2.

覆晶技術及磊晶膜轉移技術及磊晶膜轉移技術應用應用於 f氮 化鎵發光二極體發光效率提升之研究 研究生 : 李佳恩 指導教授 : 郭浩中教授 盧廷昌教授 交通大學光電工程學系 摘要 由於 f氮 化鎵基材發光元件可廣泛的應用於如指示燈 各種照明 光儲存等領域, 因此自 1960 年代以來 f氮 化鎵相關材料成為世界上各研發團體的重要研究課題之ㄧ, 且在此數十年間 f氮 化鎵材料與元件特性上有屢有重大的發現與進展 本論文主要研究目的係提供提升 f氮 化鎵發光二極體元件特性之方法 其中, 主要藉由覆晶技術及磊晶膜轉移技術製作高功率與高亮度發光二極體並研究分析此二類元件之主要特性 發光波長介於紫外光至綠光之間的 LED, 一般都是在藍寶石基板上, 以 f氮 化鎵 (GaN) 製作而成, 然而由於此型基板的低熱傳導性質及絕緣特性, 使得該類 LED 只能做低功率元件的應用 為克服此一缺點, 透過覆晶技術及磊晶膜轉移技術, 可有效的提升元件之光電特性 於覆晶型發光二極體方面, 主要開發於藍寶石基板製作微米柱陣列及幾何形狀化結構的製程技術, 另外, 再藉由磊晶技術於微米柱陣列藍寶石基板成長 f氮 化鎵磊晶層及表面粗化結構, 並針對不同的製程技術於覆晶型發光二極體的影響進行探討 在第一部分, 我們成功的利用乾蝕刻製程技術在藍寶石背面製作出微米柱陣列, 使得光萃取效率得以提升 在量測結果部分, 發光波長在 460 奈米下, 有製作微米柱陣列的覆晶發光二極體的光輸出增加了 10%~68% 相較於傳統的覆晶發光二極體 在第二部分中為了提升光功率輸出, 我們成功的利用化學濕蝕刻製程技術在藍寶石背面製作出幾何形狀化結構, 經過濕蝕刻製程技術, 我們在藍寶石背面蝕刻出 (1-106) (11-25) 和 (1-102) 的晶格面, 相較於 (0001) c 軸而言角度分別是 30 50 和 60, 深度有 100 微米左右, 這些晶格傾斜 i

面和相當厚的藍寶石視窗層對光萃取效率的提升有很大的幫助 在量測的結果部分, 在電流 350 毫安培下, 有製作幾何形狀化的覆晶發光二極體的光功率輸出增加了 55% 在第三部份, 我們更結合了磊晶技術於微米柱陣列藍寶石基板成長 f氮 化鎵磊晶層及表面粗化結構成功製作三重光散色層覆晶型發光二極體, 同時解決光由 f氮 化鎵至空氣 f氮 化鎵至藍寶石基板 及藍寶石基板至空氣之高折射係數差所造成之全反射效應, 在電流 350 毫安培下, 具三重光散色層覆晶型發光二極體之光功率輸出增加了 60% 於薄膜型發光二極體方面, 係利用晶片接合及雷射剝離技術製作薄膜型發光二極體, 藉由將成長於 sapphire 基板上之 f氮 化鎵磊晶層轉移至具高導熱性及導電性基板, 此薄膜型發光二極體可於高電流操作下保持其光輸出功率呈之線性增加 本研究主要探討表面結構對薄膜型發光二極體取光效率的影響, 藉由網狀型表面粗化結構及光子晶體表面結構的開發, 增加薄膜型發光二極體表面取光效率 在第一部分, 我們成功的將成長於微米柱陣列藍寶石基板成長 f氮 化鎵磊晶層轉移至具高導熱及導電特性之矽基板, 再利用濕蝕刻粗化技術來達到網狀型表面粗化結構效果, 同時並探討不同表面結構對元件特性的影響, 在電流 350 毫安培注入下, 具網狀型表面粗化結構之薄膜型發光二極體光功率輸出增加了 20% 在第二部分中, 我們成功的開發具光子晶體表面結構之薄膜型發光二極體, 在本論文中, 我們於薄膜型發光二極體表面製作八欉準光子晶體結構, 除了能有效增加元件之取光效率外, 此元件亦能實現一般 Lambertian 遠場對稱光型 ii

Study of Light Extraction Enhancement for GaN-based Light Emitting Diodes by Flip-Chip and Epilayer-Transfer Technology Student: Chia-En Lee Advisors: Prof. Hao-Chung Kuo Prof. Tien-Chang Lu Department of Photonics, National Chiao Tung University Abstract In the thesis, we report the flip-chip and epilayer-transferred types GaN-based LEDs for improving the light extraction efficiency of devices. In the part of flip-chip type LEDs, the light-emitting diodes (FC-LEDs) with micro-pillar-array structure, the FC-LEDs with geometric oblique sapphire structure, and FC-LEDs with triple light scattering layers were investigated. In the first part, the micro pillar arrays structure is formed on the bottom side of sapphire substrate by dry etching process to increase the light-extraction efficiency. The light output power of the FC-LED is increased by 68% for a 3.2 µm textured micro pillar on the bottom side of the sapphire substrate. Our work offers promising potential for enhancing output powers of commercial light-emitting devices. In the second part, the sapphire shaping structure is formed on the bottom side of sapphire substrate by chemical wet etching technique for light extraction purpose. The crystallography-etched facets are (1-106), (11-25) and (1-102) plane against the (0001) c-axis with the angles range between ~30 - ~60. These large slope oblique sidewalls and greatly thick sapphire windows layer are useful for enhancing light extraction efficiency. The light output power of sapphire shaping FC-LEDs (SSFC-LEDs) was increased 55 % (@ 350 ma current injection) compared to that of conventional FC-LEDs (CFC-LEDs). In the third part, the FC-LEDs with triple light scattering layers were demonstrated by conjunction with the iii

epi-growth on micro-pillar array sapphire substrate and naturally textured p-gan surface. The totally internal reflection effect could be effectively reduced between the GaN-air, GaN-sapphire, and sapphire-air interfaces. The light output of the FC-LEDs with triple light scattering layer structure was increased about 60% (@350 ma current injection). The epilayer-transferred type LEDs were fabricated by the combination of wafer bonding and laser lift-off techniques to transfer the GaN epilayer to a better thermal dissipation and conductive substrate. In this part, the epilayer-transferred type LEDs with modified surface structures were demonstrated including roughened mesh surface and photonic crystal surface structures. In the first part, the epilayer-transferred type LEDs with the roughened mesh surface were proposed by transferring the GaN epilayer which was grown on a pattern sapphire substrate to a high thermal conductivity and electrical conductive silicon substrate and roughening the mesh surface by chemical wet etching process. Under 350 ma current injection, the epilayer-transferred type LEDs with the roughened mesh surface presents a further enhancement of 20% in output power comparing with that of regular epilayer-transferred type LEDs. In the second part, the epilayer-transferred types LEDs with the 8-folds photonic crystal surface structure were demonstrated. By adopting a photonic crystal surface structure, the light extraction efficiency was effectively improved. In addition, a Lambertian far-field pattern was also realized. iv

誌 謝 博士班四年在交通大學光電工程研究所的時光裡有幸得到許多人在各方面的指導與協助 感謝指導教授郭浩中老師在求學路上的耐心指導與督促, 及盧廷昌老師對於實驗內容指導與討論許多研究工作才得以完成 還要感謝同實驗室的朱榮堂博士 黃泓文博士及王德忠博士在實驗上的協助與指導 感謝孟儒 柏孝和清華等學弟妹實驗上的幫忙以及實驗室歷屆的研究生與博士生們的幫助, 讓我的研究得以順利進行 也特別要感謝工研院光電所的蔡政達博士 聯華電子的溫子稷博士 璨圓光電的武良文博士及威力盟電子的郭家泰博士在求學期間的協助及提攜 最後, 我要感謝我的家人, 感謝他們在對我求學期間的全力支持, 讓我能專心於學業上 感謝研究生過程中所有曾經予以我協助的所有人, 有以上諸位的協助與我個人些許的努力, 此論文才得以完成 佳恩 2009 11/5 於交通大學 v

Contents Abstract (in Chinese) Abstract (in English) Acknowledgement Contents. List of tables.. List of figures. i iii v vi ix x Chapter 1 Introduction. 1 1-1 History of GaN LEDs. 1 1-2 Flip chip technology.. 2 1-3 Epilayer transfer technology... 3 1-4 Monte Carlo raytracing 4 1-5 Organization of this dissertation... 5 Chapter 2 Flip-Chip Light-Emitting-Diodes with Textured Micro Pillar Arrays (MPAFC-LEDs).. 9 2-1 Progress in flip-chip light-emitting diodes.. 9 2-2 Fabrication of MPAFC-LEDs 10 2-3 Characteristics of MPAFC-LEDs. 11 2-4 Monte-carlo ray-tracing calculations. 13 Chapter 3 Flip-Chip Light-Emitting-Diodes with Geometric Sapphire Shaping Structure (SSFC-LEDs)... 28 3-1 Progress of geometric LED shaping structure.. 28 3-2 Fabrication of SSFC-LEDs 28 3-3 Characteristics of SSFC-LEDs. 30 vi

3-4 Monte-carlo ray-tracing calculations. 32 Chapter 4 Flip-Chip Light-Emitting-Diodes with Triple Light Scattering layers... 49 4-1 Fabrication of SSFC-LEDs 49 4-2 Characteristics of SSFC-LEDs. 50 Chapter 5 Luminance Enhancement of Vertical-Injection Light-Emitting Diodes by Surface Modification.. 60 5-1 Progress in vertical-injection near-ultraviolet light-emitting diodes.. 60 5-2 Fabrication of vertical-injection near-ultraviolet light-emitting diodes with a roughened mesh-surface 61 5-3 Characteristics of vertical-injection near-ultraviolet light-emitting diodes with a roughened mesh-surface 62 5-4 Progress in ultraviolet light-emitting diodes... 64 5-5 Fabrication of vertical-injection ultraviolet light-emitting diodes with the GaN-free and surface roughness structures 65 5-6 Characteristics of vertical-injection ultraviolet light-emitting diodes with the GaN-free and surface roughness structures.. 66 Chapter 6 Vertical-Injection Light-Emtting Diodes with Photonic Crystal Surface.. 82 6-1 Progress in vertical-injection light-emitting diodes with photonic crystal surface... 82 6-2 Fabrication of vertical-injection light-emitting diodes with a vii

photonic crystal surface.... 83 6-3 Characteristics of vertical-injection light-emitting diodes with a photonic crystal surface... 84 Chapter 7 Summary... 93 References..... 95 Publication list...... 100 Curriculum Vita....... 104 viii

List of tables Table 2-1 (a) shows the material variable of the models and (b) shows the surface variable of the models 23 ix

List of figures Chapter 1 Figure 1-1 Various flip chip technologies.. 6 Figure 1-2 Structure of conventional GaN-based LED... 7 Figure 1-3 Structure of Flip chip GaN-based LED 7 Figure 1-4 Schematic drawing of laser lift-off process.. 8 Figure 1-5 Picture of TracePro software 8 Chapter 2 Figure 2-1 Schematic of fabrication steps of GaN LEDs with micro-pillar arrays 15 Figure 2-2 SEM images of Si sub-mount before flip chip bonding. (a) Top view and (b) Side view... 16 Figure 2-3 SEM image of a chip bonding on the Si sub-mount.. 17 Figure 2-4 SEM images of micro-pillar-array surface of sapphire backside with various depth and bevel angle. (a) 1.1 µm MPA, (b) 1.8 µm MPA, (c) 2.7 µm MPA, and (d) 3.2µm MPA.. 19 Figure 2-5 The current-voltage (I-V) characteristics of flat surface FC-LEDs and MPAFC-LEDs... 20 Figure 2-6 The light output power-current (L-I) curves of flat surface FC-LEDs and MPAFC-LEDs 21 Figure 2-7 Light extraction enhancement of experimental results versus different depth of MPA.. 22 Figure 2-8 Photons of (a) conventional flat surface FC-LED and (b) micro pillar-array FC-LED at a dc injection current of 350 ma... 22 Figure 2-9 Figure 4-9 (a) shows the structure of the simulated models, (b) shows the x

models in the TracePro software, and (c) is a sketch of the pattern on the backside surface of the sapphire substrate. 25 Figure 2-10 The simulation results by Monte-Carlo ray-tracing. (a) and (b) show the irradiance maps of conventional flat FC-LEDs and 3.2 µm MPAFC-LEDs, respectively.. 26 Figure 2-11 Light extraction enhancement comparison of experimental and simulation results versus different depth of MDA. 27 Chapter 3 Figure 3-1 Schematic of fabrication steps of sapphire shaping FC-LEDs... 34 Figure 3-2 Schematic drawing of the GaN SSFC-LEDs, illustrating the means by which light may be extracted from the oblique sapphire sidewall. 35 Figure 3-3 SEM images of sapphire shaping structures (a) top and (b) cross-sectional view 36 Figure 3-4 SEM images of the crystallography facets of (a) R-plane (60 ), (b) A-plane (30 ), and (c) M-plane (50 ) against (0001) c-axis, respectively... 38 Figure 3-5 SEM images of (a) CFC-LED and (b) SSFC-LED devices. 39 Figure 3-6 Photomicrographs of (a) CFC-LED and (b) SSFC-LED chips (40X40 mil) operating at 20 ma (dc) with an emission wavelength of λ p ~460 nm 40 Figure 3-7 The corresponding current-voltage (I-V) characteristics of SSFC-LEDs and CFC-LEDs.. 41 Figure 3-8 The light output power and wall plug efficiency as a function of injection current for λ p ~460 nm devices of SSFC-LEDs and CFC -LEDs 42 Figure 3-9 The normalized far-field patterns of the SSFC-LED and CFC-LED versus two directions ((1-106)-plane to (1-102)-plane, X-axis; (11-25)-plane to xi

(11-25)-plane, Y-axis), respectively.. 43 Figure 3-10 The normalized three-dimensional far-field patterns of (a) FC-LEDs and (b) SSFC-LEDs 44 Figure 3-11 The structure of the siumalted model in TracePro software 45 Figure 3-12 The candela maps of (a) CFC-LEDs and (b) 100 µm SSFC-LEDs 46 Figure 3-13 The ray-tracing images of oblique sidewall of (a) CFC-LEDs and (b) SSFC-LEDs 47 Figure 3-14 The calculated enhancement of the light extraction efficiency with the increasing of etching depth of SSFC-LEDs 48 Chapter 4 Figure 4-1 Schematic drawings of fabrication steps of FC-LEDs with triple light scattering layers: (a) Epi-growth on paatern sapphire substrate with naturally surface roughness, (b) back-side pattern formation, (c) standard chip process,and (d) chip separation and flip-chip bonding 53 Figure 4-2 Schematic drawing of the FC-LEDs with triple-light scattering layers... 54 Figure 4-3 SEM image of top surface sapphire textured layer 55 Figure 4-4 SEM image of surface morphology of pattern sapphire substrate and epi-growth on pattern sapphire substrate... 56 Figure 4-5 Naturally textured p-gan layer with different Mg-treatment 58 Figure 4-6 The corresponding current-voltage (I-V) characteristics of SSFC-LEDs and CFC-LEDs. 59 Chapter 5 xii

Figure 5-1 Diagrams of fabrication processes for regular VLEDs (a) wafer bonding, (b) laser lift-off, (c) isolation, and (d) surface roughness and bonding pad 69 Figure 5-2 Diagrams of fabrication processes for VLEDs (a) wafer bonding, (b) LLO and (c) chemical wet etching and electrode deposition.. 70 Figure 5-3 SEM images of (a) surface morphology of mesh-surface VLEDs after LLO process and (b) cross sectional view of VLEDs structure 72 Figure 5-4 SEM images of mesh-surface VLEDs after chemical etching process... 74 Figure 5-5 Room-temperature EL spectra of flat-surface and mesh-surface VLEDs.. 75 Figure 5-6 L-I-V characteristics of flat- and mesh- surface VLEDs with and without chemical wet etching process... 76 Figure 5-7 Schematic drawings of (a) conventional UV-LED and (b) GaN-free UV-VLED. 77 Figure 5-8 Room-temperature EL spectra of UV-LEDs.. 78 Figure 5-9 (a) AFM image of VLED surface after LLO process and (b)afm image of VLED surface after surface treatment process.. 79 Figure 5-10 L-I-V characteristics of conventional UV-LED, UVVLED, GaN-free UV-LED, and surface roughened GaN-free UV-LED. 80 Figure 5-11 Intensity distribution of (a) surface roughened GaN-free UV-VLED and (b) conventional UV-LED at 100 ma current injection 81 Chapter 6 Figure 6-1 Schematic drawings of VLEDs with (a)2d-pc and (b) chemical wet etching random roughness surface. 87 xiii

Figure 6-2 SEM images of the etched top n-gan formed by electron-beam lithography patterns using ICP etch process: (a) top and (b) cross sectional views. 88 Figure 6-3 L-I-V characteristics of VLEDs with and without PQC surfaces.. 89 Figure 6-4 Figure 6-4 3D far-field patterns of (a) PQC-VLED and (b)regular-vled... 90 Figure 6-5 (a) Angular distribution radiation pattern and (b) 3D far-field pattern of a PQC-VLED 91 Figure 6-6 The FDTD calculate the various lattice constant with fixed r/a=0.31 in PQC VLED of light enhancement.. 92 xiv