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

Our work is related with Nb thin film for Cu radio frequency cavities. Nb films, Nb/Cu bilayers are prepared by ion beam sputtering on Si substrate. Structural and superconducting properties were investigated. Grain size increases, lattice parameter decreases for bilayers compare to Nb films. Superconducting transition temperature increases for bilayer compare to Nb films. 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 (TC) of Nb thin films are smaller than TC of bulk Nb. The Nb-200 Å sample does not show TC 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 TC 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 TC of these films is also observed. The maximum decrease in TC 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 TC 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.