Thickness-dependent metal-to-insulator transition in La-doped SrTiO₃ thin films

The electronic and structural properties of La-doped SrTiO₃ (LSTO) films grown by molecular beam epitaxy are studied as a function of film thickness and La doping. We demonstrate that electronic transport is highly affected by the insulating dead layers inside LSTO films, whose thickness depends on La doping. As the film thickness is reduced below the dead layer thickness, an abrupt metal-to-insulator transition occurs. We attribute the existence of insulating layers to the surface structure of LSTO. The reduced conductivity of LSTO films due to dead layers can be counteracted by growing additional capping layers on top of the LSTO surface, indicating the strong relationship between insulating dead layers and surface structure of LSTO films. Synchrotron-based X-ray scattering analysis is used to test this hypothesis. Our results emphasize the importance of surface structure studies for functional oxides in the thin film limit and provide a guiding principle to fabricate oxide nanoscale devices.