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 ca rburised tantalum (TaC) filament in the HFCVD of a diamond film. An abrupt filament temperature drop is observed which is associated with the transiti on of the filament surface from poisoned to clean. By measuring the power i ncrease needed to compensate for the temperature drop at a wide range of te mperatures and corresponding CH4 concentrations, and also considering the d ifference in radiative cooling of the filament, an energy balance is establ ished. From this energy balance the production rate for H atoms by catalyti c 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 interva l. The difference in these increase factors is interpreted as suggesting th at thermal dissociation of H-2 plays a substantial role for H-atom producti on along with catalytic dissociation. (C) 1998 Elsevier Science S.A. All ri ghts reserved.