SUBJECT: Ph.D. Proposal Presentation
BY: Zhe Cheng
TIME: Friday, December 14, 2018, 2:30 p.m.
PLACE: Love Building, 210
TITLE: Thermal Energy Transport in Wide Bandgap Semiconductors and Interfaces
COMMITTEE: Dr. Samuel Graham, Co-Chair (ME)
Dr. Baratunde Cola, Co-Chair (ME)
Dr. Shannon Yee (ME)
Dr. Ching Ping Wong (MSE)
Dr. Alan Doolittle (ECE)


The maximum output power of high-electron mobility transistors is limited by high channel temperature induced by localized self-heating which degrades device performance and reliability. Chemical vapor deposited (CVD) diamond is an attractive candidate to aid in the extraction of this heat and in minimizing the peak operating temperatures of high power electronics. Due to limiting ways to integrate diamond, CVD diamond is either directly grown on or bonded with other semiconductors. The directly-grown diamond has a columnar grain structure, leading to three-dimensional anisotropic thermal conductivity. The diamond used for bonding has large grains, resulting in thermal conductivity variation near grain boundaries. The anisotropic and inhomogeneous thermal conductivity impacts the effectiveness of CVD diamond in device thermal management significantly but detailed measurements and understanding are still lacking. Additionally, the large thermal boundary resistances between diamond and other integrated materials impede heat dissipation significantly because of the large phonon density of states mismatch among them. Reducing these TBR would leverage the cooling effect of diamond. This proposal will detail efforts to (1) measure the three-dimensional anisotropic thermal conduction in directly-grown diamonds by a multi-frequency multi-spot-size time-domain thermoreflectance method; (2) map local thermal conductivity variation across grain boundaries in large grain diamonds; (3) tune thermal energy transport across diamond-silicon interfaces by graphoepitaxy. (4) probe local vibrational modes across GaN-diamond interfaces with high energy resolution monochromated EELS-STEM (HERMES). Additional proposed research includes experimentally and theoretically explore thermal transport across Ga2O3-diamond interfaces and optimizing the interface structure to facilitate heat dissipation.