Niobium nitride technology for Josephson junction devices

Over the last decades Nb/Al-AlOx/Nb multi-layers have been the primary choice for Josephson junction (JJ) devices such as SIS mixers, SQUIDs and RSFQ. Various applications require high critical-current densities jc and low sub-gap leakage. Additionally, a large gap-voltage benefits the performance of most devices. Nb/Al-AlOx/Nb technology is limited in jc due to an increasing transparency of the barrier with increasing jc, and the energy-gap of the Nb electrodes poses an upper frequency limit for SIS mixers. NbN/AlN/NbN multi-layer technology has emerged as an alternative to Nb/Al-AlOx/Nb. The upper frequency limit of NbN-based SIS mixing element significantly exceeds that of Nb, and AlN-barriers result in higher jc's at identical thicknesses as compared to AlOx. We have developed an in-situ fabrication technology for NbN/AlN/NbN multi-layers. We found a clear influence of the sputter parameters on the surface morphology of the NbN electrodes, which directly impacts on the quality of the JJs. Transport properties of JJs on different substrates are presented.