Investigation and Improvement of Efficiency Droop in GaN-based Light-emitting Diodes
|關鍵字:||氮化鎵;發光二極體;效率下降;GaN;Light-emitting diodes;Efficiency droop|
GaN-based light-emitting-diodes (LEDs) have been developed in various applications due to its widely tunable wavelength from ultraviolet to blue/green. Nevertheless, the most expected application, solid-state lighting, is still under developing, which means the state-of-the-art GaN-based LEDs should be further improved. Although the light-extraction efficiency has been significantly improved by different techniques, the internal quantum efficiency (IQE) still suffers a major obstacle, i.e., the substantial decrease in efficiency with increasing injection current, as known as efficiency droop. This behavior strongly limits the development of many specific applications which require the operation current of the GaN-based LEDs under high injection levels. In this study, we propose a method to investigate the major mechanism of efficiency droop by analyzing the temperature-dependent droop behaviors in LEDs with different quantum-well thicknesses. Within well thickness from 1.5 nm to 2.5 nm, all the LEDs show serious droop behavior at 80 K, indicating that low hole mobility is the dominant factor in efficiency droop rather than well thickness at low temperature. On the other hand, at room temperature, LED with thicker wells shows smaller droop effect, indicating that carrier transport in active region is responsible for efficiency droop. In order to modify the carrier transport in active region, we design a LED with graded-thickness multiple quantum wells (GQWs). More uniform hole distribution in active region in GQW LED is simulated and observed by APSYS software, and the experiment results show significant differences in emission spectra and improved droop behavior as compared to conventional LED. In the second part, band engineering is applied to epitaxial structure to reduce the efficiency droop. By using graded-composition electron blocking layer (GEBL), the valance-band offset at the interface of last GaN barrier and EBL is eliminated, and the hole injection and the electron confinement could be simultaneously improved. Proper degree of gradation for this Al1-xGaxN EBL not only lowers the forward voltage and series resistance, but also greatly enhances the light output power at high current density. As a result, the efficiency droop in LEDs with GEBL could be significantly reduced. Graded-composition multiple quantum barriers (GQBs) are then applied in active region to reduce the valance-band offsets at quantum barriers and enhance the transport and distribution of holes. The simulation and experiment results show a uniform hole distribution and a slight droop behavior in GQB LEDs. However, the poor spatial overlap between holes and electrons in this GQB LED results in low quantum efficiency at low current density even though it has great droop behavior. Since thoroughly improving hole transport would cause poor spatial overlap, selective carrier distribution manipulation is applied in such GQB LED, as known as selectively graded composition multiple quantum barriers (SGQBs). Simulation results show that spatial overlap between electrons and holes is higher, thus the efficiency droop and overall efficiency could be simultaneously improved. The output of this dissertation provided a great help on solving efficiency droop and realizing the solid state lighting in next generation.
|Appears in Collections:||Thesis|