Development of ${\hbox {Nb/Al-AlN}}_{\rm x}/{\hbox {Nb}}$ SIS Tunnel Junctions for Submillimeter-Wave Mixers

Nb/Al-AlN_x/Nb SIS junctions have been fabricated under various conditions of plasma nitridation. Junctions with low R_N A of ~ 10 Omega mum² have been fabricated in pure nitrogen plasma, although this process exhibited low reproducibility. A process with higher reproducibility was realized by adopting a two-step-ignition nitridation process in a composite plasma of N₂ and Ar. An exponential correlation between the nitridation time and the junction R_N A was found for the junctions with barriers formed in this new process, which suggests a linear time dependence of the barrier thickness during nitridation. SIS junctions with R_N A ~ 10 Omega mum² can be repeatedly produced using this process. Preliminary results from X-ray photoelectron spectroscopy (XPS) observations of the nitride barrier are also presented. While the N 1s XPS signal was apparently detected in the junctions with high R_N A, their intensity was reduced as R_N A the decreases.