Carrier Removal Effects in Neutron-Irradiated Lithium-Doped Silicon

Abstract

Carrier removal effects in n-type silicon as a result of neutron irradiation and subsequent heat treatment have been monitored by Hall effect and conductivity measurements at a temperature of 275°K. Samples of float-zoned, crucible, and Lopex grown (ρ=100 to 0.3Ω-cm) phosphorus-doped silicon and float-zoned and Lopex grown (ρ= 30 to 0.3Ω-cm) lithium-doped silicon were irradiated at ambient temperature with ~5-MeV neutrons and, subsequently, heat-treated between 20 and 200°C. The carrier removal rates for the lithium-doped samples, 7 to 75 cm-1 were significantly higher than the removal rates observed in the conventionally-doped samples, 6 to 12 cm-1. In addition, the lithium-doped material exhibited a strong dependence of removal rate on dopant concentration; whereas, the data for the conventional samples were relatively independent of dopant level. Upon subsequent heat-treatment, the lithium-doped samples showed continued carrier removal, while the phosphorus-doped samples tended to recover to a preirradiation condition. The enhanced carrier removal in the lithiumdoped material was found to be consistent with the removal of positively-charged lithium donors from a state of electrical activity by ion-drift to the negatively-charged defect cluster. Relevant parameters yielded by this model are: a defect cluster radius of 300A, a cluster charge of -150e, and an effective capture radius for the lithiumdrift of 2500Å. The continued carrier removal in the heattreated, lithium-doped silicon follows kinetics expected for the precipitation of lithium at the neutron-produced defect cluster sites through a diffusion-limited process. The model yields an activation energy of ~0.62 eV which compares well with the ~ 0.