Epitaxy Development in Thin Superconducting YBa 2 Cu 3 O 7 Films
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EPITAXY DEVELOPMENT IN THIN SUPERCONDUCTING YBa 2Cu 30 7 FILMS A. YEN%, L. Li", J. D. KLEIN', W. B. NOWAK", and S. F. COGAN' *EIC Laboratories,Inc., Norwood, Massachusetts 02062 "**NortheasternUniversity, Boston, Massachusetts02115 ABSTRACT Ultrathin superconducting YBa 2 Cu30 1 films were grown on (100) YSZ (yttria-stabilized-zirconia) substrates by off-axis rf magnetron sputtering at a relatively high deposition rate. The structure, orientation, and morphology of the films were examined by x-ray diffraction, reflection high-energy electron diffraction (RHEED), and scanning electron microscopy. X-ray diffraction patterns of films deposited on YSZ substrates exhibited strong c-axis alignment with the YBa 2Cu3O 7 peaks sharpening as the film thickness was increased. The degree of epitaxy apparent in RHEED photographs was found to increase dramatically as the film thickness was increased from 12 nm to 108 nm. This behavior is attributed to a nucleation and growth process in which epitaxy develops as a result of a 3 stage progression from a random to an oriented film. The fidms were in-situ superconducting, exhibiting superconducting transition temperatures, T,(0)'s, of 80 K for a 12 nm film and 88 K for a 280 nm film. However, the relatively low critical current densities (J, < 1 x 106 A/cm 2) at 77 K are probably due to a lack of in-plane epitaxy. INTRODUCTION Heteroepitaxial growth of YBa 2Cu 30 7 (YBCO) is becoming essential for device applications in microelectronics. Dimo et a! have shown that YBCO grain boundaries play a major role in limiting critical current densities in films [1]. Recently, Garrison etal observed two in-plane epitaxial states in YBCO films on YSZ substrates [2]. They have demonstrated the importance of eliminating the high angle grain boundaries and promoting in-plane epitaxy to improving J,. Off-axis magnetron sputtering has been studied extensively to optimize deposition conditions such as pressure, atmosphere and substrate temperature [3,4]. The role of deposition rate in determining the initial growth has also been studied by Zheng et a! [5]. Either a-axis or c-axis oriented films can be obtained under optimized deposition conditions. The development of epitaxy in YBCO films is normally a layer-by-layer [6] or island growth [7] process. However, it can also be achieved through other nucleation and growth mechanisms. In this paper, we report the heteroepitaxial growth of thin YBa 2Cu30 7 films on YSZ with a polycrystalline intermediate layer. We have shown that the early growth at our relatively high deposition rate is randomly oriented rather than epitaxial. However, epitaxial growth can be achieved after a critical thickness has been reached. EXPERIMENTAL The sputtering experiments were done in a diffusion pumped Microscience IBEX-2000 deposition chamber. The system was equipped with a liquid nitrogen trap and had a base pressure in the middle 10-6 Torr range. All film deposition conditions except the deposition time were identical and performed in an off-axis sputtering configuration [
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