Highlights

Influence of Substrate Grain Orientation on Heteroepitaxial Nucleation of YBa₂Cu₃O₇

Scientific Achievement

We have used electron back-scattered diffraction (EBSD) to establish a relationship between YBa₂Cu₃O₇ (YBCO) nucleation and the orientation of the underlying substrate grains during growth of YBCO on cube-textured Ni-W buffered.  Specifically, for growth of YBCO using a metal-organic deposition (MOD) process, we have found that YBCO nucleation is suppressed on Ni-W grains with a tilt angle above ˜ 8.5°.  As nucleation occurs on surface terraces, when the terrace width becomes smaller than the critical nuclei size, nucleation is suppressed.

In the MOD process, a precursor is deposited onto a substrate and then subsequently converted to YBCO by thermal processing. The Ni-W substrates are polycrystalline but highly cube-textured, with grains typically aligned to within a few degrees of one another.  Inevitably, some grains exhibit larger misorientations. In characterizing partially converted samples, we consistently observed some fraction of grains poorly covered by YBCO. Microscopy revealed that the poor YBCO coverage on these grains was a result of a suppressed YBCO nucleation. Using ESBD, we established that the grains with poor YBCO coverage always exhibited a rather high degree of tilt misorientation from the desired cube texture. In these materials, the grain misorientation can be characterized by a rotation about the surface normal of the tape and by a tilt from the surface normal.  While a rotation of a grain about the surface normal does not change the growth surface, tilt leads to a more terraced growth surface.  Moreover, the terrace ledge width decreases with increased tilt.  Since YBCO nucleation takes place on the terrace ledges for this MOD process, there can be a substantial increase nucleus free energy once the length of the ledge becomes smaller than lateral diameter of the critical nucleus.  Based upon this observation, we propose a model that explains the reduction in nucleation as the result of absence of (100) terraces large enough to accommodate the critical size for YBCO nuclei.

Significance

Our studies shed new light on thedamaging effect of the out-of-plane tilt of substrate grains on superconducting properties of YBCO layers produced by metal-organic deposition. While it is well known that substrate grain misalignment has a deleterious effect on the performance of YBCO layers by introducing high-angle grain boundaries, little is known about how the substrate grain misorientation influences the nucleation.  Our results show that the effect high out-of-plane tilt in substrate grains goes beyond the generation of high angle grain boundaries in the YBCO by influencing the growth process itself.  Our model based on nucleation on terrace ledges suggests pathways to improve nucleation by controlling nuclei size.

Performers

V. F. Solovyov, Q. Li, H. Wiesmann (Brookhaven Nat'l Lab.); D. Abraimov (Florida State U.); D. J. Miller (Argonne-MSD-EMC)