Optical constants of a series of amorphous hydrogenated silicon-carbon alloy films: Dependence of optical response on film microstructure and evidence for homogeneous chemical ordering

The optical constants (\~n=n+ik, ε₁=n²-k², ε₂=2nk) of a series of amorphous hydrogenated silicon-carbon alloy films (a-Si_1-x{C}ₓ$:H) have been determined for photon energies between 1.5 and 4.75 eV. These films have been prepared via the rf glow-discharge decomposition of ${SiH}₄$ and ${C}₂H₂. The index of refraction n at 1.5 eV increases smoothly from n=1.67 for a-C:H (x=1) to n=3.18 for a-Si:H (x=0), while the optical energy gap Eₒₚₜ reaches a maximum value of 2.68 eV for a carbon fraction of x=0.68. The films in this alloy series are proposed to be macroscopically homogeneous, while having a heterogeneous microstructure. Their optical response has been modeled, via the Bruggemann effective-medium approximation (EMA), as arising from three amorphous components: polymeric (a-CHₘ, m{~}2), graphitic (a-C), and tetrahedral (a-Si:C:H), and a void component. A Si- and C-centered tetrahedron model developed in the preceding paper has been used to predict the optical response of the amorphous tetrahedral component as a function of its composition. This EMA approach based on four components in the films gives a good description of the observed dependences of ε₁ and ε₂ on composition and provides a convincing demonstration that the appearance of the amorphous graphitic component in the films limits the attainable value of Eₒₚₜ in this alloy series as the carbon content increases. In addition, the model provides strong evidence that complete chemical ordering with homogeneous dispersion exists within the amorphous tetrahedral (a-Si:C:H) component across the entire alloy series.