Laser-based diagnostics of diamond synthesis reactors

The many existing and potential applications of diamond thin films have created a flurry of activity directed toward the understanding and optimization of diamond synthesis by chemical vapor deposition (CVD). In this thesis, laser-based diagnostic techniques are applied to investigate the gas-phase environment of two diamond CVD reactors. Degenerate four-wave mixing (DFWM) is used to probe the thin reacting boundary layer above the deposition substrate in an atmospheric pressure plasma torch. Cavity ring-down spectroscopy (CRDS) is applied to measure absolute CH₃ and CH radical concentrations and temperatures in a hot-filament CVD (HFCVD) reactor. Concentration and temperature profiles obtained by these experiments are compared with modeling predictions.; Atmospheric pressure plasmas are an attractive method for diamond thin film synthesis because the inherently high reactant densities lead to large growth rates. DFWM is a technique that enables the in situ measurements of CH and C₂ radical concentrations, vibrational and rotational temperatures, in the thin boundary layer above the diamond growth substrate.; Measurements are also performed in a HFCVD reactor to gain additional understanding of the diamond growth process. The highly sensitive CRDS technique is used to measure the rotational temperature as well as absolute CH ₃ and CH radical concentration profiles inside the HFCVD reactor during film growth. For many operating conditions, the methyl radical concentration is found to peak at a location several millimeters from the filament surface. The peak is the result the effects of steep temperature gradients and thermal diffusion, on the production and destruction of the methyl radical.; The CH rotational temperature measurements indicate that the gas temperature near the filament surface is significantly lower than the filament temperature. This temperature discontinuity is due to the breakdown of the continuum energy conduction theory at low pressures, and to the small energy accommodation coefficient between hydrogen and the filament.; Random temporal oscillations in the CH concentration are observed when the tungsten filament is replaced with a rhenium filament. The oscillations are attributed to surface reactions, and to the cyclical formation and destruction of a carbon layer on the filament surface.