Specific features of conductivity of Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>x</Subscript>Te and Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>Te single crystals

The electrical characteristics of p-type Cd_1?xZn_xTe (x=0.05) and Cd_1?xMn_xTe (x=0.04) single crystals with a resistivity of 10³–10¹⁰ Ω cm at 300 K are studied. The conductivity and its variation with temperature are interpreted on the basis of statistics of electrons and holes in a semiconductor with deep acceptor impurities (defects), with regard to their compensation by donors. The depth of acceptor levels and the degree of their compensation are determined. The problems of attaining near intrinsic conductivity close to intrinsic are discussed.     

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.     

Alternating current conductivity and electron spin resonance of the Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>x</Subscript>Te alloys

The results of measurements of frequency dependences of conductivity and electron spin resonance (ESR) of the Cd_{1 ? x} Fe_xTe alloys (0.01 ≤ x ≤ 0.05) at room temperature are presented. It is found that, in the composition range 0.003 ≤ x ≤ 0.05, a new asymmetric ESR absorption line with g ≈ 3.9 emerges, and its hysteresis manifests itself while the samples are magnetized. The line with g ≈ 3.9 is attributed to the charge state 3⁺ of the Fe atoms, which occurs in tetrahedra with three Fe atoms.     

Investigation of Multicarrier Transport in LPE-Grown Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Layers

A detailed study is presented of multicarrier transport properties in liquid-phase epitaxy (LPE)-grown n-type HgCdTe films using advanced mobility spectrum analysis techniques over the temperature range from 95 K to 300 K. Three separate electron species were identified that contribute to the total conduction, and the temperature-dependent characteristics of carrier concentration and mobility were extracted for each individual carrier species. Detailed analysis allows the three observed contributions to be assigned to carriers located in the bulk long-wave infrared (LWIR) absorbing layer, the wider-gap substrate/HgCdTe transition layer, and a surface accumulation layer. The activation energy of the dominant high-mobility LWIR bulk carrier concentration in the high temperature range gives a very good fit to the Hansen and Schmit expression for intrinsic carrier concentration in HgCdTe with a bandgap of 172 meV. The mobility of these bulk electrons follows the classic μ ~ T ^−3/2 dependence for the phonon scattering regime. The much lower sheet densities found for the other two, lower-mobility electron species show activation energies of the order of ~20 meV, and mobilities that are only weakly dependent on temperature and consistent with expected values for the wider-bandgap transition layer and a surface accumulation layer.     

Photoluminescence of Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te based heterostructures grown by molecular-beam epitaxy

Photoluminescence (PL) of Hg_{1 − x} Cd _x Te-based heterostructures grown by molecular-beam epitaxy (MBE) on GaAs and Si substrates has been studied. It is shown that a pronounced disruption of the long-range order in the crystal lattice is characteristic of structures of this kind. It is demonstrated that the observed disordering is mostly due to the nonequilibrium nature of MBE and can be partly eliminated by postgrowth thermal annealing. Low-temperature spectra of epitaxial layers and structures with wide potential wells are dominated by the recombination peak of an exciton localized in density-of-states tails; the energy of this peak is substantially lower than the energy gap. In quantum-well (QW) structures at low temperatures, the main PL peak is due to carrier recombination between QW levels and the energy of the emitted photon is strictly determined by the effective (with the QW levels taken into account) energy gap.     

Analysis of Carrier Transport in <Emphasis Type="Italic">n</Emphasis>-Type Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te with Ultra-Low Doping Concentration

Mercury cadmium telluride (HgCdTe, or MCT) with low n-type indium doping concentration offers a means for obtaining high performance infrared detectors. Characterizing carrier transport in materials with ultra low doping (N_D?=?10¹⁴ cm^?3 and lower), and multi-layer material structures designed for infrared detector devices, is particularly challenging using traditional methods. In this work, Hall effect measurements with a swept B-field were used in conjunction with a multi-carrier fitting procedure and Fourier-domain mobility spectrum analysis to analyze multi-layered MCT samples. Low temperature measurements (77 K) were able to identify multiple carrier species, including an epitaxial layer (x?=?0.2195) with n-type carrier concentration of n?=?1?×?10¹⁴ cm^?3 and electron mobility of μ?=?280000 cm²/Vs. The extracted electron mobility matches or exceeds prior empirical models for MCT, illustrating the outstanding material quality achievable using current epitaxial growth methods, and motivating further study to revisit previously published material parameters for MCT carrier transport. The high material quality is further demonstrated via observation of the quantum Hall effect at low temperature (5 K and below).     

Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Alloys: Application Considerations

The search for alternative energy sources is at the forefront of applied research. In this context, thermoelectricity, i.e., direct conversion of thermal into electrical energy, plays an important role, particularly for exploitation of waste heat. Materials for such applications should exhibit thermoelectric potential and mechanical stability. PbTe-based compounds include well-known n-type and p-type compounds for thermoelectric applications in the 50°C to 600°C temperature range. This paper is concerned with the mechanical and transport properties of p-type Pb_0.5Sn_0.5Te:Te and PbTe<Na> samples, both of which have a hole concentration of ∼1 × 10²⁰ cm⁻³. The ZT values of PbTe<Na> were found to be higher than those of Pb_0.5Sn_0.5Te:Te, and they exhibited a maximal value of 0.8 compared with 0.5 for Pb_0.5Sn_0.5Te:Te at 450°C. However, the microhardness value of 49 H_V found for Pb_0.5Sn_0.5Te:Te was closer to that of the mechanically stable n-type PbTe (30 H_V) than to that of PbTe<Na> (71 H_V). Thus, although lower ZT values were obtained, from a mechanical point of view Pb_0.5Sn_0.5Te:Te is preferable over PbTe<Na> for practical applications.     

Steady-State Electron Transport and Low-Field Mobility of Wurtzite Bulk ZnO and Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O

Steady-state electron transport and low-field electron mobility characteristics of wurtzite ZnO and Zn_1−x Mg _x O are examined using the ensemble Monte Carlo model. The Monte Carlo calculations are carried out using a three-valley model for the systems under consideration. Acoustic and optical phonon scattering, intervalley (equivalent and nonequivalent) scattering, ionized impurity scattering, and alloy disorder scattering are used in the Monte Carlo simulations. Steady-state electron transport is analyzed, and the population of valleys is also obtained as a function of applied electric field and ionized impurity concentrations. The negative differential mobility phenomena is clearly observed and seems compatible with the occupancy and effective nonparabolicity factors of the valleys in bulk ZnO and in Zn_1−x Mg _x O with low Mg content. The low-field mobilities are obtained as a function of temperature and ionized impurity concentrations from the slope of the linear part of each velocity–field curve. It is seen that mobilities begin to be significantly affected for ionized impurity concentrations above 5 × 10¹⁵/cm³. The calculated Monte Carlo simulation results for low-field electron mobilities are found to be consistent with published data.     

Metal-insulator transition in chromium-doped Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te alloys

The electrical properties of chromium-doped n-Pb_1?x Ge _x Te alloys (x = 0.02–0.13) have been studied. A decrease in the free-electron concentration and a metal-insulator transition are observed as the germanium content of alloys increases. This is due to the Fermi level pinning by the chromium impurity level and to the flow of electrons from the conduction band to the impurity level. The experimental data obtained are used to calculate, in terms of the two-band Kane dispersion law, the dependences of the electron concentration and Fermi energy on the germanium content in the alloy. The motion rate of the chromium-related level with respect to the conduction band bottom is determined and a model of variation of the electronic structure with the matrix composition is suggested.     

Modeling of the Structural Properties of Hg<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

Structural properties of Hg_1–x Cd _x Te are investigated by using first-principles calculations based on density functional theory. An energetically minimized and geometrically optimized model for Hg_1–x Cd _x Te was formulated. A virtual crystal approximation model for Hg_1–x Cd _x Te produced a poor fit to experimental lattice parameters and Vegard’s law. However, the virtual crystal approximation model provides reasonably accurate values for the band gap␣energy. An ordered alloy approximation model produced a good fit to Hg_1–x Cd _x Te lattice parameters and followed Vegard’s law. The ordered alloy approximation also produced a bimodal distribution in Hg-Te and Cd-Te bond lengths in agreement with experimental results.     

Investigation of MBE-Growth of Mid-Wave Infrared Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se

Undoped mid-wave infrared Hg_1?xCd_xSe epitaxial layers have been grown to a nominal thickness of 8–14 μm on GaSb (211)B substrates by molecular beam epitaxy (MBE) using constant beam equivalent pressure ratios. The effects of growth temperature from 70°C to 120°C on epilayer quality and its electronic parameters has been examined using x-ray diffraction (XRD) rocking curves, atomic force microscopy, Nomarski optical imaging, photoconductive decay measurements, and variable magnetic field Hall effect analysis. For samples grown at 70°C, the measured values of XRD rocking curve full width at half maximum (FWHM) (116 arcsec), root mean square (RMS) surface roughness (2.7 nm), electron mobility (6.6?×?10⁴ cm² V^?1 s^?1 at 130 K), minority carrier lifetime (～?2 μs at 130 K), and background n-type doping (～?3?×?10¹⁶ cm^?3 at 130 K), indicate device-grade material quality that is significantly superior to that previously published in the open literature. All of these parameters were found to degrade monotonically with increasing growth temperature, although a reasonably wide growth window exists from 70°C to 90°C, within which good quality HgCdSe can be grown via MBE.     

Effective electron mass in a Mn<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te system

The reflection spectra of n-Mn_xHg_1?xTe single crystals and epitaxial layers were measured at 300 K. The effective electron mass was determined for the samples with x=0.06–0.10 and an electron concentration N>6×10¹⁶ cm^?3. The calculated values of effective electron mass are close to experimental values.     

Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te epitaxial films for terahertz detectors

Liquid-phase epitaxy is used to fabricate Pb_0.8Sn_0.2Te films, undoped or doped with indium to different levels. The depth profiles of the carrier density and dopant concentration in the films are measured and examined. A uniform dopant concentration to a depth of 15 μm is obtained. Electrical-conduction inversion is observed at a temperature of 77.3 K as the doping level is varied. The liquid-phase epitaxial method is shown to be a more suitable technology for the reproducible manufacture of epitaxial films with a given carrier density, such as the ones used in terahertz detectors.     

Highly Efficient Ge-Rich Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te Thermoelectric Alloys

The search for alternative energy sources is presently at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This paper is concerned with the development of highly efficient p-type Ge _x Pb_1−x Te alloys for thermoelectric applications, using spark plasma sintering. The carrier concentration of GeTe was varied by alloying of PbTe and/or by Bi₂Te₃ doping. Very high ZT values up to ~1.8 at 500°C were obtained by doping Pb_0.13Ge_0.87Te with 3 mol% Bi₂Te₃.     

Cathodoluminescence from dilute GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> solutions (<Emphasis Type="Italic">x</Emphasis> ≤ 0.03)

Cathodoluminescence from GaN _x As_1?x layers (0 ≤ x ≤ 0.03) was measured at photon energies ranging from the intrinsic absorption edge to 3 eV at room temperature. An additional emission band was visible in the visible range of the cathodoluminescence spectra. The intensity of this band is two orders of magnitude lower than the edge-emission intensity. The photon energy corresponding to the peak of this band and its FWHM are virtually independent of x and equal to ～2.1 and 0.6–0.7 eV, respectively. This emission is related to indirect optical transitions of electrons from the L _6c and Δ conduction-band minimums to the Γ₁₅ valence-band maximum.     

Complex Refractive Indices of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O Thin Films Grown by Molecular Beam Epitaxy

The complex refractive indices of Cd _x Zn_1−x O thin films were determined by transmission spectrophotometry. Transmission spectra were modeled from 375 nm to 800 nm for samples having cadmium concentrations ranging from 2% to 77%. The transparent and absorptive regimes were fitted separately by Sellmeier and Forouhi–Bloomer models, respectively. Real refractive indices of Cd _x Zn_1−x O shift to higher values in the transparent region and the optical absorption edge shifts to longer wavelengths with increasing cadmium concentration. Spectroscopic ellipsometry was carried out on one sample from λ = 190 nm to 1.8 μm. Comparison between the two methods shows that the results are in general agreement.     

Investigating the Electron–Phonon Coupling of Molecular Beam Epitaxy-Grown Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Semiconductor Alloys

Using spectroscopic ellipsometry, the temperature-dependence of the dielectric functions of a series of Hg_1?xCd_xSe thin films deposited on both ZnTe/Si(112) and GaSb(112) substrates were investigated. Initially, for each sample, room-temperature ellipsometric spectra were obtained from 35 meV to 6 eV using two different ellipsometers. Subsequently, ellipsometry spectra were obtained from 10 K to 300 K by incorporating a cryostat to the ellipsometer. Using a standard inversion technique, the spectroscopic ellipsometric data were modeled in order to obtain the temperature-dependent dielectric functions of each of the Hg_1?xCd_xSe thin films. The results indicate that the E₁ critical point blue-shifts as a function of Cd-alloy concentration. The temperature-dependence of E₁ was fitted to a Bose–Einstein occupation distribution function, which consequently allowed us to determine the electron–phonon coupling of Hg_1?xCd_xSe alloys. From the fitting results, we obtain a value of 17 ± 2 meV for the strength of the electron–phonon coupling for Hg_1?xCd_xSe alloy system, which compares nominally with the binary systems, such as CdSe and CdTe, which have values around 38 meV and 16 meV, respectively. This implies that the addition of Hg into the CdSe binary system does not significantly alter its electron–phonon coupling strength. Raman spectroscopy measurements performed on all the samples show the HgSe-like transverse optic (TO) and longitudinal optic (LO) phonons (～?130 cm^?1 and?～?160 cm^?1, respectively) for all the samples. While there is a slight red-shift of the HgSe-like TO peak as a function of the Cd-concentration, HgSe-like LO peak does not significantly change with the alloy concentration.     

Study of the electrical properties of Cd<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te

On the basis of the temperature and field dependences of the Hall coefficient R _H , it was found that samples with a low electron density are, as a rule, compensated, and the degree of compensation changes upon thermal conversion of the conductivity of the sample to p type. For n-Cd_xHg_1?xTe, the ionization energy of the donor level was found from the temperature dependences of resistivity ρ(T): E _d =24–32 meV. For the same samples, after their thermal conversion to p type, the ionization energies of acceptors, which are related to doubly charged vacancies V _Hg ⁺⁺ , were determined: E _a =32 and 48 meV. In addition, a deep level E _t , related to an unknown amphoteric impurity, was found (E _t ?E _v ≈0.7E _g ).     

Modification of Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>x</Subscript>Te properties by low-energy ions

We present an overview concerning the modification of properties of HgCdTe solid solutions and related Hg-containing materials under surface treatment with low-energy (60–2000 eV) ion beams. The conditions for conductivity-type conversion in p-material, dose, and time dependences of the depth of the conversion layer are analyzed. The modification of electrical properties of n-type material subjected to ion-beam treatment is discussed. The suggested mechanisms of conductivity-type conversion under low-energy ion treatment of HgCdTe doped with vacancies or acceptor impurities are regarded. Properties of p-n junctions produced by this technique are reviewed, and electrical and photoelectric parameters of HgCdTe IR photodetectors fabricated by low-energy ion treatment are analyzed. Several examples of novel device structures developed with the use of the method are presented.     

Monitoring the composition of the Cd<Subscript>1 − <Emphasis Type="Italic">z</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript>Te heteroepitaxial layers by spectroscopic ellipsometry

A hardware-software complex based on a spectroscopic ellipsometer integrated into a molecular beam epitaxy installation and destined to monitor the composition of the Cd_{1 − z} Zn _z Te alloy at small values of z is described. Methodical features of determination of the composition of growing layers by the spectra of ellipsometric parameters are considered. The procedure of determination of the composition by the absorption edge that allows measuring this parameter accurate to 1.2% is developed. Problems are considered the solutions of which will allow one to increase the resolution by the composition. In particular, maintaining a stable temperature during growth is required for this purpose.