TOPOGRAPHICAL DEVELOPMENT AND MISFIT RELIEF IN LASER-ABLATED HETEROEPITAXIAL YBA2CU3O7-DELTA THIN-FILMS

Epitaxial YBa2Cu3O7-delta thin films of various thicknesses have been deposited on single crystal substrates of MgO and SrTiO3 with nominal (001) surface orientations. These films have been examined using scann ing tunnelling and atomic force microscopies to reveal the development of the surface topography. It has been shown that films on MgO exhibi t Volmer-Weber island growth but those on SrTiO3 grow in the two-dimen sional Frank-van der Merwe mode. For both systems, spiral growth featu res are apparent only for films above a certain deposit thickness indi cating that they are formed at the emergent threading segments of disl ocation half-loops introduced in response to the misfit. For films on SrTiO3, the thickness at which this occurs corresponds well with the c ritical thickness for dislocation half-loop introduction given by Matt hews' model. For films on MgO, the first spirals arise at a similar th ickness indicating that the residual misfit is similar to the total mi sfit for YBCO/SrTiO3. The spiral density increases with deposit thickn ess to a peak value before falling to a background level in both syste ms. The form of the variation corresponds well to the variation in thr eading dislocation density that would be expected for dislocation half -loop introduction followed by mutual annihilation of adjacent threadi ng segments with Burgers vector components parallel to [001] of opposi te sense.