Phonon and Electron Transport in Periodic Nanoporous Structures

Abstract

Comparing with their bulk counterparts, the nanostructured materials provide superior performance in many applications due to their unique physical properties. Therefore, a better understanding of phonon and electron transport within nanostructures is critical for their applications, including the thermoelectrics, thermal management, and electronics, etc. This dissertation is a collection of research ranging from simple but accurate thermal conductivity predictions for engineering purposes, to the understanding of physics of micro/nano-scale phonon transports. For the property estimation in engineering applications, the characteristic lengths of periodic nanoporous bulk material and thin films are extracted and derived. The presented models enable a simple but accurate thermal conductivity prediction without performing complicated phonon Monte Carlo simulation. The thermoelectric properties can also be predicted using such models. In addition, a two-step phonon mean free path modification model is developed for various thin-film based nanostructures including etched nanowires. This two-step model can incorporate different surface roughness between the thin film surface and etched surfaces. On the understanding of the physics of the phonon transport, a new approach is used to evaluate the impact of possible phonon wave effects by comparing the thermal conductivity of the same Si thin film with increased rows of nanopores as drilled with a focused ion beam. As a result, no significant wave effect is observed in the temperature range of 85 K to 300 K for measured samples.