Moore, NNC; (2012) A Classical Investigation of the Dynamics of MgO Grain Boundaries and an Ab Initio Study of Oxygen Vacancies in Amorphous SiO2. Doctoral thesis , UCL (University College London).

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Abstract

The arrangement of atoms in most ceramic materials is not perfect and point defects such as vacancies and interstitials, as well as extended defects like grain boundaries exist. In general these defects dominate the properties and processes that are important for the applications of the material. The capture and emission of charge at point defects can affect the stability of dielectrics such as those used in MOS devices. The presence of grain boundaries is also known to lower both the electric and thermal conductivity of a material. Collectively the diffusion of point defects at grain boundaries play a role in mechanisms such as creep and have also been suggested to be involved in the corrosion of metals. In this thesis simulation techniques were used to investigate properties of defects in amorphous silica and near grain boundaries in MgO. Atomistic methods were used to determine the migration barriers of defects at MgO grain boundaries, the effect of electric field on the stability of the defects, and also the effect of temperature on the structure and stability of the grain boundaries. The nudged elastic band method was used to determine the activation energy for vacancy and interstitial migration at the Σ17 {410}/[001] tilt and the Σ5 twist grain boundaries. At the tilt and the twist grain boundaries it was found that the activation energies for vacancy migration were up to 1.31 eV and 1.41 eV lower than those in bulk MgO respectively. A finite MgO film model was produced to investigate the effect of electric field on point defects at the tilt grain boundary. The electric field was added to the system by sandwiching the MgO between two layers of point charges. It was found that the field anisotropically lowers the activation energies for vacancy migration by up to 0.37 eV with respect to those determined in the absence of the field. Molecular dynamics simulations were used to investigate the effect of temperature on the stability of the tilt grain boundary and two of its metastable structures and also on the twist grain boundary. The twist grain boundary was found to have the highest entropy in the temperature range 300 - 3000 K which suggests that it may be the most commonly occurring grain boundary in MgO. An ab initio study was also carried out on the structure and electronic structure of the neutral oxygen vacancy in amorphous silica in order to investigate mechanisms associated with dielectric breakdown such as negative bias temperature instability. Contrary to published suggestions the positively charged and neutral vacancy defects studied were found to have one electron energy levels below the Si valence band which suggests that these defects do not contribute to threshold voltage shifts.

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