Layer-resolved band bending at the n−SrTiO3(001)/p−Ge(001) interface

Abstract

The electronic properties of epitaxial heterojunctions consisting of the prototypical perovskite oxide semiconductor, $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$, and the high-mobility Group IV semiconductor $p$-Ge have been investigated. Hard x-ray photoelectron spectroscopy with a new method of analysis has been used to determine band alignment while at the same time quantifying a large built-in potential found to be present within the Ge. Accordingly, the built-in potential within the Ge has been mapped in a layer-resolved fashion. Electron transfer from donors in the $n\ensuremath{-}\mathrm{SrTi}{\mathrm{O}}_{3}$ to the $p$-Ge creates a space-charge region in the Ge resulting in downward band bending, which spans most of the Ge gap. This strong downward band bending facilitates visible light, photogenerated electron transfer from Ge to STO, favorable to drive the hydrogen evolution reaction associated with water splitting. Ti 2p and $\mathrm{Sr}\phantom{\rule{0.28em}{0ex}}3d$ core-level line shapes reveal that the STO bands are flat despite the space-charge layer therein. Inclusion of the effect of Ge band bending on band alignment is significant, amounting to a $\ensuremath{\sim}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ reduction in valence band offset compared to the value resulting from using spectra averaged over all layers. Density functional theory allows candidate interface structural models deduced from scanning transmission electron microscopy images to be simulated and structurally optimized. These structures are used to generate multislice simulations that reproduce the experimental images quite well. The calculated band offsets for these structures are in good agreement with experiment.