On the role of oxygen and hydrogen in diamond-forming discharges

Abstract

Plasma emission actinometry has been used to study the mechanism by which small additions of oxygen (∼0.5%) enhance the rate of diamond deposition in a dilute (4%) CH4/H2 discharge at high temperature (900–1300 K). Increasing amounts of CH4 in the feed depress [H], while increasing the O2 concentration, up to ∼5%, produces a fivefold increase in atomic hydrogen in the discharge zone. Invoking a mechanism where diamond growth competes with the formation of an amorphous/graphitic inhibiting layer, these results and earlier studies suggest that oxygen (1) increases [H] which selectively etches amorphous/graphitic carbon, (2) accelerates reaction of this layer with molecular hydrogen, and (3) may itself act as a selective etchant of nondiamond carbon. As a result, the number of active diamond growth sites is increased and enhanced growth rates are observed. We also have grown diamond by alternating a CH4/He discharge with a H2/O2/He discharge and results are consistent with this mechanism. Instantaneous growth rates are very high (45 μm/h) and codeposition of nondiamond allotropes is less. The H2/O2/He discharge removes the inhibiting carbon layer more effectively, while the CH4/He plasma presumably is richer in diamond-forming precursors. Thermochemical calculations suggest that these precursors are likely to be C2H2, C2H, C2, or C atoms rather than methyl radicals (CH3) as others have postulated.

Formation of nanodiamonds at near-ambient conditions via microplasma dissociation of ethanol vapour

“…In addition, we detect excited H 2 from the Fulcher-a band system B600 nm ( Supplementary Fig. S3b) 20 . The confirmation of C 2 and CH radicals is particularly important for nanodiamond formation.…”

“…The confirmation of C 2 and CH radicals is particularly important for nanodiamond formation. Previous reports have shown that C 2 species initially nucleate a solid carbon cluster, and CH species form a hydrocarbon layer, promoting the nucleation of nanodiamonds 20,21 ; however, these experiments were carried out on films where the substrate may also influence nucleation 22,23 . The addition of H 2 gas results in a new feature in the OES spectra at 656 nm corresponding to the H a Balmer line (see right panel of Fig.…”

Diamond growth by chemical vapour deposition

“…Several studies have reported enhanced growth rates or quality due to oxygen addition and, more importantly, the substrate temperature can be significantly lowered when oxygen is introduced into the hydrocarbon/H 2 gas mixtures [73][74][75][76][77][78].…”

Plasma-enhanced chemical vapor deposition of nanocrystalline diamond

“…In the PE-CVD of diamond films, oxygen is added to a CH 4 /H 2 system to obtain a high growth rate and/or an improvement of the quality of the resultant films [35][36][37][38]. Several authors have discussed the role of oxygen in diamond growth.…”

“…Several authors have discussed the role of oxygen in diamond growth. Mucha et al [37] pointed out that the addition of oxygen to a CH 4 /H 2 system increases the concentration of atomic hydrogen; correspondingly, the formation of amorphous and graphitic carbon, which would otherwise inhibit diamond growth, is suppressed. From a detailed chemical kinetics model that describes both gas phase and surface processes occurring in diamond CVD, Frenklach and Wang [38] postulated that OH radicals and atomic oxygen gasify sp 2 carbon and suppress the formation of aromatic species in the gas phase.…”