Positron annihilation in neutron-irradiatedp-type Ge

Effects of neutron irradiation and subsequent thermal annealings on positron annihilation rates in p-type germanium have been studied. Two annihilation rates (4.17 ± 0.08 ± 109 sec−1 with an intensity of 98% and 7.1 ± 1.0 × 109 sec−1 with an intensity of 2%) are observed in unirradiated samples. Only the annihilation rate of the dominent component becomes smaller upon irradiation, showing that some neutron-induced defects in Ge can act as traps for positrons. The effect saturates at high neutron fluence around 7 × 105 n/cm2. The annihilation rate in saturated samples decreases further upon annealing around 200°C. The mean electron densities of the positron-sensitive defect after irradiation and after the 200°C annealing are 35% and 49% less than that in the perfect part of the crystal respectively. It is concluded that the non-planar vacancy clusters may be formed during the 200°C annealing. All positron sensitive defects disappear upon annealing below and around 350°C.