Anisotropy of ion-beam-induced self-diffusion in pyrolytic graphite

The ion-beam-induced self-diffusion in pyrolytic graphite (HPG) has been investigated. In order to look for anisotropic effects two target orientations have been chosen, one with the basal planes perpendicular to the surface (HPG-edge) and a second one with the planes parallel to it (HPG-base). Samples preimplanted with a ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ marker were bombarded with 20-keV ${\mathrm{D}}^{+}$ ions at different temperatures. The ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ depth profiles were analyzed by Rutherford backscattering (RBS) using 2.0- and 2.6-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ions. Strong effects of anisotropy are found for different target orientations despite the high damage levels. A simulation of the marker diffusion is presented which is based on radiation damage and the behavior of moving defects in graphite. The theoretical description is in good agreement with the experimental results if suitable rate constants of interstitial-vacancy recombination and vacancy-vacancy annihilation are chosen. The orientational effects are explained by an interstitial diffusion which proceeds mainly parallel to the graphite planes.