Fabrication of black aluminium thin films by magnetron sputtering

Black aluminium thin films were prepared by direct current (DC) pulsed magnetron sputtering. The N2 concentration in the Ar–N2 mixture that was used as the deposition atmosphere was varied from 0 to 10%, and its impact on the film growth and optical properties was studied. A strong change in the film growth process was observed as a function of the N2 concentration. At a specific N2 concentration of ∼6%, the Al film growth process favoured the formation of a moth-eye-like antireflective surface. This surface morphology, which was similar to the structure of a cauliflower, is known to trap incident light, resulting in films with a very low reflectivity. A diffuse reflectivity lower than 4% was reached in the ultraviolet-visible-near infrared (UV-VIS-NIR) spectral range that corresponds to a value observed for an ultrahigh absorber. We found that for the preparation of black aluminium, the nitrogen content plays an important role in film formation and the resulting film morphology.