Growth and characterization of Y-BA-CU-O high-Tc superconductor thin films

Citation

Kittl, Jorge A. (1991) Growth and characterization of Y-BA-CU-O high-Tc superconductor thin films. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/918E-BJ98. https://resolver.caltech.edu/CaltechETD:etd-06222007-150332

Abstract

Two types of growth processes of Y-Ba-Cu-O thin films were investigated: three step processes involving post deposition high temperature anneals, and in situ growth processes. Films were deposited by sequential ion beam sputtering from elemental Y, Ba and Cu targets, and characterized by x-ray diffraction, transmission and scanning electron microscopy, energy dispersive x-ray analysis, Rutherford backscattering spectrometry, and low temperature resistivity measurements. In the three step process, multilayers of ~60 Å periodicity were deposited on (001) SrTiO₃, annealed in oxygen at 850-900°C, and subsequently at 400-500°C, to obtain the superconducting YBa₂Cu₃O₇-δ phase. The films were epitaxial, predominantly single phase YBa₂Cu₃O₇-δ, with different orientations. The nucleation and growth of Y-Ba-Cu-O films deposited on (001) SrTiO₃ by magnetron sputtering from separate Y, BaF₂ and Cu sources and grown by a three step process, was investigated by transmission electron microscopy. The in situ growth of YBa₂Cu₃O₇-δ films by sequential ion beam sputtering was investigated. The films were deposited following the stacking sequence of YBa₂Cu₃O₇-δ, with the individual layer thicknesses nominally equal to one monolayer, at temperatures between 550 and 750°C. O2 was supplied during growth. Epitaxial, c-axis oriented YBa₂Cu₃O₇-δ films were obtained on MgO and SrTiO₃. The correlations between deposition parameters, and structural and electrical properties were investigated. The films had expanded c-axis lattice parameters. The superconducting transition temperatures decreased with the enlargement of the c-lattice parameter. The deposition temperature was the main parameter controlling the lattice expansion. This was later interpreted in terms of the thermally activated dissociation of O₂ at the film surface. We proposed that the expansion of the c-lattice parameter was a consequence of kinetic limitations to the incorporation of oxygen into the films during growth. This led to a consistent description of the results obtained in this work, and results reported in the literature for other in situ growth techniques. The films also presented inhomogeneous lattice distortions along the c-direction, that were larger for films with large lattice parameters. The superconducting transitions were broader for films with large inhomogeneous strains. The microstructure of films grown on several substrates (SrTiO₃, MgO, SiO₂/Si) under different growth conditions was investigated.

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