Design of hemi-urchin shaped ZnO nanostructures for broadband and wide-angle antireflection coatings.

We study theoretically and experimentally the hemi-urchin shaped zinc oxide (ZnO) nanostructures for broadband and wide-angle antireflection coatings. The antireflective characteristics of hemi-urchin shaped ZnO nanostructures, which can be formed by integrating one-dimensional (1D) nanostructures (i.e., nanorods) on the periodic 2D structural architecture, are investigated. The optimization process is performed using a rigorous coupled-wave analysis method in terms of the order of taper of Si subwavelength gratings (SWGs) as a 2D structural architecture, the geometry of Si SWGs, and the height/size of ZnO nanorods. To simply test an experimental feasibility, a hemi-urchin shaped ZnO nanostructure is fabricated by hydrothermally growing ZnO nanorods on the periodic Si SWG structure. The angle-dependent reflectance of the hemi-urchin shaped ZnO nanostructures on the Si SWG is compared with that of the vertically aligned ZnO nanorod arrays on the Si substrate. The optimized hemi-urchin shaped ZnO nanostructure can significantly improve the antireflective property by suppressing the surface reflection over a broad spectrum and a wide range of angles of light incidence, indicating a reasonable agreement with the experimental results.