Ab initio study of electronic and optical properties of Be-chalcogenides in GW approximation

An ab initio study of the structural electronic and optical absorption properties of beryllium chalcogenides (BeS, BeSe and BeTe) has been made after using pseudopotential method in generalized gradient approximation (GGA). The many-body effects have been considered in GW approximation. The results for the lattice parameters, bulk modulii and cohesive energies calculated in GGA are in excellent agreement with the experimental data. The lattice parameter increases in the order, BeS, BeSe and BeTe because of an increase in the size of the anion. The bulk modulus and cohesive energy decrease in the order, BeS, BeSe and BeTe because of the enhanced volume of the unit cell. We find that our calculated B's are in close agreement with the experimental values within 9%. For BeTe, the calculated electronic band structure along the Γ–X direction after the inclusion of many body effects is in good agreement with the experimental data. The calculated direct and indirect energy gaps in GW approximation for BeTe are also in close agreement with the experimental data. The optical absorption appears mostly in the ultraviolet region. The no-phonon optical absorption is extended towards the low-energy side monotonically with the size of the anion.