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.     

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.     

Optical properties of Hg1−xCdxTe

Optical properties of Hg_1−xCd_xTe are summarized in this study. Based on Penn-like models, the Moss relation and the Wemple and DiDomenico approach, calculations of energy gap, plasmon energy, Fermi energy, oscillator strength and electronic polarizability have been made. Comparisons are made with the data available in the literature. Details of the dependency of the properties on composition are presented.     

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.     

Structural fluctuations and randomness in GaAsAlxGa1−xAs superlattices

We discuss the use of X-ray and Raman scattering to probe structural disorder in aperiodic GaAsAl_xGa_1−xAs superlattices, including random and quasiperiodic examples. Evidence is found for the presence of monolayer-thick steps at the interfaces. The X-ray data appear far more sensitive to this type of disorder than the acoustic phonon spectra obtained by Raman scattering.     

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.     

The performance of Hg1−xMnxTe photodiodes

The intrinsic carrier concentration and carrier lifetime in Hg_1−xMn_xTe are calculated for the temperature range 77–300 K and the compositional range 0.07 ⩽ x ⩽ 0.20. The influence of different junction current components on the R₀A product of p⁺−n photodiodes at 77 K is analysed. The highest observed R₀A values are determined by the generation-recombination current in the depletion layer. The comparison between results of calculations and the experimental data shows potential possibilities for constructing higher quality Hg_1−xMn_xTe photodiodes.     

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.     

Unique features of the magnetic susceptibility of Hg1−xMnxTe1−ySey

The magnetic susceptibility of the new semimagnetic solid solutions Hg_1–xMn_xTe_1–ySe_y (0Mn = f(T) are caused by the presence within the specimen of Mn-Te-Mn-Te, Mn-Se-Mn-Se, and Mn-Te-Mn-Se type clusters, in which indirect exchange action of an antiferromagnetic character occurs among the Mn atoms by means of the Te or Se atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 60–62, April, 1991.     

Properties of Hg implanted Hg1−x Cd x Te infrared detectors

Experimental performance parameters of Hg implanted Hg_1?x Cd _x Te photovoltaic detectors are analyzed. At 77K, for 8–14 μm band, a comparison is made between performances and theoretical ultimate diffusion limits in low frequency direct detection. Experimental features are well-explained by a model based on the Auger band-to-band process for carrier recombination. Peak detectivities exceeding 10¹¹ cm Hz^1/2W^?1, external quantum efficiencies as high as 90%, and zero-bias resistance-area products better than 1 Ω·cm² have been achieved in devices with 12 μm cutoff wavelengths. In the 3–5 μm band performances are far from the diffusion limit. Notwithstanding, at 77K zero-bias resistance-area products are better than 10⁴Ω·cm² and detectivities of the order of 10¹² cm Hz^1/2W^?1 were observed at 5 μm. Predominant generation-recombination contribution are present at room temperature in 1–1.3 μm photodiodes whose detectivities, primarily limited by the Johnson noise, at 1.3 μm are higher than 10¹¹ cm Hz^1/2W^?1 at 300 K. The high frequency response of the photodiodes is also discussed. Response times as low as 0.5 ns are reached despite some limitations arising from the implanted layer sheet resistance.     

Characterization of flash evaporated Hg1−xZnxTe films

Results of structural, optical and electrical properties of flash-evaporated Hg_1−xZn_xTe films. in the composition range 0.08 ⩽ x ⩽ 1, show that the films are formed by alloying HgTe and ZnTe. The optical band gap of these films increases from 0.04 to 2.26 eV with an increase in Zn concentration from 0.08 to 1.0. The band gap variation is not linear but shows a “bowing effect” familiar to pseudobinary solid solutions. An increase in resistivity with an increase in the substrate temperature can be related to larger grain size at the higher substrate temperatures in these p-type films. The films exhibit higher conductivity for the Hg-rich films which decreases linearly as the Zn concentration is increased.     

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.     

Index of refraction of GaAsAlxGa1−xAs superlattices and multiple quantum wells

Recent research of superlattices and multiple quantum wells has generated considerable interest in the optical waveguiding properties of these structures for optoelectronic applications. As a result we present a theoretical study of the index of refraction of superlattices and determine its variation as a function of frequency and the superlattice parameters, i.e., layer width and AlAs composition. Γ-region exciton and valence-band mixing effects are included in the model. It is found that these two effects have an important influence on the value of the index of refraction and that superstructure effects rapidly decrease for energies greater than the superlattice potential barriers. Because of the quasi-two-dimensional character of the Γ-region excitons, our results indicate that the superlattice index of refraction can vary by ∼ 2% at the quantized, bound-exciton, transition energies. Overall, the theoretical results are in good agreement with the experimental data.     

The behavior of Shubinikov-de Haas oscillations in Hg1−x−yCdxMnyTe alloys

The transverse and longitudinal magnetoresistance are measured on a quaternary semimagnetic semiconductor, n-type Hg_1−x−yCd_xMn_yTe with 0.01 < x < 0.23 and y ≈ 0.14, at different temperatures. The peaks of the Landau-spin states are assigned using the selection rules already developed for HgCdTe. The Landau-spin states are modified by an exchange interaction between the Mn²⁺ ions localized spin and the band electrons, thus it becomes possible to compare directly the extent of the spin modification between the negative gap side and the open gap side of the new alloys.     

Specific features of the properties of Hg-Te lattice vibrations in Cd1 − y Hg y Te alloys enriched with CdTe

In alloys (solid solutions) Cd_{1 ? y} Hg _y Te enriched with CdTe, the properties of Hg-Te lattice vibrations are determined by a single-well lattice potential for the Hg atom located at the center of the anion tetrahedron. The HgTe-enriched alloys retain the anomalous properties of Hg-Te lattice vibrations, which are determined by a double-well lattice potential for the Hg atom located in the off-center position at low temperatures; a shallower well of the potential for the Hg atom located at the center of the anion tetrahedron is filled upon thermal activation. The polarizability of the ion pair in the TO mode of Hg-Te lattice vibrations with the Hg atom located at the center of the anion tetrahedron for the CdTe-enriched alloys considerably exceeds the polarizability of the pair with the off-center Hg atom for the HgTe-enriched alloys.     

Imperfection photoconductivity peak related to the Urbach's absorption edge in Cd1−xMnxTe and CdTe

The narrow photoconductivity (PC) peak observed as the main feature of the PC spectrum in many Cd_1−xMn_xTe and CdTe crystals is ascribed to the transitions between ionized shallow hydrogen-like acceptors and the conduction band. The shape and the position of this large extrinsic PC peak is determined by a steep Urbach's edge of the intrinsic absorption which plays a parasitic role.     

Field inversion of the sign of the hall coefficient in Hg1−xMnxTe1−ySey

In the temperature range T=77–300 K and H1–18 kOe, the dependence of the Hall coefficient (R_H) of crystals of Hg_1–xMn_xTe_1–ySe_y (0H=f(H), as well as the inversion of the sign of R_H as H increases for Hg_1–xMn_xTe_1–ySe_y (x0.1 and y=0.05) are explained by the presence in the samples of three types of charge carriers: holes, and heavy and light electrons.Chernovitskii University. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 4, pp. 11–14, April, 1994.     

Galvanomagnetic infrared-luminescence of varying-gap CdxHg1−xTe/CdTe structures

Luminescence of varying-gap epitaxial Cd_xHg_1−xTe films at T = 295 K under conditions of magnetoconcentration effect were investigated theoretically and experimentally. Field and spectral characteristics of the positive and negative luminescence were studied. The use of varying-gap structures in combination with the magnetoconcentration effect for the excitation of luminescence in narrow-gap semiconductors at high temperatures is shown to be promising. In this case, high radiation power and the possibility of controlling the luminescence spectrum are demonstrated.     

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.