Structural and optoelectronic properties of Mg substituted ZTe (Z=Zn,Cd and Hg)

Wide band gap semiconductor alloys, Mg_xZ_1−xTe (Z=Zn, Cd and Hg), are investigated over a full range of Mg compositions (0≤x≤1) using density functional theory (DFT). The variation in the lattice constant of Mg_xZ_1−xTe is linear with the composition x, and all these alloys obey Vegrd's law. The CdTe (6.50 Å) and MgTe (6.44 Å) are lattice matched compounds, therefore the lattice constant of MgCdTe decreases slightly with the concentration x, whereas the lattice constant also decreases for MgHgTe but increases for MgZnTe. It is due to the fact that Mg has larger size than Zn and smaller size than Cd and Hg. The band gap of these compounds are calculated using the modified Becke–Johnson (mBJ) exchange potential as LDA and GGA are not effective in producing the experimental band gap of a strongly correlated electron system. The calculated band gaps of these compounds cover the range 0–3.5 eV and are consistent with the experimental band gaps. The band gaps exhibit nonlinear behavior or bowing effect with the change in concentration. The frequency dependent optical properties like dielectric functions, and indices of refraction of these ternary systems are also calculated and discussed.     

The effect of the violation of Vegard's law on the optical bowing in Si1 − xGex alloys

The effect of the deviation of the lattice parameters from Vegard's rule on the optical bowing of the fundamental gap in Si_{1 − x}Ge_x semiconductor alloys is discussed. Our computations are based on the pseudopotential method. To make allowance for the chemical disorder, the virtual-crystal approximation is used, including a correction to the alloy potential. Our results show that the bowing parameter is highly improved when the lattice relaxation effect is included which indicates the importance of the lattice mismatch in the electronic band structure calculations of IV-IV semiconductor alloys.     

FP-LMTO method to calculate the structural,thermodynamic and optoelectronic properties of SixGe1−xC alloys

The structural and electronic properties of the ternary Si_xGe_1?xC alloys have been calculated using the full-potential linear muffin-tin-orbital (FP-LMTO) method based on density functional theory within both local density approximation (LDA) and generalised gradient approximation (GGA). The calculated equilibrium lattice constants and bulk moduli are compared with previous results. The concentration dependence of the electronic band structure and the direct and indirect band gaps are investigated. Using the approach of Zunger and co-workers, the microscopic origins of the band gap bowing are investigated also. Moreover, the refractive index and the optical dielectric constant for Si_xGe_1?xC are studied. The thermodynamic stability of the alloys of interest is investigated by means of the miscibility. This is the first quantitative theoretical prediction to investigate the effective masses, optical and thermodynamic properties for Si_xGe_1?xC alloy, and still awaits experimental confirmations.     

Electronic and optical properties of ZnS x Se1−x alloys

A series of calculations from first principles have been carried out to study structural, electronic, and optical properties of ZnS_xSe_1−x alloys. Our results show that the lattice constant scales linearly with sulfur composition. The imaginary parts of the dielectric function are calculated, which are in good agreement with the experimental data. We have also interpreted the origin of the spectral peaks on the basis of band structure and density of states. Additionally, we find that no bowing effect in the absorption edge is observed, unlike other II-VI semiconductor alloys.      

First-principles calculations of structural,electronic and thermal properties of Zn1−x Mg x S ternary alloys

Structural, electronic and thermal properties of Zn_1?x Mg _x S ternary alloys are studied by using the full potential-linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT). The Wu-Cohen generalized gradient approximation (WC-GGA) is used in this approach for the exchangecorrelation potential. Moreover, the modified Becke-Johnson approximation (mBJ) is adopted for band structure calculations. The dependence of the lattice constant, bulk modulus and band gap on the composition x showed that the first exhibits a small deviation from the Vegard’s law, whereas, a marginal deviation of the second from linear concentration dependence (LCD). The bowing of the fundamental gap versus composition predicted by our calculations agrees well with the available theoretical data. The microscopic origins of the gap bowing are explained by using the approach of Zunger and co-workers. Thermal effects on some macroscopic properties of Zn_1?x Mg _x S alloys are also investigated using the quasi-harmonic Debye model, in which the phononic effects are considered. As, this is the first quantitative theoretical prediction of the thermal properties of Zn_1?x Mg _x S alloys, no other calculated results and furthermore no experimental studies are available for comparison.     

纤锌矿BeO掺Cd的电子结构与能带特性研究

采用基于密度泛函理论平面波赝势方法, 对纤锌矿BeO掺Cd的Be_1-xCd_xO合金进行电子结构与能带特性研究. 结果表明: Be_1-xCd_xO的价带顶始终由O 2p电子态决定, 而导带底由Be 2s和Cd 5s的电子态决定.随着Be_1-xCd_xO合金的Cd掺杂量增加, Cd 4d与O 2p的排斥效应逐渐加强, 同时Be_1-xCd_xO的带隙逐渐变小, 出现"直接-间接-直接"的带隙转变. 为了使理论值与实验值相一致, 对Be_1-xCd_xO带隙进行修正, 并分析了纤锌矿BeO-ZnO-CdO三元合金的带隙和弯曲系数与晶格常数的关系.     

First-principles study on the electronic and optical properties of SnxGe1 − x

The electronic and optical properties of the direct band gap alloys Sn_xGe_1 − x (x = 0.000, 0.042, 0.083, 0.125, 0.167, and 0.208) have been studied by using the generalized gradient approximation in the framework of the density functional theory. The calculated lattice constants obey Vergard's law. The band structures show that the alloys have direct band gap and the band gaps can be tunable by Sn contents. The optical properties of the Sn_xGe_1 − x alloys with the physical quantities such as the complex dielectric function, the energy-loss function and the static dielectric constant, respectively, are shown to support the potential application of infrared devices in the future.     

纤锌矿BexZn1-xO合金能隙弯曲系数的第一原理研究

采用密度泛函理论框架下的第一性原理平面波超软赝势方法, 对纤锌矿Be_xZn_1-xO合金进行能隙特性、弯曲系数和结构参数的计算. 结果表明: Be_xZn_1-xO合金的能隙和弯曲系数都随Be掺杂组分的增大而增大. 通过修正Be_xZn_1-xO合金的能隙值得知其合金弯曲系数b为6.02 eV, 这与实验值接近. 纤锌矿Be_xZn_1-xO合金的能隙弯曲系数过大主要来源于体积形变和电荷转移的贡献. 文中还分析了Be_xZn_1-xO合金的晶格常数、 平均键长和平均次近邻原子距离与Be组分的关系.     

First-principles calculations of the structural, electronic and optical properties of cubic BxGa1−xAs alloys

Density functional calculations are performed to study the structural, electronic and optical properties of technologically important B_xGa_1−xAs ternary alloys. The calculations are based on the total-energy calculations within the full-potential augmented plane-wave (FP-LAPW) method. For exchange-correlation potential, local density approximation (LDA) and the generalized gradient approximation (GGA) have been used. The structural properties, including lattice constants, bulk modulus and their pressure derivatives, are in very good agreement with the available experimental and theoretical data. The electronic band structure, density of states for the binary compounds and their ternary alloys are given. The dielectric function and the refractive index are also calculated using different models. The obtained results compare very well with previous calculations and experimental measurements.     

Electronic and structural properties of ZnxCd1−xSySe1−y alloys lattice matched to GaAs and InP: An EPM study

The electronic, optical and structural properties of Zn_xCd_1−xS_ySe_1−y quaternary alloys lattice matched to GaAs and InP are studied. The electronic band structure and density of states are computed using empirical pseudopotential method. The disorder effects are included via modified virtual crystal approximation. The bandgap computed from band structures are utilized to evaluate refractive indices, dielectric constants and ionicity factors for the alloys. Among structural properties elastic constants and bulk moduli are computed by combining the EPM with Harrison bond orbital model. All possible semiconductors from the ZnCdSSe system are found to have direct bandgap. The lattice matched alloys have larger band gap and more ionic character than the lattice matched compounds.     

The structural, electronic and optical properties of CdxZn1 − xSe ternary alloys

The structural, electronic, and optical properties of Cd_xZn_1 − xSe alloys are investigated using the first-principles plane-wave pseudopotential method within the LDA approximations. In particular, the lattice constant, bulk modulus, electronic band structures, density of state, and optical properties such as dielectric functions, refractive index, extinction coefficient and energy loss function are calculated and discussed. Our results agree well with the available data in the literature.     

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.     

Universal bandgap bowing in group-III nitride alloys

The energy gaps of molecular-beam-epitaxy grown wurtzite-structure In_1−xAl_xN alloys with x≤0.25 have been measured by absorption and photoluminescence experiments. The results are consistent with the recent discovery of a narrow bandgap of ∼0.7 eV for InN. A bowing parameter of 3 eV was determined from the composition dependence of these bandgaps. Combined with previously reported data of InGaN and GaAlN, these results show a universal relationship between the bandgap variations of group-III nitride alloys and their compositions.     

Structural,electronic, optical and thermodynamic properties of NaxRb1−xH and NaxK1−xH alloys

A theoretical study of the structural, electronic, optical and thermodynamic properties of Na_xRb_1?xH and Na_xK_1?xH ternary alloys in NaCl phase has been carried out using the first-principles method. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependences on the composition of the lattice constant, band gap, dielectric constant, refractive index, Debye temperature, mixing entropy and heat capacities were analyzed for x=0, 0.25, 0.50, 0.75 and 1. The lattice constants of Na_xRb_1?xH and Na_xK_1?xH exhibit a marginal deviation from Vegard's law. A strong deviation of the bulk modulus from linear concentration dependence was observed for both alloys. We found that the composition dependence of the energy band gap is highly non linear and the large bowing coefficient for Na_xRb_1?xH is sensitive to the composition. Using the approach of Zunger and co-workers, the microscopic origins of the gap bowing were detailed and explained. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds, which is a support for those of the ternary alloys that we report for the first time.     

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.     

Ab initio calculations of structural,electronic, optical and thermodynamic properties of alkaline earth tellurides BaxSr1−XTe

Structural, electronic and thermodynamic properties of SrTe and BaTe compounds and their ternary mixed crystals Ba_xSr_1−xTe in the rock-salt structure have been studied with density functional theory (DFT), whereas the optical properties have been obtained by using empirical methods such as the modified Moss relation. The exchange-correlation potential was calculated using the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) and the local density approximation (LDA) of Teter–Pade (TP). In the present work, we used the virtual-crystal approximation (VCA) to study the effect of composition (x). The calculated lattice parameters at equilibrium volume and the bulk modulus for x=0 and x=1 are in good agreement with the literature data. Furthermore, the Ba_xSr_1−xTe alloys are found to be an indirect band gap semiconductor. In addition, we have also predicted the heat capacities (C_V), the entropy(S), the internal energy (U) and the Helmholtz free energy (F) of the parent compounds SrTe and BaTe.     

Electronic structure and optical properties of CdSexTe1−x mixed crystals

The band structure and optical properties of the CdSe_xTe_1−x ternary mixed crystals have been studied using the pseudopotential formalism under an improved virtual crystal approximation approach. Quantities such as, energy gaps, band-gap bowing parameters, electron effective mass and dielectric constants are calculated. Our results agree well with the available data in the literature. The composition dependence of all studied quantities has been expressed by quadratic polynomial forms.     

First-principles calculations of structural and electronic properties of Tl_xGa_(1-x)As alloys

The zincblende ternary alloys Tl_xGa_(1-x) As(0 x 1) are studied by numerical analysis based on the plane wave pseudopotential method within the density functional theory and the local density approximation. To model the alloys,16-atom supercells with the 2 × 2 × 2 dimensions are used and the dependency of the lattice parameter, bulk modulus,electronic structure, energy band gap, and optical bowing on the concentration x are analyzed. The results indicate that the ternary Tl_xGa_(1-x) As alloys have an average band gap bowing parameter of 4.48 eV for semiconductor alloys and 2.412 eV for semimetals. It is found that the band gap bowing strongly depends on composition and alloying a small Tl content with GaAs produces important modifications in the band structures of the alloys.     

First-principle calculations of dilute nitride GaP1−xNx alloy in zinc-blende structures

First-principle plane-wave pseudopotential calculations using the density-functional theory within the local density approximation (LDA) have been carried out to investigate the structural, electronic and thermodynamic properties of zinc-blende dilute nitride GaP_1−xN_x alloy. We have found that the unit cell volumes of GaP_1−xN_x alloys are smaller than the value calculated from the linear interpolation of GaN and GaP binary alloys and there exists a negative deviation from Vegard's law. The Ga-P and Ga-N bond lengths of GaP_1−xN_x alloys reveal a slight relaxation. The incorporation of nitrogen into GaP leads to the giant reduction of the bandgap energy. We have investigated the thermal stabilities and obtained the most stable configurations of the GaP_1−xN_x alloy. In addition, the formation enthalpies have also been calculated, which could explain the difficulty in nitrogen incorporation into GaP.     

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.