Effect of Sulfur Surface Structure on Nucleation of Oxide Seed Layers on Textured Metals for Coated Conductor Applicatio
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Effect of Sulfur Surface Structure on Nucleation of Oxide Seed Layers on Textured Metals for Coated Conductor Applications C. Cantoni, D. K. Christen, A. Goyal, L. Heatherly, G. W. Ownby, D. M. Zehner, D. P. Norton*, C. M. Rouleau, and H. M. Christen Oak Ridge National Laboratory, Oak Ridge, TN 37831 *University of Florida, Department of Materials Science and Engineering, Gainesville, FL 32611
ABSTRACT We present a study of the {100} biaxially textured Ni (001) surface and oxide seed layer nucleation by in situ reflection high-energy electron diffraction and Auger electron spectroscopy. Our observations revealed the existence of a c(2×2) superstructure on the textured Ni surface due to segregation of sulfur contained in the bulk metal. The sulfur superstructure promotes the epitaxial (002) nucleation of seed layers such as Y2O3-stabilized ZrO2 (YSZ) and CeO2 on the metal and optimizes the biaxial texture necessary for high Jc superconductors on RABiTS.
INTRODUCTION The RABiTS technology is based on the epitaxial growth of oxide buffer layers on a textured metal. This process is intrinsically complex because of the different chemical and electronic natures of the two surfaces involved. Changes in the atomistic structure and free energy of the metal surface determined by chemisorbed species can have a dramatic influence on the seed layer nucleation and ultimately determine the film crystallographic orientation. Although several groups have conducted accurate studies of the atomistic surfaces of d-band metals like Ni since 1960, these studies were not oriented towards understanding the heteroepitaxial nucleation of different oxide buffer layers on the metal. We were motivated to investigate the influence of surface physical and chemical properties on oxide films nucleation by inconsistencies in the seed layer texture of long lengths of RABiTS. In fact, continuous processing of meter-long tapes is much more sensitive to local imperfections than short-sample fabrication processes, in which poor-quality samples can be individually discarded. Although several different seed layer oxides have been successfully grown on textured Ni over the past few years by different techniques1, the previous work was based on an incomplete understanding of the Ni surface properties. In particular, the seed layer was assumed to nucleate on a clean and pure Ni surface that was obtained by annealing in forming gas before deposition1-3. Here we present a study of seed layer growth on {100} Ni conducted using a laser ablation molecular beam epitaxy (MBE) chamber equipped with a reflection high energy electron diffraction (RHEED) system, a mass spectrometer, and a pulsed KrF excimer laser (λ = 248 nm). Using in situ RHEED, it was possible to monitor continuously surface structural changes during Ni substrate annealing and oxide deposition. A chemical analysis of the textured Ni surface was conducted using Auger electron spectroscopy (AES) in a separate ultra high vacuum chamber. We show that the structure and chemical composition of the textur
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