Morphological instabilities in the low pressure synthesis of diamond
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Morphological instabilities attending the high growth rate of diamond films are examined. Pertinent literature on morphological instabilities and microstructure evolution in vapor deposited films is reviewed and theoretical treatments related to the case of diamond growth are discussed. Diamond films of various thicknesses have been synthesized utilizing the combustion flame synthesis technique involving diamond growth rates of ~ 1 /xm/min. Films of thicknesses under 20 /jm are found to be dense and the surface smoothness of such films is governed by facets on the individual crystallites that make up the film. Increasing film thicknesses, at high growth rates, results in extremely rough surfaces, the trapping of voids and discontinuities, and the incorporation of non-diamond phases in the growing film. These characteristics are typical of morphological instabilities when surface diffusion and re-evaporation processes are absent and instability is promoted by the high rate arrival of the appropriate species from the flame ambient to the surface. Factors contributing to morphological instabilities include competitive shadowing and nutrient starvation and growth anisotropy of the different crystallographic faces on individual diamond crystals. It is shown that surface temperature and the presence of oxidizing species in the flame ambient contribute to anisotropic growth of diamond crystals and hence to morphological instabilities in diamond films. An approach to avoiding these instabilities is briefly discussed.
I. INTRODUCTION The synthesis of diamond at low pressures is a current topic of intense worldwide interest as a result of the superior physical properties of this material. The feasibility of synthesizing diamond at low pressures enables the exploitation of these properties in a large range of potential applications. Diamond crystals, films, and, increasingly, thick diamond slabs are being synthesized by a variety of low pressure techniques whereby mixtures of hydrocarbons, such as methane, and hydrogen are activated with approaches such as the use of microwave energy to generate plasmas, DC glow discharges, hot filaments, combustion flames, and arc jets. Low pressure diamond synthesis is made possible by the presence of high concentrations of atomic hydrogen in the ambient. Atomic hydrogen, in the plasma ambient, suppresses the formation of graphite as well as preferentially dissolves any non-diamond phases that might form during synthesis. Broadly speaking, there are two classes of diamond synthesis processes which differ in the rates at which diamond can be synthesized. The low growth rate techniques are the plasma CVD techniques employing microwave energy,1 DC glow discharge,2 and hot filaments.3 These approaches typically permit synthesis of diamond films at growth rates of up to a few microns per hour, with growth rates being dependent upon a num384 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 2, Feb 1992
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ber of variables such as the gas pressure, the plasma energ
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