H. F. Schaake

The effect of low temperature annealing on defects, impurities, and electrical properties of (Hg,Cd)Te

Many methods for the preparation of (Hg,Cd)Te alloys rely on a low temperature processing step to convert the as‐grown p‐type material to n‐type, or to otherwise adjust the concentration of native acceptors. During this anneal, tellurium precipitates in the material are annihilated by in‐diffusing mercury, resulting in a substantial multiplication of dislocations. For substantially long anneals (>1 day at 270 °C) the depth of the p–n junction is found to vary as the square root of the anneal time and inversely as the square root of the excess tellurium concentration. Rapidly diffusing impurities such as silver are gettered out of the skin and into the remaining vacancy‐rich core. The kinetics of these processes are analyzed for self‐diffusion on the metal sublattice involving only vacancies, only interstitials, and for a mixed vacancy–interstitial model. Comparison with experimental data shows best agreement with the mixed interstitial–vacancy model.

Comparison of HgCdTe and QWIP dual-band focal plane arrays

We report on results of laboratory and field tests of dual- band MWIR/LWIR focal plane arrays (FPAs) produced under the Army Research Laboratorys Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system and by BAE Systems using QWIP technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640 X 480-pixel single-color read-out integrated circuit (ROIC) to produce a dual-band 320 X 240 pixel array. The MWIR and LWIR pixels are co-located and have a high fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using MBE-grown III-V materials. The LWIR section consisted of GaAs quantum wells and AlGaAs barriers and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two color operation) which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The relative advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

Physical and chemical properties of the anodic oxide/HgCdTe interface

In this paper we show that anodic oxide/Hg0.8Cd0.2Te interface is a two‐layered structure with a thin CdTeO3 layer next to the Hg0.8Cd0.2Te substrate and a second thicker layer of CdTeO3, TeO2, HgTeO3, and HgTe particles. For a 700‐A‐thick anodic oxide, the thin CdTeO3 layer was 30–50 A thick and the second interface layer was ∼120–150 A thick. The large positive fixed charge measured on metal‐insulator semiconductor devices which have been fabricated with the anodic oxide may be caused by a large concentration of positively charged oxygen vacancies associated with the HgTe particles in the second interface layer. The low interface trap densities are probably due to the thin CdTeO3 layer which forms a good interface with HgCdTe.

Comparison of HgCdTe and quantum-well infrared photodetector dual-band focal plane arrays

We report on results of laboratory and field tests of dual-band focal plane arrays (FPAs) in the medium-wave infrared (MWIR) and long-wave infrared (LWIR), produced under the Army Research Laboratorys Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system, and by BAE Systems using quantum-well infrared photodetector (QWIP) technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640×480-pixel single-color readout integrated circuit (ROIC) to produce a dual-band 320×240 pixel array. The MWIR and LWIR pixels are co-located and have a large fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating-frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using III-V materials grown by molecular-beam epitaxy (MBE). The LWIR section consisted of GaAs quantum wells and AlGaAs barriers, and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel, allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two-color operation), which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

High Operating Temperature MWIR detectors

The utilization of the non-equilibrium photodiode concept for high operating temperature (HOT) FPAs is discussed, both generically, and with regard to the specific example of MWIR HgCdTe. The issues of dark current, surface passivation, and 1/f noise are considered for three different architectures, namely N+/N-/P+, N+/P-/P+, and nBn. These architectures are examined with regard to possible FPA performance limitations, and potential difficulty in reduction to practice. Performance data obtained at DRS for the N+/N-/P+ and N+/P-/P+ HgCdTe architectures will be presented.

Thermal annealing studies on boron‐implanted HgCdTe diodes

Junction profiles of B+‐implanted Hg1−xCdxTe are examined using reverse bias capacitance–voltage measurements. All devices are fabricated on residual copper‐doped, low vacancy concentration, p‐type bulk material with a net doping of 1.5×1015 cm−3. The junction profiles showed the as‐implanted junctions to be abrupt with one side of the junction doped in the low‐ to mid‐1014 cm−3 range. These data and related circumstantial evidence are consistent with an n−/p junction after implantation. Type conversion in the immediate vicinity of the junction is postulated to result from indiffusion of interstitial Hg that is created by implant damage, the annihilation of group‐II site vacancies by the Hg, and the associated movement of the highly mobile copper acceptor away from the low vacancy concentration region via an interstitial gettering mechanism. An immobile background donor concentration produces the n− region in the copper denuded region. During thermal anneal treatments, grading of the junction and improvem...

Observation of a new gettering mechanism in (Hg, Cd)Te

Abstract A new gettering mechanism is proposed for substitutional impurities which diffuse by an interstitial process. In this mechanism, an externally imposed gradient of self interstitials generates a gradient in impurity interstitials leading to the segregation of fast diffusing impurities to low interstitials, high vacancy regions. Data are presented to support this model in (Hg, Cd)Te alloys, as well as explicitly rule out gettering by dislocations, by segregation to precipitates, or by enhanced solubility arising from the interaction of the impurity with a varying Fermi level.

Improved breakdown voltage in molecular beam epitaxy HgCdTe heterostructures

We present recent progress in the development of metal‐insulator‐semiconductor (MIS) heterostructure detectors grown by molecular beam epitaxy (MBE); these heterostructure films should significantly improve the available well capacity for MIS long wavelength infrared (LWIR) detectors. Recent MBE grown HgCdTe(112)Te heterostructures show a 4× increase in available charge capacity and a 2× decrease in net donor density relative to our MBE HgCdTe(001) layers. To obtain full advantage of the MIS heterostructure however, another 2.5×increase in the MWIR breakdown field (0.8–2.0 V/μm) and a 2× decrease in the MWIR donor density (1×1015–5×1014 cm−3) is necessary. Since the MWIR breakdown field does not appear to be limited by compositional micro‐inhomogeneities or by compound twin defects in the MBE HgCdTe layers, we suspect either dopant micro‐inhomogeneities or native point defects are responsible for the significant tunnel currents measured in our MWIR MBE HgCdTe.

Resonant tunneling in HgCdTe heterostructures

Resonant tunneling has been demonstrated through a variety of molecular‐beam epitaxially grown HgCdTe heterostructures. Single‐quantum‐well, double‐barrier resonant tunneling structures with a large variation in quantum well widths and barrier thicknesses were investigated. We also present results on transport through two strongly coupled superlattices, which exhibit negative differential resistance due to energy filtering by the superlattice minibands.

HDVIP for low-background-flux and high-operating-temperature applications

An overview of the DRS HDVIP architecture for realization of large-area infrared focal plane arrays (IRFPAs) is given. Improvements needed to meet more stringent application requirements are discussed and modeled. Both theoretical and experimental data are presented.