Structure and growth of hydrogenated microcrystalline silicon: Investigation by transmission electron microscopy and Raman spectroscopy of films grown at different plasma excitation frequencies

Microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition consists of variable volume fractions of amorphous phase, grain boundaries, cavities and crystalline grains. In this paper the structural properties, which strongly depend on the growth conditions, were investigated in detail by transmission electron microscopy and by Raman spectroscopy. A columnar structure parallel to the growth direction is observed for all conditions investigated. By increasing the plasma excitation frequency the crystalline volume fraction and the grain sizes are enhanced. Simultaneously an increase in the growth rate can be achieved, which is accompanied by an increasing etch rate of amorphous material. In addition, spherical voids were found predominantly in samples prepared at a low plasma excitation frequency. The growth of a porous initial layer containing a high density of 'crack-line' voids is observed when high plasma excitation frequencies are applied. These results suggest that the microcrystalline growth is governed by the preferential etching of the amorphous phase. In addition, chemical reactions have to be taken into account to explain the formation of the spherical voids.