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Physical Review Applied

Activation Energies in MoSi/Al Superconducting Nanowire Single-Photon Detectors

Daniela Salvoni, Mikkel Ejrnaes, Alessandro Gaggero, F. Mattioli, F. Martini, Halima Giovanna Ahmad, Luigi Di Palma, Roberta Satariano, X.Y. Yang, Lixing You, Francesco Tafuri, Giovanni Piero Pepe, D. Massarotti, Domenico Montemurro, L. Parlato

Published Jul 5, 2022• 15 citations
Publisher

Abstract

Superconducting nanowire single-photon detectors (SNSPDs) are receiving the interest of the scientific and industrial communities due to their unparallel high performances in the infrared. However, even though the fabrication process permits the achievement of about 98% efficiency and less than 1 cps dark-count rate, the physical mechanism inducing the detection remains unclear. It is clear however that normal core vortices play a crucial role. In this work we investigate the role of vortices in two-dimensional (2D) molybdenum silicide SNSPDs of different widths covered with an $\mathrm{Al}$ layer through the analysis of the switching current distributions from the superconducting to resistive regime, in a wide interval of temperatures from 4.5 K down to 10 mK. This analysis provides the energy scales of different mechanisms that are responsible for fluctuations and dark counts in SNSPDs. We consider two models based on vortices, the unbinding of vortex-antivortex pairs (VAPs) and vortices hopping over the edge barrier (VH) and we underline the differences among different devices made by different materials. We also estimate the energy scales of similar $\mathrm{Nb}\text{\ensuremath{-}}\mathrm{Ti}\text{\ensuremath{-}}\mathrm{N}$ and $\mathrm{Nb}\mathrm{N}$ devices and compare the results. The lower activation energies obtained for $\mathrm{Mo}\mathrm{Si}/\mathrm{Al}$ devices, explain the peculiarity of this material to work at longer wavelengths with a higher quantum detection efficiency.

DOI
10.1103/physrevapplied.18.014006

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Superconducting nanowire single-photon detectors (SNSPDs) are receiving the interest of the scientific and industrial communities due to their unparallel high performances in the infrared. However, even though the fabrication process permits the achievement of about 98% efficiency and less than 1 cps dark-count rate, the physical mechanism inducing the detection remains unclear. It is clear however that normal core vortices play a crucial role. In this work we investigate the role of vortices in two-dimensional (2D) molybdenum silicide SNSPDs of different widths covered with an Al\mathrm{Al}Al layer through the analysis of the switching current distributions from the superconducting to resistive regime, in a wide interval of temperatures from 4.5 K down to 10 mK. This analysis provides the energy scales of different mechanisms that are responsible for fluctuations and dark counts in SNSPDs. We consider two models based on vortices, the unbinding of vortex-antivortex pairs (VAPs) and vortices hopping over the edge barrier (VH) and we underline the differences among different devices made by different materials. We also estimate the energy scales of similar Nb\ensuremath-Ti\ensuremath-N\mathrm{Nb}\text{\ensuremath{-}}\mathrm{Ti}\text{\ensuremath{-}}\mathrm{N}Nb\ensuremath-Ti\ensuremath-N and NbN\mathrm{Nb}\mathrm{N}NbN devices and compare the results. The lower activation energies obtained for MoSi/Al\mathrm{Mo}\mathrm{Si}/\mathrm{Al}MoSi/Al devices, explain the peculiarity of this material to work at longer wavelengths with a higher quantum detection efficiency.
Abstract Superconducting Nanowire Single-Photon Detectors (SNSPDs) are key building blocks for photonic quantum technologies due to their ability to detect single photons with ultra-high efficiency, low dark counts and fast temporal resolution. Superconducting materials exhibiting high uniformity, large absorption cross-section and atomic-scale thickness are desirable to extend single-photon detection from the near-infrared up to the terahertz regime, where existing material choices are especially constrained. Substrate independence would further open the way to integrating detectors onto functional materials and heterostructures, enhancing performance and enabling proximal readout of a wide range of individual excitations. Here, we pattern the prototypical two-dimensional superconductor niobium diselenide (NbSe 2 ) into few-layer nanowires less than 100 nm wide and demonstrate single-photon detection at 780 nm and 1550 nm. At the same time, the dark-count rate remains below 1 Hz up to the switching current and we achieve a timing jitter below 50 ps. We estimate via a diffusive hot-spot model that materials and geometric parameters would allow a theoretical cut-off wavelength in the millimetre range, assuming state-of-the-art device parameters. Our results open up routes toward quantum-limited detectors integrated into quantum-photonic circuits and quantum devices, with the potential for novel detection capabilities and unprecedented energy sensitivity.
In this paper, we theoretically show that a multi-layer superconducting nanowire single-photon detector (SNSPD) is capable of approaching characteristics of an ideal SNSPD in terms of the quantum efficiency, dark count, and band-width. A multi-layer structure improves the performance in two ways. First, the potential barrier for thermally activated vortex crossing, which is the major source of dark counts and the reduction of the critical current in SNSPDs is elevated. In a multi-layer SNSPD, a vortex is made of 2D-pancake vortices that form a stack. It will be shown that the stack of pancake vortices effectively experiences a larger potential barrier compared to a vortex in a single-layer SNSPD. This leads to an increase in the experimental critical current as well as significant decrease in the dark count rate. In consequence, an increase in the quantum efficiency for photons of the same energy or an increase in the sensitivity to photons of lower energy is achieved. Second, a multi-layer structure improves the efficiency of single-photon absorption by increasing the effective optical thickness without compromising the single-photon sensitivity.
Single photon detection advanced rapidly in the last decade. The superconducting nanowire single-photon detector (SNSPD) is an emerging technology, which offers low dark count rates, broadband sensitivity from visible to mid-infrared wavelengths and very high quantum efficiency. All these features are appealing for future emerging applications in remote gas sensing, quantum key distribution and quantum information processing. By enclosing the detector in an optical cavity, tuned to the operating wavelength, the total system detection of the SNSPD is improved. Based on the cavity design, the detector absorption is boosted at the desired experimental wavelength. Devices have been fabricated and tested, in this thesis, with enhanced detection efficiency at near-infrared, infrared and mid-infrared wavelengths. NbTiN detectors fabricated on Ta2O5 / SiO2 distributed Bragg reflectors (DBR) for front-side coupling exhibit a system detection efficiency of ≈ 90% at λ = 940 nm for the best device measured. SNSPDs were also fabricated in NbTiN on metallic mirrors for back-side coupling for 1550 and 2333 nm experimental wavelengths. The first design exhibited a system detection efficiency of ≈ 84.5%, at λ = 1550 nm for the best device measured. The second exhibited low system detection efficiency since the device suffered from unexpectedly high dark count rate mainly due to blackbody radiation at 2.2 K. However, it was possible to measure the timing jitter of this device at 350 mK indicating a FWHM of 84 ps. Another technique to improve the total system detection efficiency of the SNSPD, is by fabricating a hairpin-shaped nanowire atop a single mode waveguide. In this design, photons travel through the waveguide and via evanescent coupling are absorbed in the nanowire. These SNSPD devices have been designed to thread along single mode Si ridge waveguides using a hairpin-shaped nanowire structure, which offers enhanced coupling and absorption efficiency via evanescent coupling. These ultra-thin superconducting MoSi detectors are precisely patterned on top of a single mode Si waveguide (600 nm) with alignment error less than 40 nm. Low temperature electrical and nano-optical characterization was carried out using a two-stage pulse tube (PT) coldhead, using an additional 4He / 3He sorption pump, low vibration characterization of SNSPD devices at 350 mK was carried out. Characterised devices exhibit low dark count rate <5 counts per second and timing jitter of ≈ 83 ps full width half maximum (FWHM) at wavelength (λ) = 1550 nm and at critical current ≈ 30 µA.
The superconducting nanowire single photon detector (SNSPD) is currently one of the best detectors with comprehensive performance in the mid-infrared wavelength, which has been widely used in quantum optics, LiDAR, and biological fluorescence detection. To develop new devices with excellent performance, the photon detection mechanism of SNSPD still needs further research. Here we find that the single quantum phase slip is the dominating mechanism leading to dark-count and photon-response events in SNSPD below the quantum temperature. We fabricate the nanowire with a length of 7.5 μm and a width of 25 nm using niobium nitride film with a thickness of 5 nm. The distribution of the superconducting switching current is measured as a function of temperature at temperatures ranging from 0.3 to 7 K. We observe single quantum phase slips in the nanowire at the temperature below 1.2 K, and we further investigate the quantitative relationship between the single quantum phase slip rate and the superconducting switching rate of the nanowire under the laser illumination. Experimental results show that the single quantum phase slip is a significant factor leading to photon response and dark counts in the nanowire below the quantum temperature. Our research helps to further reveal the photon detection mechanism of the SNSPD, laying a theoretical foundation for the development of high-performance SNSPD.
When superconducting materials work as Single Photon Detectors (SPD) they are patterned in nanostrips about 100 nm wide with a typical meander structure to increase the photosensitive area. Moreover, they are biased by a DC current close to the critical current to have higher efficiency. The critical current could be non uniform mainly in proximity of the turn bends producing an increase of the dark count events. In this work MoSi nano-strips covered with a very thin (2 nm) layer of Al, have been realized and characterized in a wide interval of temperatures from 6 K down to 7 mK to investigate the critical current as a function of the temperature and then its uniformity in the presence of sharp corners and/or turn-arounds. Finally, the counts rates were recorded by a nano-meander under irradiation at two different wavelengths λ = 650 nm and 1550 nm, respectively, and compared with the events in the dark regime.
We report on the impact of the stoichiometry of superconducting MoSi thin films on the performance of superconducting nanowire single-photon detectors (SNSPDs). Specifically, we investigate the relation between the film parameters critical temperature Tc , sheet resistance Rs and superconductor thickness d and observe a universal scaling behavior. To benchmark the performance of SNSPDs fabricated from films having different stoichiometry, we measure the bias dependent count rate curves, while the detector is illuminated with wavelengths between 780 nm and 1550 nm. The detector performance as a function photon energy for different nanowire widths reveals a linear relation between the detection current and the photon energy. Furthermore, we determine the interfacial thermal boundary conductance βββ between the superconducting thin film and the substrate, by measuring the return current of the SNSPD and find an increase of βββ with increasing Mo concentration. The highest sensitivity amongst all compared devices is achieved for Mo0.53_{0.53}0.53​Si0.47_{0.47}0.47​, with low Tc (4.1 K) and high Rs (397ΩΩΩ/sq) at a film thickness of 5.4 nm.