Development of low-noise SIS mixers with NbN technique for ALMA Band 10

Low-noise and wide-band SIS (superconductor-insulator-superconductor) mixers operating at around 1 THz are now required for the ALMA (Atacama large millimeter/submillimeter array) Band-10 receivers, which cover the frequency band from 787 to 950 GHz. SIS mixers based on Nb technique have shown excellent performances with a few times of quantum-limited noise at frequencies below 725 GHz, which is the gap frequency of Nb. However, the RF loss in Nb significantly increases due to pair breaking at frequencies above 725 GHz, and noise performances of Nb-based SIS mixers are considerably degraded. To overcome this problem, it is necessary to make SIS mixers based on superconducting materials with a larger energy gap than that of Nb. Niobium nitride (NbN) have attracted interest recently as replacements for Nb used in terahertz SIS mixers because of its high superconducting energy gap, corresponding with a gap frequency of about 1.5 THz. However, NbN is known to have a very short superconducting coherence length (xi~5 nm) and superconducting properties of NbN thin films are sensitive to its crystal structures.