The Role of Size Effects on the Thermal Conductivity of Thin Film Alloys and Superlattices

Cheaito, Ramez, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Hopkins, Patrick, Department of Mechanical and Aerospace Engineering, University of Virginia

Advancements in modern technologies have relied primarily on the miniaturization of electronic devices. As the dimensions of these devices are reduced to hundreds of nanometers, thermal management becomes a challenge. Performances are now dependent on the amount of power a device can dissipate before surpassing the temperature set by reliability requirements. Understanding thermal transport in thin film nanostructures is a key element in manufacturing devices with long lifetimes and better energy efficiencies.
The role of size effects on the behavior of heat carriers in thin film structures and across interfaces have been the focus of numerous studies over the past few decades. However, discrepancies among studies on phonon behavior obstruct the understanding of the fundamental processes governing phonon transport. On the other hand, the lack of data on electron thermal transport across interfaces and in periodic structures motivates more research in this direction. This dissertation presents thermal conductivity measurement results on four different material systems of sample thicknesses spanning three orders of magnitude to provide a deep understanding into the processes of phonon and electron thermal transport in thin film alloys and superlattices. Measurements were performed using time-domain thermoreflectance, a non-contact, optical method for the thermal characterization of bulk and thin film materials.
The effect of boundary scattering of long mean free path phonons on the thermal conductivity of thin film SiGe alloys and AlAs-GaAs superlattices is thoroughly discussed in light of the spectral contribution of these phonons to thermal transport. The interplay between short and long range boundary scattering in AlAs-GaAs superlattices is studied by systematically varying the period and film thicknesses. Phonon coherence in epitaxially grown SrTiO3-CaTiO3 superlattices is demonstrated by showing a minimum in the thermal conductivity as a function of period thickness. For electrons, the interplay between electron characteristic length and the materials' intrinsic properties is studied via measurements of the thermal interface conductance in Cu-Nb multilayers.
A major result of this dissertation is demonstrating the possibility of achieving a desired thermal conductivity by prescribing both the period and sample thickness of a superlattice, a result that has important implications on thermal management and thermal engineering applications.

PHD (Doctor of Philosophy)
thermal conductivity, phonon coherence, size effects, thin films, semiconductors, nanoscale heat transfer, superlattice, pump-probe, time-domain thermoreflectance, alloys, silicon, germanium, copper, niobium, strontium titanate, calcium titanate, aluminum arsenide, gallium arsenide, Kapitza conductance, thermal interface conductance, thermal boundary resistance, phonon scattering, diffuse mismatch model, electron transport, phonon transport, boundary scattering
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