute, Dayton, OH 45469-0178.

KEY WORDS GaAs/AlGaAs multiple quantum well (MQW), infrared detectors, intersubband absorption, quantum well infrared photodetector

We studied p-type GaAs/AlGaAs multiple quantum well (MQW) materials as a possible alternative to the current n-type GaAs/AlGaAs MQWs for infrared detection. The advantage of p-type MQWs is that absorption of infrared radiation at normal incidence is not selection rule forbidden as it is for the n-type. We have verified that significant photoresponse occurs at normal incidence in p-type MQWs. We studied changes in the photoresponse spectrum as a function of well width and temperature. The MQW heterostructures were designed to use bound to continuum intersubband absorption in the GaAs valence band and to have a peak photoresponse near 8 µm. The photoresponse spectrum was compared to the first theoretical model of the bound to continuum absorption in p-type GaAs/AlGaAs MQWs. The theoretical absorption curve was found to be in good qualitative agreement with the experimental results.

DIFFUSION

KEY WORDS Diffusion, HgCdTe, modeling, simulation

Details are provided concerning the basic point defect parameter set of the Stanford University mercury cadmium telluride process simulator (SUMerCad). The Hg interstitial and vacancy parameters are presented and justified for x = 0.2 material. In particular, values for the thermodynamic limits, diffusion coefficients, recombination rates, generation rates, and boundary conditions are specified and their determination methodology is reviewed. The parameters have been determined for overall consistency with a specific pool of experimental results which include studies of Hg self-diffusion, type-conversion, and the existence region. Our presentation will review the current state of the Hg_1-xCd_xTe modeling effort, and outline the future direction of the simulator, providing examples and discussion. Finally, some issues related to the future development of SUMerCad are discussed, including electric field effects, general boundary conditions, alternative junction formation processes, extended defects, and interdiffusion.

Process Modeling and Simulation for Hg_1-xCd_xTe. Part II: Self-Diffusion, Interdiffusion, and Fundamental Mechanisms of Point-Defect Interactions in Hg_1-xCd_xTe
JOSÉ L. MELÉNDEZ² and C.R. HELMS¹
1--Department of Electrical Engineering, Stanford University, Stanford, CA 94305. 2--Present address: Texas Instruments, Inc., P.O. Box 655936, Dallas, TX.75265.

KEY WORDS Diffusion, HgCdTe, modeling, simulation

The Hg_0.8Cd_0.2Te type-conversion, Hg self-diffusion and interdiffusion processes are analyzed in the context of a first order reaction kinetics approach. Sets of nonlinear, stiffly coupled continuity equations are presented which describe the underlying physics, and their solutions model the observed macroscopic behavior. It is demonstrated that the Frenkel pair mechanism interactions dominated by the cation sublattice, in conjunction with basic diffusive and drift properties of the ionized point defects, comprise the basic processes which effect all macroscopic phenomena discussed. Existing experimental results are reviewed and apparent discrepancies discussed. Use is made of the Stanford University mercury cadmium telluride process simulator to provide quantitative and insightful examples of important results.

Studies on the Diffusion of Zinc and Iodine into CdTe
E.D. JONES,¹ J.C. CLARK,¹ J. MALZBENDER,¹ J.B. MULLIN,² N. SHAW,³ and A.W. BRINKMAN⁴
1--School of Natural and Environmental Sciences, Coventrg University, Coventry, CVI 5FB, UK. 2--Electronic Materials Consultant, The Hoo, Brockhill Road, West Malvern, Worcester, WR14 4DL, UK. 3--DRA Electronics Division, RSRE, St Andrews Road, Great Malvern Worcester, WR14 3PS, UK. 4--Applied Physics Group, University of Durham, Durham, DH1 3LE.

KEY WORDS CdTe, defects, diffusion, HgCdTe, iodine, zinc

Studies on the diffusion of iodine and zinc into CdTe are reported. Each iodine profile was divided up into four distinct regions and described mathematically by a function consisting of the sum of four complementary error functions. When plotted on an Arrhenius graph, the diffusivities gave four straight line relationships with similar slopes and the Arrhenius parameters for the fastest component of D₀₁ = (7±3) · 10^-11cm² s^-1 and E₁ = (0.21±0.05) eV. When extrapolated down to 20°C this gave a diffusivity of 10^-14 cm² s^-1, indicating that when iodine is diffused from the vapor it is not suitable as a long term stable dopant in devices where sharp impurity profiles are required. In the case of the zinc diffusions, each profile can be divided into two parts and was fitted satisfactorily by the sum of two complementary error functions giving two values of the diffusivities: D_slow due to zinc diffusion into the slice from the vapor and D_fast due to interdiffusion between a surface layer of Zn_xCd_1-xTe formed on the slice and the remaining CdTe.

Diffusion in Mercury Cadmium Telluride--An Update
D. SHAW
Department of Applied Physics, University of Hull, Hull, HU6 7RX, UK.

KEY WORDS Diffusion, diffusivity, HgCdTe, impurities, interstitials, vacancies

The various diffusion coefficients (self-diffusivity, chemical self-diffusivity compositional diffusivity, impurity diffusivity) are defined. The conditions required for the observation of a Kirkendall effect are described. There is good agreement in the results for Hg and for Cd self-diffusion. The evidence suggests that the self-diffusivity is largely independent of P_Hg above ~300°C but shows an increase as Hg saturation is approached (for both Hg and Cd). Good agreement is also found in the Arrhenius parameters describing the movement of the p to n conversion boundary. Modeling of this diffusion boundary does, however, raise problems which are discussed. The situation in compositional interdiffusion is more complex: above ~400°C, reasonable agreement exists between various workers for large x_Hg but not for small values; below 400°C, substantial disagreement is evident. In impurity diffusion exhibits both erfc and non-erfc profiles for reasons which are unclear. Good agreement is found between In diffusivity measurements at high In concentrations: at low concentrations, the diffusivity decreases dramatically. As diffusion yields erfc profiles with good agreement again being found: a notable feature is the P_Hg dependence of the As diffusivity. Where appropriate, diffusion models are discussed.

Variation of Arsenic Diffusion Coefficients in HgCdTe Alloys with Temperature and Hg Pressure: Tuning of p on n Double Layer Heterojunction Diode Properties
D. CHANDRA, M.W. GOODWIN, M.C. CHEN, and L.K. MAGEL
Central Research Laboratories, Texas Instruments Inc., Dallas, TX 75265.

KEY WORDS As, diffusion, diode, HgCdTe, heterojunction, infrared detectors

Arsenic diffusion coefficients were measured in HgCdTe alloy films at 375, 400, and 425°C. The diffusion coefficients displayed a strong dependence on Hg pressures at all three temperatures, increasing by 1000 x with decreasing Hg pressure. The Hg pressures employed were selected to span a portion of the single phase field at each temperature. These investigations added to the diffusion measurements at 350°C reported earlier. The behavior reported at all three temperatures generally resembled the behavior observed at 350°C. For example, at 375°C, the diffusion coeffcient increased from 1.6 x 10^-14 cm²/s for a Hg overpressure of ~1.25 atm, corresponding to the Hg saturated end of the phase field, to 3.2 x 10^-12 cm²/s, when the Hg overpressure was decreased to 0.2 atm, at a point within the phase field, but still considerably removed from the Te-saturated end. For mercury pressure ranges generally within an order of magnitude from the Hg saturated end point, the transport behavior remained simple classical and the diffusion coefficients varied approximately as P_He^-3 at all three temperatures. With continued decrease in the mercury pressure, the transport behavior still remained simple classical, but the dependence of the diffusion coefficients on mercury pressure became more complex, progressing from a P_Hg^-3 dependence to a P_Hg^-1 dependence. This transition was most apparent at 425°C. With additional decrease in the Hg pressure, the transport behavior became more complex and could no longer be described by a simple classical or a monocomponent concentration independent Fickian model. This range was not studied during the present investigations. Additional progress in tuning the positioning of the p/n junction for double layer heterojunction films was possible from the results obtained. This was demonstrated by comparing diode performances obtained on heterojunctions films subjected to seleccted anneal sequences.

Enhanced Arsenic Diffusion and Activation in HgCdTe
S.H. SHIN, J.M. ARIAS, M. ZANDIAN, J.G. PASKO, L.O. BUBULAC, and R.E. DE WAMES
Rockwell International Science Center, Thousand Oaks, CA 91360.

KEY WORDS As, diffusion, HgCdTe, infrared detectors

Temperature and time dependent Hg-annealing studies for arsenic activation have been carried out on As-doped molecular beam epitaxy HgCdTe either in situ or by ion implantation to determine the extent of arsenic activation in the single layer. Enhanced As diffusion and activation in double layer heterostructures have also been investigated to further our understanding of the effects on zero bias resistance-area product (R₀A) and quantum efficiency. The results show that the arsenic activation anneal is limited by Hg self-diffusion into the HgCdTe epilayer. Using this arsenic activation process for either in situ doped arsenic or implanted arsenic, high performance p-on-n double layer heterostructure photodiodes have been demonstrated on both mesa and planar device structures.

P-Type Doping of Double Layer Mercury Cadmium Telluride for Junction Formation
L.O. BUBULAC, D.D. EDWALL, S.J.C. IRVINE, E.R. GERTNER, and S.H. SHIN
Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360.

KEY WORDS Annealing, diffusion, HgCdTe

Extrinsic p-type doping of long wavelength infrared-HgCdTe double layer heterostructure for p-on-n device application requires good control of the p-type dopant, regardless of the doping technique. The approach is to place the electrical junction ahead of the compositional interface, thus avoiding quantum efficiency reduction. This research addresses the As and P doping of HgCdTe by an implant/diffusion process. The data demonstrates an enhanced atomic diffusion process for As and P from an ion implanted source, within the single phase domain, with a diffusion rate orders of magnitude higher than the rate under Hg-saturated conditions at the same temperature. This work also reveals a new phenomenon, namely, a transition in the enhanced diffusion of both As and P from an exponential to a Gaussian redistribution. This transition is controlled by temperature at a given P_Hg. Gaussian diffusion dominates at high temperatures, T >400°C. The diffusion coefficient of the Gaussian mechanism decreases as the P_Hg increases, from D_p ~2 x 10^-11 cm²/s at P_Hg ~0.02 atm to D_p ~3 x 10^-14 cm²/ s under Hg-saturated conditions (quartz ampoute) at 440°C. The difference in the diffusion coefficients between open tube and closed tube (quartz ampoule), under nominally Hg-saturated conditions, indicates that P_Hg is undersaturated regardless of the Hg-source proximity. The deviation of P_Hg from saturation is estimated from the annealing furnace temperature profile up to a maximum of 50%. Variation of the diffusion coefficient close to Hg saturation appears to be sharply dependent on the actual P_Hg value (example: D_p ~1 x 10^-12 cm²/s in open-tube anneal vs D_p ~3 x 10^-14 cm²/s in closed tube at nominally the same temperature, T = 440°C). Comparative anneals of As and P showed faster diffusion rates for P than for As in both mechanisms.

Annealing Experiments in Heavily Arsenic-Doped (Hg,Cd)Te
H.R. VYDYANATH,¹ L.S. LICHTMANN,¹ S. SIVANANTHAN,² P.S. WIJEWARNASURIYA,² and J.P. FAURIE²
1--Aerojet Electronic Systems Division, P.O. Box 296, Azusa, CA 91702. 2--University of Illinois at Chicago, Chicago, IL 60680.

KEY WORDS Amphoteric, annealing, As, diffusion, HgCdTe, Hg partial pressure

Arsenic doped molecular beam epitaxy (MBE) (Hg,Cd)Te films were grown on (Cd,Zn)Te substrates. The concentration of arsenic was varied from 5 x 10¹⁸ cm^-3 to 1 x 10²⁰ cm^-3. After the growth, the epitaxial layers were annealed at various partial pressures of Hg within the existence region of (Hg,Cd)Te at temperatures ranging from 400 to 500°C. Hall effect and resistivity measurements were carried out subsequent to the anneals. 77K hole concentration measurements indicate that for concentrations of arsenic <10¹⁹ cm^-3, most of the arsenic is electrically active acting as acceptors interstitially and/or occupying Te lattice sites at the highest Hg pressures. At lower Hg pressures, particularly at annealing temperatures of 450°C and higher, compensation by arsenic centers acting as donors appears to set in and the hole concentration decreases with decrease in Hg pressure. These results indicate the amphoteric behavior of arsenic and its similarity to the behavior of phosphorus in (Hg,Cd)Te previously inferred by us. A qualitative model which requires the presence of arsenic occupying both interstitial and Te lattice sites along with formation of pairs of arsenic centers is conjectured.

DEVICES

KEY WORDS HgCdTe, infrared detectors, semiconductor device modeling

In this article, device modeling refers to numerical simulation of semiconductor device physics to predict electrical behavior. The silicon integrated circuit industry provides the example for the use of technology computer-aided design to simulate wafer fabrication processes, and the electrical performance of devices and circuits. This paper first reviews semiconductor device modeling in general, then as applied in work supporting the development and analysis of HgCdTe infrared detectors. Example applications of one- and two-dimensional device modeling are simulation of a bias-selectable, integrated two-color detector, and two-dimensional effects on the spectral response of a HgCdTe detector with composition grading.

Transport Studies in Narrow-Gap Semiconductors Revisited
SRINIVASAN KRISHNAMURTHY and ARDEN SHER
SRI International, Menlo Park, CA 94025.

KEY WORDS Band gap, calculation of accurate band structures, electron mobility, Fermi level, Hall coefficient, HgCdTe

Transport-related properties such as electron mobility, Hall coefficient, Fermi level, and energy gap are calculated with accurate analytical band structures, Fermi-Dirac statistics, and a full solution to the Boltzmann transport equation. These calculated values differ substantially from the ones obtained with parabolic or k·p generated band structure approximations for a Hg_0.78Cd_0.22Te alloy. A new way to analyze absorption data to extract the temperature variation of the band gap is also explained.

Metalorganic Chemical Vapor Deposition CdTe Passivation of HgCdTe
Y. NEMIROVSKY, N. AMIR, and L. DJALOSHINSKI
Kidron Microelectronics Research Center, Department of Electrical Engineering, Technion, Haifa 32000, Israel.

KEY WORDS CdTe, HgCdTe, infrared detectors, metalorganic chemical vapor deposition (MOCVD), surface passivation

CdTe epilayers are grown by metalorganic chemical vapor deposition (MOCVD) on bulk HgCdTe crystals with x ~ 0.22 grown by the traveling heater method (THM). The THM HgCdTe substrates are (111) oriented and the CdTe is grown on the Te face. The metalorganic sources are DMCd and DETe, and the growth is performed at subatmospheric pressure. Ultraviolet (UV) photon-assisted hydrogen radicals pretreatment plays a dominant role in the electrical properties of the resulting heterostructures. The requirements of a good passivation for HgCdTe photodiodes vis-a-vis the passivation features of CdTe/HgCdTe heterostructures are discussed. The effect of valence band offset and interface charges on the band diagrams of p-isotype CdTe/HgCdTe heterostructures, for typical doping levels of the bulk HgCdTe substrates and the MOCVD grown CdTe, is presented. Electrical properties of the CdTe/HgCdTe passivation are determined by capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor test devices, where the MOCVD CdTe is the insulator. It is found that the HgCdTe surface is strongly inverted and the interface charge density is of the order of 10¹²cm^-2 when the CdTe epilayer is grown without the UV pretreatment. With the in-situ UV photon-assisted hydrogen radicals pre-treatment, the HgCdTe surface is accumulated and the interface charge density is -4 · 10¹¹ cm^-2.

Investigation of Epitaxial P-p CdTe/Hg_0.775Cd_0.225Te Heterojunctions by Capacitance-Voltage Profiling
V. ARIEL,¹ V. GARBER,¹ G. BAHIR,¹ A. SHER,² and G. CINADER²
1--Department of Electrical Engineering, Technion, Haifa 32000, Israel. 2--Soreq NRC, Yavne 70600, Israel.

KEY WORDS CdTe, HgCeTe, infrared detectors, metalorganic chemical vapor deposition (MOCVD), surface passivation

We investigate the electrical properties of isotype P-p CdTe/Hg_0.775Cd_0.225Te heterojunctions grown in situ by the metalorganic chemical vapor deposition technique. The capacitance-voltage (C-V) characterization of Schottky barriers (SB) is used to study the apparent majority carrier distribution and the valence band discontinuity. The C-V characteristics of metal insulator semiconductor (MIS) devices were used to determine the interface charge density. A theoretical model suitable for analysis of graded heterodunctions was developed based on the numerical solution of Poisson's equation. The model includes an approximate description of the conduction band nonparabolicity and carrier degeneracy. We describe the procedures used in crystal growth and device fabrication for both SB and MIS structures. We demonstrated, on the basis of experimental measurements and theoretical analysis, that the valence band discontinuity in the devices studied here was 0.15 ± 0.05 eV and the fixed interface charge density was approximately (3 ± 1) · 10¹⁰ cm^-2. Also, we observed a dependence of the C-V measurements on temperature which seems to be caused by either interface traps or carrier inversion at the CdTe/HgCdTe interface.

Metalorganic Chemical Vapor Deposition of HgCdTe for Photodiode Applications
P. MITRA,¹ T.R. SCHIMERT,¹ F.C. CASE,¹ R. STARR,² M.H. WEILER,² M. KESTIGIAN,² and M.B. REINE²
1--Loral Vought Systems Corporation, P. O. Box 650003, Dallas, TX 75265. 2--Loral Infrared and Imaging Systems, Lexington, MA 02173.

KEY WORDS HgCdTe, infrared detectors, metalorganic chemical vapor deposition (MOCVD)

Metalorganic chemical vapor depositor (MOCVD) in situ growth of p-on-n junctions for longwavelength infrared (LWIR) and medium wavelength infrared (MWIR) photodiodes is reported. The interdiffused multilayer process was used for the growth of the HgCdTe junctions on CdTe and CdZnTe substrates. The n-type region was grown undoped while the p-type layer was arsenic doped using tertiarybutylarsine. Following a low temperature anneal in Hg vapor, carrier densities of (0.2-2) x 10¹⁵ cm^-3 and mobilities of (0.7-1.2) x 10⁵ cm²/V-s were obtained for n-type LWIR (x ~ 0.22) layers at 80K. Carrier lifetimes of these layers at 80K are ~ 1-2 µs. For the p-type region arsenic doping was controlled in the range of (1-20) x 10¹⁶ cm^-3. Arsenic doping levels in the junctions were determined by calibrated secondary ion mass spectroscopy depth profile measurements. Composition and doping of the p-on-n heterojunctions could be independently controlled so that the electrical junction could be located deeper than the change in the composition. The graded composition region between the narrow and wide (x = 0.28-0.30) bandgap regions are 1-2 µm depending on the growth temperature. Backside-illuminated variable-area circular mesa photodiode arrays were fabricated on the grown junctions as well as on ion implanted n-on-p MWIR junctions. The spectral responses are classical in shape. Quantum efficiencies at 80K are 42-77% for devices without anti-reflection coating and with cutoff wavelengths of 4.8-11.0 µm. Quantum efficiencies are independent of reverse bias voltage and do not decrease strongly at lower temperatures indicating that valence band barrier effects are not present. 80K R₀A of 15.9 -cm² was obtained for an array with 11.0 µm cutoff. Detailed measurements of the characteristics of the MOCVD in situ grown and implanted photodiodes are reported.

Independently Accessed Back-to-Back HgCdTe Photodiodes: A New Dual-Band Infrared Detector
M.B. REINE,¹ P.W. NORTON,¹ R. STARR,¹ M.H. WEILER,¹ M. KESTIGIAN,¹ B.L. MUSICANT,¹ P. MITRA,² T. SCHIMERT,² F.C. CASE,² I.B. BHAT,³ H. EHSANI,³ and V. RAO³
1--Loral Infrared & Imaging Systems, Lexington, MA 02173. 2--Loral Vought Systems Corporation, Dallas, TX 75266. Rensselaer Polytechnic Institute, Troy, NY 12180.

KEY WORDS Dual-band detectors, focal plane arrays, HgCdTe, infrared detectors, metalorganic chemical vapor deposition (MOCVD)

We report the first data for a new two-color HgCdTe infrared detector for use in large dual-band infrared focal plane arrays (IRFPAs). Referred to as the independently accessed back-to-back photodiode structure, this novel dual-band HgCdTe detector provides independent electrical access to each of two spatially collocated back-to-back HgCdTe photodiodes so that true simultaneous and independent detection of medium wavelength (MW 3-5 µm) and long wavelength (LW, 8-12 µm) infrared radiation can be accomplished. This new dual-band detector is directly compatible with standard backside-illuminated bump-interconnected hybrid HgCdTe IRFPA technology. It is capable of highfill factor, and allows high quantum efficiency and BLIP sensitivity to be realized in both the MW and LW photodiodes. We report data that demonstrate experimentally the key features of this new dual-band detector. These arrays have a unit cell size of 100 x 100 µm², and were fabricated from a four-layer p-n-N-P HgCdTe film grown in situ by metalorganic chemical vapor deposition on a CdZnTe substrate. At 80K, the MW detector cutoff wavelength is 4.5 µm and the LW detector cutoff wavelength is 8.0 µm. Spectral crosstalk is less than 3%. Data confirm that the MW and LW photodiodes are electrically and radiometrically independent.

Room Temperature Characterization of Hg_1-xCd_xTe P-on-n Heterostructure Photodiodes
M. ZANDLAN, J.G. PASKO, J.M. ARIAS, R.E. DE WAMES, and S.H. SHIN
Rockwell International Science Center, Thousand Oaks, CA 91360.

KEY WORDS Current-voltage characteristics, HgCdTe, infrared detectors

Measurements of 77K R₀A and 300K reverse bias dynamic impedance (R_dA) products at one volt reverse bias has been carried out to assess the degree of correlation of this figure of merit. Planar P-on-n heterostructures were grown on near lattice-matched CdZnTe substrates with Hg_1-xCd_xTe (0.20< x <0.30) by molecular beam epitaxy. These devices were passivated with CdTe and doped with indium and arsenic as n- and p-type dopants, respectively. Current-voltage characteristic of these devices exhibit thermally generated dark currents at small and modest reverse bias. We have observed that R₀A values of these long wavelength infrared P-on-n heterostructure photodiodes at 77K correlate with room temperature R_dA values. Diode arrays with high room temperature R_dA values at one volt reverse bias also have high R₀A values at 77K. Similarly, low R_dA values at room temperature indicate poor performance at 77K where deviation from diffusion current occurs at reverse bias of 0.2 to 1 volt at room temperature. The results presented here, for a small samples of devices, demonstrate that room temperature measurements of current-voltage characteristics to evaluate Hg_1-xCd_xTe (0.22< x <0.28) diode performance and array uniformity at lower temperatures can be used. This provides an acceptable criteria for further study at lower temperatures.

OPTICAL CHARACTERIZATION

KEY WORDS HgCdTe, in-situ characterization, molecular beam epitaxy (MBE), metalorganic molecular beam epitaxy (MOMBE), reflectance, photoreflectance

Epitaxial growth of Hg-based semiconductors by molecular beam epitaxy (MBE) and metalorganic MBE (MOMBE) has progressed sufficiently to shift emphasis to the control of factors limiting the yield of both materials and devices. This paper reports on an ex-situ study to evaluate the suitability of reflectance and photoreflectance (PR) as in-situ characterization techniques for the growth of CdTe and HgCdTe. Photoreflectance yields information about CdTe layers, with largest utility for doped and multi-layer structures. However, caution must be taken in interpretation of the spectra since the near-bandedge PR spectra consists of multiple transitions and the E₁ transition energy is very sensitive to the sample history. Photoreflectance appears to be of limited utility for HgCdTe single layer growth with x<0.4. However, reflectance measurements of the E₁ peak can be used to determine composition in HgCdTe single layers with an accuracy x = ±0.01, which can be useful for growth control. A tight binding model was used to calculate the E₁ peak energy as a function of bandgap for HgCdTe and HgTe/CdTe superlattices. Comparisons are made with experimental observations. Surface interdiffusion in HgTe-CdTe superlattices was probed using reflectance measurements.

Optical Properties of Undoped and Iodine-Doped CdTe
N.C. GILES,¹ JAESUN LEE,¹ T.H. MYERS,¹ ZHONGHAI YU,¹ B.K. WAGNER,² R.G. BENZ II,² and C.J. SUMMERS²
1--Department of Physics, West Virginia University, Morgantown, WV 26506. 2--EOEML, Quantum Microstructures Laboratory, Georgia Tech Research Institute, Atlanta, GA 30332.

KEY WORDS CdTe, metalorganic molecular beam epitaxy (MOMBE), photoluminescence, photoreflectance

A comprehensive study of the properties of undoped and iodine-doped CdTe structures by photoluminescence (PL) and photoreflectance (PR) is reported. Undoped bulk CdTe and iodine-doped CdTe layers grown by metalorganic molecular beam epitaxy on (100)-oriented CdTe and (211)B-oriented GaAs substrates with electron concentrations ranging from 10¹⁴ to mid- 10¹⁵ cm^-3 were included in this study. Lineshape modeling of 80K PL and PR spectra indicated the presence of both free excitor and donor-hole transitions at the higher doping levels. Strong PL and PR signals were also observed at room temperature. If only a single transition is considered for the analysis of the 300K spectra, the PL emission peak and the PR transition energy both exhibit a strong dependence on electron concentration for doped layers. However, lineshape modeling of the room-temperature spectra indicated the presence of multiple transitions consisting of free excitor and direct band-to-band transitions. The use of two transitions resulted in a constant value of bandgap over the entire range of conductivities studied. A strong correlation remained between the broadening of the PR and PL spectra and excess carrier concentration N_D- N_A. In addition, the E₁ transition energy measured by PR was found to vary dramatically with growth conditions.

A Comparison of Techniques for Nondestructive Composition Measurements in CdZnTe Substrates
S.P. TOBIN,¹ J.P. TOWER,¹ P.W. NORTON,¹ D. CHANDLER-HOROWITZ,² P.M. AMIRTHARAJ,² V.C. LOPES,³ W.M. DUNCAN,³ A.J. SYLLAIOS,³ C.K. ARD,⁴ N.C. GILES,⁵ JAESUN LEE,⁵ R. BALASUBRAMANIAN,⁶ A.B. BOLLONG,⁶ T.W. STEINER,⁷ M.L.W. THEWALT,⁷ D.K. BOWEN,⁸ and B.K. TANNER⁹
1--Loral Infrared & Imaging Systems, Lexington, MA 02173. 2--National Institute of Standards and Technology, Gaithersburg, MD 20899. 3--Texas Instruments, Dallas, TX 75265. 4--II-VI, Inc., Saxonburg, PA 16056. 5--West Virginia University, Morgantown, WV 26506. 6--Johnson Matthey Electronics, Spokane, WA 99216. 7--Simon Fraser University, Burnaby, BC, Canada. 8--University of Warwick, Coventry CV4 7AL, U.K. 9--University of Durham, Durham DH3 ILE, U.K..

KEY WORDS CdZnTe, HgCdTe, lattice constant, nondestructive measurements, photoluminescence, photoreflectance

We report an overview and a comparison of nondestructive optical techniques for determining alloy composition x in Cd_1-xZn_xTe substrates for HgCdTe epitaxy. The methods for single-point measurements include a new x-ray diffraction technique for precision lattice parameter measurements using a standard high resolution diffractometer, room-temperature photoreflectance, and low-temperature photoluminescence. We compare measurements on the same set of samples by all three techniques. Comparisons of precision and accuracy, with a discussion of the strengths and weaknesses of different techniques, are presented. In addition, a new photoluminescence excitation technique for full-wafer imaging of composition variations is described.

Investigation of Monolayer Roughness in HgTe-CdTe Superlattices
J.R. MEYER,¹ K.A. HARRIS,⁴ R.W. YANKA,² L.M. MOHNKERN,² A.R. REISINGER,² J.F. EGLER,³ K. MAHALINGAM,³ and N. OTSUKA³
1--Code 5612, Naval Research Laboratory, Washington, DC 20375. 2--Martin Marietta Electronics Laboratory, Syracuse, NY 13221. 3--School of Materials Engineering, Purdue University, West Lafayette, IN 47907. 4--Present address: II-VI, Inc., Saxonburg, PA 16056.

KEY WORDS HgTe-CdTe superlattices, photoluminescence, molecular beam epitaxy (MBE), quantum wells

Infrared photoluminescence (PL) measurements were performed on (211)-oriented superlattices with energy gaps in the range 110-495 meV. Most of the samples with thinner HgTe quantum wells displayed two PL peaks separated by h 30-65 meV (which generally increased with decreasing well thickness). Both peak energies (E_p) sometimes varied gradually with location on the surface, and in one case three peaks of approximately equal spacing were observed in some locations. The data are consistent with a model which assumes the presence of randomly distributed islands having well thicknesses varying by approximately one monolayer. We find that h and the variations of the spectra with temperature agree well with calculations based on this simple model.