A Study of Grain Boundaries in High T C Superconducting Yba 2 Cu 3 O 7-x Thin Films Using High Resolution Analytical Ste
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A STUDY OF GRAIN BOUNDARIES IN HIGH Tc SUPERCONDUCTING YBa 2Cu 30 7 - THIN FILMS USING HIGH RESOLUTION ANALYTICAL STEM D. H. Shin, Buhrman
J.
Silcox,
S.
E.
Russek,
D.
School of Applied And Engineering Physics, Ithaca, New York 14853
K.
Lathrop,
and R.
A.
Cornell University
ABSTRACT Grain boundaries in thin films of high TC YBa 2 Cu 3 OT7 superconductors have been investigated with high resolution scanning transmission electron microscope (STEM) imaging and nanoprobe energy dispersive x-ray (EDX) analysis. Atomic resolution images indicate that the grain boundaries are mostly clean, i.e., free of a boundary layer of different phase or of segregation, and are often coherent. EDX microanalysis with a 10 A spatial resolution also indicates no composition deviation at the grain boundaries. Introduction Epitaxially grown high quality films of YBa 2 CU 3OT 7. superconductors have critical current densities (Jc's) - 2-3 orders of magnitude larger than those of the bulk high Tc samples [1,2]. Random orientations of grains in bulk samples coupled with disordered, nonstoichiometric regions at the grain boundaries are thought to cause weak Josephson coupling between grains and lead to suppressed JC's [3,4]. In thin films it is thought that the grain boundaries are clean and highly oriented and that this leads to an overall increase in the Jc's. Experimental Thin films of high Tc superconducting YBa 2 Cu 307_x were grown in situ by reactive evaporation, reactive sputtering, and laser ablation on MgO and ZrO2 substrates. Details of film growth methods have been described elsewhere [5,6,7]. Films of about 3000 A in thickness were grown in oxygen partial pressures ranging from 5 x 10-4 Torr for evaporation to 0.4 Torr for laser ablation at substrate temperatures of 650-700 C. Typical Tc's for these films range from 75-85 K with critical current 2 densities from 1-5 x 106 Amp/cm at 4.2 K. Thinning of specimens for electron transmission has been done by mechanical polishing followed by argon ion-milling. The specimen was thinned at room temperature until a small hole began to appear, then the ion voltage was lowered from 6 kV to 3 kV in order to minimize the surface damage on the specimen. The specimens were then studied with a VG HB501A STEM operating at 100 kV. This STEM has a demonstrated resolution of better than 2 A in both annular dark field (with Cs=1.2mm [8]) and bright field imaging modes (with CS=0.7mm [9]). Results and Discussions As shown in the cross-section STEM micrograph of Fig. Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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Fig. 1 Cross-section STEM micrograph of a high Tc YBa 2Cu 30 7 - film grown on (100) MgO. The c-axis is perpendicular to the substrate.
FIG. 2 High resolution STEM bright field images of grain boundaries in c-axis oriented films. Films were grown by (a), (b) laser ablation, and (c) reactive evaporation. Grain boundary tilt angles are 00, 80, and 270, respectively.
a
b.C high T, YBa 2Cu 307_X film grows on (100) MgO substrate with a sharp inter
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