Design of Terahertz SIS Mixers Using Nb/AlN/Nb Junctions Integrated With All-NbTiN Tuning Circuits

Abstract

We have designed superconductor–insulator–superconductor (SIS) mixers using Nb/AlN/Nb tunnel junctions integrated with NbTiN/SiO2/NbTiN microstrip tuning circuits for the Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 (0.78–0.95 THz). In order to design the tuning circuits, we used terahertz time domain spectroscopy to experimentally derive the complex conductivity of relatively thick NbTiN films deposited on quartz substrate directly as a ground plane and on RF-sputtered SiO2 as a microstrip. Results showed that the conductivities of both films can be described by a modified Mattis–Bardeen theory. We also investigated the junction heating effects originating from quasi-particle trapping in the Nb electrodes embedded in NbTiN circuits by varying the volume of the electrodes. It was confirmed that current–voltage curves of Nb/AlN/Nb tunnel junctions with thick (∼200 nm) counter electrodes contacting NbTiN wirings showed higher gap voltages as well as reduced back-bending than those with thin (∼50 nm) electrodes. Making use of these findings, our designed SIS mixers employing Nb/AlN/Nb junctions with a current density of 15 kA/cm2 and all-NbTiN tuning circuits may cover the Band 10 with good sensitivity.