PNNL

Abstract

We have used high-energy-resolution x-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO2(002)/n-SrTiO3(001) heterojunction prepared by molecular beam epitaxy, Within experimental error, the valance band offset is zero for anatase thicknesses between 1 and 7 monolayers. The conduction band offset is also zero by virtue of the fact that both anatase and SrTiO3 exhibit the same bandgap value (~3.2 eV). In one set of experiments, the interface included a partial monolayer of fluorine remaining from the HF etch that was used to prepare the substrate. The F could not be removed without Ar ion sputtering and annealing, which in turn resulted in ~0.15 eV of band bending, indicating the presence of interfacial defects. The band offsets were measured to be approximately 0 eV as well when the F was removed. Density functional theory predicts the valence band offset for the clean interface to be 0.5 eV. Inclusion of interfacial F reduces the theoretical band offset to 0.2 eV, much closer to experiment, and suggesting that the interface dipoles created by F and sputter defects have a major effect on the band offset.