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 scanning tunnelling and atomic force microscopies to reveal the development of the surface topography. It has been shown that films on MgO exhibit Volmer-Weber island growth but those on SrTiO3 grow in the two-dimensional Frank-van der Merwe mode. For both systems, spiral growth features are apparent only for films above a certain deposit thickness indicating that they are formed at the emergent threading segments of dislocation half-loops introduced in response to the misfit. For films on SrTiO3, the thickness at which this occurs corresponds well with the critical thickness for dislocation half-loop introduction given by Matthews' model. For films on MgO, the first spirals arise at a similar thickness indicating that the residual misfit is similar to the total misfit for YBCO/SrTiO3. The spiral density increases with deposit thickness to a peak value before falling to a background level in both systems. The form of the variation corresponds well to the variation in threading dislocation density that would be expected for dislocation half-loop introduction followed by mutual annihilation of adjacent threading segments with Burgers vector components parallel to [001] of opposite sense.