Type-II Band Alignment in the $β$-Ga$_2$O$_3$/Rutile GeO$_2$ Heterojunction toward Solar-Blind Photodetection: A first-principles study

Semiconductor heterostructures capable of separating photogenerated electrons and holes have a wide range of optoelectronic applications, including photodetectors, solar cells, and photocatalysts. $β$-Ga$_2$O$_3$ and rutile GeO$_2$ are both ultrawide-bandgap semiconductors, with bandgaps of 4.85 eV and 4.68 eV, respectively. In this work, we employ first-principles calculations based on density functional theory to investigate the band alignment of the $β$-Ga$_2$O$_3$/rutile GeO$_2$ heterojunction and explore the effect of interfacial oxygen vacancy. Calculations using the PBE0 hybrid functional based on an interface model show that a type-II band alignment emerges at the $β$-Ga$_2$O$_3$/rutile GeO$_2$ interface, which facilitates the separation of photogenerated carriers. The valence band maximum of $β$-Ga$_2$O$_3$ lies 0.38 eV below that of rutile GeO$_2$, and its conduction band minimum lies 0.36 eV below. The presence of interfacial oxygen vacancy in the stable configuration leads to a reduction in the band offset. Our results suggest that the $β$-Ga$_2$O$_3$/rutile GeO$_2$ heterojunction holds significant promise for application in strictly solar-blind photodetectors.