Role of mobile hydrogen in the amorphous silicon recrystallization

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 SiH₄ and H₂ plasmas. In the steady state, a minimum exposure time to the hydrogen plasma is necessary to recrystallize the amorphous top layer (10-85 Å). It is shown that this critical time is determined by the diffusion time of some mobile H through the top a-Si:H layer. The recrystallization is discussed in relation to the diffusion of hydrogen leading to the nanovoid and broken bond formation processes.