Origin of the bipolar doping behavior of SnO from X-ray spectroscopy and density functional theory

The origin of the almost unique combination of optical transparency and the ability to bipolar dope tin monoxide is explained using a combination of soft and hard X-ray photoemission spectroscopy, O K-edge X-ray emission and absorption spectroscopy, and density functional theory calculations incorporating van der Waals corrections. We reveal that the origin of the high hole mobility, bipolar ability, and transparency is a result of (i) significant Sn 5s character at the valence band maximum (due to O 2p-Sn 5s antibonding character associated with the lone pair distortion), (ii) the combination of a small indirect band gap of ∼0.7 eV (Γ-M) and a much larger direct band gap of 2.6-2.7 eV, and (iii) the location of both band edges with respect to the vacuum level. This work supports Sn²⁺-based oxides as a paradigm for next-generation transparent semiconducting oxides.