SIS Mixers Based on NbN Techniques for ALMA Band 10

We designed, fabricated, and tested SIS mixers based on NbN techniques and estimated the specific capacitance of NbN/AlN/NbN tunnel junctions, which were fabricated by DC-magnetron sputtering, by measuring DC-SQUID resonance steps. In the mixers, NbN/AlN/NbN junctions and NbN/Si O₂/Al tuning circuits were used for investigating receiver performance at frequencies greater than 900 GHz. The specific capacitance of the junction was estimated at 120 fF/ mum² with a critical current density of 12 kA/cm² . The resonance frequencies generated in the tuning circuits linearly changed with respect to the tuning lengths up to at least 1.1 THz. We found that an input-circuit structure, which we used, is a factor that degrades receiver noise at frequencies greater than 900 GHz.