Microstructural defects in bicrystal substrates and their influence on yttrium barium copper oxide grain boundary Josephson junctions

Using a near-field scanning optical microscope (NSOM), we found non-uniform ly distributed microstructural defects near the fusion boundary of bicrysta l substrates, These defects depict themselves as circular dark spots in the optical transmission images. We attribute these optical features to near-s urface voids at the boundary previously found in transmission electron micr oscopy studies. Our results show a direct link between the presence of thes e defects and the superconducting properties of the YBa2Cu3O7 (YBCO) grain boundary Josephson junctions (GBJJs). We find that junctions that are fabri cated over highly defected regions of the bicrystal substrates have lower c ritical temperatures and currents. The positions of defects across a juncti on also play a role in determine the device characteristics. Strain fields associated with these defects are most likely responsible for affecting YBC O film growth locally and junction performance. We employ an NSOM to survey these microstructural defects in bicrystals of various tilt angles and of different materials that are commonly used for fabrication of YBCO GBJJs. W e find that these defects and their strain fields are the result of the bic rystal fusion process and are not intrinsic to the materials.