We have investigated surface morphology of YBa2Cu3O7−y thin films prepared by chemical vapor deposition (CVD) using liquid metalorganic (MO) sources on MgO(100) single crystalline substrates by atomic force microscopy (AFM). An abrupt change in the terrace width was observed at the deposition temperature of around 750 °C. An anomalous decrease in the efficiency of incorporation of the yttrium component into the film was also found above the same temperature. It suggests that the appearance of liquid phase on the growing surface and the growth mode change from the conventional vapor growth to the VLS (vapor–liquid–solid) growth mode at this temperature.
We review and comment on the various requirements that a metallic substrate has to meet to be a good candidate for the fabrication of electrodeposited BSCCO superconducting tapes. We conclude that, in the present state of the art, no metallic substrate is really ideal. Hence we have investigated in detail the use of silver-buffered nickel-based alloys that seem to be a viable alternative to pure silver tape, which is more expensive and less resistant to high temperature. The major difficulty encountered was the occurrence of holes and blisters induced in the silver layers by the oxidation of the nickel underlayer during the heat treatments performed at high temperature in open air, which according to our procedure are required to synthesize high-temperature superconducting tapes. It was found that the liquid phases, transiently present in the process during the synthesis of the precursor phases, infiltrate between the Ag layer and the substrate through these holes and strongly react with the substrate transferring the poisoned element to the superconducting film greatly reducing its superconducting properties. Hence, several routes have been explored to try and suppress hole formation. It was found that pre-oxidizing the substrate at 880 • C for 1 h in open air sufficiently lowers the hole and blister densities to allow us to synthesize good Bi-2212 tapes on pure nickel, but not on Ni 80 -Cr 20 alloys. A much more interesting solution seems to be to pre-anneal the substrate in a hydrogenous atmosphere which permits us to remove blisters and holes.
YBa2Cu3O7-δ (YBCO) thin film was fabricated by metal-organic chemical vapor deposition (MOCVD) using newly developed liquid MO-sources (MO=metal-organic). The melting points of the MO-sources are lower than those of conventional MO-sources, and they have stable vapor pressure even in the liquid state. The MO-sources contain fluorine; however, when water was added to the flowing oxygen gas, the fluorine-to-oxygen ratio of the films was reduced to less than 0.08 at% as determined by secondary ion mass spectroscopy (SIMS). The reproducibility of the composition and the superconducting properties of the prepared films was excellent compared to that of films fabricated using conventional MO-sources.
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