| 摘要: | 本論文旨在提升氮化鎵發光二極體之光萃取效率。第二章探討二維柱狀結構於氮化鎵發光二極體表面與光萃取效率的關係。吾人利用三維有限時域插分法計算俱二維柱狀結構之氮化鎵發光二極體的光萃取效率。模擬結果顯示，隨著結構深度的加深，對應的光萃取效率也隨的提高。在實驗上，吾人鋪排直徑為1.5 ?m的單層聚苯乙烯小球於氮化鎵發光二極體的表面做為蝕刻遮罩，經過乾式蝕刻後，週期為1.5 ?m的氮化鎵柱便形成於元件的表面。製作出來的氮化鎵柱高度分別為80 nm、120 nm及180 nm的發光二極體。在直流電流為20 mA注入下，不同柱高元件所對應的光功率增益分別為12.0%、19.1%及30.7%。此實驗的結果與模擬得到的結果趨勢相同。製作二維週期性表面結構於LED元件表面的取光機制可由k空間的圖形來解釋。由於氮化鎵磊晶層的折射指數大於藍寶石基板與空氣，使得從量子井所發出來的光大部份都侷限於氮化鎵層。於是吾人利用小球製作柱狀結構於氮化鎵發光二極體表面，只要滿足此式，|kn//+G|<k0(kn//為波導傳播模態之波向量、G為週期結構所提供的倒晶格向量而k0為自由空間的波向量)，則該模態的光將可以藉此週期結構被取出。製作週期結構於元件的表面時，使得直流電流為20mA注入下，元件的操作電壓有些微的提升，此電壓的提升是由於部份的p-GaN在蝕刻製程時被破壞，造成透明導電層與p-GaN的接觸電阻增加所造成。 第三章探討製作二維空氣柱結構於氮化鎵發光二極體表面與光萃取效率的關係。吾人利用三維有限時域插分法計算俱二維柱狀與孔洞表面結構之氮化鎵發光二極體的光萃取效率。模擬結果顯示，隨著結構深度的加深，對應的光萃取效率也隨的提高。二維空氣洞的製作係透過微米球能聚焦的特性來完成。將微米鋪排於已塗佈光阻之氮化鎵發光二極體表面，經過曝光及顯影的步驟後，製作出二維空氣洞結構於光阻上，再透過乾蝕刻的方式將圖案轉移到發光二極體的表面。在此研究中，吾人製作了三種不同的空氣洞深度於元件上，深度分別為80 nm、120 nm及180 nm，其空氣洞的直徑皆為900 nm。在直流電流為20mA注入下，其光功率相較於未製作結構之元件的增益分別為9.5%、20.7%及35.7%。隨著結構深度的增加，元件的光功率亦跟著提升，此結果與模擬的趨勢相仿。由於製作表面結構會破壞到p-GaN，導致元件的操作電壓有些微的提升，此電壓的提升是由於部份的p-GaN在蝕刻製程時被破壞，造成透明導電層與p-GaN的接觸電阻增加所造成。 第四章旨於製作奈米級結構於藍寶石基板，並成長氮化鎵發光二極體磊晶層於此基板上，來達到提升光萃取效率的目的。在此研究中，吾人將直徑為750 nm的二氧化矽小球鋪排於鍍有100 nm二氧化矽薄膜的藍寶石基板上，先使用乾蝕刻方法將球與球交接處孔洞下的二氧化矽薄膜移除，再利用濕蝕刻的方法來蝕刻藍寶石基板。製作來的結構為二維的柱狀結構，週期為750 nm且高度200 nm。透過成長氮化鎵發光二極體磊晶層於此基板上並製作出的發光元件，其光功率相較於成長在平面藍寶石基板的元件增強了76%。從變溫發光光譜量測的結果，俱奈米結構於藍寶石基板之發光二極體其內部量子效率增強了19.6%。由此二數值可推得俱奈米結構於藍寶石基板之發光二極體的光萃取效率增強了47%。從三維有限時域插分法計算得到俱奈米結構於基板的發光二極體在光萃取效率上有40%的提升。此模擬結果與實驗的結果有些微的差距，吾人推測，這是因為模擬時沒有考慮到氮化鎵薄膜亦存在鋸齒狀結構於與藍寶石基板的交界面，此鋸齒狀結構將會增加光的散射機率，進而提升元件的光萃取效率。從實驗的結果可以得到，氮化鎵發光二極體光功率的增強係歸究於內部量子效率與光萃取效率的提升。 In this dissertation, we focused on improving the light extraction efficiency (LEE) of the GaN-based light-emitting diodes (LEDs). In Chapter 2, we explored the dependence of rod-texturing effect on LEE of the GaN LEDs. Three-dimensional finite-difference time-domain (3D FDTD) was employed to calculate the LEE of the rod-textured LEDs. The simulation results revealed that the LEE increased with increasing the height of the rods which were formed on the surface of the LEDs. An experiment was carried out to prove the simulation results. We deposited a monolayer of polystyrene (PS) microspheres of the diameter of 1.5 ?m onto the surface of LEDs and these spheres were used to serve as etching mask to resist the dry etching process. The periodic patterns were transferred onto the surface of GaN-based LEDs. We had fabricated 3 different depths, which were 80 nm, 120 nm and 180 nm, in order to check relations between etching depth and LEE enhancement. Under 20 mA dc current injection, the output power enhancements were 12.0%, 19.1% and 30.7%. The experimental results displayed the same trend as the simulation results. The physical origin of extracting the light by fabricating the 2D periodical structure can be explained by k-space diagram. Large portion of light emitted by the active layer are guided in the GaN layer because the refractive index of GaN, 2.4, is larger than those of air and sapphire, which are 1 and 1.76, respectively. Each guided mode is featured with its own wavevector, kn//. When periodical structure is formed onto the surface of GaN, the periodical structure provides a reciprocal lattice vector, G, which can couple the kn//. Once the condition, |kn//+G|<k0, is satisfied, the guided light can be extracted into air. The forward voltages, under 20 mA dc current injection, of these rod-textured LEDs compared with the LED without surface texturing are slightly degraded. The texturing process results in ion damage in the p-GaN layer. The increase in forward voltage can be attributed to the increase in contact resistance between TLC and textured p-GaN. In Chapter 3, we simulated, fabricated and characterized the GaN-based LEDs featuring with 2D air hole structure on the surfaces. The 3D FDTD was employed to calculate the LEE enhancement of the air hole-textured LEDs with different depths on the GaN surface. We found that the LEE enhancement increased with the increasing of the texturing depth of the structure. The hole structure was first formed on the photoresist via the focusing property of the microspheres and then transferred onto the surfaces of the GaN LEDs by dry etching process. In this study, we had fabricated 3 different etching depths, 80 nm, 120 nm and 180 nm, on the GaN surfaces, and the corresponding power enhancements for different etching depths were 9.5%, 20.7% and 35.7%. The experimental results follow the same trend as the 3D FDTD simulation predicted. The forward voltages of the hole-textured LEDs were slightly increased and were attributed to the damage of p-GaN layer during dry etching process. In Chapter 4, we fabricated nano structure on the sapphire substrates and grew the GaN LED epitaxial structure on it in order to improve the light extraction efficiency. The nano structure is fabricated by using natural lithography technique. In this study, we deposited a monolayer of 750-nm-diameter SiO2 nanospheres onto the sapphire substrate which was coated SiO2 thin film with thickness of 100 nm. A dry-etching process was followed to remove the SiO2 thin film that was uncovered by the SiO2 spheres until the sapphire surface was exposed; then, the sample was wet-etched. 2D sapphire rods were finally formed on the sapphire substrate with period of 750 nm and height of 200 nm. The optical power of the nano-patterned sapphire substrate (NPSS) LED is improved by 76% compared with the LED on flat sapphire substrate (FSS). According to the temperature-dependent photoluminescence result, the internal quantum efficiency (IQE) is improved by 19.6%. Therefore, we can deduce that the LEE enhancement of the NPSS LED is 47%. We obtain 40% LEE enhancement from the 3D FDTD simulation results. The minor difference is attributed to the existence of voids in the GaN film at the sapphire/GaN interface which we do not take into concern in the simulation. These voids may enhance the scattering and further increase the LEE. According to the experimental results, we can conclude that the power improvement of NPSS LED is due to the improvement of both IQE and LEE. |
