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Magnetic Field Dependence of the Microwave Properties of Proximity Effect Nb/Al Bilayers Close to the Gap Frequency

Songyan Zhu, T. Zijlstra, C. F. J. Lodewijk, A. Brettschneider, M. van den Bemt, Brian D. Jackson, A. M. Baryshev, A. A. Golubov, T. M. Klapwijk

Published Apr 1, 2009• 0 citations

Abstract

High critical current density tunnel barriers of AlN have two effects on the operation of niobium-based SIS mixers close to the upper frequency limit of superconducting niobium: 1. the bandwidth is no longer limited by the RC time constant of the tunnel-junction, but the physical processes in the superconductors at high frequencies; 2. the devices have smaller lateral dimensions in order to provide impedance matching given the low RnA value, which requires the application of a higher magnetic field to quench the Josephson-effect through the Fraunhofer pattern. Consequently, under operating conditions the superconducting properties are no longer the equilibrium properties as realized by the fabrication conditions, but are deteriorated by the applied magnetic field. We report the measurements of I-V curves and Fourier transmission spectrum to determine and correlate the evolution of the superconducting properties in a high magnetic field. The results are compared with a calculation of the proximity effect based on the Keldysh Greens' function technique, leading to a magnetic field dependent density of states in the Al toplayer of the commonly used Nb-Al bilayer to fabricate tunnel barriers as well as forming one part of the superconducting stripline.

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Abstract — We have developed a new fabrication process for AlN barriers with excellent reproducibility and control compared to previous work, enabling the use of these barriers in SIS mixers. We report results with niobium-aluminum nitride-niobium superconductor-isolator-superconductor (SIS) mixers for 700 GHz, demonstrating a wide bandwidth behavior with Fourier Transform Spectrometer (FTS) measurements. These measurements have been performed in air and reveal that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. We also present noise temperature measurements of AlN SIS mixers that complement the FTS results. I.

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High critical current density tunnel barriers of AlN have two effects on the operation of niobium-based SIS mixers close to the upper frequency limit of superconducting niobium: 1. the bandwidth is no longer limited by the RC time constant of the tunnel-junction, but the physical processes in the superconductors at high frequencies; 2. the devices have smaller lateral dimensions in order to provide impedance matching given the low RnA value, which requires the application of a higher magnetic field to quench the Josephson-effect through the Fraunhofer pattern. Consequently, under operating conditions the superconducting properties are no longer the equilibrium properties as realized by the fabrication conditions, but are deteriorated by the applied magnetic field. We report the measurements of I-V curves and Fourier transmission spectrum to determine and correlate the evolution of the superconducting properties in a high magnetic field. The results are compared with a calculation of the proximity effect based on the Keldysh Greens' function technique, leading to a magnetic field dependent density of states in the Al toplayer of the commonly used Nb-Al bilayer to fabricate tunnel barriers as well as forming one part of the superconducting stripline.

We study, using niobium-technology, the bandwidth of SIS mixers operating at frequencies close to the energy-gap frequency. Microstriplines of niobium-silicon dioxide-niobium have different properties for the top and bottom superconductor, which we find to depend on the used fabrication process. Replacing the AlOxtunnel barrier by AlN, the bandwidth increases by 53%. The measurements, using a Fourier Transform Spectrometer (FTS) and performed in ambient air, demonstrate that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. Excellent noise temperatures are found over a full band of 600 to 720 GHz.

Modern radio astronomy demands for broadband receiver systems. For SIS mixers, this translates into objective to employ superconducting tunnel junctions with a very low RnA and low specific capacitance. The traditionally used Nb/AlOx/Nb junctions have largely approached their physical limit ofminimizing those parameters. It is commonly recognized that it is AlN-barrier junctions, which are needed for further progressing of the broadband SIS mixer instrumentation for radio astronomy. In this work, we present the progress in fabrication of high quality Nb/Al-AlN/Nb superconducting tunnel (SIS) junctions and their characterization in terms DC electric properties junctions’ specific capacitance.

Abstract — We have developed a new fabrication process for AlN barriers with excellent reproducibility and control compared to previous work, enabling the use of these barriers in SIS mixers. We report results with niobium-aluminum nitride-niobium superconductor-isolator-superconductor (SIS) mixers for 700 GHz, demonstrating a wide bandwidth behavior with Fourier Transform Spectrometer (FTS) measurements. These measurements have been performed in air and reveal that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. We also present noise temperature measurements of AlN SIS mixers that complement the FTS results. I.

Superconductor–insulator–superconductor (SIS) tunnel junctions including magnetic barriers, with structures NbN/AlNiN/NbN or NbN/MgO/NiO/MgO/NbN, were fabricated. The junctions with upper NbN layers of good crystallinity showed a gap voltage ( V g ) higher than 5 mV. In an NbN/Al 0.942 Ni 0.058 N/NbN junction, its Josephson current I c was suppressed to approximately a third of a theoretical value under no magnetic field. Therefore, it would be possible to avoid Josephson noise in magnetic barrier junctions with a much smaller magnetic field than junctions with conventional barriers. The Ni ion area density dependence of I c suppression ratio was identical for both types of junction, suggesting that the area density of magnetic ions is the factor that determines the I c suppression ratio. On the other hand, the extents of degradation of V g and increase in leakage current caused by the increase in Ni ion area density were smaller in MgO/NiO/MgO barrier junctions than in AlNiN barrier junctions. This suggests that the proximity effect of magnetic ions was weakened owing to the separation of Ni ions and NbN layers by the inserted thin MgO layers.

We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.