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

Abstract

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 CH(4)/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 CH(4)/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 approximately 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.

What [plasma used for growing] diamond can shine like flame?

“…If a filament is used, it must be held in close proximity to the substrate and maintained at a temperature >2000°C, whereupon surface-catalysed hydrogen decomposition yields H atoms. 34 The most common filament materials are W, Ta, or Re because of their high melting points. Laser and mass spectroscopic measurements of key species (including H atoms, [35][36][37][38][39][40] and CH3 radicals 37,41,42 ) in HF-activated CH4/H2 gas mixtures, coupled with thermodynamic and gas-kinetic modelling, 27,43 have yielded a fairly complete picture of the prevailing gas-phase chemistry.…”

Quantification of the atomic hydrogen flux as a function of filament temperature and H2 flow rate

“…To quantify this loss, He was used instead of H 2 , because with the latter the heat transport effects due to the gas flow and atomic hydrogen recombination cannot be separated. This procedure is similar to that reported earlier [32][33][34][35]39 where the H 2 cracking experiments were conducted first with He to measure the convective cooling of the filament due to the flowing gas. Next, the measured effect was adapted to H 2 to account for the differences between the two gases.…”

“…The flux increases exponentially with the filament temperature, which is in agreement with the previous findings regarding H 2 cracking and recombination studies. 31,39,44 Such exponential dependence on filament temperature can be explained by the H 2 dissociation at the hot filament surface being a thermally activated process. 34,35,44,45 A promising result in view of application of atomic hydrogen for cleaning surfaces (e.g., optics) is that the radical flux increases proportionally with the H 2 flow rate through the gun.…”

Wire diameter dependence in the catalytic decomposition of H2

“…Their assumption of the thermal decomposition of H 2 in the gas phase is in contradiction to the results of more recent studies. 5,6,[19][20][21] It has been shown that the catalytic decomposition on wire surfaces is the dominant source of H atoms and gas-phase reactions play only a negligible role.…”