UCLA

The enhancement of modern electronic devices' performance relies on the increased density of electronic components and the miniaturization of device sizes, in line with Moore's Law. However, this improvement often leads to concentrated heat generation within a small area, which can damage the device and limit its performance. Consequently, efficient thermal management has become critically important. My Ph.D. research focuses on the engineering of electron and phonon transport in semiconductors to fine-tune material properties, thereby reducing heat generation and improving thermal management.

My thesis is structured into five main sections. The first section investigates the vacuum field emission properties of tin selenide (SnSe) nanostructures, specifically in the form of 'nano flowers' (NFs), as an alternative electron transport mechanism that could circumvent the heat generation associated with Joule heating in solid-state devices. The ideal field emission material requires a low work function, high atomic bond energy, and sharp morphological features. SnSe NFs possess these necessary properties. Field emission measurements reveal that SnSe NFs exhibit a high electron emission current density, significant field enhancement factor, and robust emission stability, highlighting their potential for vacuum field emission applications.

The second section delves into phonon transport at the interface between boron arsenide (BAs) and gallium nitride (GaN) to enhance heat dissipation. While GaN is a wide bandgap material favored in high-power and high-voltage devices, such devices demand efficient thermal management for reliable operation. BAs has been identified as a material with superior thermal conductivity, making it a promising candidate for thermal management applications. This study integrates BAs into GaN-based electronic devices to investigate phonon transport across the GaN-BAs interface. The findings indicate a high thermal conductance at this interface, attributable to the good lattice vibration matching between BAs and GaN. Additionally, GaN-on-BAs devices show a substantially reduced hotspot temperature compared to integrated GaN devices with other high thermal conductivity substrates such as diamond or silicon carbide, achieving enhanced heat dissipation and device performance.

The third section examines the controlled doping of van der Waals materials, focusing on black phosphorus (BP). Doping can alter the electron or hole concentration in semiconductors, modifying charge transport properties. However, introducing foreign materials as dopants can also scatter phonons and decrease thermal conductivity. Therefore, precise control over doping is essential to balance high electrical performance with efficient thermal management. Especially, rational doping control remains challenging for van der Waals structures. This research demonstrates controlled doping of BP, utilizing an electrochemical approach to manage the ion intercalation of lithium and copper. This control transforms pristine black phosphorus-based resistors into n-type and p-type phosphorus-based field-effect transistors, as well as p-n junction diodes, depending on the doping parameters. Furthermore, lithium doping has been shown to enhance charge mobility by reducing neutral impurity scattering. This study proposes a novel method for tailoring the properties of van der Waals materials and provides a new platform for engineering the electron and phonon transport properties of 2D materials.

The fourth section demonstrates dynamic tuning thermal conductivity of tin selenide (SnSe). The manipulation of electricity via logic circuits has been one of the cornerstones of modern technologies. However, manipulating heat through logic units, though equally crucial, has been a longstanding challenge. A primary obstacle is to develop a simple yet effective approach to dynamically tune the thermal conductivity of materials across a broad range, which is vital for optimizing thermal management, enhancing energy harvesting efficiency, and developing thermal logic devices. This study presents a breakthrough in dynamically tuning thermal conductivity via electrical gate-tuning, achieved by modulating electron-phonon interactions in a SnSe thermal transistor. The thermal transistor attained a thermal conductivity tuning. Our analysis based on ab initio theory reveals that this modulation in thermal conductivity is primarily stems from the alignment of the SnSe chemical potential with its defect energy level, which activates the electrons of the defect states to scatter phonons. These findings present a novel method for controlling heat flow, opening up new avenues for application in various thermal technologies.

Finally, the fifth section presents thermal properties of defect doped lead selenide (PbSe). PbSe is extensive studied due to its promising thermoelectric properties. Experimental study on the thermal properties of PbSe showed sharpen longitudinal acoustic phonon mode linewidth at high temperature due to reduction of scattering phase space. This study examines the effect of vacancy-induced dislocations on the phonon dynamics of Sb-doped PbSe using inelastic neutron scattering method and diffusivity measurement. The results show sharpening phonon linewidth and enhancement of thermal diffusivity in vacancy-induced dislocations doped PbSe at 300 K. The results show an unexpected impact of defects on the thermal properties of PbSe.