Applications and non-idealities of submicron Al-AlOx-Nb tunnel junctions

TL;DR

This paper reports the fabrication of high-quality submicron Al-AlOx-Nb tunnel junctions with potential for nanoscale thermometry, highlighting non-idealities like excess sub-gap current likely due to materials interface issues.

Contribution

It introduces a fabrication technique for submicron Al-AlOx-Nb junctions and investigates their electrical properties, revealing non-ideal behaviors not explained by standard theory.

Findings

High critical temperature (7.5 K) and gap (1.3 meV) indicating high-quality junctions.

Observation of excess sub-gap current unaccounted for by standard models.

Potential materials science issues at the barrier or interface affecting performance.

Abstract

We have developed a technique to fabricate sub-micron, 0.6 um x 0.6 um Al-AlOx-Nb tunnel junctions using a standard e-beam resist, angle evaporation and double oxidation of the tunneling barrier, resulting in high quality niobium, as determined by the the high measured values of the critical temperature TC 7.5 K and the gap 1.3 meV. The devices show great promise for local nanoscale thermometry in the temperature range 1 - 7.5 K. Electrical characterization of the junctions was performed at sub-Kelvin temperatures both with and without an external magnetic field, which was used to suppress superconductivity in Al and thus bring the junction into a normal-metal-insulator-superconductor (NIS) configuration. We observed excess sub-gap current, which could not be explained by the standard tunneling theory. Evidence points towards materials science issues of the barrier or Nb/AlOx interface as the culprit.