YBCO films with induced composition gradient: an accurate exploration of the film growth characteristics

Abstract

The purpose of this study was to investigate the surface morphology and the structure of laser ablated YBa 2Cu 3O 7 (YBCO) films, as a function of cation composition in special samples presenting a wide range of compositions on the same substrate induced by off-axis geometry. This method ensures that the variations observed in the films are only influenced by the deviations from composition. The YBCO films studied had an almost linear composition gradient, which allowed the analysis in a continuous range of Cu/Ba compositions from 1.0 to 2.0 along a pseudo-binary line with Ba/Y∼1.4. The Y in excess induced the formation of Y 2O 3 inclusions in the film for all the Cu/Ba range. A dramatic change in the surface morphology, domain orientation, and c-axis parameter, explored through local X-ray diffraction measurements, was systematically observed when crossing the composition ratio Cu/Ba∼1.6, regardless of the oxygen pressure used during the deposition process. For Cu-rich compositions, Cu/Ba>1.6, the films show the presence of CuO precipitates onto a flat c-axis oriented YBCO matrix, whereas in the Cu-depleted region, Cu/Ba<1.6, a rough morphology starts to develop, associated to a progressive misorientation of the c-axis domains. The smoothest morphology, with total absence of outgrowth precipitates or deep holes, and rocking curve width below 0.1°, was obtained in a very narrow range of Cu/Ba composition ratio around 1.6. The abrupt transition from one regime to the other was related to a change in the growth mode, influenced by the balance between the formation of supplementary Cu–O planes and the presence of anti-phase boundaries. Some variations in the c-axis parameter of the films were also observed and were related to the oxygen diffusion mechanism during film preparation, and induced by the particular microstructure of the different regions.