Study of lifetimes and photoconductivity relaxation in heterostructures with Hg x Cd1 ? x Te/Cd y Hg1 ? y Te quantum wells

Carrier lifetimes in the continuum of the quantum well of a Hg _x Cd_{1 ? x} Te/Cd _y Hg_{1 ? y} Te hetero-structure were studied by terahertz pump-probe spectroscopy. It is found that the relaxation duration of the transmission signal is ??65 ps and is independent of the pump power. Such rapid relaxation in these structures is most likely determined by the interaction of holes with acoustic phonons due to a high density of states in the valence band and a larger effective mass compared with electrons. By the obtained data, the times of the interband nonradiative recombination of holes are determined. In this publication, we report the results of numerical calculation of the energy spectrum of the model structure, in which the possibility of obtaining population inversion at specified concentrations of nonequilibrium carriers is analyzed.     

Photoluminescence spectra of Cd x Hg1 − x Te quantum-well heterostructures

Photoluminescence spectra are measured for a nanoscale Cd _x Hg_{1 ? x} Te heterostructure with a single quantum well about 12.5 nm thick in the case x = 0.24. The confined energy levels and the respective rates of different optical transitions are calculated on this basis. Major radiative processes are identified.     

Interaction Between AsHg and V Hg in Arsenic-Doped Hg1−x Cd x Te

Arsenic-related defect complexes have been proven responsible for the charge-compensation effects in arsenic-doped Hg_1?x Cd _x Te, but the underlying mechanism is still unclear. In this study, we systematically investigated the interaction between arsenic donor (As_Hg) and mercury vacancy (V _Hg) versus the As_Hg–V _Hg separation in arsenic-doped Hg_1?x Cd _x Te using first-principles calculations. A new long-range interaction between As_Hg and V _Hg is found, and the related binding energies and electronic structures are calculated to reveal its coupling mechanism. Our results show that V _Hg can increase the distortion of the lattice collaboratively with As_Hg due to the different characteristics of As_Hg and V _Hg in distorting the lattice. The relaxational enhancement as well as the electrical compensation of the As_Hg donor is weakened as V _Hg moves away from As_Hg, and the underlying mechanism is revealed. In addition, a set of defect levels in the band gap generated from the donor–acceptor interaction are also shown, and the origin of these levels is explored. The results of this work are important for theoretically explaining the characteristics of complicated defect levels found in experiments.     

Quantitative Auger Electron Spectroscopic Analysis of Hg1−x Cd x Te

Auger electron spectroscopy of Hg_1?x Cd _x Te has been investigated for quantitative analysis. HgCdTe is known to be a very sensitive material which easily suffers from ion beam and electron beam interactions. Two methods were compared for accurate and reproducible quantification of the chemical composition. The first strategy was to reduce as much as possible the damage caused by surface preparation and the effect of the incident electron beam on the material. Quantification was then achieved by use of relative sensitivity factors estimated by use of calibration reference samples. This method was rejected because of unavoidable beam damage which resulted in different chemical changes for the reference CdTe and the HgCdTe sample of interest. The second strategy was to precisely control the experimental conditions to ensure reproducible degradation. Quantification was achieved by analysis of reference HgCdTe samples with different Cd composition. Successful quantification was achieved on stoichiometric material of composition of 0.2 < X _Cd < 0.3 with a X _Cd discrimination limit of ΔX _Cd = 0.02 and an analysis step of 10 nm.     

Defects Study in Hg x Cd1−x Te Infrared Photodetectors by Deep Level Transient Spectroscopy

At high temperature, infra-red focal plane arrays are limited by their performance in operability, detectivity D ^* or noise equivalent temperature difference. Trap characterization and defect studies are necessary to better understand these limitations at high temperature. In this paper, we use deep level transient spectroscopy to study electrically active defects in mercury cadmium telluride n ⁺/p diodes. The material investigated has a cut-off frequency (λ _c) of 2.5 μm at 180 K and p doping performed with mercury vacancy. Trap energy signatures as well as capture cross-section measurements are detailed. A low temperature hole trap close to midgap is observed in the range 150–200 K with an activation energy around 0.18 ± 0.025 eV. A high temperature hole trap is also observed in the range 240–300 K with an activation energy of 0.68 ± 0.06 eV. A hole capture cross-section of 10^?19 cm² is obtained for both traps. The nature of the defects and their correlation with dark current are discussed.     

Variable-Field Hall Measurement and Transport in LW Single-Layer n-Type MBE Hg1−x Cd x Te

Molecular beam epitaxy n-type long-wavelength infrared (LWIR) Hg_1?x Cd _x Te (MCT) has been investigated using variable-field Hall measurement in the temperature range from 50 K to 293 K. A quantitative mobility spectrum analysis technique has been used to determine the role of multicarrier transport properties with respect to epilayer growth on lattice-matched cadmium zinc telluride, as well as lattice-mismatched silicon (Si) and gallium arsenide (GaAs) buffered substrates. Overall, after postgrowth annealing, all layers were found to possess three distinct electron species, which were postulated to originate from the bulk, transitional (or higher-x-value) regions, and an interfacial/surface layer carrier. Further, the mobility and concentration with respect to temperature were analyzed for all carriers, showing the expected mobility temperature dependence and intrinsic behavior of the bulk electron. Electrons from transitional regions were seen to match expected values based on the carrier concentration of the resolved peak. At high temperature, the lowest-mobility carrier was consistent with the properties of a surface carrier, while below 125 K it was postulated that interfacial-region electrons may influence peak values. After corrections for x-value and doping density at 77 K, bulk electron mobility in excess of 10⁵ cm² V^?1 s^?1 was observed in all epilayers, in line with expected values for lightly doped n-type LWIR material. Results indicate that fundamental conduction properties of electrons in MCT layers are unchanged by choice of substrate.     

History of the “Detector Materials Engineering” Crystal Growth Process for Bulk Hg1−x Cd x Te

This paper reviews the history and technology of a bulk Hg_1?x Cd _x Te crystal growth process that was developed in the early 1980s at Honeywell Electro-Optics Division (presently BAE Systems, Electronic Solutions). The crystal growth process name, DME, was an acronym for the department name: Detector Materials Engineering. This was an accelerated crucible rotation technique (ACRT) vertical traveling heater method growth process. Crystal growth occurred in the pseudobinary Hg_1?x Cd _x Te system. ACRT mixing allowed the lower-density, higher-x-value Hg_1?x Cd _x Te growth nutrient in the upper region of the ampoule to replenish the depleted melt and allowed the growth of constant-x-value, higher-density Hg_1?x Cd _x Te. The material grown by this research and production growth process yielded single crystals that had improved purity, compositional uniformity, precipitate density, and reproducibility in comparison with solid-state recrystallization and other bulk Hg_1?x Cd _x Te growth techniques. Radial and longitudinal nonuniformities in x-value for Hg_1?x Cd _x Te were reduced to <0.0008/cm. The net electrically active background impurities did not exceed 1 × 10¹⁴ cm^?3. Electron mobilities in excess of 1.5 × 10⁶ cm²/V-s were observed at 77 K. Structural defects of less than 10⁴ cm^?2 were measured. Te precipitates were not observed. As a result of these material improvements, long-wavelength infrared (LWIR) photoconductive devices fabricated from DME material had highly desired performance characteristics.     

Low-temperature anomalies of the photoelectromagnetic effect in p-Cd x Hg1 − x Te due to recharging of surface states

A number of anomalies inexplicable in the context of available theoretical models are observed in the photoelectromagnetic effect in p-Cd _x Hg_{1 ? x} Te crystals. A general model of the photoelectromagnetic effect in semiconductors with a large ratio between the mobilities of electrons and holes is developed, taking into account the influence of space charge. The model allows the interpretation of all experimentally observed anomalies of the photoelectromagnetic effect, including the double sign inversion in magnetic field. Comparison of the theory with the experiment makes it possible to determine the parameters of the material. It is suggested that the space charge responsible for the anomalies of the photoelectromagnetic effect is associated with a high concentration of specific surface states (～10¹³ cm^?3) recharged when trapping nonequilibrium charge carriers.     

Structure and properties of Cd x Hg1−x Te-metal contacts

A metal-semiconductor contact is a composite structure consisting of several nanodimensional layers. The contact properties depend strongly on the technique of metal deposition. A metal forms chemical compounds with the components of Cd _x Hg_1?x Te (CMT), thus changing the properties of the surface layer. Mercury is accumulated at the interface with the metal, while tellurium is accumulated on the metal surface. The CMT compounds with metals, heats of their formation, and the Fermi level shifts are reported. The structure and properties of the interfaces between CMT and gold, silver, indium, aluminum, copper, and other metals, as well as the effect of sublayers of other metals and insulators, are described.     

Exploring the Optical Properties of Hg1−x Cd x Se Films Using IR-Spectroscopic Ellipsometry

We have used infrared spectroscopic ellipsometry to interrogate the dielectric response of a series of Hg_1?x Cd _x Se samples in a spectral range between 2000 and 40000 nm. Using a standard inversion technique, the experimental data obtained at multiple angles of incidence were modeled to deduce the dielectric function of each sample. The dielectric functions obtained for Hg_1?x Cd _x Se samples allowed us to predict the band gaps for the samples, which increase as a function of the Cd concentration. Next, we modeled the dielectric function as a collection of oscillators, each of which represented a particular transition manifested in the spectrum. The most significant result obtained from this work is the recovery of their doping characteristics from the ellipsometric data. Specifically, two Hg_1?x Cd _x Se samples with x = 0.21 and 0.28, grown on GaSb substrates, show a carrier concentration of 1.6 × 10¹⁷ and 1.8 × 10¹⁸ cm^?3, respectively. These results are particularly helpful because conventional Hall measurements cannot be used for these specific samples due to the substrates (i.e., GaSb) used to grow them being highly conductive.     

Vanadium deep impurity level in diluted magnetic semiconductors Pb1 ? x ? y Sn x V y Te

The crystal structure, Sn and V distribution over the length of single-crystal ingots, and galvanomagnetic effects in low magnetic fields (4.2 K ?? T ?? 300 K, B ?? 0.07 T) in Pb_1?x?y Sn _x V _y Te alloys (x = 0.05?0.21, y ?? 0.015) are studied. It is shown that all the samples are single-phase, while the Sn and V concentrations exponentially increase from the beginning to the end of the ingots. Upon doping with V, a decrease in the concentration of free holes and a metal-insulator transition are found. They are related to the appearance of a deep impurity level of V in the band gap, electron redistribution between the level and the valence band, and pinning of the Fermi-level to the impurity level. The shift rate of the V level relative to the conduction band bottom is determined and a diagram of the reconstruction of the electronic structure of the Pb_{1 ? x ? y} Sn _x V _y Te alloy upon varying the host composition is suggested.     

Optical transitions in Cd x Hg1 ? x Te-based quantum wells and their analysis with account for the actual band structure of the material

Quantum-confinement levels in a Cd _x Hg_{1 ? x} Te-based rectangular quantum well are calculated in the framework of the four-band Kane model taking into account mixing between the states of electrons and three types of holes (heavy, light, and spin-split holes). Comparison of the calculation results with experimental data on the photoluminescence of Cd _x Hg_{1 ? x} Te-based quantum wells suggests that optical transitions involving the conduction and light-hole bands are possibly observed in the spectra.     

On the resonant level of chromium in the rhombohedral and cubic phases of Pb1 − x − y Ge x Cr y Te alloys

The temperature dependences of the Hall coefficient (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) in Pb_{1 ? x ? y} Ge _x Cr _y Te alloys (x = 0.03–0.08, y ≤ 0.01) are studied. An increase in the absolute value of the Hall coefficient with an increase in temperature is found. This fact is indicative of a decrease in the concentration of free electrons as a result of the motion of the resonant level of chromium stabilizing the Fermi level relative to the conduction-band bottom. The temperature dependences of the Hall coefficient, in satisfactory agreement with the experimental ones, are calculated in the context of the two-band Kane dispersion law allowing for the structural phase transition upon increasing temperature. The energy position and temperature coefficients of the motion of the resonant level of chromium relative to the middle of the band gap in the rhombohedral and cubic phases are determined.     

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.     

Role of intervalley scattering in the radiative recombination in Pb1 − x Eu x Te alloys (0 ≤ x ≤ 1)

Measurements of the photoluminescence from epitaxial layers of Pb_{1 ? x} Eu _x Te alloys with 0 ≤ x ≤ 0.32 are carried out. It is found that the luminescence intensity decreases with increasing Eu content and, already for x as low as about 0.1, drops by more than one order of magnitude. No luminescence is observed for 0.2 < x ≤ 0.32. This behavior is explained by the fact that, for x ≈ 0.1, the absolute minimum in the conduction band changes from the L to X point, which results in the scattering of nonequilibrium electrons to the X valley and, thus, causes a decrease in the quantum efficiency of the emission. According to published data, for x > 0.85, optical transitions also take place with the participation of the X valley; in this case, the emission is governed by the formation of magnetic polarons. The temperature dependences of the band gap are determined for 0 ≤ x ≤ 0.11. These dependences have a wide linear region characterized by a positive dE _g /dT coefficient, which decreases with the Eu content to become negative in pure EuTe.     

Cd x Hg1 − x Te-based photodetector with the spectral characteristic controlled by the voltage

A photodetector with a spectral characteristic of the photoresponse controlled by the bias voltage is realized on the basis of the Al-n-Cd _x Hg_{1 ? x} Te heterostructure with a narrow insulator gap. The features of the spectral characteristic of the photocurrent are accounted for by the variation in the ratio between the surface and bulk components of the photocurrent upon varying the bias voltage. The possibility of simultaneous detection and control over the spectral characteristic of photosensitivity at the fundamental adsorption edge and in the short-wavelength spectral region is shown.

     

A study of galvanomagnetic phenomena in MBE-grown n-CdxHg1−x Te films

The magnetic-field dependences of the Hall coefficient and the conductivity of n-type Cd_xHg_1?x Te epitaxial structures were measured at 77 K. The structures were grown by molecular-beam epitaxy with a prescribed solid solution composition profile across the thickness. A specific feature of the obtained dependences is that the conductivity and the absolute value of the Hall coefficient decrease with an increasing magnetic field. The obtained experimental dependences can only be described in terms of a model including low-mobility electrons. It is shown that anodic oxide deposited onto the Cd_xHg_1?x Te film surface makes the concentration of low-mobility electrons higher and that of anodic fluoride lower. The possible reasons for the appearance of low-mobility electrons are discussed. The most probable sources of such electrons are surface layers and electrical microheterogeneities in Cd_xHg_1?x Te films.     

Nature of the diamagnetic maximum in the temperature dependences of the magnetic susceptibility of crystals of (Bi2 ? x Sb x )Te3 alloys (0 < x < 1)

The temperature dependences of the magnetic susceptibility ?? of crystals of (Bi_{2 ? x} Sb _x )Te₃ alloys (0 < x < 1) are studied using a SQUID magnetometer in the temperature range from 2 to 400 K with the parallel and perpendicular orientations of the vector of magnetic field strength H relative to the trigonal axis of the crystal C ₃ (H ?? C ₃ and H ?? C ₃). It is found that the diamagnetic susceptibility of the samples with x = 0.2 (Bi_1.8Sb_0.2Te₃) and x = 0.5 (Bi_1.5Sb_0.5Te₃) increases in the range from 50 K to temperatures preceding the emergence of intrinsic conductivity (250 K). It is found that the diamagnetic maximum manifests itself in the same temperature range, in which an anomalous increase in the Hall coefficient is observed. It is shown that the nature of the diamagnetic maximum is associated with the nonparabolicity of the energy spectrum of light diamagnetic holes, a decrease in whose concentration is accompanied by a decrease in their effective masses, which provides an increase in the diamagnetic susceptibility with increasing temperature. These results are confirmed by the dynamics of the temperature variation in the resonance frequency of plasma oscillations of free charge carriers.     

Character of Interaction in the SnSb2Te4–SnBi2Te4 System and the Thermoelectric Properties of (SnSb2Te4)1 – x(SnBi2Te4)x Solid Solutions

Semiconductors - For the first time, the character of the interaction of components along the cut of SnSb2Te4–SnBi2Te4 is studied by various physicochemical methods in a wide temperature...     

Fermi level pinning and electrical properties of irradiated CdxHg1−x Te alloys

The composition dependence of the local neutrality level E _lnl in Cd_xHg_1?x Te alloys has been studied theoretically. It is shown that for compositions with x<0.47 the E _lnl level lies within the conduction band, and at x>0.47 it lies in the upper half of the band gap. The identification of the calculated E _lnl values with the limiting position of the Fermi level (F _lim) in Cd_xHg_1?x Te irradiated with high-energy particles suggests that irradiation is responsible for the n ⁺-or n-type conduction in the material.