Plasma enhanced chemical vapor deposition of amorphous, polymorphous and microcrystalline silicon films

The plasma processes and growth reactions involved in the deposition of amorphous, polymorphous and microcrystalline silicon thin films are reviewed. The reference being a-Si:H deposition through surface reactions of SiH₃ radicals, we study the growth of microcrystalline silicon films produced by the layer-by-layer and standard hydrogen dilution techniques. We show that subsurface reactions play a key role, particularly during the incubation phase where hydrogen is responsible for the formation of a porous layer in which nucleation takes place. The evolution of the film properties is related to the long range effects of hydrogen. Coming back to a-Si:H deposition, we further consider the deposition at low substrate temperature (<200°C) and pressure (<5 Pa) where the role of ions is dominant and at deposition rates where powder formation takes place. We propose that rather than a drawback, nanoparticle formation in silane plasmas might be considered as a potential for obtaining new silicon films. We address in particular the deposition of polymorphous silicon consisting of an a-Si:H matrix with silicon nanocrystallites produced in the gas phase. Despite their heterogeneity polymorphous silicon films have improved transport properties and stability with respect to a-Si:H.