薄膜式氮化鎵發光二極體之亮度提升; Light Enhancement of Thin-GaN Light Emitting Diodes

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题名:	薄膜式氮化鎵發光二極體之亮度提升;Light Enhancement of Thin-GaN Light Emitting Diodes
作者:	林京亮;Ching-Liang Lin
贡献者:	化學工程與材料工程研究所
关键词:	發光二極體;氮化鎵;雷射剝離;晶圓鍵合;歐姆接觸;聚苯乙烯;蜂窩狀結構;honeycomb structure;ohmic contact;GaN;Poly-styrene;wafer bonding;Light Emitting Diodes;Laser lift-off
日期:	2008-06-04
上传时间:	2009-09-21 12:24:34 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	氮化鎵(GaN)材料發展在近二十年內有了相當大的突破，特別是將氮化鎵以化學氣相沈積的方式成長在藍寶石(Sapphire)基板上，可以得到單晶，且直接能隙的氮化鎵，藉由摻雜銦(In)可調變氮化鎵發光波長自紫外光區到藍光，目前藍光發光二極體主要的材料即是氮化鎵為主的材料，也因為得到了藍光氮化鎵材料，使得利用藍光發光二極體配合黃光螢光粉或其他波長轉換材料，將發光二極體用於白光照明用途的可能性大大提升。照明用途的氮化鎵發光二極體必須要具備效率高的特性，也就是發射出的光功率與消耗的電功率比值要愈高愈好。此外，氮化鎵發光二極體在高電功率操作時，散熱的問題將變得非常棘手，由於傳統式(Conventional)發光二極體結構是以導熱係數低的藍寶石為基板，造成傳統式發光二極體溫度極高而有光電特性退化的議題。因此，在本研究中，元件的製作結構為薄膜式發光二極體結構，此結構利用晶圓鍵合技術以及雷射剝離技術，將氮化鎵薄膜自磊晶的藍寶石基板轉移到導熱係數較高的矽基板，由於散熱特性較佳，所以薄膜式發光二極體已經被視為發展高亮度氮化鎵發光二極體的主要結構。在本研究中，將對此結構的兩個重要的議題做探討：(1)設計適合使用於薄膜式發光二極體製程的金屬反射層及歐姆接觸系統以及(2)提高光自薄膜式發光二極體的表面萃取效率。在製作薄膜式發光二極體時，所需用的晶圓鍵合技術是一個高溫高壓的製程，又矽基板對藍光波段是一個非透明基板，在設計P型氮化鎵的接觸電極時，必須同時設計一反射層。因此在本研究中，用鎳金鎳鋁(p-GaN/Ni/Au/Ni/Al)以及銀鋁合金系統(p-GaN/Ni/Ag(Al))當作P型氮化鎵之高熱穩定性的金屬反射系統，後者的金屬系統是以Ag(Al)合金降低P型氮化鎵接觸電阻與反射率在高溫鍵合製程中銀的聚集以及電性退化的狀況。其中以鎳金鎳鋁的系統中，接觸電阻可以在500 ℃熱處理之後維持在10-2 Ω-cm2，藍光波段的反射率也可以維持在60 %，此高熱穩定性P型氮化鎵接觸電極相當適合用於薄膜式氮化鎵發光二極體結構。由於氮化鎵折射率與空氣的折射率的差異相當大，產生相當嚴重的全反射現象，使得光在氮化鎵材料中產生後，不容易被萃取到空氣中。在本研究第二階段中，利用聚苯乙烯(Poly-styrene)球當作模板，在薄膜式發光二極體之N型氮化鎵表面，以溶膠凝膠法製作氧化鋁以及氧化矽蜂窩狀結構，增加薄膜式發光二極體的發光功率分別為35 %以及19 %。再者，藉由討論光在此系統的光萃取行為與氧化物蜂窩狀結構如何增益薄膜式發光二極體的光萃取效率，並提出光功率增加的理論公式。 GaN-based materials have leaped to a brand new stage in the past two decades. The single crystalline and direct band-gap GaN film can be grown on the sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The wavelength of the emitting light from GaN ranges from ultra-violate (UV) to blue light region by doping various indium content. Nowadays, the material of the blue light emitting diodes is based on the GaN material. Pumping phosphors or other wavelength converter by blue light, the white light can be generated. Hence, the GaN material is the key material for white solid-state lighting. For the solid-state lighting applications, the GaN-based LED operates under a high electric power. Under such a high operation power, the heat dissipation is a critical issue. The sapphire substrate of the conventional LED has a poor thermal conduction. Also, the degradation of the electric and optical property would be very serious due to the high operation temperature. Therefore, in this study, the thin-GaN LED device is produced by the wafer bonding process and the laser lift-off process, which are used to transfer GaN thin film from the sapphire substrate to a better thermal conductive Si substrate. Owing to the better thermal dissipation, the thin-GaN LED structure is a very promising candidate for developing high-power GaN LED. Two main topics of the studied thin-GaN LED structure in this work: (1) Design a suitable p-GaN contacts and reflector for thin-GaN LED structure. (2) Increase the light extraction efficiency of thin-GaN LED. In thin-GaN LED process, the wafer bonding process is necessary and it is a high temperature and high pressure process. Furthermore, the Si substrate is a non-transparent material for the blue light region. So, the p-type GaN contact should consist of an ohmic contact layer and a reflector as well. It is very important to develop a high thermally stable p-GaN contact. In this study, the Ni/Au/Ni/Al p-GaN contacts and Ni/Ag(Al) p-GaN contacts are investigated. These two thermally stable p-GaN contacts can reduce the degradation of the specific contact resistance and the reflection upon the thermal process. The specific contact resistance of the Ni/Au/Ni/Al p-GaN contact keeps on the order of 10-2 Ω-cm2 after 500 ℃ annealing. The reflectance of the Ni/Au/Ni/Al metal scheme is 60 % after 500 ℃ annealing. This high thermally stable Ni/Au/Ni/Al p-GaN contact is very suitable for the thin-GaN LED structure. Another critical issue is the low light extraction efficiency due to large refraction index difference between GaN and air. The light emitted from the active layer in GaN is significantly trapped in the GaN epi-layer, and a serious total internal reflection occurs. In this study, the aluminum oxide and silicon oxide honeycomb structure are produced on the n-GaN emitting surface by poly-styrene spheres template and sol-gel method. The aluminum oxide and silicon oxide honeycomb structures capping on the n-GaN surface can increase the external quantum efficiency by 35 % and 19 %, respectively. The mechanism of increasing light out-put by the oxide honeycomb structure would be discussed.
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