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.