EXPERIMENTAL MEASUREMENTS AND NUMERICAL SIMULATIONS OF THE GAS-COMPOSITION IN A HOT-FILAMENT-ASSISTED DIAMOND CHEMICAL-VAPOR-DEPOSITION REACTOR

Molecular-beam mass spectroscopy was used to measure the gas compositi on near a growing diamond surface in a hot-filament-assisted chemical- vapor-deposition reactor. The dependencies of the gas composition on c hanges in (1) the carbon mole fraction in the reactor feed X(C), (2) t he identity of the inlet carbon source (CH4 versus C2H2), and (3) the surface temperature T-S, were studied. For X(C) less than or equal to 0.02, the gas composition appeared to be nearly independent of the ide ntity of the inlet hydrocarbon source and depended only on the C/H rat io in the feed gas. At higher values of X(C), catalytic poisoning of t he hot filament resulted in different product distributions in these t wo systems. Increasing the surface temperature affected changes in the hydrocarbon composition; the dependencies of the CH3 and C2H2 mole fr actions on T-S can each be characterized as having an activation energ y of 3+/-1 kcal/mol. Surprisingly, the H-atom mole fraction was indepe ndent of T-S. These results suggest that reported temperature sensitiv ities of film growth properties are primarily due to changes in the ki netics of surface processes rather than changes in the gag composition near the surface. A numerical model of the process is presented. In t he study of the compositional change as a function of X(C), the code g ives good prediction for the methane case but grossly underestimates t he methane and methyl concentrations for the acetylene case. The H-ato m mole fraction is predicted to increase by X7 if the H destruction pr obability on the diamond surface is expected to have an activation ene rgy of 7.3 kcal/mol. Good agreement with experimental data can be obta ined, however, if H loss by lateral transport to the walls is taken in to account. (C) 1994 American Institute of Physics.