Amorphous and polycrystalline silicon (poly-Si) films, deposited by an electron cyclotron resonance plasma-enhanced chemical vapor deposition system at 120 degrees C, have been investigated. All films have been grown with either hydrogen or argon dilution. Using the films with the hydrogen dilution, the effect of rf (13.56 MHz) substrate bias has also been studied. Analysis with x-ray diffraction shows that films grown with Ar dilution and no rf bias do not show any crystallinity while the corresponding films deposited with H-2 dilution and no rf bias contain a significant amount of the crystalline phase. With only a 3:1 H-2 to silane ratio, highly crystallized films can be grown at 120 degrees C. In the presence of rf (13.56 MHz) substrate bias, there is a decrease of crystallinity in films. It has been found from,cross-sectional transmission electron microscopy that films deposited without rf bias develop a very uniform columnar structure whereas films made with rf bias develop a closely packed, continuous but more amorphous structure. It is found that these changes caused by rf bias affect the photoluminescence intensity.