自我複製式光子晶體與微反射鏡陣列應用於發光二極體以提升光汲取效率之研究

Light extraction enhancement for InGaN/GaN LED by auto-cloned photonics crystal and micro mirror array

黃承揚(Chen-yang Huang)

指導教授 ： 趙煦

國立清華大學/電機資訊學院/光電工程研究所/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00255

摘要

本論文提出幾種結構來改善氮化鎵發光二極體之光汲取效率，主要藉由自我複製式光子晶體(auto-cloned photonic crystal，簡稱APhC)，與微反射鏡陣列結構(micro mirror array，簡稱MMA)，來提升發光二極體之光汲取效率。
使用有限時域差分法(finite difference time domain，簡稱FDTD)，模擬自我複製式光子晶體發光二極體，與微反射鏡陣列發光二極體之光汲取效率分析。藉由光子晶體之反射與繞射特性，可回收發光二極體背部光線，同時導正因為全反射特性無法出光的光線進入發光二極體出光錐體內(escape cone)。
以薄膜成長理論詳述自我複製式光子晶體的成長機制。模擬薄膜材料在沉積與蝕刻相互調配下，可將薄膜堆疊成多層鋸齒狀分佈，即為自我複製式光子晶體結構。同時，實驗製鍍Ta2O5/SiO2多層膜堆疊在圖案化藍寶石基板上(pattern sapphire substrate)，調變射頻偏壓(radio frequency，簡稱RF)功率，可影響鋸齒狀堆疊特性，此實驗結果與模擬可以相互呼應。操作優化後之射頻偏壓功率30~45W，可堆疊61層之Ta2O5/SiO2鋸齒狀多層膜，仍能維持鋸齒狀角度與形狀結構。
製作自我複製式光子晶體發光二極體(auto-cloned photonic crystal LED，簡稱APhC LED)，藉由光子晶體之反射與繞射特性回收背部光線，提升光汲取效率。自我複製式光子晶體發光二極體相較於傳統發光二極體有97%光提升效果，也比背部製作反射鏡的發光二極體(Bragg reflector LED，簡稱BR LED)，提升22%光汲取效果。此種結構優點在於光子晶體可以用薄膜濺鍍方式大面積製作於發光二極體之背部，且不侵入磊晶層結構，亦不干擾晶粒電性製程，即可單純提升發光二極體出光亮度。
以橫向磊晶技術(Epitaxial lateral overgrowth，簡稱ELOG)埋崁微反射鏡陣列於發光二極體內部，提升發光二極體光汲取效率。在此之前，需測試陣列式薄膜對於高溫磊晶環境的耐受程度。結果可知週期6m、間距3m、10層之Ta2O5/SiO2多層膜微反射鏡，經過1200oC高溫耐受實驗後不會有任何剝落與損壞，且仍能維持高反射率。可將微反射鏡陣列相融於磊晶製程條件當中。
具有內埋式反射鏡陣列之發光二極體，相較於一般發光二極體具107%光汲取效率，也比圖案化SiO2陣列發光二極體(patterned SiO2 array LED，簡稱P-SiO2 LED)提升36.4 %的光汲取效果。此結構具有三項特點來提升出光亮度的功能。第一利用反射鏡當作阻擋層(mask)，抑制磊晶線差排缺陷(threading dislocation defect)向上延伸至主動發光層，提升內部量子效應。第二反射鏡陣列埋堪於GaN內部，如同將反射元件放置在離主動發光區不到3m的距離內，直接反射背部光線朝向正面出光。第三該三角形陣列排置方式兼具繞射特性，可將發光二極體內部因為全反射侷限之光線，藉由多階繞射光束導正至出光錐體內。
本篇論文研究，主要以光學薄膜製作自我複製式光子晶體與微反射鏡陣列。並藉由該元件的反射特性，讓發光二極體背部光線反射朝向正面出射。同時繞射特性破壞內部全反射效應，取出被侷限在發光二極體內部之光線，提升出光亮度。

關鍵字

自我複製式光子晶體 ； 微反射鏡陣列 ； 發光二極體 ； 有限時域差分法 ； 橫向磊晶

並列摘要

In this dissertation, we present several types of light extraction enhancement structures on the backside of light emitting diodes (LEDs) to enhance the efficiency of light output. The first one is auto-cloned photonics crystal (APhC) on the backside of the sapphire wafer of the LED substrate, and the second one is micro mirror array (MMA) structure which was embedded in the gallium nitride (GaN) LED mainbody by the fabrication process of epitaxial lateral overgrowth (ELOG).
The first section of the dissertation is related to research of the APhC. Based on the theory of thin film growth, we simulated the growth of the auto-cloned Ta2O5/SiO2 multi-layer photonic crystal with a lateral saw-tooth period under the mechanism of deposition and etching. Ion-beam-sputter (IBS) was applied to deposit the films and RF-bias etching was applied simultaneously with the IBS on the Ta2O5 film. Both simulation and experiment results showed that the quality of the auto cloning can be optimized and well controlled by the RF-bias power. There exists an intermediate power range, within this range, the drop of peak to valley height variation of the saw-tooth profile can be reduced significantly to achieve high degree of auto-cloning. Analysis showed that simultaneous deposition and etching at the proper RF-bias power on the Ta2O5 has the capability to compensate the flattening effect of the SiO2 deposition such that the saw-tooth surface profile can be maintained.
In the second section of the dissertation, we introduce the fabrication of three dimensional (3-D) APhC of Ta2O5/SiO2 multi-layers on the backside of the sapphire wafer that has InGaN/GaN multi-quantum wells (MQWs) LED on the front side. 94% light extraction enhancement in comparison to the LED without APhC was obtained. Electrical properties of the LED did not affected by the APhC and its fabrication process. Experimental evidences showed that light extraction enhancement mechanism is in two aspects: for rays that are emitted from the source and incident at lower angle of incidence to the APhC, the APhC acts as a high reflector; for rays incident at higher angle of incidence to the APhC, first order diffracted light from the APhC appears, the diffracted light is concentrated around the surface normal and is therefore capable of escaping.
In the third section of the dissertation, we propose a light extraction enhancement structure by using the heat-resistive dielectric MMA embedding in the ELOG GaN. Taking advantages of reducing dislocation density by ELOG together with the capability of diffraction and high reflectance of the patterned structure from the MMA, higher light output power for the LED can be expected. The MMA of Ta2O5/SiO2 dielectric multi-layer with the mirror diameter of 3m and the array period of 6m was fabricated on c-plane sapphire substrate. ELOG of GaN was applied to the MMA that was deposited on both sapphire and sapphire with 2.56m GaN template. The MMA was subjected to 1200oC high temperature annealing and remained intact with high reflectance in contrast to the continuous multi-layer for which the layers have undergone severe damage by 1200oC annealing. The result implies that our MMA is compatible to the high temperature MOCVD growth environment of GaN.
In the final section of the dissertation, we propose fabrication of MQWs InGaN/GaN LEDs, 300m  300m chip size, with Ta2O5/SiO2 dielectric multi-layer MMA embedded in the ELOG GaN on the c-plane sapphire substrate. MQWs InGaN/GaN LEDs with ELOG embedded patterned SiO2 array (P-SiO2) of the same dimension as the MMA were also fabricated for comparison. Dislocation density was reduced for the ELOG samples. 75.2% light extraction enhancement for P-SiO2-LED and 102.6% light extraction enhancement for the MMA-LED were obtained over the standard LED. We demonstrated that the trapped lights can be redirected from the MMA by multiple-diffraction to escape from the LED. Therefore, the light extraction can be enhanced.

並列關鍵字

auto-cloned photonic crystal ； micro mirror array ； light emitting diode ； finite difference time domain ； epitaxial lateral overgrowth

參考文獻

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