The dependence of the optical and structural properties of amorphous and polycrystalline FeSi2 layers fabricated by ion beam sputter deposited (IBSD) FeSi2 and ion beam mixing (IBM), have been characterised. Amorphous and polycrystalline FeSi2 were fabricated using IBSD of Fe and Si at deposition temperatures from room temperature (80 °C) up to 700 °C and post-anneals between 300 and 700 T. Optical absorption measurements revealed direct band gaps for all layers including the amorphous layers from 0.891 - 0.947 eV. Little effect on the optical properties was found for amorphous layers annealed below 500 °C. The band gap value and absorption coefficient only significantly increased upon annealing above 500 °C, coinciding with the transformation from the amorphous to crystalline , B-phase. The deposition temperature was seen to affect the crystallinity of the as-deposited thin films, and vary the optical and structural properties within the layers significantly. An increase in deposition temperature resulted in a decrease of the band gap energies and an increase in photo-absorption by an order of magnitude. Using ion beam mixing, we have demonstrated formation of semiconducting silicides possessing direct band gap energies between 0.89 - 1.43 eV via low energy process conditions on multi-layer structures of Fe - Si. RBS was successfully employed to determine the level of silicidation formed using the extracted depth profiles. Mixing was enhanced upon increasing either the irradiation temperature or the ion fluence. The increase in the silicidation was greater when irradiating similar structures at a higher fluence than at a higher temperature. Optically, the increased silicidation did not affect the band gaps, with values of 0.89 - 1.03 eV upon irradiation of structures with 4- or more layers. Comparison of the effective absorption coefficients a} of structures revealed that although there is higher silicidation within the higher fluence structures, a larger absorption coefficient a was observed for higher temperature mixing. The formation of the FeSi2 at higher temperatures allows the amorphous structure to form with more short-to mid-range order thus exhibiting stronger and sharper absorption.