近年來，氮化鎵系列藍光發光二極體的發光效率隨著磊晶技術的發展已持續增加到約20(流明/瓦)左右，但此發光效率和傳統照明系統(100流明/瓦)相比仍低了許多。我們可藉由改善藍光發光二極體的光取出方式來增加其發光效率，製作高亮度的藍光發光二極體。
在本研究中，我們利用覆晶技術(Flip-Chip technique)、反射鏡、光阻微透鏡(Photoresist Microlenses)及熱電致冷器來增加氮化鎵多重量子井發光二極體的光取出效率。並以電激發光(Electroluminescence)光譜及空間功率分布計量測發光二極體的發光強度。藉由變溫之電流-電壓(I-V)特性量測，我們亦針對氮化鎵多重量子井發光二極體在不同偏壓下的載子傳輸機制進行討論。
由實驗結果可得知，具有鋁/二氧化矽(Al/SiO2)反射鏡的背面發光二極體，其在90度角的發光強度為3.28μW，比正面發光二極體的強度(2.73μW)來的大許多。在背面具有光阻微透鏡(折射係數，n =1.62)結構的發光二極體可提高光取出效率達1.2倍。光取出效率的增加是由於背面發光二極體金屬吸光的效應減少以及在氮化鎵/空氣介面的Fresnel’s傳輸損失降低之緣故。
最後，我們製作一個具有增強光取出結構的發光二極體，其發光強度較傳統藍光發光二極體增加1.25倍。因此，我們可利用此結構的發光二極體製作成陣列，獲得一高光取出效率及高亮度的照明系統。

In recent years, even though the light output of GaN-based LED continues to increase, the brightness (~20 lm/W) is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs.
In this study, we utilize flip-chip technique, photoresist microlenses, reflectors and thermoelectric cooler to increase the light extraction of GaN MQW LED. Electroluminescence (EL) and power angular distribution are used to measure the light output intensity of LED. From temperature dependent current-voltage (I-V-T) characteristics, the charge carrier transport mechanisms at different biased regions are also investigated.
In the results, back emission of LED with SiO2/Al reflector has maximum light intensity ( 3.28μW ) , which is higher than front emission one ( 2.73μW ) in vertical emitting area ( at 90 angles). LED with P.R. microlenses (refractive index, n=1.62) on backside could improve the light extraction of LED (about 1.2 times) as well. The enhancement of light output is duo to the reduction of light absorption from the metal contact and Fresnel’s transmission losses at GaN (n=2.4)/air (n=1) interface.
Finally, we fabricate a high brightness LED with above light enhancement design. EL intensity of LED is increased about 1.25 times than conventional one. Therefore, we can manufacture a LEDs array with above designs to obtain high light output for future solid-state illumination.

CONTENTS………………………………………………………………I
LIST OF FIGURES…………………………………………………..…III
LIST OF TABLES………………………………………………..….......V
ABSTRACT………...………………………………………………......VІ

CHAPTER 1 INTRODUCTION
1.1 Evolution and Applications of Light Emitting Diodes…………… ..1
1.1.1 Evolution of Light Emitting Diodes…………………………..2
1.1.2 Prospects of White LEDs Lighting……………………………3
1.2 Structure and Problems of GaN Blue Light Emitting Diodes………4
1.2.1 Structure of GaN Blue Light Emitting Diodes………………..4
1.2.2 Problems of GaN Blue Light Emitting Diodes for White
Lighting……………………………………………………….6
1.3 Enhancement of Light Extraction of GaN MQW Light Emitting
Diodes………………………………………………………………8
1.3.1 Microlens……………………………………………………..9
1.3.2 Flip-Chip Technology……………………………………….10
1.3.3 Thermoelectric Cooler…………………………..……….........12

CHAPTER 2 EXPERIMENT
2.1 Equipment for Various Temperatures Measurements……………..14
2.2 Equipment of Power Angular Distribution Measurements………..15
2.2.1 Fabrication of Reflector……………………………………...16
2.2.2 Fabrication of Edge Emission LED………………….………17
2.3 Refractive Layers for Electroluminescence Enhancement………...17
2.4 Photoresist Microlenses for Electroluminescence Enhancement........18
2.5 Enhancement of Light Extraction of LEDs………………………..19
CHAPTER 3 RESULTS AND DISCUSSION
3.1 Temperature-dependent Characteristics of LEDs…………...............20
3.1.1 Current-Voltage Characteristics—Basic Theory…………...........21
3.1.2 Current-Voltage Characteristics of GaN MQW LED……........23
3.1.3 Electroluminescence Measurements……………………….25
3.1.4 Photoluminescence Properties of GaN films……………….26
3.2 Power Angular Distribution of GaN MQW LEDs with Different Reflectors… …………………………………………………….....30
3.2.1 LEDs with Al/SiO2 Reflector…………………………….....31
3.2.2 LEDs with SiO2/Al Reflector………………………………...32
3.2.3 Edge Emission LEDs…………………………………………34
3.3 Electroluminescence Measurements of LEDs with Different Refraction Index Layers……………………………………….....35
3.3.1 Front Emission LED with TiO2 Refraction Layer…………35
3.3.2 Front Emission LED with TiO2/SiO2 Refraction Layers …36
3.3.3 Back Emission LED with SiO2 Refraction Layer…………36
3.4 Electroluminescence Measurements of GaN MQW LEDs with
Photoresist Microlenses…………………………………………..37
3.4.1 Effect on Baking Treatment of Photoersist Microlenses…...37
3.4.2 Effect on Geometry of Photoresist Microlenses…………...38
3.5 High Brightness GaN MQW LEDs……………………………….. 38

CHAPTER 4 CONCLUSIONS…………………………………….40

FIGURES……………………………………………………….......42~74
TABLES………………………………………………………….75~77
REFERENCES……………………………………………….......78~83

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