Antireflective gradient-refractive-index material-distributed microstructures with high haze and superhydrophilicity for silicon-based optoelectronic applications

Abstract

We fabricate gradient-index (n) material-based microstructures, i.e., magnesium fluoride (MgF₂, n ∼ 1.37) film-coated SU8 ultraviolet curable polymer (n ∼ 1.59) microcones (MCs) with tapered architectures, on silicon (Si, n ∼ 3.9) substrates using a soft imprint lithography method for high-performance Si-based optoelectronic applications. The effects of various geometry parameters (i.e., height, filling ratio, and period) of conical MCs on the SU8 film/Si structure including different thicknesses of MgF₂ films on the reflectance properties are investigated by a theoretical analysis using a rigorous coupled-wave analysis simulation. For the fabricated samples, their optical characteristics and surface wetting behaviors are also explored. The MgF₂ film-coated SU8 MCs on Si substrates (i.e., MgF₂/SU8 MCs/Si) exhibit a superhydrophilic surface with very low water contact angles of <10° and reduce the surface reflection of the bare Si over a wide wavelength of 350–1100 nm, showing the lower average reflectance (R_avg) and solar weighted reflectance (R_sw) values of ∼14% and ∼12.1%, respectively, (i.e., R_avg ∼ 15.1% and R_sw ∼ 13% for the SU8 MCs/Si and R_avg ∼ 38.5% and R_sw ∼ 38% for the bare Si). Furthermore, the MgF₂/SU8 MCs on glasses also show strong light scattering in transmissions at wavelengths of 350–1100 nm, which indicates an average haze ratio of ∼89.8%, while maintaining high total transmissions. Both the measured and calculated reflectance spectra showed similar results.