We show that heavy C doping of poly-Si (poly-Si:C) layers in the range up to similar to 16% allows one to modify film morphology, crystallographic structure and optical properties also causing a crystalline to amorphous phase transition. Poly-Si:C films were grown on SiO2 surfaces at 700-800 degreesC by lamp-heated low-pressure chemical vapor deposition (LPCVD). With the help of spectroscopic ellipsometry (SE) measurement using a harmonic oscillator (HO) model, we find drastic changes in the spectrum indicating decreasing amounts of the polycrystalline phase with increasing C incorporation into the films at 700 degreesC. SE using the Bruggeman effective medium approximation (BEMA), transmission electron microscopy (TEM) and X-ray diffraction (XRD) results indicate a sharp phase transition from polycrystalline to amorphous caused by increased C incorporation at 700 and 750 degreesC. At 800 degreesC, the phase transition is broadened. If similar to 16% of C was incorporated, a part of poly-Si:C still remains. No crystalline silicon carbide was observed. The C atoms were partially incorporated on substitutional sites in the poly-Si:C at each temperature. The surface roughness of poly-Si:C grown at 700 and 750 degreesC decreases with increasing C incorporation. Especially at 700 degreesC, films with low surface roughness can be produced. This surface roughness change was detected at the same C incorporation as the phase transition. For 800 degreesC, a non-monotonic behavior of the roughness as a function of C incorporation was detected.