Heat Conduction in Complicated Nanostructures: Experiments and Theory

The thermal conductivity (k) of a semiconducting nanostructure is dramatically reduced from the bulk value due to boundary and interfacial scattering of energy carriers (phonons). The theoretical understanding of such nanoscale thermal phenomena is based on measurements of relatively simple nanostructures, such as thin films or nanowires. However, qualitatively new heat transfer mechanisms may emerge in more complicated nanostructures such as etched silicon nanomeshes or arbitrarily anisotropic thin films. New theoretical tools are needed to predict k of these nanostructures, and new experimental nanoscale temperature mapping tools would resolve questions about the dominant nanoscale mechanisms. In addition, nanothermometry techniques could be used to improve the thermal performance of technologies utilizing complicated nanostructures, which range from data storage devices to light-emitting diodes and microelectronics. In this dissertation, I develop experimental, computational, and analytical tools to answer fundamental questions about heat transfer in complicated nanostructures. I begin by demonstrating two nanothermometry techniques in the scanning transmission electron microscope (STEM) utilizing temperature-dependent thermal diffuse scattering. Temperature mapping of a Joule-heated silicon carbide device in the STEM shows the path forward towards ultrahigh spatial resolution temperature mapping of complicated nanostructures. Then, I describe how phonon ray tracing simulations quantify the boundary scattering reduction of k in complicated nanostructures. Comparing these simulation results with collaborator’s k measurements reveals that thermal phonons behave like incoherent particles rather than like coherent waves in silicon nanomeshes, which are membranes with periodically etched holes. Lastly, I derive solutions of the Boltzmann transport equation for phonon transport in arbitrarily aligned anisotropic thin films, and use these solutions to extend a well-known bulk Onsager relation for anisotropic heat conduction into the boundary scattering regime. In summary, further research using these experimental and theoretical techniques can answer long-standing fundamental thermal questions and can be leveraged in the design of energy-efficient lighting technologies and improved data storage devices