The microstructural evolution of hydrogenated amorphous silicon‐carbon (a‐Si1−xCx:H) alloy thin films with optical gaps of ∼1.95 eV has been characterized by real time spectroscopic ellipsometry versus hydrogen dilution of the reactive gases (CH4+SiH4) used in plasma‐enhanced chemical vapor deposition. As the H2/(CH4+SiH4) flow ratio is increased to 24, the monolayer‐scale features of nucleation and growth suggest an enhancement in the diffusion length of the film precursors on the substrate and film surfaces, leading to an increase in the surface structural stability and bond‐packing density of the final material. We suggest a causal connection between the monolayer‐scale processes and the ultimate photoelectronic properties of the a‐Si1−xCx:H, which also improve with H2 dilution over the same range.

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