The interaction of hydrogen atoms and molecules with diamond and silicon surfaces is important in several important applications. Two areas that we are interested in are: 1) tribology (molecular level friction) and 2) the role of H atoms in silicon chemical vapor deposition (CVD). In the tribology area, H atoms can be used to tie off dangling bonds, which otherwise form bonds between adjacent surfaces, and lead to resistance to sliding the surfaces by each other. Processes which are important in understanding molecular level friction include barriers to addition of H/H2 to the surface and barriers to migration of H atoms on the surface. In the silicon CVD area, we have studied the process of H2 elimination from the 100 surface of silicon.
Cluster models for the three surfaces of diamond are shown in Fig. 1. The unrelaxed 100 surface has carbene like surface carbon atoms; however, for the relaxed surface these dimerize to give rows of surface dimers and there is a significant amount of p bonding between the radical orbitals of the dimer. The 110 surface has zig-zag rows of carbon atoms with a dangling bond on each carbon atom. These dangling bonds are hybridized away from each other and thus interact less strongly than for the 100 surface. Finally, the 111 surface has surface C atoms arranged in a triangular pattern and the surface dangling bonds are well separated from each other (second nearest neighbor distance) leading to almost no interaction between adjacent dangling bonds. These qualitative features may be quantified by computing the overlap of adjacent dangling bonds in a GVB(pp) calculation. The overlaps are 0.462, 0.292, and 0.016 for the diamond 100, 110, and 111 surfaces, respectively. (Note that these are with a 6-31G basis set and all the overlaps would be increased for a larger basis set, but these results clearly indicate the trends.) As discussed elsewhere (S. P. Walch and R. C. Merkle, Nanotechnology, in press.) the different overlaps between adjacent radical orbitals result in different reactivity for the various surfaces. For example, in the reaction with a carbene, the diamond 111 surface behaves like a radical, but the diamond 100 surface behaves like a p bond. Based on the GVB(pp) overlaps, the 110 surface is expected to be somewhere in between the 100 and 111 surfaces in reactivity.
For the diamond surfaces, we have computed barriers for adding H and H2 to the surface and barriers to migration of H atoms on the surface. For addition of H2 we have looked at three cases. The first is addition of an H2 to a non dimerized C atom (carbene center) as on the diamond 100 surface. The second is addition of an H2 in a symmetrical (Woodward-Hoffman forbidden sense). The third is a dissociative recombination (or abstraction of an H from H2 by the surface). We have also looked at barriers to migration of H atoms on the surface. We have obtained energetics for a large number of equilibrium structures for various numbers of H atoms bonded to the various surfaces. From these calculations we obtain thermochemistry and barrier heights for the important equilibrium and rate processes involved in tribology of diamond.
Fig. 1. Cluster models for the 100, 110, and 111 surfaces of diamond.
Stephen P. Walch
ELORET, 690 W. Fremont Ave, Suite 8, Sunnyvale, CA 94087-4202