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The optical properties of polycrystalline silicon thin films prepared by low‐pressure chemical vapor deposition were studied by spectroscopic ellipsometry. The surface roughness and the microstructure of the films were investigated by combining the theory of Ohlidal and Lukeš with the Bruggeman effective‐medium approximation. Furthermore, a line‐shape analysis of the dielectric function, as well as a method to describe the dielectric function, was used to discuss the effect of the surface roughness and that of the crystallite size and grain boundaries in these materials. The results of this analysis show that the parameters that mainly control the growth mode in these materials, at a temperature of 630 °C, are the silane pressure and the growth rate. The different surface morphologies and the growth of a native oxide on the material surface seem to be associated with the growth mode of the film. The effect of surface roughness on the measured data must be taken into account even at energies as low as 2 eV, while the presence of a native oxide overlayer influences the measured data at higher energies. A reasonable agreement with recent electron microscopy studies on these materials was found.