Unusual bandgap bowing in highly mismatched ZnOS alloys: Atomistic tight-binding band anti-crossing model

Abstract

An atomistic band anticrossing (BAC) model is developed and used to study “unusual bowing” in energy bandgap and its dependence on the material composition in minority O anion-alloyed ZnS (ZnS1−xOx) and minority S anion-alloyed ZnO (ZnO1−xSx) highly mismatched alloys. For dilute O in ZnS1−xOx, it is found that the bandgap decreases as the O composition is increased. A “down-shift” in the conduction band edge (CBE) of host ZnS, which arises from an interaction between the CBE and the localized O defect state, is identified as the root cause. However, the reduction in bandgap as a function of dilute S composition in the ZnO1−xSx alloy follows an “up-shift” in the valence band edge (VBE) of host ZnO, which arises from an interaction between the VBE and the localized S defect state. The BAC model captures the E+ and E− splitting in the sub-bands, which are found to be an admixture of the extended CBE (VBE) of ZnS (ZnO) and the localized O (S) state. A fully atomistic 8-band sp3-spin tight-binding basis set is used to construct the Hamiltonian for the wurtzite host materials as well as their alloy supercells. For alloy supercells, a strain is computed via the valence force-field formalism using Keating potentials. The O and S energy states are found to be approximately 199 meV below the CBE of ZnS and 190 meV above the VBE of ZnO, respectively. Overall, the calculated energy bandgaps using the BAC model are in good agreement with corrected local density approximation (LDA+U) calculations and experimental results.