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Wide-band ‘black silicon’ with atomic layer deposited NbN

Antireflection surfaces are often utilized in optical components to reduce undesired reflection and increase absorption. We report on black silicon (b-Si) with dramatically enhanced absorption over a broad wavelength range (250–2500 nm) achieved by applying a 10–15 nm conformal coating of NbN with atomic layer deposition (ALD). The improvement is especially pronounced in the near infrared (NIR) range of 1100–2500 nm where absorption is increased by >90%. A significant increase of absorption is also observed over the ultraviolet range of 200–400 nm. Preceding NbN deposition with a nanostructured ALD Al2O3 (n-Al2O3) coating to enhance the NbN texture was also examined. Such texturing further improves absorption in the NIR, especially at longer wavelengths, strong absorption up to 4–5 μm wavelengths has been attested. For comparison, double side polished silicon and sapphire coated with 10 nm thick NbN exhibited absorption of only ∼55% in the NIR range of 1100–2500 nm. The results suggest a positive correlation between the surface area of NbN coating and optical absorption. Based on the wide-band absorption, the presented NbN-coated b-Si may be an attractive candidate for use in e.g. spectroscopic systems, infrared microbolometers.