Towards saturation of detection efficiency in superconducting single-photon detectors at 4.2 K using local helium ion irradiation
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Superconducting single-photon detectors (SSPDs) are the leading detectors in terms of high-speed single-photon counting and high detection efficiency (DE). One factor that limits the DE is the critical current Ic, which is the maximum current before the superconductor switches to the normal state. Increasing device’s bias current towards the Ic can improve the DE. However, the device’s Ic is reduced due to constriction and current crowding at the edges of the wire. Typically, this is caused by fabrication defects. Locally suppressing superconductivity at these defects can potentially lessen the occurrence of current crowding. In this thesis, we used the beam from the helium ion microscope (HIM) and measured the Ic to observe the effects of locally irradiating specific areas on a SSPD wire. Due to the HIM’s small spot size and high collimation, we can control the superconducting gap precisely at the center and edges of the wire. Suppressing the edges can potentially reduce current crowding and increase the device’s critical current while suppressing the center can improve detection sensitivity for photons incident at that location. Our results showed that the irradiated devices had reduced Ic compared to unirradiated devices for both cases. We then extend this method of local suppression of superconductivity to explore an alternative method of fabricating SSPDs by directly writing the device on the superconducting thin film. This can enable the fabrication of devices without the use of lithography resist. In our experiment, we fabricated a 3 μm wire using optical lithography that was disconnected at the center and connected it by writing a single 1 μm wire with the He+ ion beam. We measured the Ic for samples with and without the 1 μm wire pattern and observed that the Ic decreased as we increased the ion dose. Overall, this work aims to contribute to the continuing investigation of the detection mechanism for SSPDs and the improvement of nanofabrication methods using the HIM.