Boron-doped and undoped a-Si1-x: H, for x approximate to 0.5, films were prepared by a plasma-enhanced-chemical-vapor-deposition from methyltrichlorosilane as the main precursor. Optical absorption spectra of these films demonstrate three characteristic peaks at similar to1.6, 2.0-2.2 and 2.5-2.8 eV in agreement with other experimental measurements. In order to account for the observed peculiarities of the spectra, the atomic and electronic structures of a-SiC were investigated using both molecular dynamics simulations based on Tersoff's potential and the recursion method. The results of the model band structure calculations revealed three main peaks in the gap (similar to2.45 eV), originated from homonuclear Si-Si bonds, five-fold (T-5), anomalous four-fold (T-4a), three-fold (T-3) coordinated defects and normal four-fold (T-4n) coordinated atoms which are the nearest neighbors of these defects. The optical absorption peaks are assumed to be formed as a consequence of electronic transitions between the valence band (VB), the three gap peaks and the conduction band (CB). Boron doping effects upon the optical spectra was not revealed.