High-Tc Superconductor Multilevel Structures formed with Ta205 Spacer Layers
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HIGH-Tc SUPERCONDUCTOR MULTILEVEL STRUCTURES FORMED WITH Ta205 SPACER LAYERS RABI S. BHATTACHARYA*, P.B. KOSEL**, AND T. PETERSON*** *Universal Energy Systems, Inc., Dayton, OH 45432-1894 **University of Cincinnati, Dept. Electrical & Computer Eng., Cincinnati, OH 45221 ***Wright Patterson Air Force Base, WRDC/MLPO, Dayton, OH 45433
ABSTRACT An important class of electronic devices could be realized with current state-of-the-art high-Tc superconductor (HTSC) thin films if a method were found for the formation of multilevel structures separated by thin dielectric layers. It has been found that such structures can be formed with tantalum oxide as the spacer layer because of its high melting point (1870°C) and good chemical stability at the high temperatures commonly used to anneal thin films of YBaCuO superconductor. Good results have been achieved with thin films prepared by electron beam (EB) sequential deposition from Y, BaF 2 and Cu targets onto oriented (100) strontium titanate substrates and oxygen furnace annealing at 850°C to promote the epitaxial alignment and growth of the superconducting crystalline phase. Two-layer structures have been formed by interposing a tantalum oxide barrier deposited by EB evaporation between two successive layers of HTSC films. The transition temperatures of both layers were checked by resistance measurements and found to display superconductivity after annealing. However, the top (second) layer was found to always show a reduced abruptness of the transition characteristic thereby indicating, perhaps, a lesser degree of epitaxial alignment since it makes no direct contact with the substrate surface.
HTSC FILM PREPARATION The emphasis of this work has been to investigate the possibily of extending certain existing HTSC material preparation techniques to the production of simple yet useful electronic devices. The devices considered here were vertical Josephson junctions which require a barrier layer formed between two planar thin films of HTSC. The HTSC material chosen for this work was YBaCuO and two alternative deposition methods were investigated: (a) electron beam sequential evaporation from Y, BaF 2 and Cu sources [1], and (b) magnetron sputtering from a preformed target [2]. Crystalline strontium titanate of (100) surface orientation was used as the substrate for most of this work [3]. In all cases it was found that the best film characteristics were obtained only after the subtrates had received a thorough clean in an RCA I solution (NH4 OH:H 2 0 2 :H 2 0 = 1:1:5) at 80°C for about 10 minutes [4]. In the electron beam deposition method sequential layers of Y, BaF 2 and Cu were put down with thicknesses set to produce the final YBaCuO film stoichiometry. After deposition the films were first annealed at 750°C in dry 02 for 1 hour, then activated at 850°C for 1 hour in wet 02, and finally cooled in dry 02 at a rate of 2°C/min [5]. Rutherford backscattering (RBS) analysis was used to identify the final film stoichiometry as 1:2:3:7 of Y:Ba:Cu:O with a uniformity of about ±5%
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