Rutile GeO₂: an ultra-wide-band-gap semiconductor with ambipolar doping

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. 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 Sb_Ge, As_Ge, and F_O as possible donors with low ionization energies and propose growth conditions to avoid charge compensation by native acceptor-type defects. Acceptors such as Al_Ge have relatively large ionization energies (0.45 eV) due to the formation of localized hole polarons. Yet, we find that the co-incorporation of Al_Ge with H_i can increase the solubility limit of Al and enable hole conduction in the impurity band. We also calculate electron (153.6 cm²V^-1s^-1) and hole mobilities (4.7 cm²V^-1s^-1) of r-GeO₂ at 300 K, suggesting r-GeO₂ has outstanding electronic properties that can compete with the state-of-the-art UWBG semiconductors such as β-Ga₂O₃. We will also discuss on our recent experimental progress on thin-film growth and electrical characterization of r-GeO₂.