Gap states in the electronic structure of SnO₂ single crystals and amorphous SnO_x thin films

The electronic structure of a SnO₂ single crystal is determined by employing resonant photoelectron spectroscopy. We determine the core level, valence band, and X-ray absorption (XAS) data and compare these with those of amorphous SnO_x thin films. We find similar properties concerning the data of the core levels, the valence band features, and the absorption data at the O1s edge. We find strong signals arising from intrinsic in-gap states and discuss their origin in terms of polaronic and charge-transfer defects. We deduce from the XAS data recorded at the Sn3d edge that the Sn4d¹⁰ ground state has contributions of 4d⁹ and 4d⁸ states due to configuration interaction. We identify localized electronic states depending on the strength of the 4d-5s5p interaction and of the O2p-to-Sn4d charge-transfer processes, both appear separated from the extended band-like states of the conduction band. For the amorphous SnOx thin films, significant differences are found only in the absorption data at the Sn3d-edge due to a stronger localization of the in-gap states.