Submicrometer electrical imaging of grain boundaries in high-Tc thin-film junctions by laser scanning microscopy

High-resolution spatially resolved study of electrical inhomogeneities in high-T-c thin-film junctions on bicrystal substrates has been carried out. A laser beam has been focused into a submicrometer spot on the surface of the sample and induced an increase of its local temperature. Due to bolometric or thermo-electric effects in the heated region, the change of a voltage, Delta V, across the junction has been developed and it has been measured as a function of the beam position (x,y). The Delta V(x, y) images of YBa2Cu3O7-x grain-boundary junctions made from c-axis and tilted c-axis thin films on bicrystal substrates have been obtained. In spite of the difference in the symmetry of the bolometric and thermo-electric Delta V(x,y) images across the grain boundary, the correlation between these two images along the grain-boundary has been shown. Due to an odd symmetry of thermo-electric images Delta V(x,y) across the grain boundary, it is possible to locate the position of a grain boundary in high-T-c junctions with an improved resolution as low as 0.1 mu m. Within this accuracy, the deviations of a grain boundary in thin films from a bicrystal plane in the substrate have been demonstrated in grain-boundary junctions made from c-axis YBa2Cu3O7-x films and the absence of this faceting has been shown for grain-boundary junctions made from tilted c-axis films. (C) 1998 Elsevier Science B.V.