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Native defects

In contrast to silicon, for which many theoretical studies were done [6,7,8,9,10] concerning defect creation and their annealing, there are only very few attempts in theoretical simulations on diamond implantation. Specific simulations of damage in material induced by ion implantation can be divided into two categories: the most commonly used TRIM code [11,12] and the molecular dynamics technique. The TRIM code is a Monte Carlo program that calculates the trajectories of the atoms involved in the irradiation event, with few input parameters like the displacement energy, the kind of material and of implanted atoms involved in the process, and the initial kinetic energy. It provides good estimates of the damage distribution, especially at low temperature, and appears to be very useful in plaining the dopant profile in experimentation. However, it cannot account for dynamic annealing that may induce diamond to graphite transition or vacancy-interstitial recombination.

In the molecular dynamics technique (see below), the forces on each atom are derived from an interatomic classical or quantum potential model. The position and the velocity of each atom are then calculated from the Newton equation of motion, evaluated at each time step. A molecular dynamic study of a single carbon atom radiation damage in diamond was performed by W. Wu and S. Fahy [13], using the Tersoff potential [14]. They restricted their investigation on the damage threshold energy necessary to displace a single atom from its lattice site. Although only atomic displacement along axis of high symmetry (<100>, <110>, and <1111> axis) were investigated in this work, the threshold energy calculated may provide fruitful information on the probability for an atom to be disrupted from its lattice site, and therefore, enables an estimation of the density of defects ceated. R. Smith [15] simulated the bombardment of diamond with the same computational method limiting his analysis to the ejection mechanisms (carbon self-sputtering). H. P. Kaukonen et al [16] investigated the growth of diamond-like films by the deposition of energetic carbon atoms on a low temperature diamond substrate, using the molecular dynamics technique applied to the Tersoff potential [14].

In B. A. Pailthorpe work [17], the main goal was to study the behavior of the (111) surface during the collision of 1-100 eV carbon atoms, at low temperature. Molecular dynamics simulations were carried out, using a Stillinger-Weber potential [18] parameterized to sp3 bonding configurations only. S. Uhlmann etal [19] performed similar calculations, but used a density functional based tight-binding approach. The two last works focus more directly on surface and near-surface phenomena, with a very small number of defects involved.

During the MSc research of D. Saada [20], the formation of point defects in diamond under ion-impact and the relaxation of the disrupted crystal were studied by molecular dynamics simulations, with the Tersoff potential [21]. The <100>split-interstitial, which has a bonding configuration similar to graphite, was found to be the most stable defect, with a formation energy of 16.5 eV (section 6.2 in ref. [20]). The displacement energy for Frenkel pair creation, Ed, was found to be 52 eV (section 6.3 in ref. [20]), in agreement with experiment [22]. The structure of the disrupted diamond lattice was also studied (section 6.4 in ref. [20]). The damaged diamond for this study was created by knocking a bulk atom from its site with an initial kinetic energy EK>Ed, at 0 K. The defects created were therefore rapidly frozen in, the annealing effects being minimized by the low temperature process. It was found that the <100> split interstitial defect is consistently formed for a wide range of initial kinetic energies. Moreover, regions rich in sp2-like (graphitic) bonds were identified in the vicinity of the defect. However, till now, no theoretical study was undertaken to investigate the effects of ion implantation in diamond, that should account for crystal reconstruction induced by dynamic annealing, as done in silicon, for instance [6,7,8,9,10].


next up previous contents
Next: Dopants Up: Theoretical work Previous: Theoretical work
David Saada
2000-06-22