BA OXIDES - CORE LEVEL BINDING-ENERGIES AND DEFECT-RELATED FERMI LEVEL PINNING

X-ray photoemission spectroscopy has been used to measure core level and valence band energies for Ba oxides prepared by oxidation of Ba metal and evaporation of Ba in an activated oxygen ambient. We demonstrate that previously-observed negative binding energy shifts for oxidized Ba relative to metallic Ba are due to changes in the position of the Fermi level in the energy gap of the oxide. Shifts of the Ba and O core levels ranging from -0.4 to + 0.5 eV were induced by changing the amount of disorder in an oxide surface by sputtering or annealing without changing the chemical composition of the surface. Movement of EF toward the valence band maximum was correlated with increasing disorder. Apparent shifts of up to -1.6 eV with respect to the metal were obtained when Ba was evaporated in an activated oxygen ambient. These results are related to the unusual core level binding energies observed in the high temperature superconductors, particularly Ba in YBa2Cu3O7-x. © 1990.