LAUSR

By contact of platinum catalyst with Si wafers immersed in H2O2 plus HF solutions, ultra-low reflectance Si surfaces can be produced. The ultra-low reflectance results from the formation of a nanocrystalline Si (nc-Si) layer with ∼300 nm thickness. A porosity-graded optical model consisting of 19 sublayers for the nc-Si layer, which is determined from SEM and TEM observations, and the fitting of ellipsometric spectra, is found to well explain the optical properties. The dielectric function of each sublayer is calculated from the Bruggeman effective medium approximation using porosity and the dielectric functions of Si and ∼5 nm Si nanocrystals. The reflectance calculated from the optical model shows a good agreement with the experimental result in the wide wavelength region between 300 and 800 nm. This good agreement demonstrates that the ultra-low reflectance of the nc-Si/Si structure results from the refractive index monotonically increasing with the depth.