X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY OF THIN AG OVERLAYERS ON YBA2CU3O7-X - EFFECTS OF OXYGEN ANNEALING

X-ray photoelectron spectroscopy was used to examine vapor deposited Ag overlayers on YBa2Cu3O7-x thin films and cleaved single-crystal surfaces (x=1), including the effects of subsequent heat treatment in oxygen. The core level spectra of the present tetragonal crystals were found to closely resemble those previously reported for nominally orthorhombic superconducting samples. Minor differences, notably an increased fraction of Cu in the +1 oxidation state and a slightly higher binding energy of the Ba 3d levels, may be attributed to the lower oxygen content of the tetragonal semiconducting phase. Deposition of Ag on clean monocrystal surfaces was found to cause some disruption of CuO bonds at the Ag/YBa2Cu 3O7-x interface. We also find weak indications of surface band bending (i.e., Schottky barrier formation) from a uniform shift in the measured binding energies of the substrate core levels upon Ag deposition. Heat treatment in pure oxygen (1 h at 500 °C) of Ag-coated superconducting thin films was found to reduce efficiently the amount of superficial contaminant phases, initially present on the air-exposed thin-film surfaces, and to cause diffusion of Ag to a depth of ∼20 nm into the volume of the material.