Smoothing of rough a-Si:H surfaces by ion-assisted deposition and sputter erosion

We examine the smoothing of initially rough, amorphous silicon surfaces by sputter erosion and ion-assisted surface diffusion during deposition. We have found both to be able to reduce surface roughness, although, we believe, by apparently different mechanisms. For the case of ion sputter erosion, preferential removal of porous over dense material regions initially occurs that actually increases the surface roughness; at some point, surface roughness decreases from preferential sputtering of peaks over valleys. Ion-enhanced deposition is believed to increase surface diffusion of film precursor species, allowing them access to high sticking coefficient sites such as those found in valleys. Effects of these smoothing processes on the material near the surface is also different as ion sputtering results in an optically porous material relative to a dense surface cap resulting from ion-assisted deposition.