Optical Emission from C₂^– Anions in Microwave-Activated CH₄/H₂ Plasmas for Chemical Vapor Deposition of Diamond

摘要

Visible emission from C(BΣ) anions has been identified underlying the much strongerSwan band emission from neutral C(dΠ) radicals (henceforth C* and C*, respectively) in MW-activated C/H/(Ar) plasmas operating under conditions appropriate for the chemical vapour deposition (CVD) of diamond. Spatially resolved measurements of the C* and C* emissions as functions of the C/H/(Ar) ratio in the input gas mixture, the total pressure, and the applied MW power, together with complementary 2-D(, ) plasma modelling, identifies dissociative electron attachment (DEA) to CH radicals in the hot plasma as the dominant source of the observed C* emission. Modelling indicates substantially higher concentrations of CH anions (from analogous DEA to CH) in the near-substrate region, but also suggests that the anion number densities will typically be 3–4 orders of magnitude lower than those of the electrons and partner cations, i.e. mainly CH and CH. The identification of negatively charged carbon-containing species in diamond CVD plasmas offers a possible rationale for previous reports that nucleation densities and growth rates can be enhanced by applying a positive bias to the substrate.