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

Abstract

We have fabricated Cu-AlOx-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 exhibits a strong temperature dependence, rendering them suitable for thermometry from ∼0.1 K all the way up to the superconducting transition temperature of the NbN layer, which was here ∼11 K, but could be extended up to ∼16 K in our PLD chamber. Our data fit well to the single particle NIS tunnel junction theory, with an observed proximized superconducting gap value ∼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. © 2013 American Institute of Physics.