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AIP conference proceedings

Simulation of the cut-off wavelength of superconducting single-photon detectors based on WSi thin films for systems for optical space communications

Konstantin M. Moiseev, Д.Д. Васильев, Selbi Hydyrova

Published Jan 1, 2019• 0 citations
Publisher

Abstract

Superconducting single-photon detectors (SNSPD) are receivers in optical systems for space communications and exhibit high quantum efficiency and count rate. The receiver in the optical system of atmospheric communication should provide highly efficient detection of radiation with a wavelength of 1.55 µm. The cut-off wavelength of SNSPD is the maximum wavelength of radiation at which high detection efficiency is maintained and depends on the geometrical parameters of the sensitive element of the detector and the thickness and structure of the thin film of superconducting material parameters. In this work, we simulate the cut-off wavelength depending on the thickness of the superconducting film WxSi1-x and the stoichiometric composition with a nanowire width of 100 nm and a ratio of the bias current to the critical 0.7. It is shown that the largest value of the cut-off wavelength λc = 3.06 µm corresponds to a film thickness of 5 nm and a Si content of 25%. With a film thickness of 7 nm, detection with the required wavelength of 1.55 µm is provided over the entire investigated range of the stoichiometric composition of the film: when the Si content is from 15 to 25%.

DOI
10.1063/1.5133246

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Superconducting single-photon detectors (SNSPD) are receivers in optical systems for space communications and exhibit high quantum efficiency and count rate. The receiver in the optical system of atmospheric communication should provide highly efficient detection of radiation with a wavelength of 1.55 µm. The cut-off wavelength of SNSPD is the maximum wavelength of radiation at which high detection efficiency is maintained and depends on the geometrical parameters of the sensitive element of the detector and the thickness and structure of the thin film of superconducting material parameters. In this work, we simulate the cut-off wavelength depending on the thickness of the superconducting film WxSi1-x and the stoichiometric composition with a nanowire width of 100 nm and a ratio of the bias current to the critical 0.7. It is shown that the largest value of the cut-off wavelength λc = 3.06 µm corresponds to a film thickness of 5 nm and a Si content of 25%. With a film thickness of 7 nm, detection with the required wavelength of 1.55 µm is provided over the entire investigated range of the stoichiometric composition of the film: when the Si content is from 15 to 25%.
We have developed the deposition technology of WSi thin films 4 to 9 nm thick with high temperature values of superconducting transition (T c ~4 K). Based on deposed films there were produced nanostructures with indicative planar sizes ~100 nm, and the research revealed that even on nanoscale the films possess of high critical temperature values of the superconducting transition (T c ~3.3-3.7 K) which certifies high quality and homogeneity of the films created. The first experiments on creating superconducting single-photon detectors showed that the detectors' SDE (system detection efficiency) with increasing bias current (I b ) reaches a constant value of ~30% (for X=1.55 micron) defined by infrared radiation absorption by the superconducting structure. To enhance radiation absorption by the superconductor there were created detectors with cavity structures which demonstrated a practically constant value of quantum efficiency >65% for bias currents I b >0.6-I c . The minimal dark counts level (DC) made 1 s -1 limited with background noise. Hence WSi is the most promising material for creating single-photon detectors with record SDE/DC ratio and noise equivalent power (NEP).
The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) have been studied systematically on detectors made from thin NbN films with different chemical compositions. Reduction of the nitrogen content in the 4 nm thick NbN films results in a decrease of the dark count rates more than two orders of magnitude and in a red shift of the cut-off wavelength of the hot-spot SNSPD response. The observed phenomena are explained by an improvement of uniformity of NbN films that has been confirmed by a decrease of resistivity and an increase of the ratio of the measured critical current to the depairing current. The latter factor is considered as the most crucial for both the cut-off wavelength and the dark count rates of SNSPD. Based on our results we propose a set of criteria for material properties to optimize SNSPD in the infrared spectral region.
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.
Abstract 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 T c , sheet resistance R s 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 β between the superconducting thin film and the substrate, by measuring the return current of the SNSPD and find an increase of β with increasing Mo concentration. The highest sensitivity amongst all compared devices is achieved for Mo 0.53 Si 0.47 , with low T c (4.1 K) and high R s (397 Ω /sq) at a film thickness of 5.4 nm.
To measure light down to the single photon level, superconducting nanowire single photon detectors (SNSPDs) have become the go-to devices, due to their near unity detection efficiency [1] , high-speed and low noise. An important figure of merit is the internal detection efficiency (IDE). To achieve a saturated IDE in the infrared, wire widths of the order 100 nm are typically required. Nevertheless, recent experiments have shown that functional detectors can be fabricated with wire widths up to 3 µm [2] – [4] . A higher silicon content of the films combined with a smaller film thickness enables wider wires. Increasing the wire width allows for bigger fabrication tolerances which enables the fabrication with optical lithography.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) made of ultrathin δ -NbN films have been widely applied in both visible and infrared wavelengths. For mid-infrared (MIR) wavelengths, SNSPDs made of tungsten silicide films with a lower critical temperature were reported up to 9.9 μ m wavelength. In this study, we demonstrate the potential of NbN-SNSPDs for use in MIR applications. SNSPDs made of γ -Nb 4 N 3 films (critical temperature of 5.1 K for 6.5 nm thick films) instead of δ -NbN films were fabricated. The dependence of the normalized detection efficiency on the bias current indicated a clear trend toward a saturated plateau for wavelengths up to 2145 nm. The calculated cut-off wavelengths indicated the possibility of using γ -Nb 4 N 3 SNSPDs for longer MIR wavelengths.