NbN-MgO-NbN tunnel junctions integrated in aluminum strip lines for terahertz quasiparticle mixers

NbN tunnel junctions are of great interest for THz heterodyne receivers because their large gap voltage of V-gap approximate to 5 mV implies an upper frequency limit of 4 Delta/h approximate to 2.6 THz as compared to 1.4 THz of Nb. However, due to the high ac losses in NbN films for frequencies above the NbN gap frequency, 2 Delta/h approximate to 1.3 THz, NbN matching circuits cannot be used. Lower losses are achieved with normal metals such as Al. Both for waveguide mixers and quasioptical mixers, quartz substrates are preferred. We report on a room-temperature fabrication process for NbN-MgO-NbN junctions with surfaces below 1 mu m(2) integrated in Al strip lines on quartz substrates. Appropriate plasma conditions for NbN-film deposition are obtained through the use of a second Nb target which acts as a selective nitrogen pump. NbN films on quartz substrates with and without an intermediate layer of Al have T-c > 15 K and rho(N) = 100 - 130 mu Omega.cm. Nb buffers should prevent the possible formation of insulating AIN at the Al-NbN interfaces. We fabricated the first NbN junctions sandwiched between Al layers. Sputtered Al were employed. Critical-current densities of up to 60 kA/cm(2) and subgap resistances R-sg approximate to 5.R-N were achieved. The I-V curves show various structures below the gap voltage. We discuss here the observation of a distinct steep step at eV = 2 Delta/3.