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.     

Plasma resonance in Pb1 − x Ag x Te alloys

From the data of reflectance measurements for Pb_{1 ? x} Ag _x Te (x = 0.001–0.007) alloy single crystals, the hole concentration and conductivity are calculated as functions of the Ag content. It is established that, for all of the samples, the plasma minimum is in the infra-red region and shifts to shorter wavelengths, as the Ag content is increased. In this case, the hole concentration and conductivity increase.     

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.     

Electrical properties of Pb1 − x Mn x Te single crystals with an excess of tellurium

The effect of excess Te atoms (as high as 0.5 at %) and thermal treatment at 473 K for 120 h on the electrical conductivity σ, the thermopower coefficient α, and the Hall coefficient R of Pb_0.96Mn_0.04Te single crystals in the temperature range ～77–300 K is investigated. It is shown that excess atoms of tellurium predominantly act as acceptor impurity centers at low concentrations in unannealed samples and form antisite defects at relatively high concentrations (0.05 at % or higher) being located mainly in vacancies of the lead sublattice, and decrease the hole concentration. As a result of annealing, certain lattice defects (for example, deformational) are healed, and the accommodation process for Te atoms at lead-sublattice vacancies is intensified. These processes substantially affect the values of the electrical parameters, their temperature dependences, as well as the sign of the thermopower and Hall coefficients of the samples.     

Magnetic susceptibility of Bi2 − x Sb x Te3 (0 < x ≤ 1) crystals in the intrinsic conductivity region

Temperature dependences of the magnetic susceptibility of Bi₂Te₃-Sb₂Te₃ with 10, 25, and 50% of Sb₂Te₃ are experimentally studied using an MPMS MultiVu SQUID magnetometer (Quantum Design, USA) in the temperature range from 2 to 400 K with regard to the anisotropy of the magnetic susceptibility. The behavior of the magnetic susceptibility at temperatures above 250 K is analyzed where the electron system of the crystals passes to the nondegenerate state. It is shown that the temperature dependence of the magnetic susceptibility in the intrinsic conductivity region can be described within the framework of the Pauli and Landau-Peierls approaches.     

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.     

Effect of annealing on the electrical properties of Pb1 − x Mn x Te single crystals with excess tellurium

The electrical conductivity σ, Hall coefficients R, and thermopower α of Pb_0.96Mn_0.04Te (Te) single crystals annealed at 573 K for 120 h are investigated. It is shown that, in contrast to unannealed samples, the investigated samples exhibit n-type conductivity and metal-type dependences σ(T) in the temperature range 77–300 K. It is suggested that, upon annealing, a portion of the excess tellurium atoms occupy vacancies in the lead sublattice with the formation of new vacancies in the tellurium sublattice of the samples.     

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.     

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.     

Effect of surface orientation of CdxHg1−x Te crystals on the properties of surface anodic oxides

Electron spectroscopy for chemical analysis was used to study the effect of surface orientation of Cd_xHg_1?x Te crystals (x=0.19) on the composition of anodic oxide. It is found that anodic oxide compositions on the (111) and (100) crystal surfaces are identical and change gradually from the substrate to the outer surface, whereas the oxide on the (110) surface contains an interlayer of unoxidized Te. This result correlates with some properties known for Cd_xHg_1?x Te oxides.     

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.     

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.     

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.     

Hopping carrier transport in epitaxial V:TiO2 − x layers

The electrical conductivity of TiO₂ layers heavily doped with vanadium has been measured as a function of temperature in the range 77?C300 K. It is shown that, in the case of doping with vanadium, thin epitaxial layers exhibit 3D variable-range hopping conductivity according to Mott??s law. The conductivity parameters, i.e., the average hopping energy and distance and the density of states near the Fermi level, are calculated.     

Superstructured ordering in Al x Ga1 − x As and Ga x In1 − x P alloys

Epitaxial heterostructures produced on the basis of Al _x Ga_{1 ? x} As and Ga _x In_{1 ? x} P ternary alloys by metal-organic chemical vapor deposition are studied. The composition parameter x of the alloys was ～0.50. By X-ray diffraction studies, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy, it is shown that superstructured ordered phases with the stoichiometry composition III_{1 ? η}III_{1 + η}V₂ can be formed. As a consequence of this effect, not only does the cubic crystal symmetry change to the tetragonal type in the new compound, but also the optical properties become different from those of disordered alloy with the same composition.     

Resistivity variation of semi-insulating Cd1−xZnx Te in relationship to alloy composition

We investigated the resistivity variation of semi-insulating Cd_1−xZn_xTe used as room temperature nuclear radiation detectors, in relationship to the alloy composition. The resistivity and the zinc composition were determined using leakage current measurements and triple axis x-ray diffraction lattice parameter measurements, respectively. While the zinc content of the nominally x_Zn∼0.1 ingot varied monotonically according to the normal freezing behavior with an effective segregation coefficient of k_eff=1.15, the resistivity was found to vary non-systematically throughout the ingot. Furthermore, the “expected” relationship of higher zinc content with higher resistivity was not always observed. For example, wafer regions of x_Zn∼0.12 and x_Zn∼0.08 exhibited resistivity values of ∼10¹⁰ and ∼10¹¹ Ω·cm, respectively. In general, the experimental resistivity values can be explained by calculated values which take into account a compensating deep level defect and various electron and hole mobility values. The relative influence of the parameters that govern the resistivity (n,p, μ_e, and μ_h) are quantitatively investigated.     

Preparation of Uncapped CdSe x Te1−x Nanocrystals with Strong Near-IR Tunable Absorption

Semiconducting nanocrystals with near-infrared (NIR) photosensitivity are appealing materials for application as photodetectors and in medical diagnostics. Herein, we report the preparation of composition-tunable, uncapped CdSe _x Te_1?x (x = 0 to 1) nanocrystals by simple mechanical alloying. The resulting ternary CdSe _x Te_1?x (x = 0.25, 0.5, 0.75) nanocrystals with average sizes smaller than 10 nm have zincblende crystal structure, instead of the wurtzite structure commonly obtained by wet chemical routes, and show strong NIR absorption even beyond 1400 nm. While a linear relationship between the lattice parameter and the chemical composition (Se/Te ratio) is observed, indicating the formation of homogeneous alloys, the bandgap energy of the three ternary samples is found to be substantially lower than that of binary CdSe or CdTe nanocrystals, and lower than any ternary CdSeTe reported so far. Existence of a small number of tellurium metal defects in the CdSe _x Te_1?x (x = 0.25, 0.5, 0.75) nanocrystals is confirmed by x-ray diffraction and Raman spectroscopy. Both the optical bowing effect and tellurium metal-induced defects of the mechanically alloyed samples are believed to cause the strong NIR photosensitivity.     

Thermal Conductivity in Bi1−x Sb x Solid Solutions

Bi_1?x Sb _x solid solutions have attracted much attention as promising low-temperature thermoelectric materials. Previously, we observed distinct extrema in the isotherms of the transport and mechanical properties of polycrystalline Bi_1?x Sb _x and attributed their presence to the transition from diluted to concentrated solid solutions and to the reconstruction of the energy band structure under increasing Sb concentration. The goal of the present work is a detailed study of the concentration dependences of the thermal conductivity λ for Bi_1?x Sb _x polycrystalline solid solutions (x = 0 to 0.09) in the temperature range of 170 K to 300 K. It is established that the λ(x) dependences exhibit a nonmonotonic behavior: in certain concentration ranges an anomalous increase in λ with increasing x is observed. It is shown that the concentration dependences of the thermoelectric figure of merit calculated on the basis of the measured λ values are also nonmonotonic. The obtained data represent additional evidence in favor of our assumptions stated earlier about a significant effect of electronic phase transitions observed in Bi_1?x Sb _x solid solutions on the concentration dependences of their thermoelectric properties. These results should be taken into account when developing new Bi_1?x Sb _x -based materials.     

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.