Electronic-structure and thermodynamic properties of ZnS1-xSex ternary alloys from the first-principles calculations

In this paper, the composition-dependent electronic-structure and thermodynamic properties for hexagonal wurtzite (WZ) and face-centered cubic zincblende (ZB) ZnS1-xSex ternary alloys have been studied utilizing first-principles pseudopotential plane-wave self-consistent field calculations. Special quasi-random structure method was applied to generate disordered structures of 16-atom supercell for x = 0.25, 0.5 and 0.75. We obtained a direct band-gap at Gamma point and a shrinking tendency of the band-gap with increasing Se content in both phases. The reasons for the band-gap shrinkage have been discussed based on the analysis of electronic density of states and band offsets Delta-sol method was applied to correct the band gaps calculated by density functional theory. The Delta-sol corrected band-gaps are in good agreement with experimental band-gaps. The thermodynamic properties were calculated by the cluster expansion method. It turns out that, the stretching forces decrease considerably and linearly with the bond length, while the bending forces are dependent insignificantly on the bond length. The phase diagrams reveal that the lattice vibration effects can influence the critical temperature T-c and the solubility of S- and Se-rich ZnS1-xSex alloys, but the influence is not obvious for ZB-ZnS1-xSex. Based on the calculated phase diagrams without lattice vibration effects, the T-c for WZ- and ZB-ZnS1-xSex are 559 K and 421 K, respectively. When the vibration enthalpy is included, the T-c values of both phases are lowered to 543 K and 418 K, respectively. And if the vibration entropy is also taken into account, the T-c values of both phases are further lowered to 529 K and 415 K, respectively. With inclusion of lattice vibration effects, the ZnS1-xSex alloy starts a phase transition from ZB to WZ at a temperature of 1600 K, and the solid solubility of Se increases with the temperature rise.