DIAMOND FILM GROWTH IN A FLOWTUBE - A TEMPERATURE-DEPENDENCE STUDY

Data are presented on the rate of diamond film growth in a flowtube at uniform temperature and pressure, and on the rate of H-atom decay in the tube during growth. Diamonds are grown by adding methane to a flow of atomic hydrogen in a carrier gas. The study covers temperatures from 600 to 900-degrees-C, and a range of methane and H-atom concentrations. The profile of diamond growth rate in the tube is a measure of the gas-phase rise and decay of chemical species required for growth. A rudimentary computer model, fitted to the measured H-atom loss rates, is used to compare the gas-phase chemistry with the profiles. Using an assumed rate law for diamond growth, reasonable agreement with the data can only be obtained by including rapid, nonproductive wall loss of more than 95% of the methyl radicals. The data also suggest that methane (or methyl) increases the loss rate of H atoms, and that the converse is true: H atoms increase the loss rate of methyl radicals. Both processes are too fast to be accounted for by gas-phase chemistry. The model comparison yields an estimate for an activation energy of 19 +/- 10 kcal for the heterogeneous growth process. This number is meaningful only if the assumed rate law is correct.