Ab-initio study of fundamental properties of ternary ZnO_{1- x}S_x alloys by using special quasi-random structures

We employed the special quasi-random structure (SQS) approach to investigate the structural, electronic and optical properties of the binary compounds (ZnO and ZnS) and their alloys ZnO_1-xS _x (x = 0.25, 0.50, 0.75) in the rock salt (RS) and zinc blende (ZB) phases. All the calculations were performed using full-potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the frame work of density functional theory (DFT). The DFT functionals, Wu-Cohen generalized gradient approximation (WC-GGA), Perdew-Burke-Ernzerhof GGA (PBE-GGA) and modified Becke-Johnson GGA (mBJ-GGA) were applied to calculate the exchange-correlation energy and band gap energies. In structural optimization, the predicted results of lattice constant and bulk modulus have shown a non-linear behavior with the selected doping concentrations. From the study of band structures, it has been noticed that the compounds and their alloys under study has metallic behavior in RS phase except at x = 0 and 0.25 concentrations (indirect band gaps), whereas semiconducting behavior is obvious in ZB phase with increasing doping order (direct band gaps). The total and partial densities of states (TDOS and PDOS) have been examined to deeply study the electronic properties. Finally, by incorporating the basic optical properties, we discussed the dielectric function, refractive index, optical reflectivity, the absorption coefficient and optical conductivity in terms of incident photon energy up to 20 eV.