Nanocrystalline hydrogenated silicon (nc-Si:H) thin films were deposited at room temperature by plasma enhanced chemical vapor deposition (PECVD): a mixture of SiH4 and H-2 was introduced into the evacuated reaction chamber. When the H-2 flow rate was low, the density of Si-H-3 bonds was high in the films. In particular, when the H-2 flow rate was 50 sccm, similar to2 nm-sized nanocrystallites were present in the films; a large number of Si clusters consisting of Si-H-3 and Si-H-2 bonds were present in the amorphous matrix of the films. When the H-2 flow rate was high, e.g. 100 sccm, a large number of Si-H bonds contributed to the passivation of the surface of the large volume of nanocrystallites; a small number of Si clusters consisting of Si-H-2 bonds were present in the amorphous matrix. The relative fraction of the Si-H-3 and Si-H-2 bonds in the amorphous matrix varied sensitively with either the H-2 flow rate or post-deposition annealing temperature. The variation was associated with the change in the intensity as well as the wavelength of the main PL peaks, indicating the change in the total volume as well as the size of the nanocrystallites in the films. (C) 2004 Elsevier B.V. All rights reserved.