Ambipolar doping in SnO

SnO is a promising oxide semiconductor that can be doped both p- and n-type, but the doping mechanisms remain poorly understood. Using hybrid functionals, we find that native defects cannot account for the unintentional p-type conductivity. Sn vacancies are shallow acceptors, but they have high formation energies and are unlikely to form. Unintentional impurities offer a more likely explanation for p-type doping; hydrogen is a likely candidate, and we find that it forms shallow-acceptor complexes with Sn vacancies. We also demonstrate that the ambipolar behavior of SnO can be attributed to the high position of the valence-band on an absolute energy scale.