| Traditionally, there is no scheme for modeling continuous random network (CRN) configurations of amorphous silicon (a-Si) that was not based upon the use of inter-atomic potentials, and molecular dynamics or Monte Carlo. Such schemes are known as forward modeling schemes. In this dissertation, we demonstrate how a reverse modeling scheme, known as reverse Monte Carlo (RMC), which employs information from experiments and appropriate a priori network topology constraints, can be used to model realistic atomistic configurations of a-Si without necessarily using inter-atomic potentials. The RMC approach is flexible and we have used it to form new models of a-Si that are consistent with fluctuation electron microscopy (FEM) experiments (which measures the amount of medium range order in a material), which the conventional CRN model of a-Si fail to satisfy.;Defects in covalent materials display a host novel properties and fully understanding their behavior is important from a fundamental and technological point of view. We systematically studied the localization of dangling bond defect electron states in silicon by performing ab initio static lattice calculations. Using defected models of amorphous and crystalline silicon, and a localized basis density functional Hamiltonian, we studied the dependence of wave function and spin localization on exchange-correlation functionals and localized basis sets. We observed that the minimal basis set tends to overestimate measures of localization, and we came to the conclusion that to accurately represent of the localization dangling bond defect electron states, a larger basis set is necessary. Then, we added thermal disorder to the underlying topological disorder of the lattice and, assuming the harmonic approximation, showed that the electron-phonon coupling is large for localized defect electron states. We deduced analytic expressions connecting a static property, that is, wave function localization [gauged by the inverse participation ratio (IPR)] and a dynamic property, that is, mean square thermal fluctuations of the electronic eigenvalues, for localized electron states. In particular, both the variance of the electronic eigenvalues and the square of the electron-phonon coupling are linearly related to the IPR of the localized states. We verified this from first principles thermal molecular dynamics simulations. |
