films

We investigate the metallic surface state in ultra-thin films of c-axis oriented BaTiO₃ by angle resolved photoemission spectroscopy (ARPES). We find Fermi surface contours derived from the Ti 3d t_2g conduction band, with broad spectral features due to enhanced quasi-particle scattering. Oxygen vacancies created in the X-ray beam spot allow for reversible in situ doping control. Up to carrier densities as high as 2 × 10²¹ cm⁻³, they retain a charge state of 2+, preventing the formation of localized defect states and charge carrier trapping. Sub-surface oxygen vacancy migration at T > 285 K quenches the surface state. Our findings challenge the implementation of non-volatile room temperature applications based on non-stoichiometric BaTiO₃.