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

Abstract

We have fabricated Cu/AlOx-Al/Nb normal-metal/insulator/superconductor tunnel junction devices with a high value of the superconducting gap (up to ∼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.