臺灣博碩士論文加值系統

本論文旨在針對利用雷射剝離與電鍍鎳金屬基板技術所製作之氮化鎵系列垂直結構發光二極體(VLEDs)進行光輸出功率與發光效率之改善。相較於傳統水平結構藍寶石基板發光二極體(Regular LEDs)，VLEDs因導電基板與較厚且較易電流擴散之n-GaN在結構上方之故，擁有散熱佳、短電流傳導路徑、較少的電流叢聚效應(Current crowding effect)與單電極而發光面積大等優點。然而，為要早日達到VLEDs可應用於一般照明，VLEDs仍需持續改善增進其光析出效率。
首先，為要改善本實驗室先前所製作之VLEDs光析出效率，本研究提出一種利用準分子雷射結合濕蝕刻粗化VLEDs之n-GaN所製造出半球圓弧突起物附加角錐形狀提昇VLEDs光析出之方法。相較於表面無任何粗化VLEDs，兩階段粗化之VLEDs光輸出功率在操作電流350與750 mA分別增加95%與95.1%。光輸出功率增加的原因包括兩階段粗化後n-GaN表面積增加則發光面積與不規則自半導體射出之光子亦增加。
其次，為了要再進一步增進VLEDs光析出效率，除兩階段表面蝕刻外，我們亦以水熱法垂直成長氧化鋅(ZnO)奈米柱於已被粗化之n-GaN上。相較於僅僅使用化學蝕刻之VLEDs，兩階段粗化加上氧化鋅奈米柱之VLEDs (300?300 ?m2)於操作電流20與100 mA光輸出效率分別增加29與41%，順向電壓分別下降0.18與0.2 V。光輸出效率增加之原因歸功於(1)兩階段蝕刻所造成半球型突起物與眾多六角錐增加之表面發光面積(2)氧化鋅奈米柱折射係數(n=2-2.1)介於n-GaN (n=2.5)與空氣(n=1)中間，亦可有效減少光子自內往外射出時於半導體與空氣界面發生全反射的機會。
此外，我們亦利用雷射蝕刻在n-GaN表面生成氧化鎵(GaOx)薄膜與p-GaN面蒸鍍由二氧化矽(SiO2)與二氧化鈦(TiO2)組成之布拉格反射鏡技術來製作VLEDs元件。相較於傳統以鋁(Al)為反射層且無氧化鎵薄膜之VLEDs，表面具氧化鎵薄膜與使用布拉格反射鏡技術之VLEDs (chip size: 1 mm2)於操作電流350與750 mA光輸出效率分別增加68與51%。我們認為光輸出效率增加之原因可歸於(1) n-GaN表面經雷射蝕刻形成之半球型突起物增加光子射出角度，(2) n-GaN表面經雷射蝕刻分解出的鎵與空氣中氧結合成為氧化鎵薄膜附著於半球型突起物上，由於氧化鎵折射係數(n~1.89)介於n-GaN與空氣中間，亦可有效減少光子自內往外射出時於半導體與空氣界面發生全反射的機會，(3)布拉格反射鏡雖具有極高反射率但卻無法導電，蒸鍍於p-GaN之後，產生了電流阻障(Current blocking)的效應而有效地將電流擴散至VLEDs元件四周，使得電流平均分配於元件之主動層而非集中於金屬電極四周，進而增加了光析出效率。
最後，本研究亦提出運用銦鋅氧化物(Indium Zinc Oxide, IZO)透明導電膜作為n-GaN表面之電流擴展層，期使表面經兩階段粗化之VLEDs具有更好的電流擴散及更高的發光效率。相較於傳統表面僅使用濕蝕刻粗化之VLEDs，兩階段粗化並覆蓋IZO透明導電膜之VLEDs於操作電流350與750 mA光輸出效率分別增加79.3與65.1%，且順向電壓亦降低0.2 V。此結果顯示，除兩階段蝕刻增加元件表面積及IZO接觸面積之效益外，IZO透明導電膜具有優異電流擴展能力且其折射係數(n=1.9-2)介於n-GaN與空氣中間，亦增加了光子析出角度，對元件之光輸出功率之改善有極大助益。

In this study, efforts to enhance the light extraction and power conversion efficiency of vertical-structured GaN-based light-emitting diodes (VLEDs) employing nickel electroplating and laser lift-off (LLO) techniques were proposed. As compared to conventional lateral GaN-based LEDs with sapphire substrate (abbreviated as regular LEDs), VLEDs have good heat dissipation, shorter conduction path, less current crowding effect, and larger effective area through a conducting substrate and improved current spreading through the top of the n-GaN epilayer. However, the light-extraction efficiency of VLEDs must still be improved for general lighting.
First, we aimed to further improve the light-extraction efficiency of regular VLEDs beyond previous work. A two-step roughening process that uses a KrF excimer laser and KOH chemical etching for the n-GaN layer surface of VLEDs to yield circular protrusions with hexagonal cones atop for light extraction enhancement is demonstrated. As compared to VLEDs without surface roughening, increases in light output power (Lop) of about 95% at 350 mA and 95.1% at 750 mA were achieved for VLEDs roughened using the proposed two-step etching scheme. Such improvements could be attributed to the proposed method creates a twofold surface roughness to maximize the angular randomization of photons at the emission surface and significantly increases the surface area of VLEDs.
Second, to further improve the performance VLEDs, two step surface roughening followed by the hydrothermal growth of vertically-aligned ZnO nanorods on the top n-GaN surface were also investigated and discussed. As compared to that of the VLEDs (300?300 ?m2 in chip size) with surface wet etching only, the formation of curved protrusions and ZnO nanorods on the n-GaN surface enables a typical increase in Lop by 29% at 20 mA and 41% at 100 mA with a decrease in forward voltage (VF) from 3.24 V to 3.06 V at 20 mA and 3.9 V to 3.7 V at 100 mA, respectively. A cumulative effect of curved protrusions, hexagonal cones, and vertically-aligned ZnO nanorods through effectively reducing the effective thickness of the n-GaN layer, increasing surface emission area, and enhancing the escape probability of photons was responsible for these improvements.
Next, the GaN-based VLEDs with a roughened GaOx film atop n-GaN layer via KrF laser irradiation and a TiO2/SiO2 distributed Bragg reflector (DBR) were fabricated and investigated. As compared to regular VLEDs with Al reflector and without roughened GaOx film, the proposed VLEDs with a chip size of 1 mm2 show a increase in Lop by 68% at 350 mA and 51% at 750 mA, which is attributed to enhanced reflectivity and current blocking capability from the DBR layer, the surface roughening with circular GaN protrusions, and the formation of a surface GaOx film from KrF laser irradiation.
Finally, a transparent and low-resistant indium-zinc oxide (IZO) film was proposed to serve as a current spreading layer (CSL) for VLEDs. The performance of VLEDs with two step surface texturing and a high transparent indium-zinc oxide (IZO) film was investigated. Compared with the regular VLEDs, the textured GaN layer that can disperse with the angular distribution of photons at the GaN/IZO interface, and the IZO film play the role of CSL to decrease in forward voltage and also reduce the Fresnel reflection. The fabricated VLEDs with the proposed surface roughening scheme revealed an increase in Lop by 79.3% and 65.1% at 350 mA and 750 mA, respectively, and showed a relatively low forward voltage as compared to that of regular VLEDs.

Abstract (in Chinese) I
Abstract (in English) III
Acknowledgements VI
Contents VII
Table Captions XI
Figure Captions XII
Chapter 1 Introduction
1-1 Overview of GaN-based LEDs 1
1-1.1 Important properties of GaN 1
1-1.2 Major issue of fabricating GaN 2
1-1.3 Overview of blue GaN-based LEDs 4
1-2 Thesis organization 6
References 8
Chapter 2 Challenges for improving light extraction efficiency of vertical-structured GaN-based LEDs
2-1 Issues of external quantum efficiency in LEDs 14
2-2 Substrate engineering 17
2-2.1 Wafer bonding 17
2-2.2 Metal electroplating 19
2-2.3 Laser lift-off 21
2-3 High reflectivity, thermal stable and low resistance contacts to p-GaN 23
2-4 Surface roughening on the n-GaN layer 24
References 26
Chapter 3 Enhanced light output of vertical GaN-LEDs with two-step surface roughening using KrF laser and chemical wet etching
3-1 Introduction 34
3-2 Device fabrication 35
3-3 Results and Discussion 37
3-3.1 Surface morphology 37
3-3.2 EDS analysis 39
3-3.3 AFM analysis 40
3-3.4 Ray-tracing simulation 40
3-3.5 Electrical and optical characteristics 42
3-4 Summary 44
References 45
Chapter 4 Enhanced light output of vertical GaN-LEDs with surface roughening using KrF laser and ZnO nanorods
4-1 Introduction 59
4-2 Device fabrication 61
4-3 Results and Discussion 62
4-3.1 Etching rate of KrF laser 62
4-3.2 Surface morphology 63
4-3.3 Electrical and optical characteristics 64
4-4 Summary 65
References 67
Chapter 5 Enhanced light output of vertical GaN-LEDs with TiO2/SiO2 reflector and roughened GaOx surface film
5-1 Introduction 77
5-2 Device fabrication 79
5-3 Results and Discussion 81
5-3.1 Surface morphology 81
5-3.2 EDS analysis 81
5-3.3 Electrical and optical characteristics 82
5-4 Summary 84
References 85
Chapter 6 Enhanced performance of vertical GaN-LEDs with two-step surface roughening and indium-zinc oxide layer
6-1 Introduction 95
6-2 Device fabrication 97
6-3 Results and Discussion 99
6-3.1 Surface morphology 99
6-3.2 Electrical and optical characteristics 100
6-4 Summary 101
References 102
Chapter 7 Conclusions and Future Study
7-1 Conclusions 111
7-2 Suggestions for Future Study 112
Publication Lists 114
Vita 118

Chapter 1
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Chapter 2
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Chapter 3
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Chapter 4
[4.1]S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku, “Room-temperature continuous-wave operation of InGaN multi-quantum-well structure laser diodes with a lifetime of 27 hours, Appl. Phys. Lett., vol. 70, no. 11, pp. 1417-1419, Mar. 1997.
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Chapter 5
[5.1] E. F. Schubert and J. K. Kim, “Solid-State Light Sources Getting Smart, Science, vol. 308, pp. 1274-1278, May 2005.
[5.2] C. J. Humphreys, “Solid-State Lighting, MRS Bulletin, vol. 33, pp. 459-471, Apr. 2008.
[5.3] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, “Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting, J. Display Technol., vol. 3, no. 2, pp. 160-175, June 2007.
[5.4] S. J. Wang, K. M. Uang, S. L. Chen,Y. C. Yang, S. C. Chang, T. M. Chen, and C. H. Chen, “Use of patterned laser liftoff process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes, Appl. Phys. Lett., vol. 87, p. 011111-1, 2005.
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[5.6] E. S. Fred, “ Light-Emitting Diodes, Cambridge Univ. Press, Cambridge, 2003.
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Chapter 6
[6.1]E. F. Schubert and J. K. Kim, “Solid-State Light Sources Getting Smart, Science, vol. 308, pp. 1274-1278, 2005.
[6.2]M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting, J. Display Technol., vol. 3, no. 2, pp. 160-175, 2007.
[6.3]H. Kim, K. K. Kim, K. K. Choi, H. Kim, J. O Song, J. Cho, K. H. Baik, C. Sone, and Y. Park, Design of high-efficiency GaN-based light emitting diodes with vertical injection geometry, Appl. Phys. Lett., vol. 91, p. 023510-1, 2007.
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[6.9]S. J. Wang, K. M. Uang, S. L. Chen, Y. C. Yang, S. C. Chang, T. M. Chen, and C. H. Chen, “Use of patterned laser liftoff process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes, Appl. Phys. Lett., vol. 87, p. 011111-1, 2005.
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[6.11]A. Motayed, M. Jah, A. Sharma, W. T. Anderson, C. W. Litton and S. N. Mohammad, “Two-step surface treatment technique: Realization of nonalloyed low-resistance Ti/Al/Ti/Au ohmic contact to n-GaN, J. Vac. Sci. Technol. B, vol. 22, no. 2, pp. 663-667, 2004.
[6.12]H. W. Jang, J. K. Kim, J. L. Lee, J. Schroeder, and T. Sands, “Electrical properties of metal contacts on laser-irradiated n-type GaN, Appl. Phys. Lett., vol. 82, no. 4, pp. 580-582, 2003.