Stephen Redman - PhD Thesis
PhD Thesis, July 1999
A diamond-growing hot filament chemical vapour deposition reactor has been designed, built and shown to produce good quality diamond at rates typical of this process. Two gas phase species important to diamond growth, namely hydrogen atoms and methyl radicals, have been probed in this reactor using 2+1 REMPI spectroscopy. A detection scheme, which involves the collection of the ions produced in the 2+1 REMPI on a negatively-biased platinum probe wire, has been devised that allows spatial profiling of these key species. H atoms were detected using 243.1 nm radiation that is resonant at the 2-photon energy with the n=2, n=1 transition. The lineshapes so obtained are predominantly Doppler broadened, provided care is taken to avoid the various saturation effects that have been identified. Analysis of the profiles gives a measure of the local gas temperature and the relative H atom number density. CH3 radicals were detected using 333.4 nm radiation that is resonant at the 2-photon energy with the 3pz 2A2", X2A2" transition. Analysis of the origin band contour allowed determination of relative CH3 radical number densities (once certain temperature dependent factors have been corrected for) and rotational temperatures. The temperature so deduced agrees very well with the translational temperatures deduced from the H atom Doppler broadened profile under comparable conditions.
The variation of relative H atom number density and gas temperature were measured as a function of H2 pressure and flow rate, filament temperature and filament distance in pure H2 at a pressure of 20 Torr. These measurements suggested that in this reactor: (i) the dominant H atom formation mechanism is H2 bond fission on the hot filament surface with kinetics that are zero order with respect to H2 pressure; (ii) diffusion id the dominant transport mechanism; (iii) the key role of the filament is to provide an efficient means by which H2 molecules can attain the filament temperature; (iv) H atoms exist in super-equilibrium concentrations at all positions away from the filament. Both temperature and number density profiles were in good accord with other work. The heterogeneous recombination of H atoms on a diamond surface was detected and the measured profiles compared well with results of a simulation of this reactor. The similarity between the spatial profiles of CH3 radicals measured in CH4/H2 and C2H2/H2 gas mixtures suggests that CH3 radicals may be produced predominantly from C2H2 in the gas phase, possibly via a C2H3 intermediate.