Diamond growth experiments were carried out by a microwave plasma assisted CVD technique in various gas mixtures of CH4(0–100%)/02/H2. The phase diagram obtained by this study shows that a diamond growth region exists. With addition of more than 5% O2 in reactant gases, diamond particles could be included in amorphous or graphitic carbon films even using CH4/O2 gas mixtures. Faceted diamond films were obtained if the oxygen gas concentration [O2] was approximately more than half the methane gas concentration [CH4] ([O2]>[CH4]/2). However, no films were grown when [O2] exceeded half of [CH4] plus 7% ([O2]>[CH4]/2+7%). These results corresponded to the observations by plasma emission spectroscopy. Though oxygen etches carbon films and decomposes methane by forming carbon monoxide, oxygen rarely reacts with hydrogen in a film growth region.
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