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

We describe laser and mass spectroscopic methods, and related modelling stu dies, that have been used to unravel details of the gas phase chemistry inv olved in diamond chemical vapour deposition (CVD) using both H/C (i.e. hydr ocarbon/H-2) and H/C/O (e.g. CO2/CH4) gas mixtures, and comment on the rela tive advantages and limitations of the various approaches. In the case of t he more extensively studied hydrocarbon/H-2 systems we pay particular empha sis to investigations (both experimental, and 2- and 3-dimensional modellin g) of transient species like H atoms and CH3 radicals, their spatial distri butions within the reactor and the ways in which these distributions vary w ith process conditions, and the insight provided by such investigations int o the chemistry underpinning the diamond CVD process. These analyses serve to highlight the rapid thermochemical cycling amongst the various hydrocarb on species in the reactor, such that the gas phase composition in the vicin ity of the growing diamond surface is essentially independent of the partic ular hydrocarbon source gas used. Such applies even to the case of hot fila ment activated C2H2/H-2 gas mixtures, for which we show that CH, radical fo rmation (hitherto often presumed to involve heterogeneous hydrogenation ste ps) can be fully explained in terms of gas phase chemistry. Diamond growth using H/C/O-containing gas mixtures has traditionally been discussed in ter ms 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., 19 91, 1, 1). Detailed studies of microwave activated CO2/CH4 gas mixtures, ac companied by simpler zero-dimensional thermochemical modelling of this and numerous other H/C/O-containing input gas mixtures, provide a consistent ra tionale for the 'no growth', 'diamond growth' and 'non-diamond growth' regi ons within the H-C-O atomic phase composition diagram.