A THERMOCHEMICAL MODEL FOR DIAMOND GROWTH FROM THE VAPOR-PHASE

Several models have been proposed for diamond growth. This paper describes a thermochemical or "chemical pump" model. By supplying external energy (radicals, ions, excited molecules) to the surface of a solid, a metastable state may be reached. In systems with different polymorphs the phase stability may then be changed. In the carbon system this means that diamond is more stable than graphite under the special conditions used in diamond vapour growth processes. The external energy is then mainly supplied to the carbon surface. For a surface with a graphite structure and unsaturated bonds, atomic hydrogen adds external energy to the system and simpler hydrocarbons are formed and desorb from the surface. The diamond surface represents a more stable structure than the graphite surface in an environment of superequilibrium hydrogen. This means that superequilibrium atomic hydrogen chemically pumps the graphite surface to an energy state which is less stable than the diamond surface. From a chemical pump model, temperature-composition stability diagrams were calculated for conditions of relevance for diamond growth processes. The diagrams agreed well with experimental observations.