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 (ñ=n+ik, ${\epsilon }_1$=$n^2$-$k^2$, ${\epsilon }_2$=2nk) of a series of amorphous hydrogenated silicon-carbon alloy films (a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{C}}_{\mathrm{x}}$: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∼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 ${\epsilon }_1$ and ${\epsilon }_2$ 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.