Methods for enhancing and tuning phonon transport at solid-solid interfaces

The continued miniaturization of modern electronics and the associated increase in the density of on-chip components has brought with it a number of challenges in managing thermal properties and dissipation of thermal energy. The transition to device components with characteristic lengths on the micro- and nano-scale has necessitated the further refinement of relevant energy transport models and theories. These subsequent theories are distinct from former macroscopic and continuum approaches, and are grounded in knowledge of fundamental energy carrier behaviour at nanometer-length and sub-picosecond time scales. The interface between materials has become the dominant thermal resistance governing broader thermal properties in nanoscale solids, stemming from the increase in the number of material interfaces which exist in high density, nanostructured devices. In order to ensure reliability, efficiency, and operating performance, there is an increasing need for a better understanding of phonon mediated transport across interfaces as well as methods to predict and tune interfacial thermal properties. The ability to independently control thermal properties at the nanoscale, while retaining the functional properties (i.e., electrical, optical, and magnetic) necessary for broader device operation, remains a major engineering challenge.

This thesis addresses these challenges with a focus on energy transport at interfaces between materials including non-metals and semiconductors where quantized lattice vibrations (phonons) are the dominant energy carriers. Building on the knowledge of the factors influencing phonon mediated transport at interfaces, a novel method is proposed to not only enhance phonon transport at interfaces, but to provide tunability as well. The proposed enhancement mechanism exploits the dependence of interfacial phonon transport on the relative mismatch in vibrational properties of each material comprising a solid-solid interface in addition to localized vibrational states via a mixing layer characterized by atomic diffusion. Simulation in the form of molecular dynamics is used to explore the major hypothesis of this thesis by investigating the effects of altering vibrational This thesis addresses these challenges with a focus on energy transport at interfaces between materials including non-metals and semiconductors where quantized lattice vibrations (phonons) are the dominant energy carriers. Building on the knowledge of the factors influencing phonon mediated transport at interfaces, a novel method is proposed to not only enhance phonon transport at interfaces, but to provide tunability as well. The proposed enhancement mechanism exploits the dependence of interfacial phonon transport on the relative mismatch in vibrational properties of each material comprising a solid-solid interface in addition to localized vibrational states via a mixing layer characterized by atomic diffusion. Simulation in the form of molecular dynamics is used to explore the major hypothesis of this thesis by investigating the effects of altering vibrational properties near an interface, such that the vibrational mismatch at a solid-solid interface is effectively bridged and interface conductance is enhanced. A systematic study of the pertinent design parameters explores the ability to enhance and tune phonon transport at both ideal (epitaxial) and non-ideal (disordered) interfaces. The following salient results are found: (1) an interfacial film with mediating vibrational properties may enhance thermal boundary conductance by up to 25% in ideal (epitaxial) interface structures, and 50% when considering less aggressive disordering at interfaces. (2) The source of enhancement is partially attributed to perturbed phonon density of states in the monolayers near to the interfacial film, which exhibit a massive redistribution of modes which enhance elastic phonon-phonon scattering pathways across the interface. (3) In contrast to macroscopic energy transport theory, the observed enhancement demonstrates that additional material along the direction of net flux can in fact increase the conductance across the structure in question. Equivalently, this implies that the addition of more material forming an interfacial film -can remove thermal resistance from a nanostructure. (4) The degree of enhancement is found to have an inverse parabolic and symmetric functional dependence on the vibrational spectrum of the interfacial film. In so much as the vibrational properties of the interfacial film can be chosen, the enhanced structure can be tuned to exhibit selective degrees of enhancement.