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

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.