NBCN JOSEPHSON-JUNCTIONS WITH ALN BARRIERS

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 (almost-equal-to 20 angstrom). 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/AIN/NbCN junctions with V(m) values (at 3 mV) up to 27 mV. These results demonstrate a fabrication technology compatible with realistic closed cycle refrigerator operating temperatures.