Hard X-ray Photoelectron Spectroscopy Investigation of Off-Stoichiometric LaAlO3 interfaces with SrTiO$_{3}$

LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) are both insulators, yet the interface between TiO$_{2}$-terminated STO and LAO is a high mobility conductor when the LAO film is above a critical thickness. The origin of this conductivity is under debate; possible explanations for conductive interfaces include chemical intermixing, oxygen defects, and charge redistribution arising from the built-in potential due to the polar LAO layers on the non-polar STO surface. Recently, interfacial conductivity has been found to depend on the stoichiometry of the LAO film, with Al-rich samples providing conductive interfaces, while stoichiometric and La-rich samples do not. Here, hard x-ray photoelectron spectroscopy (HAXPES) and variable kinetic energy XPS (VKE-XPS) have been used to investigate the interface of 10 unit cell films of La-rich, Al-rich, and stoichiometric LAO on STO.