A high efficiency superconducting nanowire single electron detector

Abstract

We report the detection of single electrons using a Nb 0.7 Ti 0.3 N superconducting wire deposited on an oxidized silicon substrate. While it is known that this device is sensitive to single photons, we show that it also detects single electrons with kilo-electron-volt energy emitted from the cathode of a scanning electron microscope with an efficiency approaching unity. The electron and photon detection efficiency map of the same device are in good agreement. We also observe detection events outside the active area of the device, which we attribute to sensitivity to backscattered electrons. © 2010 American Institute of Physics. ͓doi:10.1063/1.3506692͔The versatility of superconducting nanowires as single particle detectors relies on their sensitivity to the minute amount of energy required to locally induce a resistive transition. From this point of view, the latest achievements involving the detection of organic molecules 1 and photons in the infrared range 2 all derive from early experiments with ␣-particles in the million electron volt range. 3 In order to go beyond the optical resolution limit, the scanning electron microscope ͑SEM͒ working at low temperature, 4 proved useful. This technique enabled the visualization of the real size of the hot spot caused by a detection process. 5 However, the best achieved spatial resolution is limited by thermal diffusion to about 1 m and single electron detection has not been demonstrated. In this paper, we show single electron detection using a superconducting nanowire. Our superconducting single electron detector ͑SSED͒ offers a high spatial and timing resolution and we compare the electron detection efficiency map with a photon detection efficiency map, measured on the same device.The fabrication process of our superconducting nanowire has been described before. 6 It consists of a 100 nm wide, 500 m long, and 6 nm thick wire of Nb 0.7 Ti 0.3 N. The wire is folded into an 10ϫ 10 m 2 area, with a separation of 100 nm between adjacent detecting branches. One end of the wire is grounded whereas the other end is connected to a cryogenic coaxial cable used to inject a current through the structure. We measure a dc critical current I c =10 A at 4.2 K and I c = 5.2 A at 8 K. Our experimental setup consists of a cryogenic SEM. 7 The detector is mounted on a cold translation stage at T = 8 K under the electronic beam of an SEM. The SEM current I b is controlled and can be measured with a picoammeter ͑measurement uncertainty 10%͒. The energy of the incident electrons E e can be varied between 5 and 30 keV.To block low frequency 1 / f noise we use a dc-block, through which we can only bias the wire with pulses of current amplitude I and duration t d = 800 ns ͑see inset of Fig. 1͒. Each pulse is reflected by the circuit. When the current is on, the detection of an electron triggers a short pulse at the output of the system ͑duration of a few nanoseconds, see inset of Fig. 1͒. The change in baseline is caused by the limited bandwidth ͑0.1 to 1000 MHz͒ of the amplification of the o...

Vortex ratchet effects in a superconducting asymmetric ring-shaped device

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