Theory of the charged cluster formation in the low pressure synthesis of diamond: Part I. Charge-induced nucleation
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Theory of the charged cluster formation in the low pressure synthesis of diamond: Part I. Charge-induced nucleation Hyun M. Janga) Department of Materials Science and Engineering, and Laboratory for Physics/Chemistry of Dielectric Materials, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
Nong M. Hwang Korea Research Institute of Standards and Science, P.O. Box 102, Daedok Science Town, Daejon 305-600, Republic of Korea (Received 11 August 1997; accepted 3 March 1998)
Based on several experimental observations, Hwang et al. recently proposed “the charged cluster model” [J. Cryst. Growth, 162, 55 –68 (1996)] to disentangle the “puzzling thermodynamic paradox” encountered in the gas-activated chemical vapor deposition (CVD) of diamond. Many unusual phenomena observed in the CVD diamond process can be successfully approached by the charged cluster model. However, there are a couple of important subjects still unsolved quantitatively. The first question is connected with the main driving force for this unusual nucleation in the gas phase. The second issue is related to the difference in the thermodynamic stability between graphite and diamond for a nanometer-sized cluster during the growth. In this study, we have theoretically examined the thermodynamic driving forces for the charge-induced nucleation, in general, and have applied this idea to the nucleation of the charged carbon-atom cluster. It was shown that the short-range ion-induced dipole interaction and the ion-solvation electrostatic effect (Born term) were mainly responsible for this unusual nucleation in the gas phase. The theoretical analysis presented in this article is quite generic and, thus, can be applied to any process that involves the charge-induced nucleation.
I. INTRODUCTION
Since the successful synthesis of diamond in the gasactivated chemical vapor deposition (CVD) process1–3 was reported, the process was intensively studied.4,5 The underlying principle, however, is not understood well, and there has been much debate as to the mechanism of the CVD diamond process.5,6 The most popular explanation is the atomic hydrogen hypothesis suggested by the pioneering Russian scientists.1,2 This hypothesis is based on the observation that the activated hydrogen etches graphite at rates orders of magnitude faster than diamond. Similar observations were reported by Angus and co-workers,7 Setaka,8 and Saito and co-workers.9 In addition, the hypothesis is supported by experimental observations that diamond can be synthesized with graphite as the only source of carbon, and that diamond is deposited with simultaneous etching of graphite.10,11 These well-confirmed experimental observations seem to be regarded as evidences of the atomic hydrogen hypothesis.
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Author to whom correspondence should be addressed. J. Mater. Res., Vol. 13, No. 12, Dec 1998
http://journals.cambridge.org
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However, this hypothesis has a critical handicap of contradicting th
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