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

摘要

${rm Nb/Al}{hbox{-}}{rm AlN}_{rm x}/{rm Nb}$ SIS junctions have been fabricated under various conditions of plasma nitridation. Junctions with low $R_{rm N}A$ of $sim 10 Omega mu {rm m}^{2}$ 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 ${rm N}_{2}$ and Ar. An exponential correlation between the nitridation time and the junction $R_{rm N}A$ 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 $R_{rm N}Asim 10 Omega mu{rm m}^{2}$ 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 $R_{rm N}A$, their intensity was reduced as the $R_{rm N}A$ decreases.