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IEEE Transactions on Applied Superconductivity

SIS Mixers Based on NbN Techniques for ALMA Band 10

Mahoto Takeda, Yoshinori Uzawa, Zhen Wang

Published Jun 1, 2007• 7 citations
Publisher

Abstract

We designed, fabricated, and tested SIS mixers based on NbN techniques and estimated the specific capacitance of NbN/AlN/NbN tunnel junctions, which were fabricated by DC-magnetron sputtering, by measuring DC-SQUID resonance steps. In the mixers, NbN/AlN/NbN junctions and NbN/Si O2/Al tuning circuits were used for investigating receiver performance at frequencies greater than 900 GHz. The specific capacitance of the junction was estimated at 120 fF/ mum2with a critical current density of 12 kA/cm2. The resonance frequencies generated in the tuning circuits linearly changed with respect to the tuning lengths up to at least 1.1 THz. We found that an input-circuit structure, which we used, is a factor that degrades receiver noise at frequencies greater than 900 GHz.

DOI
10.1109/tasc.2007.898568

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We designed, fabricated, and tested SIS mixers based on NbN techniques and estimated the specific capacitance of NbN/AlN/NbN tunnel junctions, which were fabricated by DC-magnetron sputtering, by measuring DC-SQUID resonance steps. In the mixers, NbN/AlN/NbN junctions and NbN/Si O2/Al tuning circuits were used for investigating receiver performance at frequencies greater than 900 GHz. The specific capacitance of the junction was estimated at 120 fF/ mum2with a critical current density of 12 kA/cm2. The resonance frequencies generated in the tuning circuits linearly changed with respect to the tuning lengths up to at least 1.1 THz. We found that an input-circuit structure, which we used, is a factor that degrades receiver noise at frequencies greater than 900 GHz.
We report on the fabrication and properties of high current density NbN/AlN/NbN tunnel junctions for the application of superconducting SIS mixers in the submillimeter wave regions. Junctions having current densities as high as 50 kA/cm/sup 2/ exhibited a good Josephson tunneling behaviour, excellent terahertz response, and sensitive heterodyne mixing properties. Accurate values of junction capacitance were estimated by measuring dc-SQUID resonant steps for the design of tuning circuits. The measured specific capacitance gives values in the range of 20-200 fF//spl mu/m/sup 2/ for the current density range of 1-100 kA/cm/sup 2/. The NbN/AlN/NbN tunnel junctions were integrated with a NbN thin film antenna to investigate the terahertz responses and mixing tests in a quasi-optical testing system. Photon-assisted tunneling steps were clearly observed on the I-V curve with irradiation up to 1.02 THz, and a low-noise heterodyne mixing was demonstrated in the 300-GHz band.
We have designed, fabricated, and tested superconductor-insulator-superconductor (SIS) mixers based on Nb/AlN/NbN twin tunnel junctions for waveguide receivers operating in a frequency range of 790-950 GHz. Electromagnetic simulations and measurement results of the mixer performance are presented. The junctions have a high gap voltage of 3.15 mV and a high current density of about 30 kA/cm2, providing a wide receiver band, which was confirmed by Fourier transform spectrometer (FTS) and noise temperature measurements. The corrected receiver noise temperature varies from 240 K at low frequencies to 550 K at the high end of the band. The influence of the SIS junction heating on its characteristics has been studied and compared to another similar high current density technologies. The heating does not have a critical impact on the mixer performance.
We have designed and fabricated waveguide superconductor-insulator-superconductor (SIS) receivers at 870 GHz with NbN/AlN/NbN tunnel junctions on an MgO substrate. A waveguide probe was incorporated into the mixer chip for a waveguide-to-microstrip transition. As the tuning circuit for the SIS mixer, we designed a resonant distributed junction whose length is one wavelength and critical current density is 20 kA/cm/sup 2/. The mixing properties were calculated based on the quantum theory of mixing. The SSB mixer noise temperature should be less than 100 K and conversion gain larger than -8.0 dB in the frequency range from 840 to 920 GHz. We also observed the resonance steps of the fabricated SIS mixer. The tuning frequency was slightly higher than the designed frequency.
We have theoretically and experimentally investigated submillimeter-wave receiver performances employing NbN/ALN/NbN tunnel junctions with high current density and high energy. gap. The receiver noise based on parameters for our typical NbN SIS junction were analyzed using Tucker's quantum theory of mixing, which showed that DSB receiver noise temperatures of below 3hv/K B can be achieved by using the NbN SIS junctions for frequencies up to 2 THz. An experimental receiver has been designed and tested for the 1 THz band. The mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, a NbN twin-slot antenna, and NbN tuning circuits. The prepared NbN/A1N/NbN junction size was about 0.5 !mut), and the current density was about 30 kA/cm 2 (o)CJRN = 11 at 1 THz). Although the junctions showed good dc I-V characteristics with a small sub-gap leakage current, the receiver noise temperature measured by the standard Y-factor method was about 2700 K(DSB) at 761 GHz, which is much larger than the theoretical prediction. The relative high noise performance may be caused by the large RF losses in the tuning circuit of polycristalline NbN on the SiO.
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.
The authors report on the fabrication and testing of all-refractory NbN/MgO/NbN SIS (superconductor-insulator-superconductor) tunnel junctions for use as high-frequency mixers. Progress in the development of techniques for the fabrication of submicron-area tunnel junctions is described. Junction structures which have been investigated include mesa, crossline, and edge geometries. Using reactive sputtering techniques, NbN tunnel junctions with critical currents in excess of 10/sup 4/ A/cm/sup 2/ have been fabricated with V/sub m/ values as high as 65 mV and areas down to 0.1 mu m/sup 2/. Specific capacitance measurements on NbN/MgO/NbN mesa-type tunnel junctions give values in the range 60-90 fF/ mu m/sup 2/. These SIS tunnel junctions have been integrated with antennas and coupling structures for mixer tests in a waveguide receiver at 207 GHz. Preliminary mixer results are reported.>