Untersuchung der Wachstumsmechanismen von mikrokristallinem Silicium mittels Rastertunnelmikroskopie und Rasterkraftmikroskopie

The formation of crystalline nuclei and the growth of microcrystalline material in hydrogenatedsilicon films were investigated. By using plasma-enhanced chemical vapour depositionfrom silane diluted in hydrogen low-temperature films were deposited onto oxidized crystallinesilicon, hydrogen-passivated silicon, hydrogenated amorphous silicon and polycrystallinegraphite. As-grown films were characterized by scanning tunneling microscopy and spectroscopyas well as by scanning force microscopy, while transmission electron microscopy andRaman spectroscopy were applied ex situ.Two-phase films are presented in which crystallites nucleate near the substrate and evolveinto conically shaped features of the size of several 100 nm while being surrounded by anamorphous matrix. Hydrogen promotes the nucleation in a way that a reduction of the silaneconcentration in the source gas significantly increases the nucleation rate.Regarding the growth of crystallites, results can be explained on the base of the assumptionthat there is a state near the thermal equilibrium including the surface of the growing film andits vicinity. Here, driving forces for the growth are the Gibbs free energy of crystallization andthe difference of the chemical potential between the plasma and the film. Regarding the nucleationof crystallites, differences in the effect of hydrogen for the different substrate materialscan be understood in the frame of a surface model where nuclei are assumed to be formeddirectly on top of the substrate. The slope by which the nucleation rate decreases as a functionof the silane concentration turns out to depend on the efficiency by which hydrogen generatesnucleation sites at the substrate