The production of vacancies, via the 3MeV electron irradiation of 6H-type material at room temperature, was studied by using positron lifetime spectroscopy; combined with annealing experiments. It was found that the trapping rates of positrons in the vacancies increased linearly with the fluence in the initial stages of irradiation. Following this linear increase, the trapping rates were found to be proportional to the square root of the fluence. The linear and non-linear fluence dependences of the trapping rate were explained in terms of a reduction in the number of vacancies, due to recombination with interstitials during irradiation. The positron trapping rate for an admixture of Si vacancies and divacancies exhibited a tendency to saturate in the higher fluence range. The trapping rate of C vacancies decreased after reaching a maximum. These results were explained in terms of a shift in the Fermi level, due to irradiation. It was found that, in lightly irradiated specimens, an annealing stage was observed which was caused by recombination between close vacancies and interstitials. Such an annealing stage was not observed in heavily irradiated specimens. The differing results were explained in terms of a reduction in the number of interstitials, due to recombination with vacancies and the long-range migration of interstitials to sinks during irradiation.

A.Kawasuso, H.Itoh, T.Ohshima, K.Abe, S.Okada: Journal of Applied Physics, 1997, 82[7], 3232-8