Deposition Mechanism and Model

Diamond has been deposited from a large variety of precursors which include, besides methane, aliphatic and aromatic hydrocarbons, alcohols, ketones, and solid polymers such as polyethylene, polypropylene and polystyrene (a special case, deposition with halogens, is reviewed in Sec. 2.5). 

These substances generally decompose into two stable primary species the methyl radical (CH3) and acetylene (C2H2).t J The methyl radical is considered the dominant species and the key compound in generating the growth of CVD diamond, at least in the hot-filament deposition process (see Sec. 3.4).I K I Direct deposition from acetylene, although difficult experimentally, has been accomplished, with a marked increase in the crystallinity of the diamond deposit.f °i 

Deposition Modei. A two-step deposition model was recently presented by Frenklash and Spear.f In the first step, the diamond surface is activated by the removal of a surface-bonded hydrogen ion by atomic hydrogen as follows  

In the second step, the activated surfece-carbon radicai reacts with the carbon-hydrogen species (acetylene as a monomer uniQ in the gas phase to become the site for carbon addition  

The model is consistent with experimental observations and should provide a useful guideline forfuture experiments. A similar model has been proposed that is based on the addition of a methyl group to one of the carbons followed by atomic hydrogen abstractionfromthe methyl group.l iiis] 

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