Self-organized, effective medium Black Silicon for infrared antireflection

•Silicon antireflection structures yield transmittances of 97% in the 3–5 μm range.•Black Silicon antireflection structures are statistical and self-organized.•ICP-RIE fabrication technique is maskless and applicable to curved substrates.•Structure optical properties are not affected by temperature variations.•An effective medium criterion serves as design rule.

Statistical Black Silicon antireflection structures for the mid-infrared spectral region, fabricated by Inductively Coupled Plasma Reactive Ion Etching, are investigated. Upon variation of etch duration scaling of the structure morphologies is observed and related to the optical losses in specular transmittance. By means of statistical morphology analysis, an effective medium criterion for the examined structures is derived that can be used as a design rule for maximizing sample transmittance at a given wavelength. To obtain Black Silicon antireflection structures with elevated bandwidth, an additional deep-etch step is proposed and demonstrated.