Tantalum and niobium films have been examined with respect to their superconducting properties. The films were deposited both by sputtering in a high vacuum station and by electron beam evaporation in an ultra high vacuum system. The transition temperatures of sputtered films are lower than the respective values of the pure bulk metals presumably due to interstitial impurities. Reactively sputtered TaC , TaN , and NbN films were found to be superconducting, but no superconducting transition was found for Ta 2 N films. Bulk Ta 2 N and TaN have no superconducting transition down to 1.5 °K, and the appearance of a transition in the TaN film is probably related to a new fcc crystal structure of the TaN films. Electron beam evaporated tantalum and niobium films of a few thousand angstrom thickness have superconducting transition temperatures of 4.3° and 9.1°K, respectively, which are comparable to those of the pure bulk metals of 4.5° and 9.5°K. The purest evaporated tantalum and niobium films, however, show critical fields which are an order of magnitude higher than bulk values, probably due to imperfections and strain.
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Gerstenberg et al. (1964) studied this question.