Bioinspired Parabola Subwavelength Structures for Improved Broadband Antireflection

Sample preparation: Semi-insulated, single-side polished GaAs (100) wafers (Wafer Technology Ltd., UK) were cleaned with acetone, methanol, and deionized water prior to the fabrication process. To remove native oxides, they were immersed in a concentrated HCl solution for 1 min, followed by a 3% NH 4OH solution for 30 s. To obtain a 200-nm-thick PR layer, AZ 5206-E (AZ Electronic Materials Ltd., USA) was diluted with AZ 1500 thinner. Exposure dose profiles of laser light were recorded on the resist using a laser-interference lithography system with a Lloyd's mirror and an argon laser (λ=363.8 nm) as a light source. The periodicity of the pattern was adjusted by controlling the incident angle of the laser. For an array of hexagonally arranged elliptical patterns, the substrate was rotated by 60°. The exposed resist was developed using AZ 300 MIF 2.38%. The thermal reflow process was carried out on a Shamal hotplate (HHP-411, Iuchi Seiei Dou Co., Japan) at 200°C for 40 s to make a lenslike PR shape. To transfer the pattern of lens-shaped PR to the GaAs substrate, a dry-etch process was conducted by using an ICP etcher (Plasmalab system 100, Oxford Instrument Co., UK). SiCl4 and Ar were used as process gases, the RF power was 100 W, and the process temperature was 20 °C. The amount of SiCl4 gas was varied from 2.5 to 7.5 sccm to modify the shape of the SWSs, and the process pressure was also changed from 2 to 50 mTorr to increase the height of the SWSs. Characterization: The samples were characterized by fieldemission SEM (S-4700, Hitachi, Japan) with an operating voltage of 10 kV. Reflectance spectra of the samples were measured by UV/Vis-near-IR spectrophotometry (Cary 5000, Varian, USA) in the wavelength range of 300-2000 nm. Reflectance calculation: The RCWA method was used to calculate the reflectance of SWSs by using commercial software (DiffractMOD 3.1, RSoft Design Group, USA). In the RCWA calculation, cone- and parabola-shaped SWSs with sixfold hexagonal symmetry were modeled using linear and paraboloid tapered profiles in the radial direction. The dispersion of GaAs was considered to obtain the exact result at each wavelength. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.