Distribution of Pores in a-Si_1-xC_x:H Thin Films

The aim of this paper is to compare the optical, compositional and morphological properties of a-Si_{1 - x}C_x: H films deposited by plasma enhanced chemical vapour deposition (PECVD) using different mixtures of silane (SiH₄) and methane (CH₄) under minimum attainable deposition pressure. Films deposited at lower silane flow present a higher carbon content and larger optical gap. The morphology of the films was investigated by small-angle X-ray scattering (SAXS) using two different light sources: (i) conventional tube and (ii) synchrotron radiation. The analysis of the data from both experiments was performed in order to determine a size distribution for spherical pores. The results obtained with both light sources are consistent: the increase in the CH₄ concentration implies broader size distribution functions, with an increase of the pore size up to 10 nm. Larger pores are found in films deposited at lower silane flow. For all samples, the density of the smaller pores dominates the size distribution. The relative microvoid density is not proportional to the carbon concentration but presents a maximum for the low carbon content films.