Fabrication and characterization of Nb/Al-AlN /Nb superconducting tunnel junctions

TL;DR

This paper presents a new fabrication process for Nb/Al-AlN/Nb superconducting tunnel junctions using plasma nitridation, resulting in uniform barriers with high quality and well-characterized electrical properties.

Contribution

The study introduces a plasma nitridation method for forming AlN barriers in superconducting tunnel junctions, enabling uniform barriers with a wide resistance range and detailed characterization.

Findings

Achieved uniform tunnel barriers with $R_n A$ down to 3 Ω·μm².

Junctions exhibit high quality with $R_j/R_n \,\ge 25$.

Validated capacitance calibration through SIS mixer performance.

Abstract

We report a Nb/Al-AlN /Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron-cyclotron-resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with $R_n A$ down to $\approx 3,\Omega,\mu\mathrm{m}^2$. Junctions maintain excellent quality, exhibiting $R_j/R_n \ge 25$ at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room-temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance $C_s$ versus $R_n A$ is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted $C_s$.