Influence of hydrogen dilution on surface roughness development of a-Si:H thin films grown by remote plasma deposition

Hydrogenated amorphous silicon (a-Si:H) films are grown at different hydrogen dilutions. For high dilutions we observe a discrepancy in the surface roughness analysis between real-time spectroscopic ellipsometry (RTSE) and atomic force microscopy (AFM) measurements. With RTSE a much higher roughness layer thickness is measured than with AFM. Additionally we observe what appears to be a strong roughening phase in the first 30-50 nm of film growth for all conditions. A mechanism that involves the formations of a hydrogenrich overlayer and etching of higher hydrides in this overlayer is suggested. This mechanism explains the difference in RTSE roughness layer thickness evolution. At higher hydrogen dilutions, we obtain a thicker overlayer dominated by lower hydrides. RTSE interprets this overlayer as additional surface roughness and consequently overestimates the surface roughness for higher dilutions. The initial strong roughening phase is related to the built-up of this over-layer. In this phase, we obtain much denser films compared to the bulk film. The presence of columnar growth at higher dilutions is attributed to reduced surface mobility under these conditions (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim