NbCN Josephson junctions with AlN barriers

Abstract

Niobium carbonitride (NbCN) Josephson circuits can operate over a wider temperature range than either niobium or niobium nitride circuits. Higher operating temperature places NbCN technology more comfortably within the range of closed cycle refrigerators, a key factor in aerospace applications. We have fabricated tunnel junctions from NbCN films with transition temperatures up to 18 kelvin. High quality NbCN tunnel junction fabrication generally requires low stress films with roughness less than the barrier thickness (=20 A). We have developed scanning tunneling microscopy as a tool for measuring and optimizing film smoothness. Junctions formed in situ with AlN tunneling barriers show reproducible I-V characteristics. Unlike NbN, NbCN oxidizes readily, enabling junction definition by a modified SNAP process. This SNAP lithography technique successfully produced NbCN/AlN/NbCN junctions with Vm values (at 3 mV) up to 27 mV. These results demonstrate a fabrication technology compatible with realistic closed cycle refrigerator operating temperatures.

Monophasic Nanostructured Powders of Niobium, Tantalum, and Hafnium Carbonitrides Synthesized by a Mechanically Induced Self‐Propagating Reaction

“…Their excellent properties under hostile environments are particularly interesting. Nowadays, the interest in niobium carbonitride as electronic or superconducting material is growing due to potential applications as field emitter tips and arrays, diffusion barriers in integrated circuits, or Josephson junctions 4–7 …”

Characterization of NbN/AlN/NbN tunnel junctions fabricated without intentional heating

“…of the growth, NbN films have a very complicated surface morphology with the large concentration of the sharp peaks. They lead to the formation of the shorts in the tunnel barrier and degradation of the junction's characteristics (increase subgap leakage current and a voltage interval in the vicinity of 2Ale where the increase of the quasiparticle current occur, suppression of the gap voltage) [8].…”