POSITRON-ANNIHILATION INVESTIGATION OF VACANCY AGGLOMERATION IN ELECTRON-IRRADIATED FLOAT-ZONE SILICON

Positron-annihilation experiments which combine lifetime and Doppler b roadening measurements have been done on 10-MeV electron-irradiated fl oat-zone silicon. After irradiation, a lifetime of 305 ps is observed at 300 K, decreasing to 290 ps at 30 K, and the positron trapping rate decreases strongly with increasing temperature. The Doppler measureme nts yield, when coupled with lifetime data, a defect S value 6.7% larg er than that for the bulk which is nearly twice the value hitherto cla imed for divacancies. Isochronal annealing of the 1.8-mu m infrared ab sorption band is accompanied by a significant change in the defect S v alue to 3.8% larger than for the bulk. Surprisingly, the trapping rate at 50 K decreases only by 20% during the annealing out of the 1.8-mu m infrared absorption, and the positron lifetime stays essentially con stant. Loose vacancy complexes (a ''sponge'' defect) consisting of dis cernible monovacancies are suggested to be formed upon annealing as an intermediate step in the clustering of vacancies.