Diamond growth using carbon monoxide as a carbon source
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Polycrystalline diamond films were produced in a microwave plasma assisted CVD reactor using CO as the carbon source gas. Reactor exhaust gas compositions were determined by mass spectrometry using 2-10% CO and 0-1.5% O2 in H2 feed gas mixtures. The chemistry involved in the gas phase is similar to that which occurs when diamond is grown using hydrocarbons as carbon source gases. A chemical mechanism for the oxidation of CH4 in flames appears to be applicable to this system. Addition of O2 to the reactor feed gas results in increased growth rates for low addition levels possibly due to activation of the diamond surface, while lower growth rates result at high addition levels due to oxidation of carbon from the surface and depletion of diamond growth precursors in the gas phase. The chemical reactions that take place in the plasma are similar to those that occur in flames and hot filament reactors, indicating that the plasma acts to induce reactions that are normally associated with high temperature combustion processes. I. INTRODUCTION Diamond films have been synthesized by chemical vapor deposition (CVD) using a number of different techniques, including hot filament assisted CVD, plasma assisted CVD, acetylene flame CVD, and DC plasma jet CVD.1"3 Usually, diamond is grown from gas mixtures containing a small amount of hydrocarbon (0.1-3%) in hydrogen. Beneficial effects of introducing oxygen into the reactor feed gas have been reported by several groups4"14 and include improved film quality, lower substrate deposition temperatures, enhanced H atom concentrations in the gas phase, and enhanced etching of non-diamond phases during deposition. The most common method of incorporating oxygen into the reactor system is addition of O2 directly to a hydrocarbon/ hydrogen feed gas. Several research groups have demonstrated the feasibility of synthesizing diamond by CVD using organic compounds containing oxygen in the feed.15"17 A number of groups have synthesized diamond using CO as the carbon source gas.18"23 It has been suggested that diamond deposition from CO occurs via a chemical reaction mechanism that is unlike that occurring when a hydrocarbon is used as the carbon source gas. 18 Many details concerning the exact nature of the chemistry occurring in the gas phase and at the gas/ surface interface in diamond deposition remain unknown at this time. However, once the chemistry of diamond deposition is known, CVD systems may be optimized based on first principles, and fundamental limitations on important material parameters such as growth rate and crystal quality may be identified. In systems using "'Present address: Crystallume, Menlo Park, California 94025.
CH4 as the hydrocarbon source gas, CH 3 may be the primary growth precursor for diamond in CVD, although C2H2 is present at high enough concentrations to account for observed growth rates.24~35 CO is a major reaction product in systems when O2 is used in the reactor feed gas5'14'36; however, there appears to be no correlation between increases in CO production an
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