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IEEE Transactions on Applied Superconductivity

NbTiN/SiO/sub 2//Al tuning circuits for low-noise 1 THz SIS mixers

Brian D. Jackson, N. N. Iosad, Gert J. de Lange, Andrey M. Baryshev, W. M. Laauwen, J. R. Gao, T. M. Klapwijk

Published Mar 1, 2001• 25 citations
Publisher

Abstract

Waveguide SIS mixers in which Nb/Al-AlO/sub x//Nb tunnel junctions are integrated with NbTiN/SiO//sub 2//Al tuning circuits are shown to yield receiver noise temperatures as low as 565 K at 970 GHz. Analyzing the noise and gain of one such receiver, it is shown that the NbTiN ground plane is low-loss (<0.6 dB) at 970 GHz. These results are in good agreement with results obtained previously with a quasi-optical mixer incorporating a similar tuning circuit. A decrease in sensitivity above 1 THz is attributed to increasing loss in the NbTiN.

DOI
10.1109/77.919429

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Applied Physics Letters•Dec 20, 1999•81 citations
Waveguide SIS mixers in which Nb/Al-AlO/sub x//Nb tunnel junctions are integrated with NbTiN/SiO//sub 2//Al tuning circuits are shown to yield receiver noise temperatures as low as 565 K at 970 GHz. Analyzing the noise and gain of one such receiver, it is shown that the NbTiN ground plane is low-loss (<0.6 dB) at 970 GHz. These results are in good agreement with results obtained previously with a quasi-optical mixer incorporating a similar tuning circuit. A decrease in sensitivity above 1 THz is attributed to increasing loss in the NbTiN.
Waveguide SIS mixers are presented in which Nb/Al-AlOx/Nb tunnel junctions are integrated with both NbTiN/SiO2/NbTiN and NbTiN/SiO2/Al tuning circuits. The mixers with a NbTiN/SiO2/NbTiN tuning circuit yield relatively low receiver noise temperatures between 700 and 830 GHz, but their sensitivity drops significantly above ~ 850 GHz. In contrast, low-noise heterodyne mixing near 1 THz is observed in mixers incorporating a NbTiN/SiO2/Al tuning circuit. In particular, an uncorrected receiver noise temperature of 585 K is measured at 970 GHz. From an analysis of the receiver sensitivity and gain, it is determined that the loss in the NbTiN ground plane is low (≤ 0.6 dB) below 970 GHz. A decrease in receiver sensitivity above 1 THz is attributed to an onset of rf loss in the NbTiN.
The low-noise performance of a waveguide SIS mixer with NbN/AlN/NbN tunnel junctions and an NbN/MgO/NbTiN tuning circuit on an MgO substrate was demonstrated at frequencies above the gap frequency of Nb. To design the mixer, the complex refractive index of the MgO substrate was measured by using a THz time-domain spectrometer. The superconductive properties of NbTiN films on MgO were examined in detail. An uncorrected receiver noise temperature of 262 K (DSB) at 820 GHz was achieved in an IF bandwidth of 4-12 GHz at a bath temperature of 4.2 K. By comparing the measured and calculated conversion gains, the loss in the NbN/MgO/NbTiN tuning circuit was estimated to be 0.98 dB at 810 GHz.
Heterodyne mixers incorporating Nb SIS junctions and NbTiN-SiO/sub 2/-Al microstrip tuning circuits offer the lowest reported receiver noise temperatures to date in the 0.8-0.96- and 0.96-1.12-THz frequency bands. In particular, improvements in the quality of the NbTiN ground plane of the SIS devices' on-chip microstrip tuning circuits have yielded significant improvements in the sensitivity of the 0.96-1.12-THz mixers relative to previously presented results. Additionally, an optimized RF design incorporating a reduced-height waveguide and suspended stripline RF choke filter offers significantly larger operating bandwidths than were obtained with mixers that incorporated full-height waveguides near 1 THz. Finally, the impact of junction current density and quality on the performance of the 0.8-0.96-THz mixers is discussed and compared with measured mixer sensitivities, as are the relative sensitivities of the 0.8-0.96- and 0.96-1.12-THz mixers.
Low-noise heterodyne mixing at 1 THz is demonstrated in a quasioptical mixer incorporating Nb superconductor–insulator–superconductor tunnel junctions and a NbTiN/SiO2/Al tuning circuit. Receiver noise temperatures as low as 250 K at 850 GHz, 315 K at 980 GHz, and 405 K at 1015 GHz are measured—a factor of 2 improvement in sensitivity versus state-of-the-art 1 THz receivers, which incorporate normal metal tuning circuits. An analysis of the receiver sensitivity at 980 GHz demonstrates that NbTiN is low loss up to ∼1 THz.
We present the development of a low noise 1.2 THz and 1.4 THz SIS mixers for heterodyne spectrometry on the Stratospheric Observatory For Infrared Astronomy (SOFIA) and Herschel Space Observatory. This frequency range is above the limit for the commonly used Nb quasi particle SIS junctions, and a special type of hybrid Nb/AlN/NbTiN junctions has been developed for this project. We are using a quasi-optical mixer design with two Nb/AlN/NbTiN junctions with an area of 0.25. The SIS junction tuning circuit is made of Nb and gold wire layers. At 1.13 THz the minimum SIS receiver uncorrected noise temperature is 450 K. The SIS receiver noise corrected for the loss in the LO coupler and in the cryostat optics is 350-450 K across 1.1-1.25 THz band. The receiver has a uniform sensitivity in a full 4-8 GHz IF band. The 1.4 THz SIS receiver test at 1.33-1.35 THz gives promising results, although limited by the level of available LO power. Extrapolation of the data obtained with low LO power level shows a possibility to reach 500 K DSB receiver noise using already existing SIS mixer.
We have developed a low-noise 850 GHz superconductor–insulator–superconductor quasiparticle mixer with NbTiN thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. The mixer uses a quasioptical configuration with a planar twin-slot antenna feeding a two-junction tuning circuit. At 798 GHz, we measured an uncorrected double-sideband receiver noise temperature of TRX=260 K at 4.2 K bath temperature. This mixer outperforms current Nb SIS mixers by a factor of nearly 2 near 800 GHz. The high-gap frequency and low loss at 800 GHz make NbTiN an attractive material with which to fabricate tuning circuits for SIS mixers. NbTiN mixers can potentially operate up to the gap frequency, 2Δ/h∼1.2 THz.