Ion Beam Mixing of High-T c Superconductor Components
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ION BEAM MIXING OF HIGH-T,
SUPERCONDUCTOR COMPONENTS.
P. Borgesen* and D. A. Lilienfeld* *) Department of Materials Science and Engineering #) National Nanofabrication Facility Cornell University Ithaca, NY 14853 ABSTRACT The design of the necessary multilayer structures for producing superconducting thin films by ion beam mixing methods requires, among others, the knowledge of the individual (binary) mixing rates. In order to measure these, various combinations of Y, Ba, Cu, and Bi were irradiated with 600 keV Xe-ions at 80K and 300K. The systems exhibited a wide range of mixing behaviors which are also of fundamental interest. Ba and Cu readily formed the BaCu phase, and further mixing with Cu progressed only via binary collision mechanisms. At 80K Cu and Y were rapidly mixed in any ratio by thermal spikes, whereas a Cu rich sample rapidly formed the Cu6 Y phase at 300K. Ba could not be mixed into Y or a Y-Cu mixture. Finally, irradiation of polycrystalline layers of Cu and Bi apparently lead to rapid motion of Bi along grainboundaries at both temperatures. INTRODUCTION The discovery of high temperature superconductors has motivated an interest in a wealth of new systems for which the fundamental ion-solid interaction parameters are unknown. As part of an ongoing effort to produce high-Ta superconducting thin films by ion beam mixing, as well as an interest in basic ion-solid interactions, the behaviour of the various components during mixing is
being investigated.
Until now, the elements studied most thoroughly are those of the Y-Ba-Cu-O system. These materials appear to be quite unfavourable for the production of superconducting films by ion beam mixing. Originally, it was hoped that an appropriately designed multilayer sample would be mixed sufficiently during the implantation of the necessary oxygen, but it appears that substantial pre-mixing, for instance by implantation of Ba, is needed [I]. However, a study of the basic materials properties seems to suggest that the newer compounds, such as Bi-Ca-Sr-Cu-0, might be more suitable for this approach. Least promising would seem to be the mixing of Cu and Bi, which are miscible in the liquid, but immiscible in the solid phase. Attempts were made to measure basic mixing rates for combinations of the elements Y, Ba, Cu, and Bi. For most of these, the design of samples that will survive transfer to implanter and analysis chamber is a major problem. For instance only relatively thin films of Ba and Y buried in a more stable material (e.g. Cu) could be studied. The measurement of quantitative mixing efficiencies was complicated by the formation and stability of phases such as Cu6 Y and BaCu. EXPERIMENT Films were deposited on Si0 2 substrates in an ion pumped system by electron beam evaporation. The base pressure was _10-7 Torr. Yttrium and barium films were covered by a Cu film for protection. Even with the Cu protective layer the Y and Ba thicknesses were limited as thicker films would have caused rapid deterioration of the samples upon exposure to air (see bel
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