Investigations of the gas phase chemistry in a hot filament CVD reactor operating with CH4/N2/H2 and CH4/NH3/H2 gas mixtures

We report a combination of experimental and theoretical studies of hot filament (HF) surface effects and the gas-phase chemistry prevailing in HF activated CH4/N-2/H-2 and CH4/NH3/H-2 gas mixtures which provide some rationale for the observed low nitrogen doping levels in diamond films grown from such gas mixtures. The experimental studies involve use of resonance enhanced multiphoton ionisation (REMPI) to monitor relative H atom and CH3 radical number densities in a HFCVD reactor as a function of filament temperature and N-2/CH4 and NH3/CH4 gas mixing ratios. With NH3, contrary to N-2, clear depletion of both H atom and CH3 radical number densities are observed. The experimental observations are successfully reproduced using a previously developed 3-D model of HFCVD reactors. Three-dimensional (3-D) model calculations for C/H/N gas mixtures show significant N atom production due to successive 'H-shifting' reactions NHx + H double left right arrow NHx-1 + H-2 (x=1-3). N atom densities reach 5 x 10(13) cm(-3); their reaction with CH3 radicals accounts for the observed depletion of the latter and results in eventual production of HCN. (C) 2002 Elsevier Science B.V. All lights reserved.