Bruggeman effective medium approach for modelling optical properties of porous silicon: comparison with experiment

While numerous works deal with the properties and applications of porous silicon (PS), some of the related topics are not complete or could be investigated from different aspects. The main objective of this paper is to provide novel information associated with the optical properties of nano- and meso-PS by studying the variation of the effective refractive index (n(eff)) and the relative dielectric constant (epsilon(r,eff)) as a function of porosity. For this purpose various PS samples were prepared by electrochemical etching of p and p(+)-type silicon wafers in order to form silicon supported nano-PS and free-standing meso-PS layers, respectively. The experimental effective optical parameters (n(eff), epsilon(r, eff)) of the meso-PS films, were determined from the transmission spectra and the Bragg law. While, in the case of nano-PS layers, we applied the Goodman method, and deduced the values of n(eff) and epsilon(r, eff) from the Bruggeman's effective medium approximation (EMA). In the EMA calculation, the PS structure was considered as being a physical combination of three distinct phases formed by silicon, silicon dioxide and voids with a convenient volume fraction. A good agreement between theory and experiment was found in the case of silicon-supported nano-PS for all porosities. However, for free-standing meso-PS, the theory does not well fit the experimental results for porosities lower than 50% and higher than 70%. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.