ay the word diamond to most people, and they think of things like engagement rings, the Crown Jewels, and certain James Bond movies. However, diamond has a less glamorous side as well. The properties
of diamond make it useful in a wide variety of applications and potential applications that range from coatings to cutting tools to microelectronic devices. Natural diamonds are not suitable for many
of these applications like those that employ diamond as well-adhering, high surface area films. For many years, synthetic diamonds could be created only in very high-pressure reactors, under conditions
like those found deep under the surface of the Earth where natural diamonds are born. The problem is that high-pressure methods are expensive, cumbersome, and not amenable to the production of high
surface area diamond films.
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| An illustration of the height of carbon atoms that result from simulations of diamond film growth. This figure is from simulations at conditions that favor layered growth of the film. |
Over the past several years, low-pressure chemical vapor deposition (CVD) has emerged as a potential alternative route to synthetic diamond. The chief advantages of CVD-based methods are low cost and
flexibility with respect to the form of diamond that is deposited. A major hurdle to the development of a commercially viable diamond CVD process is that it has proved difficult to control diamond film
microstructure. Microstructure‹the shape and size distributions of the crystallites in a polycrystalline film‹is important because it has a profound influence on the properties and therefore the
usefulness of synthetic diamond.
Researchers in the Chemistry Department at the University of Minnesota have developed a new approach for investigating how microstructure develops during diamond CVD. Using computer resources provided
by the Supercomputing Institute, graduate student Ron Brown has come up with a way to simulate diamond growth under conditions like those in a diamond CVD reactor. This work was done under the direction
of Associate Professor Jeffrey Roberts and Fellow of the Supercomputing Institute Christopher Cramer. What¹s new about Ron¹s work is its focus on microstructure development rather than the overall
growth rate and in the way it combines high-level electronic structure calculations,
kinetic modeling, and Monte Carlo simulation methods.
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| An illustration of the height of carbon atoms that result from simulations of diamond film growth. This figure is from a simulation that leads to a rougher appearance. |
The starting point of this work is a scheme of 69 different chemical reactions,
each of which accounts for one of the important steps in the conversion of the
gas-phase methyl radical (CH3) to solid
diamond. A unique feature of the kinetic scheme is its inclusion of reactions that allow for the migration of carbon-containing fragments across a growing diamond surface. The rate parameters for many
of these reactions were obtained from literature sources, but those of others were obtained using high-level quantum chemical calculations. Once the reaction rates were determined for the CVD
conditions of interest, they were used to simulate the accumulation of diamond on a 30 x 30 array of carbon atoms. The figures shown here summarize the results of two such simulations. In these figures,
which are called false-color topographic maps, the color scale is defined so that the lighter and darker shades indicate positions on the array where more and less carbon has accumulated, respectively.
One of the maps shows a much less even color distribution than the other, due to the formation of "pits" (the dark regions) and "peaks" (the light regions) during diamond deposition. It turns out that
the appearance of pits and peaks correlates with the formation of certain types
of crystallite surface structure, and thus with the development of
microstructure.
The immediate goal of the Chemistry Department team is to determine why certain sets of CVD conditions favor the development of one type of microstructure over another. To that end, they already have
shown that surface migration reactions play a critical role in the formation of pits and peaks in a simulated diamond film. Future simulations will explore a wider range of growth conditions so different
types of CVD reactors may be compared directly. Also, the simulation scheme will be refined to encompass a wider variety of diamond growth mechanisms. |
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