Broadband and wide-angle antireflective characteristics of nanoporous anodic alumina films for silicon-based optoelectronic applications

Abstract

Nanoporous anodic alumina (NPAA) films are fabricated on silicon (Si) surfaces by the anodization of aluminum (Al). Effect of the thickness of initial Al films on the antireflective characteristics of NPAA films consisting of random pore structures is investigated in the wavelength range of 300–1,100 nm at incident light angles (θ inc) of 3°–70°, together with theoretical analysis using a rigorous coupled-wave analysis method. The reflectance strongly depends on the thickness and porosity of NPAA films, resulting from anodization parameters such as applied voltage, anodization time, and pore widening time. For the obtained NPAA film at the initial Al thickness of 100 nm and the applied voltage of 30 V, the surface reflectance is reduced over a wide wavelength region of 300–1,100 nm at normal incidence of ~3°, exhibiting the relatively lower average reflectance (R avg) of ~19 % (i.e., R avg ~ 39 % for the bare Si substrate). The lower angle-dependent reflective properties are observed compared to the bare Si substrate at θ inc = 20°–70° for unpolarized light. The calculated reflectance results of NPAA films show similar trends to the measured data.