Morphology of thin superconducting Nb films

Abstract

Experimental results are presented that show correlations between the superconducting transition temperatures (Tc) and the crystalline morphology in very thin Nb films (∠60 nm thick). Variations of vapor deposition rates, substrate temperatures, and pressure during electron beam evaporation lead to different film morphology and grain sizes. Using transmission electron microscopy, diffraction and Tc measurements, it was possible to follow some of the changes in superconducting characteristics effected by the resulting different grain sizes. Broader superconducting transitions and lower Tc’s were generally found for films with average grain sizes ≲10 nm. Average grain sizes of less than 10 nm appear to give lower Tc and a broader transition. Single crystal Nb films showed, in general, a sharp superconducting transition at 9.1 K, the expected value for bulk Nb.