Optical properties and critical points in ordered Be xZn 1− xSe alloys

Abstract

We have investigated the electronic and optical properties of ordered Be x Zn 1− x Se alloys using a 8-atom supercell for compositions x = 0.0 , 0.25 , 0.50 , 0.75 and 1.0. We report ‘ state-of-the-art’ calculations within generalized gradient approximation (GGA) and Engel-Vosko’s corrected generalized gradient approximation (EVGGA) using the full potential linear augmented plane wave (FPLAPW) method as implemented in the WIEN2K code. To see the effect of disorder, alloys are also modeled following the special quasirandom structure (SQS) approach. The calculated lattice constants scale linearly with composition (Vegard’s law). Dielectric functions for different compositional alloys are calculated for 8-atom cubic supercell and chalcopyrite structure corresponding to [0 0 1] superlattice which show good qualitative agreement when compared with experiment. The calculated band gaps are fitted with a quadratic equation E g ( x ) = a x 2 + b x + c . We find that there is a direct to indirect band gap crossover at x = 0.53 compared with the measured value x = 0.46 . The position of critical points (CP’s) E 0 + Δ 0 , E 1 and E 2 show good agreement with the experimental data. The difference between the calculated band gap and measured band gap decreases with increasing concentration of Be. The valence band maxima and conduction band minima are dominated by Se-3p and Zn-4s/Be-2p states for x = 0.0 and 1.0, respectively.