Structural and superconducting properties of ion beam sputtered Nb thin films and Nb/Cu bilayers

Abstract

We present the results of a study of structural and superconducting properties of polycrystalline Nb thin films (200 Å, 300 Å, 400 Å, 700 Å and 1000 Å) and Nb/Cu bilayers (300 Å/300 Å and 400 Å/300 Å) prepared on Si substrates by ion beam sputtering at room temperature. The thicknesses, roughnesses at the surfaces and interfaces were determined by X-ray reflectivity whereas the grain sizes were determined from grazing incidence X-ray diffraction and transmission electron microscopic studies. The superconducting transition temperature ( T C ) of Nb thin films are smaller than T C of bulk Nb. The Nb-200 Å sample does not show T C down to 2.3 K. The average size of the grains varies from 42 Å for Nb-200 Å sample to 69 Å for Nb-1000 Å sample. Our results show that the T C in these polycrystalline films is not only limited by its thickness but also by the size of the grains. The Nb films deposited in situ on the Cu layer (Nb/Cu) show a marginal increase in average sizes of the grains as compare to their respective values in Nb films of same thicknesses. As a result a marginal increase in T C of these films is also observed. The maximum decrease in T C due to oxygen intake during deposition should be about 0.5 K from its bulk value (9.28 K). We have attributed the large decrease in T C in our case on the basis of decrease in the Debye temperature and density of states at the Fermi level for Nb thin films as compared to their respective values for bulk Nb.