Superconducting niobium nitride thin films are used for a variety of photon detectors, quantum devices, and superconducting electronics. Most of these applications require highly uniform films, for instance, when moving from single-pixel detectors to arrays with a large active area. Plasma-enhanced atomic layer deposition (ALD) of superconducting niobium nitride is a feasible option to produce high-quality, conformal thin films and has been demonstrated as a film deposition method to fabricate superconducting nanowire single-photon detectors before. Here, we explore the property spread of ALD-NbN across a 6-in. wafer area. Over the equivalent area of a 2-in. wafer, we measure a maximum deviation of 1% in critical temperature and 12% in switching current. Toward larger areas, structural characterizations indicate that changes in the crystal structure seem to be the limiting factor rather than film composition or impurities. The results show that ALD is suited to fabricate NbN thin films as a material for large-area detector arrays and for new detector designs and devices requiring uniform superconducting thin films with precise thickness control.
We demonstrate and characterize first superconducting nanowire single-photon detectors (SNSPDs) made from atomic layer-deposited (ALD) NbN layers. To assess the suitability of these films as a detector material, transport properties of bare films and bridges of different dimensions and thicknesses are investigated. Similar ratios of the measured critical current to the depairing current are obtained for micro-bridges made from ALD and sputtered NbN films. Furthermore, we characterized the single-photon response for 5 and 10 nmthick nanowire detectors. A 100 nm-wide straight nanowire with a length of 5 µm exhibits saturated count-rate dependencies on bias current and a cut-off wavelength in the near-infrared range. The ALD technique could open up the possibility to fabricate NbN-based detectors on the wafer scale and to conformally cover also non-planar surfaces for novel device concepts.
We demonstrate a GHz-gated operation of resonator-coupled superconducting nanowire single-photon detectors suitable for synchronous applications. In comparison with conventional dc-biased nanowire detectors, this method prevents the detector from latching and can suppress dark counts and background noise. Using a gating frequency of 3.8 GHz and a fast, synchronized laser diode, we show that the detector's operation point follows the oscillating current and its detection efficiency depends on the relative frequency and phase of the bias and modulated optical signal. The obtained experimental results are in good agreement with simulations, showing that the duty cycle of a gated detector can be adjusted in a wide range in the case of a pronounced saturation of the current-dependent detection efficiency. This operation mode could be suitable for applications such as quantum key distribution and time-of-flight laser ranging.
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