First principles calculations of ZnS:Te energy levels

Abstract

A comprehensive density-functional calculation for the ZnS:Te isoelectronic impurity state is presented. We deploy a charge patching method that enables us to calculate systems containing thousands of atoms. We found that the impurity state is only weakly localized, and previous calculations using 64-atom cells were severely unconverged. Our calculated impurity binding energy agrees with experimental photoluminescence excitation spectrum. We have analyzed the impurity wave function in both real space and the reciprocal space, and in terms of the host bulk valence bands. We have also calculated the Stokes shifts and Jahn-Teller effects. We found small Stokes shift compared to the experimental results, which might indicate the limitations of the current method. We also calculated the ${\mathrm{Te}}_n$ clusters and their impurity states. We found six (counting spin) bound states inside the band gap of ZnS for all $1<~n<~4.$ We obtained pressure coefficients for ${\mathrm{Te}}_n,$ all close to the value of bulk ZnS. This is consistent with the fact that the impurity states of ${\mathrm{Te}}_n$ consist almost entirely of the bulk valence bands of ZnS. Finally, we have calculated the effects of spin-orbit coupling for the impurity state eigenenergies.