Hollow cores and step-bunching effects on (001)YBCO surfaces grown by liquid-phase epitaxy

The knowledge of growth parameters and their influence on the surface morphology is fundamental to improve the surface flatness of high-T(c) superconducting films. YBa(2)Cu(3)O(7-x) (YBCO) layers were grown on (1 1 0)NdGaO(3) substrates by liquid-phase epitaxy (LPE). The morphology of YBCO spirals grown on a c-layer were investigated. For single dislocations with large Burgers vector b, the interstep distance increases linearly with the step height. For complex dislocations having b>1, the interstep distance decreases as a function of the factor epsilon, as predicted by the BCF theory. The step density over this surface remains constant, which is an indication for diffusion-limited growth. Spirals with very small interstep distances are usually observed on vapor-grown layers. This can be explained by a much higher driving force, estimated to be (typically) Delta G=37 000 J/mol for laser-ablated films and for LPE Delta G((Delta T=2.5))=285 J/mol. At equilibrium, the hollow cores of the spirals should be equal to Frank's radius. The measurements of the cores agree fairly well with the theory, for the available values of surface tension and shear modulus. (C) 1998 Elsevier Science B.V. All rights reserved.