VACANCY PRODUCTION BY 3 MEV ELECTRON-IRRADIATION IN 6H-SIC STUDIED BYPOSITRON LIFETIME SPECTROSCOPY

The vacancy production in 6H-SiC by 3 MeV electron irradiation at room temperature was studied using positron lifetime spectroscopy combined with annealing experiments. It was found that the trapping rates of p ositrons in vacancies increased linearly with the fluence in the initi al stage of irradiation. After the linear increase, the trapping rates were found to be proportional to the square root of the fluence. The linear and nonlinear fluence dependences of the trapping rates are exp lained by the reduction of vacancies due to recombination with interst itials during irradiation. The positron trapping rate for the admixtur e of silicon vacancies and divacancies showed a tendency to saturate i n the higher fluence range. The trapping rate for carbon vacancies dec reased after reaching a maximum. These results are explained in terms of the shift of the Fermi level due to the irradiation process. It was found that, for the lightly irradiated specimen, an annealing stage c aused by recombination between close vacancies and interstitials was o bserved. However, such an annealing stage was not observed when using a heavily irradiated specimen. These different results are explained a s the reduction of interstitials due to the recombination with vacanci es and long-range migration of interstitials to sinks during irradiati on. (C) 1997 American Institute of Physics.