Structural and optical properties of nitrogen and oxygen doped a-C:H coatings

Abstract

The deposition and characterization of hydrogenated amorphous carbon films (a-C:H) obtained by radio-frequency plasma enhanced chemical vapor deposition on silicon and on polycarbonate are reported. Small amounts of N 2 and O 2 were added to a CH 4–He gas mixture in order to promote the doping of the films. A systematic study of the coating structure and optical and mechanical properties as a function of gas phase composition and applied radio-frequency power is also reported. X-ray photoelectron spectroscopy (XPS) was used to quantify the amount of nitrogen and oxygen incorporation, while the characterization of sp 2 carbon bonding was performed by Raman spectroscopy. Ellipsometry and UV–Vis–NIR transmission measurements were exploited to relate the refractive index and absorption of light in the visible region to the film composition and the deposition process parameters. The associated variation of the coating hardness was checked by nanoindentation measurements. The results obtained show that oxygen incorporation allows the achievement of an increased film transparency in the visible spectral range, while nitrogen dilution of the methane precursor in the plasma is responsible for the deposition of films that have a higher absorbance in the visible, due to an increased size of graphitic domains. The addition of both doping elements in a small percentage allows the preservation of the characteristic high hardness of a-C:H coatings.