Shallow Valence Band of Rutile GeO${}_2$ and P-type Doping

Abstract

GeO${}_2$ has an $\alpha$-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-GeO${}_2$ polymorph with a 4.6 eV band gap has a surprisingly low $\sim$6.8 eV ionization potential, as predicted from the band alignment using first-principles calculations. Because of the short O$-$O distances in the rutile structure containing cations of small effective ionic radii such as Ge${}^{4+}$, 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 $1.5\times 1.0\times 0.8$ mm${}^3$ large rutile GeO${}_2$ single crystals and confirm the thermal stability for temperatures up to $1021\pm 10{}^{\circ }$C. X-ray fluorescence spectroscopy shows the inclusion of unintentional Mo impurities from the Li${}_2$O$-$MoO${}_3$ flux, as well as the solubility of Ga in the r-GeO${}_2$ lattice as a prospective acceptor dopant. The resistance of the Ga- and Mo-codoped r-GeO${}_2$ single crystals is very high at room temperature, but it decreases by 2-3 orders of magnitude upon heating to 300 ${}^{\circ }$C, which is attributed to thermally-activated p-type conduction.

References in corpus (2)

• Rutile GeO2: an ultrawide-band-gap semiconductor with ambipolar doping
• Crystal structure built from a GeO${}_6$-GeO${}_5$ polyhedra network with high thermal stability: $\beta$-SrGe${}_2$O${}_5$

Cited by in corpus (1)

• Perspective: Towards the predictive discovery of ambipolarly dopable ultra-wide-band-gap semiconductors: the case of rutile GeO${}_2$