Carbon-doped b-FeSi2 films synthesized by ion implantation are investigated with the aim to fabricate high-quality semiconducting b-FeSi2 layer on silicon substrate. According to transmission electron microscopy cross-section observations, carbon-doped films, with homogeneous thickness and smooth b/Si interface, have higher quality than binary Fe-Si films. In particular, annealing at 500 °C ~ 700 °C leads to the formation of a flat and continuous b-type silicide layer. Improved thermal stability of the b phase is also found. Optical emission spectroscopy measurements show that the doping influences only slightly the band gap values.