| dc.description.abstract | GaN crystal has the piezoelectric characteristic, the producing piezoelectric field would cause the energy level tilting in the InGaN MQWs, which is named as quantum-confinement Stark effect. Energy level tilting causes some disadvantages, i.e., the carrier confinement ability decay, the carrier spatial separation (reducing carrier recombination). The carrier spatial separation also causes a carrier screening effect with a direction opposite to the piezoelectric field, which could ease the quantum-confinement Stark effect. To explore the relationship between the piezoelectric field and the carrier screening effect, and their effects on the optical performance of the GaN-based LEDs. We adjust the process temperature of the die-attachment process, to vary the residual compressive stress in the GaN epilayer. We note that the higher temperature of the die-attachment process, the more compressive stress of the GaN LEDs can be released. By this concept, the residual compressive stress in the GaN epilayer could be controlled. In this Ph. D. study, we first propose that the CTE mismatch (between the GaN epilayer and the sapphire substrate) causes a CTE-induced piezoelectric field, which dominates the energy levels distortion in the InGaN MQWs of the GaN-based LEDs. However, the decreasing in the CTE-induced piezoelectric field does not have a significant effect on the optical performance of the GaN-based LEDs. One of the major reasons could be that the carrier screening effect could compensate the CTE-induced piezoelectric field. And, we realize that the magnitude of the carrier screening field depends on the piezoelectric field induced energy level tilting and the carrier concentration. Thus, the carrier screening field is a function of the CTE-induced piezoelectric field and applied electric field, which can be described as . Two major conclusions are: (1) the CTE-induced piezoelectric field dominates the initial energy level tilting in the InGaN MQWs; (2) the carrier screening effect is the major internal electric fields that influences the optical property of the GaN-based LEDs. | en_US |