Origin of positive charge generated in thin SiO₂ films during high-field electrical stress

The characteristics of electron capture in a 131-angstroms silicon dioxide after hot-hole injection have been studied, which have been compared with those after high-field Fowler-Nordheim (FN) electron injection. After hole injection from the silicon substrate into the oxide, positive charges were accumulated in the oxide and electrons could be captured even at low oxide fields only under the positive gate polarity. The charge centroid of the captured electrons was near the substrate-SiO₂ interface. The low-field electron capture can be explained based on the electron tunneling from the substrate into the positive charge and neutral trap centers created near the substrate-SiO₂ interface. In order to investigate the initial stage of the oxide degradation due to high-field FN stress, electrons were injected from the gate and the charge fluence was selected to be -1.0 C/cm². After the high-field stress, positive charges appeared in the oxide and electrons were captured only under the positive gate polarity by the positive charge and neutral trap centers, which were distributed near the interface. These facts are explained on the basis of the model describing that hole injection and trapping are the dominant causes for the generation of the positive charge centers during high-field FN stress.