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Summary We applied the molecular dynamics technique (MD) to model the creation of damage in diamond at 0 K, and its subsequent annealing. The use of the Tersoff potential to describe the interatomic interaction was justified by calculating the graphitization of diamond at 2500 K, and then comparing our results with those obtained by ab initio MD on an identical sample. It was found that the conversion of fourfold to threefold coordinated atoms progressed into the slab concurrent with the graphitization of the surface layers, until complete graphitization of the whole sample was achieved. The fact that our results compare qualitatively very well with those of the ab initio MD demonstrates that the Tersoff potential is suitable for describing graphitization of diamond at high temperature.

We created the damage region in diamond by displacing carbon atoms of the bulk with an initial kinetic energy of 416 eV. By calculating the variation of the mean coordination number of the sample with time, we could evaluate the life-time of the thermal spike created by the bombardment. A value of 0.2 ps was found, in good agreement with other estimations. This life-time should determine the cooling rate of the quenching from the melt for the simulation of amorphous carbon formation.

The heavily damaged sample was obtained by the energetic displacement of twelve atoms toward the same region in the crystal. With an increasing number of successively displaced atoms, the damaged volume and the number of threefold coordinated atoms increased, due to bond breakage during the damage cascade. The radial distribution function revealed that the mean bond length is shortened by the bombardment events, toward the graphite bondlength, and that the shortest bonds are those between threefold coordinated atoms. However, bonds between fourfold coordinated atoms are lengthened. A very sharp peak appeared in the radial distribution function, centered at 2.1 Å. It has been attributed to the second nearest neighbors of threefold coordinated atoms.

Two distinct regions could be identified in the heavily damaged sample: the core of the damage, which contains clusters of threefold coordinated atoms, and its periphery, where isolated point defects are present. Under annealing at 3000 K for 20 ps, atoms in these two distinct regions behaved differently. The atoms in the heavily damaged core were sometimes found to diffuse several lattice sites while those of the periphery exhibited local relaxation only. In the core, the number of fourfold coordinated atoms increased with annealing time, due to local relaxations or to atomic diffusion followed by vacancy-interstitial recombinations, leading to a more diamond-like region. The mean bond length, in this region, is shifted toward the diamond bond length, as obtained from the radial distribution function. Furthermore, the sharp peak that appeared following the damage creation (centered at 2.1 Å), almost completely disappeared upon annealing.

In the periphery of the damage, the number of threefold coordinated atoms increased, and graphitic planes were created, preferentially oriented along the <111> directions. This behavior was not found for a lightly damaged diamond region, created by the energetic dislodgement of just one carbon atom, in which case the number of threefold coordinated atoms in the sample remained constant during the annealing. At a higher temperature it was found that 70 % of the threefold coordinated atoms created by the bombardment, transformed back to fourfold coordinated atoms. Thus, the low density of defects, together with the high pressure applied by the diamond matrix, prevented the damage structure from evolving toward a more graphitic geometry, and favored the recovery of sp3 bonds when annealing is applied.

The tight binding model is used to show that neutral vacancies and <100> split-interstitials in diamond are a source of n-type defects. These defects could therefore contribute to compensation in doped diamond, and may be responsible for the difficulties accounted in the doping process.

A new stable site for a hydrogen interstitial in diamond was found, with an energy lower than that of H at the bond-centered site. The site is called the ET (equilateral triangle) site. Hydrogen at this new site was found to create a dangling bond that induces a mid-gap state. This H related configuration is consistent with the results of EPR (electron paramagnetic resonance) of hydrogen containing CVD (chemical vapor deposition) diamond. We associate this site with the experimentally found anomalous muonium, Mu$^\star$, in diamond. The motion of hydrogen in diamond was found to involve bond rearrangement, and can be described as a coupled-barrier diffusion. The complex composed of the two H atoms at adjacent ET sites was found to be more stable than complexes proposed previously, and when formed, it greatly reduce the possibility of diffusion of H in diamond.


next up previous contents
Next: Bibliography Up: No Title Previous: Motion of hydrogen in
David Saada
2000-06-22