Surface layers on superconducting Yhyphen;Bahyphen;Cuhyphen;O films studied with xhyphen;ray photoelectron spectroscopy

摘要

Xhyphen;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 laserhyphen;deposited superconducting films that have nearly atomically flat surfaces for which quantitative analysis formalisms exist. The chemical compositions of highTc(90 K) and highJc(106A/cm2) Yhyphen;Bahyphen;Cuhyphen;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 takehyphen;off angles and different escape depths lsqb;using Althinsp;Kagr; (1486.6 eV) and Mgthinsp;Kagr; (1253.6 eV) excitationsrsqb;, 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 80percnt; of the detected signal originates in the bulk. The surface layer contains Cu2+and oxygen with a photoelectron binding energy of 531 eV.