Dynamic model of trapping-detrapping in SiO2

Abstract

The field and time dependence of charge carriers trapping under different charge injection conditions, is studied in this work, using the dc hot electron injection technique. It is shown that the trapping characteristics converge to field-dependent quasisaturation values. Variation of the trapping levels, due to change of the oxide field magnitude, are obtained in both directions and exhibit complete reversibility. These results, which cannot be explained by the first-order conventional trapping model, are consistent with a dynamic trapping-detrapping model. According to this model, quasisaturation of trapping characteristics is obtained when the trapping and detrapping processes are balanced. The occupation of the traps under steady-state conditions is therefore field dependent. The same model also describes the generation of positive charge under high-field injection conditions. This phenomenon is shown to be related to ionization of localized states in the SiO2 forbidden gap. The implications of the dynamic model suggest the need for a reevaluation of the present characterization methods for trapping in oxide layers.