Band anticrossing in highly mismatched group II-VI semiconductor alloys

We have successfully synthesized highly mismatched Cd_1−yMn_yO_xTe_1−x alloys by high-dose implantation of O ions into Cd_1−yMn_yTe crystals. In crystals with y>0.02, incorporation of O causes a large decrease in the bandgap. The bandgap reduction increases with y; the largest value observed is 190 meV in O⁺-implanted Cd_0.38Mn_0.62Te. The results are consistent with the band anticrossing (BAC) model, which predicts that a repulsive interaction between localized states of O located above the conduction-band edge and the extended states of the conduction band causes the bandgap reduction. A best fit of the measured bandgap energies of the O-ion-synthesized Cd_1−yMn_yO_xTe_1−x alloys using the BAC model for y<0.55 suggests an activation efficiency of only ∼5% for implanted O in Cd_1−yMn_yTe.     

Magneto-optical Studies of Spin Phenomena in CdMnTe Doped with Co and Cr

We investigate the magneto-optical and magnetic properties of two quaternary diluted magnetic semiconductor alloys, Cd_1−x−y Mn _x Cr _y Te and Cd_1−x−y Mn _x Co _y Te, with fixed Mn concentration x ∼ 0.37 and, respectively, with concentrations of Cr in the range 0 < y < 0.07 and Co in the range␣0 < y < 0.009. The introduction of Cr and Co leads to very different behaviors, including the occurrence of ferromagnetic order in the case of Cd_1−x−y Mn _x Cr _y Te. We discuss the possible origins leading to the observed behaviors.     

Vanadium-Doped Cadmium Manganese Telluride (Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te) Crystals as X- and Gamma-Ray Detectors

CdMnTe offers several potential advantages over CdZnTe as a room- temperature gamma-ray detector, but many drawbacks in its growth process impede the production of large, defect-free single crystals with high electrical resistivity and high electron lifetimes. Here, we report our findings of the defects in several vanadium-doped as-grown as well as annealed Cd_1−x Mn _x Te crystals, using etch pit techniques. We carefully selected single crystals from the raw wafer to fabricate and test as a gamma-ray detector. We describe the quality of the processed Cd_1−x Mn _x Te surfaces, and compare them with similarly treated CdZnTe crystals. We discuss the characterization experiments aimed at clarifying the electrical properties of fabricated detectors, and evaluate their performance as gamma-ray spectrometers.     

Evaluation of Mn Uniformity in CdMnTe Crystal Grown by the Vertical Bridgman Method

Cd_1−x Mn _x Te is a typical diluted magnetic semiconductor, as well as substrate for the epitaxial growth of Hg_1−x Cd _x Te. In this paper, the homogeneity of a Cd_1−x Mn _x Te (x = 0.2) single-crystal ingot grown by the vertical Bridgman method was studied. The crystal structure and quality of the as-grown ingot were evaluated. Near-infrared (NIR) transmission spectroscopy was adopted to develop a simple optical determination of the Mn concentration in the as-grown ingot. A correlation equation between cut-off wavelength λ _co from NIR transmission spectra and Mn concentration by inductively coupled plasma atomic emission spectrometry (ICP-AES) was established. Using this equation, we investigated the Mn concentration distribution in both the axial and radial directions of the ingot. It was found that the segregation coefficient of Mn in the axial direction of the ingot was 0.95, which is close to unity. The Mn concentration variation in the wafers from the middle part of the ingot was 0.001 mole fraction. All these results proved that homogeneous Cd_0.8Mn_0.2Te crystals can be grown from the vertical Bridgman method.     

Transport phenomena in n-MnxHg1−x Te/Cd0.96Zn0.04Te epitaxial films

The results of an experimental study of samples of Mn_xHg_1−x Te films grown by liquid-phase epitaxy on a Cd_0.96Zn_0.04Te substrate are presented. It shows that, as a result of the diffusion of cadmium from the substrate, a Cd_xMn_yHg_1−x−y Te film with a variable band-gap layer is formed close to the 〈epitaxial-film〉-substrate interface. The appearance of this variable band gap is revealed by the transport phenomena. The temperature dependence of the band gap E _g (T) is determined in a linear approximation on T from the results of a theoretical analysis of the temperature dependences of the free-carrier concentration and mobility. It is shown that averaging the semiempirical dependences for the ternary compounds with the extreme compositions, using the virtual-crystal approximation, can produce large errors when determining E _g (T) in a specific semiconductor. Fiz. Tekh. Poluprovodn. 31, 268–272 (March 1997)     

Relaxation processes in conductivity of Cd1 − x Mn x Te crystals (0.02 < x < 0.55)

The results of studying the temperature dependences of resistivity and the Hall coefficient in crystals of Cd_{1 ? x} Mn _x Te alloys (0.02 < x < 0.55) are used to gain insight into the conductivity relaxation processes observed in the temperature range 200–420 K in Cd_{1 ? x} Mn _x Te crystals with the manganese content x > 0.06. An anomalous isothermal variation in the hole concentration by an amount from half to three orders of magnitude occurs in these crystals. It is shown that relaxation of resistivity in the crystals under consideration can be provided by quasi-chemical reactions with involvement of uncontrolled Cu impurity and defects of CdTe crystal structure.     

HgCdZnTe quaternary materials for lattice-matched two-color detectors

As the number of bands and the complexity of HgCdTe multicolor structures increases, it is desirable to minimize the lattice mismatch at growth interfaces within the device structure in order to reduce or eliminate mismatch dislocations at these interfaces and potential threading dislocations that can degrade device performance. To achieve this we are investigating the use of Hg_1−x−yCd_xZn_yTe quaternary alloys which have an independently tunable lattice constant and bandgap. Lattice matching in Hg_1−x−yCd_xZn_yTe structures can be achieved using small additions of Zn (y<0.015) to HgCdTe ternary alloys. We have investigated some of the basic properties of Hg_1−x−yCd_xZn_yTe materials with x≈0.31 and 0≤y≤0.015. The quaternary layers were grown on (112)CdZnTe substrates using MBE and the amount of Zn in the layers was determined from calibrated SIMS measurements. As expected, the lattice constant decreased and the bandgap increased as Zn was added to HgCdTe to form Hg_1−x−yCd_xZn_yTe. Hall-effect results for both n-type (In) and p-type (As) Hg_1−x−yCd_xZn_yTe layers were very similar to HgCdTe control samples. We have also utilized x-ray rocking curve measurements with (246) asymmetric reflections as a novel sensitive technique to determine the correct amount of Zn needed to achieve lattice matching at an interface. MWIR/LWIR n-p-n two-color triple-layer heterojunction structures were grown to evaluate the effects of minimizing the lattice mismatch between the widest bandgap p-type collector layer, using Hg_1−x−yCd_xZn_yTe, and the HgCdTe MWIR and LWIR collector layers and compared to structures that did not incorporate the quaternary. Sequential mode two-color detectors were fabricated using a 256 × 256, 30 μm unit cell design. There were several interesting findings. Macro defects predominantly affected the LWIR band (Band 2) operability and had little effect on the MWIR band (Band 1). The incorporation of Hg_1−x−yCd_xZn_yTe p-type collector layers had little effect on MWIR detector performance, but overall the LWIR performance was generally better. These initial detector results indicate that the use of Hg_1−x−yCd_xZn_yTe alloys in multicolor detector structures are potentially promising and should be pursued further.     

A study of polycrystalline Cd(Zn,Mn)Te/Cds films and interfaces

Polycrystalline films of Cd_1-x Zn _x Te (x = 0–0.4) and Cd_1-x Mn _x Te (x = 0–0.25) were grown by MBE and MOCVD, respectively, on CdS/SnO₂/glass substrates to investigate their feasibility for solar cell applications. The compositional uniformity and interface quality of the films were analyzed by x-ray diffraction, surface photovoltage, and Auger depth profile measurements to establish a correlation between growth conditions and lattice constant, atomic concentration, and bandgap of the ternary films. MBE-grown polycrystalline Cd_1-x Zn _x Te films showed a linear dependence between Zn/(Cd + Zn) beam flux ratio, Zn concentration in the film, and the bandgap. Polycrystalline Cd_1-x Zn _x Te films grown at 300° C showed good compositional uniformity in contrast to compositionally non-uniform Cd_1-x Mn _x Te films grown by MOCVD in the temperature range of 420–450° C. The MBE-grown Cd_1-x Zn _x Te interface also showed significantly less interdiffusion compared to the MOCVD-grown Cd_1-x Mn _x Te/CdS interface, where preferential exchange between Cd from the CdS layer and Mn from the Cd_1-x Mn _x Te film was observed. The compositional uniformity of MOCVD-grown polycrystalline Cd_1-x Mn _x Te films grown on CdS/SnO₂/glass substrates was found to be a strong function of the growth conditions as well as the Mn source.     

Chemical–Mechanical Polishing of CdTe and ZnxCd1?xTe Single Crystals by H2O2(HNO3)–HBr–Organic Solvent Etchant Compositions

Chemical–mechanical polishing of CdTe and Zn _x Cd_1−x Te single-crystal surfaces by bromine-evolving compositions based on aqueous solutions of H₂O₂(HNO₃)–HBr–solvent has been investigated. The dependences of the chemical–mechanical polishing rate on the dilution of the base polishing etchant for various organic components have been determined. The surface condition after such polishing has been investigated using profilometry. The polishing etchant compositions for CdTe and Zn _x Cd_1−x Te single-crystal surfaces and the chemical polishing conditions have been optimized.     

Vapor phase equilibria in the Cd1−xZnxTe alloy system

Cd_1−xZn_xTe compounds of different compositions have been prepared at temperatures ranging from 400 to 1000°C by annealing elemental Te in sealed quartz ampoules, in an atmosphere comprising vapors of Cd and Zn whose partial pressures were varied by varying the composition of the binary Cd_1−yZn_y alloys which provided the Cd and Zn vapors in these annealing experiments. The chemical compositions of the resulting Cd_1−xZn_xTe compounds have been analyzed using electron probe microanalytical techniques. Results indicate that presence of a 0.5%Zn along with Cd in a closed or semi-closed system may prove to be beneficial in preventing decomposition and/or formation of a metal/non metal phase during annealing of Cd_0.96Zn_0.04 Te substrates. Using the thermodynamic data in the literature for the binary Cd_1−yZn_y alloys and with the assumption that the activities of the Cd and Zn components are weakly dependent on temperature, the partial pressures of Cd and Zn in equilibrium with the Cd_1−xZn_xTe compounds at various temperatures have been evaluated.     

Effect of successive implantation of Ag+(Cu+) and Xe+ ions on the recombination properties of CdxHg1−x Te crystals

The effect of successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions on the recombination properties of Cd_xHg_1−x Te (0.2<x<0.3) crystals has been investigated. It is shown that after implantation of ions of one chemical element, followed by diffusion thermal annealing at temperatures below 150–200 K, recombination through local levels lying 30±5 meV below the conduction band bottom dominates. Successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions followed by diffusion thermal annealing changes the course of the temperature dependence of the lifetime of the nonequilibrium charge carriers. It was determined that for Cd_xHg_1−x Te crystals with x⋍0.20–0.25 in the temperature interval 700–200 K the lifetime of the nonequilibrium charge carriers is low (τ<0.15 μs) and does not depend on the temperature. For Cd_xHg_1−x Te crystals with x⋍0.3 recombination of nonequilibrium charge carriers occurs through two types of levels: in the temperature range 140–200 K — deep levels E _t1⋍E _c −51 meV and at lower temperatures (77–140 K) — through shallower levels E _t2⋍E _c −(16±2) meV. Fiz. Tekh. Poluprovodn. 31, 786–789 (July 1997)     

Dispersion of the refractive index of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 1) alloy layers below the absorption edge

The transmittance spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤x ≤ 0.1) alloy layers are exploited to study the dispersion of their refractive index in the spectral range from 650 to 8000 cm⁻¹ (below the absorption edge). The refractive index and the position of the absorption edge as functions of the composition parameter of the alloys are determined at two temperatures, 80 and 295 K. The refractive index is calculated in the context of the classic wave concepts of propagation of electromagnetic radiation. The experimentally determined dispersion dependences are described by the empiric Sellmeier expression of the second order. From analysis of the transmittance of the layers, it follows that the band gap of the epitaxial Pb_{1 − x} Eu _x Te alloys increases with increasing temperature at x < 0.5 and decreases at x > 0.5.     

Effect of laser irradiation on the photoconductivity and noise in n-Cdx Hg1−x Te single crystals

The effect of nanosecond laser radiation on the photoconductivity and 1/f noise in Cd_xHg_1−x Te crystals has been investigated. It is shown that laser irradiation decreases the photosensitivity of the samples and produces a short-wavelength shift of the maximum and the long-wavelength edge of the photoconductivity spectrum. The intensification of 1/f noise and the increase in its frequency are due to a laser-induced increase in the defect density in the material. Fiz. Tekh. Poluprovodn. 31, 820–822 (July 1997)     

Dependence of the properties of Cd1−x ZnxTe crystals on the type of intrinsic point defect formed by oxygen

Coordinated investigations of cathodoluminescence spectra, microstructure, specific resistance and presence of oxygen in Cd_1−x Zn_xTe crystals are used to identify how the electrical properties and degree of perfection of the crystal lattice of these materials are affected by the form in which oxygen is present as an intrinsic point defect. It is found that oxygen, which is the main background impurity in Cd_1−x Zn_xTe, forms different types of point defects at different positions in the lattice, depending on the ratio [Cd]/[Zn]. The optimum composition for making detectors of ionizing radiation, for which the crystal resistance is highest, is Cd_0.77Zn_0.23Te. Fiz. Tekh. Poluprovodn. 33, 569–573 (May 1999)     

Temperature dependence of the optical properties of Hg1−xCdxTe

The alloy composition of Hg_1−xCd_xTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg_1−xCd_xTe by spectroscopic ellipsometry (SE). The spectral data of Hg_1−xCd_xTe with x = 1,0.235, and 0.344 were obtained from room temperature to 800Kin the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E₀, E₀+Δ₀, E₁, E₁+Δ₁, E₂(X), and E₂(Σ). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.     

Electronic structure, absorption coefficient, and auger rate in HgCdTe and thallium-based alloys

The band structures, absorption coefficients, and Auger recombination rates in narrow-gap alloys HgCdTe, InTIP, InTlAs, and InTlSb in the zinc blende structure, along with those of GaAs, are calculated using a hybrid pseudopotential and tight-binding method. The composition-dependent band gaps of the thallium-based alloys are reported along with those of several other semiconductor alloys. Within 50 meV from the absorption edge, the absorption coefficient of In_xTl_1−xP is found to have about the same magnitude as that of Hg_xCd_1−xTe and GaAs, while that of In_xTl_1−xAs and In_xTl_1−xSb is much smaller. In agreement with previous theories, the calculated Auger lifetimes in Hg_0.78Cd_0.22Te with unit or k • p overlap agree very well with experiments. Among the thallium alloys studied, the Auger lifetimes are longest in In_0.33Tl_0.67P, but still shorter than those in Hg_0.78Cd_0.22Te by an order of magnitude. In addition, realistic overlaps produce lifetimes one to two orders of magnitude larger than those observed.     

Photoconductivity spectral characteristics of semiconductors with exponential fundamental absorption edge

An analysis has been performed of photoconductivity spectral characteristics of semiconductors with an exponential fundamental absorption edge as functions of the surface recombination rate and sample thickness. It is shown, in particular, that in crystals of Cd_xHg_1−x Te (x≈0.2) the spectral position of the photoconductivity maximum over a wide range of values of these parameters can be used, with an error not exceeding 1%, to determine the effective band gap and, consequently, the composition of the material. Fiz. Tekh. Poluprovodn. 33, 1295–1299 (November 1999)     

Thermoelectric Characteristics in MBE-Grown HgCdTe-Based Superlattices

We present a study on the thermoelectric properties of n-type Hg_0.75Cd_0.25 Te/Hg_0.7Cd_0.3Te superlattices (SLs). This material system was chosen because HgCdTe is the primary material used in high-performance infrared imaging applications. HgCdTe-based devices can be directly grown on Hg_1−x Cd _x Te/Hg_1−y Cd _y Te SL coolers using advanced growth methods such as molecular-beam epitaxy (MBE), making the monolithic integration of infrared sensors and thermoelectric elements possible. Also, the thermoelectric figure of merit ZT for Hg_0.75Cd_0.25Te/Hg_0.7Cd_0.3Te SLs is predicted to reach values of 2.09, more than two times greater than that achieved in the best thermoelectric devices based on bulk Bi₂Te₃. This large ZT is due to the unique and superior electrical and thermal properties of the HgCdTe system, which has not yet been experimentally explored in any great depth as a thermoelectric material. We used a Riber 32P MBE system equipped with a Hg valved cell, reflection high-energy electron diffraction, infrared pyrometer and in situ spectroscopic ellipsometry to grow the thermoelectric structures. MBE was chosen as a growth technique since it allows for the lowest growth temperature compared with other methods, which limits interdiffusion at the interfaces, thereby allowing for a precise control over electrical and thermal properties. Thermal devices were fabricated using standard photolithography and etching techniques. Thermal properties were evaluated using a differential technique. A thermal conductivity of 0.82 ± 0.07 W/m K and a Seebeck coefficient of 811 ± 150 μV/K were measured. Using a measured value of 0.017 Ω cm for the resistivity, an upper bound ZT of 1.4 is estimated. An erratum to this article can be found at     

Multi-Mode behavior of optical phonons in II-VI ternary and quaternary alloys

Zone-center optical phonons of tetrahedrally coordinatedII-VI semiconductor ternary (AB_1-xC_x) and quaternary (AB_1-x-yC_xD_y) alloys display a variety of distinct multi-mode behavior patterns. A brief review of the modified random element isodisplacement model, which can satisfactorily account for the composition dependence of the frequencies of the zone-center optical phonons, is presented; in this model, one uses the macroscopic parameters of the binary end members and their LO-TO phonon frequencies. When a constituent is present in extreme dilution in a binary (or in a ternary), e.g., C in AB (or in AB_1-y D_y), one sees either a local or a gap mode associated with it. For higher concentrations, one can see two (three) LO-TO phonon pairs for the ternary (quaternary) in the first order Raman spectrum or in the infra-red deduced from an analysis of the reststrahlen bands, when the masses of B and C differ significantly but are lighter than that of A. We illustrate these features in the Raman spectra of Cd_1-xMg_xTe and Cd_1-x-y Mg_xMn_yTe. Also shown are the infra-red spectra of the local mode of Mg²⁺ and Mn²⁺ in Cd Te, where their isotopic nature is clearly manifested.     

Defect reduction in Hg1−xCdxTe grown by molecular beam epitaxy on Cd0.96Zn0.04Te(211)B

A study on preparation of Cd_0.96Zn_0.04Te(211)B substrates for growth of Hg_1−xCd_xTe epitaxial layers by molecular beam epitaxy (MBE) was investigated. The objective was to investigate the impact of starting substrate surface quality on surface defects such as voids and hillocks commonly observed on MBE Hg_1−xCd_xTe layers. The results of this study indicate that, when the Cd_0.96Zn_0.04Te(211)B substrates are properly prepared, surface defects on the resulting MBE Hg_1−xCd_xTe films are reduced to minimum (size, ∼0.1 m and density ∼500/cm²) so that these MBE Hg_1−xCd_x Te films have surface quality as good as that of liquid phase epitaxial (LPE) Hg_1−xCd_xTe films currently in production in this laboratory.