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

Abstract

We have successfully fabricated Cu-AlOx-Al-NbN normal metal-insulator-superconductor (NIS) tunnel junction devices, using pulsed laser deposition (PLD) for NbN film growth, and electronbeam 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 ∼ 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 fits well to the single particle NIS tunnel junction theory, with an observed proximised 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.

Superconducting tantalum nitride-based normal metal-insulator-superconductor tunnel junctions

“…Clearly, the data shows that the contact resistance is low <1X/junction (four orders of magnitude less than the tunneling resistances of the second and third type devices, as will be shown later), and that the general behavior is that of a good NS contact, although several resonance features are seen, possibly originating from multiple Andreev reflections. 23 The resonance features were not observed in similar devices using NbN electrodes, 11 however, the NS contact resistance in NbN devices was actually orders of magnitude higher for unknown reasons.…”

“…From the fits, we also get the temperature dependence of the superconducting gap D, which was seen to follow the simple BCS theory well, in contrast to the NbN x based devices 11 which exhibited stronger modifications due to proximity effect. 29 At 0.18 K, the measured D was $0.9 meV, about four times higher than a typical Al film gap, indicating that the Al layer is well proximized by the TaN x .…”

“…13 Furthermore, we have already fabricated NbN x based NIS junctions, using PLD for the growth of NbN x films, and electron beam lithography (EBL), reactive ion etching (RIE), and shadow angle evaporation for the device fabrication, with an ex-situ thermally oxidized Al barrier. 11 These two advances were combined here to develop Cu-AlO x -Al-TaN x NIS tunnel junctions.…”

“…However, to achieve operation at higher temperatures, one must switch to materials with higher superconducting transition temperatures (T C ) than the T C of Al at $1.2 K, as T C limits the maximum range of thermometry, and cooling power drops strongly above T $ 0.4T C . 1 Recently, we fabricated Nb (T C $ 8 K) 10 and NbN x (T C $ 12 K) 11 based NIS devices and demonstrated an order of magnitude increase of the thermometry range, and an observation of some electronic cooling in the Nb device. 10 However, as the optimal operational temperature for cooling for those type of devices is around 3.5-5 K, one should also develop NIS devices with a T C in the intermediate range between Al and Nb/NbN devices.…”

“…For the second device type, the method previously developed for the NbN x NIS junction fabrication with AlO x tunnel barriers has been used. 11 First, 40 nm thick Al islands of size 6 lm  6 lm were evaporated on top the TaN x electrodes, followed by in-situ oxidation at room temperature in 50 millibars of O 2 for 4 min, to grow the AlO x tunnel barriers. Then, without breaking the vacuum, a 100 nm thick Cu strip of width 0.5 lm was evaporated to form the connection between two TaN x electrodes (separated by a distance of 15 lm) so that a series connection of two Cu-AlO x -Al-TaN x NIS tunnel junctions (SINIS) is formed [ Fig.…”