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Polycrystalline silicon (poly‐Si) thin films prepared by recrystallization of α‐Si either with annealing or ion implantation and annealing have been studied with the optical techniques of spectroscopic ellipsometry (SE) and Raman spectroscopy (RS), as well as with transmission electron microscopy (TEM). A detailed analysis of the SE dielectric function ϵ(ω) with the effective‐medium theory (EMT) has demonstrated the formation of a silicon oxinitride layer on the top of the poly‐Si films and a rms surface roughness of about 10 Å. The experimental ϵ(ω) data have been also analyzed with the second‐derivative technique in order to study the effect of the different ion fluencies on the films’ properties and the results are compared with those from TEM. Based on the presented experimental findings we propose the following: (a) The ion‐implanted and annealed samples exhibit grain sizes several times larger than those of the only annealed samples; (b) implantation with low ion energies and concentrations induces a disordered material similar to that obtained by annealing; and (c) the mechanism that is responsible for the decrease of the lifetime of the excited carrier is mainly the scattering at the microcrystallite boundaries. Furthermore, the study of the Raman TO vibrational modes reveals the existence of compressive stress in the Si substrate and tensile stress in the poly‐Si films. For the latter, results from both RS and SE suggest that the average stress amounts to 2–3 kbar. Finally, the use of a broadening‐dependent dielectric function as reference instead of that of c‐Si in the EMT analysis, as a consequence of the finite grain size, leads to acceptable structural characteristics similar to those obtained by electron microscopy and RS.