Optical constants of a hydrogenated amorphous carbon film

The real and imaginary parts, n and k, of the complex index of refraction of a hydrogenated amorphous carbon (a-C:H) film have been determined for photon energies between 1.45 and 4.9 eV from measurements of reflectance R and transmittance T. Both n and k, and consequently ε1 and ε2 (the real and imaginary parts of the dielectric constant), show a considerable variation with subsequent annealing of the a-C:H film up to Ta=750 °C, with the most rapid changes occurring for Ta=450 °C and above. The optical gap Eopt is observed to decrease from 2.2 eV to 0 as the film is annealed, signaling the development of graphitic short range order in the film. Using an effective medium approximation to model the optical properties of the film, we find that the as-deposited film contains amorphous diamond-like, graphitic, and polymeric components. With annealing, the amorphous diamond-like and polymeric components decrease, the amorphous graphitic component grows, and a void component appears. We discuss the local order present in the as-deposited and annealed film.