Investigation Of YBa 2 Cu 3 O 7-gd Films Grown on Vicinally-Polished MgO Substrates
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INVESTIGATION OF YBa2Cu307-8 FILMS GROWN ON VICINALLY-POLISHED MgO SUBSTRATES S. K. Streiffer, B. M. Lairson, and J. C. Bravman Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 ABSTRACT X-ray diffraction and transmission electron microscopy have been used to study the morphology and defect structure of YBa2Cu30"-8 films on [001] MgO substrates polished away from the [001] crystallographic axis towards either [010] or [110] by angles ranging from 10 to 200. On the [010] off-axis substrates it was found that growth of the YBCO film was highly influenced by the surface normal of the substrate with only very weak alignment to the substrate [001] direction. All [010]-angled samples maintained Tc's of 86K. Angles of 1', 2.50, and 50 toward [010] produced films with large mosaic spread parallel to the direction of the angle, as high as 40 FWHM for the 50 sample. Critical current densities of these films remained as high as those of films grown on on-axis substrates, e.g. 4.4x10 7 A/cm 2 at 4.2 K and zero applied field. Larger off-axis angles led to samples with reduced mosaic spread but still alignment to the surface normal, and reduced Jc's of 2x10 7 A/cm 2 and lxl05 A/cm 2 for 100 and 200 angles, respectively. TEM investigation and in-plane x-ray examination of the 50, 10', and 200 samples show good in-plane texture except for the 200 sample, and increased granularity with angle. Films on substrates polished off-axis towards [110] have degraded Tc's and Jc's for all angles.
INTRODUCTION Under proper deposition conditions it is possible to grow highly oriented YBa2Cu3O7 (YBCO) on [001] MgO single crystals 1 , despite a lattice mismatch of about 8.6% during high temperature, low pressure in-situ growth. In light of the highly anisotropic transport properties of this and other oxide superconductors, an understanding of this growth 2 on such a poorly lattice-matched substrate is useful for the production of technologically useful YBCO thin films on MgO. In addition, the nature, density, and control of planar boundaries at the intersection of different nuclei of the growing film is an interesting area for investigation because of the small coherence length of YBCO 3 . For instance, one would expect to see influences on the electrical properties of the superconducting state even for narrow boundaries, because such boundaries may strongly suppress the order parameter in the region near them 4 . One phenomenon possibly related to such effects is the lowering of intergranular critical currents due to artificially induced grain boundaries 5 . Epitactic growth may be investigated by deposition onto vicinally polished substrates, which is expected to produce microstructures different than those found in films deposited onto standard substrates. This technique is commonly employed in the growth of polar semiconductors on nonpolar substrates (for example, GaAs on Si) where it has been shown to greatly reduce the number of antiphase boundaries by controlling ledge heights on the substrate surfa
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