On the mechanism of H atom production in hot filament activated H{sub 2} and CH{sub 4}/H{sub 2} 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{sub 2}, and H{sub 2}, and in CH{sub 4}/H{sub 2} gas mixtures of relevance to diamond growth by HF chemical vapor deposition, as functions of filament temperature T{sub fil} (in the range of 1800-2700 K) and gas pressure p (in the range of 10{sup -2}-100 Torr). In the cases of H{sub 2} and the CH{sub 4}/H{sub 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{approx}10-20 Torr and thereafter to remain constant or, at the highest T{sub 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{sub 2} to a two-step mechanism involving H atom formation by dissociative adsorption at bare (S{sup *}) sites and by desorption at hydrogenated (SH) sites on the HF surface.