We used argon and helium gases to dilute silane to deposit silicon thin films by plasma-enhanced chemical vapor deposition (PECVD). Obtained films were characterized by Raman spectroscopy, spectroscopic ellipsometry, and atomic force microcopy (AFM). The three techniques revealed that the films obtained with argon dilution are composed of nanocrystallites embedded in an amorphous matrix. The grain size and the crystalline fraction increases with argon fraction in gas mixture. The optical gap increases from 1.91 to 2.02 eV when the dilution rate varies from 25 to 70%. The refractive index increases also with the dilution rate indicating a more compact film. AFM images show that a smoother surface is obtained with high dilution. All the results obtained from films deposited with helium dilution have the feature of an amorphous phase in contrary of those obtained with argon dilution where a nanocrystalline phase appears. These results show that these two inert gases have different effects on silicon film's growth kinetic.