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Effect of thermal conductance on the performance of superconducting nanowire single-photon detectors (SNSPDs)

Nicolò Petrini

Published Oct 25, 2019• 0 citations
OpenAlex

Abstract

The hotspot dynamics is one of the key process involved in the detection mechanism in superconducting nanowires single photon detectors (SNSPDs). The energy delivered by the photon absorption creates a non-equilibrium region where Cooper pairs break into quasi-particles, which undergo a thermalization process involving hot-electron–phonon interactions within the superconducting film. Eventually, phonons dissipate the excess energy into the substrate and relax to the base temperature. Hence, intrinsic reset time, count rate and possibly other parameters are conceivably affected by thermal coupling between the substrates and the superconducting film. Thus, a detailed understanding of the thermal boundary conductance is crucial for achieving the best possible performances. Even though recent studies have demonstrated that the substrate plays an important role in defining the hotspot relaxation time, no systematic study on the influence on detector performance metrics has been carried out yet. In this work, we investigated the effect of different substrates on SNSPDs performance. Meandered detectors and single nanowires with various dimensions have been successfully fabricated on ultrathin NbN film sputtered on six substrates: SiO2, Al2O3, SiC, SiN, AlN and GaN. We characterized the deposited films measuring Tc, ρ, ΔTc and RRR. Lattice mismatch is known to play an important role in determining the resulting film quality. SiO2 and SiN samples, in which the mismatch is pronounced, showed lower Tc and higher ρ compared to the other substrates. We extracted the critical and retrapping current densities (Jc, Jr respectively) for all the nanowire widths, observing a peculiar behavior in SiO2 and SiN samples which needs further investigation. Optical response was characterized in terms of reset time (τ), dark count rate (DCR) and photon count rate (PCR) with a 1550 nm wavelength laser at 3K. Single photon regime was verified in all the measured detectors. Reset time has been confirmed to be limited by kinetic inductance Lk for all the substrates. DCR behavior appeared to be correlated to the difference in film properties. Only detector fabricated on SiN and SiO2 substrates showed saturation in the photon count rate, while the highest count rate was achieved by Al2O3 sample. The different behavior of photon counts response on the substrates suggests a correlation with the measured thermal conductance β and needs further study. Nevertheless, the comparison between the extracted thermal boundary conductances and the theoretical ones highlighted relevant discrepancies between theory and experimental data. In conclusion, devices fabricated on NbN deposited on different substrates showed a peculiar detector photo-response behavior. A deeper evaluation of NbN disorder grade and a better thermal characterization of the substrate-thin film interface are needed as future work.

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The hotspot dynamics is one of the key process involved in the detection mechanism in superconducting nanowires single photon detectors (SNSPDs). The energy delivered by the photon absorption creates a non-equilibrium region where Cooper pairs break into quasi-particles, which undergo a thermalization process involving hot-electron–phonon interactions within the superconducting film. Eventually, phonons dissipate the excess energy into the substrate and relax to the base temperature. Hence, intrinsic reset time, count rate and possibly other parameters are conceivably affected by thermal coupling between the substrates and the superconducting film. Thus, a detailed understanding of the thermal boundary conductance is crucial for achieving the best possible performances. Even though recent studies have demonstrated that the substrate plays an important role in defining the hotspot relaxation time, no systematic study on the influence on detector performance metrics has been carried out yet. In this work, we investigated the effect of different substrates on SNSPDs performance. Meandered detectors and single nanowires with various dimensions have been successfully fabricated on ultrathin NbN film sputtered on six substrates: SiO2, Al2O3, SiC, SiN, AlN and GaN. We characterized the deposited films measuring Tc, ρ, ΔTc and RRR. Lattice mismatch is known to play an important role in determining the resulting film quality. SiO2 and SiN samples, in which the mismatch is pronounced, showed lower Tc and higher ρ compared to the other substrates. We extracted the critical and retrapping current densities (Jc, Jr respectively) for all the nanowire widths, observing a peculiar behavior in SiO2 and SiN samples which needs further investigation. Optical response was characterized in terms of reset time (τ), dark count rate (DCR) and photon count rate (PCR) with a 1550 nm wavelength laser at 3K. Single photon regime was verified in all the measured detectors. Reset time has been confirmed to be limited by kinetic inductance Lk for all the substrates. DCR behavior appeared to be correlated to the difference in film properties. Only detector fabricated on SiN and SiO2 substrates showed saturation in the photon count rate, while the highest count rate was achieved by Al2O3 sample. The different behavior of photon counts response on the substrates suggests a correlation with the measured thermal conductance β and needs further study. Nevertheless, the comparison between the extracted thermal boundary conductances and the theoretical ones highlighted relevant discrepancies between theory and experimental data. In conclusion, devices fabricated on NbN deposited on different substrates showed a peculiar detector photo-response behavior. A deeper evaluation of NbN disorder grade and a better thermal characterization of the substrate-thin film interface are needed as future work.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) wherein ultrathin films are fabricated on Si substrates are greatly affected by lattice mismatch between the thin film and the substrate. A buffer layer can be used to reduce such lattice mismatch or optimize the strain in the film, thereby improving device performance. We prepared and optimized Nb 5 N 6 as a buffer layer and found that it considerably improved the properties of NbN films on Si substrates. The zero-resistance critical temperature ( T C0 ) of a 3 nm thick NbN film with a 20 nm thick buffer layer was 10.3 K. SNSPDs with Nb 5 N 6 -buffered NbN films were fabricated and compared with normal devices; the fabricated devices had high hysteresis current and low timing jitter. Furthermore, we investigated the thermal diffusion process of the device based on the hysteresis current and hotspot relaxation time and found that Nb 5 N 6 buffer layers enhance the thermal coupling between the superconducting film and substrates. The relaxation time of buffered SNSPD was 14.2 ps, which was shorter than that of nonbuffered SNSPD by 17.8 ps. These effects explain the performance improvement observed in the case of the buffered devices.
Superconducting nanowire single photon detectors (SNSPDs) based on Si substrates have demonstrated excellent performance, such as high efficiency, low dark count rate, short reset time and low timing jitter. But due to the lattice mismatch between NbN and Si substrate, the performance of film is limited. To this end, Nb5N6layer is fabricated as the buffer layer between the NbN film and Si substrate, and that will reduce the mismatch and optimize superconducting properties of NbN films. The 6nm-thick film with a 65 nm-thick buffer layer shows the zero resistance critical temperature (Tc0) of 13.27 K, which is 5 K higher than that without buffer. Based on the NbN film on the buffered substrate, we fabricated SNSPD devices demonstrate critical current (IC) of 65 μA which is 5 ~ 6 times higher to the detectors without buffer. Besides, the experiment results prove that the buffer layer reduces the kinetic inductance, which fasten the recovery of nanowire.
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 Hotspot relaxation time (τ th ) is one of the essential parameter which defines the maximum count rate of superconducting nanowire single-photon detectors (SNSPDs). We studied the τ th for NbN-based SNSPDs on various substrates using the two-photon detection method based on the pump-probe spectroscopy technique. We observed that τ th strongly increased with increasing bias current in the two-photon detection regime. In addition, the minimum hotspot relaxation time (τ th )min was not significantly affected by the bath temperature; this is different from the previous observations reported for WSi SNSPDs. In addition, a strong dependency of (τ th )min on the substrate was found. The minimum (τ th )min was 11.6 ps for SNSPDs made of 5.5-nm-thick NbN on MgO (100), whereas the maximum (τ th )min was 34.5 ps for SNSPDs made of 7.5-nm-thick NbN on Si (100). We presented a direct correlation between the values of τ th and degrees of disorder of NbN films grown on different substrates.
The hotspot relaxation time (τth) was systematically studied on superconducting nanowire single-photon detectors (SNSPDs) made from ultrathin NbN films with different substrates and chemical compositions. The τth values were measured to be between 9.2 ps and 41.1 ps at the bath temperature of 2.15 K. The temperature dependence of τth ∝ TC−1.5 was observed, which indicated a dominant electron-phonon scattering relaxation mechanism in disordered NbN films. In addition, the increase in τth results in a higher latching probability in NbN-based SNSPDs with the same electrical response time (τe). The experimental results confirmed that the devices suffered latching when the ratio of the electric and thermal relaxation time was less than 133. This observation is an interesting reference for the design and process optimization of SNSPDs.
Achieving both high internal efficiency and fast reset times at elevated temperatures remains challenging due to limited understanding of how film properties govern superconducting nanowire single-photon detector (SNSPD) performance. We demonstrate that epitaxial NbN films on sapphire enable simultaneous high efficiency and rapid response. We fabricate and characterize SNSPDs based on these films deposited via DC magnetron sputtering on c-cut sapphire. High-quality epitaxial growth preserves a low electron diffusion coefficient and promotes strong electron-phonon coupling, yielding a high critical temperature and efficient hotspot formation in the dirty limit. X-ray diffraction and transmission electron microscopy confirm epitaxial alignment and lattice order. Nanowires of 20 nm width exhibit saturated internal efficiency at 1550 nm wavelength and short reset times at 4.2 K, enabled by lattice matching and high thermal conductance of the sapphire interface. Ab initio modeling reproduces photon count rates, linking device performance quantitatively to film properties such as diffusivity and electron-phonon coupling.