Contribution of plasma generated nanocrystals to the growth of microcrystalline silicon thin films

We report on a particular growth mechanism for microcrystalline silicon thin films deposited using SiF₄/H₂/Ar mixtures. The structure of the films is studied by in situ ellipsometry and atomic force microscopy. The measurements show that growth starts directly from crystallites of various sizes without any detectable amorphous tissue. With increasing deposition time, we observe the formation of columns which grow laterally until their coalescence and the obtaining of a continuous film. Based on plasma impedance measurements, combined with the trapping of particles on a cold substrate, we attribute the early presence of crystallites to the sticking of plasma produced nanocrystals on the substrate, and the columnar growth to the preferential sticking on the top and beside the nanocrystals. Accordingly, modifying the surface of silicon nitride substrates by various plasma treatments allows one to change the density of sticking centers. It therefore gives us a way to control the size of the columns, which grow laterally from the initial crystallites. This is a new way to optimize devices where carriers flow parallel to the substrate, such as thin-film transistors (TFTs).