高功率氮化鎵發光二極體結構特性之研究

Abstract

近年來，發光二極體的發展著重於照明上的應用，因此高注入高效率發光二極體的技術發展儼然成了重要的課題，而發展過程中，除了製程技術的提升外，所遇到最大的問題便是在高電流注入下產生之效率下降(efficiency droop)的問題。 因此本研究期望透過元件製程結構與磊晶基板的設計改善效率下降的問題。第一部分，發光二極體製程結構設計，此部分分別研究傳統面上型發光二極體 (Face up LED, FU-LED)以及覆晶式發光二極體(Flip chip LED, FC-LED)。由實驗結果可得FC-LED相較於FU-LED於1A操作電流下，發光效率 droop 現象提升了29%，並且利用二極體理想因子β與Power law證實了電流聚集(current crowding)對droop效率的影響性是由於FC-LED出光與電極處於元件不同端，因此其P型氮化鎵上覆蓋導電金屬做反射與電流擴散層，相較於FU-LED之P型氮化鎵上僅使用氧化铟锡(ITO)作電流擴散層，因此使得FC-LED晶粒於高功率注入下仍具有均勻之電流分布，降低了電流聚集的效應，同時改善歐傑電子復合(Auger recombination)及電子溢流(leakage current)，而減少了效率下降的問題。 第二部分，利用同質磊晶技術，比較氮化鎵基板(FS-GaN)與藍寶石基板(PSS)製作FC-LED。實驗結果得到，FS-GaN FC-LED於1A操作電流擁有無效率下降的現象(no droop)，這是由於氮化鎵基板具有較佳之磊晶品質與較高的熱傳導能力，減少熱累積與非輻射複合，在高電流注入獲得更高的發光效率。

Nowadays, light emitter device (LED) has been widely used in the solid-state lighting, so the development of high-power LED become very important. Besides, the reduction of efficiency droop plays a critical role in high-power operating. In this thesis, we expect to improve efficiency droop problem by LED structures and substrate design. First, we demonstrated two type LED structures, including the flip chip LEDs (FC-LED) and the traditional face up LEDs (FU-LED) with the chip size of 45x45 mil. In the experiment, FC-LED exhibited a maximum external quantum efficiency about 68.2% and the efficiency droop was reduced from 56% to 27% at the injection current of 1A, comparing with FU-LED. We investigate efficiency droop effect by using ideal factorβ and power law (i.e. L∝I^S). Because he FC-LED has Ag metal as conductive and reflective layer that benefits the current spreading. Therefore, FC-LED reduces efficiency droop effect and improve Auger recombination and current leakage. The second part, we utilized the homoepitaxial growth on the free-standing GaN substrate (FS-GaN) to fabricate flip chip LED (FC-LED). The experiment results show that FS-GaN FC-LED has greater efficiency with no droop at injection current of 1 A, because the better epitaxial quality and thermal conduction. We get excellent emission efficiency due to the decreasing of thermal budget and non-radiative recombination.