  The electronic properties of epitaxial heterojunctions consisting of the
prototypical perovskite oxide semiconductor,n-SrTiO3 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-SrTiO3 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, photo-generated electron transfer from Ge to
STO, favorable to drive the hydrogen evolution reaction associated with water
splitting. Ti 2p and Sr 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 ~0.4 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 multi-slice simulations that reproduce the experimental images quite
well. The calculated band offsets for these structures are in good agreement
with experiment.
