Departmental News

Mr. Xuefeng Li

Posted: October 24, 2023

Date: Thursday, October 27, 2023

Examination Time: 11:00 AM Mountain Time (US and Canada) 

Location: CHTM, Room #103

Dissertation Committee:

Dr. Daniel Feezell, Committee Chair, Electrical and Computer Engineering Department

Dr. Andrew Armstrong, Sandia National Laboratories

Dr. Ganesh Balakrishnan, EPSCoR Director and Electrical and Computer Engineering Department

Dr. Payman Zarkesh-Ha, CHTM Director and Electrical and Computer Engineering Department

Dr. Tara Drake, Physics and Astronomy Department


Solid-state lighting has achieved significant success over the past two decades, but the low quantum efficiency of green LEDs remains a barrier to full red-green-blue (RGB) displays in numerous applications. Combating efficiency reduction in longer-wavelength LEDs requires understanding the relative roles of intrinsic effects (e.g., wave-function overlap, carrier-current density relationship, phase-space filling (PSF)) vs. extrinsic effects (e.g., material degradation due to increased defect density, compositional inhomogeneities, etc.). A systematic study of the carrier dynamics in InGaN/GaN LEDs is very important for understanding the origin of the green gap and for providing solutions to improve the efficiency of the LEDs.

In this dissertation, several techniques, such as small-signal electroluminescence (SSEL), deep-level optical spectroscopy (DLOS) and lighted capacitance-voltage (LCV), were applied to LEDs grown under state-of-the-art growth conditions using metal-organic chemical vapor deposition (MOCVD). The effect of indium composition was examined to study the origin of the lower quantum efficiency in green LEDs compared with blue and cyan LEDs. The impact of the deep-level defect density on various non-radiative recombination mechanisms was also investigated, and the role of each recombination mechanism was analyzed separately. Furthermore, the effect of quantum well (QW) thickness was studied to determine the optimized value and tradeoffs between various design parameters. Finally, a novel multiple carrier lifetime model was developed to study carrier dynamics in InGaN/GaN multiple-quantum-well (MQW) LEDs with non-uniform carrier distribution. This model can also be applied to micro-LEDs and other LEDs with similar carrier dynamics behavior. The studies mentioned above will contribute to a better understanding of carrier dynamics in green InGaN/GaN LEDs and inform approaches for improving quantum efficiency in LEDs that already possess state-of-the-art growth quality.



Xuefeng Li received his B.S. degree in Physics from Harbin Institute of Technology in 2017 and his M.S. degree in Optical Science and Engineering from University of New Mexico in 2019. His current research focuses on the study of carrier dynamics in InGaN/GaN LEDs.