Design and fabrication of broadband ultralow reflectivity black Si surfaces by laser micro/nanoprocessing

Abstract

Light collection efficiency is an important factor that affects the performance of many optical and optoelectronic devices. In these devices, the high reflectivity of interfaces can hinder efficient light collection. To minimize unwanted reflection, anti-reflection surfaces can be fabricated by micro/nanopatterning. In this paper, we investigate the fabrication of broadband anti-reflection Si surfaces by laser micro/nanoprocessing. Laser direct writing is applied to create microstructures on Si surfaces that reduce light reflection by light trapping. In addition, laser interference lithography and metal assisted chemical etching are adopted to fabricate the Si nanowire arrays. The anti-reflection performance is greatly improved by the high aspect ratio subwavelength structures, which create gradients of refractive index from the ambient air to the substrate. Furthermore, by decoration of the Si nanowires with metallic nanoparticles, surface plasmon resonance can be used to further control the broadband reflections, reducing the reflection to below 1.0% across from 300 to 1200 nm. An average reflection of 0.8% is achieved. Using lasers to process silicon is a convenient way of creating textured surfaces that exhibit broadband anti-reflection properties. Yang and co-workers from Singapore and China used laser interference lithography and etching to create arrays of high aspect ratio, thin silicon nanowires, which they then decorated with metallic nanoparticles. The researchers say that the low reflectivity of their ‘black silicon’ is due to the nanowires' light-trapping properties and low refractive index, combined with surface plasmon resonances associated with the nanoparticles. Measurements indicate that the reflectivity of the treated surface was less than 1% across the ultraviolet, visible and near-infrared spectral regions (300–1200 nm). Because laser processing can pattern surfaces rapidly, the researchers believe that their scheme could be a cost-effective way of manufacturing large-area broadband anti-reflection coatings.