Role of lanthanum aluminate composition in interface formation with strontium titatnate

While LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) are both insulators, the interface between the two materials is conductive when the LAO is above a critical thickness and the STO is TiO$_2$-terminated; the origin of this conductivity is widely debated. It has also been demonstrated that the conductive interface depends on the composition of the LAO film: conductive interfaces were only found to occur for La:Al $<$ 0.97. Hard x-ray photoelectron spectroscopy has been performed on ten unit cell thick Al-rich (La:Al = 0.9), stoichiometric (La:Al = 1), and La-rich (La:Al = 1.1) LAO films deposited on STO to elucidate the role of LAO composition in the interfacial structure. A small built-in potential was observed in the Al-rich film compared to the stoichiometric and La-rich films, as determined from valence-band broadening. The stoichiometric and La-rich films were also found to have La-enrichment at the interface, while the Al-rich film did not. These results combined with first-principles calculations demonstrate the role that defects in the LAO film play in the LAO/STO interfacial structure.