Si-based thin films, including mu c-Si, Si1-xGex and Si1-xCx alloys, have been deposited by plasma enhanced chemical vapor deposition (PECVD) using SiF4:H-2:He, SiF4:GeH4:H-2 and SiF4:CH4:H-2 plasmas, respectively. When SiF4 is used as Si-precursor, it is found that a low flux of CH4 or GcH(4) results in incorporation of C and Ge in alloys as high as 30%. Correlations between microstructure and optical properties of films are investigated using spectroscopic ellipsometry. The role of fluorine atoms in the growth chemistry and material microstructure is discussed. (c) 2005 Elsevier B.V. All rights reserved.