Electronic structure of indium oxide using cluster calculations

We report first-principles electronic-structure calculations of In2O3 using the discrete variational X alpha 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.