次世代基板成長高效率氮化鎵系列發光二極體之研究

Abstract

在本研究中，我們利用 metal-organic chemical vapor deposition (MOCVD) 將氮化鎵系列之發光二極體成功的成長在火山口狀圖形化藍寶石基板以及內嵌中空空氣孔洞及二氧化矽之氮化鎵奈米柱基板。 在火山口狀圖形化藍寶石基板上成長的發光二極體 (CPSS-LEDs) 研究中，從穿隧電子顯微鏡 (Transmission electron microscopy, TEM) 拍攝的圖片中我們可以明顯的指出缺陷(dislocation)有著彎曲倒像在圖形化基板側壁的趨勢。 而從上視的陰極光偵測系統所擷取之圖可以發現 CPSS-LEDs 有著較均勻的發光圖形。除此之外，從掃描式電子顯微鏡所 (Scanning electron microscopy, SEM)拍之截面圖可以發現在圖形化基板之上有著甜甜圈形狀的空氣孔洞。此空氣孔洞有藉由光散射效應進而增加光萃取效率之可能性，這是由於空氣本身和氮化鎵磊晶層有著極大的折射率差異，導致光從氮化鎵欲進入空氣時有著極小的全反射角，進而使的光能夠反射而使萃取效率提升。 再來我們對光強度、電流對電壓作圖來研究其電學特性可以發現成長在半球型圖形化基板 (Hemisphere patterned sapphire substrate, HPSS) 和成長在火山狀圖形化基板 (CPSS) 之發光二極體相對於成長於一般藍寶石基板之發光二極體 (C-LEDs)在二十毫安培操作電流下，光強度有著百分之二十以及百分之三十二之提升。名為 Tracepro 的光學模擬軟體在我們想要進一步了解光萃取效率提升的目標下被使用。從光學模擬的結果中，在主動區設定10毫瓦強度之發光源之下，成長於三種基板上之發光二極體分別在遠場之積分球上收到3.57毫瓦、5.41毫瓦以及5.69毫瓦之光強度。此結果也和我們的實驗結果趨勢吻合。 在我們成功成長發光二極體結構於CPSS上的基礎上，我們開始考慮將發光二極體成長於奈米等級的結構之上。為了將發光二極體的效率提高已達到更好的應用目的，我們想藉由奈米等級的結構進一步的降低缺陷密度以及提高光萃取效率來達成我們的目的。最後，我們選擇了氮化鎵奈米柱的基板來達到我們的需求。 在發光二極體成長於氮化鎵奈米柱基板的研究中，我們可以從掃描式電子顯微鏡 (SEM) 中明顯的觀察到微米級的空氣孔洞和奈米級的二氧化矽在奈米柱之間和在奈米柱及再成長之氮化鎵磊晶層之間。空氣孔洞及二氧化矽和氮化鎵磊晶層的折射率差可以有效的提高光萃取效率。而從穿隧式電子顯微鏡 (TEM) 所截取的圖中可以發現，藉由nanoscale epitaxial lateral overgrowth (NELOG) 效應使得線缺陷彎曲而隱沒在空氣孔洞以及二氧化矽中。進一步地，我們利用室低溫的變強度光學量測 (PDPL) 來定義內部量子效率來驗證磊晶品質的提升。 最後，藉著Finite-difference time domain (FDTD) 模擬來進一步的驗證光萃取的效率。從光學量測和光學模擬的結果中，我們得知可以有效的提高光萃取效率。而從穿隧式電子顯微鏡 (TEM) 所截取的圖中可以發現，藉由nanoscale epitaxial lateral overgrowth (NELOG) 效應使得線缺陷彎曲而隱沒在空氣孔洞以及二氧化矽中。進一步地，我們利用室低溫的變強度光學量測 (PDPL) 來定義內部量子效率來驗證磊晶品質的提升。 最後，藉著Finite-difference time domain (FDTD) 模擬來進一步的驗證光萃取的效率。從光學量測和光學模擬的結果中，我們得知內部量子效率和光萃取效率分別有了 15 % 和 44 %的提升。

In this research, the high performance GaN-based light-emitting diodes (LEDs) growth on crown-shaped pattern sapphire substrates (CPSS) and GaN nanorods template with embedded microscale air voids and SiO2 nanomask by metal–organic chemical vapor deposition (MOCVD) were demonstrated. From transmission electron microscopy (TEM) image, we can clearly observed the dislocation bending. And the plan-view cathodoluminescence (CL) image shows the light uniformity of CPSS-LEDs. Besides, by cross-sectional scanning electron microscopy (SEM), donut-shaped air voids on the top of the crown PSS have been observed. These air voids is consider to enhance the light scattering effect and further enhance the light extraction due to the huge difference reflective index between air voids and GaN epilayer. From the power-current-voltage (L-I-V) characteristic, we can figure out the light output power enhancement of hemisphere patterned sapphire substrate (HPSS) LEDs and CPSS-LEDs compared with conventional LEDs (C-LEDs)is about 20 % and 32.1 % under 20 mA, respectively. For the purpose to prove light extraction enhancement, the Tracepro simulation have been used. According to the simulation result, we find out the output power of three sample are 3.57 mW, 5.41 mW and 5.69 mW where we give the total emission power is 10 mW. This result is well agree with our L-I measurement. Based on the research of CPSS-LEDs, we start to considering the nano structure inside the epitaxial layer. For the wish to push the efficiency of nitride-based LEDs reach the application needed, we expect the nano structure can reduce more dislocation and enhanced higher light extraction due to density of patterned. Finally, the GaN nanorods template have been demonstrate to satisfy our purpose. Microscale air voids and SiO2 nanomask were clearly observed at the interface between GaN nanorods (NRs) and the overgrown GaN layer by scanning electron microscopy (SEM). The difference reflective index between air voids (SiO2) and GaN can increase the light extraction efficiency due to additional total reflection. The transmission electron microscopy (TEM) images show the threading dislocations were suppressed by nanoscale epitaxial lateral overgrowth (NELOG). To quantify the internal quantum efficiency (IQE), power dependent photoluminescence measurement (PDPL) at roomtemperature and low temperature has been used. And the finite-difference time domain (FDTD) simulation was also be used to calculate the light extraction efficiency. From the PL measurement and FDTD simulation, we estimate the IQE and LEE enhancement are 15% and 44%.