beta-FeSi2 embedded in a Si matrix was prepared by ion beam synthesis (IBS). Two step implantation, with energies 60 and 20 keV, of two different doses of the iron ions, 5 x 10(15) and 5 x 10(16) cm(-1), was performed. After the implantation, the samples were subjected to rapid thermal annealing (RTA) at 900 degrees C. The crystal structure of the resulting material was studied using cross-sectional transmission electron microscopy (XTEM), including high-resolution electron microscopy (HREM). The comparison of the XTEM images with the initial iron ions implantation profiles, simulated by SRIM (Stopping and Range of Ions in Matter) demonstrate that the process of IBS, followed by RTA, preserves the initial implantation profile, implying a negligible Fe atoms diffusion velocity in comparison with the one of the chemical reaction between Fe and Si. The XTEM images show that continuous beta-FeSi2 layers are fabricated when there is a stoichiometric region in the initial implantation profile. Fe concentration lower than the stoichiometric one in the whole implantation range results in formation of beta-FeSi2 nanocrystallites embedded in the Si matrix. The behavior of the absorption coefficient energy dependences, obtained from the optical transmittance and reflectance measurements, reflects the different crystal structures forming in the two types of samples.