Ellipsometric investigation of nitrogen doped diamond thin films grown in microwave CH_4/H_2/N_2 plasma enhanced chemical vapor deposition

The influence of N2 concentration (1%–8%) in CH_4/H_2/N_2 plasma on structure and optical properties of nitrogen doped diamond (NDD) films was investigated. Thickness, roughness, and optical properties of the NDD films in the VIS–NIR range were investigated on the silicon substrates using spectroscopic ellipsometry. The samples exhibited relatively high refractive index (2.6 ± 0.25 at 550 nm) and extinction coefficient (0.05 ± 0.02 at 550 nm) with a transmittance of 60%. The optical investigation was supported by the molecular and atomic data delivered by Raman studies, bright field transmission electron microscopy imaging, and X-ray photoelectron spectroscopy diagnostics. Those results revealed that while the films grown in CH_4/H_2 plasma contained micron-sized diamond grains, the films grown using CH_4/H_2/(4%)N_2 plasma exhibited ultranano-sized diamond grains along with n-diamond and i-carbon clusters, which were surrounded by amorphous carbon grain boundaries.