SUBJECT: Ph.D. Dissertation Defense
BY: Zhe Cheng
TIME: Friday, November 1, 2019, 11:00 a.m.
PLACE: Marcus NanoTech Building, 1116
TITLE: Thermal Energy Transport Across Ultrawide and Wide Bandgap Semiconductor Interfaces
COMMITTEE: Dr. Samuel Graham, Chair (ME)
Dr. Baratunde Cola (ME)
Dr. Ching-Ping Wong (MSE)
Dr. Alan Doolittle (ECE)
Dr. Shannon Yee (ME)


The development of ultrawide and wide bandgap semiconductors enables a variety of applications in power or RF electronics, including energy infrastructure, wireless communication, self-driving cars, and radar systems for defense. With the increasing power and frequency of these applications, Joule-heating induced hot-spots in the device channel degrade the device performance and reliability. Thermal management of these devices plays a very important role in achieving stable device operation and long lifetime, and correspondingly improving energy efficiency and reducing cost. The grand challenge of thermal management of power and RF electronics lies placing the hot-spot area f GaN/AlGaN and Ga2O3 devices close to heat sinks or heat spreaders with small thermal resistance and low stress. Thermal boundary resistance accounts for a large or even dominant part of the total thermal resistance in these devices. This thesis studied the TBC of five technologically important interfaces: GaN-SiC, GaN-diamond, diamond-Si, (Al0.1Ga0.9)2O3-Ga2O3, Ga2O3-diamond. This thesis shows a new method (room-temperature surface-activated bonding technique) to integrate ultrawide and wide bandgap semiconductors with high thermal boundary conductance. The bonding of GaN with SiC and single crystal diamond was demonstrated to achieve high TBC to facilitate excellent heat dissipation of power and RF electronics. This results will also push forward the development of 3D microchips packing in which thermal management is one of the key challenges. Additionally, nanoscale graphoepitaxy is used to enhance both the thermal conductivity of CVD diamond and Si-diamond TBC significantly. The temperature-dependence of thermal conductivity of β-(Al0.1Ga0.9)2O3/Ga2O3 superlattices were measured and significantly reduced thermal conductivity (5.7 times reduction) at room temperature was observed comparing with bulk Ga2O3. The estimated minimum TBC of β-(Al0.1Ga0.9)2O3/Ga2O3 interfaces is found to be larger than the Ga2O3 maximum TBC, which shows that some phonons could transmit through several interfaces as possible evidence of phonon coherence. To address the challenges of thermal management of Ga2O3-based devices, Ga2O3 was integrated with single crystal diamond by exfoliation-transferring and ALD-growth.