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A methyl radical mechanism is described for the growth of diamond on its (110) surface. The growth of diamond in a hot filament reactor is modeled using a simple first order steady state approximation and calculated methyl radical concentrations are found to agree well with recent measurements. Steady state local equilibrium is assumed and used to estimate the mole fraction of reactive sites at the growth surface, giving a value of ~6 X 10-4 at 1200 K and 50 Torr (0.066 atm) of 1% CH4 in H2. A simple first order thermochemical model of hot filament assisted deposition is described and methyl radical concentration estimates found to be in reasonable agreement with recent experimental results. The rate of methyl radical addition onto diamond (110) is calculated using these estimates and, assuming incorporation is as fast or faster than addition, gives a growth rate of ~0.8 micrometer/hr, in good agreement with the experiment. It is proposed that the cubo- cotahedral habit and <100> texture commonly seen in much CVD diamond is due to the <110> being the preferred low index growth rate axis. The reported rotation of polycrystalline texture toward a <100> axis, seen with changes in temperature and feed gas composition, is ascribed to variations in the relative rates of growth by this mechanism along [110] and [011]. Absolute reaction rate theory is sued to estimate a rate constant for methyl radical addition at sterically unhindered tertiary carbon sites. A 'loose' transition state model predicts a rate constant of 1012 to 1013 cm3 mol-1 sec-1. If a much 'tighter' transition state is assumed, the predicted value is reduced to 1010 to 1011 cm3 mol-1 sec-1. A 'loose' transition state model is preferred as this gives a result consistent with empirical rate constants for many radical recombinations and is consistent with a simple calculation based steady state approximation. It is argued that the highest rate of growth commonly occurs along <110> as the (110) surface represents a relatively unhindered tertiary carbon surface to which, at most, only two methyl radicals need be added to propagate the diamond lattice.
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