Surface crystallographic structure compatibility between substrates and high T c (YBCO) thin films
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A review and some case studies on the interatomic distances of candidate substrates in comparison to the high Tc superconductor (HTSC) phase YBa2Cu307_5 (YBCO) is presented, in an attempt to enhance the basis for substrate selection for YBCO film epitaxy. This preliminary study was carried out by examining a variety of interatomic distances in the structure rather than merely matching the lattice parameters. Interatomic structure matching planes of selected YBCO orientations in contact with substrates were identified. The surface termination of the substrate was found to be a crucial parameter in determining the oriented or epitaxial growth. Possible composition dependence of the orientation of the films at the nucleation stage was also anticipated depending on the comparison. Several currently most commonly used substrates are discussed in some detail.
I. INTRODUCTION There is great interest worldwide in many research groups focused on growing high quality thin films of high Tc superconductors (HTSC), using a whole variety of available deposition techniques. The selection of useful substrate materials is of first importance and is subjected to a number of constraints such as thermal and chemical stability under the conditions used in the film process and use, reasonable lattice constant matching, thermal compatibility in terms of thermal expansion matching over the temperature range of film processing and annealing down to the operating temperature (90 K), physical properties (for instance, the dielectric constant and loss) suitable for specific use, commercial availability, and, if possible, low cost. An enormously valuable and comprehensive survey of recent experimental work has been provided by Perrin and Sergent.1 In the general case most such films are referred to as "epitaxial films." It is arguable that this term is misapplied in many cases.2 The understanding of the term epitaxy has come a long way, and detailed crystallographic guidelines for successful epitaxy are still to be sorted out. At the beginning of this century, without the help of x-ray diffraction, scientists knew that epitaxy was more likely to occur if the molecular volumes of the two intergrowing alkali halides were nearly equal (i.e., low misfit).3 The discovery of x-ray diffraction made it possible to examine the effect of the geometry of crystal structure on the phenomenon. It was found largely for growth from liquid solutions that "epitaxy" (oriented growth) occurs only when it involves the parallelism of two lattice planes that have networks of identical or quasi-identical structure and of closely similar spacing.4 1644 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 7, Jul 1994
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The experiments indicated that epitaxy occurs only if the "misfit" between "lattices" of substrate and growing phase is no more than about 15%. 4 This vague belief about epitaxy remains prominent to this day. A classic introduction to the history of epitaxy was given by Pashley.5 Since the time of Royer's pioneering work, p
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