Niobium nitride-based normal metal-insulator-superconductor tunnel junction microthermometer

TL;DR

This paper reports the fabrication of NbN-based NIS tunnel junctions with strong temperature-dependent conductance suitable for precise thermometry from 0.1 K up to 11-16 K, demonstrating good theoretical fit and potential for electronic cooling.

Contribution

The study introduces a novel NbN-based NIS microthermometer with high superconducting gap and demonstrates its temperature-dependent conductance for thermometry applications.

Findings

Device operates effectively from 0.1 K to 11-16 K.

Conductance fits well to NIS theory with a 1 meV gap.

High tunneling resistance limits cooling but benefits thermometry.

Abstract

We have successfully fabricated micron-scale Cu-AlO$_{x} $-Al-NbN normal metal-insulator-superconductor (NIS) tunnel junction devices, using pulsed laser deposition (PLD) for NbN film growth, and electron-beam lithography and shadow evaporation for the final device fabrication. The subgap conductance of these devices exhibit a strong temperature dependence, rendering them suitable for thermometry from $\sim$ 0.1 K all the way up to the superconducting transition temperature of the NbN layer, which was here $\sim 11$ K, but could be extended up to $\sim 16$ K in our PLD chamber. Our data fits well to the single particle NIS tunnel junction theory, with an observed proximised superconducting gap value $\sim $ 1 meV for a 40 nm thick Al overlayer. Although this high value of the superconducting energy gap is promising for potential electronic NIS cooling applications as well, the high value of the tunneling resistance inhibits electronic cooling in the present devices. Such opaque barriers are, however, ideal for thermometry purposes as self-induced thermal effects are thus minimized.