臺灣博碩士論文加值系統

本論文中，為了改善氮化鎵系發光二極體之光取出效率(light extraction efficiency)，吾人研製一系列具有特殊邊牆結構之高品質氮化鎵系發光二極體。分別提出了奈米材料之應用與元件製造技術，其中包含特殊邊牆結構、利用快速對流沉積法研製奈米級粗糙化背鍍反射鏡、混合式特殊邊牆之奈米級粗糙化背鍍反射鏡以及複合式二氧化矽微奈米球抗反射保護層填補於特殊邊牆之奈米級粗糙化背鍍反射鏡，有效提升氮化鎵系發光二極體之光電轉換效率，優化氮化鎵系發光二極體之性能。本論文對氮化鎵系發光二極體元件之光電特性，及各種複合式特殊邊牆結構之製程皆有深入且詳細的研究與探討。

首先，吾人利用感應耦合電漿離子(inductively coupled plasma, ICP)蝕刻製程，製備出具有特殊邊牆結構之氮化鎵系發光二極體，並可在不影響電性特性之下，改善元件邊牆粗糙度，有效降低內部全反射(total internal reflection, TIR)與增加光散射(light scattering)機會。光輸出功率(light output power)與光通量(luminous flux)分別提升18.3%以及18.4%，可有效改善光電轉化效率。
其次，將具自組裝偽六角緊密堆積(pseudo-hexagonal close packing)陣列之二氧化矽奈米球單層結構，塗佈至藍寶石基板背面當作蝕刻硬遮罩，利用乾蝕刻轉印出具凸半球陣列之圖騰於藍寶石背基板，再利用熱蒸鍍依序背鍍上鋁金屬反射鏡(aluminum metal mirror)，藉由粗糙化凸半球圖騰之反射鏡結構，減少元件背部封裝金屬之光吸收並增加光散射的機會，有效增加光取出效率，相較於傳統高功率氮化鎵系發光二極體，在不影響元件之電性特性下，仍可提升光輸出功率與光通量達30.7%以及32.0%。

此外，以前章之特殊邊牆結構，結合具凸半球陣列之背鍍反射鏡，形成混合式特殊邊牆之凸半球陣列背鍍反射鏡，改善元件邊牆及基板粗糙度，有效降低內部全反射，並反射由主動區發出之背向光以增進光散射，有效提高光取出效率。相較於傳統高功率氮化鎵系發光二極體，在不影響元件之電性特性下，仍可提升光輸出功率與光通量達55.8%以及49.3%。

最後，探討經由快速對流沉積法，塗佈混合式二氧化矽微米球抗反射保護層結構於具有特殊邊牆之凸半球陣列背鍍反射鏡，而混合式二氧化矽微米球結構，可在不影響電性特性下，進一步改善元件表面粗糙度，同時形成漸層式折射率(graded refractive index)結構，有效降低內部全反射之效應，使主動層發出之光子有更高機率散射至結構之外，提升光輸出功率與外部量子效率高達54.2%以及54.4%。本研究論文中所研製的高品質氮化鎵系發光二極體，皆可有效提升光電轉換效率，在商業應用上相當具有潛力。

In this thesis, to enhance light extraction efficiency (LEE), a series of GaN-based light-emitting diodes (LEDs) with specific sidewall structures are fabricated and studied. The device fabrication process and nanomaterials applications, including fabrication of specific sidewall structures, GaN-based LEDs with a nanoscale textured backside reflector fabricated via rapid convection deposition (RCD), GaN-based with hybrid structure of textured sidewalls and a nanoscale textured backside reflector, and GaN-based LEDs with textured sidewalls, a nanoscale textured backside reflector, and a passivation layer are proposed to improve wall-plug efficiency (WPE). The optical and electrical properties of these GaN-based LEDs are studied and discussed. In addition, the fabrication of various combined textured sidewalls structures is discussed in detail.

GaN-based LEDs with textured sidewall structures fabricated via inductively coupled plasma (ICP) etching process are studied. The textured sidewall structures improve sidewall roughness without degrading electrical properties. Thus, the total internal reflection (TIR) is effectively reduced and the light scattering probability is increased. Compared with a conventional GaN-based LED, at 350 mA, the studied device exhibits 18.3% and 18.4% enhancements in light output power (LOP) and luminous flux, respectively. Therefore, performance can be improved by employing textured sidewall structures.

A monolayer of self-assembled pseudo-hexagonal close-packing SiO2 nanospheres was coated onto the backside of a sapphire substrate as a hard mask. Using a dry ICP etching process, hemispherical patterns were transferred onto the backside of the sapphire substrate. Then, an aluminum (Al) metal mirror was deposited using a thermal evaporator. Due to the presence of textured hemispherical patterns, downward photons emitted from the active region toward the textured backside reflector reflected and scattered for light extraction rather than being absorbed by the package metal. Compared with a conventional GaN-based LED, at 350 mA, the studied device exhibits 30.7% and 32.0% enhancements in LOP and luminous flux, respectively. In addition, textured sidewall structures were combined with the hemispherical pattern backside reflector. This combination improved the roughness of sidewalls and the sapphire substrate. Thus, the TIR is effectively reduced. Downward photons emitted from the active region are scattered and redirected in arbitrary directions for light extraction. The probability of light scattering is also increased. Compared with a conventional GaN-based LED, at 350 mA, the studied device exhibits 55.8% and 49.3% improvements in LOP and luminous flux, respectively, without degrading electrical properties.

Finally, the combination of a hybrid SiO2 microsphere (MS) antireflection passivation layer deposited on the textured sidewall structures and a hemispherical backside reflector fabricated via RCD was studied. The hybrid SiO2 MS antireflection passivation layer further improved surface roughness and light scattering without degrading electrical properties. The enhanced light extraction of GaN-based LEDs can be attributed to a graded-refractive-index structure. Due to a reduction of TIR, photons emitted from the active region more easily escape from the inside of LEDs. Compared with a conventional LED, at 350 mA, the studied device exhibits 54.2% and 54.4% improvements in LOP and external quantum efficiency (EQE), respectively. The leakage current is also reduced without degrading the electrical properties.

The proposed modifications improve the performance of GaN-based LEDs. High-performance GaN-based LEDs are expected to compete with traditional light sources in solid-state lighting applications.

Contents
Abstract……………………………………………IV
Table Captions……………………………………………XVIII
Figure Captions……………………………………………XIX
Chapter 1. Introduction
1-1. History and Development of GaN-based LEDs……………………………1
1-2. Problems of GaN-based LEDs……………………………………………3
1-3. Review of Textured Sidewall Structures……………………………………4
1-4. Review of Micro-/ Nano-sphere Structures………………………………5
1-5. Thesis Organizations……………………………………………5
Chapter 2. Fabrication of GaN-based LEDs with Specific Textured Sidewalls
2-1. Introduction……………………………………………7
2-2. Fabrication Processes of LED Devices…………………………………………8
2-2-1. LED Wafer Cleaning Process……………………………………………8
2-2-2. Devices Structure and Fabrication……………………………………………8
2-3. Experimental Results and Discussion…………………………………………10
2-3-1. Surface Morphology……………………………………………10
2-3-2. Electrical Properties……………………………………………10
2-3-3. Optical Properties……………………………………………11
2-3-4. Light Emission Mapping……………………………………………14
2-3-5. Far-filed pattern……………………………………………14
2-4. Summary……………………………………………15
Chapter 3. Improved Performance of GaN-based LEDs with Textured Sidewalls and ICP-transferred Nanohemispherical Backside Reflector
3-1. High-power GaN-based LEDs with ICP-transferred Nanohemispherical Backside Reflector via Rapid Convection Deposition……………………………………………16
3-1-1. Introduction……………………………………………16
3-1-1-1. Introduction……………………………………………16
3-1-1-2. Rapid Convective Deposition……………………………………………18
3-1-2. Fabrication Processes of LED Devices…………………………………19
3-1-2-1. LED Wafer Cleaning Process……………………………………………19
3-1-2-2. Devices Structure and Fabrication……………………………………19
3-1-3. Experimental Results and Discussion……………………………………21
3-1-3-1. Surface Morphology……………………………………………21
3-1-3-2. Atom Force Microscope……………………………………………22
3-1-3-3. Reflectivity……………………………………………22
3-1-3-4. Electrical Properties……………………………………………23
3-1-3-5. Optical Properties……………………………………………24
3-1-3-6. Light Emission Mapping……………………………………………27
3-1-3-7. Far-field pattern……………………………………………27
3-1-4. Summary……………………………………………28
3-2. High-power GaN-based LEDs with Textured Sidewalls and ICP-transferred Nanohemispherical Backside Reflector……………………………………………29
3-2-1. Introduction……………………………………………29
3-2-2. Fabrication Processes of LED Devices…………………………………30
3-2-2-1. LED Wafer Cleaning Process……………………………………………30
3-1-2-2. Devices Structure and Fabrication……………………………………30
3-2-3. Experimental Results and Discussion……………………………………32
3-2-3-1. Surface Morphology……………………………………………32
3-2-3-2. Atom Force Microscope……………………………………………34
3-2-3-3. Electrical Properties……………………………………………34
3-2-3-4. Optical Properties……………………………………………35
3-2-3-5. Light Emission Mapping……………………………………………38
3-2-3-6. Far-field pattern……………………………………………39
3-2-4. Summary……………………………………………40
Chapter 4. Improved Performance of GaN-based LEDs with Textured Sidewalls, Nanohemispherical Backside Reflector, and SiO2 Microsphere/Nanoparticle Passivation Structures
4-1. Introduction……………………………………………41
4-1-1. Introduction……………………………………………41
4-1-2. Mechanisms of Anti-reflection……………………………………………43
4-2. Fabrication Processes of LED Devices………………………………………44
4-2-1. LED Wafer Cleaning Process……………………………………………44
4-2-2. Devices Structure and Fabrication…………………………………………45
4-3. Experimental Results and Discussion…………………………………………48
4-3-1. Surface Morphology……………………………………………48
4-3-2. Transmittance……………………………………………49
4-3-2. Atom Force Microscope……………………………………………49
4-3-3. Electrical Properties……………………………………………50
4-3-4. Optical Properties……………………………………………51
4-3-5. Light Emission Mapping……………………………………………55
4-3-6. Far-field pattern……………………………………………55
4-4. Summary……………………………………………56
Chapter 5. Conclusion and Prospects
5-1. Conclusion……………………………………………58
5-2. Prospects……………………………………………61
References……………………………………………62
Tables……………………………………………78
Figures……………………………………………82

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