Electron-stimulated oxygen ion diffusion in thin oxide films

Diffusion of oxygen ions in electrically stressed polycrystalline and amorphous oxide films facilitates the dielectric breakdown of complementary-metal-oxide-semiconductor devices and the performance of resistive random access memories. Abrupt changes of resistance in response to electrical stress are hallmarks of correlated electron and ion dynamics and a manifestation of structural transformations of oxide films. We will present the experimental evidence of structural dynamics of SiO_x and HfO_x films under electrical stress. This dynamics is caused by the creation and diffusion of oxygen ions under electron injection and bias application conditions, as revealed by Density Functional Theory with non-local density functionals. Depending on the Fermi level position in the system, the calculated barriers for vacancy diffusion in the bulk of m-HfO₂ are 2.4 and 0.7 eV for neutral and doubly positively charged vacancies, respectively, and are reduced towards grain boundaries, which serve as sinks for oxygen vacancies. The diffusion of interstitial oxygen ions in m-HfO₂ has barriers of the order of 0.5 eV. In amorphous SiO₂, an average diffusion barrier for neutral vacancy diffusion was found to be about 4.6 eV, but at Fermi level positions above 6.6 eV with respect to the top of the valence band, vacancies are charged with average barriers for diffusion of negative and doubly negatively charged vacancies of 2.7 eV and 2.0 eV, respectively. Interstitial oxygens are negatively charged and diffuse by producing double bridge configurations with diffusion barriers on the order of 0.2 eV. These barriers are strongly affected by an external electric field. Using these data, we propose a qualitative mechanism for initial stages of dielectric breakdown and electroforming in thin oxide films.