Microstructure of device‐related YBCO thin films

A preliminary brief review is given on various kinds of microstructure defects in view of their possible effects on flux pinning, junction behaviour, and film growth. Then, transmission electron microscope investigations are reported concerning thin films of YBCO on LaAlO3 (laser‐deposited), on SrTiO3 and MgO (hollow‐cathode sputtered), and on a Mg‐Ti‐spinel buffer layer on MgO. The usual ‘‘c‐epitaxy’’ with the typical twin lamellae has been thoroughly observed, also in case of the buffer layer and within the edge region of photolithographically patterned films where the twins provide conditions of superconductivity. Grains (islands) in the mm size region of a 45° rotated c‐epitaxy were observed in YBCO films on MgO (001). Furthermore, much smaller inhomogeneities of some 1 nm size—possibly nuclei of the ‘‘a‐epitaxy’’—were found which have to be considered with respect to flux poinning.