We present a spectroscopic ellipsometry study of the layer-by-layer growth of intrinsic and doped microcrystalline silicon layers on hydrogenated amorphous silicon films. Particular attention has been paid to the effects of the porosity of the a-Si:H substrates, process temperature, hydrogen plasma treatment time, and doping on the mu c-Si growth. Our results show that it is possible to produce thin (approximate to 20 nm) and highly crystallized films (crystalline fraction greater than or equal to 96%). Highly conductive p- and n-doped mu c-Si films have also been obtained. The most striking result concerning mu c-Si growth is that atomic hydrogen modifies the a-Si:H substrates on which mu c-Si films are produced. This modification of the a-Si:H substrate can extend over 50 nm in depth. The extent and amplitude of this modification are discussed in terms of the process conditions and substrate film type (dense or porous a-Si:H).