High T c Superconductor Films Grown by the Liquid Phase Epitaxial Technique
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HIGH Tc SUPERCONDUCTOR FILMS GROWN BY THE LIQUID PHASE EPITAXIAL TECHNIQUE C. S. Yang and A. S. Yue Department of Materials Science and Engineering, School of Engineering and Applied Science, 6532 Boelter Hall, University of California, Los Angeles, California 90024-1595 ABSTRACT High Tc superconducting YBa 2 Cu 3 0 7 films have been prepared by the Liquid Phase Epitaxial (LPE) technique. The x-ray diffraction patterns displayed a high degree of (00/) preferred orientations with c-axes perpendicular to the film surfaces. The in-situ grown YBa 2 Cu 30 7 films showed zero-resistivity at 76°K. Examination of the film cross sections by SEM revealed a growth mechanism. INTRODUCTION Although high Tc superconductor films can be prepared by several different techniques, such as three-element electron beam co-deposition[l], three-element co-deposition using magnetron sputtering[2], molecular beam epitaxy[3], sputtering from a single oxide target[4] and laser ablation of a sintered target[5], the approaches are expensive for commercialization. Furthermore, it is difficult to control stoichiometric compositions of these films produced by these techniques, because vapor pressures of different elements vary with a fixed substrate temperature during film deposition. On the other hand, the liquid-phase-epitaxial (LPE) technique is an inexpensive process and can produce films with stoichiometric compositions rather easily. A possible disadvantage is that in LPE growth, a solvent is present and it can be trapped in the grown crystalline film during decantation. In this paper, we will demonstrate how YBa 2Cu 30 7 (YBCO) films can be deposited on A12 0 3 substrates by the LPE growth technique. The quality of these LPE-grown films were characterized in terms of x-ray diffraction patterns and SEM. Their superconducting properties were measured in terms of critical temperature, Tc and critical current density, Jc. HYPOTHETICAL TERNARY PHASE DIAGRAMS FOR LPE GROWTH Consider the growth of a LPE film from a hypothetical ternary phase diagram ABC as shown in Fig. 1 which is similar to most of the high Tc superconductor ternary systems. Components A,B, and C are completely soluble in each other in the liquid state. Components A and B form a compound D; A and C form a compound F by a peritectic reaction; B and C form a simple eutectic with no solid solubilities of B in C and C in B; compound D has a maximum melting point, i.e., it forms a eutectic with A and B. Curves eCP1,M1P1,P1P 2 ,e 4 P2 ,P2 T,e 2T and e 3 T are the projections of the space curves of the ABC ternary system on an isothermal section. These space curves are analogous to valleys between hills. A melt at any point of any space curve is doubly saturated with two solid phases. To prepare a high Tc superconductor film by the LPE growth technique from a ternary system, it is necessary that its pseudo-binary phase diagram containing the superconducting phase that is to be grown must be known. Let us construct a vertical section a-b of the ternary system ABC as given in Fig. 2 repre
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