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Hydrogenated amorphous silicon thin films were prepared by d.c. plasma enhanced chemical vapour deposition (PECVD) of helium diluted silane. Gas mixtures containing different helium to silane flow-rate ratios have been used to produce these films. The films have been analysed using optical transmission spectroscopy, infrared transmission spectroscopy and X-ray diffraction. The X-ray diffraction results clearly indicate the presence of two phases in the material: microcrystalline and amorphous phase when the helium to silane flow-rate ratio was between two and four. However, further helium dilution resulted in a purely amorphous film structure as in films produced from the discharge of pure silane. The optical properties, hydrogen content and microstructure parameter of the films were obtained from the optical and infrared transmission spectra of these films. The effects of the appearance of the microcrystalline phase on these properties were also investigated.