Optical constants of an a-Si1−xCx:H film

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Abstract
The real and imaginary parts n and k of the index of refraction of a carbon‐rich hydrogenated amorphous silicon‐carbon alloy film (a‐Si1−xCx:H, x∼0.87) have been determined for photon energies between 1.5 and 4.75 eV from measurements of reflectance R and transmittance T. The optical energy gap parameter Eopt has been determined from the energy dependence of ε2=2nk, the imaginary part of the dielectric constant. Depending on assumptions involving the energy dependencies of valence‐ and conduction‐band densities of states and optical matrix elements, values of Eopt ranging from 1.65 to 2.5 eV have been obtained for the as‐deposited film (Td=250 °C). Independently of such assumptions, Eopt is observed to decrease to zero as the film is annealed. These results are consistent with enhanced tetrahedral coordination and demonstrate improved thermal stability for this silicon‐carbon alloy film as compared to an hydrogenated amorphous carbon film. The density of the film increased from 1.51 to 1.88 g/cm3 with annealing up to 850 °C.