Electronic structure of spinel oxides: zinc aluminate and zinc gallate

The electronic structure of zinc aluminate (ZnAl2O4) and that of zinc gallate (ZnGa2O4) were studied by the self-consistent tight-binding linearized muffin-tin orbital method with the atomic sphere approximation. The calculated results predict these zinc-based spinel oxides to be direct-gap materials. The direct gap at 0 is found to be 4.11 eV for ZnAl2O4 and 2.79 eV for ZnGa2O4. With reference to the calculated band gap of 5.36 eV for MgAl2O4, the systematic decrease in the gap is attributed to the presence of 3d orbitals of Zn and Ga and the associated p–d hybridization in the upper valence band of zinc aluminate and gallate. Comparison of the contour maps of the electron localization function of ZnAl2O4 and ZnGa2O4 with that of MgAl2O4 clearly shows the bonding to be less ionic in the zinc-based spinel oxides. Finally, the calculations yield a smaller electron effective mass for zinc gallate as compared to that for zinc aluminate, suggesting a higher mobility of electrons in gallate.