Effects of epitaxial strain on the growth mechanism inYBa2Cu3O

Abstract

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...

Interface engineering and strain in YBa2Cu3O7−δthin films

“…The observation of a reduced c-axis for ultrathin films is in good agreement with results for sputter deposition [36,37], but contradict earlier results by pulsed laser deposition [20]. An important point is the SrRuO 3 cap layer in these experiments, which was not present in the other PLD experiments.…”

Spin diffusion versus proximity effect at ferromagnet/superconductorLa0.7Ca0.3MnO3∕YBa2Cu3OPeña1, Visani2, García‐Barriocanal3 et al. 2006Phys. Rev. BSelf Cite541251View full textAdd to dashboardCite

“…We have used the PBCO buffer since aside from providing a better growth as discussed above, it has been reported that YBCO single films directly grown on STO substrates have smaller T c values as compared with samples grown in PBCO buffers. 34 Both samples show the critical temperature to decrease when the thickness of the superconducting layer is reduced. However, while YBCO/LCMO/STO bilayers show broad tails for the smallest YBCO thicknesses pointing to a 35,36 as a result of epitaxial strain relaxation or dimensionality effects, but the decrease is more pronounced for samples with ferromagnetic layers as a result of the strong F/S interplay observed previously in trilayers and superlattices.…”

Interface barriers for flux motion in high-temperature superconducting superlattices

“…This can be understood considering that YBCO layers in the superlattice are fully decoupled, and thus the observed behavior is in fact the same that could be expected for a single YBCO layer of identical thickness and perfect surfaces. Unfortunately in practice YBCO single films five unit cells thick exhibit depressed T c values ͑60 K͒ compared to superlattices ͑80 K͒, resulting from surface imperfections and exposure to ambient conditions, 30 and thus comparison of the dissipation properties is meaningless. BL barrier effects are not observed in the coupled superlattice since the relevant length scale to compare with a 0 is sample thickness ͑200 nm͒.…”