The plasma deposition of nanocrystalline silicon thin films is usually performed under a high flux of atomic hydrogen and hydrogenated chemical species. The growth mechanisms are investigated using the layer‐by‐layer deposition of dense nanocrystalline silicon, obtained at 250 °C by alternating SiH4and H2plasmas. In the steady state, a minimum exposure time to the hydrogen plasma is necessary to recrystallize the amorphous top layer (10–85 A˚). It is shown that this critical time is determined by the diffusion time of some mobile H through the topa‐Si:H layer. The recrystallization is discussed in relation to the diffusion of hydrogen leading to the nanovoid and broken bond formation processes. ©1995 American Institute of Physics.