We used Adachi's model dielectric function (MDF) to characterize porous silicon thin films of systematicaly changing nanocrystal size. Since both the surface and interface roughnesses have to be taken into account, and the E₀, E₁, and E₂ critical point (CP) features are all
described by a combination of several lineshapes (two-dimensional CP, excitonic, damped harmonic oscillator) with numerous parameters in each layer, the number of fitted parameters were reduced by parameter coupling and selecting sensitive parameters for the fit. The porous
silicon layers were prepared by electrochemical etching of p-type single-crystal Si (c-Si) of varying dopant concentration resulting in systematically changing morphology and nanocrystal (wall) sizes in the range of 2-25 nm. The broadening parameters of the interband transitions
in the measured photon energy range correlate with the long-range order in the crystal, and, as a result, with the nanocrystal size. The advantage of this method over the robust and simple effective medium approximation (EMA) presented recently (Petrik et al., Appl. Surf. Sci.
253 (2006) 200) using a composition of voids and c-Si with a nanocrystalline Si reference is that the combined EMA+MDF multilayer method of this work provides a more detailed description of the material and layer structure. Error bars and parameter correlations
have to be investigated thoroughly, because of the increased number of fitted parameters.

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