NbN-MgO-NbN junctions prepared on room-temperature quartz substrates for quasiparticle mixers

Abstract

NbN tunnel junctions are of great interest for THz heterodyne receivers because their large gap voltage of 5 mV yields an upper frequency limit of 4/spl Delta//h=2.4 THz for quasiparticle mixing. AC losses in NbN films, however, imply that a NbN matching circuit can be used at most to the NbN gap frequency, 2/spl Delta//h=1.2 THz. Another issue is the poor thermal conductivity of NbN films which complicates heat extraction from the junction. One solution to these problems are hybride mixer chips in which NbN junctions are integrated into appropriate superconducting or normal metal matching circuits. Both for waveguide mixers and quasioptical mixers quartz substrates are the first choice. We report on a room-temperature process for NbN-MgO-NbN junction fabrication which permits the realization of hybride mixer chips on quartz substrates. Appropriate plasma conditions for NbN film deposition are created by employing a second Nb target as a selective nitrogen pump. NbN films on quartz substrates without and with intermediate layers of Nb, Al, Al/SiO/sub 2/, and Nb/SiO/sub 2/ have critical temperatures above 15 K and normal state resistivities from 100 to 130 /spl mu//spl Omega/cm. In the first step, NbN junctions with nominal areas of 0.36 /spl mu/m/sup 2/ and current densities of 10 kA/cm/sup 2/ at 5.5 mV were integrated into Nb films forming the tuning circuit and a dipole antenna. The lowest receiver noise temperature in the 350 GHz range was 245 K double side band.