SUBJECT: Ph.D. Dissertation Defense
BY: Sukwon Choi
TIME: Wednesday, June 26, 2013, 3:00 p.m.
PLACE: MRDC Building, 4211
TITLE: Stress Metrology and Thermometry of AlGaN/GaN HEMTs Using Optical Methods
COMMITTEE: Dr. Samuel Graham, Chair (ME)
Dr. Yogendra Joshi (ME)
Dr. Ting Zhu (ME)
Dr. Alan Doolittle (ECE)
Dr. Shyh-Chiang Shen (ECE)
Dr. Eric Heller (AFRL)


The development of state-of-the-art AlGaN/GaN high electron mobility transistors (HEMTs) has shown much promise for advancing future RF and microwave communication systems. However, their full commercialization still carries concern over device reliability. Part of this issue is the mechanical stresses induced in the devices as well as the self-heating, which limits device performance. Thus, quantification of stress and temperature in AlGaN/GaN HEMTs is of great importance. Micro-Raman spectroscopy and micro-photoluminescence were utilized to quantify the residual stress in AlGaN/GaN HEMTs. Through the use of these optical characterization methods, mapping of the vertical and lateral stress distributions in the device channels was performed. Results show that stress can be influenced by the substrate material as well as patterned structures including metal electrodes and passivation layers. A Raman thermometry method was developed which offers both experimental simplicity and high accuracy in temperature results. By utilizing this technique, it was revealed that under identical power dissipation levels, the bias conditions (combination of Vgs and Vds) alter the heat generation profile across the conductive channel and thus influence the degree of device peak temperature. A combined analysis using Raman spectroscopy, coupled electro-thermo-mechanical simulation, and electrical step stress tests was conducted to investigate the link between performance degradation and the evolution of total stress in devices. It was found that not only the inverse piezoelectric effect but also the substrate induced residual stress and operational themoelastic stress in the AlGaN layer play a major role in determining the onset of mechanically driven device degradation. The optical characterization methods developed in this study show the ability to reveal unprecedented relationships between temperature/stress and device performance/reliability. They can be used as effective tools for facilitating improvement of the reliability of future AlGaN/GaN HEMTs.