Characterization of Epitaxial Superconducting YBacuO Thin Films with Three Different Orientations
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CHARACTERIZATION OF EPITAXIAL SUPERCONDUCTING YBaCuO THIN FILMS WITH THREE DIFFERENT ORIENTATIONS X.K. Wang, D.X. Li, S.N. Song, J.Q. Zheng, R.P.H. Chang, and J.B. Ketterson Materials Research Center and Science & Technology Center for Superconductivity, Northwestern University, Evanston, IL 60208
Abstract Epitaxial thin films of YBaCuO were prepared by multilayer deposition from Y, Cu, and BaF2 sources with: (1) the a-axis perpendicular to (100)SrTiO 3; (2) the c-axis perpendicular to (100)SrTiO 3 ; and (3) the [1101 axis perpendicular to (110)SrTiO 3. XRD patterns as well as SEM and HREM images confirm that the films are highly oriented, essentially epitaxial. Both the a-axis oriented and the c-axis oriented films exhibit zero resistance at 91K. The [1101 oriented film shows the sharpest transiton with a transition width of 1K and zero resistance at 85K. The zero field critical current density, JC, determined magnetically, is in excess of 107A/cm 2 at 4.4K and 1.04 x 106A/cm 2 at 77K for the c-axis oriented film; for the a-axis oriented film we obtained 6.7 x 106A/cm 2 at 4.4K and 1.2 x 105A/cm 2 at 77K. The orientation dependence of the critical current density in the basal plane of the a-axis oriented film was studied. The largest Jr's occur along the in-plane < 100> axes of the substrate. A primary goal of most high TC thin film efforts is to increase the critical current.[1" 4l It is known that critical current density is affected by stoichiometry, the atomic arrangement at grain boundaries, intergrain coupling, alignment of the crystal grains, and the critical current anisotropy.1 5] Accordingly, it is expected that samples consisting of grains with the desired stoichiometry, with good intergrain contact and a high degree alignment will show high value of Jc- Therefore the preparation and characterization of essentially epitaxial thin films with well defined but different orientations should be of considerable interest. We have prepared epitaxial, oriented thin films of YBaCuO with: (1) the a-axis perpendicular to (100)SrTiO (100)SrTiO
3
3
(a-axis oriented); (2) the c-axis perpendicular to
(c-axis oriented); and (3) the [110] axis perpendicular to (110)SrTiO
3
([110] axis oriented). Our films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution electron microscopy (HREM), conventional four probe resistivity, and magnetization measurements. Thin films, with an artificial-superlattice structure, were deposited from three electron guns containing Y, BaF., and Cu in an atmosphere of 5 x 10-5 Torr of 02. The substrates, maintained at 450"C, were sequentially exposed to the three sources to create a multilayer film. The most important requirement for high quality films is to achieve strict Mat. Res. Soc. Symp. Proc. Vol. 169. •1990 Materials Research Society
680
stoichiometry in order to avoid secondary phase formation and to minimize interdiffusion between the substrate and the film. The composition of a multilayer film can be controlled simply by adjusting
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