Defect Structure and Amorphization of Graphite Irradiated by D+ and He+

The defect structure and amorphization of 4.0 fJ (25 keV) D+ and He+ irradiated highly-oriented pyrolytic graphite have been systematically investigated in terms of irradiation dose and temperature. The graphite irradiated by either D+ or He+ is amorphized at a different critical dose, but the calculated critical dpa is the same in both cases, indicating no dominant chemical effect of the implanted particles. The damage structures appeared on the irradiated surface are characterized into the following three types; (a) lenticular openings, originating from gas accumulation in between the basal planes, (b) twins owing to the stress due to the depth dependent elongation, and (c) bubbles and blisters appeared after the amorphization. Raman spectroscopy distinguished three linear relationships between the peakwidth of 1580 cm−1 and the peak intensity ratio of I1355/I1580. These relations respectively represent (1) accumulation of defects in the basal plane, (2) turbulence and disordering of the basal planes, and (3) amorphization accompanying relaxation of accumulated stress. The evolution of the defect structure and the mechanism of the amorphization are discussed in terms of the change of bonding nature and the stability of defect structures.