Collision reactions of CH radical on diamond and their effects on the carbon film growth

Abstract

The growth mechanism of hydrogenated carbon films in plasma-enhanced chemical vapor deposition (PECVD) is complicated and much attention has to be paid to it for the unique properties of carbon films. In this paper molecular dynamics simulations are carried out to illustrate the collision behaviors of CH radical on the clear and hydrogenated diamond (111) surface with varying incident energy (from 1.625 to 65 eV), aiming at the growth mechanism of hydrogenated carbon film by PECVD. Our simulations show that the behaviors of incident CH radical can be divided into adsorbing, rebounding, reaction releasing one H atom and reaction releasing two H atoms, while the reaction releasing one H2molecule rarely occurs. At low incident energy, selective adsorption of CH at unsaturated surface C site is the dominated growth mechanism since no reactions can conduct. Such growth model results in films with rough surface, high hydrogen fraction, and loose structure. As the incident energy increases, two chemical reactions that one releases one H atom and the other releases two H atoms are important. Caused by these reactions, the saturated C site in the surface will be transferred into unsaturated one, so that it can further adsorb subsequently incident CH radicals. The occurrence of these reactions makes films grow more uniformly, leading to the smoothness and dense structure of the films. The hydrogen fraction in the films will be reduced by these reactions. To confirm the above growth mechanism, the carbon film growth from CH radicals are then simulated. The film obtained with low energy (3.25 eV) CH radicals is found to be loose, rough, and have many carbon chains with adsorbed hydrogen atoms on the surfaces, while the film produced with high energy (39 eV) radicals are dense, smooth and the chains on the surfaces are short and have less hydrogens. On the other hand, most of the C atoms in the films deposited with low energy have one H atom as coordination, while for high energy most of C atoms in the films have no H atom as coordination. These observations agree well with the proposed growth mechanism. The destruction effects caused by the incident CH radicals are also analyzed based on the variation of the sp2-C and sp3-C in the films.