氮化鎵發光二極體之光萃取效率分析與晶片設計

The Light Extraction Analysis and Chip Design for GaN-based Light-Emitting Diodes

李宗憲(Tsung-Xian Lee)

指導教授 ： 孫慶成

國立中央大學/理學院/光電科學研究所/博士(2008年)

摘要

LED光萃取效率受到許多因素限制，這些因素彼此相互獨立，又相互影響。基於此，我們以蒙地卡羅光追跡法為基礎，提出一套模擬LED光萃取特性的定量分析模型，研究氮化鎵LED光萃取與光損耗之機制。此外，我們也利用封裝實驗驗證蒙地卡羅方法之模擬結果的精準度。
在本論文中，共有五項主題進行探討，包含主動層吸收與光子循環效應、晶片幾何尺寸、電極與電流分佈、封裝以及微結構等。首先，我們分析主動層吸收對光萃取效率之影響，並在模擬中考慮光子循環效應，結果顯示擁有光萃取結構的LED對主動層吸收的敏感度比一般LED高。其次，我們分析LED幾何尺寸對光萃取效率之影響，結果顯示晶片塑形與圖案式基板適用於小晶片；而表面粗糙化的薄膜氮化鎵結構則適用於大晶片，其中藍寶石基板的建議厚度為50~100um。接著，我們分析電極與電流分佈對光萃取效率之影響，結果顯示光子易被p型電極所阻擋並吸收，為了避免此效應發生，整合高反射率電極、電流阻檔層以及底部反射鏡粗化等技術可有效提升光萃取效率。我們亦探討封裝對光萃取效率之影響，從光學的
觀點，圖案式基板對覆晶接合提升光萃取效率最有效益，而封裝透鏡尺寸必須比晶片大2.5倍才可以有效將光完全萃取出來。另外，增加封裝材料的折射率比增加光萃取結構更有顯著效益。最後，為了增加光萃取效率，我們針對微結構對光萃取效率之影響進行分析與優化，結果顯示微結構主要是藉由光來回反射循環的方式獲得高光萃取效率，因此元件底部的反射率是微結構提升光萃取效率的重要關鍵。

關鍵字

封裝 ； 電流分佈 ； 微結構 ； 光萃取效率 ； 發光二極體 ； 氮化鎵

並列摘要

The light extraction efficiency of light-emitting diodes (LEDs) is limited by many effects. For this reason, we present a quantitative analysis model for LED light extraction characteristics based on Monte Carlo ray tracing method, According to this model, the light extraction and loss mechanisms in GaN-based LEDs is studied. We furthermore verify the validity of the Monte Carlo simulation results by packaging experiments at low temperature.
In the thesis, there are five topics is discussed in detail. First at all, we analyze the issue of absorption in active layer, the simulation also take into account the effect of photon recycling. According to the simulation results, the light extraction efficiency of LED with light extraction structure is more sensitive to active layer absorption than that of typical LED. Secondly, the realistic geometry of chip scale is analyzed. According to the simulation results, the structure of chip shaping and patterned substrate is suitable for small chip, and the ThinGaN LED with Surface texture is suitable for large chip. Besides, the suggested thickness for sapphire substrate is 50~100um. Then we analyze the issue of contact and current spreading. The results show that the light extraction efficiency decreases due to the shadow effect of the p-contact while the current crowds near the p-contact. In order to overcome this effect, the light extraction efficiency can be greatly improved by integrating high reflectivity contact, current blocking layer and diffused bottom mirror. We also compare the light extraction efficiency of LED packaging with bare chip. From the optical point of view, the patterned substrate should be regarded as the most effective way in enhancing light extraction efficiency in an encapsulated flip-chip LED. Besides, the diameter of encapsulant lens must be larger 2.5 times than chip size, and to increase the encapsulant’s refractive index usually is more effective at improving light extraction efficiency than light extraction structure. Finally, in order to improve light extraction efficiency, the LED with micro-structure is analyzed and optimized. The simulation results show that the light extraction efficiency is improved by multiple reflections and reflected scattering in a LED chip, so that increasing reflectivity of the bottom mirror is an important key factor.

並列關鍵字

Package ； Current Spreading ； Micro-Structure ； Light Extraction Efficiency ； Light-Emitting Diode ； GaN

參考文獻

[1]A. Zukauskas, M. S. Shur, and R. Caska, Introduction to Solid-state Lighting, John Wiley & Sons, New York (2002).

[3]E. F. Schubert and J. K. Kim, “Solid-state light sources becoming smart,” Science 308, 1274-1278 (2005).

[5]N. Holonyak and S. F. Bevacqua, “Coherent (visible) light emission from Ga(As1-xPx) junctions,” Appl. Phys. Lett. 1, 82-83 (1962).

[6]A. A. Bergh and P. J. dean, Light emitting diodes, Clarendon Press, Oxford (1976).

[7]C. P. Kuo, R. M. Fletcher, T. D. Osentowski, M. C. Lardizabal, M. G., Craford, and V. M. Robbins, “High performance AlInGaP visible light emitting diodes,” Appl. Phys. Lett. 57, 2937-2939 (1990).