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

Abstract

Visible emission from C(BΣ) anions has been identified underlying the much stronger Swan 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 vapor 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(r, z) plasma modeling, identifies dissociative electron attachment (DEA) to CH radicals in the hot plasma as the dominant source of the observed C* emission. Modeling not only 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.

Spatially Resolved Optical Emission and Modeling Studies of Microwave-Activated Hydrogen Plasmas Operating under Conditions Relevant for Diamond Chemical Vapor Deposition

“…The experiment employed upgraded versions of our previously described custom designed MW PACVD reactor 54 and optical emission imaging setup. 55 One key upgrade to the former is the way that process gas is introduced at the top of the reactor. The previous two diametrically opposed inlet pipes have been replaced by eight symmetrically arranged 4 mm inlets in order that the input gas flow better approximates the cylindrical symmetry assumed in the modelling.…”

Combined Spatially Resolved Optical Emission Imaging and Modeling Studies of Microwave-Activated H2/Ar and H2/Kr Plasmas Operating at Powers and Pressures Relevant for Diamond Chemical Vapor Deposition

“…[12][13][14][15] Larger additions of Ar (and other noble gases) substantially affect the relative intensities of the various features in the optical emission spectrum of the plasma, as a result of changes in gas temperature and the plasma chemistry. [16][17][18][19][20][21] Successful CVD of high-quality diamond depends on the availability of suitable concentrations of both H atoms and CHx (x = 0-3) radicals, particularly CH3 radicals, in the gas phase adjacent to the growing surface. H atom production is dominated by thermal dissociation of H2 in the hot plasma region, so the benefits of replacing some H2 in the process gas mixture by, for example, Ar might not be obvious.…”

Dissociative Electron Attachment in C2H via Electronic Resonances

“…2 is commonly occurring in many high-energy environments 38,[46][47][48] , the mechanism of its formation is not fully understood. From the observed spatial distribution and its variation with changes in process conditions, the experimentalists deduced 38 that both ground-and excited-state C − 2 (X 2 Σ + g and B 2 Σ + u , respectively) are formed directly via DEA to C 2 H:…”

“…In this contribution, we employ EOM-CC methods [25][26][27] augmented with the complex absorbing potential (CAP) 24,36,37 to interrogate electronic structure and DEA in C 2 H − . This study is motivated by the observation of electronically excited C − 2 (detected via fluorescence) in carbon-rich plasmas 38 , such as microwave activated plasma in chemical vapor deposition (CVD) of diamond. C 2 H − is also relevant for astrochemistry, along with other molecular anions detected in interstellar medium and planetary atmospheres, such as CN − , C 2n H − , C 2n+1 N − , benzonitrile, etc [39][40][41][42][43][44][45] .…”