Properties of NbN thin films deposited on ambient temperature substrates

The preparation of cubic NbN films by reactive dc magnetron sputtering is described. These superconductive films are deposited at a sufficiently low temperature (<90 °C) that photoresist liftoff techniques and can be used in fabricating Josephson junctions. The superconducting transition temperature has been measured as a function of gas composition and pressure. It reaches a maximum of 14.2 K at 15% N₂-85% Ar and 1.06 Pa total pressure. The resistivity ratio of these films is close to unity. Structural studies by transmission electron microscopy and electron diffraction show that films 100-nm thick or less are randomly oriented, and noncolumnar with a crystallite diameter of 5 nm and a lattice parameter of 4.46 Å, which is significantly higher than the bulk value for cubic NbN. The films are dense with void diameters no larger than 0.7 nm. Films 300-400 nm thick show a small degree of texturing in x-ray studies with a Read camera. Auger analysis shows a monotonic increase in the N/Nb ratio with increase in the N₂/Ar ratio in the sputtering ambient up to 30% N₂. From 30% to 50% N₂ in the sputtering mixture the N/Nb ratio is constant. Small amounts of carbon impurity are found in all films.