We use spectroscopic ellipsometry to investigate crystallization of amorphous Ge thin films by thermal annealing Of SiO2/a-Ge/SiO2 trilayer structures. We study the influence of both film thickness and annealing temperature on the effective dielectric functions of the Ge films, which are related to the film micro- and narrostructures. For annealing temperatures below 900 degrees C, all films remain continuous and consist of mixtures of amorphous and nanocrystallized Ge. The crystallite sizes can be estimated from the observed energy blueshift of the E-1 interband transition. Samples annealed at 900 degrees C display dielectric function spectra which differ from a bulk-like behavior. This suggests a variation in optical properties which is correlated to formation of discontinuous films of Ge nanocrystals. (c) 2008 Elsevier B.V. All rights reserved.