Nanoscale structure and transport : from atoms to devices

Abstract

Nanoscale structures present both unique physics and unique theoretical challenges. Atomic-scale simulations can find novel nanostructures with desirable properties, but the search can be difficult if the wide range of possible structures is not well understood. Electrical response and other non-equilibrium transport phenomena are measured experimentally, but not always simulated accurately. This thesis presents four diverse applications that demonstrate how first-principles calculations can address these challenges. Novel boron nanotube structures with unusual elastic properties are presented. Internal degrees of freedom are identified that allow longitudinal stress to be dissipated without changing the tube's diameter, leading to high lateral stiffness. Self-trapped hole structures in amorphous silicon dioxide are investigated in order to connect the behavior of hole currents to atomic-scale structures. Calculations on a paired-oxygen analogue to the ... center show that such a configuration does not result in a metastable trapped-hole state. A novel method to enable first-principles mobility calculations in ultrathin silicon-on-insulator (UTSOI) structures is presented and applied to interface roughness scattering in transistor channels. Self-consistent potentials and accurate wavefunctions and band structures allow for a direct link between measured electrical response and atomic structure. Atomic-scale interface roughness is shown to be an important limit on mobility at high carrier densities. At low carrier densities, such short-wavelength roughness results in qualitatively different mobility behavior than gradual UTSOI channel thickness fluctuations.

(cont.) An effective Hamiltonian technique to calculate short-time, non-equilibrium fluctuations in quantum devices is developed. Applications to quantum dots and resonant tunneling diodes show that temporal fluctuations are reproduced well.