A model was proposed in order to explain irradiation-induced amorphization. The model assumed that the single-vacancy concentration saturated at the irradiation temperature, that the single-vacancy concentration contained regions which gradually transformed to disordered regions whose accumulation led to amorphization, and that the transformation rate was proportional to the single-vacancy concentration. By using the saturation level as a fitting parameter, this model could explain the dose- and temperature-dependences of the Raman spectra of 25keV He+-bombarded highly-oriented pyrolytic graphite. Moreover, Arrhenius plots of the saturation values (which indicated the critical doses for amorphization) corresponded well with the results of a previous transmission electron microscopic study which had revealed activation energies of 0.036eV below about 573K and 0.25eV above about 573K. By analyzing the chemical kinetics of the steady-state, activation energies of 0.14 and 0.86eV, respectively, were obtained for single and di-interstitial migration. The stored energy was attributed to a significant accumulation of di-interstitials, below about 573K, which originated in a reduced annihilation with vacancies, due to the presence of a barrier. By extending the theory to a quasi-steady state, with collapsed line formation and loop growth, dimensional changes before amorphization could also be explained. This model was expected to provide insight into the graphitization process and the formation mechanism of various C clusters.

K.Niwase: Physical Review B, 1995, 52[22], 15785-98