In this work we present a detailed investigation of Si surface passivation obtained by a PECVD double dielectric layer, composed of intrinsic hydrogenated amorphous silicon-carbon (a-SiCx:H), followed by a silicon nitride (SiNx). The double layers have been deposited on p-and n-type of mono- and multi-crystalline silicon wafers. IR spectra have been carried out to evaluate the structure of a-SiCx:H layers on monocrystalline wafers. The passivation effects have been studied performing the following measurements: the photoconductance decay, to measure contactlessly the effective lifetime of passived mono and multi Si wafers; the capacitance voltage profile of Al/SiNx/Si, Al/a-SiCx:H/Si and Al/SiNx/a-SiCx:H/Si MIS structures, to estimate the field effect at the dielectric/silicon interface and individuate the passivation mechanism on silicon surfaces. It has been found that the mechanism of the surface passivation depends on the doping type of the silicon wafer. Indeed from C-V measurements it has been realized that the great amount of positive charge within the SiNx is able to promote an inversion layer if it is deposited on a-Si-x:H/Si p-type and an accumulation if it is grown on a-SiCx:H/Si n-type. (C) 2007 Elsevier B.V. All rights reserved.