具有奈米級微結構表面之高亮度垂直式氮化鎵發光二極體元件製作-總計畫暨子計畫一：利用磊晶結構優化改善高功率發光二極體的效率下降(I)

Abstract

在節能省碳的意識逐漸高漲，如何更有效率的使用能源成了未來最重要的研究主題，綠色產品方興未艾。白光發光二極體由於具有節省能源、環保、高壽命與堅固耐用等優點，因此目前中、美、日、韓歐洲等國家均透過國家型計畫，投入大量資金及人員全力積極發展相關技術與應用。白光發光二極體在固態照明的應用上尚未完全普 及，最主要的因素有下列幾點：(1) 效率太低、(2) 藍寶石基板散熱問題、(3) 高電流操作下效率驟降等問題。在高功率照明燈具的應用之下，發光二極體面臨到了幾個嚴重的考驗，其中最重要的分別為高注入下的效率下降以及元件散熱。從實驗以及模擬結果中可發現，氮化鎵發光二極所面臨的最大問題就是電子與電洞的遷移率(約為10 cm2/Vs以及800 cm2/Vs)相差了將近兩個數量級，會使得電子與電洞在主動層中傳遞的速度不成比例，這樣的現象容易造成電子溢流而效率下降，亦或是載子濃度太高而發生Auger復合。而欲解決效率下降的問題最主要的方式還是針對發光二極體的主動層結構做優化設計。本項子計畫將透過磊晶結構優化改善高功率操作下的效率下降的問題，內部量子效益將透過量子井/位能障結構設計及採用無極性材料來達成。為了早日實現固態照明，兩年期┌具有奈米級微結構表面之高亮度垂直式氮化鎵發光二極體元件製作┘整合型計畫目標為採用垂直結構的元件並使用散熱較好的基板，並結合其他子計畫所開發的垂直型發光二極體製程，提出一種新穎奈米柱狀結構化的氮化鎵模板，預計在此一計畫中嘗試使用化學性濕蝕刻剝離法來取代雷射剝離製程，此方法把氮化鎵薄膜轉移到了高導電性、高散熱性的新基板上，同時表面的奈米結構可增加表面光取出進一步提升元件特性發展更適合高功率固態照明的發光二極體。

For a more energy-efficient and CO2-reducing era, the eye-catching "Green Products" are coming and still-growing. Eco-friendly white-light LED has shown great promise and benefits for energy-saving, improved durability. The LED-related techniques and applications are fully supported by the countries around the world including China, the United States, Japan, Korea and European Union. Today, white modules are not available to all. To be commercialized, some pressing problems hindering further advances in solid state lighting includes: (1) Low Quantum Efficiency, (2) Poor Heat Dissipation of Sapphire Substrate and (3) Current Droop. We believe that the most challenging obstacle is the efficiency droop which inhibits development for high brightness and high power devices. It was recognized that electron overflow and poor hole injection efficiency result in seriously rollover, as a monotonically decreases with further increase in current. To defeat Droop, a sub-project “Efficiency Droop in High Power Light-emitting Diodes by Optimization of Epitaxial Structure” is suggested. We try to alter the materials and surface orientations in LED active region to surpass the deleterious droop effect. Furthermore, we will propose a simple approach to successfully separate GaN film from its substrate without harming the material quality. In the integrated project, a navel chemical lift-off process based on embedded nano-rods template is introduced to form the vertical type LEDs. Specifically, the nanostructure texture surface of GaN epilayer is believed to improve the light extraction efficiency of III-nitride LEDs.