Kelvin Probe Force Microscopy Study of LaAlO3/SrTiO3 Heterointerfaces

Surface potential distributions in ultra-thin (0.8-3.9 nm) LaAlO3 layers deposited on SrTiO3 substrates are studied by Kelvin probe force microscopy. It is found that the potential distribution evolves from island-like to a homogeneous one with increasing LaAlO3 thickness. It is suggested that the observed islands are caused by a locally enhanced concentration of mobile charge carriers at the interface that is, in turn, related to non-stoichiometry of the layers with thickness below 4 unit cells. Transition to a homogeneous potential distribution with increasing LAO thickness (≥ 4 unit cells) corresponds to the formation of a quasi-2-dimensional electron gas. The results agree with a percolation model explaining the insulator-to-metal transition that occurs at the LaAlO3/SrTiO3 heterointerface.