NbN Josephson and Tunnel Junctions for Space THz Observation and Signal Processing

摘要

Active superconducting circuits based on picoseconds switching, i.e., THz oscillating, Josephson tunnel junctions are expected to find a large domain of applications in spatial sub-millimeter wave and FIR detectors as well as in satellite based wideband signal and data processors. In the first case, SIS (superconductor- insulator-superconductor) tunnel junctions are required, while in the second case SNS (superconductor-normal metal-superconductor) self- shunted junctions are preferred. We present the advantages of the nitride junction technology currently developed at CEA-Grenoble, based on high- performance MTS (medium temperature superconductor) NbN films in both SIS and SNS junctions. In the SIS case the device performances rely on a sputter deposited and post-annealed, only 0.5-0.7 nm thick, dielectric MgO barrier. This leads to junctions with a Josephson critical current in the range of 10-25 kA/cm2, and a large superconducting energy gap (> 5 meV) associated with a low sub-gap quasi-particle leakage current. These parameters are suitable for low "noise temperature" heterodyne mixers, local oscillators, and integrated receivers in the 0.8 to 1.4 THz frequency range. In the SNS case, a new reactively sputtered TaXN barrier material has been developed. The obtained junctions have a characteristic Josephson frequency above 350 GHz. This can be easily extended above 800 GHz by barrier parameter engineering, and reducing the junction size by using conventional submicron lithographic techniques. Such NbN SNS junctions have been shown to be suitable for large scale integration in nitride-based multilayer Rapid Single Flux Quantum logic circuits operating at 10K. Both NbN SIS and SNS junction circuits can be combined on-chip in new ultra-wide band and low dissipation front-end SOC functions operating near 10K. In addition, they can be eventually interfaced with higher temperature semiconductor stages for space telecom applications.