Local characterization of Y-Ba-Cu-O thin films

Abstract

A high-resolution spatially resolved study of electrical inhomogeneties in high-Tc thin films by Laser Scanning Microscopy has been carried out. A laser beam was focused onto a submicrometer spot on the surface of the thin film microbridge resulting in an increase in its local temperature. Bolometric or thermo-electric effects in the heated region result in a voltage change across the sample that has been measured as a function of the beam position. These two electrical images have been compared with optical images formed by subtracting two conventional images taken with polarized light. The correlation between the three images strongly suggest that interfaces between grains with different twin boundary orientations are more resistive than interfaces between grains with the same twin boundary orientation.

Laser Scanning Microscopy Studies on Detwinned<tex>$rm YBa_2rm Cu_3rm O_7-delta$</tex>Thin Films

“…A thermoelectric voltage is obtained by inducing a temperature gradient in the film using the focused laser beam. Any electrical defects, such as a grain or a bi-crystal junction will produce a response with odd-symmetry [3], [4], [10], [11] meaning that the voltage obtained on one side of the boundary will be equal in magnitude, but opposite in sign to the one obtained on the other side of the boundary. This mode, therefore, provides a method to identify the location of defect boundaries.…”

“…By scanning the beam it becomes possible to determine the sample's local electrical characteristics. The technique is well suited for determining the electrical homogeneity of a superconducting thin film [3], [4], [9]- [11].…”

“…This reduces the overall current carrying capacity of the film. The image obtained by rastering the beam can be used to determine the homogeneity of local current carrying capacity of the film [3], [4]. The second mode allows for a thermoelectric measurement that is carried out on an unbiased bridge.…”

“…Boundaries inside a grain were the least resistive, followed by boundaries between grains with the same microtwin orientation. The most resistive boundaries were those between grains with a perpendicular microtwin orientation [3], [4]. The extent of the microtwinning in thin films has made it difficult to study the transport properties of individual twin boundaries as the length scales involved are inaccessible to the LSM technique.…”