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Global Symposium on Millimeter-Waves

Comparison of the THz Direct-Detection Behaviors of Nb and NbN Superconducting Tunnel Junctions

Xiao-Fang Shen, Xuesong Cao, Jing Li, Sheng‐Cai Shi

Published Apr 1, 2008• 3 citations
Publisher

Abstract

Superconducting SIS (Superconductor-Insulator-Superconductor) tunnel junctions are promising application prospect in the incoherent detection of astronomy observation. In this paper, the direct detection performances of Nb and NbN based superconducting SIS junctions, which cover about 250 GHz and 140 GHz centered at 600 GHz respectively, are characterized at a bath temperature of 4.2 K. The experiments show that Nb and NbN based SIS junctions have a current shot noise of 2.5×10-12and 3.1×10-12A/radicHz, a current responsivity of 303 and 187 A/W (corresponding to the quantum efficiency of 0.67 and 0.43) respectively. Thus their respective NEPs are 8.4×10-15and 1.7×10-14W/√Hz.

DOI
10.1109/gsmm.2008.4534567

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 29, 2007•3 citations
Superconducting SIS (Superconductor-Insulator-Superconductor) tunnel junctions are promising application prospect in the incoherent detection of astronomy observation. In this paper, the direct detection performances of Nb and NbN based superconducting SIS junctions, which cover about 250 GHz and 140 GHz centered at 600 GHz respectively, are characterized at a bath temperature of 4.2 K. The experiments show that Nb and NbN based SIS junctions have a current shot noise of 2.5×10-12and 3.1×10-12A/radicHz, a current responsivity of 303 and 187 A/W (corresponding to the quantum efficiency of 0.67 and 0.43) respectively. Thus their respective NEPs are 8.4×10-15and 1.7×10-14W/√Hz.
In THz regime, NbN Superconductor - Insulator - Superconductor (SIS) mixers are potentially applicable to astronomy and atmospheric research. We have developed a 0.5-THz waveguide SIS mixer incorporating twin NbN/AlN/NbN junctions and NbN/MgO/NbN tuning circuit. In this paper, we compare it with a 0.5-THz Nb SIS mixer, which has been successfully used on POST telescope. The design and fabrication parameters of the two mixers are introduced, and the noise temperatures are measured. In addition, the direct detection performance is analyzed, such as the shot noise, the current responsivity and the noise equivalent power. The detailed results are presented.
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.
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.
Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 x 10(-13) W/root Hz around 5 K and 1.5 x 10(-12) W/root Hz at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.
The terahertz band, a unique segment of the electromagnetic spectrum, is crucial for observing the cold, dark universe and plays a pivotal role in cutting-edge scientific research, including the study of cosmic environments that support life and imaging black holes. High-sensitivity superconductor–insulator–superconductor (SIS) mixers are essential detectors for terahertz astronomical telescopes and interferometric arrays. Compared to the commonly used classical Nb/AlO x /Nb superconducting tunnel junction, the Nb/AlN/NbN hybrid superconducting tunnel junction has a higher energy gap voltage and can achieve a higher critical current density. This makes it particularly promising for the development of ultra-wideband, high-sensitivity coherent detectors or mixers in various scientific research fields. In this paper, we present a superconducting SIS mixer based on Nb/AlN/NbN parallel-connected twin junctions (PCTJ), which has a bandwidth extending up to 490 GHz–720 GHz. The best achieved double-sideband (DSB) noise temperature (sensitivity) is below three times the quantum noise level.
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.