Flexible stainless steel foil as a substrate for superconducting Y-Ba-Cu-O films
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I. INTRODUCTION
Since the discovery of the new high Tc superconductor YiBa2Cu3O7_.c, much effort has been devoted to fabricating thin films of this material. The twodimensional nature of the high Tc material and the large current-carrying capability of the oriented films in addition to the feasibility of integration with the existing semiconductor technology, make Y-Ba-Cu-O thin films attractive for fundamental research as well as for applications. Due to the complicated perovskite structure of the YiBa2Cu3O7-x superconductor, growth of high quality thin films is best carried out on single crystal substrates with a close lattice match. Currently, the most popular single crystal substrates for superconducting thin film growth today are SrTiO3, MgO, and ZrO2. Even though films grown on these substrates are known to show superior superconducting properties,1"4 they are by no means the most practical substrates. As Si proves to be a technologically important substrate for devices, relatively cheap, strong, and flexible metallic substrates such as stainless steel may prove to be important for flexible superconducting interconnectors and power storage devices. Growth of superconducting "123" films on most of the metallic substrates, including Cu, is hindered by the large thermal expansion mismatch between the substrate and the superconducting film. However, due to the relatively close thermal expansion coefficients of stainless steel and Y-Ba-Cu-O, stable superconducting films can be grown without microcracks at the interface. The major concern in obtaining a superconducting film on stainless steel is the degradation of the superconducting properties due to iron diffusion into the film. If the deposited film has to be annealed at high temperatures in oxygen to obtain superconductivity, the large diffusion of iron will completely destroy the superconductivity of the film. J. Mater. Res., Vol. 5, No. 4, Apr 1990
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Therefore, an in situ low temperature process is desired to grow superconducting films on stainless steel. While deposition of Y-Ba-Cu-O films on stainless steel with a MgO buffer layer has been reported previously,5 critical currents of these films have not been discussed. We are reporting here the successful growth of asdeposited superconducting Y-Ba-Cu-O films on stainless steel, utilizing a low temperature plasma assisted laser deposition technique (PLD). II. EXPERIMENTAL
The PLD process has been discussed in previous publications.6'7 A schematic of the laser evaporation chamber is shown in Fig. 1. This turbo-pumped vacuum system can attain a base pressure of about 5 x 10~6 Torr. The evaporation source is a pellet of Y1Ba2Cu3O7-x pressed into about 90% density. An ArF excimer laser focused on the rotating pellet produces a molecular beam of the material, which forms a stoichiometric film on a heated substrate placed above. Evaporation is carried out in about 20 m Torr oxygen back pressure. By applying a +300 V dc voltage to the middle electrode, an oxyg
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