Mixing Properties of NbN-Based SIS Mixers With NbTiN Wirings

We report on the mixing properties of waveguide SIS mixers with epitaxial NbN/AlN/NbN junctions and NbN/SiO₂/NbTiN microstrips on MgO substrates. The superconducting transition temperature and 20-K resistivity of NbTiN film on the SiO₂ dielectric were 14.7 K and 175 muOmega cm, respectively. The junction tuning circuit was composed of an NbN/SiO₂/NbTiN microstrip and two parallel-connected NbN/AlN/NbN junctions 0.9 mum in diameter. The critical current density and normal-state resistance of the fabricated NbN junction were 13 kA/cm² and 22 Omega, respectively. The heterodyne response was measured using 295- and 77-K blackbody sources, a local oscillator (LO) source, and a 25-mum-thick Kapton film as a beam splitter. An intermediate frequency (IF) amplifier with a bandwidth of 4-12 GHz was used in the measurements. The receiver noise temperature corrected for losses in the beam splitter and in the vacuum window of the cryocooler was 370 K at 860 GHz and was comparable to that of all-NbN mixes with NbN/MgO/NbN microstrips.