Volatile two-dimensional electron gas in ultrathin BaTiO3 films

We investigate the metallic surface state in ultrathin films of BaTiO3 by angle-resolved photoemission spectroscopy. We find Fermi-surface contours derived from the Ti 3d t(2g) conduction band, similar as in SrTiO3 but with broader spectral features due to enhanced quasiparticle scattering. Oxygen vacancies created in the x-ray beam spot allow for reversible in situ doping control up to surface carrier densities as high as 1014 cm(-2), but vacancy migration into the subsurface at T >= 285 K quenches the surface state. Our analysis suggests that the charge state of oxygen vacancies in ultrathin films is predominantly 2+, which limits charge-carrier trapping and the formation of localized defect states.