Electronic structure of indium oxide using cluster calculations

We report first-principles electronic-structure calculations of In₂O₃ using the discrete variational Xα method on model clusters. The computation has been made up to the model cluster composed of 163 atoms in order to see the size effect. The In-O bond is found to be predominant in In₂O₃, and both O-O and In-In bonds are much weaker. Antibonding interaction between O-2p with nearly filled In-4d orbitals near the top of the valence band is noticed. Valence-band structure by XPS is well reproduced by the calculation. Unoccupied In-5sp orbitals show wide spatial distribution over the third In shell. Direct interaction between In-5sp orbitals is found to be important in the excited states. When an oxygen vacancy is present, a vacancy level appears in between the band gap. The vacancy level is composed of In-5sp orbitals hybridized with O-2p orbitals, which exhibits a strong In-In bonding interaction. The occupation of the vacancy level due to the localization of electrons to the oxygen vacancy thus results in the reinforcement of the In-In bond strength. This is suggested to be the electronic mechanism for the stability of the oxygen vacancies in the In₂O₃ crystal.