Date of Completion

7-28-2015

Embargo Period

7-27-2015

Keywords

intrinsic breakdown, quantum mechanics, dipole, Monte Carlo

Major Advisor

Steven A. Boggs

Associate Advisor

Ramamurthy Ramprasad

Associate Advisor

S. Pamir Alpay

Field of Study

Materials Science

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

A first principles quantum mechanical method for estimating intrinsic breakdown field of insulating materials has been implemented based on an average electron model which assumes that breakdown occurs when the average electron energy gain from the electric field exceeds the average energy loss to phonons. The approach is based on density functional perturbation theory and on the direct integration of electronic scattering probabilities over all possible final states, with no adjustable parameters. The computed intrinsic breakdown field for many prototypical materials over a range of elemental compositions and crystal structures compares favorably with experimental data. This model also provides physical insight into the material properties that affect breakdown. The introduction of dipoles into a polymer can enhance breakdown field as a result of dipole-induced scattering which tends to “cool” hot electrons and thereby inhibits impact ionization. A theoretical analysis of electron scattering by dipoles and phonons is presented which explains temperature dependence of the breakdown field on the basis of the dominant scattering process as a function of temperature. Electron mobility calculations in non-polar and polar polymers produce a quantitative correlation between chemical composition and intrinsic breakdown field. Calculation of dipole scattering limited electron mobility can be used to assess the effect of dipole scattering on the intrinsic breakdown field of polymers. The problem of hot electron transport and energy loss in insulators at high electric fields is of interest in related to aging of dielectrics. Monte Carlo (MC) simulations provide the basis for a study of hot electron transport at high electric fields in thin polyethylene (PE) films with nanocavities based on energy loss to phonons computed using computational quantum mechanics. The electron trajectories, probability densities, and spatial evolution of the electron energy distribution are presented. Electrons with energy greater than the bandgap (8.8 eV) trigger impact ionization, which can cause avalanche breakdown, while electrons with energy greater than 3-4 eV can cause degradation through bond cleavage. In the presence of nanocavities, high field aging is likely to occur in the immediate vicinity of nanocavities.

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