Rate limitation in low pressure diamond growth

We performed diamond deposition experiments from a gas phase containing H₂, CH₄, and sometimes CO, using a microwave plasma ball reactor operating at 400 mbar pressure. The molybdenum substrates were stamped with a suitable tool to form a number of flattened cones on its surface. A strong preference for crystal growth on top of the cones was observed. Numerical calculations were used to solve the underlying thermal conduction and diffusion problems. At the substrate, the flow of the active species entering by diffusion from the bulk of the gas phase was balanced by those leaving the system due to incorporation in the crystals. Comparison with the experiments showed that at least 10% of the active species striking the surface are incorporated. Thus, the limitation of diamond growth in our investigation lies in gas phase transport and not in incorporation difficulties at the growing surface.