Ultrafast electro-thermal transport through nanoscale electronic materials and interfaces
Keywords: Engineering.
Mechanical Engineering.

Although silicon-based nanofabrication technology has satisfied computational demands for decades, the aggressive scaling of complementary metal oxide semiconductor (CMOS) technology to sub-5 nm geometries poses challenges that must be addressed at the materials level. One example is tuning electro-thermal transport in metal nanostructures to enhance the transfer of information and the dissipation of heat in integrated circuits. The manipulation of these pathways can be further optimized by integrating low-temperature passivation materials with varying thermal conductivities. Furthermore, the emergence of photonic interconnects presents an opportunity for the integration of electro-optic components that rely heavily on the movement, transfer, and recombination of charge carriers within photosensitive materials. All the above are governed by the fundamental limits of physical transfer mechanisms in semiconductors, bringing electron and phonon engineering --the control of heat and charge carriers in materials-- to the forefront of CMOS hardware design. This work explores the fundamental mechanisms and limits of electron-phonon transport in four individual material systems which can comprise different parts of a broader, electro-thermally optimized electronic system using primarily time-domain thermoreflectance (TDTR) and scanning ultrafast electron microscopy (SUEM) as probes. First, we discuss the electro-thermal characterization of iridium (Ir) as an emerging metal for high aspect ratio nanostructures on account of its favorable resistivity scaling with thickness. The exceptionally defect-free metal films offer minimal confounding microstructural effects and allow the probing of thermal anisotropy and cross-plane quasi-ballistic thermal transport in epitaxial Ir(001) interposed between Al and MgO(001). Such effects reveal a transition between three dominant cross-plane thermal transport mechanisms which include electron dominant, phonon dominant, and electron-phonon energy conversion dominant regimes at different thicknesses. Finally, we develop a phenomenological model that correctly describes the dominant transport regimes, providing insight into the thickness-dependent interplay between carriers in metals as well as enabling quick evaluation and potential scalability to broader material systems. Next, we describe defect-modulated thermal transport in sputtered aluminum nitride (AlN) thin films for enabling wide-bandgap (WBG), high-temperature, and high-power electronic devices deposited at back-end of the line (BEOL) compatible temperatures (< 500 C). By controlling the sputtering gas composition during deposition using varied ratios of inert Ar with reactive N2, we demonstrate how the thermal transport can be engineered in AlN films ranging from 100 nm to 1.7 um in thickness deposited on both Si(111) and c-Al2O3. Thermal conductivity measurements are correlated with microstructural, electrical, and compositional analyses to investigate thermal scattering mechanisms that are elucidated via analytical models based on the Boltzmann Transport Equation taking into account atomic-scale Al defects that vary over an order of magnitude. A nearly 3X thermal conductivity contrast and record-high thermal conductivity values are reported, the insights for which can be optimized for application-specific strategic heat spreading or thermal confinement. Finally, we shift our focus to present fundamental work on bulk indium arsenide (InAs) and gallium arsenide (GaAs) for low-bandgap electro-optical readout and sensing using the nascent technique known as SUEM. Using a novel implementation of SUEM, we resolve the transient electron densities and surface-sensitive potentials of both materials, exposing fundamental limitations on the carrier transport due to surface trap states, which is remarkable given the data were obtained by probing transient surface states at saturation, a condition not generally accessible with other techniques. Moreover, the resulting dynamics confirmed two fundamental time limits to carrier transport relevant for single-shot high-bandwidth and multi-pulse applications