Size Effects in Thin Films ofV3Ge, Nb, and Ta

An enhancement of the critical field of ${\mathrm{V}}_3$Ge, Nb, and Ta has been achieved by reducing the effective coherence length of these materials. This change in the coherence length was obtained in Nb and Ta by changing the grain size and therefore, the mean free path of the bulk material. Such a study yields also an experimental estimate of the coherence length of the bulk material which is in fair agreement with theoretical calculations. Thin films of ${\mathrm{V}}_3$Ge, Nb, and Ta have been obtained in thicknesses ranging from 200 000 to 100 Å. The ${\mathrm{V}}_3$Ge films were produced by the hydrogen reduction of the mixed chlorides and by a new method called getter sputtering. The Nb and Ta films were sputtered. In all cases, MgO wafers were used as substrates. The critical temperatures of the films were approximately that of the bulk, although the thinner films always displayed slightly lower critical temperatures. The transition field of the films, measured by a resistance technique, was found to increase as the thickness of the films decreased. In conjunction with the transition field increase, a corresponding increase in the residual resistivity was observed, which implies a reduction in the mean free path. The departure of the transition magnetic field from the bulk value occurred at such large thicknesses (a few 10 000 Å) that such a size effect cannot be explained in terms of the film thickness becoming comparable with the coherence length. This effect can be interpreted in terms of the very small grain size produced by sputtering films which, in turn, leads to a mean free path well below 100 Å. The presence of a sufficiently small grain size provides a means to produce negative surface energy superconductors with decreasing mean free path and consequently increasing $\kappa$ (the Ginzburg-Landau parameter), without the addition of alloying elements.