Oxygen annealing of ex-situ YBCO/Ag thin-film interfaces

摘要

The resistivity of YBCO/Ag interfaces has been measured for different oxygen annealing temperatures for a series of ex-situ fabricated thin-film contacts having sizes from 16 μm×10 μm down to 4 μm×4 μm. The interface resistivity began to decrease after annealing at 10 minutes in one atmosphere oxygen. After annealing at 400°C, the contact resistivity decreased by several orders of magnitude to the 10-7 range. The 500-nm thick Ag layer showed surface diffusion and agglomeration for annealing temperatures above 400°C; this temperature thus represents a practical limit for oxygen annealing the YBCO/Ag interface system for more than 10 minutes. Rapid cooling of the chip after annealing led to a severe loss of critical current density in the YBCO layer, which could be restored by reannealing and cooling at a slower rate of 50°C/min. The relative shape of the conductance-vs.-voltage characteristics of the YBCO/Ag interface were essentially unaltered by oxygen annealing; the overall parabolic shape, superconducting gap features, and magnetic-scattering zero bias anomaly remained constant, even though the contact conductance increased by several orders of magnitude. These data suggest main reduction in interface resistivity enhancement of the effective contact area, not a change in interface conduction mechanism.