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IEEE Transactions on Applied Superconductivity

Comparison of Hot-Spot Formation in NbC and NbN Single-Photon Detectors

Y. Korneeva, M. Sidorova, Alexander V. Semenov, S. I. Krasnosvobodtsev, Кирилл Владимирович Мицен, Alexander A. Korneev, Чулкова Галина Меркурьевна, Gregory N Gol'tsman

Published Feb 11, 2016• 8 citations
Publisher

Abstract

We report an experimental investigation of the hot-spot evolution in superconducting single-photon detectors made of disordered superconducting materials with different diffusivity and energy downconversion time values, i.e., 33-nm-thick NbN and 23-nm-thick NbC films. We have demonstrated that, in NbC film, only 405-nm photons produce sufficiently large hot spot to trigger a single-photon response. The dependence of detection efficiency on bias current for 405-nm photons in NbC is similar to that for 3400-nm photons in NbN. In NbC, large diffusivity and downconversion time result in 1-D critical current suppression profile compared with the usual 2-D profile in NbN.

DOI
10.1109/tasc.2016.2526009

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We report an experimental investigation of the hot-spot evolution in superconducting single-photon detectors made of disordered superconducting materials with different diffusivity and energy downconversion time values, i.e., 33-nm-thick NbN and 23-nm-thick NbC films. We have demonstrated that, in NbC film, only 405-nm photons produce sufficiently large hot spot to trigger a single-photon response. The dependence of detection efficiency on bias current for 405-nm photons in NbC is similar to that for 3400-nm photons in NbN. In NbC, large diffusivity and downconversion time result in 1-D critical current suppression profile compared with the usual 2-D profile in NbN.
In superconducting single-photon detectors (SSPD), the efficiency of local suppression of superconductivity and hotspot formation is controlled by diffusivity and electron-phonon interaction time. Here, we selected a material, 3.6-nm-thick MoNx film, which features diffusivity close to those of NbN traditionally used for SSPD fabrication, but with electron-phonon interaction time an order of magnitude larger. In MoN∞detectors, we study the dependence of detection efficiency on bias current, photon energy, and strip width, and compare it with NbN SSPD. We observe nonlinear current-energy dependence in MoNx SSPD and more pronounced plateaus in dependences of detection efficiency on bias current, which we attribute to longer electron-phonon interaction time.
The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) have 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 decrease of the dark count rates more than two orders of magnitude 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.
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.
We have experimentally studied the supercurrent-assisted formation of the resistive state in nano-structured disordered superconducting Nb(N) films after absorption of a single optical to near-infrared photon. The efficiency of the resistive state formation has a pronounced spectral cut-off; corresponding threshold photon energy decreases with the bias current. Analysis of the experimental data in the framework of the refined hot-spot model suggests that the quantum yield for near-infrared photons increases with the photon energy. Relaxation of the resistive state depends on the photon energy making the phenomena feasible for the development of energy resolving single-photon detectors.
Superconducting nanowire single photon detectors (SNSPD) functionality is based on the local suppression of the superconducting order parameter upon photon absorption, causing a change in resistance which can be detected as a DC voltage pulse. Finding the size and the profile of the superconducting order parameter suppression area (the so-called “hot-spot”) experimentally is a challenging task. Here, we report the results of a quantum detector tomography on micron-length SNSPDs. The high internal detection efficiency of our SNSPDs allowed us to extract single and double photon efficiencies from the count rate vs radiation power dependence and extract hot-spot interaction length. We investigated a series of SNSPD samples made of NbN films with different sheet resistance and a MoSi film, with various widths of the stripe. The experimental results confirmed that regardless of the material or the film resistance, the hot-spot interaction length coincides with the strip width which is a promising feature for making a 2-photon counter with high fidelity.