Surface passivation and electronic structure characterization of PbTiO3 thin films and Pt∕PbTiO3 interfaces

Abstract

The surface electronic structure, postdeposition surface passivation, and Schottky barrier height in contact with Pt of PbTiO3 thin films on (001) SrTiO3 were investigated by x-ray photoemission spectroscopy (XPS). Angle-resolved XPS analysis shows that an ∼10-Å-thick surface layer which consists of lead carbonate and lead oxide exists on high-quality PbTiO3 epitaxial films, although the layer can be removed by postdeposition aqueous HNO3 etching. Electronic states associated with this defective surface layer determine the position of the surface Fermi level relative to the band edges of the PbTiO3 film. In situ XPS measurements were carried out during the Pt deposition on as-grown and HNO3-treated PbTiO3 films. The Pb 4f, Ti 2p, and O 1s peaks were observed to shift to higher binding energies during the in situ Pt deposition, consistent with metallization-induced band bending. Although the initial Fermi energies for both Pt-uncoated as-grown and HNO3-treated PbTiO3 differ by ∼0.3eV, the postmetallization Fermi energy lies at 2.4eV above the valence-band maximum after 2 ML (monolayers) of the Pt deposition for both samples. These results suggest that the Fermi level is pinned by interface defect states because the resulting Pt∕PbTiO3 electron Schottky barrier (∼1eV) is substantially smaller than the value derived from recent electronic structure calculations (1.45eV). Consistent with this observation, angle-resolved XPS results indicate that the (001) surface of both as-deposited and HNO3-treated PbTiO3 films decomposes during the initial stages of the Pt deposition and that metallic Pb diffuses into the Pt layer during the Pt deposition, even at room temperature. The presence of the metallic Pb and the resultant formation of a defective interface layer at the Pt∕PbTiO3 (001) interface apparently produce the observed Fermi energy pinning.