On the mechanism of H atom production in hot filament activated H₂ and CH₄/H₂ gas mixtures

This article reports systematic measurements of the power utilization by Ta (and Re) hot filaments (HFs) operating in a poor vacuum, in pure He, N-2, and H-2, and in CH4/H-2 gas mixtures of relevance to diamond growth by HF chemical vapor deposition, as functions of filament temperature T-fil (in the range of 1800-2700 K) and gas pressure p (in the range of 10(-2)-100 Torr). In the cases of H-2 and the CH4/H-2 gas mixtures, the power consumption studies are complemented by in situ measurements of the relative H atom densities [H] near the HF-which are seen to maximize at p similar to 10-20 Torr and thereafter to remain constant or, at the highest T-fil, to decline at higher p. These (and many previous) findings are rationalized by a companion theoretical analysis, which reduces the complex array of chemisorption and desorption processes that must contribute to the HF-surface mediated dissociation of H-2 to a two-step mechanism involving H atom formation by dissociative adsorption at bare (S-*) sites and by desorption at hydrogenated (SH) sites on the HF surface.