A model for minority carrier lifetime variation in the oxide–silicon structure following 253.7 nm ultraviolet irradiation

Abstract

The effective minority carrier lifetime, in the silicon wafer covered with different oxides, is found to increase, or decrease, or decrease and then increase, following ultraviolet (UV) light irradiation. Evidence is presented of injection of UV-generated electrons from the silicon substrate into the oxide. Subsequent trapping occurs at the outer oxide surface for dry or native oxides, but mainly in the bulk of the oxide, in the case of wet or chemical vapor deposited oxides. Recognizing that the lifetime is determined predominantly by carrier recombination at the silicon–silicon oxide interface, and that this recombination rate is controlled by silicon surface band bending, a simple model, based on the postirradiation shift in the location of the Fermi level in the silicon at the interface, is shown to be able to reconcile the apparently conflicting reports in the lifetime behavior. The location of the pre-irradiation Fermi level is determined by the initial oxide charge, assumed to be positive, and also by the interface states.