A major shortcoming of ultrawide-bandgap (UWBG) semiconductors is unipolar doping, in which eithern-type orp-type conductivity is typically possible, but not both within the same material. For UWBG oxides, the issue is usually thep-type conductivity, which is inhibited by a strong tendency to form self-trapped holes (small polarons) in the material. Recently, rutile germanium oxide (r-GeO2), with a band gap near 4.7 eV, was identified as a material that might break this paradigm. However, the predicted acceptor ionization energies are still relatively high (∼0.4 eV), limitingp-type conductivity. To assess whether r-GeO2is 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 TiO2and SnO2strongly trap holes at acceptor impurities, consistent with previous work. Self-trapped holes are found to be unstable in r-SiO2, a metastable polymorph that has a band gap near 8.5 eV. Group-III acceptor ionization energies are also found to be lowest among the rutile oxides and approach those of GaN. Acceptor impurities 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-SiO2has the potential to exhibit the most efficientp-type conductivity when compared to other UWBG oxides.
Keywords: first-principles calculations; p-type conductivity; p-type doping; ultrawide-bandgap oxides.
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