Synchrotron radiation photoemission studies of ultra-thin Yb diffusion barriers at the interface between Mercury-Cadmium-Telluride semiconductors and Ag overlayers show that the interlayers act as effective diffusion barrier only after thicknesses of 10-15 Å are reached. Studies of interlayer morphology by means of photoemission from physisorbed Xe indicate that effective diffusion barriers are consistent with a model in which a continous Yb-Te reacted layer is covered by an Yb-rich layer with high alloying enthalpy for Hg.

Growth of long wavelength infra-red mercury cadmium telluride films by liquid phase epitaxy has usually yielded films of inferior electrical properties as evidenced by Hall mobilities lower than theoretical values by factors of 5 or more at 77°K. In addition the Hall behavior over the entire temperature range did not follow classical patterns. A systematic series of investigations was conducted to improve the electrical performance of these films by four methods: i) growth of HgCdZnTe films, where a portion of the cadmium was replaced by zinc, ii) growth at temperatures > 550°C, iii) growth at a slow rate on a CdZnTe substrate following ‘Cleaning’ in the melt of the substrate surface and iv) doping the film by controlled levels of indium. The first method did not lead to any improvements in the Hall behavior. In addition, the films grown displayed varying dislocation densities. The second method led to a small but definite increase in the yield of non-anomalous or classical films (5%). The third method yielded films with classical or non-anomalous Hall behavior about 20% of the time. The last method consistently led to films with classical Hall behavior. This was accomplished with indium doping levels at 1.5× 1014/cm3. Preliminary data indicate that it will be possible to go to still lower doping levels while maintaining classical Hall behavior. The improvement in electrical properties of these epifilms can be attributed to the reduction or elimination of type inhomogeneities known to degrade Hall mobilities in films grown by liquid phase epitaxy.

A comprehensive study of all materials parameters influencing metal-insulator-semiconductor (MIS) properties of n-type HgCdTe films grown by liquid phase epitaxy from tellurium rich melts was conducted. When the epitaxy process was optimized to grow films free of inclusions and terracing, the first indications of the MIS properties to be expected could be obtained from the temperature dependence of the Hall electron mobility.

Films displaying an anomalous dependence of the Hall mobility on temperature yielded non-classical low frequency MIS properties with little or no measurable ‘dark’ storage times (< 2×10−6 second). The MIS performance of these films appeared relatively independent of other materials parameters; remaining, for example, virtually unaffected by the film dislocation density changing between 5×104 and 2 × 106/cm2.

Films displaying a classical dependence of the Hall mobility on temperature yielded drastically improved classical high frequency MIS properties. For these films, the MIS performance appeared almost exclusively dependent on dislocation density levels as long as the donor density remained lower than 1×1015/cm3. The dark storage time of classical films increased continuously with decreasing dislocation density levels, rising to 100µsec for a dislocation density level of 1 × 105/cm2 for materials with a 77°K cutoff wavelength of 10.5 µm. A simple monotonic relationship could be established between the MIS performance parameters and the dislocation density over the entire measurement span: from 5×104 to 2.5×106/cm2.

Low temperature (<60° C) processing technologies that avoid potentially damaging processing steps have been developed for devices fabricated from II-VI semiconductor epitaxial layers grown by photoassisted molecular beam epitaxy (MBE). These low temperature technologies include: 1) photolithography (1 µm geometries), 2) calibrated etchants (rates as low as 30 Å/s), 3) a metallization lift-off process employing a photoresist profiler, 4) an interlevel metal dielectric, and 5) an insulator technology for metal-insulator-semiconductor (MIS) structures. A number of first demonstration devices including field-effect transistors and p-n junctions have been fabricated from II-VI epitaxial layers grown by photoassisted MBE and processed using the technology described here. In this paper, two advanced device structures, processed at <60° C, will be presented: 1) CdTe:As-CdTe:In p-n junction detectors, grown in situ by photoassisted MBE, and 2) HgCdTe-HgTe-CdZnTe quantum-well modulation-doped field-effect transistors (MODFETs).

The last two years has seen rapid development in the growth of mercury cadmium telluride material for use in far infrared detectors. This paper will briefly review the progress before this period, and will focus on recent developments in these materials.

The emphasis will be on the direct alloy growth of HgCdTe material by organometallic vapor phase epitaxy (OMVPE).

Before an organometallic compound can be used as an MOVPE precursor, certain basic properties need to be known and one of the most important of these is vapour pressure. A technique for the measurement of vapour pressures using an MOVPE system designed for precursor assessment is described. Results on the assessment of di-is-propyl-telluride, di-n-propyl-telluride, di-allyl-telluride, methyl-allyl-telluride and di-t-butyl-telluride are presented.

The structural and electrical properties of heteroepitaxial HgCdTe/CdZnTe/GaAs/Si were evaluated using high-resolution x-ray diffraction techniques and Hall-effect measurements as a function of temperature. Significant tilting of the layers was found for both {100} and {111} CdZnTe layers grown on misoriented {100}GaAs/Si substrates, consistent with the interpretation of a low-angle tilt boundary being formed at the interface to relieve the large lattice mismatch between the layers. The GaAs layer is in a state of biaxial tension before and after the growth of the CdZnTe layers. The x-ray FWHM of HgCdTe layers grown by LPE on these substrates was found to be reduced from that of the MOCVD-grown CdZnTe buffer layer due to both an annealing effect during LPE growth and to the increased distance of layer surface from the defective CdZnTe/GaAs interface. Hall-effect mobility for {100}HgCdTe layers was nearly identical to that of layers grown on bulk CdZnTe substrates. High-quality heterojunction infrared detectors have been fabricated using these materials.

HgTe:ZnTe superlattices have been grown by thermal MOVPE at temperatures down to 325°C. At this temperature, interdiffusion is sufficiently low to make superlattice periods as low as 45A practicable. The results of theoretical calculations of the electronic structure of these materials are also reported. These show that the electronic structure may be significantly different to that found for the HgTe:CdTe system due to the large biaxial compression present in the HgTe well layers.

A series of modulation-doped small-band-gap HgTe-Hg1−xCdxTe superlattices (SLs) have been grown by photoassisted MBE. N-type or p-type conductivity was obtained in the modulation-doped samples by incorporating either In or As dopant atoms in the barrier layers, respectively. The electrical and optical properties that these new multilayered quantum well structures display will be discussed.

In this paper, we studied the magneto-field emission from HgTe/Cdre superlattices with graded structure and compared the same with the bulk specimens of the constituent materials. It is found that the field emitted current density increases both with electron concentration and magnetic field in an oscillatory manner. The field emission in HgTe/CdTe SLi is greater than that of the same from constituent materials the theoretical results are in agreement with the experimental observation as reported elsewhere.

We have grown high-mobility LWIR HgCdTe thin films on CdTe substrates, using molecular beam epitaxy (MBE). The structural, optical, and electrical properties of these epilayers were determined by SEM, DCRC, FTIR, and Hall effect measurements. For films of 10 to 11 µm thick and composition X value ranging from 0.152 to 0.172, the highest mobility observed was 7.5 × 105 cm2 /V-sec, and the FWHMs of the rocking curves were 75 to 110 arcsec. We also have carried out the temperature-dependent EXAFS study of HgCdTe.

Free-carrier induced third-order optical nonlinearities can be both large and fast in narrow-gap semiconductors. We have studied a variety of mechanisms in bulk semiconductors and heterostructures using CO2 lasers and found third-order susceptibilities as large as 2×10−3 esu with picosecond relaxation times. These mechanisms saturate at much higher intensities than do slower mechanisms and hence induce huge modulations of the dielectric function. In addition, most of these processes are nonresonant so they do not require the exact matching of material parameters and are relatively insensitive to temperature.

We report an experimental study of the nonlinear optical properties of HgTe-CdTe superlattices grown by MBE and Pb1−x,SnxSe grown by hot wall epitaxy. Nondegenerate four-wave mixing has been employed to measure third-order nonlinear susceptibilities at 10.6 µm as a function of temperature, laser intensity, and difference frequency. The nonlinearity is believed to be due to modulation of the free carrier temperature and density by the optical beams. The HgTe-CdTe results are compared to theoretical calculations based on free carrier contributions to the susceptibility, and the agreement with experiment is quite good. In 1-µm-thick Pb1−xSnxSe layers, multiple internal reflections of the light within the sample is found to enhance the nonlinear signal by an order of magnitude.

The present understanding of the carrier transport properties in Hg-based super-lattices such as the HgTe-CdTe system is reviewed. Novel features in the calculated superlattice band structures and their implications for macroscopic transport properties are discussed. Nearly all of the main experimental results are qualitatively consistent with a large valence band offset, but are difficult to explain if the offset is small. The valence band offset controversy thus appears to have been largely resolved.

We report a detailed magneto-transport investigation of HgTe-CdTe multiple quantum wells. The data yield the first experimental confirmation of a theoretically-predicted “negative-gap” semiconducting region in wide-well Hg-based heterostructures. Gaps of up to 19 meV are observed.

The Quantum-Confined Stark Effect in II-VI semiconductor coupled quantum wells is studied theoretically. It is found that because of the difference in localization of the wavefunctions of the heavy hole and the electron subbands involved, large band gap shifts can be induced by an external electric field for quantum wells with zero-field band gaps in the spectrum region from 0.4 µm to 12 µm. Several potential device applications based on this effect are proposed.

Much of the work in diluted magnetic semiconductors (DMS) in past years has focused on the alloys such as Cd1−xMnxTe. Recently, there has been an increasing accent on DMS alloys containing the transition metal ions Fe2+ and Co2+. We review here the properties of these new materials and compare them with the alloys. We also examine the novel opportunities afforded by the molecular beam epitaxy of epilayers and heterostructures containing the “metastable” zinc-blende phase of Cd1−xMnxSe.

CdTe/Cd1−xMnx Te superlattices with well/barrier thicknesses ranging from 20Å to 300Å were examined via plan and cross section TEM in analytical instruments capable of EDX and CL. Misfit dislocations, inclined dislocations and stacking faults were observed with a net density of around 109cm−2.

It was observed that the dislocations originated near the substrate/epilayer interface and that stacking faults appeared preferrentially on the pair of {111}B (tellurium terminated) planes in the growth direction rather than on the {111}A planes.

It is suggested that this is due to a difference in mobility between a and P dislocations originating at or near the substrate.

A study of Ga and In outdiffusion into Cd1−xMnxTe (0≥ x ≥ .70) epitaxial layers grown on (111)GaAs and (001)lnSb was carried out. The layers were grown by pulsed laser evaporation and epitaxy on substrates held at temperatures below 310 °C. The structural quality of the layers was examined using x-ray diffraction and transmission electron microscopy. A tendency toward precipitation of Ga at the near surface region of (11 1)CdMnTe grown on (11 1)GaAs, usually less than 300 nm wide, has been observed with secondary ion mass spectroscopy. Similar results were observed for the migration behaviour of In in (001)CdTe grown on (001)InSb. The ion imaging revealed that both In and Ga accumulate near the surface at localized spots, up to about 10 pm in diameter. The concentration of the spots is in the range of 104 - 106 cm−2. The Ga- and In-rich channels sometimes extend over the whole sample. Annealing at temperatures as low as 400 °C for 2 h significantly increases the concentration of the Ga spots and the average concentration of Ga in the films to above 1016 cm−3. Low-temperature photoluminescence data obtained for annealed samples do not indicate any structural deterioration typical for heavily doped Bridgman grown samples. A sharp neutral-donor bound-exciton transition (D°,X) is observed for samples with 0 < x < .10.

We present a photoluminescence study of the excitons and electron-to-acceptor (e-°) transitions in Cd1−xMnxTe (x = 0.05 and 0.15) under hydrostatic pressure at 15K. We investigate the changing magnetic and Coulombic binding energies of the e-° transition under pressure. We find that the e-° binding energy increases with pressure for x = 0.15 where the magnetic term due to the acceptor bound magnetic polaron is large, while it decreases for x = 0.05. We also obtain the pressure coefficients of the excitonic and acceptor related transitions.