Optical properties of nanocrystalline diamond/amorphous carbon. composite films

Thin nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films were prepared by microwave plasma chemical vapor deposition (MWCVD) from methane/nitrogen gas mixtures. The investigation of the basic films properties (crystallinity, morphology, composition, structure, etc.) showed that the layers are composed of diamond nanocrystallites with a size of 3-5 nm, which are embedded in an amorphous carbon matrix. The ratio of the two fractions in the composite films is close to unity. Here, we report on the characterization of these films with regard to their optical properties and compare them with those of polycrystalline diamond (PCD) films also prepared by MWCVD. Optical reflectivity and total light scattering were measured in the range from 200 to 1000 nm using a spectrophotometer, while a null-type ellipsometer was used for the ellipsometric characterization of the samples. The measured spectra were mathematically simulated in order to obtain the refractive index and the extinction coefficient. The NCD/a-C films have refractive indices between 1.8 and 1.9 in the visible region; their extinction coefficient increases from the IR to the UV region, reaching a value of about 0.1. In case of PCD films, the refractive index varies between 2.1 and 2.3 due to dispersion, while the extinction coefficient is close to zero in the whole measured spectral range. Finally, the optical characteristics are correlated with the basic film properties (topography, structure, nature of the matrix, etc.).