We report on the effects of high hydrogen dilution on the optical (refractive index, absorption coefficient and band gap) and electrical (carrier concentration, Hall mobility and dark conductivity) properties of boron-doped nc-Si:H thin films grown by PECVD on glass substrates through X-ray diffraction, Raman scattering, transmission and Hall measurements at room temperature. Wavelength-dependent refractive indices and absorption coefficients are obtained through a modified four-layer-medium transmission model based on the envelope method. The carrier concentration, Hall mobility and dark conductivity are related to the boron-doping effect and the crystallinity of the ne-Si:H samples. The hydrogen dilution ratio is found to increase basically both the incorporation efficiency of the boron dopant atoms and the crystallinity. The results are discussed in the framework of the growth mechanism for the boron-doped nc-Si:H taking into account the etching effect of atomic hydrogen and the doping effect of boron. (C) 2003 Elsevier B.V. All rights reserved.