Optimisation of NbN thin films on GaAs substrates for in-situ single photon detection in structured photonic devices

Abstract

We prepare NbN thin films by DC magnetron sputtering on [100] GaAs substrates, optimise their quality and demonstrate their use for efficient single photon detection in the near-infrared. The interrelation between the Nb:N content, growth temperature and crystal quality is established for 4−22nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6±0.2K for a film thickness of 22 ± 0.5nm and 10.2 ± 0.2K for 4 ± 0.5nm thick films that are suitable for single photon detection. The optimum growth temperature is shown to be ∼ 475 • C reflecting a trade-off between enhanced surface diffusion, which improves the crystal quality, and arsenic evaporation from the GaAs substrate. Analysis of the elemental composition of the films provides strong evidence that the δ-phase of NbN is formed in optimised samples, controlled primarily via the nitrogen partial pressure during growth. By patterning optimum 4nm and 22nm thick films into a 100nm wide, 369µm long nanowire meander using electron beam lithography and reactive ion etching, we fabricated single photon detectors on GaAs substrates. Time-resolved studies of the photo-response, absolute detection efficiency and dark count rates of these detectors as a function of the bias current reveal maximum single photon detection efficiencies as high as 21 ± 2% at 4.3 ± 0.1K with ∼ 50k dark counts per second for bias currents of 98%I C at a wavelength of 950nm. As expected, similar detectors fabricated from 22nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates ≤ 3cps. The maximum lateral extension of a photo-generated hotspot is estimated to be 30±8nm, clearly identifying the low detection efficiency and dark count rate of the thick film detectors as arising from hotspot cooling via the heat reservoir provided by the NbN film. PACS numbers: 74.78.-w 74.25.Gz 78.67.Uh 85.25.Oj 85.25.-j 42.50.-p

On-chip time resolved detection of quantum dot emission using integrated superconducting single photon detectors

“…This expectation is confirmed by the data presented in the inset of figure 2a that shows the direct normal incidence response of the detector at 940 nm when raster scanning the laser spot with a low power density of 0.4 W/cm 2 . A clear maximum is observed when the laser spot is incident on the detector and, by measuring the normal incidence count rate and carefully calibrating the incident photon flux onto the detector, we estimated the top-illumination quantum efficiency to be ~ 0.001% at 940 nm, in good accord with previous measurements1921 and expectations for a 10 nm thick NbN film. These observations clearly indicate that the signal detected when exciting on the waveguide arises from QD PL emitted into the waveguide mode and guided to the SSPD whereupon it is evanescently absorbed by the SSPD.…”

“…For the operation conditions of the SSPD, as described in the methods section, the integrated detector shows a negligible dark count rate < 10 cps and a top-illumination detection efficiency of 0.001% for light at 940 nm, as expected for the relatively thick 10 nm NbN film21. This efficiency was estimated from the registered single photon events divided by the number of photons incident upon the active device area19. The active area of the device is given by the fractional areal coverage of NbN − 80 nm wide nanowires with a separation of 170 nm corresponding to a fill-factor of 32%.…”

“…Since the bias current flowing through the nanowire is slightly sub-critical (), the local heating arising from single photon absorption results in the breakup of Cooper pairs, local switching of the nanowire to a normal conducting state and a measurable voltage pulse in the external readout circuit. Such SSPDs provide very high single photon detection efficiencies1416171819, low dark count rates20, sensitivity from the visible to the IR21 and picosecond timing resolution2223. The possibility to integrate superconducting single photon detectors (SSPDs) onto dielectric114 and plasmonic2 waveguides results in a drastic increase of the absorption length for incoming photons, pushing the single-photon detection efficiency towards unity.…”

A carrier relaxation bottleneck probed in single InGaAs quantum dots using integrated superconducting single photon detectors

“…To directly observe such relaxation bottleneck effects, superconducting single photon detectors (SSPDs) are suitable due to their near unity quantum efficiency and picosecond timing resolution 10,11 . Building up on recent progress in this field 12-14 , we developed highly efficient 15,16 NbNSSPDs on GaAs 17 and demonstrated the monolithic integration of InGaAs QDs as single photon emitters together with waveguides and detectors on a single chip 18 . In this letter, we compare photoluminescence (PL) dynamics recorded from a single dot with confocal off-chip detectors with on-chip PL using integrated SSPDs that provide temporal resolution better than 70 ps.…”

Highly oriented, free-standing, superconducting NbN films growth on chemical vapor deposited graphene

“…1 Much of the work on superconducting single photon detectors (SSPD) has been based on NbN thin films, [2][3][4] since NbN has a relatively high superconducting transition temperature, T C ∼16 K, and a short superconducting coherence length of a few nanometers and fast energy relaxation time. For increased detector efficiency by integrating SSPD with optical structures, NbN growth on non-conventional substrates like GaAs, 5,6 3C-SiC, 7 and Si 8 has been explored. Increased efficiency of terahertz detection is achieved by thermal isolation of HEBs by depositing NbN films on air bridges formed by thin membrane of Si 3 N 4 , 9 to improve the quality of these films alternative materials for buffer layer are being explored.…”