Exploring the Extreme Limits of Thermal Conduction in Solids via Steady-State Thermoreflectance (SSTR)

The primary objective of this dissertation is to study the extreme limits of thermal conduction in solids via the pump-probe technique steady-state thermoreflectance (SSTR). This recently-developed optical technique operates at a time and length scale previously inaccessible by conventional laser-based thermometry techniques, e.g., time-domain thermoreflectance (TDTR), frequency-domain thermoreflectance (FDTR), and laser flash analysis (LFA). These unique features enable SSTR to circumvent many limitations of the existing laser-based thermal measurement systems. For instance, the thermal penetration depth of SSTR can be significantly higher compared to TDTR and FDTR, while still remaining sensitive to nanoscale resistances and thermal transport in solids with length scales inaccessible by LFA. Additionally, using Fourier’s law, SSTR can directly measure the thermal conductivity of any material without prior knowledge of its heat capacity. These capabilities make SSTR an ideal technique for investigating the fundamental transport mechanisms in different materials as well as measuring the thermal properties of challenging geometries. Inspired by this, I use SSTR to study the thermal properties of buried films and substrates, in-plane and cross-plane thermal conductivity of thin film materials, total thermal resistance of multilayered geometries, and high-temperature thermal conductivity of new-types of materials. The material candidates chosen for the projects include high-quality aluminum nitride films, organic-inorganic hybrid metalcone films, copper-tungsten nanomultilayers, high-entropy diborides, and perovskite chalcogenides. Each of the discussed projects fit a geometry consideration that is highly challenging to measure via the more traditionally used thermometry techniques. The measured thermal properties of these systems increase the fundamental understanding of electron and phonon transport mechanisms that is only enabled by the unique length scales and sensitivities of SSTR, which we analyze and discuss in detail in this dissertation.