Sub-micron normal-metal/insulator/superconductor tunnel junction thermometer and cooler using Nb

TL;DR

This paper reports the fabrication of sub-micron Nb-based tunnel junctions that function effectively as thermometers across a broad temperature range and demonstrate direct electronic cooling, with performance influenced by Al layer geometry.

Contribution

The study introduces a novel Nb-based tunnel junction device with high superconducting gap and demonstrates its dual use for thermometry and electronic cooling, highlighting fabrication and geometric effects.

Findings

High superconducting gap up to 1 mV achieved.

Effective thermometry from 100 mK to 6 K demonstrated.

Evidence of direct electronic cooling near the gap edge.

Abstract

We have successfully fabricated Cu/AlOx-Al/Nb normal-metal/insulator/superconductor tunnel junction devices with a high value of the superconducting gap (up to $\sim 1$ mV), using electron-beam lithography and angle evaporation techniques in the sub-micron scale. The subgap conductance of these junctions shows the expected strong temperature dependence, rendering them suitable for thermometry all the way from 100 mK to 6 K. In addition, some direct electronic cooling of the normal metal was also seen at bias values near the gap edge. The device performance was strongly influenced by the details of the Al layer geometry, with lateral spilling of the aluminium giving rise to strong extra subgap features, and the thickness of Al layer affecting the proximised superconducting gap value of the superconducting Al/Nb bilayer.