Aspect-ratio-dependent ultra-low reflection and luminescence of dry-etched Si nanopillars on Si substrate

The Si nanopillars with high aspect ratio were fabricated by dry-etching the thin SiO2-covered Si substrate with a rapidly self-assembled Ni nanodot patterned mask. Aspect-ratio-dependent ultra-low reflection and anomalous luminescence of Si nanopillars are analyzed for applications in all-Si based lighting and energy transferring systems. The Si nanopillars induce an ultra-low reflectance and refractive index of 0.88% and 1.12, respectively, at 435 nm due to the air/Si mixed structure and highly roughened surface. The reflectance can be <10% with a corresponding refractive index of<1.80 between 190 and 670 nm. Lengthening the Si nanopillars from 150 ± 15 to 230 ± 20 nm further results in a decreasing reflectance, corresponding to a reduction in refractive index by Δn/n = 18% in the visible and near-infrared wavelength region. After dry-etching an Si wafer into Si nanopillars, the weak blue-green luminescence with double consecutive peaks at 418-451 nm is attributed to the oxygen defect (O2-)-induced radiation, which reveals less relevance with the ultra-low-reflective Si nanopillar surface.