Valence Band Anti-Crossing Analysis of Dilute Sulfur in ZnOl-xSxAlloys

ZnO and related alloys are promising materials for application in photonics, electronics, piezoelectric nanogenerators and electromechanical devices. In this work, a Valence Band Anti-Crossing (VBAC) model is developed to analyze the energy bandgap of ZnO binary compound when it is lightly alloyed with anion sulfur (S) material. Minority anion alloy ZnOl-xSxexhibits unusual bowing of energy bandgap compared to cation alloying. The energy bandgap decreases dramatically from 3.37 eV to approximately 2.65 eV as the S composition increases to 100%. The main reason of the energy bandgap reduction is found to be the increase (up-shift) of the valence band edge (VBE) due to interaction between ZnO's extended VBE and the localized S defect energy state. The VBE of ZnO host material spilt into two sub-bands, the upper band E_ and the lower band E+, when the S atoms replace the O atoms. The overall computational model is based on a coupling of the Hamiltonian of the supercell based on a fully-atomistic 8-band Sp3tight-binding basis set including spin orbital interaction and the Valence Force- Field (VFF) model using Keating potentials for strain calculations.