Ellipsometric study of crystallization of amorphous Ge thin films embedded in SiO2

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 nanostructures. For annealing temperatures below 900 °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 E1 interband transition. Samples annealed at 900 °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.