xCdxTe Films Research Articles

PT-Symmetric Microwave Photoconductivity in Heterostructures Based on the Hg1 −xCdxTe Topological Phase

The PT-symmetric photoconductivity has been detected for the first time in microwave-irradiated heterostructures based on thick Hg1 −xCdxTe films with the CdTe content x corresponding to the topological phase although the magnetic field symmetry (T symmetry) and the symmetry in the positions of potential contact pairs (P symmetry) are not conserved separately. The microwave photoconductivity in similar heterostructures based on the trivial Hg1 −xCdxTe phase is both P- and T-symmetric.

Radiofrequency Photoconductivity in Hg1−xCdxTe Based Heterostructures

In this paper, we report on observation of a new effect—asymmetric in a magnetic field photoconductivity stimulated by radiofrequency pulses in heterostructures based on thick Hg1−xCdxTe films with composition x < 0.16 corresponding to the topological phase. At certain values of the excitation frequency, the photoconductivity may even change sign when the magnetic field is reversed. Possible mechanisms responsible for the effect appearance are discussed.

Calculation of Multiply Charged States of Impurity-Defect Centers in Epitaxial Hg1 –xCdxTe Layers

A method for calculating the states of multivalent donors and acceptors in Hg1 –xCdxTe materials is developed. The ionization energies of deep acceptor and donor centers in epitaxial Hg1 –xCdxTe films are calculated. The calculation method takes into account the influence of both the valence band and the conduction band on the states of impurity-defect centers. The calculations of energies for the levels of tetravalent acceptors and donors associated with crystalline structure defects indicate the intercenter nature of lines observed previously in the photoluminescence spectra of Hg1 –xCdxTe films.

Electrical properties of the Hg0.7Cd0.3Te films grown by MBE method on Si(0 1 3) substrates

The electrical properties of undoped and doped with indium Hg1−xCdxTe (x ≈ 0.3) films grown by the MBE method on Si (0 1 3) were investigated. In as-grown films, staking faults and trending dislocations with densities of ∼106 cm−2 and ∼107 cm−2, respectively, are observed. Annealing films under conditions causing the formation of mercury vacancies leads to drastic decrease of the staking faults (<104 cm−2). Filling vacancies leads to a decrease in the contribution of Shockley-Read recombination and to an increase in the lifetime. The dominant generation-recombination level in the as-grown Hg1−xCdxTe/Si(0 1 3) is the level associated with the vacancies. The magnetic field dependences of the Hall effect in the magnetic field range of 0.05–1.0 T at 77 K were explained by the fact that, in the films, there are two types of electrons with high and low mobilities.

Characterization of HgCdTe Films Grown on Large-Area CdZnTe Substrates by Molecular Beam Epitaxy

Recent advances in growth of Hg1−xCdxTe films on large-area (7 cm × 7.5 cm) CdZnTe (CZT) substrates is presented. Growth of Hg1−xCdxTe with good uniformity on large-area wafers is achieved using a Riber 412 molecular beam epitaxy (MBE) tool designed for growth of Hg1−xCdxTe compounds. The reactor is equipped with conventional CdTe, Te, and Hg sources for achieving uniform exposure of the wafer during growth. The composition of the Hg1−xCdxTe compound is controlled in situ by employing a closed-loop spectral ellipsometry technique to achieve a cutoff wavelength (λco) of 14 μm at 78 K. We present data on the thickness and composition uniformity of films grown for large-format focal-plane array applications. The composition and thickness nonuniformity are determined to be <1% over the area of a 7 cm × 7.5 cm wafer. The films are further characterized by Fourier-transform infrared spectroscopy, optical microscopy, and Hall measurements. Additionally, defect maps show the spatial distribution of defects generated during the epitaxial growth of the Hg1−xCdxTe films. Microdefect densities are in the low 103 cm−2 range, and void defects are below 500 cm−2. Dislocation densities less than 5 × 105 cm−2 are routinely achieved for Hg1−xCdxTe films grown on CZT substrates. HgCdTe 4k × 4k focal-plane arrays with 15 μm pitch for astronomical wide-area infrared imagers have been produced using the recently developed MBE growth process at Teledyne Imaging Sensors.

Effect of Lattice Mismatch on HgCdTe LPE Film Surface Morphology

A new type of crystal defect, which we call a rough structure, is reported in this work. The rough structure appears in large lattice mismatch regions whereas a normal surface appears in the regions where there is a small lattice mismatch on the same substrate. Experiments proved that under normal liquid-phase epitaxy growth conditions, the appearance of a rough structure is related to the lattice mismatch between the substrate and the film. Statistical data indicated that for Hg1−xCdxTe films with different Cd compositions x, a rough structure appeared on the film surface when the lattice mismatch was outside the range of 0.02–0.11%. The rough structure may result from the misfit dislocations resulting from strain relaxation. When there was a small surface crystal orientation deviation from (111), dense growth ripples appeared instead of the rough structure. A super-flat surface sometimes appeared inside the rough structure regions.

Hg1−xCdxTe vapor deposition on CdZnTe substrates by Closed Space Sublimation technique

Closed Space Sublimation (CSS) technique has been studied to deposit Hg1−xCdxTe polycrystalline films on CdZnTe substrates at the improved pressure–temperature conditions. The experimental results on film characterization suggest that the CSS optimal conditions are the argon atmospheric pressure (1013mbar) and the deposition temperature in the range of 500–550°C. These conditions provide macro-defect free Hg1−xCdxTe films with the uniform size and surface distribution of polycrystals.

Impact of the nanosecond volume discharge in atmospheric pressure air on the distribution of the surface potential of epitaxial HgCdTe

In this paper we present the results of our research of the impact of nanosecond volume discharge on the electronic properties of the near-surface region of epitaxial Hg1−xCdxTe films. We show that the distribution of the surface potential and, as a consequence, the material composition of the individual crystal grains that form V-defects possess a complex structure and contain regions with elevated content of both mercury and cadmium. The volume discharge treatment of the film surface leads to a decrease of the mercury content in individual crystal grains compared to the bulk of Hg1−xCdxTe epitaxial film. This indicates a higher mercury desorption rate from the V-defect region.

Influence of composition of the near-surface graded-gap layer on the admittance of metal-insulator-semiconductor structures based on graded-gap MBE n-Hg1−xCdxTe in wide temperature range

AbstractInfluence of the CdTe content in a near-surface graded-gap layer on the admittance of MIS-structures fabricated on the basis of heteroepitaxial Hg1−xCdxTe (x = 0.22–0.23 and 0.31–0.32) films grown by molecular beam epitaxy was investigated in a wide temperature range. It is shown that a temperature drop from 77 K to 8 K results in a decrease of hysteresis of the capacitance-voltage (C-V) characteristics and a decrease of frequencies which corresponds to a high-frequency behaviour of C-V characteristics of MIS-structures based on n-HgCdTe (x = 0.22–0.23) with and without graded-gap layersand also for MIS-structures based on n-HgCdTe (x = 0.31–0.32). Temperature dependences of the resistance of the epitaxial film bulk and differential resistance of the space-charge region (SCR) in strong inversion mode were studied. The experimental results can be explained by the fact that for MIS-structures based on n-HgCdTe (x = 0.22–0.23) with the graded-gap layers and for MIS-structures based on n-HgCdTe (x = 0.31–0.32), the differential resistance of SCR is limited by Shockley-Read generation at 25–77 K. Differential resistance of SCR for MIS-structures based on n-HgCdTe (x = 0.22–0.23) without the graded-gap layers is limited by tunnelling through deep levels at 8–77 K.

Spectra and kinetics of THz photoconductivity in narrow-gap Hg1–xCdxTe (x < 0.2) epitaxial films

Investigation into far infrared photoconductivity (PC) in narrow-gap epitaxial bulk Hg1–xCdxTe (x < 0.2) films grown by molecular beam epitaxy and chemical vapor deposition techniques is presented. A broadband of photosensitivity in terahertz region is found at 4.2 and 77 K. Some long-wavelength peculiarities of spectra are discovered and their origins are discussed. We also study PC relaxation process with nanosecond time resolution. It is found that carrier relaxation is non-radiative and measured lifetimes show that some of the structures are applicable for detecting in very long-wavelength infrared range.

Magnetic-field-induced change in optical transmission spectra of Hg1 − x Cd x Te epitaxial films

The optical transmission spectra of epitaxial Hg1 − xCdxTe films with a gradient of the band gap have been studied experimentally. The possibility of transforming the spectra after exposure of the samples to a homogeneous magnetic field has been demonstrated. This effect is possibly caused by the dependence of the magnetization of the Hg1 − xCdxTe films on the composition gradient of the solid solution.

Influence of near-surface graded-gap layers on electrical characteristics of MIS-structures based on MBE grown HgCdTe

AbstractThe paper examines influence of near-surface graded-gap layers on electrical characteristics of MIS-structures fabricated on heteroepitaxial Hg1−xCdxTe films grown by molecular beam epitaxy (MBE). Two types of insulators, i.e., two-layer SiO2/Si3N4 and anodic oxide films were used. As it is seen from the depth and width of the valley on the C-V characteristics, the capacitance is found to vary in a wide range, in contrast to the structures without graded-gap layers. It is shown that the graded-gap layer under MIS-structures with x = 0.22 effectively reduces the tunnelling generation via deep levels and increases a lifetime of minority carriers in the space charge region and its differential resistance. The properties of the HgCdTe-insulator interfaces are studied.

Effect of post-implantation annealing on the current-voltage characteristics of IR photodiodes based on p-HgCdTe

Current-voltage characteristics of IR photodiodes and distributions of charge carriers in n+-n−-p-structures based on vacancy p-doped Hg1 − xCdxTe films with x = 0.22 are examined. Three-dimensional numerical modeling of the distribution of charge carriers and current-voltage characteristics during photodiode annealing is performed. The calculations predict that large tunnel currents in diodes after implantation can result from an elevated (more than 1015 cm−3) concentration of donors in the n−-layer, which enhances tunneling by decreasing the thickness of the space charged region of the n-p-junction, and also from a small (less than 3 µm) depth of the p-n-junction.

CdTe as a passivating layer in CdTe/HgCdTe heterostructures

CdTe/Hg1 − xCdxTe heterostructures are studied. In the structures, CdTe is used as a passivating layer deposited as a polycrystal or single crystal on a single-crystal Hg1 − xCdxTe film. The film and a passivating layer were obtained in a single technological process of molecular beam epitaxy. The structure of passivating layers was studied by reflection high-energy electron diffraction, and the effect of the structure of the passivating layer on the properties of the active layer was studied by X-ray diffractometry. Mechanical properties of heterostructures were studied by the microhardness method. Electrical and photoelectrical parameters of the Hg1 − xCdxTe films are reported.

Layered structure formation in Hg1−xCdxTe films after high-frequency sonication

Layered structure formation in Hg1−xCdxTe films after high-frequency sonication

Photoelectrical characteristics of MIS structures on the basis of graded-band-gap n-HgCdTe (x = 0.21–0.23)

Voltage, frequency, and temperature dependences of photo-emf are experimentally studied in the MIS HgCdTe/SiO 2 /Si 3 N 4 and HgCdTe/AOF structures. The MIS structures were produced on the basis of graded-bandgap Hg 1−x Cd x Te films grown by molecular-beam epitaxy on GaAs substrates. It is found that the subsurface graded-band-gap layers do affect the photoelectrical characteristics of MIS structures. The mechanisms limiting the differential resistance in the space-charge region at various temperatures are revealed.

Effects of a-Si:H resist vacuum-lithography processing on HgCdTe

A vacuum-compatible process for carrying out lithography on Hg1−xCdxTe and CdTe films was previously demonstrated. It was shown that hydrogenated amorphous silicon (a-Si:H) could be used as a dry resist by projecting a pattern onto its surface using excimer laser irradiation and then developing that pattern by hydrogen plasma etching. Pattern transfer to an underlying Hg1−xCdxTe film was then carried out via Ar/H2 plasma etching in an electron cyclotron resonance (ECR) reactor. Despite the successful demonstration of pattern transfer, the possibility of inducing harmful effects in the Hg1−xCdxTe film due to this vacuum lithography procedure had not been explored. Here we present structural and surface compositional analyses of Hg1−xCdxTe films at key stages of the a-Si:H vacuum lithography procedure. X-ray diffraction double crystal rocking curves taken before and after a-Si:H deposition and after development etching were identical, indicating that bulk structural changes in the Hg1−xCdxTe film are not induced by these processes. Cross-section transmission electron microscopy studies show that laser-induced heating in the 350 nm thick a-Si:H overlayer is not sufficient to cause structural damage in the underlying Hg1−xCdxTe surface. In vacuo surface analysis via Auger electron spectroscopy and ion scattering spectroscopy suggest that the hydrogen plasma development process produces Hg-deficient surfaces but does not introduce C contamination. However, after ECR plasma etching into the Hg1−xCdxTe film, the measured x value is much closer to that of the bulk.

Mobility of minority charge carriers in p-HgCdTe films

Temperature dependences of electron mobility in p-Hg1−xCdxTe films (x=0.210–0.223) grown by molecular beam epitaxy are investigated. In the temperature range 125–300 K, mobility was found by the mobility-spectrum method, and for the range 77–125 K, it was found using a magnetophotoconductivity method suggested in this study. The method is based on the measurement of the magnetic-field dependence of photoconductivity. The magnetic field is parallel to the radiation and normal to the sample surface. The electron mobility is determined using the simple expression μ n [m2/(V s)]=1/B H [T]. Here, B H is the induction of the magnetic field corresponding to a half-amplitude of the photoconductivity signal under zero magnetic field. In the temperature range 100–125 K, the results obtained by the magnetophotoconductivity and mobility-spectrum methods coincide. For the samples investigated, the electron mobility at 77 K is in the range 5–8 m2/(V s).

RF sputtered HgCdTe films for tandem cell applications

Polycrystalline Hg1−xCdxTe films were investigated for their potential as bottom cells of a CdTe-based tandem solar cell. The films were deposited by RF sputtering from a cold pressed target containing 30% HgTe + 70% CdTe. The as-deposited films were highly resistive with (111) preferred orientation and a bandgap of ∼1.0 eV. Various thermal treatment schemes were investigated under different conditions of ambient and temperature to reduce the resistivity. The film properties were analyzed using infrared transmission spectra, energy dispersive X-ray spectra and X-ray diffraction. N doped p-HgCdTe films were also prepared by reactive sputtering in a N2/Ar ambient. P-n junction solar cells were fabricated with CdS films as the heterojunction partner. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation and characterization of monolithic HgCdTe/CdTe tandem cells

ABSTRACTA prototype monolithic HgCdTe/CdTe superstrate tandem cell has been fabricated by RF sputtering, comprising a CdTe/CdS top cell, a ZnTe:N/ZnO:Al interconnect junction and a HgCdTe/CdS bottom cell. The Hg1−xCdxTe film as the bottom absorption layer was deposited by RF sputtering with 70% or 85% Cd content in the Hg1−xCdxTe magnetron target. Hg1−xCdxTe films with band gap from 0.98 eV to 1.45 eV were obtained by controlling the deposition temperature. CdCl2 thermal treatments were used to improve the Hg1−xCdxTe film electrical properties. A nitrogen-doped ZnTe film combined with an aluminium (Al) doped ZnO film formed a good interconnect junction. Results of Voc = 0.99 V and Jsc = 2.1 mA/cm2 were obtained in the best such tandem cell at one sun (AM1.5).