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arXiv:0712.3295 · Optics Express

High efficiency NbN nanowire superconducting single photon detectors fabricated on MgO substrates from a low temperature process

Francesco Marsili, David Bitauld, Andrea Fiore, Alessandro Gaggero, F. Mattioli, Roberto Leoni, M. Benkahoul, Francis Lévy

Published Jan 1, 2008• 64 citations
arXivHTMLPDF

Abstract

We demonstrate high-performance nanowire superconducting single photon detectors (SSPDs) on bN thin films grown at a temperature compatible with monolithic integration. NbN films ranging from 150 nm to 3 nm in thickness were deposited by dc magnetron sputtering on MgO substrates at 400 degrees C SSPDs were fabricated on high quality NbN films of different thickness (7 to 3 nm) deposited under optimal conditions. Electrical and optical characterizations were performed on the SSPDs. The highest QE value measured at 4.2K is 20% at 1300 nm.

DOI
10.1364/oe.16.003191

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We demonstrate high-performance nanowire superconducting single photon detectors (SSPDs) on bN thin films grown at a temperature compatible with monolithic integration. NbN films ranging from 150 nm to 3 nm in thickness were deposited by dc magnetron sputtering on MgO substrates at 400 degrees C SSPDs were fabricated on high quality NbN films of different thickness (7 to 3 nm) deposited under optimal conditions. Electrical and optical characterizations were performed on the SSPDs. The highest QE value measured at 4.2K is 20% at 1300 nm.
We demonstrate efficient nanowire superconducting single photon detectors (SSPDs) based on NbN thin films grown on GaAs. NbN films ranging from 3 to 5 nm in thickness have been deposited by dc magnetron sputtering on GaAs substrates at 350 °C. These films show superconducting properties comparable to similar films grown on sapphire and MgO. In order to demonstrate the potential for monolithic integration, SSPDs were fabricated and measured on GaAs/AlAs Bragg mirrors, showing a clear cavity enhancement, with a peak quantum efficiency of 18.3% at λ=1300 nm and T=4.2 K.
High sensitivity ultrafast nanowire superconducting single photon detectors (SSPD) in the near infrared wavelength range have been fabricated with ultrathin (3.5nm) NbN films grown on R-plane sapphire substrates by dc reactive magnetron sputtering in Ar+N2mixture. These results show for the first time that high performance NbN SSPDs can be realized on different substrates and at lower deposition temperature than previously reported, and opens the way to integration with advanced solid state optical structures. SSPDs have been fabricated by a two mask process using electron beam lithography and reactive ion etching on 3.5nm thick NbN films deposited under optimal conditions on MgO.
We report on the fabrication and characterization of high performance nanowire superconducting single photon detectors (SSPD) on MgO substrates. The deposition of the superconducting material (NbN) has been performed by magnetron sputtering at 400°C. This deposition temperature is low enough to be compatible with the fabrication on traditional optical materials like GaAs. First, SSPD meanders covering a surface of 5 × 5 µm2 were characterized. We measured quantum efficiencies up to 20% at 1300 nm. Then, we used a straight 250 µm-long NbN nanowire to perform a spatially resolved measurement of the efficiency. Both efficiency dips and peaks were observed. The dips were correlated to lithographic defects, as confirmed by scanning electron microscopy. Conversely, no evidence of a lithographic origin of efficiency peaks was found.
We report on the performance of large area NbN nanowire superconducting single-photon detectors (SSPDs). 20×20μm2 area SSPDs with 80 and 100nm linewidths and 50% fill factor were fabricated in 4-nm-thick NbN films grown on single-crystal MgO substrates. The high quality of the devices was verified by electrical and optical testing and compares favorably to measurements of 10×10μm2 area SSPDs. Measurements of kinetic inductance versus bias current indicate that the constriction density is low. The fiber-coupled detection efficiency of the devices was 0.4%–3.5% at 100Hz dark count rate.
A very promising way to increase the detection efficiency of nanowire superconducting single-photon detectors (SSPDs) consists in integrating them with advanced optical structures such as distributed Bragg reflectors (DBRs) and optical waveguides. This requires transferring the challenging SSPD technology from the usual substrates, i.e. sapphire and MgO, to an optical substrate like GaAs, on which DBRs and waveguides can be easily obtained. Therefore, we optimized the deposition process of few-nm thick superconducting NbN films on GaAs and AlAs/GaAs-based DBRs at low temperatures (substrate temperature T S = 400 °C), in order to prevent As evaporation. NbN films ranging from 150 to 3 nm in thickness were then deposited on single-crystal MgO, GaAs, MgO-buffered GaAs and DBRs by current-controlled DC magnetron sputtering (planar, circular, balanced configuration) of Nb in an Ar+N 2 plasma. 5.5 nm thick NbN films on GaAs exhibit T C = 10.7 K, Δ T C = 1.1 K and RRR = 0.7. The growth of such high quality thin NbN films on GaAs and DBRs has never been reported before.
We report on superconducting single-photon detectors (SSPDs) based on NbN nanowires and their application to single-photon measurements at telecom wavelengths. The detectors are realized by patterning ultra thin (3 - 5 nm-thick) NbN films into meanders of 100 nm-wide wires. The films are deposited by DC magnetron sputtering at relatively low temperatures (≈500 °C) on MgO and GaAs substrates (as compared to the approach of high-temperature deposition on sapphire). In 5 nm-thick films, TCsas high as 10 K and 11 K have been obtained on GaAs and MgO, respectively (Fig. 1). SSPDs have been fabricated by a two-mask process using electron beam lithography and reactive ion etching on 3.5nm thick NbN films deposited under optimal conditions on MgO. The detectors are 5 × 5 μm2in size, and composed of a 100 μm long, 100 nm wide nanowire folded in a meander pattern with filling factor of 40%. Typical room temperature resistivity ρ is of 190 μΩ·cm, and I-V curves at 4.2 K show a critical current density JCof about 6 MA·cm-2, which are state of the art values. Quantum efficiency (QE) was measured illuminating the detectors mounted in a liquid He bath through an optical fiber. Quantum efficiencies >10% are measured at 4.2 K, comparable to best results on sapphire substrates. In particular, SSPDs show a large increase in sensitivity, temporal resolution and maximum counting frequencies as compared to InGaAs avalanche photodiodes.