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

• Published in: Journal of applied physics 2013-04, Vol.113 (14)
• Main Authors: Reithmaier, G., Senf, J., Lichtmannecker, S., Reichert, T., Flassig, F., Voss, A., Gross, R., Finley, J. J.
• Format: Article
• Language: English
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.4800838

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.