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.     

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.     

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.     

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.     

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.     

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.     

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.

     

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.     

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.     

Avalanche Mechanism in p +-n −-n + and p +-n Mid-Wavelength Infrared Hg1−x Cd x Te Diodes on Si Substrates

Hg_1−x Cd _x Te mid-wavelength infrared (MWIR) p ⁺-n ⁻-n ⁺ and p ⁺-n ⁻ avalanche photodiodes (APDs) with a cut-off of 4.9 μm at 80 K were fabricated on Si substrates. Diode characteristics, avalanche characteristics, and excess noise characteristics were measured on two devices. Temperature-dependent diode and avalanche characterization was performed. Maximum 3 × 10⁶ Ω cm² and 9 × 10⁵ Ω cm² zero-bias resistance times active area (R ₀ A) products were measured for the p ⁺-n ⁻-n and p ⁺-n devices at 77 K, respectively. Multiplication gains of 1250 and 410 were measured at −10 and −4 V for the p ⁺-n ⁻-n ⁺ and p ⁺-n APDs at 77 K, respectively, in the front-illumination mode with the help of a laser with an incident wavelength of 632 nm. The gains reduce to 200 and 50 at 120 K, respectively. The excess noise factor in all APDs was measured to be in the range of 1 to 1.2.     

Phase formation upon the interaction of thin films in the system Yb1 − x Sm x Te-As2Te3

Phase formation upon the interaction of thin Yb_{1 ? x} Sm _x Te (x = 0.02) and As₂Te₃ films obtained through their simultaneous and successive evaporation is studied. It is shown that the Yb_{1 ? x} Sm _x As₄Te₇ phase is formed upon interaction of the thin films at a temperature of 473 K and the Yb_1?x Sm _x As₂Te₄ phase at 503 K.     

1～3微米光谱区用(Hg,Cd)Te光电二极管激光接收器

各种组分的(Hg,Cd)Te灵敏高速光电二极管探测器已制成,用于1～3微米的光谱区。这些探测器工作于室温,量子效率为40～70％,比探测度由1.3微米波长的3×10~(11)厘米·赫~(1/2)/瓦到3微米波长的5×10~9厘米·赫~(1/2)/瓦。冷却光电二极管探测器可改进性能。在77°K下2.2微米波长的比探测度测得为2×10~(12)厘米·赫~(1/2)/瓦,接近背景限。对探测器-前置放大器组件进行了响应时间测量,用的是0.9、1.06及1.54微米的脉冲辐射。响应时间约为10毫微秒,似乎与波长无关,但受RC乘积和放大器频带宽的限制。若光电二极管接一频带很宽的放大器,脉冲响应就被RC之积限制到0.5毫微秒,并测得另一时间常数(约为5毫微秒),据说它是由储存的载流子的渡越时间决定的。     

Electrochemical Deposition and Characterization of Thermoelectric Ternary (Bi x Sb1?x )2Te3 and Bi2(Te1?y Se y )3 Thin Films

Thermoelectric thin films of the ternary compounds (Bi _x Sb_1?x )₂Te₃ and Bi₂(Te_1?y Se _y )₃ were synthesized using potentiostatic electrochemical deposition on gold-coated silicon substrates from aqueous acidic solutions at room temperature. The surface morphology, elemental composition, and crystal structure of the deposited films were studied and correlated with preparation conditions. The thermoelectric properties of (Bi _x Sb_1?x )₂Te₃ and Bi₂(Te_1?y Se _y )₃ films, i.e., Seebeck coefficient and electrical resistivity, were measured after transferring the films to a nonconductive epoxy support. (Bi _x Sb_1?x )₂Te₃ thin films showed p-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively large negative potential with composition close to Bi_0.5Sb_1.5Te₃. In addition, Bi₂(Te_1?y Se _y )₃ thin films showed n-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively small negative potential, having composition close to Bi₂Te_2.7Se_0.3. In contrast to Bi₂Te_2.7Se_0.3 thin films, an annealing treatment was required for Bi_0.5Sb_1.5Te₃ thin films to achieve the same magnitude of power factor as Bi₂Te_2.7Se_0.3. Therefore, Bi₂Te_2.7Se_0.3 thin films appear to be good candidates for multilayer preparation using electrochemical deposition, but the morphology of the films must be further improved.