Dissertation Defense, Hongping Zhao

"Enhancement of Internal Quantum Efficiency and Optical Gain for Nitride Light-Emitting Diodes and Laser Diodes"

Energy efficiency and renewable energy technologies have significant importance for achieving sustainable energy systems in modern society. Lighting accounts for more than 22% of the total electrical energy usage in US, and technologies based on solid state lighting (SSL) utilizing semiconductor-based material has tremendous promise to replace the existing lighting devices. As compared to traditional incandescent and fluorescent lamps, SSL is more energy-efficient, reliable, and environmentally-friendly. Once widely used, SSL could lead to the decrease of worldwide electricity consumption for lighting by >50% and reduces total electricity consumption by >10%. The U.S. Department of Energy describes SSL as a pivotal emerging technology that promises to fundamentally alter lighting in the future. Rapid progress in SSL research and development has resulted in the advent of light emitting diodes (LEDs) for general lighting applications. Two major challenges for current state-of-art III-nitride based LEDs are 1) ‘green gap’ issue in InGaN quantum well light-emitting diodes, and 2) ‘efficiency droop’ issue in III-Nitride LEDs resulting in output power quenching at high current injection.
In this dissertation, novel approaches to address the major issues related to state-of-the-art nitride LEDs are investigated, in particular related to 1) engineering of InGaN nanostructure active layers for achieving high internal quantum efficiency and minimal efficiency droop in nitride LEDs, and 2) the use of surface plasmon approach for increasing the radiative efficiency in nitride LEDs. Specifically, the dissertation covers the following topics: 1) novel InGaN-based quantum well structures such as staggered InGaN QW, type-II InGaN-GaNAs QW, strain-compensated InGaN-AlGaN QW, and InGaN-delta-InN QW with enhanced matrix element for achieving high radiative efficiency; 2) the use of lattice-matched InGaN-AlInN QW-barrier structure for suppressing the efficiency-droop in nitride LEDs; and 3) the use of surface plasmon dispersion engineering to achieve wide – spectrum tuning of the Purcell peak enhancement of the radiative recombination rate for InGaN quantum well LEDs.
To understand the device physics for III-Nitride compound semiconductor based heterostructure / nanostructure, a comprehensive self-consistent numerical model based on 6-band k•p method has been developed. The model allows the calculation of the optical properties of III-Nitride semiconductor nanostructures via calculating the energy dispersion relation, spontaneous recombination rate, and optical gain spectrum. In addition, the model allows prediction of device parameter values such as threshold current density of laser diodes and power conversion (wallplug) efficiency of LEDs.

Committee Members:
Prof. Nelson Tansu (PhD Advisor)
Prof. Filbert Bartoli
Prof. Yujie Ding
Prof. Volkmar Dierolf
Prof. Helen Chan
Prof. Sushil Kumar