X-ray photoemission studies of the metal-insulator transition in LaAlO3/SrTiO3 structures grown by molecular beam epitaxy

LaAlO3/SrTiO3 interfaces grown by molecular beam epitaxy show a metal-insulator transition at a critical film thickness of four unit cells of LaAlO3 on TiO2-terminated SrTiO3 substrates. This transition had previously been observed in LaAlO3 films grown by pulsed laser deposition, where defects related to the growth process have been suggested as playing a role in the interface behavior. X-ray photoemission was used to examine the band offsets and look for an electric field across LaAlO3 for a range of film thicknesses on both SrO- and TiO2-terminated SrTiO3 substrates. These results are compared to the predictions of the polar catastrophe model for the emergence of a metallic interface in this system. We do not find the predicted electric field in the LaAlO3 or any dependence on substrate termination. While the observation of metal-insulator transitions in LaAlO3/SrTiO3 structures grown by different techniques points to an intrinsic effect, the absence of an electric field in the LaAlO3 layer is not consistent with the polar catastrophe model of interface metallicity.