Photoemission study of the metallic state of lightly electron-doped SrTiO3

The behavior of the metallic state of lightly electron-doped SrTiO3 has been studied using angle-integrated ultra-violet photoemission spectroscopy (UPS) and angle-resolved photoemission spectroscopy (ARPES). Two states in the bulk band gap of stoichiometric SrTiO3 are shown by doping electron carriers: a metallic state with a sharp Fermi cut-off and a broad state centered at 1.3 eV below the Fermi level. By slight oxygen exposure, the UPS spectra showed a rapid decrease in the spectral intensity of the metallic state followed by a sudden energy shift of the valence band. These changes are explained in terms of the band-bending effect due to the formation of a thick charge depletion layer when the adsorbed oxygen atoms attract the free electrons from the bulk to the surface. Normal-emission photoelectron spectra before the oxygen exposure showed that the spectral intensity of the metallic state is remarkably enhanced at the photon energy of 28 and 45 eV whereas its energy position is almost independent of the photon energy. Off-normal-emission spectra showed that the spectral intensity of the metallic state is drastically reduced with increasing emission angle. From a comparison between the calculated band structure and ARPES results, we conclude that the metallic state is qualitatively explained by the Fermi liquid behavior based on the band theory