Polytypism in ZnS, ZnSe, and ZnTe: First-principles study

Abstract

In this work, we are focused on the influence of relativistic effects on the physical properties of ZnX , CdX and HgX compounds and study of Polytypism in ZnX (X= S, Se, Te) materials. The objective of the first part is to show the influence of relativistic effects on the structural, electronic and optical properties of these materials studied in the zinc-blende structure. The study is performed using linearized augmented plane wave method as implemented in the WIEN2k code. We show principally that stabilization (destabilization) of s, p * (p ) orbitals reduces the lattice parameter, gap energy and the effective masses of the II-VI compounds. However, there is a high spin orbit coupling for the heavy compounds. In addition, the use of Engel Vosko (EV) and the modified Beck Johnson (mBJ) potentials as a correction of the terms of exchange and correlation allowed us to have energy gaps in good agreement with the experimental results. In the second part, we report results of first-principles calculations based on the projector augmented wave (PAW) method to explore the structural, thermodynamic, and electronic properties of cubic (3C) and hexagonal (6H, 4H, and 2H) polytypes of II-VI compounds : ZnS, ZnSe, and ZnTe. For the structural parameters, we found good agreement between the calculated and experimental lattice constants a, while a perfect agreement is found for the ratio c/a for the 2H polytype. The lattice parameter a is found to decrease slightly with increasing hexagonality, while the lattice constant ratio is found to increase weakly with hexagonality. The ANNNI model with up to third-nearest neighbor layer interactions provides a good description of the preference for the 3C polytype and stacking faults in cubic materials. The quasiparticle band structures of ZnS, ZnSe, and ZnTe polytypes have been obtained within the recently developed LDA-1/2 method including spin-orbit interaction. The results show very good agreement with the available experimental data for band gaps, spin-orbit splitting energies, and crystal-field splittings. Furthermore, we found a relative weak (strong) dependence of band gap (crystal-field splitting) with hexagonality, while the spin-orbit splitting is practically not influenced by the polytype. The comparison with measured effective masses shows good agreement with the computed values. In addition, we demonstrate the importance of the spin-orbit interaction for the dispersion and the splittings of the bands around the BZ center and, hence, explain the chemical trend and the symmetry-induced mass splitting.