We report results obtained from FTIR and TEM measurements carried out on silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) from silane diluted with hydrogen. The hydrogen content, the microstructure factor, the mass density and the volume per Si-H vibrating dipoles were determined as a function of the hydrogen dilution. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (mc-Si:H). With increasing dilution the transition from amorphous to microcrystalline phase appears faster and the average mass density of the films decreases. The mc-Si: H films are mixed-phase void-rich materials with changing triphasic volume fractions of crystalline and amorphous phases and voids. Different bonding configurations of vibrating Si-H dipoles were observed in the a-Si:H and mc-Si:H. The bonding of hydrogen to silicon in the void-and vacancy-dominated mechanisms of network formation is discussed. (C) 2007 Elsevier B.V. All rights reserved.