MICROSTRUCTURE DEFECTS IN BICRYSTAL SUBSTRATES AND THEIR INFLUENCE ON YBa2Cu3O7 GRAIN BOUNDARY JUNCTIONS

Julia W. P. Hsu, University of Virginia

Using a near-field scanning optical microscope (NSOM), we found submicron-size microstructural defects distributed non-uniformly along the fusion boundaries of bicrystal substrates. These defects depict themselves as circular dark spots in the optical transmission images, and are most likely near-surface voids. Surveying bicrystals of various tilt angles and of different materials that are commonly used for fabrication of YBa2Cu3O7 (YBCO) grain boundary Josephson junctions shows that these defects are always present, but the density and distribution vary depending on the material. We believe that these defects are the result of the bicrystal fusion process and are not intrinsic to the materials. We also established a direct link between the presence/absence of these defects in the substrate and the superconducting characteristics of the YBCO junctions. Strain fields associated with these defects are most likely responsible for affecting YBCO film growth locally and junction performance.

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