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

TL;DR

This study optimizes NbN thin films on GaAs for near-infrared single photon detection, demonstrating high efficiency and low dark counts in nanowire detectors with detailed analysis of growth conditions and film properties.

Contribution

It presents a comprehensive optimization of NbN film growth on GaAs and demonstrates their application in high-performance single photon detectors with detailed characterization.

Findings

Optimized NbN films achieve critical temperatures up to 12.6K.

Detectors show up to 21% efficiency at 950nm wavelength.

Thinner films have higher efficiency and dark count rates.

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\pm0.2K for a film thickness of 22\pm0.5nm and 10.2\pm0.2K for 4\pm0.5nm thick films. The optimum growth temperature is shown to be ~475{\deg}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 \delta-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, 369\mu m 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\pm2% at 4.3\pm0.1K with ~50k dark counts per second for bias currents of 98%Ic 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\pm8nm, 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.