Towards p-type conductivity in ultrawide-bandgap oxides

A major shortcoming of ultrawide-bandgap semiconductors is unipolar doping, in which either n-type or p-type conductivity is possible, but not both. For ultrawide-bandgap oxides, the issue is usually p-type conductivity, which is inhibited by a strong tendency to form self-trapped holes (small polarons). Recently, rutile germanium oxide (r-GeO₂), with a band gap near 4.7 eV, was proposed to break this paradigm. However, the predicted acceptor ionization energies are still relatively high (~0.4 eV), limiting p-type conductivity. To assess whether r-GeO₂ is an outlier due to its crystal structure, the properties of a set of rutile oxides are calculated and compared. Hybrid density functional calculations indicate that rutile TiO₂ and SnO₂ strongly trap holes at acceptor impurities, but self-trapped holes are found to be unstable in r-SiO₂, a metastable polymorph with an 8.5 eV band gap. Group-III acceptor ionization energies are also found to be lowest among rutile oxides. Furthermore, acceptors have sufficiently low formation energies to not be compensated by donors such as oxygen vacancies, at least under O-rich limit conditions. Based on the results, it appears that r-SiO₂ has the potential to exhibit the most efficient p-type conductivity when compared to other UWBG oxides.