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S. Saiello Dip, di Ingegneria dei Materiali e della Produzione, Universitd Federico II di Napoli, Italy

U. Scotti di Uccio and M. Valentino Dip. Scienze Fisiche dell'Universitd Federico II di Napoli, Italy (Received 3 August 1993; accepted 4 October 1994)

High-quality YBCO thin films have been grown by Inverted Cylindrical Magnetron Sputtering (ICMS) on LaAlO3(100), SrTiO3(100), SrTiO3(110), and MgO(lOO) substrates. Transition temperatures of oaxis films exceed 90 K, and transition widths are within 1 K. Critical currents range up to 5 X 106 A/cm 2 at 77 K. Structural and morphological features analyzed by x-ray diffraction and scanning electron microscopy, respectively, are found to be strongly dependent on film orientation and deposition temperature. In order to understand such dependence, a simple interpretation is proposed in terms of Gibbs energies and growth dynamics of the nucleation process.

I. INTRODUCTION After the discovery of 90 K superconductivity in the YBCO compound,1 a great amount of work has been devoted worldwide to the realization of YBCO films with the best attainable transport properties. This task has been achieved in several laboratories, at least as far as dc properties are concerned. In order to obtain samples that are suitable for technological and scientific applications, and to optimize rf properties, careful control of structural and morphological features is also needed. Two especially important problems that we need to deal with are (i) control of crystal orientation and (ii) minimization of surface defects. (i) It is well known that different film orientations may be needed in view of different applications. Samples with the c-axis perpendicular to the substrate plane [(001) orientation] are necessary for all applications that require excellent transport properties (like high critical current densities Jc and low microwave surface resistance Rs). Films with the c-axis parallel to the substrate plane [(100) and (110) oriented] may be instead required (as a consequence of the coherence length anisotropy) in some configurations of tunnel and Josephson junctions. Besides that, it has been proven that surface morphology and other film structural features, like twins, grain boundaries, and other defects, are closely connected to orientation. The design of a number of planar devices, such as bicrystal or step Josephson junctions (see Ref. 2), is based on the control of the crystal growth. (ii) Different kinds of surface defects3"6 are found on YBCO samples. Surface morphology is essentially J. Mater. Res., Vol. 10, No. 1, Jan 1995

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determined by the shape of the single crystal grains and by the density and size of the outgrowths present on the surface; those two features are dependent on deposition conditions and on the orientation of the sample. The presence of such defects represents a serious problem in the development of any technology requiring multilayer deposition because the presence of surface roughness on a layer will cau