SUBMILLIMETER SIS MIXERS USING HIGH CURRENT DENSITY Nb/AlN/Nb TUNNEL JUNCTIONS AND NbTiN FILMS

We are currently exploring ways to improve the performance of SIS mixers above 700 GHz. One approach is to use NbTiN in place of Nb for all or some of the mixer circuitry. With its high gap frequency and low losses demonstrated up to 800 GHz, it should be possible to fabricate an all-NbTiN SIS mixer with near quantumlimited noise performance up to 1.2 THz. Using a quasioptical twin-slot two-junction mixer with NbTiN ground plane and wiring and hybrid Nb/AlN/NbTiN junctions, we measured an uncorrected receiver noise temperature of TRX 500K across 790– 850 GHz at 4.2 K bath temperature. Our second approach is to reduce the RC product of the mixer by employing very high current density Nb/AlN/Nb junctions. By using these we will greatly relax the requirement on tuning circuits, which is where substantial losses occur in mixers operating above the Nb gap frequency. These junctions have resistance-area products of RNA 5:6 m2, good subgap to normal resistance ratios, Rsg=RN 10, and good run-to-run reproducibility. From FTS measurements we infer that !RNC = 1 at 270 GHz in these junctions. This is a substantial improvement over that available using Nb/AlOx/Nb technology. The sensitivity of a receiver incorporating these high current density mixers is TRX = 110K at 533 GHz using a design for lower Jc mixers, which is close to the best we have measured with lower Jc Nb/AlOx/Nb mixers.