Superconductivity in Nb-Sn Thin Films of Stoichiometric and Off-Stoichiometric Compositions

Binary Nb-Sn thin film samples were fabricated and characterized in terms of their composition, morphology, and superconducting properties. Nb-Sn was magnetron-sputtered onto heated R-plane sapphire substrates at 700^°C, 800^°C, and 900^°C, using a custom-built heater assembly. Samples were cut into strips, where each strip has a unique composition. For a subset of the samples, Nb-Sn was selectively etched away at an etching rate of 6 ± 1 nm/s using an aqueous solution of 3 vol.% hydrofluoric and 19 vol.% nitric acid. The sample composition was investigated with a scanning electron microscope with an X-ray energy dispersive spectroscopy detector. Surface and cross-section morphologies were investigated using scanning electron microscopy and scanning transmission electron microscopy, revealing a dense columnar poly-crystalline grain structure. X-ray diffraction measurements indicate a highly textured film that is (100) oriented out-of-plane and random in-plane. The critical temperature Tc (ranging from 9.8 to 17.9 K), critical magnetic field μ₀H_c2 (ranging from 12.5 to 31.3 T), residual resistivity ratio (RRR), and normal state resistivity ρ₀ were measured and found to be broadly consistent with literature data on bulk Nb₃Sn.