Model for Short-Term Annealing of Neutron Damage in p-Type Silicon

Abstract

A model is presented for the short-term annealing of a damage claster resulting from neutron radiation in p-type silicon. The damage cluster is assumed to be a disordered core imbedded in an otherwise undisturbed lattice, as suggested by Gossick. The disordered core is characterized in this model by two defects. The dominant defect immediately after the damaging event is assumed to be the vacancy, which is represented by a two-level defect (donor-acceptor) in the forbidden gap. The density of this defect decreases with time as a function of the densities of its different charge states. The second defect is the divacancy which is also represented by a two-level defect in the forbidden gap. The density of the divacancies increases as the vacancies anneal. The analysis is made for a single, spherically symmetric cluster located at the center of a sphere of radius R of undamaged material. The problem is solved by the PN code which approximates the exact continuity, Poisson's, and generation-recombination equations in finite difference form. Annealing factors are obtained as a function of time by simulating a steady, uniform ionization rate throughout the sphere R. An effective lifetime for the volume R is calculated as the instantaneous average density of excess minority carriers in the sphere divided by the ionization rate. Time histories of the annealing factors are compared to experimental data obtained from shortcircuited solar cells.