Modeling of Thin-Film Diamond Growth
L. A. Curtiss, D. Gruen, D. Horner, and P. Redfern, Argonne National Laboratory
Research Objectives
This project involves a fundamental theoretical study of diamond thin-film growth from fullerene precursors to understand why nano-crystalline diamond results from growth in plasmas containing little hydrogen. At Argonne National Laboratory, we have grown extremely smooth nano-crystalline diamond films, with crystallite sizes in the range 3-10 nm, in experiments involving chemical vapor deposition following fragmentation of fullerene in a microwave discharge.
Computational Approach
Ab initio molecular orbital and density functional theory is being used to investigate reaction mechanisms involved in thin-film growth on (110) and (100) diamond surfaces, with the carbon dimer as the growth species. The diamond surface is modeled using clusters of carbon atoms. The reaction energies and barriers are calculated at various levels of theory. At the highest level is G2 theory, which is used for model molecule reactions. Density functional theory is used for the large cluster models.
Accomplishments
Density functional calculations of growth on the (110) diamond surface using carbon dimer C2 as the growth species found that the various steps were energetically very favorable, without requiring the participation of atomic hydrogen and with small activation barriers. The C2 is found to insert into the CH bonds in this mechanism. Similar calculations on the (100) surface have used clusters that model the unhydrided and monohydride surfaces, respectively. The results indicate that on the monohydride surface, the C2 inserts into the CH bond and growth is similar to that for the (100) surface. In contrast, for the bare (100) surface, the C2 insertion is found to occur into the CC double bonds with no barrier. This has important implications for diamond growth.
Significance
The results of these reaction-mechanism studies indicate that lack of hydrogen on the surface can lead to insertion into a CC double bond and a possible new nucleation site.
Figure showing insertion of C2 into a bare (100) diamond surface that leads to a nucleation site for a new diamond crystallite.
The disparate nucleation rates of diamond crystallites grown in hydrogen-rich vs. hydrogen-poor microwave plasmas (leading to nano-crystalline diamond in the latter case) are accounted for qualitatively by these results.
Publications
D. A. Horner, L. A. Curtiss, and D. M. Gruen, "A theoretical study of the energetics of insertion of dicarbon (C2) and vinylidene into methane C-H bonds," Chemical Physics Letters 233, 243 (1995).
P. Redfern, D. A. Horner, L. A. Curtiss, and D. M. Gruen, "Theoretical studies of growth of diamond (110) from dicarbon," Journal of Physical Chemistry 100, 11654 (1996).
D. M. Gruen, L. A. Curtiss, P. C. Redfern, and L. C. Lin, "Nucleation of nanocrystalline diamond by fragmentation of fullerene precursors," Proceedings of the Symposium on Fullerenes: Chemistry, Physics, and New Directions XI: 193rd Meeting of the Electrochemical Society (in press, 1998).