Characterization of thermal and hydrodynamic phonon transport using transient thermoreflectance




Jeong, Jihoon

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Thermal management becomes a significant challenge due to generating more power per unit volume in developing high-power density micro-/nano-sized electronics. The critical feature size of nano- or micro-electronics has already reached less than 10 nm. The thermal conductivity of the semiconducting, dielectric, and metallic materials used in fabricating these microelectronics can be reduced significantly at a small dimension because of increased boundary scattering of phonons and electrons, known as the heat carriers in solids. The size effects on the thermal transport property are considered one of the most fundamental problems for thermal management, which needs to be understood thoroughly for next-generation electronic devices' rational design. This work seeks to advance the understanding of how heat travels in solid materials. Utilizing precise methodologies for characterizing thermophysical properties is the most critical job. Transient thermoreflectance (TTR) and grating imaging techniques are developed to measure thermal conductivity in both lateral and horizontal directions. These techniques are employed to study materials from bulk to nanostructure, including thin-film and nanoparticles, to be characterized. A new regime of phonon transport between ballistic and diffusive transports, called hydrodynamic phonon transport, is observed with a novel method of heat pulse measurement. With thermal characterization using the TTR, four bulk materials, including glass, sapphire, GaAs, and Si are examined for interfacial thermal resistance and cross-plane thermal conductivity. Nanostructures are also tested, such as MoS2 thin-films over a wide range of thicknesses and digital alloy samples of GaAs/InAs. Cu nanoparticles' effective thermal properties are also measured, including heat capacity, density, and thermal conductivity to improve laser sintering technology in the additive manufacturing industry. Thermal characterization using the grating imaging technique, the in-plane thermal conductivity is measured in GaAs/AlAs superlattices and Cu nano-films on Si substrate, representing the bulk and layered structures, respectively. A new regime of phonon transport, hydrodynamic, is observed in graphite using optical heat pulse measurement. The measurement results reveal transient lattice cooling near the adiabatic center of a ring-shaped pump beam at temperatures between 80 and 120 K.


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