臺灣博碩士論文加值系統

本論文中，為了改善氮化鎵(GaN)系發光二極體(light-emitting diodes, LEDs)之光萃取效率(light extraction efficiency)及內部量子效率(internal quantum efficiency, IQE)，我們有效地運用具有自組裝(self-assemble)特性的二氧化矽奈米球，並分別提出了三種改善方法及元件製程，縝密地進行材料分析與元件量測，深入討論其光電特性與磊晶品質之改善結果。
為了有效改善高功率發光二極體之光萃取效率，我們提出利用內嵌一二氧化矽奈米球單層結構來形成三維光子晶體背鍍反射鏡結構，首先將具自組裝偽六角緊密堆積(pseudo-hexagonal close packing)陣列之奈米球單層結構塗佈至藍寶石基板背面，並背鍍分佈式布拉格反射鏡(distributed Bragg reflector, DBR)及鋁金屬反射鏡(aluminum metal mirror)，利用奈米球之圓球外形使得背鍍反射鏡形成三維結構，改變反射光之光路徑降低內部全反射(total internal reflection, TIR)；除此之外，由於奈米球單層結構於球與球之間會形成空隙，會導致似光子晶體(photonic crystal)的空氣孔洞(air voids)陣列的產生，因此元件內部之光散射(light scattering)效應可有效地提升，令光子有更多的機會由逃脫角錐散射至元件外，進而大幅地改善光萃取效率。相較於不具有背鍍反射鏡的傳統LED，此三維光子晶體背鍍反射鏡結構在350 mA的操作電流下，不但使光輸出功率增加了136%，而且相對於僅背鍍混和反射鏡 (DBR + Al mirror)，其光輸出功率亦增加了23.6%，而且由於此結構製作於元件之背面而不在電流路徑上，更具有不影響元件電性特性之優點。
不過由於二氧化矽奈米球結構於藍寶石基板的背面僅以凡德瓦力(Van der Waals forces)附著，因此附著力不佳易導致背鍍反射鏡脫落，故本論文中亦研究改以二氧化矽奈米球當作蝕刻遮罩，利用感應耦合電漿(inductively coupled plasma, ICP)進行乾性蝕刻達到圖形轉印之效果，形成具偽六角緊密堆積凸半球陣列之圖騰於藍寶石基板之背面，並背鍍分佈式布拉格及鋁金屬反射鏡，形成三維背鍍反射鏡結構以達到增加散射效應，並改善二氧化矽奈米球附著力不佳的問題。由於此結構係利用乾蝕刻方式轉印奈米球之凸半球陣列圖形，故不具有似光子晶體空氣孔洞陣列於結構中。相較於不具有背鍍反射鏡的傳統LED，在350 mA的操作電流下，其之光輸出功率之改善仍高達116%，而且相對於僅背鍍混和反射鏡 (DBR + Al mirror)，其光輸出功率亦增加了14.1%，且有效增加三維背鍍反射鏡結構製程良率。
此外，由於氮化鎵與藍寶石基板間晶格不匹配(lattice mismatch)與相差過大的熱膨脹係數(thermal expansion coefficient)，導致在磊晶成長過程中會有大量地線差排(threading dislocation)產生，在此論文的最後我們研究利用具有自組裝偽六角緊密堆積陣列的奈米球單層結構來製作奈米級圖形化基板並將其應用至氮化鎵發光二極體中。與此部分我們先利用乾蝕刻的方式成功地將二氧化矽奈米球單層結構轉印至藍寶石基板上形成具偽六角緊密堆積奈米級凸半球陣列之圖形化藍寶石基板(nanosphere-pattern sapphire substrate)，此奈米級圖形化基板可有效地降低氮化鎵磊晶層中的線差排密度，大幅提升磊晶品質，減少非輻射復合(nonradiative recombination)熱消耗，進而改善發光二極體的發光效率。在20 mA的操作電流下其輸出功率有高達31.4%之提升，且順向導通電壓與逆向漏電流皆較小，其可能是因為磊晶品質可藉由此奈米級凸半球陣列之圖形化藍寶石基板大幅改善。

In order to improve light extraction efficiency (LEE) and internal quantum efficiency (IQE) of GaN-based light-emitting diodes (LEDs), the self-assembled SiO2 nanosphere monolayer structure is introduced and studied. Three new nanosphere monolayer-related approaches are proposed in this thesis. The optical and electrical properties as well as related material analyses were also studied and discussed.
An interesting approach to improve light extraction efficiency of high power GaN-based LEDs by the use of a three dimensional-photonic crystal (3D-PhC) backside reflector is studied. A 3D-PhC backside reflector is formed by coating a self-assembled SiO2 nanosphere monolayer between the hybrid reflector and backside of sapphire substrate. Firstly, a self-assembled pseudo-hexagonal close packing SiO2 nanosphere monolayer was coated on the backside of sapphire substrate. Then, a distributed Bragg reflector (DBR) and an aluminum (Al) metal mirror were deposited subsequently. By inserting SiO2 nanospheres, hemispherical patterns could be transferred to the deposited reflector. Hence, photons could be redirected into arbitrary angles for light extraction by the transferred concave surface. Total internal reflection (TIR) could be limited. In addition, due to the presence of gap between SiO2 nanospheres, the PhC-like air void arrangement is formed after the deposition of backside reflector. Hence, light scattering effect could be effectively improved. This certainly gives photons more opportunities to find the escape cones. As compared with a conventional LED (without backside reflector), at 350 mA, the studied device exhibits 136% enhancement in light output power without the degradation of electrical properties. Notably, the studied device exhibits 23.6% enhancement in light output power as compared with the LED with a planar hybrid (a DBR + an Al metal mirror) backside reflector.
In chapter 3, since SiO2 nanospheres are pulled and adhered on the backside of sapphire substrate by Van der Waals forces, the adhesion between SiO2 nanospheres and sapphire substrate is poor. Hence, in order to alleviate this problem, a new approach to form a 3D backside reflector by using SiO2 nanspheres as a hard mask and inductively coupled plasma (ICP) dry etching process to transfer pseudo-hexagonal close packing hemispherical arrangement patterns on the backside of sapphire substrate. Then, a DBR and an Al metal mirror were deposited subsequently, as mentioned above. Due to the hemispherical arrangement patterns on the backside of sapphire substrate, hemispherical patterns could be transferred to the deposited reflector. Hence, photons could be redirected into arbitrary angles for light extraction by the transferred concave surface. However, since hemispherical patterns structure was formed by ICP process, the PhC-like air void arrangement was not present. As compared with a conventional LED (without backside reflector), at 350 mA, the studied device still exhibits 116% enhancement in light output power. Notably, the studied device exhibits 14.1% enhancement in light output power as compared with the LED with a planar hybrid (a DBR + an Al metal mirror) backside reflector. Moreover, the product yield could also be improved, due to the enhanced adhesion between backside reflector and sapphire substrate
In addition, due to the large difference in lattice mismatch and thermal expansion coefficient between GaN and sapphire, many threading dislocations (TDs) on GaN epitaxial layers would be induced. For this reason, the employment of nanospheres-pattern sapphire substrate (NS-PSS) is studied in this work. This NS-PSS is successfully fabricated by using SiO2 nanspheres as the hard mask and inductively coupled plasma (ICP) dry etching process to transfer hemispherical patterns, which exhibits pseudo-hexagonal close packing arrangement, on the sapphire substrate. The use of NS-PSS could effectively decrease threading dislocation density, which reduces undesirable nonradiative recombinations, and enhance light output power. As compared with a conventional LED, at 20 mA, the studied device exhibits 31.4% enhancement in light output power. And forward voltage and leakage current could also be reduced. These results could be attributed to that the crystalline quality of GaN epitaxial layer is effectively improved by using a NS-PSS.

Abstract
Figure Caption
Chapter 1. Introduction 1
1-1. History and development of blue LEDs 1
1-2. The problem of GaN-based LEDs 2
1-3. Development of high power GaN-based LEDs 3
1-4. Synthesis techniques of nanospheres 3
Chapter 2. High power GaN-based LEDs with a three dimensional-
photonic crystal (3D-PhC) backside reflector 5
2-1. Introduction 5
2-1-1. Introduction 5
2-1-2. Development of backside reflector 6
2-2. Fabrication processes of LED devices 7
2-2-1. LED wafer cleaning process 7
2-2-2. Devices structure and fabrication 7
2-3. Experimental results and discussion 10
2-3-1. Investigations of high power GaN-based LEDs with a
3D-PhC backside reflector 10
2-3-2. Electrical and optical characterizations 12
2-4. Summary 16
Chapter 3. High power GaN-based LEDs with a three dimensional (3D) backside reflector by using patterned substrate 39
3-1. Introduction 39
3-2. Fabrication processes of LED devices 40
3-2-1. LED wafer cleaning process 40
3-2-2. Devices structure and fabrication 41
3-3. Experimental results and discussion 43
3-4. Summary 45
Chapter 4. Improved performance of GaN-based LEDs with SiO2 nanospheres pattern sapphire substrate 57
4-1. Introduction 57
4-1-1. Introduction 57
4-1-2. External quantum efficiency issues of GaN-based LEDs 58
4-1-3. Threading dislocations of GaN-based LEDs 58
4-2. Fabrication processes of LED devices 59
4-2-1. LED wafer cleaning process 59
4-2-2. Devices structure and fabrication 60
4-3. Experimental results and discussion 61
4-3-1. Material analysis of GaN-based LEDs with NS-PSS 61
4-3-2. Electrical and optical characterizations 64
4-4. Summary 68
Chapter 5. Conclusion and prospect 89
5-1. Conclusion 89
5-2. Prospect 91
Reference 95

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