This letter reports a low noise submillimeter-wave mixer using NbN tunnel junctions integrated in Nb matching circuits. The double side band receiver noise temperature was 245 K at 345 GHz. Plasma conditions for NbN film deposition on quartz substrates at room temperature are created by using a second Nb target as a selective nitrogen pump. Electrodes for tunnel junctions with critical temperatures above 15 K and normal state resistivities in the range from 130 to 160 μΩ cm were obtained. This permits integrating NbN junctions into normal metal or non-NbN superconducting matching circuits, which is of great interest for THz mixers.

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We study noise and conversion gain in the SIS mixers with NblMgOlNbN junctions at the millimeter and submillimeter wavelengths in order to estimate the possibility to extent the low noise operation towards the theoretical limit of 4iVh=2.4 THz. W e present as preliminary result the lowest achieved receiver noise with NbN SIS junctions of 65 K with conversion gain of -6 dB at 162 GHz and 230 K receiver noise with conversion gain of -10 dB a t 306 GHz. The junction area is 2 km2 and Josephson critical current density is 1.4 KA/cm*. The optimum RNOC of NbN junction for mixers is estimated as 600 GHz/v. In preliminary tests the junctions RNOC is 10 -20 time superior to the optimum and a significant improvement of NbN SIS mixers may be expected. The noise sources in the NbN SIS junctions are studied and a possible mixer sensitivity improvement is discussed. The thermal properties of the SIS NbN are studied. The frequency limit of the low loss integrated tuning structure in NbN is estimated as at least as 1.5 THz.