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The iron silicides samples were formed by annealing of iron films evaporated onto silicon wafers and capped with either amorphous silicon or SiOx thin overlayers. Semiconducting FeSi2 phase is formed by annealing at the temperatures from 550 degrees to 850 degrees. The optical properties of the FeSi2 layers have been deduced from reflectance and transmittance measurements carried out in the temperature range of 77-380K. The spectral dependence of the absorption coefficient favors direct allowed transitions with forbidden energy gap of 0.87eV at the room temperate. The application of a simple three-parameter semiempirical formula to the temperature dependence of the direct energy gaps leads to the following best fit parameters: the band gap at zero temperature Eg(0) equals 0.895 +/- 0.0037eV, the dimensionless coupling parameter S equals 2.01 +/- 0.27, and the average phonon energy. By examining all the reported triplets of parameters for (beta) -FeSi2 fabricated by different techniques and thermal processes, an obvious discrepancy can be found for the lattice coupling parameter and average phonon energy, although the bandgaps at 0K are very similar. Unlike the theoretical prediction and the earlier reported result our results do not show any evidence of a particularly strong electron-phonon interaction, which would give the ow carrier mobilities. From optical model for the thin film-substrate system we found the index of refraction to be 5-5.9 in the photon energy interval from 0.65 eV to 1.15 eV. There is also indication of an additional higher-energy absorption edge at 1.05 eV. (beta) - FeSi2 seems to be an intriguing material where states with energies near the band edges permit ambiguous interpretation of the character of transitions. We also present the formulae for the reflectance and transmittance for the system thin film on substrate that better reflect the experimental conditions.
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