Catalytic dissociation of hydrogen on a tantalum carbide filament in the HFCVD of diamond

New experimental results are presented which describe the behaviour of a carburised tantalum (TaC) filament in the HFCVD of a diamond film. An abrupt filament temperature drop is observed which is associated with the transition of the filament surface from poisoned to clean. By measuring the power increase needed to compensate for the temperature drop at a wide range of temperatures and corresponding CH4 concentrations, and also considering the difference in radiative cooling of the filament, an energy balance is established. From this energy balance the production rate for H atoms by catalytic dissociation is obtained. It is found to increase from a level of about 3.8 x 10(20) cm(-2) s(-1) at 2200 degrees C to about 1.6 x 10(21) cm(-2) s(-1) at 3000 degrees C. This increase, by a factor of 4.5, is somewhat lower than the increase in growth rate observed over the same temperature interval. The difference in these increase factors is interpreted as suggesting that thermal dissociation of H-2 plays a substantial role for H-atom production along with catalytic dissociation. (C) 1998 Elsevier Science S.A. All rights reserved.