Direct and defect-assisted electron tunneling through ultrathin SiO2 layers from first principles

We perform first-principles matrix Green's function calculations to study the coherent charge tunneling through ultrathin SiO(2) layers in metal-oxide-semiconductor devices. The tunneling behavior is analyzed within the atomistic picture based on the overlap of Si-induced gap states in the oxide region. We find that, while interface roughness defects such as suboxide-bonds and protruded O atoms only weakly affect the tunneling current, a network of oxygen vacancies composed of Si-Si bonds across the oxide layer drastically increases the gate leakage current due to the defect-assisted tunneling. We show that the formation of such percolation paths is energetically favorable in the nonequilibrium situation, and even the oxygen divacancy is enough to result in the dielectric breakdown for ultrathin oxide layers.