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.

Abstract

The optical constants (\~n=n+ik, ${\ensuremath{\epsilon}}_{1}$=${n}^{2}$-${k}^{2}$, ${\ensuremath{\epsilon}}_{2}$=2nk) of a series of amorphous hydrogenated silicon-carbon alloy films (a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{C}}_{\mathrm{x}}$:H) have been determined for photon energies between 1.5 and 4.75 eV. These films have been prepared via the rf glow-discharge decomposition of ${\mathrm{SiH}}_{4}$ and ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$. 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 ${E}_{\mathrm{opt}}$ 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-${\mathrm{CH}}_{\mathrm{m}}$, m\ensuremath{\sim}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 ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$ on composition and provides a convincing demonstration that the appearance of the amorphous graphitic component in the films limits the attainable value of ${E}_{\mathrm{opt}}$ 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.