Thickness dependence of Jc for YBCO thin films prepared by large-area pulsed laser deposition on CeO2-buffered sapphire substrates

We evaluated for the first time the thickness dependence of the critical current density (J(c)) of micrometre thick YBCO films on CeO2-buffered sapphire. YBCO films were successfully grown in microcrack-free form up to a thickness of similar to 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 BaY2O4 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 parallel to 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.