以奈米壓印技術製作高磊晶品質與光萃取效率氮化鎵發光二極體

Abstract

在本研究中，我們利用 Nanoimprint Lithography (NIL) 製作二氧化矽圓錐狀圖型基板以及內嵌中空空氣孔洞模板，並成功成長成氮化鎵系列之發光二極體。 在二氧化矽圓錐狀圖型基板上成長的發光二極體 (SiO2-LED) 研究中，從式電子顯微鏡 (Transmission electron microscopy, TEM) 拍攝的圖片中我們可以明顯的指出二氧化矽圓錐狀圖型側壁有橫向的堆疊錯誤(stacking faults)，這些橫向的堆疊錯誤(stacking faults)能夠有效的抑制直向的缺陷(threading dislocation)。此二氧化矽圓錐狀圖型有藉由光散射效應進而增加光萃取效率之可能性，這是由於二氧化矽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。這也在TracePro的模擬中得到驗證。 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藍寶石圓錐型圖形化基板 (cone-shaped patterned sapphire substrate, CPSS) 和成長在二氧化矽圓錐狀圖形化模板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-LED)在二十毫安培操作電流下，光強度有著百分之四十一以及百分之六十三之提升。 在前一個部分，我們成功的使用奈米壓印技術在藍寶石基板上製作出二氧化矽圓錐狀圖型。在這個研究的基礎下，接著我們想將奈米壓印技術應用在未摻雜氮化鎵磊晶層，且為了更進一步提升光萃取效率，使用了長方體空氣孔洞的結構，來達到我們的需求。 在發光二極體成長於長方體空氣孔洞模板的研究中，我們可以從穿隧式電子顯微鏡 (TEM) 中明顯的觀察到次微米級的空氣孔洞氮化鎵磊晶層之中。空氣孔洞氮化鎵磊晶層的折射率差可以有效的提高光萃取效率。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。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 20.6 % 和 20.3 %的提升。最後我們對光強度、電流對電壓作圖來研究其電學特性，可以發現成長成長在長方體空氣孔洞模板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-LED)在二十毫安培操作電流下，光強度有著百分之四十五之提升。

In this research, the high performance GaN-based light-emitting diodes (LEDs) growth on SiO2 crown-shaped pattern substrates and cubic airvoids by Nanoimprint Lithography (NIL) were demonstrated. In the first part, we successfully transferred the patterns of a cone-shaped patterned sapphire substrate (CPSS) into SiO2 layer to fabricate a cone-shaped SiO2 patterned template by using Nanoimprint Lithography (NIL). The GaN-based light-emitting diodes (LEDs) were grown on this template by metal-organic chemical vapor deposition (MOCVD). The transmission electron microscopy (TEM) images suggested that the stacking faults formed near the cone-shaped SiO2 patterns during the epitaxial lateral overgrowth (ELOG) can effectively suppress the threading dislocations, which results in an enhancement of internal quantum efficiency. The Monte Carlo ray-tracing simulation revealed that the light extraction efficiency of the LED grown on cone-shaped SiO2 patterned template can be enhanced as compared with the LED grown on CPSS. As a result, the light output power of the LED grown on cone-shaped SiO2 patterned template outperformed the LED grown on CPSS.

At the previous research, we transferred the cone-shaped pattern into sapphire substrate. Based on the studies of SiO2-LEDs, we started to transfer the imprint pattern from sapphire substrate to undoped-GaN. To improve the light extraction efficiency of nitride-based LEDs, we expect the cubic airvoids structure in undoped-GaN can reflect more downward light from MQW. Finally, the cubic airvoids template has been demonstrated to satisfy our purpose. In the second part, we successfully transferred the cubic patterns into uGaN layer to fabricate a cubic airvoids patterned template by using Nanoimprint Lithography (NIL). Submicro-scale airvoids were clearly observed in undoped-GaN by transmission electron microscopy (TEM). The difference reflective index between air voids (n=1) and GaN (n=2.45) can increase the light extraction efficiency due to additional total reflection. To quantify the internal quantum efficiency (IQE), power dependent photoluminescence measurement (PDPL) at room temperature 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 20.6% and 20.3%.