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

We have designed superconductor-insulator-superconductor (SIS) mixers using Nb/AlN/Nb tunnel junctions integrated with NbTiN/SiO
₂
/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 SiO
₂
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/cm
²
and all-NbTiN tuning circuits may cover the Band 10 with good sensitivity.