Role of electron and hole trapping in the degradation and breakdown of SiO2 and HfO2 films

We investigated possible mechanisms for correlated defect production in amorphous (a) SiO2 and HfO2 films under applied stress bias using ab initio simulations. During bias application, electron injection into these films may lead to the localization of up to two electrons at intrinsic trapping sites which are present due to the natural structural disorder in amorphous structures. Trapping two electrons weakens Si-O and Hf-O bonds to such an extent that the thermally activated creation of Frenkel defects, O vacancies and O2- interstitial ions, becomes efficient even at room temperature. Bias application affects defect creation barriers and O2- interstitial diffusion. The density of trapping sites is different in a-SiO2 and a-HfO2. This leads to qualitatively different degradation kinetics, which results from different correlation in defect creation in the two materials. These effects affect TDDB statistics and its dependence on the film thickness.