We report on the growth mechanism of YBa 2 Cu 3 O 7Ϫx . Our study is based on an analysis of ultrathin YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ layers in c-axis-oriented YBa 2 Cu 3 O 7Ϫx /PrBa 2 Cu 3 O 7 superlattices. We have found that the release of epitaxial strain in very thin YBCO layers triggers a change in the dimensionality of the growth mode. Ultrathin epitaxially strained YBCO layers with thickness below 3 unit cells grow in a blockby-block two-dimensional mode that is coherent over large lateral distances. Meanwhile, when the thickness increases and the strain relaxes, layer growth turns three-dimensional, resulting in rougher layers and interfaces. DOI: 10.1103/PhysRevB.66.174514 PACS number͑s͒: 74.76.Bz, 68.65.Cd, 74.80.Dm
INTRODUCTIONIn recent years, thin films of complex ionic oxides have played a significant role in the discovery and understanding of important new phenomena in condensed matter physics. Basic studies or important related applications in various fields like high-T c superconductivity, colossal magnetoresistance, etc., require the growth of ultrathin films with controlled composition and morphology. In this respect, the specific growth mode seriously influences the final film structure and morphology, and thus can deeply condition its physical properties. Growth mechanisms are determined by the chemical composition, type of chemical bonding, etc., and therefore they may be intrinsically different when going from semiconductors to metals or ionic oxides. While the questions of which is the growth mechanism and which is the minimum growth unit have been satisfactorily addressed in semiconductors or metals, in the case of ionic oxides, the growth process is still not fully understood due, probably, to the complexity of the unit cell. Experiments directed at the analysis or application of the physical properties of those ultrathin layers require an accurate control of the growth process at the atomic scale. In fact, many studies fail because the chemical and physical disorder at the interfaces ͑interdif-fusion and roughness͒ determined by the particular growth mechanism may invalidate the conclusions obtained for ultrathin layers.
1,2It has been reported that in the initial stages, the growth of c-axis-oriented YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ films takes place quite smoothly, in a two-dimensional ͑2D͒ mode.3-5 From analysis of superlattices with a noninteger number of YBa 2 Cu 3 O 7Ϫx cells, in a recent Letter we have reported evidence for a two-dimensional block-by-block growth mechanism in which the growth units are single unit cells. 6 We use the term ''block-by-block'' in the sense that the high substrate temperature (900°C) provides the necessary mass transport for complete unit cell blocks to nucleate. We showed that it is possible to obtain laterally coherent oneunit-cell-thick layers. On the other hand, a three-dimensional growth mechanism around screw dislocations has been reported for thicker films, leading to a rough terracelike surface morphology. 7,8 In fact this 3D growth has been recently show...