探討與改進氮化鎵發光二極體中的效率下降現象

Abstract

因為氮化鎵發光二極體擁有可調變發光波長從紫外到藍綠光的特性，已被廣泛的應用。儘管如此，最受期待的應用――固態照明，仍然在發展中，說明了現階段的氮化鎵發光二極體仍有很大的進步空間。透過各種手法，光萃取效率已經被大幅的提升，但內部量子效率仍然面臨很大的障礙，例如效率會隨著操作電流提高而下降，這現象又被稱為效率下降。這個現象極度地限制了發光二極體往高電流操作的應用。 在本研究中，我們利用不同量子井厚度發光二極體的變溫實驗來探討效率下降的主因。在量子井厚度從1.5奈米到2.5奈米的範圍中，所有的發光二極體在80 K時都有嚴重的效率下降，顯示了低溫下的低電子遷移率是造成效率下降的主因而非量子井厚度；另一方面，在室溫時較厚的量子井具有較小的效率下降，這些結果顯示了主動層中的載子遷移是造成效率下降主要的原因。為了改善主動層中的載子傳輸行為，我們設計了具有漸變厚度多重量子井(GQW)的發光二極體。APSYS模擬結果顯示這種發光二極體的主動層中有較均勻的電洞分布，並且實驗結果也顯示了明顯不同的發光波長變化以及效率曲線。 在第二部分中，我們在磊晶結構中應用了能帶工程化的概念來降低效率下降。首先，利用漸變成分電子阻擋層(GEBL)消除在最後氮化鎵量子能障與電子阻擋層間的價電帶偏移，進而同時增加電洞注入以及電子侷限能力。在適當的漸變成分下，不但可以降低操作電壓以及串聯電阻，更能夠大幅提升高電流操作時的光輸出功率。因此，GEBL發光二極體的效率下降現象也大幅減輕。 接著，我們將漸變成分多重量子能障(GQBs)應用於主動層中來縮小量子能障層的價電帶偏移量，進而增強電洞的傳輸以及分布。模擬與實驗結果指出GQB發光二極體擁有均勻的電洞分布以及輕微的效率下降。然而，因為較差的在載子空間分布，在低電流時GQB發光二極體的量子效率反而是較差的。如GQB一般全面增加電洞的傳輸會造成較差的載子空間分布，因此我們應用了選擇性的載子分布控制，又稱作選擇性漸變成分多重量子能障(SGQBs)。模擬結果顯示這樣的結構能提高電子與電洞的空間重疊性，因此能同時改善效率下降以及總體效率。 期許這篇論文能幫助解決效率下降的現象並實現下一世代的固態照明。

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.