The Role of Electronic and Vibrational Scattering on Thermal Transport Across Multiple Interfaces in Nanostructures

Rapid miniaturization and faster working speeds in electronic devices has led to challenges in their thermal mitigation. However, thermal conductivity of the constituent materials in the nano-devices, even though a very critical parameter in their design, is mostly overlooked and the thermal performance of the device is mainly controlled through packaging techniques. However, it would be advantageous for a diverse spectrum of technologies, which ultimately fail due to either high density of interfaces or due to high operating frequencies, to control and tune thermal properties at the submicron length scale and femto-to-picosecond time scales. Therefore, a comprehensive understanding of how heat flows across nanosystems that are mostly limited by interfacial thermal transport would prove to be quintessential for the design of various electronic devices. For example, the contribution of electronic thermal resistance across metal/semiconductor interfaces has been a subject of debate for the past several decades; understanding the intrinsic electron-electron, electron-phonon and electron-boundary scattering mechanisms in these devices could potentially lead to transistors with higher operating frequencies. Moreover, with the advent of new fabrication technologies, the role of phononic-driven thermal resistances across hybrid interfaces of inorganic/organic materials and amorphous semiconductor superlattices (SLs) with high density of interfaces has largely been unexplored.

It is the goal of this work to comprehensively explore interfacial thermal transport in these material systems by understanding the fundamental scattering mechanisms of the energy carriers, which dominate thermal transport at various length and time scales. In this regard, a combination of time-domain thermoreflectance (TDTR) experiments and non-equilibrium molecular dynamics (NEMD) simulations will be implemented to achieve these goals.

The nonequilibrium between electrons and phonons at metal/semiconductor interfaces in high frequency microelectronic devices serves as a bottleneck to heat transfer in these devices. To understand this phenomena, TDTR is used to probe the highly non-equilibrium condition that is induced due to pulse absorption by thin Au films deposited on various dielectric substrates. For electron temperature excursions of < 3,500 K in Au, it is shown that while the increase in the excited carrier density increases the e-p coupling in the metal, the bond strength between the metal and the substrate dictates the energy transfer rate across the interface during this highly non-equilibrium time regime. Furthermore, at time scales when the electrons have fully thermalized with the lattice, electron-phonon coupling does not influence the phonon-driven thermal boundary conductance.

Another phenomena that lead to device failure are the phonon-driven thermal boundary resistance in multilayered semiconductors used in technologies such as thermoelectric power generation or phase change memory devices. TDTR is used to simultaneously measure the thermal conductivities and heat capacities of hybrid inorganic/organic superlattices and amorphous superlattices. Furthermore, atomistic simulations are implemented to shed light on the intrinsic scattering mechanisms dictating heat flow in these material systems. The goal of these studies is to demonstrate that through judicious choice of individual period thicknesses and the understanding of interfacial thermal resistances in these multilayers, new strategies can be developed to engineer material systems with user defined thermal transport.