Early Nucleation of Diamond in a Combustion Flame
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EARLY NUCLEATION OF DIAMOND IN A COMBUSTION FLAME
Michael T. McClure, Jesko A. von Windheim, Jeffrey T. Glass, and John T. Prater* North Carolina State University, Raleigh, NC *Army Research Office, Research Triangle Park, NC
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ABSTRACT The early stages of a gas phase pretreatment for diamond nucleation in an oxy-acetylene flame were investigated. The pretreatment involved a low oxygen-to-acetylene ratio (Rf = 0.93) performed at 15 mm from the torch on a scratched Si substrate. The nature of the carbon species deposited was analyzed using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. The pretreatment process produced diamond crystals after 30 seconds and a complete film in the center of the deposition area after 180 seconds. Furthermore, the pretreatment process kept the initial Si0 2 layer less than the thickness generated by the control conditions, (Rf = 0.97, d = 10mm). The success of the pretreatment was attributed to changes in the flame structure and chemistry from the control conditions.
INTRODUCTION The invention of the combustion flame diamond synthesis technique by Hirose in 1988[1] generated considerable interest in the field of diamond research because of its low start up cost and high growth rate. However, a disadvantage is that the diamond growth has been limited to an annular ring when the flame direction is normal to the substrate.[2,3] To circumvent this problem, researchers have tilted the flame relative to the substrate[4] and/or used a low gas ratio pretreatment step[5,6] before operating under normal growth conditions. When the low 0 2 :C2 H2 ratio (Rf) pretreatment step was used, the researchers found an increase in the nucleation density leading to continuous film deposition in the center of the flame front.[5,6] One explanation given[5] for the success of the pretreatment involved the deposition of a diamond-like carbon layer, which was speculated to provide a higher concentration of hydrogen in the growth region and to be a highly defected structure providing a large number of nucleation sites. Alternatively, Windheim and Glass[6] suggested that the pretreatment step primarily suppresses oxide formation on the substrate surface and allows frozen equilibrium reactions to proceed. Because high nucleation density and film growth throughout the flame front are important for improved efficiency and future scale-up of the combustion flame technique, further investigation into the nature of the pretreatment process is desirable. Specifically, determining both the chemical nature of the pretreatment deposits, and the minimum time for complete coverage, are essential for understanding the process. Furthermore, investigation of the competing reactions of oxide formation and diamond nucleation should provide valuable insight into the diamond nucleation process.
EXPERIMENTAL The combustion flame system used for this research[6] was a simple welding torch with a size 2 tip. MKS mass flow controllers (MFC) were used to maintain a total gas flow of 4
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