rf characterization of thermally diffused superconducting niobium nitride

We have prepared niobium nitride by a thermal diffusion technique of nitrogen into bulk niobium at 800 °C. The resulting NbN layer has been characterized at both low and high frequencies. We obtained transition temperature Tc=11.5 K, ‘‘penetration depth’’ λ0=85 nm at 9 GHz, and surface resistance Rs evaluated at 5.5 K and 9 GHz of 2.8×10−5 Ω. Moreover, this high-frequency Rs measured for thermally diffused NbN is better than the Rs of Nb, and is comparable to the Rs of sputtered NbN. Our diffusion technique is well adapted to all kinds of shapes. In addition, improved diffusion conditions allow Tc as high as 16 K.