Density functional study on structural and optoelectronic properties of cubic Mg_xZn_1−xS_ySe_1−y semiconductor quaternary alloys

In the case of technologically important Mg_xZn_1−xS_ySe_1−y quaternary alloys, structural and optoelectronic properties have been calculated with density functional theory (DFT)-based full-potential linearised augmented plane-wave (FP-LAPW) approach. The Perdew–Burke–Ernzerhof generalised gradient approximation (PBE-GGA) for structural properties and both the modified-Becke–Johnson (mBJ) and Engel and Vosko GGA (EV-GGA) for optoelectronic properties are employed to calculate the respective exchange-correlation potentials. Each specimen within the Mg_xZn_1−xS_ySe_1−y quaternary system is a direct band-gap (Γ-Γ) semiconductor. The lattice constant decreases, while bulk modulus and band gap increase nonlinearly with increasing anionic (S) concentration y at each cationic (Mg) concentration x. On the other hand, nonlinear increment in lattice constant and band gap, but decrement in bulk modulus is found with increase in cationic concentration x at each anionic concentration y. Calculated contour maps for lattice constants and energy band gaps would be useful in fabricating new quaternary alloys with preferred optoelectronic features. Optical properties of the specimens within the Mg_xZn_1−xS_ySe_1−y quaternary system show several interesting features. Chalcogen-p→Zn-5s, 4p and chalcogen-p→Mg-4s, 4p optical excitations contribute intense peaks in each ε2(ε) spectrum. The composition dependence of each calculated zero-frequency limit shows opposite trend, while each calculated critical point shows similar trend of composition dependence of band gap. Moreover, calculations suggest the possibility of growth of several cubic Mg_xZn_1−xS_ySe_1−y quaternary specimens on GaAs and InP substrates. (author)