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.
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.
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 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.
We study the reset dynamics of niobium (Nb) superconducting nanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is, therefore, essential, and we demonstrate experimentally how this is achieved. We compare our results to studies of reset and latching in niobium nitride SNSPDs.
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.
We investigated the suitability of AlN as a buffer layer for NbN superconducting nanowire single-photon detectors (SNSPDs) on GaAs. The NbN films with a thickness of 3.3-20 nm deposited onto GaAs substrates with an AlN buffer layer demonstrate a higher critical temperature, critical current density, and lower residual resistivity in comparison to films deposited onto bare substrates. Unfortunately, the thermal coupling of the NbN film to the substrate weakens. SNSPDs made of 4.9-nm-thick NbN films on buffered substrates (in comparison to detectors made from NbN films on bare GaAs) demonstrate three orders of magnitude lower dark count rates and about ten times higher detection efficiency at 900 nm being measured at 90% of the critical current. The system timing jitter of SNSPDs on buffered substrates is 72 ps, which is 36 ps lower than those on bare substrate. However, a weaker thermal coupling of NbN nanowire to the buffered substrate leads to a latching effect at bias currents > 0.97 IC.
We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K (superconducting temperature transition TC). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electron–phonon scattering time τe–ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time τes decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, τe–ph and τes, fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10(±2) and 38 ps at 10.5 K, respectively. The obtained value of τe–ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at TC can reach 16(+4/−3) GHz if one eliminates the bolometric phonon-heating effect.