  Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to
advance electronic devices as device performance improves superlinearly with
increasing gap. Ambipolar doping, however, has been a major challenge for UWBG
materials as dopant ionization energy and charge compensation generally
increase with increasing band gap and significantly limit the semiconductor
devices that can currently be realized. Using hybrid density functional theory,
we demonstrate rutile germanium oxide (r-GeO2) to be an alternative UWBG (4.68
eV) material that can be ambipolarly doped. We identify SbGe, AsGe, and FO as
possible donors with low ionization energies and propose growth conditions to
avoid charge compensation by deep acceptors such as VGe and NO. On the other
hand, acceptors such as AlGe have relatively large ionization energies (0.45
eV) due to the formation of localized hole polarons and are likely to be
passivated by VO, Gei, and self-interstitials. Yet, we find that the
co-incorporation of AlGe with interstitial H can increase the solubility limit
of Al and enable hole conduction in the impurity band. Our results show that
r-GeO2 is a promising UWBG semiconductor that can overcome current doping
challenges and enable the next generation of power electronics devices.
