Ellipsometric monitoring of the growth of Ge layers on Si(100) by pyrolysis of GeH4 at substrate temperatures of 400, 450 and 500 degrees C was performed by using a single wavelength ellipsometer based on the four detector photopolarimeter. By coupling ellipsometric monitoring with X-ray photoelectron spectroscopy surface analysis, the changeover between bi-dimensional and tri-dimensional growth modality at 450 degrees C was found at Ge coverage of about 1 ML. The recorded Psi-Delta data were analysed by using a growth model based on the Effective Medium Approximation (EMA) in order to derive the fraction of voids upsilon and the thickness of the EMA layer h(EMA). The values found for u were consistent with the clustered morphology observed by Atomic Force Microscopy (AFM) analysis of the samples. A qualitative model for the island evolution and for the growth rate of the equivalent Ge layer as a function of the substrate temperature was derived from the calculated upsilon and h(EMA), in agreement with the indications given by AFM and Rutherford backscattering spectroscopy film characterisation.