SPECIFIC HEAT AND NONEQUILIBRIUM ASPECTS OF VAPOR DEPOSITION GROWTH OF A15 SUPERCONDUCTORS (NIOBIUM-TIN).

摘要

Thin films of high transition temperature superconductors of the A15 crystal structure have been prepared by electron beam coevaporation. The specific heat of these films has been measured between 1.5 and 30 K. The transition to the superconducting state was not uniformly sharp as would be expected from these presumably metallurgically stable materials. Instead, it was found to range in width from a low of .3 K for stoichiometric Niobium-Tin to a high of 6 K for Niobium-Tin prepared off-stoichiometry.; The systematics of this broad transition were probed by varying the deposition parameters, such as composition, substrate temperature, deposition rate, and background gases, by examining different A15's, by changing the means of deposition from evaporation to sputtering, and by annealing. The width of the transition is found to be dependent on both deposition temperature and on the microstructure of the film.; The systematics of the transition width strongly suggest that its source is a compositional inhomogeneity which develops at the film surface during the growth. The dependence on temperature indicates that the inhomogeneity is not a bulk, equilibrium effect. The dependence on microstructure suggests that the inhomogeneity is caused by a grain-orientation dependent surface effect. In particular, a model for a new type of surface segregation applicable in a growing film where the surface mobility is orders of magnitude greater than the bulk mobility is proposed to explain the results. This model, if correct, is believed to have general application to vapor phase film growth.