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

Abstract

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 μ0Hc2 (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.