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arXiv:1212.2038 · Journal of Applied Physics

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

G. Reithmaier, J. Senf, S. Lichtmannecker, T. Reichert, F. Flassig, A. Voss, R. Gross, J. J. Finley

Published Apr 10, 2013• 26 citations
arXivHTMLPDF

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 4–22 nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6 ± 0.2K for a film thickness of 22 ± 0.5 nm and 10.2 ± 0.2 K for 4 ± 0.5 nm 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 4 nm and 22 nm thick films into a 100 nm 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 ± 0.1 K with ∼50 k dark counts per second for bias currents of 98%IC at a wavelength of 950 nm. As expected, similar detectors fabricated from 22 nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates ≤3 cps. The maximum lateral extension of a photo-generated resistive region is estimated to be 30 ± 8 nm, 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.

DOI
10.1063/1.4800838

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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 4–22 nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6 ± 0.2K for a film thickness of 22 ± 0.5 nm and 10.2 ± 0.2 K for 4 ± 0.5 nm 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 4 nm and 22 nm thick films into a 100 nm 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 ± 0.1 K with ∼50 k dark counts per second for bias currents of 98%IC at a wavelength of 950 nm. As expected, similar detectors fabricated from 22 nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates ≤3 cps. The maximum lateral extension of a photo-generated resistive region is estimated to be 30 ± 8 nm, 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.
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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.
We investigated the suitability of AlN as a buffer layer for NbN superconducting nanowire single-photon detectors (SNSPDs) on GaAs. The NbN films with a thickness of 3.3-20 nm deposited onto GaAs substrates with an AlN buffer layer demonstrate a higher critical temperature, critical current density, and lower residual resistivity in comparison to films deposited onto bare substrates. Unfortunately, the thermal coupling of the NbN film to the substrate weakens. SNSPDs made of 4.9-nm-thick NbN films on buffered substrates (in comparison to detectors made from NbN films on bare GaAs) demonstrate three orders of magnitude lower dark count rates and about ten times higher detection efficiency at 900 nm being measured at 90% of the critical current. The system timing jitter of SNSPDs on buffered substrates is 72 ps, which is 36 ps lower than those on bare substrate. However, a weaker thermal coupling of NbN nanowire to the buffered substrate leads to a latching effect at bias currents > 0.97 IC.
Abstract. We describe the collective fabrication and characterization of superconducting single photon detectors (SSPD) made of a superconducting NbN meander, suitable for near-infrared applications. The NbN layer acting as the photon absorber is deposited epitaxially on a three inch diameter R-plane Sapphire heated substrate by DC-Magnetron sputtering from a Nb target in a reactive mixture of nitrogen and argon gas. Very thin NbN (~4 nm) films superconducting up to 12 K with a rather large critical current density are in-situ covered by a 1.4 nm thick room temperature sputtered AlN layer protecting NbN from oxidation. Two reliable patterning processes has been developed successfully, one based on electron beam lithography, the second based on selective NbN film anodization under an AFM tip. The filling factor obtained with 150 nm wide stripes in the meander pixel is about 0.5 with e-beam lithography and improved up to 0.8 with AFM patterning. In order to achieve an improved optical coupling of the SSPD by using wafers integrating ion implanted IR waveguides, growth studies of NbN layers have been focused on R-plane Sapphire and silicon substrates. We present optical and superconducting properties of NbN layers by FTIR, ellipsometry and STM.
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.
Ultra-thin epitaxial NbN layers are a key component of Superconducting Single Photon infrared Detectors. Efforts devoted to the layer growth aim at improving their critical temperature and critical current density, while keeping their thickness close to 5 nm and Tc above 10 K, which insure a large bandwidth, large SNR detection at 4K. Choice of substrate is critical: for both applications, MgO wafers and R-plane sapphire are usually considered as best substrates to grown onto. However, growing NbN on M-plane orientation of sapphire wafer, 3 inch in diameter, can help improving the film quality and fabrication yield. NbN thin films were grown by reactive DC magnetron sputtering at about 600°C and passivated by an AlN layer 1.5nm thick deposited in-situ at room temperature. Growth on M-plane is shown to be better than on other sapphire orientations, including R-plane: NbN layer critical temperature reaches 13.3 K, uniform on the wafer, for a film thickness of 4.4nm measured by X-ray reflectivity. Transport properties of NbN grown on those various substrates have been correlated to their crystallographic microstructure, examined by both symmetric and asymmetric X ray diffraction. Observation of diffraction peaks has given insight on the disorientation of the NbN film.