Shallow Valence Band of Rutile GeO₂ and P-type Doping

Abstract

GeO2 has an α-quartz-type crystal structure with a very wide fundamental band gap of 6.6 eV and is a good insulator. Here, we find that the stable rutile-GeO2 polymorph with a 4.6 eV band gap has a surprisingly low ∼6.8 eV ionization potential, as predicted from the band alignment using first-principles calculations. Because of the short O–O distances in the rutile structure containing cations of small effective ionic radii such as Ge4+, the antibonding interaction between O 2p orbitals raises the valence band maximum energy level to an extent that hole doping appears feasible. Experimentally, we report the flux growth of 1.5 × 1.0 × 0.8 mm3 large rutile GeO2 single crystals and confirm the thermal stability for temperatures up to 1021 ± 10 °C. X-ray fluorescence spectroscopy shows the inclusion of unintentional Mo impurities from the Li2O–MoO3 flux, as well as the solubility of Ga in the r-GeO2 lattice as a prospective acceptor dopant. The resistance of the Ga- and Mo-codoped r-GeO2 single crystals is very high at room temperature, but it decreases by 2–3 orders of magnitude upon heating to 300 °C, which is attributed to thermally activated p-type conduction.