Native point defects play an important role in HgCdTe. Here we discuss some of the relevant mass action equations, and use recently calculated defect formation energies to discuss relative defect concentrations. In agreement with experiment, the Hg vacancy is found to be the dominant native defect to accommodate excess tellurium. Preliminary estimates find the Hg antisite and the Hg interstitial to be of comparable densities. Our calculated defect formation energies are also consistent with measured diffusion activation energies, assuming the interstitial and vacancy migration energies are small.

We report on progress in the monolithic integration of a metal-semiconductor-metal (MSM) detector and transimpedence amplifier and of a photoconductive detector (PCD) with a metal-semiconductor field effect transistor (MESFET) in Hg1-xCdxTe. The layers of CdTe/n-type Hg1-xCdxTe were grown by MOCVD on semi-insulating GaAs substrates (2° misoriented 100). Fabrication of the devices was by an FET planar process; with a standard lift-off used to form Schottky metallization on both the interdigitated electrodes of the MSM detector (2μm width, 2μm spacing) and the gate of the MESFETs (5μm length, 100μm width). The MSM photodetectors exhibited breakdown voltages in the range 60 to 80V, a dark current of 1 Ona at 5V bias, and responsivities of > 1.0 A/W measured at 40V using CW 1.3um illumination. The integrated devices have been characterised by electrical and micro RBS techniques; the results were found to be strongly dependent on the stoichiometric x ratio of the Hg1-xCdxTe. This initial work demonstrates the suitability of Hg1-xCdxTe/GaAs structures in the fabrication of integrated optoelectronic circuits.

HgCdTe grown by photon-assisted molecular beam epitaxy is now suitable for use in high performance detector fabrication. These are the preliminary results for infrared detectors which have been fabricated in HgCdTe grown using this technique at GE. The detectors were fabricated using a modified Hg-diffused diode process. In addition, the first high quantum efficiency infrared detectors based on the HgTe-CdTe superlattice material system is presented as an example of the sophisticated structures obtainable with photon-assisted molecular beam epitaxy. The superlattice detectors exhibited quantum efficiencies as large as 66% (at 140K) at the peak wavelength of 4.9μm and an average quantum efficiency over the 3–5μm waveband of 55%.

The techniques, Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) have been used to investigate compositional and thickness uniformity of Hg1-xCdxTe (MCT) grown on GaAs substrates by Metal Organic Chemical Vapour Deposition (MOCVD). Composition and thickness variations are reported for orientations perpendicular and parallel to gas flow in the MOCVD reactor. Crystalline quality of the MCT layer was also determined by RBS channelling analysis.

Indium doping of mercury cadmium telluride, grown by organometallic epitaxy, has been accomplished using trimethylindium (TMIn) as the dopant source. Layers, grown by the Direct Alloy Growth (DAG) process, exhibited a linear doping vs TMIn partial pressure characteristic over the 5 × 1016 to 3 × 1018 cm−3 range. A maximum doping concentration of 5 × 1018 cm−3 was obtained in these layers.

The optical band edge in these layers was observed to move to higher energy with increased doping. It is shown that this is caused by the Burstein-Moss shift of the bandedge with doping, and that the Cd fraction is independent of the doping concentration.

Epitaxial thin films of the dilute magnetic semiconductor Hg1-xMnxTe have been grown by MOVPE on (100) GaAs substrate with or without buffer layers. Deposition took place in a horizontal, atmospheric pressure reactor at temperatures in the range 350–400° C, using tricarbonyl (methyl cyclopentadienyl) manganese, di-isopropyl tellurium and elemental mercury. Buffer layers consisted of thin layers of ZnTe and an upper thick layer (∼1μm) of CdTe. The good crystallinity was confirmed by RHEED and double crystal x-ray diffraction with rocking curve widths of 500–600 arc. sec. for non-buffered layers, and 315 arc. sec. for buffered layers. TEM investigations show that layers grown on buffered substrates had improved microstructure defect content. Electrical transport measurements revealed that as-grown layers were ptype with Hall mobilities in excess of 0.1 m2V−1s−1.

Most CdTe epilayers grown by OMVPE have been deposited on two low index orientations: {111}Te and {100}. In this study we have examined the OMVPE of CdTe as a function of deposition temperature on CdTe substrates having the four low index orientations ({111}Te, P111}Cd, {110}, and {100}) and on the {211}Te and {211}Cd orientations. Results are presented for deposition rate, surface morphology, and structural quality of the epilayers.

HgCdTe crystals have been grown from CdTe seeds by the travelling heater method (THM). Three different kinds of interfaces between the growing crystal and the seed were found: a sharp planar interface; a non-planar interface which is caused by meltback near the ampoule walls; a diffuse zone, where an interface cannot be discerned. A correlation was found between the initial growth parameters and the interface structure. Microprobe analysis revealed a boundary layer which, in the case of the planar interface, had an unusual shape. This layer had a thickness varying from 400 to 800 microns. It consisted mostly of HgTe, and contained holes as defects.

The newly developed Hot Wall Beam Epitaxy was used for the growth of (100)CdTe and (100)Cd1-yZnyTe layers (y ∼ 0.04) on 1 inch (100)GaAs wafers. The structural properties of the epilayers were investigated by means of high resblution x-ray rocking curve measurements. The results demonstrate the high crystalline perfection of the layers. The CdTe and Cd1-yZnyTe/GaAs wafers were used as substrates for the growth of Hg1-xCdx Te by close space vapor phase epitaxy. The layers show excellent homogeneity. Across a 1 inch wafer the composition x varies by ±0.0026, and the width of the rocking curve (FWHM) by ±10arcsec. Values of the FWHM as low as 50arcsec for Hg1-xCdxTe (x = 0.32) on Cd1-yZnyTe/GaAs were obtained. Linear arrays of planar photovoltaic detectors were fabricated. Arrays of 32 elements show high uniformity of the cutoff wavelength and the responsivity. The variation of the cutoff wavelength is ±1.5% (±0.04μm) and the variation of the responsivity is ±3%.

Intrinsic carrier concentrations of narrow-gap Hg1-xCdxTe alloys (0.17≦ x ≦0.30) have been calculated as a function of temperature between 0 and 300 K by using the new nonlinear temperature dependence of the energy gap obtained previously by twophoton magneto-absorption measurements for samples with 0.24≦ x ≦0.26. We report here experimental values for Eg (x,T) for samples with x = 0.20 and 0.23 obtained by one-photon magneto-absorption measurements. These data confirm the validity of the new Eg (x,T) relationship for these : values. In this range of composition and temperature, the energy gap of mercury cadmium telluride is small, and very accurate values are needed for the gap to obtain reliable values for the intrinsic carrier density. Large percentage differences exist between our new calculations and previous values for ni at low temperatures. Even at 77 K, differences approaching 10 percent exist, confirming the importance of using the new ni results for materials and device characterization and a proper understanding of device operation in long-wavelength materials.

Transmission electron microscopy has been used to assess the defect contents of the various layers and interfaces in (CdHg) Te heterostructures. Examination of cross sectional specimens of these materials suggests that the density of misfit dislocations at the interfaces is related to the layer thicknesses, and that the high density of dislocations which are generated at the GaAs/CdTe interface are effectively prevented from penetrating into the CdHgTe epilayer by a 3um thick buffer layer. The majority of the dislocations in the layers were found to have a Burgers vector b = a/2<110> and either lie approximately parallel or inclined at an angle of ∼ 60° to the interfacial plane.

The performance of HgCdTe photodiodes, realized for the purpose of thermal imaging in the 8–12μm atmospheric window, is often limited by traps and by the tunneling mechanism associated with them. This mechanism dominates the leakage current and the dynamic resistance in diodes operated in reverse bias above 1OOmV and at temperatures below 77K. In usual working conditions, namely low reverse bias and a temperature of 77K, the presence of traps and the tunneling mechanism associated with them, degrades the performance of the diodes. Although the diffusion mechanism dominating the leakage current and the dynamic resistance in diodes operated under the above conditions is of major concern, the presence of the traps should not be ignored. The traps result in shorter electron lifetime in the bulk as well as in the generation of 1/f noise, which degrades the device's figures of merit such as NETD. Therefore, a thorough understanding of the trapassisted tunneling mechanism is a prerequisite to studying its impact on the device performance.

In this paper we present a model which describes the connection between the leakage current associated with the traps and the trap characteristics: concentration, energy level and capture cross-section. The validity of the model is confirmed by the good fit between measured and calculated characteristics of the diodes.

Hg1-xCdxTe layers, grown by the organometallic vapor phase epitaxy (OMVPE), are p-type with carrier concentrations around 4 × 1016/cm3 due to the Group II vacancies in them. Following a Hg saturated anneal at 220°C, these layers become n-type with carrier concentrations around 4 × 1014/cm3. In order to fabricate p-n junction diodes, Hg1-xCdxTe layers were grown with a 0.5–0.8 μm thick CdTe cap. By opening windows in this CdTe cap, the underlying Hg1-xCdxTe Te layer was annealed in a selective manner, thus forming planar p-n junctions. The CdTe cap, which is used as the diffusion barrier for Hg during the selective anneal, also served as the junction passivant for the photodiodes. Details of device fabrication and characterization are presented in this paper.

This paper reports recent results on two-layer P-on-n LPE HgCdTe heterojunction photodiodes with cutoff wavelengths beyond 19μm. These results demonstrate the potential of photovoltaic HgCdTe detectors to satisfy the detector requirements of advanced NASA satellite instruments out to wavelengths of 17μm.

Arrays of miniature focusing optics located at the focal plane can improve the performance of focal plane systems. By more completely collecting the light at the focal plane and concentrating it into a smaller spot size on the detector plane, the photodetector area can be substantially reduced. Increased gamma radiation hardening and noise reduction result from the decrease in photodetector surface area. Binary optics technology, a process for fabricating large arrays of diffractive optical elements, is especially attractive for infrared materials. In this paper, diffractive Fresnel microlens arrays containing over six thousand F/0.9 lenslets are patterned in the surface of CdTe substrates by successive photolithographic and Ar+ ion-beam-etching steps. Results on smaller arrays of monolithically integrated binary-optics lenslets with II-VI detectors, demonstrating enhanced photodetector responsivities, are presented for the first time.

We have grown HgCdTe/CdTe multiple quantum wells by molecular beam epitaxy which show room temperature photoluminescence and sharp absorption steps at mid-infrared wavelengths. Quantitative chemical mapping, performed by transmission electron microscopy, indicates minimal interdiffusion during growth. Annealing experiments performed at higher temperatures show that the interdiffusion coefficient is a strong function of the depth of the interface below the surface. Absorption spectra have been accurately modeled with a square well/envelope function approach. The films have been used to passively mode lock color center lasers and produce pulses as short as 120 fsec near 2.7 μm.

An extremely simple and inexpensive technique (AMEBA) for the thermal treatment of Hg containing specimens which permits short or long time annealing in a Hg rich atmosphere is described. It is based on the immersion of the sample, properly protected by proximity caps in, or above, a hot mercury bath. The sample assembly is such that it permits Hg vapors, but not the liquid, to reach the specimen's surface.

The usefulness of AMEBA in improving the electrical and structural properties of as-grown Hg1-xCdxTe (x = 0.21) and in removing ion implantation related damage as well as electrically activating B implants in various p-type HgCdTe samples is demonstrated. All the data presented show that AMEBA treatment yields results which are comparable or superior to those obtainable by convensional annealing methods.

The interface between Hg1-xCdxTe(0 ≦ x ≦ 1) and Hg1-yCdyTe(0 ≦ y ≦ 1) epitaxial layers of different composition (x ≠ y) is unstable with regard to the intermixing of the Hg and Cd cations within the Group II sublattice. This phenomenon may give rise to long-term stability problems in HgTe-(Hg,Cd)Te superlattices and composition grading between (Hg,Cd)Te absorber layers and CdTe buffer or passivation layers in epitaxial infra red detectors. In this paper, a novel approach to the inhibition of interdiffusion in these systems is discussed. This involves the growth of an intervening ZnTe barrier layer at the heterointerface between two (Hg,Cd)Te layers. Initial results are presented which indicate the effectiveness of this technique in reducing interdiffusion in an experimental heterostructure grown by MOVPE. Some possible applications in a variety of HgTe-based long wavelength devices are discussed.

The structural properties of LPE-grown HgCdTe on heteroepitaxial MOCVD-grown CdZnTe/GaAs/Si substrates were evaluated using high-resolution x-ray diffraction techniques and TEM. Large tilts {up to 4°} between CdZnTe layers and GaAs/Si substrates are a general characteristic of this heteroepitaxial system and are are attributed to the interaction of closely spaced misfit dislocations that arrange to form a tilt boundary. Either {112}CdTe or {552}CdTe can be grown on {112}GaAs/Si; the {552} was shown to result from a first-order twinning operation of {112}. Lamnella {111} microtwins in {111}CdZnTe/{100}GaAs/Si substrates, measured by x-ray techniques, are not readily propagated into the LPE-grown HgCdTe layer. The x-ray FWHM of the LPE HgCdTe is typically at least a factor of two lower than that of the Si-based substrate from annealing and due to the increased thickness of the layer; both mechanisms promote dislocation interaction and annihilation. High performance MWIR and LWIR HgCdTe 128×128 hybrid focal plane arrays were fabricated on these Si-based substrates. An array average of ROAj = 17.8 ohmcm2 for a cutoff wavelength of 10.8 μm at 78K was demonstrated.

We Present the new class of infrared Photodetectors based on the lead-tin tellurides doped with group III impurities. The Persistent photoconductivity effect appearing in these materials provides the opportunity of internal signal integration resulting in the considerable increase in signalto- noise ratio. The integration characteristic time may be changed by means of the operating temperature or alloy composition variation. Even if the integration time is higher than the operation time required there exists an opportunity to quench quickly (∼10−5s) the Persistent photoconductivity. In some regime of quenching the effect of giant quantum efficiency stimulation has been observed. Both the bulk crystal and thin film technologies of the Photodetector Production are developed.