Control of surface electronic structure of high T/sub c/ superconducting films for Josephson junctions and electron spectroscopy

A high-density atomic oxygen beam generated by thermal nonequilibrium high-temperature plasma has been used to control the surface nature of high-T/sub c/ superconducting YBCO epitaxial films and Cu-1234 ceramics. We report characterization of the electronic structure of the cleaned surfaces by in-situ ultraviolet and X-ray photoemission spectroscopy (UPS,XPS) and study of the relationship between the surface natures controlled and transport properties of multilayer structures fabricated over them. For both kinds of materials, exposure of the surface at a temperature of 300/spl sim/500/spl deg/C to the atomic oxygen beam yields highly ordered surface structures and a metallic feature with high electron density of states at the Fermi level. Owing to the improvement, the contact resistance between a cleaned YBCO[100] film and in-situ deposited gold, and a cleaned Cu-1234 and gold were found to be in the range of 10/sup -10//spl sim/10/sup -9/ and /spl sim/10/sup -8/ /spl Omega/.cm/sup 2/, respectively. The realization of an intrinsic Au/YBCO interface resulted in direct observation of the superconducting proximity effect in the gold layer from the YBCO. The control of the electrical connection also led to an enhancement of interlayer superconducting coupling in Pb/Ag/Au/YBCO[100] junctions. A positive correlation was found between the critical current density of the junction J/sub c/ and the height of photoelectron Fermi edge of YBCO, which corresponds to the electron density of states at the surface. A high J/sub c/, of the order of 10/sup 4/ A/cm/sup 2/, was achieved on the junctions fabricated on the YBCO surface with a Fermi edge height of 10% relative to that of Cu 3d UPS-signal.