Microwave plasma-activated chemical vapour deposition of nitrogen-doped diamond, II: CH₄/N₂/H₂ plasmas

We report a combined experimental and modelling study of microwave-activated dilute CH₄/N₂/H₂ plasmas, as used for chemical vapour deposition (CVD) of diamond, under very similar conditions to previous studies of CH₄/H₂, CH₄/H₂/Ar and N₂/H₂ gas mixtures. Using cavity ring-down spectroscopy, absolute column densities of CH(X, v=0), CN(X, v=0) and NH(X, v=0) radicals in the hot plasma have been determined as functions of height, z, source gas mixing ratio, total gas pressure, p, and input power, P. Optical emission spectroscopy has been used to investigate, with respect to the same variables, the relative number densities of electronically excited species, namely H atoms, CH, C₂, CN and NH radicals, and triplet N₂ molecules. The measurements have been reproduced and rationalised from first principles by 2-D (r, z) coupled kinetic and transport modelling, and comparison between experiment and simulation has afforded a detailed understanding of C/N/H plasma-chemical reactivity and variations with process conditions and with location within the reactor. The experimentally-validated simulations have been extended to much lower N₂ input fractions and higher microwave powers than were probed experimentally, providing predictions for the gas-phase chemistry adjacent to the diamond surface and its variation across a wide range of conditions employed in practical diamond-growing CVD processes. The strongly bound N₂ molecule is very resistant to dissociation at the input MW powers and pressures prevailing in typical diamond CVD reactors, but its chemical reactivity is boosted through energy pooling in its lowest-lying (metastable) triplet state and subsequent reactions with H atoms. For a CH₄ input mole fraction of 4%, with N₂ present at 1–6000 ppm, at pressure p = 150 Torr and with applied microwave power P = 1.5 kW, the near-substrate gas-phase N atom concentration, [N]_ns, scales linearly with the N₂ input mole fraction and exceeds the concentrations [NH]_ns, [NH₂]_ns, and [CN]_ns of other reactive nitrogen-containing species by up to an order of magnitude. The ratio [N]_ns/[CH₃]_ns scales proportionally with (but is 10²–10³ times smaller than) the ratio of the N₂ to CH₄ input mole fractions for the given values of p and P, but [N]_ns/[CN]_ns decreases (and thus the potential importance of CN in contributing to N-doped diamond growth increases) as p and P increase. Possible insights regarding the well-documented effects of trace N₂ additions on the growth rates and morphologies of diamond films formed by CVD using MW-activated CH₄/H₂ gas mixtures are briefly considered.