DEPENDENCE OF THE GAS-COMPOSITION IN A MICROWAVE PLASMA-ASSISTED DIAMOND CHEMICAL-VAPOR-DEPOSITION REACTOR ON THE INLET CARBON SOURCE - CH4 VERSUS C2H2

Molecular beam mass spectrometry was used to measure the gas-phase composition near a growing diamond surface in a microwave plasma-assisted chemical vapor deposition reactor. The dependencies of the gas composition on changes in: (1) the carbon mole fraction in the reactor feed, X(C); (2) the identity of the inlet carbon source (CH4 versus C2H2); and (3) the surface temperature, T-S, were studied. Unlike the hot-filament case, the gas composition was independent of the identity of the inlet hydrocarbon source over the entire range of X(C). As expected from gas-composition measurements, films grown using either hydrocarbon exhibited similar growth rates, morphology and Raman spectra, The gas composition was also generally insensitive to changes in surface temperature suggesting that reported temperature sensitivities of film growth properties are primarily due to changes in the kinetics of surface processes rather than changes in the gas composition near the surface. For X(C) less than or equal to 0.01, the gas compositions measured in this study and in our previous studies conducted in a hot filament-assisted chemical vapor deposition reactor are quantitatively similar implying similar gas-phase chemistry in both reactors when sufficient atomic hydrogen is present. However, attempts to simulate the gas composition in the plasma environment using a numerical model based only on neutral-neutral gas-phase chemistry were generally unsuccessful over a wide range of X(C).