Uncoupling crystal growth and nucleation in the deposition of diamond from the gas phase
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P. Ascarelli and S. Fontana Istituto di Metodologie Avanzate Inorganiche, Area delta Ricerca del CNR, C.P.10, 1-00016 Monterotondo Scalo, Italy (Received 5 June 1991; accepted 5 March 1992)
Diamond deposits of well-separated particles have been obtained by the hot filament CVD technique on Si(100) wafers. Particle counting in SEM images and determination of their linear dimensions require a separate study of growth rates and of nucleation densities as a function of time, substrate temperature (500 °C-950 °C), gas phase composition (0.5-2% CH4 in H 2 ), and total pressure (15-76 Torr). It is shown that recent models proposed for the growth process can successfully be applied if proper consideration is given to the high catalytic activity of the growing diamond surface for the heterogeneous recombination of gaseous H-atoms. This fast reaction controls the H-atom concentration at the surface and couples growth rates and nucleation densities via the gas phase.
I. INTRODUCTION When diamond is deposited from the gas phase under conditions that ensure that crystalline diamond is the major component of the deposit and that this deposit is made of well-separated particles, i.e., no continuous film is formed, it is possible by particle counting and measuring in properly magnified SEM images of the deposits, to determine both the density of nucleation N (particles cm"2) and the distribution of the linear dimensions d (cm) of the growing crystallites. Growth rates and nucleation densities can thus be analyzed independently as a function of the operational parameters of the deposition process in an attempt to uncouple them for separate examination. In general, current theories of diamond deposition from the gas phase refer to overall growth data (g cm"2) which involve the product pd3N, with p diamond's density, a much more complex variable to be handled, as will become apparent from the present work. Our analysis will show that d and N can be studied separately as a function of time, under variable conditions of temperature, pressure, and of composition of the gas phase. This analysis will also show that a simplified treatment based on the general scheme proposed by Frenklach and Spear1 and developed by Frenklach and Wang in more recent work2'3 is able to predict observed growth rates in the whole range of experimental conditions explored, provided proper consideration is given to the possibility of a high catalytic activity of the growing diamond surface for the heterogeneous recombination of 1778 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 7, Jul 1992 Downloaded: 13 Apr 2015
gaseous H atoms. Under present experimental conditions a high value of the recombination coefficient for H atoms, 7, entrains a concentration gradient of the H atoms between the bulk of the gas and the growing surface. The deposition process, which crucially depends on the concentration of the H atoms at the surface, becomes diffusion controlled not because deposition is fast but as a consequence of the very fast recombination reaction r
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