Band alignment in LaAlO 3 /SrTiO 3 oxide heterostructures inferred from hard x-ray photoelectron spectroscopy

We present a detailed study of the electronic structure and band alignment in LaAlO${}_{3}$/SrTiO${}_{3}$ oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO${}_{3}$ film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO${}_{3}$ film and the SrTiO${}_{3}$ substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO${}_{3}$/SrTiO${}_{3}$ heterostructures.