Microstructures of a -axis oriented YBCO films made by hybrid plasma sputtering
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Microstructures of a-axis oriented YBa2Cu3O7-x films made by newly developed dc 100 MHz hybrid plasma sputtering were investigated using transmission electron microscopy (TEM). The films deposited on (110) NdGaO 3 and (100) SrTiO3 substrates were found to grow in a perfect epitaxial fashion and with clear interface. The plan view of the TEM image showed that both films were comprised of two kinds of grains having the c axis aligning along two perpendicular directions in the plane with equal probability. The structures of the grain boundary, however, were found to be very different for the two films from the plan views. The film on NdGaO 3 showed a lot of twist boundaries, while the film on SrTiO3 consisted of many symmetrical tilt boundaries and basal-plane-faced tilt boundaries. The type of grain boundary is determined by the anisotropic growth rates of the film between c direction and a-b direction.
I. INTRODUCTION High quality epitaxial YBa 2 Cu 3 0 7 - x (YBCO) films with the a axis normal to the film surface are expected to be promising candidates for a Josephson junction, due to a larger coherence length along the a-b plane. These films also offer new opportunities for investigating the anisotropic properties of this material. We have already reported the preparation of pure aaxis YBCO films with excellent crystallinity, using hybrid plasma magnetron sputtering.1 A very small value of 0.027° was achieved as a full width at half maximum (FWHM) in the rocking curve measurement through the (200) peak, and a ^ m i n evaluated from the Ba signal behind the surface peak in the Rutherford backscattering (RBS) channeling measurement was confirmed to be as low as 2.0%. These values, as compared against the 0.026° FWHM of the rocking curve for (110) NdGaO 3 substrate and the ^ m i n of 3.5% for YBCO single crystal,2 indicate that the films made by the hybrid plasma sputtering technique have almost perfect crystallinity. However, it was found to be difficult to improve the superconductivity of the aaxis YBCO film while maintaining such an excellent crystallinity. From the results of Raman spectroscopic studies, Sodtke and Miinder3 have reported that a stable disorder structure within the O sublattice is a cause of the reduced Tc. Although many other studies using transmission electron microscopy (TEM) have also been carried out to understand the superconductivity of pure a-axis films from the viewpoint of the grain boundaries,4'5 the relationship between the crystallinity and the superconductivity still seems to be under debate. In this paper, we report on the microstructures of a-axis YBCO films grown on (110) NdGaO 3 and (100) SrTiO3 substrates by hybrid plasma sputtering J. Mater. Res., Vol. 10, No. 4, Apr 1995
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through TEM observations. The cross-sectional TEM micrographs for both films showed clear interface and atomic ordering, corresponding to the C u - 0 chain. The plan view of TEM micrographs showed well-aligned structure with the c axis being in plane al
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