Thermal Engineering of Interfaces in Nanotechnology: A Study of Alloys, 2D Materials, and Ferroelectric Oxides

The more electronic devices scale down into the nanoscale regime to achieve higher performance, the more important interfaces become to dissipating the increasing heating power generated through device operation, which negatively affects device performance and can even lead to critical failures of the technology. Phonon mediated transport across metal-nonmetal interfaces has been identified as an especially limiting thermal bottleneck. While the addition of metal layers at the interface with better vibrational matching to the nonmetal has been shown to significantly enhance interfacial thermal transport, in the context of device applications such an enhancement may not always be fully realized.
In this dissertation I will first present an overview of phonons, frequency domain thermoreflectance, and different nanotechnologies in order to demonstrate the importance of improving thermal transport at metal-nonmetal interfaces for device applications. Next, I will describe my work (in collaboration with Professor Andrew Gellman’s lab at Carnegie Mellon) on how the interdiffusion of extremely thin layers of Au on Cu progressively inhibits thermal transport across the metal-Al2O3 interface. I will then demonstrate how the experimental data was used to derive an analytical model to predict our unique diffusion profiles as a function of time, metal thicknesses, and permeabilities of the metal-metal interface to mass transport. Additionally, I will show how this analytical model can be utilized as an input to the Diffuse Mismatch Model to predict how interdiffusion lowers interfacial thermal conductance. The second research project I will describe focuses on developing contacts to 2D materials, with the objective of being superior both thermally and electrically. This work was also performed in collaboration with Professor Gellman’s lab. This study is the first to investigate alloys as contacts to a 2D material, where my 2D material of interest was graphene which has some of the best-known material properties. Through this study, I determined that ~10 atomic percent palladium in nickel results in the largest reported thermal conductance of 114 MW/m2K to monolayer graphene supported by SiO2. I will also present some of my preliminary data (in collaboration with Professor Feng Xiong’s lab/his student Yanhao Du at the University of Pittsburgh) to determine whether this alloy composition will be a higher performance electrical contact. This ongoing effort indicates the critical importance of using graphene with clean surfaces to reducing contact resistances.
For the final two sections of this thesis, I will describe my thermal conductivity measurements of the single crystal ferroelectric oxides SrTiO3 and PbTiO3 (performed in collaboration with Professor Fransicso Rivadulla’s Lab at the University of Santiago de Compostela) which will demonstrate the surprising power of Ferroelectric Domain Walls (FEDWs) to significantly reduce the thermal conductivity of PbTiO3 and SrTiO3 by ≈60% at room temperature and a further ≈65% reduction from 275 K down to 225 K and temperatures below. Through these measurements I demonstrate that the thermal interfaces of FEDWs have incredible potential for future studies to actively control thermal conductivity. Through the datasets presented in this dissertation, I experimentally answer important questions regarding thermal transport across interfaces with particular relevance to nanotechnology applications, and also open the door for interesting new questions to be answered by future research in this area.