Unravelling aspects of the gas phase chemistry involved in diamond chemical vapour deposition

We describe laser and mass spectroscopic methods, and related modelling studies, that have been used to unravel details of the gas phase chemistry involved in diamond chemical vapour deposition (CVD) using both H/C (i.e. hydrocarbon/H-2) and H/C/O (e.g. CO2/CH4) gas mixtures, and comment on the relative advantages and limitations of the various approaches. In the case of the more extensively studied hydrocarbon/H-2 systems we pay particular emphasis to investigations (both experimental, and 2- and 3-dimensional modelling) of transient species like H atoms and CH3 radicals, their spatial distributions within the reactor and the ways in which these distributions vary with process conditions, and the insight provided by such investigations into the chemistry underpinning the diamond CVD process. These analyses serve to highlight the rapid thermochemical cycling amongst the various hydrocarbon species in the reactor, such that the gas phase composition in the vicinity of the growing diamond surface is essentially independent of the particular hydrocarbon source gas used. Such applies even to the case of hot filament activated C2H2/H-2 gas mixtures, for which we show that CH, radical formation (hitherto often presumed to involve heterogeneous hydrogenation steps) can be fully explained in terms of gas phase chemistry. Diamond growth using H/C/O-containing gas mixtures has traditionally been discussed in terms of an empirically derived H-C-O atomic phase composition diagram (P. K. Bachmann, D. Leers, H. Lydtin and D. U. Wiechert, Diamond Relat. Mater., 1991, 1, 1). Detailed studies of microwave activated CO2/CH4 gas mixtures, accompanied by simpler zero-dimensional thermochemical modelling of this and numerous other H/C/O-containing input gas mixtures, provide a consistent rationale for the 'no growth', 'diamond growth' and 'non-diamond growth' regions within the H-C-O atomic phase composition diagram.