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IEEE Transactions on Applied Superconductivity

Shot Noise in NbN/AlN/NbN Superconducting Tunneling Junctions

Dong Liu, Jie Hu, Ming Yao, Jing Li, Sheng‐Cai Shi

Published Nov 7, 2016• 1 citation
Publisher

Abstract

With sensitivity approaching the quantum limit, superconductor-insulator-superconductor (SIS) mixers play an important role in radio astronomy and atmospheric science at millimeter and submillimeter wavelengths. As one of the intrinsic noise sources in superconducting tunneling junctions, shot noise is still not well understood, particularly for those of relatively high energy gap (e.g., NbN/AlN/NbN). In this paper, we mainly study the multiple Andreev reflection (MAR) enhanced shot noise of two different NbN SIS junctions (i.e., junction array and long junction) as well as their temperature dependence. Barrier strength Z is taken into consideration for theoretical analysis of effective charge of the MAR effect based on the Blonder-Tinkham-Klapwijk (BTK) theory with the Andreev clusters method. It has been found that the effective charge of the MAR effect is inversely proportional to temperature, the barrier strength Z and transparency T are proportional to temperature. Detailed measurement results and analysis will be presented.

DOI
10.1109/tasc.2016.2625758

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With sensitivity approaching the quantum limit, superconductor-insulator-superconductor (SIS) mixers play an important role in radio astronomy and atmospheric science at millimeter and submillimeter wavelengths. As one of the intrinsic noise sources in superconducting tunneling junctions, shot noise is still not well understood, particularly for those of relatively high energy gap (e.g., NbN/AlN/NbN). In this paper, we mainly study the multiple Andreev reflection (MAR) enhanced shot noise of two different NbN SIS junctions (i.e., junction array and long junction) as well as their temperature dependence. Barrier strength Z is taken into consideration for theoretical analysis of effective charge of the MAR effect based on the Blonder-Tinkham-Klapwijk (BTK) theory with the Andreev clusters method. It has been found that the effective charge of the MAR effect is inversely proportional to temperature, the barrier strength Z and transparency T are proportional to temperature. Detailed measurement results and analysis will be presented.
With a gap frequency of 1.4 THz, all-NbN Superconductor-Insulator-Superconductor (SIS) junctions are potentially applicable to sensitive heterodyne mixers for THz astronomy and atmospheric research. In this paper, we report the design, fabrication, and performance characterization of a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and an NbN/MgO/NbN tuning circuit with no impedance transformers. The noise performance of two NbN SIS junctions with different tuning inductances is measured, compared with the simulated results without and with the multiple Andreev reflection (MAR) effect taken into account, and also compared with the video detection response. In addition, the IF-output-power stability and aging effect of fabricated NbN SIS junctions are investigated.
We study the behavior of NbN SIS junctions as radiation detectors with emphasis on the shot noise generated at voltages below the gap voltage. The intrinsically large subgap current of NbN junctions is carried by pinholes with a conduction attributed to multiple Andreev reflection, leading to transported charges q much greater than e. Using this charge enhancement mechanism we explain the junction shot noise characteristics in the unpumped case as well as in the pumped case. The measured mixer noise temperature in the pumped case is more than twice that calculated with standard Tucker theory. The measured double side band noise temperatures are 1450 K at 600 GHz and 2800 K at 950 GHz.
We measured shot noise and submillimeter-wave response in a superconducting NbN tunnel junction that had a subharmonic gap structure on the current-voltage (I-V) curve. We found that the observed effective charge, defined from the noise-current ratio, tends to a steplike function of voltage. In the presence of submillimeter-wave radiation of frequency v, novel step structures spaced by hv/2e below and above the half-gap voltage clearly appeared on the I-V curve, overlapping the ordinary photon-assisted tunneling steps spaced by hv/e. Observation of these features provides clear evidence that coherent multiple Andreev reflection processes occur in the NbN tunnel junction with low barrier transparency.
Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (Delta v/v<10(-6)). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (Δν/ν<10-6). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.