The Influence of Tte Substrate Surface on the Nucleation and Growth of Superconducting Thin Films
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THE INFLUENCE OF THE SUBSTRATE SURFACE ON THE NUCLEATION AND GROWTH OF SUPERCONDUCTING THIN FILMS S. N.BASU, N.BORDES, A.ROLLETT, M.COHEN and M.NASTASI Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT The quality of epitaxially grown superconducting thin films is dependent on the nature of the substrate surface. In this study, the surface of oriented SrTiO3 single crystals was modified by mechanical polishing, thermal annealing and chemical etching. A study of the YBa2Cu3O7-x thin films grown on these substrates showed that high quality superconducting films were obtained when the substrate surface was highly oriented in the direction and was also microscopically smooth. INTRODUCTION An important potential application of high temperature superconducting films is in electronic devices. One of the major problems with these films is their small critical current density as compared to metallic superconductors. It has been demonstrated that high critical current densities can be achieved in YBa2Cu3O7-x films (hereon referred to as YBCO) grown on SrTiO3 substrates, if the film can be grown epitaxiatly, or with a high degree of texture, with the c-axis of the film being perpendicular to the substrate surface (c-oriented) [1]. It is therefore very important to understand the factors influencing the nucleation and epitaxial growth of these films. The quality of the substrate plays a key role in determining the quality of the film grown on it. This paper presents the results of a study on the influence of the SrTiO3 substrate on the nucleation and epitaxial growth of superconducting YBCO thin films. EXPERIMENTAL The surface of various oriented single crystal SrTiO3 substrates were modified using mechanical, thermal and chemical treatments. The quality of these substate surfaces were characterized by Rutherford Backscattering (RBS) analysis, by measuring the channeling yield in the direction using 2.2 MeV He+ ions. The surface roughness of the substrates was examined by profilometer scans. Thin films were deposited on these substrates by electron beam coevaporation of Y, BaF2 and Cu under a vacuum of 5X1 0-8 torr. During deposition, the substrates were at room temperature. The films were then annealed for 1 hour at 7500C in dry flowing 02, followed by a 1 hour anneal at 850 0C in 'wet' 02 (02 bubbled through deionized water). The films were then slowly furnace cooled to room temperature. A profilometer trace showed these annealed films to be about 15ooA thick. High energy backscatterring analysis, using 8.8 MeV a particles [2] showed the composition of the film to be Y0.9Ba2.1Cu2.807. 0 . The films were characterized using 4point resistivity measurements, high energy channeling at 8.8Mev and Scanning Electron Microscopy (SEM). The volume of c-oriented to a- (and b) oriented material (c/a ratio) in the films were estimated from X-ray measurements in a texture goniometer. The films were also studied in cross-section by Transmission Electron Microscopy (TEM). RESULTS AND DISCUSSION Six different oriented substrates
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