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

Abstract

Silicon films were deposited at high rate by reactive magnetron sputtering in a plasma mixture of H 2 and Ar. The hydrogen pressure ratio r H= P H2/ P tot was varied in percentage from 0% to 100%. For r H 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 r H, the crystalline degree improves, whereas the apparent density deteriorates for r H⩾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 `speed' 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 annealing' and effects of diffusing H species.