SURFACE-LAYERS ON SUPERCONDUCTING Y-BA-CU-O FILMS STUDIED WITH X-RAY PHOTOELECTRON-SPECTROSCOPY

X-ray photoelectron spectroscopy (XPS) examines only a thin surface layer (<5 nm) that may not be representative of the bulk. We separated the information from the surface and bulk by using laser-deposited superconducting films that have nearly atomically flat surfaces for which quantitative analysis formalisms exist. The chemical compositions of high Tc (90 K) and high Jc (>106 A/cm2) Y-Ba-Cu-O films on SrTiO3 (001) substrates were examined. From the relative intensities of the surface and bulk components of the Ba(3d) and Ba(4d) spectra taken at different take-off angles and different escape depths [using Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) excitations], we have determined the nonsuperconducting surface layer thickness to be 1 nm and the layer composition to be BaCuO2. The surface layer thickness for a superconducting film only 8 nm thick was also 1 nm. By detecting the substrate Ti signal through this film, and ruling out a high density pinholes, we provide evidence that the XPS data contain information from the superconducting phase. A polycrystalline pellet scraped in vacuum had a surface layer only 0.4 nm thick. Since typical photoelectron escape depths are about 2 nm, about 80% of the detected signal originates in the bulk. The surface layer contains Cu2+ and oxygen with a photoelectron binding energy of 531 eV.