Origin of surface roughness for c-axis oriented Y-Ba-Cu-O superconducting films

One problem with the growth of high quality c-axis oriented YBa 2Cu3O7-x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c-axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c-axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a-axis films. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.