Unraveling Nanoscale Thermal Transport in Multilayered Semiconductors

Abstract

A thorough comprehension and control of thermal transport in nanoscale thermoelectric, microelectronic and optoelectronic devices is crucial since it is paramount to their optimum performance. The fundamental understanding of how phonons move and the physical mechanisms behind nanoscale thermal transport, however, remain poorly understood. In this thesis, we move beyond thermal conductivity calculations and provide a rigorous and comprehensive physical description of thermal phonon transport in layered nanostructures by solving the Boltzmann transport equation and extending the Beckman-Kirchhoff surface scattering theory with shadowing to precisely describe phonon-interface interactions. We commence with analyzing periodic layered nanostructures called superlattices. We explicate in-plane thermal energy distribution in Si-Ge, GaAs/AlAs and their alloy-based superlattices by segregating it into different modes based on their trajectories. We provide a rigorous structural, microscopic, spectral and finite-sized analysis of thermal transport characteristics. Next, we examine cross-plane thermal conduction in GaAs/AlAs and their alloy-based superlattices. We present a comprehensive study of superlattice thermal transport, including structure-property relations, spectral and modal descriptions, and contrast it with in-plane heat conduction thereby explaining the resultant anisotropy in III-V semiconductor superlattices. We uncover the phonon injection mechanism which provides novel pathways in modulating thermal conduction of specific layers within layered nanostructures. We use that to examine thin film-on-substrate, a ubiquitously found architecture in nanostructured optoelectronic devices. We observe an unconventional behavior of thermal conductivity variation with film thickness i.e. for Al0.1Ga0.9As thin films grown over GaAs substrate and Ge thin film on Si substrate, we find an increased thermal conductivity with decreasing thickness. We fundamentally investigate interfacial coupling through analyzing the transmission coefficient and its variation on the phonon frequency and interfacial roughness. We study thermal conductivity enhancement through a 2D visualization of the spatial distribution of heat through mapping phonon MFPs and their corresponding thermal conductivity contributions. We also include an elementary discussion on thermal phonon wave effects and classify the kinds of wave effects that can occur in a superlattice, namely, quantum confinement and thermal band gaps. We elaborate upon the mechanisms which cause them and provide strategies to enhance the possibility of their occurrence in layered nanostructures. The results and insights in this thesis advance the fundamental understanding of heat transport in layered nanostructures and the prospects of rationally designing thermal systems with tailored phonon transport properties and providing inputs for thermal management which is a crucial component in enhancing the performance of nanostructured devices.