Surface passivation of p-type crystalline silicon wafers by means of phosphorus-doped hydrogenated amorphous silicon carbide films [a-SiCx(n):H] has been investigated. Particularly, we focused on the effects of layer thickness on the c-Si surface passivation quality resulting in the determination of the fixed charge density, Qf, within the a-SiCx(n):H film and the fundamental recombination of holes, Sp0. The main result is that surface recombination velocity decreases with film thickness up to 40 nm and then saturates. The evolution of the interface parameters indicates that Qf could be located in a layer less than 10 nm thick. In addition, Sp0 increases with thinner films probably due to different hydrogenation and saturation of interface dangling bonds during forming gas annealing.