NbN Superconducting Single-Photon Detectors Prepared on Single-Crystal MgO Substrates

We report the fabrication of NbN superconducting single photon detectors (SSPDs) and the development of a receiver system operating in a standard transport helium dewar. The NbN- SSPDs consisted of a 3.5 to 6.7-nm-thick NbN ultrathin film meander line and a 150-nm-thick NbN thin-film coplanar waveguide line. The NbN ultrathin films were deposited on single-crystal MgO substrates by reactive dc-magnetron sputtering to grow the NbN ultrathin films epitaxially, and the films were formed so that they had 80 to 200-nm-wide meander lines, which covered a 20 times 20 or 50 times 50 mum² area. All devices demonstrated good superconductivity, e.g., a device with a 3.9-nm-thick and 80-nm-wide line had a T_c of 10.8 K, an I_c of 19 muA, and a J_c of 5.9 times 10⁶ A/cm², which meant there was no serious deterioration in superconductivity compared with unprocessed films. We describe a reliable connection between an optical fiber and the detector, a setup which was used for first tests with pulsed light.