Influence of shallow core-level hybridization on the electronic structure of post-transition-metal oxides studied using soft X-ray emission and absorption

The influence of shallow core-level hybridization on the electronic structure of the post-transition metal oxides ZnO, CdO, In₂O₃, and SnO₂ has been investigated using high-resolution soft x-ray emission and absorption spectroscopies. Synchrotron radiation excited O K_α emission spectra provide a direct measure of the O $2p$ partial density of states and shallow core-level hybridization for this series of transparent conducting materials and reveal significant mixing of O $2p$ and shallow-core metal d states for ZnO, CdO, and In₂O₃. The experimental data are compared with local density approximation and tight-binding band structure calculations and with previous experimental determinations of direct and indirect band gaps. Rocksalt CdO, bixbyite In₂O₃, and rutile SnO₂ all adopt structures with metal cations in sites with locally centrosymmetric coordination. This prevents hybridization of O $2p$ states with metal $4d$ states at the zone center, but mixing away from {Γ} leads to indirect band gaps for CdO and In₂O₃. A revised value for the lowest indirect band gap in CdO is proposed and the overall trends in the band gap are discussed in terms of the separation between O $2p$ and metal $4d$ states. The experimental investigation has been extended to study the effects of Sn doping in In₂O₃.