Control of meandering grain boundary configurations in YBa 2 Cu 3 O y bicrystal thin films based on deposition rate
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Control of meandering grain boundary configurations in YBa2 Cu3 Oy bicrystal thin films based on deposition rate X. F. Zhang and D. J. Miller Materials Science Division and Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, Illinois 60439
J. Talvacchio Westinghouse Science and Technology Center, Pittsburgh, Pennsylvania 15235 (Received 8 November 1995; accepted 4 June 1996)
Changing the film deposition rate is shown to be one way to influence the meandering configurations of grain boundaries (GB’s) formed in YBa2 Cu3 Oy (YBCO) bicrystal thin films. The magnitude and wavelength of the meander in YBCO films deposited at two different rates have been characterized by transmission electron microscopy (TEM) and statistically quantified. It has been found that the meander becomes more uniform and considerably less rough in films deposited at lower rates compared to that observed in films deposited at higher rates. A mechanism for the formation of the meandering GB is proposed based on heterogeneous nucleation and three-dimensional (3D) island growth together with overgrowth of the YBCO films across the substrate grain boundary. The different island sizes and tendency for overgrowth induced by changing the film deposition rate are believed to play important roles in controlling the meandering GB configuration. The possible effects of meandering configurations on transport properties are discussed.
I. INTRODUCTION
It is well known that the weak link behavior of grain boundaries in high transition temperature (Tc ) superconductors contributes to a lower critical current density (Jc ) in polycrystalline samples compared with the Jc carried within individual grains. In order to characterize the transport properties and microstructures of individual grain boundaries (GB’s), Dimos et al. prepared artificial YBa2 Cu3 Oy (YBCO) GB’s by depositing epitaxial YBCO films on [001] tilt SrTiO3 bicrystal substrates.1 A consistent reduction in Jc versus increased bicrystal misorientation angle was observed and attributed to dislocation arrays and other microstructural features associated with GB regions.1 Later studies on YBCO bicrystals by Babcock et al. suggested the possibility of a hole depletion zone along the YBCO GB when the misorientation angle was large (28± and 31±), and the weak link effect measured for these grain boundaries was attributed to this hole depletion.2 The factors affecting Jc across GB’s in polycrystalline YBCO materials were analyzed from the microstructural point of view by Zhu et al.3 Clean grain boundaries free of impurity phases, full oxygenation and good connections between CuO2 planes across the GB were identified as critical factors.3 All of these results suggest a close relationship between transport properties and boundary microstructures in YBCO GB’s. In order to establish the link between the structure of the boundary and the transport properties across 2440
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J. Mater. Res., Vol. 11,
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