Optical and photoelectrical properties of Hg1−xCdxTe/CdTe epitaxial films with graded band gap

The infrared transmission and photoconductivity spectra of HgCdTe epitaxial films with graded band gap were investigated both theoretically and experimentally. Theoretical calculations were performed in the framework of the WKB approximation. The composition profile has been obtained from a fitting procedure. In order to reduce the total number of fitting parameters as well as to improve accuracy of this procedure the differential of the transmission versus photon energy curves was used. The best fit was obtained for an exponential composition gradient.     

Photoresponse of Hg1−xCdxTe n-channel MISFETs

Photogenerated currents at 77 K, when the Semitransparent gate of an Hg_1−xCd_xTe(x = 0.205) n-channel MISFET is illuminated with IR radiation of varying intensities, have been evaluated theoretically. Two different processes of excitation are considered, namely, electron-hole pair excitation across the bandgap in the depletion layer of the field-induced junction and excitation of carriers to the conduction band from surface states lying near the middle of the bandgap. The photocurrent, in this case, is primarily due to the latter process. For the sake of comparison, the drain-source current without illumination has also been calculated as a function of the gate voltage.     

Far-infrared detection with Hg1−xCdxTe

We report detection in liquid He cooled n−Hg_1−xCd_xCd_xTe at wavelengths between 140 and 1200 μm. With the mole fraction, x, of Cd <0.4m the alloy behaves as an electron bolometer with similar detectivity, but much improved bandwidth, compared with n-InSb. With x > 0.48 the detection process is extrinsic photoconductivity with an energy gap of a few meV. Lattice absorption severely limits detection in the wavelength range 220–340 μm.     

Theoretical investigations of the structural,electronic and optical properties of Hg1−xCdxTe alloys

The structural, electronic and optical properties of mercury cadmium telluride (Hg_1?xCd_xTe; x = 0.0, 0.25, 0.5, 0.75) alloys are studied using density functional theory within full-potential linearized augmented plane wave method. We used the local density approximation (LDA), generalized gradient approximation (GGA), hybrid potentials, the modified Becke–Johnson (LDA/GGA)-mjb and Hubbard-corrected functionals (GGA/LDA + U), for the exchange-correlation potential (E_ex). We found that LDA functional predicts better lattice constants than GGA functional, whereas, both functionals fail to predict the correct electronic structure. However, the hybrid functionals were more successful. For the case of HgTe binary alloy, the GGA + U functional predicted a semi-metallic behaviour with an inverted band gap of ?0.539 eV, which is closest to the experimental value (?0.30 eV). Ternary alloys, however, are found to be semiconductors with direct band gaps. For the x = 0.25 and 0.50, the best band gaps are found to be 0.39 and 0.81 eV using LDA-mbj functional, whereas, the GGA-mbj functional predicted the best band gap of 1.09 eV for Hg_0.25Cd_0.75Te alloy, which is in a very good agreement with the experimental value (1.061 eV). The optical properties of the alloys are obtained by calculating the dielectric function ?(ω). The peaks of the optical dielectric functions are consistent with the electronic gap energies of the alloys.     

The realization of ion-implanted Hg1−xCdxTe photovoltaic arrays

A review of the main aspects of the realization of PV Hg_1−xCd_xTe arrays is presented. Results obtained on coating this material are reported and related to the carrier concentration of the bulk. Changes in the electrical properties, produced by ion implantation, are also reported and discussed.     

Intrinsic carrier concentration in Hg1−xCdxTe

The effective mass of heavy holes has been determined on the basis of simultaneous analysis of the Hall coefficient and conductivity data obtained in the temperature region 100–300 K on well characterized p-type Hg_1–x Cd_xTe (x0·2) samples. Its value is 0·7m₀. The calculation of intrinsic carrier concentration for 0·19 x0·3 and 50 Kg T 300 K has been carried out using the above value of the effective mass of holes, Hansen's expression for the band gap and momentum matrix element from magneto-optical measurements.     

Nonlinear optical effects in Hg1−xCdxTe1

The alloy semiconductor Hg_1−xCd_xTe has widespread use in modern infrared detection equipment. By adjusting the ratio of Hg to Cd, the energy gap can be tailored to any desired value between 0 eV (x = 0.17) to 1.4 eV (x = 1.0). For values of x near 0.2 the electron effective mass is small, giving rise to a large spin-level splitting factor, or ɡ-value. We have examined two nonlinear optical effects which depend upon the high ɡ-value found in small gap Hg_1−xCd_xTe. These are the spin-flip Raman laser and 4-photon mixing. In each case we employ CO₂ lasers as the input source. In order to achieve resonant enhancement, we adjust the energy gap so that the absorption edge at the temperature of operation, 4 K, is near 10 μm; the required composition is Hg_0.77Cd_0.23Te.During operation as a spin-flip Raman laser, we observe 1st Stokes, 2nd Stokes and anti-Stokes signals, with a basic tunability of 3.8 cm⁻¹/kG for the 1st Stokes radiation. In the 4-photon mixing mode, we see resonant production of an output signal at frequency ω₄ = 2ω₂ − ω₁, where ω₂ and ω₁ are the frequencies of the two CO₂ pump lasers. We also observe under some conditions 6 photon mixing, i.e. ω₆ = 3ω₂ − 2ω₁.     

Topographic methods of studying defects in Hg1−xCdxTe crystals

In this paper, three topographic methods of studying defects in Hg_1−xCd_xTe crystals are described: the back-reflection method with a “whiter” beam, the scanning-reflection method with monochromatic radiation, and the glancing-incidence method with a “white” beam. Unique results obtained by these methods are shown: topographs as a referable criterion of various defects are laid down and explanation of the topographs is carried out theoretically. A set of topographs with reversed contrasts is made and the microorientation differences among the mosaic blocks are determined. Topographs of the intensive strain field in the core region of ingots prepared by the quench-recrystallization method are made and the mechanism of crystal growth is interpreted according to these topographs. In addition, typical topographs made by the three methods respectively, are shown.     

Photo-electronic phenomena in narrow gap Hg1−xCdxTe

The photo-electronic properties of Hg_1−xCd_xTe grown by molecular beam epitaxy (MBE) or by liquid phase epitaxy (LPE) were investigated using Fourier transform transmission spectroscopy, Fourier transform photoluminescence measurements, spectroscopic ellipsometry (SE), as well as magneto-optics and magnetic-field-dependent Hall studies. The investigation was carried out from liquid helium to room temperatures in the infrared band up to 10 μm. Some important impurities and defects states, including As, Sb, Ag, Fe impurities and Hg vacancy as well as their complexes in Hg_1−xCd_xTe, were carefully studied. We obtained the energy levels of the impurity states and their optical, electric and magnetic behaviors. By SE measurement, a number of very useful parameters, such as the real and the imaginary part of dielectric constant, gap energies corresponding to important critical points, were extracted. Mobility spectra and multi-carrier fitting procedure were used to separate the contributions to the measured mobility from the light holes, heavy holes, and electrons. As a result, the sign change of transverse conductivity component with applied magnetic-field is explained according to multi-carrier process.Hg_1−xCd_xTe (MCT) is one of the most important infrared materials, which is subjected to intensive studies. Its optical and electrical properties are widely used for the fabrication of high performance photoconductive and photovoltaic detectors. Some of characteristics that directly affect device performance, such as impurities, defects, as well as the lifetime of the minority carriers, remain as the major concern. Recently, the quality of the MCT material grown by MBE and LPE has been improved and accurate control over the doping levels for several dopants have been realized [1], [2], [3]. Following the progresses made in material preparation and doping, we are able to study the material systematically.In this paper, we report the recent progress made on the investigation of the electrical and optical properties of both doped and undoped MCT in our laboratory using Fourier transform spectroscopy (FTIR), photoluminescence, magneto-photoconductivity, transport measurement, as well as SE.     

The refractive index dispersion of Hg1−xCdxTe by infrared spectroscopic ellipsometry

The refractive indices of Hg_1−xCd_xTe (x=0.276, 0.309, and 0.378) bulk samples in the region below, in, and above the fundamental band gap have been measured by infrared spectroscopic ellipsometry at room temperature. A refractive index peak, in which the corresponding energy equals approximately the band gap energy, is observed for each refractive index spectrum with different compositions. Above the band gap, the refractive index drops quickly near the gap, then decreases slowly as photon energy increases. The refractive index n above the band gap is found to follow the Sellmeier dispersion relationship n²(λ)=A+B/λ²+C/λ⁴+D/λ⁶ as a function of the wavelength of light λ.     

Optical homogeneity analysis of Hg1−xCdxTe epitaxial layers: How to circumvent the influence of impurity absorption bands?

Optical absorption and photoluminescence spectroscopies are standard tools for analysis of HgHg_1−xCd_xTe epitaxial layers in terms of homogeneity of the mole-fraction (x). For technological relevant layer thicknesses of ∼10 μm, both techniques may show dissimilar results, in particular if doped layers are investigated. This is due to defect levels, which impact to the results obtained by both techniques in different ways. We systematically investigate this behavior by analyzing two sets of HgCdTe layers, one set intrinsically doped by Hg-vacancies, the other extrinsically doped by arsenic (As). A model is outlined and applied to the experimental results, which consistently explains even non-monotonous temperature-shifts of the spectra. Eventually, guidelines for optical homogeneity tests are given. While transmission measurements are most reliable, when carried out at low temperature, where the defect level are frozen out, photoluminescence provides best results at ambient temperature, where band-states are increasingly populated. Both approaches help to reveal intrinsic material properties.     

Phase differences between quantum oscillations of the magnetoresistance and the hall effect in Hg1−xMnxTe and Hg1−xCdxTe

A Landau-level broadening-dependent phase shift has been observed between the Shubnikovde Haas oscillations of the magnetoresistance and the Hall effect in a series of Hg_1−xMn_xTe and Hg_1−xCd_xTe samples. The phase shift varies between 0 and 90° and appears not to be influenced by the exchange interaction between the Mn²⁺ ions and the carriers. The results are in good agreement with the theoretical predictions for short range scattering potentials.     

Experimental evidence for the third level (А+) of Hg vacancy in Hg1−xCdxTe

Mercury vacancy in Hg_1-xCd_xTe is not a two-level (as it was supposed until now), but a three-level acceptor. A third, most shallow (1–1.5 meV) level (А⁺ state) appears due to a capture of a third hole by a neutral acceptor, after the two deeper vacancy levels (A^? and А⁰ states) are already occupied by holes. Due to a capture of nonequilibrium holes by neutral mercury vacancies (under radiation) a positive space charge region arises near an irradiated surface. This causes the anomalies of photoelectromagnetic effect, observed in р-Hg_1?xCd_xTe at T < 10–12 K.     

Optical properties of Li1−xNb1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of LiNbO₃ is reported. Li_1?xNb_1?xEu_2xO₃ solid solutions exist over a very limited range of europium concentrations (x ≤ 0.01). The solid solutions were characterized by X-ray powder diffraction and density measurements. Within the range of compositions of the solid solutions, the optical properties (emission and excitation spectra) of the Eu³⁺ ions have been characterized. The obtained optical data indicate that two equally abundant europium luminescent species exist in the stoichiometric speciments; these correspond to the Eu³⁺ ions occupying the Li⁺ and Nb⁵⁺ sites, in agreement with the replacement mechanism which is inferred from density measurements. Other Eu³⁺ luminescent species which appear to be closely related with lithium deficiency were found to exist in nonstoichiometric samples, in addition to those which are present in the stoichiometric materials.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Optical and electrical properties of flash-evaporated Pb1−xHgxSe films

Results of optical and electrical properties of flash-evaporated Pb_1−xHg_xSe films, in the composition range 0.07 ⩽ x ⩽ 0.93, show that the alloy films are formed by alloying of PbSe and HgSe. The optical band gaps of films grown on substrates at 25 and 100°C decrease linearly from 0.20 to 0.09 eV and from 0.15 to 0.065 eV, respectively, as the Hg concentration in the films is increased from 0.07 to 0.93. Activation energy values, as obtained from conductivity measurements, suggest that between 225 and 400 K the conduction results mainly from thermally-generated charge carriers, whereas at the lower temperatures (<225 K) impurity conduction dominates. Irreversible changes in the properties of the films are observed at > 400 K.     

A general optical characterization method for determining the composition of Hg1−xCdxTe samples

The rate of variation of transmission as a function of frequency or energy exhibits a maximum in the fundamental absorption region of the alloy semiconductor Hg_1−xCd_xTe. The maximum steepness occurs in the tail region of the absorption edge and its frequency can be correlated analytically with the composition. From the experimental measurements of room temperature IR transmission on both bulk grown and epilayers of Hg_1−xCd_xTe, it is shown that an analysis of the transmission curve at (dT_r/dE)_max leads to the determination of composition and compositional uniformity in the given sample.     

Analysis of the R0A product in n+-p Hg1−xCdxTe photodiodes

The influence of different junction current components (diffusion current for radiative and Auger 7 recombination mechanisms, tunneling and depletion layer currents) on the R₀A product of n⁺-p -Hg_1−xCd_xTe photodiodes is considered. The considerations are carried out for the 77–300 K temperature region and 1–15 μm cutoff wavelength. Optimum doping concentrations in the p-type region of n⁺-p abrupt junctions are determined, taking into account the influence of the tunneling current and of a fixed surface charge density of the junction passivation layer. Results of calculations are compared with experimental data reported by many authors. An attempt is made to explain the discrepancy between theoretical calculations and experimental data.     

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnS_xSe_1–x polycrystalline thin films were prepared by evaporation in vacuum of 10^–5 Torr. The molecular fractionx varied in the region ox1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficient) were determined in the wavelength range 300–1600nm. A plot representing ₂=f(hv) shows that the ZnS_xSe_1–x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+. The variation in eitherE orE+ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.