First-principles study of the structural,electronic and optical properties of the cubic triangular quaternary ZnxCdyHg1-x-yTe alloys under hydrostatic pressure

In the present computational study, we have explored the structural, electronic and optical properties of ZnTe, CdTe and HgTe binary compounds and their ternary alloys Zn_xCd_1-xTe, Zn_xHg_1-xTe and Cd_xHg_1-xTe as well as their ordered quaternary Zn_xCd_yHg_1-x-yTe alloys using the full potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory. We have numerically estimated the total energies, the lattice parameters, the bulk moduli and their first pressure derivative using the generalized gradient approximation (GGA). The band structure is computed using the modified Becke-Johnson (TB-mBJ) approximation. Results of our study show a nonlinear dependence of the composition on the lattice constant, bulk modulus and band gap for the binary and ternary compounds as well as for the quaternary alloys. Additionally, the dielectric function, the refractive index and the loss energy were also reported. The pressure effect on the band gap energy and optical properties were also investigated and reported. Our results are in good agreement with experimental values and theoretical data available in the literature.     

Low-frequency optical lattice vibrations in Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te alloys

The lattice reflection spectra of the Hg_{1 ? x} Cd _x Te (x = 0.06–0.70) alloys measured in the low-frequency range of optical vibrations (the region of the anomalous mode of Hg-Te vibrations in HgTe) at room temperature are interpreted. The low-frequency modes observed at frequencies of 98, 105, and 112 cm^?1 for all compositions of the Hg_{1 ? x} Cd _x Te alloy are assigned to the modes of Hg-Te vibrations, as was previously done for modes of Cd-Te vibrations in the quasi-molecular approximation. According to the double-well potential model for the Hg atom in the crystal lattice of the alloy, the Hg atom either can occupy the center of the anion tetrahedron or can be located in the off-center position. The fundamental strong mode of Hg-Te vibrations at a frequency of about 120 cm^?1 (at T = 300 K) corresponds to the vibrations of the off-center Hg atom, and the low-frequency vibration modes correspond to the vibrations of the Hg atom located at the center of the anion tetrahedron.     

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.     

Ion-plasma treatment of Cd1 ? x Zn x Te (x ～ 0.04) single crystals and application of antireflection diamond-like carbon films

The influence of ion-plasma treatment of Cd_{1 ? x} Zn _x Te (x ?? 0.04) single crystals, which precedes the deposition of an antireflection coating (a diamond-like a-C: H: N film) on the optical properties of the resulting structure, is investigated. Treatment is carried out in an argon, nitrogen, and hydrogen plasma. As follows from the optical properties of the a-C: H: N/Cd_{1 ? x} Zn _x Te, the structure treated in an argon plasma offers the highest transmission. A model of the optical system is proposed that takes into account the formation of transition layers after ion-plasma treatment and the effect of the type of ion on their characteristics. The absorption spectra of the a-C: H: N/Cd_{1 ? x} Zn _x Te system are analyzed, and bands assigned to the stretching and bending vibrations of C-H bonds, as well as to C-C and C=O bonds, are revealed. The presence of these bonds should be taken into account in designing and synthesizing these optical coatings.     

Effects of in situ thermal annealing on the structural, optical, and electrical properties in Hg0.7Cd0.3Te epilayers grown on CdTe buffer layers

Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) transmission, and Hall effect measurements were performed to investigate the structural, optical, and electrical properties of as-grown and in situ-annealed Hg_0.7Cd_0.3Te epilayers grown on CdTe buffer layers by using molecular beam epitaxy. After the Hg_0.7Cd_0.3Te epilayers had been annealed in a Hg-cell flux atmosphere, the SEM images showed that the surface morphologies of the Hg_0.7Cd_0.3Te thin films were mirror-like with no indication of pinholes or defects, and the FTIR spectra showed that the transmission intensities had increased in comparison to that of the as-grown Hg_0.7Cd_0.3Te epilayer. Hall-effect measurements showed that n-Hg_0.7Cd_0.3Te epilayers were converted to p-Hg_0.7Cd_0.3Te epilayers. These results indicate that the surface, optical, and electrical properties of the Hg_1 − xCd_xTe epilayers are improved by annealing and that as-grown n-Hg_1 − xCd_xTe epilayers can be converted to p-Hg_1 − xCd_xTe epilayers by in situ annealing.     

Magnetic field dependence of the energy levels in CdTe-Cd(Mn)Te double quantum well structures

We show that if the valence band offset between CdTe and Cd_1-xMn_xTe alloys is not too large, magneto-photoluminescence experiments performed on CdTe-Cd_1-xMn_xTe-Cd_1-yMn_yTe double quantum wells should yield direct informations on this offset: the structure changes from a type I configuration (where electron and hole are mostly localized within CdTe layer) prevailing at zero magnetic field to a type II configuration (electron in CdTe layer, hole in Cd_1-yMn_yTe layer) at large magnetic field.     

Application of reflection-absorption spectroscopy to quaternary solid solutions

In the present work the author describes a method for investigating phonon spectra of thin films of semiconductor solid solutions that is based on reflection-absorption spectroscopy in the far IR region. Analysis of spectra is carried out for two radiation polarizations (of the s- and p-type) and for the following experimental conditions: the spectral region 30–250 cm⁻¹ and an angle of incidence of 45°. By comparing theoretical and experimental spectra for quaternary solid solutions of Zn_xCd_yHg_1−x−yTe (ZCMT), the frequencies of LO- and TO-phonons are found and the three-mode behavior of the phonon spectra of ZCMT is established. Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998. Institute of Physics, Pedagogical University, 16A, Reitan Str., Rzeszów, 35–310, Poland. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 669–671, September–October, 1999.     

A model of enhanced STM current due to semiconductor optical absorption

We present a simple model for the change in tunneling current between a semiconductor surface and a metal tip under spectroscopic illumination in a scanning tunneling microscope. This model predicts a sharp increase in the tunneling current due to the increase in the conduction band carrier density when the photon energy exceeds the optical band gap. The tunneling current for a large diffusion length has a more pronounced onset than for a small length. Our model should provide, when combined with experiments, a method of determining localized effective stoichiometry, and therefore provides a localized alternative to the use of optical absorption measurements. Our theoretical tunneling current versus photon energy curves are in good qualitative agreement with the existing experimentally measured curves for Si, GaAs, and InP obtained by Qian and Wessels. In addition, we have examined the effects of temperature, surface recombination velocity, and degeneracy on our theoretical results for the Hg_1−xCd_xTe, Hg_1−xZn_xTe, and Hg_1−xZn_xSe ternary narrow gap semiconductor systems.     

Terahertz spectroscopy of quantum-well narrow-bandgap HgTe/CdTe-based heterostructures

The energy spectra of quantum-well narrow-bandgap Hg_{1 ? y} Cd _y Te/Cd _x Hg_{1 ? x} Te heterostructures have been studied. The dependences of the effective cyclotron mass on the density (in classical magnetic fields) and the transition energy (in quantizing fields) have been obtained from the cyclotron resonance measurements. These dependences confirm the near-linear dispersion law for the electrons with small mass at the band bottom (the minimum cyclotron mass measured is 0.003 m ₀). The interband photoconductivity of the CdHgTe-based structures with the long-wavelength photoresponse edge lower than 6 meV has been demonstrated.     

Photoluminescence of HgCdTe heterostructures with multiple quantum wells

An analysis of experimental data obtained by various authors via recording infrared (IR) photoluminescence (PL) in heteroepitaxial structures based on solid Cd _x Hg_1?x Te (CHT) solutions with multiple quantum wells is presented. A theoretical analysis of optical transition energies is conducted based on a self-consistent solution to the Poisson and Schröbinger equations for a quantum well. Experimental data obtained at different temperatures, pump powers, and excitation wavelengths are analyzed.     

Electro-Optic Coefficient Measurements for Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> Te Single Crystals at 1550 nm Wavelength

Nonlinear optical materials of the type Zn _x Cd_1-x Te single crystals, where x = 0.0, 0.2, 0.4, 0.6, 0.7, 0.8, and 1.0, have been grown by the Bridgman method, using a vertical furnace. We have investigated the electro-optic (E-O) coefficient and refractive index of Zn _x Cd_1-x Te single crystals at optical communication wavelength (1550 nm). In the case of CdTe crystal, the E-O coefficient was 15.5 × 10^-12 m/V, which is the biggest among the E-O coefficients of Zn _x Cd_1-x Te crystals. The E-O modulation signal was very big in low frequency range (DC-100kHz), but the signal amplitude became smaller as we increased frequency above 100 kHz. We also found the acousto-optic modulation at CdTe single crystals.     

Existence of a Cu3Au-type ordered structure in CdxZn1−xTe epilayers grown on (100) GaAs substrates

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the spontaneously ordered structure in Cd_xZn_1−xTe epilayers grown on GaAs (100) substrates. The SADP showed superstructure reflections with symmetrical intensity, and the high-resolution TEM (HRTEM) micrographs showed doublet periodicity in the contrast of the {100} lattice planes. The results of the SADP and the HRTEM measurements showed a Cu₃Au-type ordered structure was formed in the Cd_xZn_1−xTe epilayer. The present results can help improve understanding of the Cu₃Au-type ordered structures in Cd_xZn_1−xTe epilayers grown on GaAs substrates.     

Raman scattering and optical normal vibrations of the Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>x</Subscript>Se crystal lattice

The nonresonant spectra of the Raman scattering in Zn_1?xCd_xSe crystals (0≤ x≤1) are investigated. The mode doublet was found to gradually shift with increasing x from the longitudinal-transverse frequencies of ZnSe to the longitudinal-transverse frequencies of CdSe. Furthermore, an additional branch of weak modes was observed between the doublet components. The frequencies of this branch are shown to correspond to the frequencies of the cadmium impurity vibrations in the ZnSe lattice at x=0 and the zinc impurity vibrations in the CdSe lattice at x=1. The concentration dependences of the modes and atomic displacements are analyzed by using the isodisplacement model that includes the interaction between ZnSe-and CdSe-like vibrations. It is concluded that a one-mode behavior with the formation of an additional weak branch in the longitudinal-transverse splitting region is characteristic of this system.     

Magnetic-field-induced change in optical transmission spectra of Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te epitaxial films

The optical transmission spectra of epitaxial Hg_{1 − x} Cd _x Te 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 Hg_{1 − x} Cd _x Te films on the composition gradient of the solid solution.     

Ab-initio investigations of structural, electronic, magnetic and optical properties of ferromagnetic Cd1-xMnxTe

We report ab-initio calculations of the structural, electronic, magnetic and optical properties of the alloy Cd_1-xMn_xTe as a function of the Mn concentration ‘x’. Ab-initio calculations are based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice constants of the Cd_1-xMn_xTe alloys exhibit Vegard's law downward bowing parameter. For the minority spin channel the Fermi level shifts toward higher energy with the value of ‘x’ in Cd_1-xMn_xTe. The spin-exchange splitting energy Δ_x(d) increases with increasing ‘x’ in Cd_1-xMn_xTe and the values of p-d exchange splitting energy Δ_x(pd) of Cd_1-xMn_xTe show that the effective potential for the minority spin is more attractive than that for the majority spin. The values of exchange constants N₀α and N₀β obtained for Cd_1-xMn_xTe are in agreement with the reported data. The magnetic moment per Mn atom reduces from its free space charge value of 5μ_B to around 4μ_B due to p-d hybridization and this results into an appearance of small local magnetic moments on the non-magnetic Cd and Te sites. The absorption threshold shifts toward higher energy and the static refractive index decreases with the increasing value of ‘x’ in Cd_1-xMn_xTe.     

Analysis of some magnetic properties of diluted magnetic semiconductors including long-range interactions

Specific heat, susceptibility and magnetization of four diluted magnetic semiconductors (Cd_1−xMn_xTe, Cd_1−xMn_xSe, Hg_1−xMn_xTe and Hg_1−x^- Mn_xSe) are analysed with a model which includes long-range interactions. Calculations based on an extended version of a nearest neighbour pair approximation yield a fair agreement with the experimental data. No adjustment of the statistical distributions of magnetic ions is found to be necessary.     

Rearrangement of the vibration spectrum of Zn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se solid solutions with varying composition

Experimental lattice reflection spectra of Zn_{1 ? x} Cd _x Se bulk crystals are interpreted and analyzed for compositions x = 0.08, 0.21, and 0.30 crystallizing in the cubic zinc blende structure and x = 0.45, 0.65, and 0.80 crystallizing in the hexagonal wurtzite structure. The lattice vibration modes for all the compositions are divided, according to their frequency, into ZnSe- and CdSe-like vibrations. On a phase transition at x > 0.3, a marked jump (7–8 cm^?1) in the frequency of lattice modes is observed, but the largest changes take places in the mode oscillator strengths. However, the summed oscillator strength of the ZnSe- and CdSe-like vibration modes changes only slightly during the phase transition, which corresponds to the double-mode type of rearrangement of the vibration spectrum of Zn_{1 ? x} Cd _x Se solid solutions with varying x. An analysis of the lattice modes is accompanied by the comparison with the parameters of the lattice modes of Zn_{1 ? x} Cd _x Se (x = 0–0.55) epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy.     

Coupled subband-phonon resonances in the far-infrared reflection from Hg1−xCdxTe surfaces

Subband resonances coupled to the lattice polarization are observed in the far-infrared reflection from surfaces of Hg_1?xCd_xTe. A substantial enhancement of signal amplitude occurs for frequencies just above ω_LO. At such a frequency the resonances show a large splitting of the collective-mode energy ?_nm and the single-particle mode energy E_nm.     

Variations of the band gap of Hg1 ? x Cd x Te solid solutions in response to a magnetic field

The optical transmission spectra of Hg_{1 ? x} Cd _x Te epitaxial layers with a magnetization gradient have been investigated experimentally. The magnetization gradient has been artificially created by the opposite arrangement of the poles of the magnets. The possibility of transforming the shape of the spectral dependences of the optical transmission of the HgCdTe films after their exposure to a magnetic field has been demonstrated. Assumptions about the mechanism underlying this phenomenon have been made.     

Influence of doping of mercury atom(s) on optoelectronic properties of binary cadmium chalcogenides - A density functional theory based investigation with different exchange-correlation functionals and including spin-orbit coupling

Influence of doping of mercury atom(s) on optoelectronic properties of binary cadmium chalcogenides have been investigated theoretically by designing the mercury doped cadmium chalcogenide ternary alloys in B3 phase at some specific Hg-concentrations and studying their optoelectronic properties using DFT based FP-LAPW methodology. The structural properties are computed using WC-GGA, while spin-orbit coupling included electronic and optical properties are computed using TB-mBJ, EV-GGA, B3LYP and WC-GGA exchange-correlation functionals. In addition, electronic properties of mercury chalcogenides are calculated precisely using the GGA+U functionals. The concentration dependence of lattice parameter and bulk modulus of each of the Hg_xCd_1−xS, Hg_xCd_1−xSe, Hg_xCd_1−xTe alloy systems show almost linearity. For each of the alloy systems, band gap decreases almost linearly with increase in Hg-concentration in the unit cell and contribution from charge exchange to the band gap bowing is larger than that from for each of the volume deformation and structural relaxation. Also, covalent bonding exists between different constituent atoms in each compound. Optical properties of each specimen are computed from their spectra of dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity, optical absorption coefficient and energy loss function. Several calculated results have been compared with available experimental and other theoretical data.