Temperature dependence of the annihilation of positrons in Si containing divacancies and quadrivacancies

Positron-lifetime and Doppler-broadening measurements have been performed on Si samples containing divacancies and quadrivacancies in the temperature range 10 to 286 K. The trapping rate is found to increase with decreasing temperature and its behavior is described in terms of a cascade model of positron trapping. The positron lifetime in divacancies decreases from 320 ps at 286 K to 270 ps at temperatures less than 150 K. The lifetime in quadrivacancies is higher (430 ps) at 286 K but attains the same value as in divacancies at low temperatures. Both the lifetime and Doppler-broadening data suggest that the nature of the trapped state varies with temperature and evidence indicating partial dissociation of the divacancies is presented.