Optical Emission from C 2 - Anions in Microwave-Activated CH 4 /H 2 Plasmas for Chemical Vapor Deposition of Diamond.

Visible emission from C2 - (B2 Σu + ) anions has been identified underlying the much stronger Swan band emission from neutral C2 (d3 Πg ) radicals (henceforth C2 - * and C2 *, respectively) in MW-activated C/H/(Ar) plasmas operating under conditions appropriate for the chemical vapor deposition (CVD) of diamond. Spatially resolved measurements of the C2 - * and C2 * 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(r, z) plasma modeling, identifies dissociative electron attachment (DEA) to C2 H radicals in the hot plasma as the dominant source of the observed C2 - * emission. Modeling not only indicates substantially higher concentrations of C2 H- anions (from analogous DEA to C2 H2 ) 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 C2 H2 + and C2 H3 + . 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.