Deposition mechanisms and properties of oxygenated carbon nitride films from rf discharges of acetylene, nitrogen, oxygen and argon mixtures

Oxygen-containing carbon nitride films were deposited in rf discharges of C2H2, N2, O2 and Ar mixtures. Actinometric optical emission spectroscopy was used to determine relative concentrations of the species H, O, CH, CN and CO in the discharge as a function of the proportion of oxygen in the feed, using Ar as an actinometer. The Ar emission intensity decreases as the proportion of O2 in the feed increases, indicating that O2 has a `cooling effect' on the discharge. The variations in the concentrations of CH and CN species, two possible precursors of film formation, showed different behaviors. The concentration of CH increases monotonically as the proportion of O2 in the feed is increased, but the concentration of CN shows a maximum at an oxygen to acetylene ratio of about 0.15, which coincides with the maximum film deposition rate. This strongly suggests that the CN species is the main precursor of film formation. Infrared reflection spectroscopy revealed the presence of CN, CH, NH and CO functionalities in the deposited films. The presence of such species and their relationship to deposition via gas-phase and surface reactions are outlined. The optical properties of the films were investigated using transmission and reflection ultraviolet–visible spectroscopy. At increasing proportions of oxygen in the gas feed, the refractive index decreases while the optical gap increases. Free radicals were investigated using electron paramagnetic resonance. Unpaired spin concentrations of the order of 1020 cm−3 were measured in the films a short time after deposition. These large spin concentrations are common in films prepared by plasma-enhanced chemical-vapor deposition (PECVD) but decrease at relatively high rates with time. Possible mechanisms accounting for the high spin concentrations and for the disappearance of the free radicals are discussed.