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

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...