Thickness Dependence of the Critical-Current Density and its Relation to Near-Interface Crystal Imperfections in Fluorine-Free-MOD YBCO Films

We have studied the thickness (t) dependence of the critical-current density (
J
_c
) of epitaxial YBCO films grown by fluorine-free MOD to understand the origin of strong depletion of
J
_c
by film thickening. We have grown more than thirty samples with t varied from 0.2 to 1.4 μm, and obtained a detailed
J
_c
-vs-t relation with good statistics. It was found that
J
_c
is strongly suppressed by increasing t, and the rate of suppression is changed at around t=0.4 μm. Numerical fitting of the data revealed that the
J
_c
-t curve above 0.4 μm is well reproduced by a t-inverse-type function. We discuss implications of these observations comparing with cross-sectional TEM results, which show a characteristic vertical distribution of microstructural disorder. This study is based on a half-photolytic F-free MOD process using a uv-excimer lamp which we have recently developed for fabrication of micron-order-thick epitaxial YBCO films.