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IEEE Workshop on Low Temperature Electronics (WOLTE)

Superconducting Molybdenum Silicide nanostrips for single photon detectors

L. Parlato, Daniela Salvoni, Halima Giovanna Ahmad, D. Massarotti, Domenico Montemurro, Raffaella Ferraiuolo, Luigi Di Palma, Roberta Satariano, Francesco Tafuri, Giovanni Piero Pepe, F. Mattioli, Alessandro Gaggero, Francesco Martini, Mikkel Ejrneas

Published Apr 12, 2021• 1 citation
Publisher

Abstract

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.

DOI
10.1109/wolte49037.2021.9655699

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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.
Abstract Superconducting nanostrip single photon detectors have emerged as the highest performing single-photon detectors; however, the possibility to use superconducting microstrip as single photon detectors is very appealing both to use them as larger areas detectors and for an easier technology in the manufacturing. The aim of this work is to test the photoresponse in liquid helium dewar of 9 nm thick MoSi covered with a very thin (2 nm) layer of Al, in two different configurations: nanomeanders and microstrips. We demonstrate that MoSi/Al microstrips can work as photodetectors also at T = 4.2 K. We also compare the dark count rate of the microstrip and the nanowire, confirming the lower noise for the microstrips also at 4.2 K.
Single photon detectors (SPD) are largely used when the signal to be measured is particularly low or if high accuracy, timing resolution and low noise are required. About twenty years ago, a new type of SPD based on superconducting materials appeared, and today these devices present the best performances in terms of efficiency, noise, counting rate, detectable wavelength, dead time, and timing jitter. In this work, we first describe the working principle of Superconducting Nanostrips Single Photon Detectors (SNSPDs) and provide a brief overview of the main properties and applications. Then, we present the results of the fabrication and characterization of Molybdenum Silicide SNSPDs with a cover layer of a thin Aluminium film (MoSi/Al). MoSi has already been adopted to fabricate SNSPDs, and in this work, we prove that the additional Al layer enhances the operating conditions without compromising the detection likelihood and leaving unaltered the properties of the device, even over a temporal window of two years and after several thermal cycles.
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 $\mathrm{Nb}\text{\ensuremath{-}}\mathrm{Ti}\text{\ensuremath{-}}\mathrm{N}$ 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.
Superconducting nanowire single-photon detectors (SNSPDs) are the leading technology for low noise, high efficiency infrared single-photon detection. The basic SNSPD consists of a nanowire patterned in an ultrathin superconducting thin film, which is cooled below its critical temperature and biased close to its critical current. The absorption of a single-photon creates a resistive region, triggering a fast output voltage pulse which can be readily amplified and registered. The excellent performance of SNSPDs at near-infrared and telecommunications wavelengths has led to their adoption in important applications such as quantum secure communications, single-photon spectroscopy and single-photon LIDAR. A clear challenge for the SNSPD community is to extend the spectral range of SNSPDs into the mid infrared, and to improve material uniformity to enable the realization of large area arrays for multimode or free space coupling. The aim of this work is to evaluate potential materials for next generation mid-infrared SNSPD arrays. In this work, thin films of polycrystalline NbN and amorphous MoSi have been optimized to test the uniformity of a multipixel array configuration composed of 8 nanowire meander structures covering 10 um x 10 um area, 100 nm width and 50% filling factor. The 8-pixels SNSPD arrays have been patterned on 8 nm thickness NbN grown on high resistivity silicon (HR Si) substrate at room temperature and at 800 °C exhibiting respectively 4.4 K and 7.3 K as mean critical temperature across the pixels. The 8- pixels SNSPD array patterned on 8 nm thickness MoSi cooling the HR Si substrate to -180 °C has exhibited a mean critical temperature of 3.2 K across the pixels. Optical properties have been measured by an attenuated 1550 nm laser diode source delivered by single mode optical fibre at a controlled distance from the chip in order to broadly illuminate the array. The optical properties have been studied only for the 8 nm thickness NbN SNSPD array grown at room temperature has demonstrated uniform optical properties across pixels exhibiting similar saturation of the internal efficiency over a large bias, similar dark count rate and similar timing jitter (about 137 ps) across pixels. In the single photon regime at 1550 nm, pixels 4 and 6 of the 8 nm thickness NbN SNSPD array exhibit 28.4% and 4.7% pixel detection efficiency as measured at the bias current 95% of their respective critical current at 2.2 K.
We demonstrate saturated internal detection efficiency at 1550 nm wavelengths for meander-shaped superconducting nanowire single-photon detectors made of 3 nm thick MoSi films with widths of 1 and 3 μm and active areas up to 400 × 400 μm2. Despite hairpin turns and a large number of squares (up to 104) in the device, the dark count rate was measured to be ∼103 cps at 99% of the switching current. This value is about two orders of magnitude lower than the results reported recently for short MoSi devices with shunt resistors. We also found that 5 nm thick MoSi detectors with the same geometry were insensitive to single near-infrared photons, which may be associated with different levels of suppression of the superconducting order parameter. However, our results obtained on 3 nm thick MoSi devices are in good agreement with predictions in the frame of a kinetic-equation approach.
We present our studies on ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs are patterned by electron beam lithography from 4-nm thick NbN film into meander-shaped strips covering square area of 10/spl times/10 /spl mu/m/sup 2/. The advances in the fabrication technology allowed us to produce highly uniform 100-120-nm-wide strips with meander filling factor close to 0.6. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, an avalanche of excited hot electrons and the biasing supercurrent, jointly produce a picosecond voltage transient response across the superconducting nanostrip. The SSPDs are typically operated at 4.2 K, but they have shown that their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by optical absorption of our 4-nm-thick NbN film. For 1.55 /spl mu/m photons, QE was /spl sim/20% and decreases exponentially with the increase of the optical wavelength, but even at the wavelength of 6 /spl mu/m the detector remains sensitive to single photons and exhibits QE of about 10/sup -2/%. The dark (false) count rate at 2 K is as low as 2 /spl times/ 10/sup -4/ s/sup -1/, what makes our detector essentially a background-limited sensor. The very low dark-count rate results in the noise equivalent power (NEP) as low as 10/sup -18/ WHz/sup -1/2/ for the mid-infrared range (6 /spl mu/m). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for the other, lower-T/sub c/ superconductors with the narrow superconducting gap and low quasiparticle diffusivity. The use of such materials will shift the cutoff wavelength towards the values even longer than 6 /spl mu/m.