Lusche, R., Semenov, A., Ilin, K., Siegel, M., Korneeva, Y., Trifonov, A., et al. (2014). Effect of the wire width on the intrinsic detection efficiency of superconducting-nanowire single-photon detectors. J. Appl. Phys., 116(4), 043906 (1 to 9).
Abstract: A thorough spectral study of the intrinsic single-photon detection efficiency in superconducting TaN and NbN nanowires with different widths has been performed. The experiment shows that the cut-off of the intrinsic detection efficiency at near-infrared wavelengths is most likely controlled by the local suppression of the barrier for vortex nucleation around the absorption site. Beyond the cut-off quasi-particle diffusion in combination with spontaneous, thermally activated vortex crossing explains the detection process. For both materials, the reciprocal cut-off wavelength scales linearly with the wire width where the scaling factor agrees with the hot-spot detection model.
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Vodolazov, D. Y., Manova, N. N., Korneeva, Y. P., & Korneev, A. A. (2020). Timing jitter in NbN superconducting microstrip single-photon detector. Phys. Rev. Applied, 14(4), 044041 (1 to 8).
Abstract: We experimentally study timing jitter of single-photon detection by NbN superconducting strips with width w ranging from 190 nm to 3μm. We find that timing jitter of both narrow (190 nm) and micron-wide strips is about 40 ps at currents where internal detection efficiency η saturates and it is close to our instrumental jitter. We also calculate intrinsic timing jitter in wide strips using the modified time-dependent Ginzburg-Landau equation coupled with a two-temperature model. We find that with increasing width the intrinsic timing jitter increases and the effect is most considerable at currents where a rapid growth of η changes to saturation. We relate it with complicated vortex and antivortex dynamics, which depends on a photon’s absorption site across the strip and its width. The model also predicts that at current close to depairing current the intrinsic timing jitter of a wide strip could be about ℏ/kBTc (Tc is a critical temperature of superconductor), i.e., the same as for a narrow strip.
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Zolotov, P. I., Semenov, A. V., Divochiy, A. V., Goltsman, G. N., Romanov, N. R., & Klapwijk, T. M. (2021). Dependence of photon detection efficiency on normal-state sheet resistance in marginally superconducting films of NbN. IEEE Trans. Appl. Supercond., 31(5), 1–5.
Abstract: We present an extensive set of data on nanowire-type superconducting single-photon detectors based on niobium-nitride (NbN) to establish the empirical correlation between performance and the normal-state resistance per square. We focus, in particular, on the bias current, compared to the expected depairing current, needed to achieve a near-unity detection efficiency for photon detection. The data are discussed within the context of a model in which the photon energy triggers the movement of vortices i.e. superconducting dissipation, followed by thermal runaway. Since the model is based on the non-equilibrium theory for conventional superconductors deviations may occur, because the efficient regime is found when NbN acts as a marginal superconductor in which long-range phase coherence is frustrated.
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Zolotov, P., Semenov, A., Divochiy, A., & Goltsman, G. (2021). A comparison of VN and NbN thin films towards optimal SNSPD efficiency. IEEE Trans. Appl. Supercond., 31(5), 1–4.
Abstract: Based on early phenomenological ideas about the operation of superconducting single-photon detectors (SSPD or SNSPD), it was expected that materials with a lower superconducting gap should perform better in the IR range. The plausibility of this concept could be checked using two popular SSPD materials – NbN and WSi films. However, these materials differ strongly in crystallographic structure (polycrystalline B1 versus amorphous), which makes their dependence on disorder different. In our work we present a study of the single-photon response of SSPDs made from two disordered B1 structure superconductors – vanadium nitride and niobium nitride thin films. We compare the intrinsic efficiency of devices made from films with different sheet resistance values. While both materials have a polycrystalline structure and comparable diffusion coefficient values, VN films show metallic behavior over a wide range of sheet resistance, in contrast to NbN films with an insulator-like temperature dependence of resistivity, which may be partially due to enhanced Coulomb interaction, leading to different starting points for the normal electron density of states. The results show that even though VN devices are more promising in terms of theoretical predictions, their optimal performance was not reached due to lower values of sheet resistance.
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Sidorova, M., Semenov, A. D., Hübers, H. - W., Ilin, K., Siegel, M., Charaev, I., et al. (2020). Electron energy relaxation in disordered superconducting NbN films. Phys. Rev. B, 102(5), 054501 (1 to 15).
Abstract: We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product qTl (qT is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14–30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/τe−ph∼Tn with the exponents n≈3.2–3.8. We found that in this temperature range τe−ph and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9–17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.
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