Electronic charges and electric potential at LaAlO3/SrTiO3 interfaces studied by core-level photoemission spectroscopy

We studied LaAlO3/SrTiO3 interfaces for varying LaAlO3 thickness by core-level photoemission spectroscopy. In Ti 2p spectra for conducting n-type interfaces, Ti3+ signals appeared, which were absent for insulating p-type interfaces. The intensity of the Ti3+ signals was significantly higher than that expected for the transport carrier densities, indicating that part of the carriers at the interfaces are localized. The Ti3+ signals increased with LaAlO3 thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO3 thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects and/or adsorbates providing charges to the interface even below the critical thickness.