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.     

Tight-binding studies of the electronic band structure of GaAlN and GaInN alloys

The semi-empirical tight-binding (TB) approach with an sp³s^* orbital basis is used to investigate the electronic band structure of cubic Ga_1-xAl_xN and Ga_1-xIn_xN alloys. The spin–orbit splitting in the Hamiltonian and first- and second-neighbour interactions are considered to explore the gap evolution as a function of the concentration x. The TB parameters used in the studies were obtained within the virtual crystal approximation using those of the binary compounds AlN, GaN, and InN. The binary parameters are calculated by applying an empirical TB Hamiltonian taking into account first- and second-neighbour interactions. A direct to indirect gap transition is obtained for Ga_1-xAl_xN at x=0.60. In contrast, the Ga_1-xIn_xN alloys exhibit direct-gap characteristics as a consequence of being formed by the direct-gap binary compounds GaN and InN. PACS 71.20.Nr; 71.23.-k; 71.55.Eq     

First-principles calculations of the structural,electronic, optical and thermal properties of the BNxAs1–x alloys

The present paper aims to study the structural, electronic, optical and thermal properties of the boron nitride (BN) and BAs bulk materials as well as the BN_xAs_1–x ternary alloys by employing the full-potential-linearised augmented plane wave method within the density functional theory. The structural properties are determined using the Wu–Cohen generalised gradient approximation that is based on the optimisation of the total energy. For band structure calculations, both the Wu–Cohen generalised gradient approximation and the modified Becke–Johnson of the exchange-correlation energy and potential, respectively, are used. We investigated the effect of composition on the lattice constants, bulk modulus and band gap. Deviations of the lattice constants and the bulk modulus from the Vegard’s law and the linear concentration dependence, respectively, were observed for the alloys where this result allows us to explain some specific behaviours in the electronic properties of the alloys. For the optical properties, the calculated refractive indices and the optical dielectric constants were found to vary nonlinearly with the N composition. Finally, the thermal effect on some of the macroscopic properties was predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account.     

First-principles investigation of the structural,optoelectronic and thermodynamic properties of InAsxP1-x ternary alloys under hydrostatic pressure effect

The main objective of our work is the study of structural, optoelectronic and thermodynamic properties of InAs_xP_1-x alloys in the zinc-blende structure using the full potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). Different exchange correlation potentials were used, as well as the local density approximation (LDA) and the generalized gradient approximation (GGA) parameterized by Perdew–Burke–Ernzerhof (PBE-GGA) and PBE sol-GGA of Perdew, to estimate structural properties such as lattice parameters, the bulk modulus and its first pressure derivative. For electronic properties, the Tran-Blaha modified Becke–Johnson potential (TB-mBJ) was used for density of states (DOS) and band structure calculations. The results show that the compounds of interest are semiconductors with direct band gaps for the full range of x compositions and that the optical band gap decreases from 1.58 to 0.41 eV with increasing As concentrations. The obtained results show a good agreement with experimental and theoretical data found in the literature. In addition, we have investigated the dielectric function as well as the refractive index and the reflectivity. The electronic and optical properties were studied under hydrostatic pressure (P = 0, 5, 10, 15, 20, and 25 GPa), and it was found that the band gaps of the binary compounds change from a direct to an indirect harmonic Debye model was used, which takes into account the effect of pressure P and temperature T on the lattice parameter, to explore the heat capacity, the Debye temperature and the entropy under pressures ranging from 0 to 20 GPa and temperatures ranging from 0 to 1200 K.     

The kinetics of solidification of Al-Si eutectic alloys under high pressure

Abstract

The kinetics of crystallization of eutectic alloys Al_100-xSi_x (χ=12, 18, 26 at.%) was investigated under pressures p=0.5, 2.5, 4.5 GPa. The values of supercooling and average grain size of silicon crystals were determined for alloys quenched from melt under different pressures with the cooling rate 10³ K/s. The data obtained were used to evaluate quantitatively the pressure dependences of surface tension (between melt and crystal) and activation energy of crystal growth which, in turn, have made it possible to determine the relative change of nucleation frequency and of the rate of crystal growth with pressure.

The possibility is shown, based on the investigation of mechanical properties of the samples obtained under high pressure, for improving the strength and the plasticity of A1-Si alloys by means of high pressure-high temperature treatment.     

Structural and optical properties of Al1-xInxN epilayers on GaN template grown by metalorganic chemical vapor deposition

This paper reports that Al1 xInxN epilayers were grown on GaN template by metalorganic chemical vapor de-position with an In content of 7%-20%. X-ray diffraction results indicate that all these Al1 xInxN epilayers have a relatively low density of threading dislocations. Rutherford backscattering/channeling measurements provide the exact compositional information and show that a gradual variation in composition of the Al1 xInxN epilayer happens along the growth direction. The experimental results of optical reflection clearly show the bandgap energies of Al1 xInxN epilayers. A bowing parameter of 6.5 eV is obtained from the compositional dependence of the energy gap. The cathodoluminescence peak energy of the Al1 xInxN epilayer is much lower than its bandgap, indicating a relatively large Stokes shift in the Al1 xInxN sample.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

Computational investigations of the band structure and thermodynamic properties of calcium-doped BaS using the FP-LAPW approach

In this work, the full-potential linearized augmented plane wave (FP-LAPW) method was used to calculate the structural, electronic, thermal and thermodynamic properties of BaS and CaS compounds and their ternary mixtures, Ba_1?xCa_xS. The local-density approximation (LDA), the Wu-Cohen generalized gradient approximation (WC-GGA) and the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) were used as the exchange-correlation potential. Moreover, the modified Becke–Johnson approximation was also used for the band structure calculations. We examined the composition effect on the lattice constants, bulk modulus and band gap. The microscopic origins of the band gap bowing were characterized in detail using the approach of Zunger and colleagues. Pressure and temperature effects on the lattice parameter, heat capacity, Debye temperature, Grüneisen parameter, and thermal expansion coefficient were predicted using the quasi-harmonic Debye model. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing, ?H_m, and the phase diagram. It was shown that these alloys are stable at high temperature.     

Structural and electronic properties of wurtzite,zincblende and rocksalt Al1 − xInxN ternary alloys at ambient and high pressure

ABSTRACT

Although AlInN is originally a wurtzite structure, zincblende and rocksalt are other potential phases. It will be interesting to have a comparative study of the physical properties of this compound in various phases. A DFT-based study of wurtzite, zincblende and rocksalt phases of AlInN alloys is carried out. Structural (lattice parameter, bulk modulus) and electronic properties (energy band gap, and electron effective mass) of the Al_{1??? x}In_xN alloys are investigated, at ambient pressure, throughout the whole range of indium contents for all considered phases. High pressure effects on the studied parameters are also examined, with the phase transition pressures computed for different values of In concentrations, and compared with available data. Structural density functional calculations are performed with Perdew–Burke–Ernzerhof gradient-corrected functional for solids (PBEsol), while electronic structure is computed with the modified Becke–Johnson (TB-mBJ) potential exchange to ensure a better accuracy of calculated the band gaps. Alloy randomness is taken into account using a special quasi-random structure.     

Energy gaps,charge distribution and optical properties of AlxIn1−xSb ternary alloys

The electronic and optical properties of Al_xIn_1−xSb ternary alloys have been investigated using a pseudopotential approach within the virtual crystal approximation. The effect of alloy disorder on the studied properties has been examined and found to be weak. The extent of the direct-to-indirect band gap transition is found to occur at x = 0.73. Our results agree well with those reported in the literature. Trends in bonding and ionicity are discussed by means of the electron charge distribution. The present study may be a useful information for mid-infrared inter band cascade lasers applications and other antimonide device structures.     

First-principles study of structrural and corrected band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO systems

A first-principles method based on density functional theory(DFT),a generalized gradient approximation(GGA),and a projector-augmented wave(PAW) are used to study the structual and band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO(0 ≤x≤1) ternary alloys.By taking into account all of the possible structures,the band gaps of Zn1-xCdxO and Zn1-xMgxO alloys are corrected and compared with experimental data.     

Structural and electronic properties of GaN x As1−x alloys

The structural and electronic properties of cubic GaN _x As_1−x with N-concentration varying between 0.0 and 1.0 with step of 0.25 were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have used the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used for the band-structure calculations. The structural properties of the binary and ternary alloys were investigated. The electronic band structure, total and partial density of states as well as the electron charge density were determined for both the binary and their related ternary alloys. The energy gap of the alloys decreases when we move from x=0.0 to 0.25; then it increases by a factor of about 1.8 when we move from 0.25 to 0.5, 0.75 and 1.0 using EVGGA. For both LDA and GGA moving from x=0.0 to 0.25 causes the band gap to close, showing the metallic nature of the GaN_0.25As_0.75 alloy. When the composition of N moves through x=0.25, 0.5, 0.75 and 1, the band gap increases.     

Predictions of mechanical and thermodynamic properties of Mg17Al12 and Mg2Sn from first-principles calculations

Using first-principles calculations, we predict mechanical and thermodynamic properties of both Mg₁₇Al₁₂ and Mg₂Sn precipitates in Mg–Al–Sn alloys. The elastic properties including the polycrystalline bulk modulus, shear modulus, Young’s modulus, Lame’s coefficients and Poisson’s ratio of both Mg₁₇Al₁₂ and Mg₂Sn phases are determined with the Voigt–Reuss–Hill approximation. Our results of equilibrium lattice constants agree closely with previous experimental and other theoretical results. The ductility and brittleness of the two phases are characterized with the estimation from Cauchy pressure and the value of B/G. Mechanical anisotropy is characterized by the anisotropic factors and direction-dependent Young’s modulus. The higher Debye temperature of Mg₁₇Al₁₂ phase means that it has a higher thermal conductivity and strength of chemical bonding relative to Mg₂Sn. The anisotropic sound velocities also indicate the elastic anisotropies of both phase structures. Additionally, density of states and Mulliken population analysis are performed to reveal the bonding nature of both phases. The calculations associated with phonon properties indicate the dynamical stability of both phase structures. The temperature dependences of thermodynamic properties of the two phases are predicted via the quasi-harmonic approximation.     

Optical properties of TlGaxIn1-xSe2-layered mixed crystals (0.5 ≤ x ≤ 1) by spectroscopic ellipsometry,transmission, and reflection measurements

The layered semiconducting TlGa_xIn_1-xSe₂-mixed crystals (0.5?≤?x?≤?1) were studied for the first time by spectroscopic ellipsometry measurements in the 1.2–6.2?eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index, and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The effect of the isomorphic cation substitution (indium for gallium) on critical point energies in TlGa_xIn_1-xSe₂ crystals was established. Moreover, the absorption edge of TlGa_xIn_1-xSe₂ crystals have been studied through the transmission and reflection measurements in the wavelength range of 500–1100?nm. The analysis of absorption data revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps decrease with the increase of indium content in the studied crystals.     

A convenient band-gap interpolation technique and an improved band line-up model for InGaAlAs on InP

The band-gap energy and the band line-up of InGaAlAs quaternary compound material on InP are essential information for the theoretical study of physical properties and the design of optoelectronics devices operating in the long-wavelength communication window. The band-gap interpolation of In_1−x−y Ga _x Al _y As on InP is known to be a challenging task due to the observed discrepancy of experimental results arising from the bowing effect. Besides, the band line-up results of In_1−x−y Ga _x Al _y As on InP based on previously reported models have limited success by far. In this work, we propose an interpolation solution using the single-variable surface bowing estimation interpolation method for the fitting of experimentally measured In_1−x−y Ga _x Al _y As band-gap data with various degree of bowing using the same set of input parameters. The suggested solution provides an easier and more physically interpretable way to determine not only lattice matched, but also strained band-gap energy of In_1−x−y Ga _x Al _y As on InP based on the experimental results. Interpolated results from this convenient method show a more favourable match to multiple independent experiment data sets measured under different temperature conditions as compared to those obtained from the commonly used weighted-sum approach. On top of that, extended framework of the model-solid theory for the band line-up of In_1−x−y Ga _x Al _y As/InP heterostructure is proposed. Our model-solid theory band line-up result using the proposed extended framework has shown an improved accuracy over those without the extension. In contrast to some previously reported works, it is worth noting that the band line-up result based on our proposed extended model-solid theory has also shown to be more accurate than those given by Harrison’s model.     

Structural,elastic, electronic and thermal properties of M2SbP (M = Ti,Zr and Hf)

The structural, elastic, electronic and thermal properties of M₂SbP (M = Ti, Zr and Hf) were studied by means of a pseudo-potential plane-wave method based on the density functional theory within both the local density approximation and the generalised gradient approximation. The optimised zero-pressure geometrical parameters, i.e. the two unit cell lengths (a, c) and the internal coordinate (z), were in good agreement with available experimental and theoretical data. The effect of high pressure, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than along c-axis. The anisotropic independent elastic constants were calculated using the static finite strain technique. Numerical estimations of the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature for ideal polycrystalline M₂SbP aggregates were performed in the framework of the Voigt–Reuss–Hill approximation. The calculated band structures show that all studied materials are electrical conductors. Analysis of the atomic site projected densities showed that the bonding is of covalent–ionic nature with the presence of metallic character. The density of states at the Fermi level is dictated by the transition metal d–d bands; the Sb element has little effect. Thermal effects on some macroscopic properties of M₂SbP were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the volume expansion coefficient, heat capacity and Debye temperature with pressure and temperature in the ranges 0–50 GPa and 0–2000 K were obtained successfully.     

First principles study on structural,electronic and optical properties of Ga1−xBxP ternary alloys (x = 0, 0.25, 0.5, 0.75 and 1)

The structural, electronic and optical properties of GaP, BP binary compounds and their ternary alloys Ga_1?xB_xP ($x=0.25$, 0.5 and 0.75) have been studied by full-potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory (DFT) as implemented in WIEN2k package. Local density approximation (LDA) and generalized gradient approximation (GGA) as proposed by Perdew–Burke–Ernzerhof (PBE), Wu–Cohen (WC) and PBE for solid (PBESol) were used for treatment of exchange-correlation effect in calculations. Additionally, the Tran–Blaha modified Becke–Johnson (mBJ) potential was also employed for electronic and optical calculations due to that it gives very accurate band gap of solids. As B concentration increases, the lattice constant reduces and the energy band gap firstly decreases for small composition x and then it shows increasing trend until pure BP. Our results show that the indirect–direct band gap transition can be reached from $x=0.33$. The linear optical properties, such as reflectivity, absorption coefficient, refractive index and optical conductivity of binary compounds and ternary alloys were derived from their calculated complex dielectric function in wide energy range up to 30 eV, and the alloying effect on these properties was also analyzed in detail.     

Electronic properties of multinary alloys

Summary A model version of the coherent-potential approximation (CPA) is outlined to evaluate the fundamental energy gapsΓ _15v -Γ _1c ′,Γ _15v -L _1c ,Γ _15v -X _1c of a multinary solid solution. A comparison of theoretical results with the transition energy values obtained from direct interpolation of the corresponding values of ternary alloys is presented for some composition of the solution Al_1−x−y Ga _x In _y As at fixed lattice constants (matching the lattice constants of InP and AlAs). Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

X-ray diagnostics of semiconductor heterostructures: Some achievements and perspectives for development

Possibilities of new approaches for the interpretation of X-ray scattering data to characterize heterostructures with nanosized layers are demonstrated. These approaches are based on both simultaneous mathematical analysis of the experimental data of X-ray rocking curves from several crystallographic planes of the sample and joint processing of the experimental data of X-ray diffraction and reflectometry. Measurements are taken using the same device with the advanced technique of experiments. In the approaches proposed, the volume of the experimental information on one sought parameter increases significantly, which provides a greater reliability of reconstructing the structural parameters of the object under study. A mathematical apparatus is developed along with the software to solve the inverse problem with a large amount of input data and sought parameters. The proposed approaches are tested on nanosized Al_xGa_1-xAs/In_yGa_1-yAs and In_xAl_1-xAs/In_yGa_1-yAs heterostructures grown on GaAs and InP substrates, respectively. It is shown that the complex X-ray method allows a reliable determination of relationships between the real structure and electrophysical properties, and thus an improvement of the technology for producing these objects with predicted properties.