ROLE OF THE NUCLEATION STEP IN THE GROWTH-RATE OF DIAMOND FILMS

We have studied separately the influence of the substrate temperature and of the methane concentration in the gas phase either on the nucleation step or on the whole growth process. This study was achieved using a separate substrate heater which allows us to decouple the substrate temperature and the plasma parameters. The nucleation density is shown to change abruptly in a narrow temperature range before stabilizing, while the overall growth rate increases monotically when the substrate temperature is raised. For lower substrate temperatures, the growing process is limited by the nucleation step. Depending on the methane concentration in the gas phase, the nucleation density is found to increase up to 1% CH4 before becoming constant with a value close to 8 x 10(8) cm-2 for higher methane concentrations. The dependence of the nucleation density on the substrate temperature can be explained by changes in the atom mobilities and sticking coefficients in one hand, and by the formation of amorphous carbon and carbide phase on the other hand.