Evolution of the surface roughness with the hydrogen partial pressure for high deposition rate of nanocrystalline silicon films

Silicon films were deposited at high rate by reactive magnetron sputtering in a plasma mixture of H2 and Ar. The hydrogen pressure ratio rH=PH2/Ptot was varied in percentage from 0% to 100%. For rH below 5%, a transition from amorphous to crystalline Si takes place as evidenced by X-ray diffraction (XRD) and Raman spectroscopy measurements. In parallel, the apparent density, as deduced from X-ray reflectivity measurements, shows a stepped decrease. This phase transition arises because of the critical role of H radicals during the deposition process. For higher values of rH, the crystalline degree improves, whereas the apparent density deteriorates for rH⩾80%. The surface roughness, determined by atomic force microscopy, correlates inversely with the apparent density. This behaviour suggests that the deposition rate is high and would then exceed the `speedʹ of network relaxation. Such a rough surface would be the counterpart of the high deposition rate, even though the compactness and high crystallinity of the film bulk would result from some `chemical annealingʹ and effects of diffusing H species.