STRUCTURE AND COMPOSITION OF GRAIN-BOUNDARY DISLOCATION CORES AND STACKING-FAULTS IN MOCVD-GROWN YBA2CU3O7-X THIN-FILMS

The dislocation cores of a low-angle grain boundary in MOCVD-grown YBa2Cu3O7-x have been studied by high-resolution electron microscopy and image simulation. It was found that the low-angle boundary consists of a wall of discrete edge dislocations separated by relatively perfect lattice matching regions. Lattice reconstruction has been observed at the dislocation cores. The dislocation cores appear to be Cu-rich, with a core radius of about 1 nm. These observations are used to discuss the transition from strong to weak coupling behavior across grain boundaries. Stacking faults in the a-c and b-c planes have been observed for the first time in the MOCVD-grown YBa2Cu3O7-x thin films. HREM image analysis indicates that the stacking faults contain an extra Cu-O layer. The thickness of the stacking faults is about 1.6 (100) interplanar spacing, or 0.6 nm, which is smaller than the coherence length in a-b plane. Thus, the stacking faults are not expected to strongly affect superconducting properties.