Activation Energies in MoSi/Al Superconducting Nanowire Single-Photon Detectors

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 $\mathrm{Al}$ 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 $\mathrm{Nb}\mathrm{N}$ devices and compare the results. The lower activation energies obtained for $\mathrm{Mo}\mathrm{Si}/\mathrm{Al}$ devices, explain the peculiarity of this material to work at longer wavelengths with a higher quantum detection efficiency.