We report the results of aninsituspectroscopic ellipsometry study concerning the substrate dependence of the evolution of microcrystalline silicon films deposited by alternating amorphous silicon deposition and hydrogen plasma treatment. The evolution of the composition of the films during growth, up to thicknesses of ∼100 nm, indicates that besides etching, the diffusion of atomic hydrogen efficiently promotes the growth (and/or nucleation) of buried crystallites. Moreover, the evolution of the films strongly depends on the nature of the substrate. This substrate selectivity is discussed in terms of initial growth processes. The effect of the hydrogen plasma well below the film surface, which produces the thickness‐dependent film composition, along with the substrate selectivity, may be of prime importance in technological applications of microcrystalline silicon. ©1995 American Institute of Physics.