The Role Of Hydrogen Atoms

In 1968, Henry Hibshman suggested that hydrogen may be used to stabilize diamond growth during the pyrolysis of carbonaceous gases. In 1971, John Angus began to duplicate Eversole's process using a hot filament process. This time he tried adding hydrogen gas to speed up the diamond growth. In 1976, Derjaquin starting experimenting the CVD growth of diamond by glow discharge. Again, he found that hydrogen addition was beneficial to his process. 1n 1981, Setaka first introduced microwave plasma for diamond growth using a high proportion of hydrogen gas. In 1988, Hirose found that acetylene flame could grow diamond film in air. This was the first time that vacuum chamber was not used to grow CVD diamond. 1n 1988, Norton acquired Technion's arc jet technology and began to depositing thick diamond films (the author was responsible for this acquisition).

The modern CVD processes for growing diamond films are dependent on using hydrogen atom as catalyst. Hydrogen atoms can be obtained by dissociation of hydrogen gas at high temperature. Because the dissociation efficiency is low, most CVD processes rely on diluting methane gas (or other carbonaceous gas) by adding an overwhelming proportion of hydrogen gas (e.g., 99%).

The roles of hydrogen atoms are two folds. First, they can maintain the four valence electrons of carbon atoms in pseudo sp3 configurations so when they join together, they will form tetrahe-dral bonds of diamond. Second, they can gasify graphite to form methane if graphite is formed. In other words, hydrogen atoms can promote diamond formation and at the same time they may prevent graphite from being formed (Fig. 11.16).

The ability for hydrogen atoms to catalyze the diamond formation lies in the same principal of "touch and go" as described above for the case of high-pressure synthesis. Just as carbon atoms must loosely attach to molten metal atoms so they can freely regroup to form diamond, so are carbons atoms must loosely attach to hydrogen atoms so they can readily link to deposit diamond. In either case, carbon atoms can maintain their sp3 configuration and attach easily to the growing diamond because of the constant arouse of metal solvent atoms (e.g. high pressure synthesis) or hydrogen solvent atoms (CVD growth).

Hydrogen is more effective to catalyze carbon to form diamond than molten transition metals. Hence, the threshold temperature of diamond growth for the CVD process is lower than that for the high-pressure method. For example, CVD diamond is typically deposited at a temperature of about 900°C, whereas high-pressure diamond is synthesized around 1300°C. This difference in synthesis temperatures reflects the difference in activation energies. The

Figure 11.16. The carbon atoms formed by decomposing sp3 configured methane may be coerced to maintain such configuration if abundant hydrogen atoms can be nearby to act like touch-and-go. When sp3 configured carbon atoms encounter one another, they will become part of diamond lattice. On the other hand, if graphite is formed, then hydrogen atoms can gasify it by combined graphite atoms to form methane again.

Figure 11.16. The carbon atoms formed by decomposing sp3 configured methane may be coerced to maintain such configuration if abundant hydrogen atoms can be nearby to act like touch-and-go. When sp3 configured carbon atoms encounter one another, they will become part of diamond lattice. On the other hand, if graphite is formed, then hydrogen atoms can gasify it by combined graphite atoms to form methane again.

activation energies for hydrogen-assisted CVD processes are about 0.09 eV; and for metal catalyzed processes, 0.13 eV.

In 1980s, Japanese invented several CVD processes that may grow metastable diamond at high deposition rates. Specifically, hydrogen molecules are decomposed by various means that heat reaction gases to high temperatures. The heating could be achieved by hot filament, microwave discharge, electric arc, or acetylene flame. It was found that the growth rate of diamond films is proportional to the square of the concentration of hydrogen atoms (Goodwin, 1993). As the concentration of hydrogen atoms tends to increase with increasing temperature of the gaseous phase, so is the growth rate of the diamond deposition and the quality of film formed.

During a CVD process, the concentration of hydrogen atoms determines the relative growth rate between diamond and graphite (Fig. 11.17).

Based on the above description, hydrogen atoms play a key role by keeping a dynamic balance between methane and decomposed carbon atoms. With the touch-and-go relationship, carbon

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