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, In ₂O ₃, and SnO ₂ 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 In ₂O ₃. 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 In ₂O ₃, and rutile SnO ₂ 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 F leads to indirect band gaps for CdO and In ₂O ₃. 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 In ₂O ₃.