Band structure of indium oxide: Indirect versus direct band gap

P. Erhart, A. Klein, R. G. Egdell, and K. Albe
Physical Review B 75, 153205 (2007)
doi: 10.1103/PhysRevB.75.153205
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The nature of the band gap of indium oxide is still a matter of debate. Based on optical measurements the presence of an indirect band gap has been suggested, which is 0.9 to 1.1 {eV} smaller than the direct band gap at the Γ point. This could be caused by strong mixing of O 2p and In 4d orbitals off Γ. We have performed extensive density functional theory calculations using the LDA+U and the GGA+U methods to elucidate the contribution of the In 4d states and the effect of spin-orbit coupling on the valence band structure. Although an indirect band gap is obtained, the energy difference between the overall valence band maximum and the highest occupied level at the Γ point is less than 50 meV. It is concluded that the experimental observation cannot be related to the electronic structure of the defect free bulk material.