Boron doping: B/H/C/O gas-phase chemistry; H atom density dependences on pressure and wire temperature; puzzles regarding the gas-surface mechanism

Experimental and modeling studies of the gas-phase chemistry occurring in dilute, hot filament (HF) activated B2H6/CH4/H2 gas mixtures appropriate for growth of boron-doped diamond are reported. The results of two-dimensional modeling of heat and mass transfer processes and the B/H/C chemistry prevailing in such HF activated gas mixtures (supplemented by reactions involving trace O2 present as air impurity in the process gas mixture) are discussed and compared with measurements of B atom densities as functions of the hot wire temperature Tw and distance from the wire. Most of the B2H6 molecules that diffuse from the cool, near-wall regions into the hot, near wire region are thermally decomposed (yielding two BH3 molecules as primary products) and then converted into various ‘active’ B-containing species like B, BH and BH2 — some of which are able to accommodate into the growing diamond film. H-shifting reactions BHx + H ↔ BHx − 1 + H2 enable rapid inter-conversion between the various BHx (x = 0–3) species and the BHx source is limited by diffusional transfer of B2H6. H atoms play several key roles — e.g. activating the process gas mixture, and driving inter-conversions between the various HxByCzOz′ species. We show that the Tw and gas pressure dependences of the H atom production rate (by H2 dissociation on the HF surface) can be accommodated by a simple gas-surface reaction model.