Solid State Thermionic Devices: Effects of Asymmetry, Defects, and Electron-Phonon Coupling on Thermionic Transport

The miniaturization of electronic devices leads to the rapid increase of heat power densities. The management of high heat power densities in nanoscale devices is a significant scientific and engineering challenge. In microelectronics and nanoelectronics, the self-heating effects significantly reduce both transistor efficiency and lifetime, and in a very large-scale integrated circuit (VLSI), the heat generation and thermal management become one of the bottlenecks to further improve clock speed and make smaller feature sizes. Moreover, the recent development of embedded systems in an internet of things viewpoint will require local and on-chip thermal management abilities. Solid-state thermionic (SSTI) coolers integrated with these devices are among the few viable options for addressing some of these issues. The same SSTI devices with the same design can also be used as heat to electrical power generators for applications such as wearable electronics. The primary objective of this dissertation is to theoretically design highly efficient SSTI devices based on two-dimensional (2D) van der Waals (vdW) heterostructures. This work also theoretically investigates the effect of asymmetry of the electrode, electron-phonon interaction, and defects on the SSTI device performance. The size effect and the significance of electron-phonon interaction in nanoscale thermionic devices are evaluated by knowing the mean-free path of electrons in the bulk version of the 2D semiconductor material used as the channel in the SSTI device. To evaluate the effect of electron-phonon interaction, the highly accurate electron-phonon scattering rates of the bulk form of the 2D semiconductor channel are computed from the first-principles. The calculated electron-phonon scattering rates are then utilized to investigate the effect of electron-phonon interaction on electron transport of the same bulk semiconductor from full first-principles calculations. In addition, in this work, the first SSTI device based on 2D vdW heterostructures is fabricated and characterized in collaboration with experimental groups. The figure of merit of the fabricated device is measured using a hybrid technique that combines thermoreflectance and cooling curve measurements. Finally, as a separate route, this work theoretically and experimentally investigates how polymorphism in Bi2Se3 allows it to be tuned for unique thermoelectric properties.