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

Abstract

Nb/Al-AlN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb SIS junctions have been fabricated under various conditions of plasma nitridation. Junctions with low R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> A of ~ 10 Omega mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Ar. An exponential correlation between the nitridation time and the junction R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> A ~ 10 Omega mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> A, their intensity was reduced as R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> A the decreases.