Surface studies of the reactivity of methyl, acetylene and atomic hydrogen at CVD diamond surfaces

The chemical reactions occurring at a CVD diamons surface exposed to methyl and acetylene species have been studied under high vacuum conditions, primarily using isotope scrambling, reactive beam scattering and computer simulation techniques. The influence of a H/D atom flux on the processes observed has also been studied. Methyl radicals are observed to adsorb readily on hydrogen-free and hydrogenated diamond surfaces. Their main reaction pathway involves thermal decomposition in the absence of surface hydrogen: on hydrogenated surfaces this thermal decomposition pathway is largely inhibited, and recombinative desorption of methane is observed. H-atom irradiation of surface methyl species results in both addition and abstraction reactions of the surface methyl to form gaseous methane and bound CH2 species, respectively. The respective reaction probabilities are measured to be 0.01 and 0.05 for the two reaction channels under the experimental conditions employed. Methane etching from CVD diamond during H-atom irradiation alone is also noted. Marked differences are seen between the surface chemistry of methyl and acetylene on diamond. In particular, acetylene shows a vanishingly low adsorption probability on the H-free diamond surface at 300 K, which only increases if the substrate is heated to high temperatures or irradiated with H atoms. Thermally driven disproportionation or hydrogen-atom promoted desorption gave rise to the evolution of propane under these extreme conditions. The observations are discussed in the light of current knowledge of CVD diamond growth. © 1998 Elsevier Science B.V