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 $1.1\mathrm{eV}$ smaller than the direct band gap at the $\Gamma$ point. This could be caused by strong mixing of $\mathrm{O}2p$ and $\mathrm{In}4d$ orbitals off $\Gamma$. We have performed extensive density functional theory calculations using the LDA+U and the GGA+U methods to elucidate the contribution of the $\mathrm{In}4d$ 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 $\Gamma$ point is less than $50\mathrm{meV}$. It is concluded that the experimental observation cannot be related to the electronic structure of the defect free bulk material.

• Received 27 November 2006

DOI:https://doi.org/10.1103/PhysRevB.75.153205