Chemistry and resistance at metal contacts to YBa2Cu3O7 high Tc superconducting thin films

X‐ray photoelectron spectroscopy (XPS) and electrical characterization have been used to investigate metal contacts to thin films of superconducting YBa2Cu3O7−x (YBCO). The metals studied are Au, Pt, Pd, Sn, and Ti, which cover a wide range of physical properties including reactivity with oxygen and bulk resistivity. XPS was also used to investigate the sputter cleaning and subsequent heating of the YBCO films prior to contact formation. Contacts for electrical study were defined by a shadow mask, while contacts studied by XPS were formed by sequential deposition of several A of metal. XPS of the O 1s, Cu 2p, and the various metal core levels confirm that the more reactive metals cause more disruption at the metal/YBCO interface. These observations of interface chemistry are correlated with contact resistivity measured at room temperature and at 77 K. We find that although Au, Pt and Pd have similar contact resistivity at room temperature, only the Au contacts show a large decrease in contact resistance a...