Broadening of hot-spot response spectrum of superconducting NbN nanowire   single-photon detector with reduced nitrogen content
  The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) has 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 more than two orders of magnitude decrease of the dark count rates 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. 
D. Henrich, S. D\"orner, M. Hofherr, K. Il'in, A. Semenov, E. Heintze,   M. Scheffler, M. Dressel, and M. Siegel
cond-mat.supr-con physics.ins-det