Influence of film thickness on structural properties of microcrystalline silicon films

When microcrystalline silicon is used as intrinsic layer of a thin film solar cell, the thickness must be several μms because of the lower optical absorption coefficient compared with amorphous silicon. Therefore, we prepared microcrystalline silicon films up to 3 μm in thickness by a hydrogen-radical CVD method and examined the film thickness dependence of their structural properties. The deposition rate increased as film thickness became thicker. The obtained microcrystalline silicon films indicated [111] preferential orientation and the grain size was about 30 nm. The surface roughness of microcrystalline silicon films was observed using atomic force microscopy (AFM). The crystallinity as well as the rms roughness was increased with an increase in film thickness. The average diameter of projection divisions measured by AFM was around 90-270 nm, and much larger than the grain size. This result suggests that a projection division was formed by more than several crystalline grains.