Two series of silicon films on c-Si substrates with different thicknesses are deposited by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD). The structures of samples are investigated by spectroscopic ellipsometry (SE) with fixed angle. The results show that the films are all amorphous for the first series of samples. In such a case an abrupt a-Si:H/c-Si heterojunction is formed which is beneficial for the passivation of the interface of HIT solar cell. For amorphous silicon films, the fitting result is acceptable by using Tauc-Lorentz Genosc model. For the second series, the films are of epitaxial Si at the initial deposition stages, the amorphous fraction increases with the increase of thickness. When the film thickness reaches a critical value of 46 nm, a transition to pure amorphous phase occurs. The epitaxial film shows excellent fitting by using the effective medium approximation (EMA) model under the assumption of the mixture of c-Si phases and a-Si:H. However, we obtain better fitting result by using a three-layer model, whose bulk layer is divided into EMA layer and a-Si layer after the transition to pure amorphous phase. This study indicates that the SE analysis, operated in different models, is effective to characterize different structures of silicon films on c-Si substrate.