Use of liquid precursors for diamond chemical vapor deposition - The effects of mass transport and oxygen

Thermal plasma chemical vapor deposition of diamond-utilizing liquid feedstock injection has been shown to yield higher mass deposition rates, larger crystal size, and thicker films when compared to the use of gaseous feedstock for equivalent operating conditions. Increased mass transport of the activated precursor species across the substrate diffusion boundary layer and the presence of oxygen in liquid precursors are investigated as potential reasons for the observed results. Comparisons of the various precursor systems investigated in this study are based on crystal size and film thickness as a function of radial postion, area of deposit, total mass deposition rate, and the observed liquid precursor droplet trajectories within the deposition chamber using a laser strobe video system. The results indicate that the mass transport in both the liquid and gaseous precursor systems is greatly improved by the use of an inert carrier gas. Further, the use of a liquid versus a gaseous precursor does not seem to result in higher total deposition rates when the operating conditions for both have been optimized. Finally, the presence of oxygen in the liquid feedstock system is found to be at least partly responsible for the increased growth rate, which is observed when comparing the plain hydrocarbon precursor cases with the oxygenated liquid precursor case.