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 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.