Crystalline Niobium Carbide Superconducting Nanowires Prepared by Focused Ion Beam Direct Writing.

Superconducting planar nanostructures are widely used in applications, e.g., for highly sensitive magnetometers and in basic research, e.g., to study finite size-effects or vortex dynamics. In contrast, 3D superconducting nanostructures, despite their potential in quantum information processing and nanoelectronics, have been addressed only in a few pioneering experiments. This is due to the complexity of fabricating 3D nanostructures by conventional techniques like electron beam lithography, and to the scarce number of superconducting materials available for direct-writing techniques, which enable the growth of 3D free-standing nanostructures. Here we present a comparative study of planar nanowires and free-standing 3D nanowires fabricated by focused electron-/ ion (Ga+)-beam induced deposition (FEBID/FIBID) using the precursor Nb(NMe)(N- t-Bu). FEBID nanowires contain about 67at% C, 22at% N and 11at% Nb, while FIBID samples are composed of 43at% C, 13at% N, 15.5at% Ga and 28.5at% Nb. Transmission electron microscopy shows that FEBID samples are amorphous, while FIBID samples exhibit a fcc NbC polycrystalline structure, with grains of about 15-20 nm in diameter. Electrical transport measurements show that FEBID nanowires are highly resistive, following a variable-range-hopping behavior. In contradistinction, FIBID planar nanowires become superconducting at Tc ≈ 5 K. Remarkably, the critical temperature of free-standing 3D nanowires is as high as Tc ≈ 11 K, which is close to the value of bulk NbC. In conclusion, FIBID-NbC is a promising material for the fabrication of superconducting nanowire single photon detectors (SNSPD) and for the development of 3D superconductivity with applications in quantum information processing and nanoelectronics.