Overdamped niobium-nitride junctions for 10 K operation

We have fabricated overdamped NbN/NbNx/NbN Josephson junctions with the IcR product of 0.64 mV and the critical current density Jc of 1.2 kA/cm/sup 2/ at 10 K. The NbNx barriers are formed by plasma nitridation of the surface of the superconductive NbN base electrodes. Although the junctions show small-hysteretic characteristics at 4.2 K, they change to overdamped ones with increasing temperature. We obtained the over-damped junctions with the Jc ranging from 0.05 kA/cm/sup 2/ to 3.3 kA/cm and the IcR product ranging from 0.11 mV to 0.64 mV at 10 K. The junction characteristics are discussed from a viewpoint of the dependence of Ic on temperature. On the other hand, small hysteretic characteristics at 4.2 K enable us to evaluate Ic spread using a series of junctions. The standard deviation, 1-4 was 2.9% for a series of 64 junctions with an area of 5/spl times/5 /spl mu/m/sup 2/ at 4.2 K.