Rate limitation in low pressure diamond growth
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We performed diamond deposition experiments from a gas phase containing H 2 , CH 4 , and sometimes CO, using a microwave plasma ball reactor operating at 400 mbar pressure. The molybdenum substrates were stamped with a suitable tool to form a number of flattened cones on its surface. A strong preference for crystal growth on top of the cones was observed. Numerical calculations were used to solve the underlying thermal conduction and diffusion problems. At the substrate, the flow of the active species entering by diffusion from the bulk of the gas phase was balanced by those leaving the system due to incorporation in the crystals. Comparison with the experiments showed that at least 10% of the active species striking the surface are incorporated. Thus, the limitation of diamond growth in our investigation lies in gas phase transport and not in incorporation difficulties at the growing surface.
I. INTRODUCTION Diamond is an interesting material due to its unique combination of mechanical, electrical, and optical properties. Its hardness and low friction coefficient are advantageous for mechanical tools, its high thermal conductivity makes it the best material available for heat sinks, its high carrier mobility and large band gap favor it as a material for electronic devices, and its optical properties, low absorption over a wide range of wavelengths, and luminescence in the blue region of the visible light spectrum are of interest for optical and optoelectronic applications. This is only a partial listing of the numerous promising properties of this material.1 As a high pressure phase, diamond is thermodynamically unstable under ambient conditions with respect to graphite,1'2 but it has been well established that it can be grown metastably under low pressure conditions by a large number of experimental methods. These methods will not be reviewed here in detail (e.g., heated filament,3 microwave CVD,4 RF plasma,5 DC jet,6 and welding torch7). Surprisingly, the linear growth rate of diamond varies over a range of about four orders of magnitude (from some tenths of a /xm to 930 /im/h 6 ). The gas phases from which growth occurs normally consist of hydrogen and a hydrocarbon (CH4, C 2 H 2 ), sometimes with additions of oxygen or a rare gas. The gas composition seems not to be rate determining. On the other hand, high rate depositions appear to be possible only at the high pressure side of all of these methods (ranging from about 1 to 1000 mbar) and that the activation of the gas phase by energy input should be high.8 There is much discussion in the literature regarding the mechanism of diamond growth from the gas phase. The nature of the gas species (molecule or radical) that acts as precursor for incorporation in the growing crys934
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 4, Apr 1992
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tal, especially, is still unclear. Two candidates are CH3 (Ref. 9) and C2H2 (or C 2 H x ), 10 ' n although under certain conditions even CH4 seems to be the involved species.12 A number of inv
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