Thickness dependence of J_c for YBCO thin films prepared by large-area pulsed laser deposition on CeO_2 -buffered sapphire substrates

摘要

We evaluated for the first time the thickness dependence of the critical current density (J_c) of micrometre thick YBCO films on CeO_2-buffered sapphire. YBCO films were successfully grown in microcrack-free form up to a thickness of approx 1.6 mu m by large-area pulsed laser deposition. J_c was found to decrease exponentially with YBCO thickness. Results suggest that the reduction in J_c with film thickness can be attributed to an evolving film microstructure as a function of thickness, as well as a corresponding change in the defect structures responsible for flux pinning. It was observed that film porosity and roughness increased with film thickness due to the growth and encroachment of the BaY_2O_4 phase. To clarify the flux pinning mechanism, we measured the angular dependence of J_c for films of different thicknesses and correlated this with the defect structure as revealed from the etch pit method and atomic force microscopy (AFM) observations. An unusually prominent J_c peak was observed when H c, which is due to correlated extended defects parallel to the c-axis of YBCO. Examination of the film microstructure revealed two defect types that give rise to the J_c peak: linear defects in the form of screw and edge dislocations, and planar defects possibly in the form of stacking faults. The density of linear defects decreased with film thickness whereas that of the planar defects increased considerably. From the behaviour of J_c with film thickness, these results suggest that linear defects may be more effective pinning centres than planar defects, or that an overabundance of planar defects may offset the increase of J_c for thick films.