Gas phase processes in chemical vapor deposition of diamond*

Gas phase processes in chemical vapor deposition (CVD) of diamond are not well studied and its formation is mostly understood as resulting from surface kinetics phenomena. To investigate this, we modified the standard experimental setup of H₂/CH₄ plasma-assisted CVD in a 2.45 GHz reactor to decouple substrate effects from gas phase processes by a) placing a metal disc above the substrate, physically separating it from the plasma, particles formed in the gas phase flow through a pinhole in the separator disc to the substrate; b) actively cooling the substrate holder and placing it at varying distances from the plasma core.

Material characterization of the collected samples shows the presence of nanodiamond with varying particle sizes strongly corroborating that nanodiamond self-nucleates and grows in the plasma.  The nucleation of diamond over graphite is hypothesized to be due to the local high pressure resulting from the capillary effect acting on the nucleus. Its growth to the final particle size and the subsequent expulsion from the plasma is shown to depend on varying levels of thermophoretic force acting on it. Activation energy analysis indicates that gas phase kinetics of nanodiamond growth follow the same rate-limiting reactions as surface kinetics of single crystal diamond (SCD) growth. A H₂/CH₄ plasma model is used to obtain various plasma parameters and reactive species distributions revealing details of possible reaction zones in the plasma for SCD versus nanodiamond formation in the same reactor.