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. Tunnel junctions have been fabricated from NbCN films with transition temperatures up to 18 K. High-quality NbCN tunnel junction fabrication generally requires low stress films with roughness less than the barrier thickness ( approximately=20 AA). Scanning tunneling microscopy was developed 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 V/sub m/ values (at 3 mV) up to 27 mV.