FF1E.7.pdf
CLEO 2017 © OSA 2017
High-Operating-Temperature Superconducting Nanowire Single Photon Detectors based on Magnesium Diboride Angel E. Velasco1, Daniel P. Cunnane1, Simone Frasca2, Thomas Melbourne3, Narendra Acharya3, Ryan Briggs1, Andrew D. Beyer1, Matthew D. Shaw1, Boris S. Karasik1, Matthäus A. Wolak3, Varun B. Verma4, Adriana E. Lita4, Hiroyuki Shibata5,6, Masataka Ohkubo7, Nobuyuki Zen7, Masahiro Ukibe7, Xiaoxing Xi3, Francesco Marsili1 1) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 2) University of Pisa, ITA 3) Department of Physics, Temple University, Philadelphia, PA, 19122 USA 4) National Institute of Standards and Technology, Boulder, CO, 80305 USA 5) Kitami Institute of Technology, JPN 6) NTT Basic Research Laboratories, JPN 7) National Institute of Advanced Industrial Science and Technology, JPN Corresponding author:
[email protected]
Abstract: We report on optically sensitive 15 nm thick, 100 nm wide MgB2 nanowires in the operating-temperature range 4 - 11 K. OCIS codes: (040.3780) Low light level, (040.5570) Quantum detectors
1. Introduction Tungsten silicide (WSi) superconducting nanowire single photon detectors (SNSPDs) offer cutting edge performance in the near infrared [1]: near unity detection efficiency (>90%), low intrinsic dark counts (~1 cps), low reset times (~50 ns), max count rates of 10 Mcps, low jitter (~50-100 ps), and a spectral range from visible to mid-IR [2]. However, WSi is a low temperature superconductor (LTS) and the optimal operating conditions are at sub-Kelvin temperatures where cryogenics is complex, expensive and resource intensive. Our goal is to reduce the cryogenic requirements of SNSPDs by developing detectors based on a high critical temperature (TC), metallic superconductor. We developed detectors based on magnesium diboride (MgB 2), which has a bulk critical temperature of 39 K [3], to increase the operating temperature of SNSPDs greater than 20 K where the cryogenics is simpler, smaller, and cheaper. Although MgB2 SNSPDs with single-photon sensitivity at 10 K have been demonstrated, those devices were fabricated with 10 nm thick films grown by molecular beam epitaxy (MBE) which had TC ~ 18 K [4-6]. The superconducting properties of MBE films quickly degrade with decreasing thickness. Increasing the operating temperature requires thicker films [7]. Here we report on MgB2 SNSPDs fabricated from 15 nm MBE films which were photosensitive at λ = 1550 nm. The higher transition temperature (TC ~ 22 K) of the film increased the operating temperature to 11 K. However, afterpulsing was present for T > 5 K. 2. Experimental Results We fabricated MgB2 nanowires with 90-400 nm widths in a meander and bridge geometry with a 15 nm thick film. The active area of the meander varied between 1 μm × 1 μm and 2 μm × 2 μm. The bridges were ~ 30 μm in length. The photoresponse of the nanowires were characterized as a function of temperature (T = 4 – 12 K) and optical wavelengths 1550 nm and 635 nm. The devices that were optically sensitive at λ = 1550 nm were 90 nm-100 nm wide. The fabrication of the detectors started with a15 nm thick film of MgB2 on sapphire passivated with 5 nm AlN. We fabricated the nanowires by electron beam lithography and the gold pads by optical lithography. An SEM image of a typical 100 nm meander structure is shown in figure 1a. Figure 1b shows the measured TC (21.95 K) and transition width ΔT (3.35 K) for the thicker devices. We defined TC as the temperature at which the resistance dropped to 50% of the value at 40 K. The width ΔT was defined as the temperature range where the resistance was 10% to 90% of the resistance value at 40 K. We measured the bias dependence of the system dark count rate (SDCR) and photon count rate (PCR) at different temperatures. The devices were illuminated with a 1550 nm and 635 nm continuous wave laser through a telecom wavelength, single mode optical fiber and diffused over a 20 mm × 20 mm area. An additional 6.5 μH inductor was added in series to the devices to prevent latching [8]. The SDCR was measured with the fiber optic setup connected to the cryostat but with the laser turned off. PCR was calculated by subtracting the DCR from the measured count rates when the light was coupled to the detectors. Figure 1c shows the SDCR versus bias current IB at 4 K (navy), 5 K (dark yellow), 6 K (magenta), 7 K (cyan), 8 K (blue), 9 K (green), 10 K (red), and 11 K (black). Above 11 K the device did not detect photons nor produce dark counts. At 1550 nm, we controlled the polarization of the light reaching the detector and measured the dependence of PCR on polarization. Varying the polarization, we measured a maximum and minimum PCR (PCRmax and PCRmin).
FF1E.7.pdf
CLEO 2017 © OSA 2017
Figure 1d shows the ratio PCRmax/PCRmin. The sharp decline at T ~ 5 K marks the onset of afterpulsing. Although the devices could detect photons at λ = 1550 nm up to 11 K, afterpulsing was present at higher temperatures.
a
2 µm
b s K TC = 21.95 ∆T = 3.35 K
c
d
Figure 1. a) An SEM of a 100 nm meander with an active area of 2×2 µm2. b) The resistance versus temperature of a 100 nm wide, 15 nm thick nanowire. The film had TC ~ 21.9 K measured at 50% the resistance at 40 K (red line) and a transition width ΔT = 3.35 K measured where the resistance was 10% to 90% of the resistance at 40 K. c) SDCR versus temperature 4 K (navy), 5 K (dark yellow), 6 K (magenta), 7 K (cyan), 8 K (blue), 9 K (green), 10 K (red), and 11 K (black). The fiber setup was connected to the cryostat and the laser was off. d) The ratio of the maximum and minimum PCR is shown versus temperature. The ratio is greater than unity up to 11 K. The sharp decline at T > 5 K was due to the onset of afterpulsing. The polarization measurements were taken at IB/ISW ~ 97.5%.
3. Conclusion We have demonstrated a 100 nm wide, 1 μm × 1 μm MgB2 meander that was optically sensitive at 1550 nm up to 11 K. However, the device had the anomalous behavior of afterpulsing at T > 5 K with an electrical time constant of ~ 130 ns. The onset of afterpulsing resulted in a sharp decrease in the polarization sensitivity at T > 5 K. The thicker films increased the region of optical sensitivity by 1 K with respect to Ref [6]. Further work includes 1) investigating the afterpulsing present at higher temperatures by varying the kinetic inductance and 2) increasing the operating temperature by fabricating narrower nanowires with thicker films [9]. The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. 4. References [1] F. Marsili et al., “Detecting Single Infrared Photons with 93% System Efficiency,” Nature Phot. 7 (3), 210-214 (2013). [2] F. Marsili et al., “Efficient single photon detection from 500 nanometer to 5 micron wavelength,” Nanolett. 12 (9) 4799-4804 (2012). [3] J. Nagamatsu et al., “Superconductivity at 39 K in magnesium diboride”, Nature 410, 63-64 (2001). [4] H. Shibata et al., “Single-photon detection using magnesium diboride superconducting nanowires,” App. Phys. Lett. 97 (21), 212504-212503 (2010). [5] H. Shibata et al., “Fabrication of MgB2 Nanowire Single-Photon Detector with Meander Structure,” App. Phys. Express 6, 023101 (2013). [6] H. Shibata et al., “Fabrication of a MgB2 nanowire single-photon detector using Br2-N2 dry etching” App. Phys. Express 7, 103101 (2014). [7] H. Shibata et al., “Ultrathin MgB2 films fabricated by molecular beam epitaxy and rapid annealing” Supercond. Sci. Technol. 26, 035005 (2013). [7] C. Zhuang et al., “Surface morphology and thickness dependence of the properties of MgB 2 thin films by hybrid physical-chemical vapor deposition,” Supercond. Sci. Technol. 23, 055004 (2010). [8] A. J. Annunziata et al., “Reset dynamics and latching in niobium superconducting nanowire single-photon detectors” J. Appl. Phys. 108, (2010). [9] F. Marsili et al., “Single-Photon Detectors Based on Ultranarrow Superconducting Nanowires,” Nano Lett. 11, 2048-2053 (2011).
