大尺寸LED晶片Efficiency Droop之光電熱效應研究;Optical-Electrical-Thermal Effect on Efficiency Droop in Large Size Light Emitting Diode Chips

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題名:	大尺寸LED晶片Efficiency Droop之光電熱效應研究;Optical-Electrical-Thermal Effect on Efficiency Droop in Large Size Light Emitting Diode Chips
作者:	蕭仲博;Hsiao,Chung-Po
貢獻者:	機械工程學系
關鍵詞:	氮化鎵;發光二極體;大尺寸晶片;效率衰退;GaN;LED;large size chip;Efficiency Droop
日期:	2014-07-29
上傳時間:	2014-10-15 17:19:27 (UTC+8)
出版者:	國立中央大學
摘要:	近年來，InGaN/GaN 多重量子井藍光或綠光LED已是固態照明中的主流，然而Efficiency Droop 是InGaN-based LED中相當關鍵的問題。Droop的現象是在高電流注入的情況下，內部量子效率會有極大的衰退，這會使LED元件的光功率輸出下降。在本研究中，我們利用有限元素法及蒙地卡羅法建立一套光電熱耦合數值模擬模型。透過電流守恆及能量守恆，我們可以了解載子在活化層內復合及漏電流之行為，並且定義其熱源方程及內部光分布。接著利用光追跡方法以內部光分布作為出光光源進行模擬。最後可以探討電流分布對接面溫度、電效率、內部量子效率、光萃取效率以及光功率之影響。本研究探討大尺寸垂直與側向高功率晶片在光電熱交互影響下現象之Droop現象。由實驗可知側向結構的接面溫度上升較快且有更嚴重的Droop。透過模擬我們可以了解漏電流是造成Droop的主要機制，並且在高功率操作下升溫的過程由漏電流主導。此外，電流壅塞也對量子效率的影響及大，事實上，電流分布是啟動Droop的關鍵。受到電流壅塞的影響，活化層的內部量子效率會在Droop發生前後有完全相反的分布。利用光學模擬，在光萃取效率也會有衰退的趨勢，其主要機制為活化層吸收效應。雖然電流分布在垂直結構較為優異，但受到電流壅塞的區域影響，電極遮蔽及吸收效應也更為強烈。這故造成了活化層具有較小的吸收係數但光萃取效率卻有較大的衰退。 ;Recently, InGaN/GaN multi-quantum well (MQW) of blue or green light emitting diodes (LEDs) has attracted great interest in solid-state lighting. However, the efficiency droop restricts LEDs ability under the high power operation which is the critical issue. The droop makes the internal quantum efficiency has a great drop. In this study, two commercial LED structures are investigated which are vertical and lateral chip respectively. The finite element method (FEM) and Monte Carlo statistics method are applied to the optical-electrical-thermal coupled numerical model. Through the current and energy conservation, we can define every carrier behavior in active layer and heat generation. We also can obtain the light source from the calculated IQE to start the ray tracing. Then we analyze the interaction of current spreading effect on electrical efficiency, internal quantum efficiency, light extraction efficiency, junction temperature and light output power. The experiment shows the more nonlinear increase of junction temperature and severer droop in lateral structure. By the simulation, the carrier leakage is the main mechanism from the nature of materials which induces the droop. Furthermore, current crowding also affects the quantum efficiency. In fact, current spreading is the key to drive the droop from external. In the final part, by the optical simulation, the light extraction efficiency also plays an important role of efficiency droop, which is caused by the absorption effect. Although the current crowding of the vertical structure degree is lower, the stronger absorption by n-pad results in the greater efficiency droop, even the absorption coefficient is smaller than lateral structure. Based on these analyses of the simulation, the better LED device is expected to improve.
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