Wide angle and broadband antireflection properties for a silicon nanotip array

Abstract

Biomimetic structures provided important clues for nano-synthesis in pursuit of enhanced performances. Here, we report a wide angle and broadband antireflection is observed on a 6-inch silicon nanotip array (SiNTs) substrate fabricated using a single step electron cyclotron resonance plasma etching technique. This subwavelength structure consists of the SiNTs with apex and bottom diameter of ~5 nm and ~200 nm, respectively, length of ~1600 nm and density of 10⁹/cm². This aperiodic array of SiNTs with geometry designed in the sub-wavelength level to demonstrate a low hemispherical reflectance of < 1% in the ultraviolet to infrared region. The antireflection property holds good for a wide angle of incidence and both, s and p, forms of polarizations of light. The effective refractive index distribution related to the structure of SiNTs is built. The equivalent three-layered thin films with gradient refractive index can be applied in interpretation of the low reflection phenomenon. The equivalent admittance of the system is shown to be near that of air even the wavelength is varied from 400 nm to 800 nm (or angle of incidence is varied from 25 to 70 degree). The configuration to have broadband and wide-angle antireflection is different from the previous design because the equivalent rare film adjacent to air in our case is much thinner than the requirement proposed by J. A. Dobrowolski. This near ideal antireflection property suggests enhanced performances in renewable energy, and electro-optical devices in defense applications.