發光二極體功率與亮度的提升

Abstract

本論文主要研究目的是提供一些方法來提升磷化鋁鎵銦與氮化鎵發光二極體之元件特性。其中，主要是藉由晶圓接合技術製作高功率與高亮度發光二極體並探討使用不同晶圓接合機制對其之發光二極體元件特性之影響。 發光頻率介於黃光至紅光之間的LED，一般都是在砷化鎵基板上，以磷化鋁鎵銦(AlGaInP)製作而成，然而由於此型機板的低熱傳導性質，使得該類LED只能做低功率元件的應用。為克服此一缺點，透過晶圓接合(wafer bonding)的技術，可在銅或碳化矽基板上製作高功率的AlGaInP LED。銅基板LED這種作法需要在AlGaInP LED結構與銅基板間加入一層氧化銦錫(ITO)膜，並與以加熱，發現在500℃下加熱30分鐘，銅尚不會穿透ITO層，而樣品又可以順利黏合。銅為基板的高功率LED可以在八倍於砷化鎵基板紅光LED的注入電流(injection current)下操作，對於LED而言，銅由於具有較高的熱導率及較低的熱阻抗，因此產生的熱量比砷化鎵少，這使得銅基板LED的操作電流可以達到800 mA，而其發光峰值強度可達1230 mcd。於室溫下連續以20 mA的電流操作500個小時後，其發光強度衰減的幅度不到5%。此外，以碳化矽為基板且具有金屬鏡面的紅光發光二極體，在20 mA的注入電流下操作其發光強度可達傳統砷化鎵基板紅光LED的3.2倍；其操作電流也優於砷化鎵基板LED可以達到650 mA。 於氮化鎵系列發光二極體方面，也成功的利用膠接合法與雷射分離技術將氮化鎵磊晶層轉移至2吋之矽基板上，藉由反射鏡面與表面粗化的配合在20 mA的注入電流下操作其垂直型以矽為基板的藍光LED的光輸出功率可高於傳統藍寶石基板LED的20 %；其操作電流280 mA也優於傳統基板LED (180 mA)。其垂直型矽基板藍光LED之發光圖形較藍寶石基板對稱，且經過1000小時的元件壽命測試仍保有高的穩定性。此外，運用表面粗化、晶圓接合與雷射分離技術製作一具有雙面粗化(於p-GaN 和undoped-GaN)的新結構氮化鎵發光二極體，發現在20 mA的注入電流下操作其正向與背向的發光強度分別為傳統LED(p-GaN與undoped-GaN均無粗化)的2.77和2.37倍。另外，也可藉由改變基板與發光層間之undoped-GaN的表面粗糙度進而調整LED的發光角度與光輸出功率。最後，氮化鎵發光二極體利用雙面粗化並運用銀鏡面於藍寶石基板上之特性將被提出。

The primary objective of this dissertation is provided some approaches to enhance the performance of AlGaInP and InGaN-GaN LED. It could use wafer-bonding technology to transfer AlGaInP or GaN thin film on the high conductivity substrate. Furthermore, InGaN-GaN LED with roughening both the p-GaN surface and the undoped-GaN surface by double transfer methods and applying a mirror coating to the sapphire substrate have also demonstrated. LED emitting in the yellow to red region of the spectrum are typically made from AlGaInP and based on a GaAs substrate. But due to the low thermal conductivity of the substrate, these LED are limited to low-power applications. To overcome this drawback, high-power AlGaInP LED with Cu and SiC substrates using wafer bonding is fabricated. For Cu-substrate LED, this involves placing an indium-tin-oxide (ITO) film between the AlGaInP LED structure and the Cu substrate and then applying heat. It is found that Cu did not penetrate the ITO layer when samples were bonded at 500°C for 30 mins. Copper-substrate LED can operate at an injection current which is eight times higher than that of traditional GaAs-substrate LED. Copper’s higher thermal conductivity and lower thermal resistance compared with GaAs means that less heat is generated in the LED. This allows the Cu-substrate LED to operate a current of up to 800 mA and reach a peak luminous intensity of 1230 mcd. The degradation of the intensity was less than 5% after 500 hours running at 20 mA at room temperature. Furthermore, the luminous intensity of SiC-substrate LED with mirror system is 3.2 times higher than that of conventional LED at the injection current of 20 mA. The saturation current could reach at 650 mA, which is better than that of GaAs-substrate LED. For the GaN-based LED, vertical InGaN-GaN LED epitaxial films are successfully fabricated on a 50mm Si substrate using glue bonding and laser lift-off technology. It is found that the light output of the vertical InGaN LED chip exceeded that of the conventional sapphire-substrate LED by about 20% at an injection current of 20 mA. The vertical InGaN LED operate at a much higher injection forward current (280 mA) than were sapphire substrate LED (180 mA). The radiation pattern of the vertical InGaN LED is more symmetrical than that of the sapphire substrate LED and the vertical InGaN LED remain highly reliable after 1000 h of testing. Furthermore, a new structure of InGaN LED with double roughened (p-GaN and undoped-GaN) surfaces is fabricated by surface-roughening, wafer-bonding and laser lift-off technologies. It is found that the frontside luminance intensity of double roughened LED was 2.77 times higher than that of the conventional LED at an injection current of 20 mA. The backside luminance intensity is 2.37 times higher than that of the conventional LED. Moreover, the effect of the roughness of the undoped-GaN layer on the performance of double roughened LED is also investigated. It is found as the root mean square (rms) roughness of undoped-GaN layer increase from 18.6 to 146.7 nm, the output power increase from 7.2 to 10.2 mW, and the view angle decrease from 133.6° to 116°, respectively. Finally, the performance of an InGaN LED with a roughened undoped-GaN surface and a silver mirror on the sapphire substrate is also investigated.