氮化鎵發光二極體成長於奈米柱模板之特性研究

Abstract

在本論文中，我們成功地利用分子束磊晶技術將氮化鎵奈米柱模板成長於藍寶石基板上，並利用有機金屬氣相沉積法將高效率發光二極體成長在奈米柱模板上。從側向的掃描式電子顯微鏡影像中我們可以清楚觀察到奈米等級的空氣孔洞被包埋在奈米柱及成長其上的氮化鎵磊晶層之間，而這很有可能會使多重量子井內部發出的光藉由光散射的方式而提升了它在光萃取上的表現。 而從穿透式電子顯微鏡影像中我們也可以發現奈米等級的空氣孔洞可以使氮化鎵磊晶層在奈米等級側向成長的過程中更進一步降低穿透錯位密度，另外，從拉曼光譜的分析結果中我們發現將氮化鎵磊晶層成長在氮化鎵奈米柱模板上能夠有效地降低磊晶層內部應力的產生，而這也有可能降低氮化銦鎵/氮化鎵量子井內之量子侷限史塔克效應的產生。 除此之外我們藉由變功率光激發螢光分別在低溫及室溫的表現去研究氮化銦鎵/氮化鎵多重量子井發光二極體的物理機制，結果發現氮化銦鎵/氮化鎵多重量子井發光二極體之內部量子效率會隨著雷射激發功率的強弱而變化，主是受到量子井內之非輻射複合中心以及量子井內之量子侷限史塔克效應(Quantum confined Stark effect, QCSE)與能帶填滿效應(Band filling effect)所影響。 最後，藉由室、低溫變功率光激發螢光和二維FDTD軟體的模擬，我們可以估計奈米柱發光二極體相較於一般發光二極體而言在內部量子效率上大約有24%的提升，而在光萃取效率上大約有48%的提升。另一方面，奈米柱發光二極體相較於一般發光二極體而言在20 mA的操作電流下其發光的光強大約提升了71%，而這也和我們前面對於內部量子效率和光萃取效率的估計吻合。

In this study, we presented high performance GaN-based light emitting diodes (LEDs) using a GaN nano-pillars (NPs) structure grown on c-plane sapphire substrate by integrating RF-plasma molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). By cross-sectional scanning electron microscopy (SEM), nano-scale air voids were clearly observed at the interface between GaN NPs and the overgrown GaN layer. It can be considered to increase the light extraction efficiency due to the light scattering. The transmission electron microscopy (TEM) images suggest the air voids between GaN NPs introduced during nanoscale epitaxial lateral overgrowth (NELOG) of GaN can suppress the threading dislocation density. Moreover, Raman spectrum demonstrated that the strain of the GaN layer grown on GaN NPs was effectively eliminated, this might result in the reduction of quantum-confined Stark effect (QCSE) in InGaN/GaN quantum wells. Besides, we intended to investigate the physical mechanisms of excitation power dependent PL in InGaN/GaN MQWs LEDs at 15 K and 300 K. We confirmed the variation of internal quantum efficiency (IQE) with increasing excitation power is due to the coulomb screening of quantum confined Stark effect (QCSE) and band-filling effect in InGaN/GaN MQWs. Moreover, the nonradiative recombination had been taken into account at lower excitation power. Finally, with low temperature PL measurement and 2D FDTD simulation, we can estimate the enhancement of IQE and light extraction should be 24% and 48%, respectively. On the other hand, the NP-LEDs exhibit great enhancement of the light output 71% at 20mA compared to the C-LEDs and this is close to the results of IQE measurement and light extraction simulation.