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研究生： 林增興
Lin, Tseng-Hsing
論文名稱： 薄膜覆晶結構與垂直結構高效率藍光氮化鎵-基發光二極體之研製及其光輸出功率改善研究
Improving the Light Output Power of Vertically-Structured and Thin-Film Flip-Chip GaN-Based High Power Light-Emitting Diodes
指導教授： 王水進
Wang, Shui-Jinn
學位類別： 博士
Doctor
系所名稱： 電機資訊學院 - 微電子工程研究所
Institute of Microelectronics
論文出版年： 2016
畢業學年度： 105
語文別： 英文
論文頁數： 99
中文關鍵詞： 發光二極體 、垂直結構 、覆晶 、表面出光 、金屬基板 、階梯 、電極設計 、雷射剝離 、雷射燒蝕 、溝槽
外文關鍵詞： light-emitting diodes, vertical-structured, flip-chip, thin-film, metallic-substrate, laser lift-off, surface roughening, trench etching, nanostructure, laser ablation
相關次數： 點閱：102  下載：4 
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    • 本論文除提出一種於氮化鎵-基(GaN-based)藍光發光二極體 (light-emitting diode, LED)之磊晶晶圓之磊晶表面上，製備歐姆接觸層、反射層、阻擋層及附著層之多層金屬結構，利用晶圓接合鎢銅基板製程或電鍍鎳金屬基板製程，以及利用準分子雷射 (KrF laser) 進行藍寶石(sapphire)基板剝離製程，成功製備具金屬基板之薄膜覆晶結構發光二極體(thin-film flip-chip LED, TFFC-LED)及垂直結構發光二極體 (vertically-structured LED, VLED)外，亦另於LED之磊晶表面進行粗化製程，降低磊晶與空氣間之全反射角，進行光輸出功率之改善。相較於傳統水平結構藍寶石基板發光二極體(regular LED)，TFFC-LED與VLED因採用高反射率之反射層結構及厚導電金屬基板(50~80 m)改善p型GaN (p-GaN)電流擴散能力不佳之缺點，亦同時提升LED元件導熱能力、降低串聯電阻、減少電流叢聚效應(current crowding effect)與降低電極遮蔽率增加發光面積等優點。
    為進一步提高LED於固態照明應用範圍，本研究亦提出多種表面出化技術，以增強TFFC-LED及VLED光析出效率。首先，利用柵欄形狀與樹枝形狀電極圖案設計，以及利用感應耦合電漿(inductively coupled plasma, ICP)乾蝕刻製程，於VLED之n型GaN (n-GaN)表面製備圖案化二次階梯表面形貌，提出一個能有效改善電流擴散的VLED結構。此方法所製備之VLED相較於傳統的VLED，於操作在350 mA (700 mA)電流下，其最佳光輸出效率增加33.3 (38.4)%。
    其次，為增進VLED光析出效率，本研究開發一種圖案化壕溝蝕刻及圖案化p型電極。此壕溝蝕刻穿透VLED發光層，將VLED切割成多顆微晶粒亦並聯成同一個元件，藉由壕溝蝕刻增加VLED側面壁之面積，提升側面出光面積。於相同元件面積1×1 mm2條件下（包含蝕刻溝槽），相較傳統大面積單顆VLED，利用4顆微晶粒並聯之VLED於操作在350 mA (700 mA)電流下，其最佳光輸出效率增加9.0% (12.1%)。
    此外，為進一步使用壕溝蝕刻製程增進VLED光析出效率，本研究開發一種利用低成本水熱法製程及黃光製程，於VLED之n-GaN表面成長圖案化氧化鋅奈米線作為蝕刻遮罩，藉由ICP乾蝕刻製程，成功製備具奈米結構之圖案化溝槽粗化結構。因此溝槽並未穿透VLED發光層，對元件的內部量子效應並無損害。實驗結果顯示，藉由溝槽以及奈米粗化結構，確可增加VLED側面以及正面出光面積，降低GaN磊晶與空氣間之全反射角，相較於未進行粗化製程之VLED，此方法所製備之VLED於操作在350 mA (700 mA)電流下，其最佳光輸出效率增加37.6% (33.9%)。
    最後，本研究提出一種利用準分子雷射燒蝕製程，接續熱KOH濕蝕刻粗化TFFC-LED之未摻雜GaN (u-GaN) 所製造出半球圓弧突起物附加六角錐形狀，增加TFFC-LED正面出光面積，降低GaN磊晶與空氣間之全反射角。相較於未進行粗化製程之TFFC-LED，於操作電流350 mA (700 mA)，雷射脈衝150 pulses之兩階段粗化之TFFC-LED光輸出功率增加13.08% (12.81%)。
    本論文所開發適用於GaN-based VLED與及TFFC-LED之電流擴散結構與表面粗化結構之製程，具簡易與快速之優點，不需另購多餘昂貴機台，有效提升LED之光輸出功率及光轉換效率，預期可加速GaN-based LED於固態照明應用。

    The dissertation aims at improving the light output power (LOP) and wall-plug efficiency (WPE) of vertical-structured and thin-film flip-chip GaN(gallium nitride)-based LEDs (VLEDs and TFFC-LEDs). The thin-GaN structure and metallic-substrate of VLEDs and TFFC-LEDs were fabricated via nickel electroplating, CuW substrates wafer bonding and laser lift-off (LLO) techniques. As compared with traditional lateral-structured GaN-based LEDs with sapphire substrate (abbreviated as regular LEDs), VLEDs and TFFC-LEDs have many advantages. It includes a conducting substrate prepared on the surface of p-type GaN (p-GaN) layer, which provides a better heat dissipation, lower series resistance, less current crowding effect, lower shading ratio of contact electrode, and larger effective emission area. Top of the n-GaN epilayer with low sheet resistance (~1×10-3 Ω∙cm) and thick thickness (1~3 μm) can improved current spreading and protect the active layer (light emitting layer, multi quantum well) of LED during surface roughing process with chemical etching.
    To further increase external emission efficiency, different surface roughening technique were also proposed for the fabrication of VLEDs and TFFC-LEDs in this study. First, an efficient current spreading design of ladder-type surface roughening through the use of an inductively coupled plasma (ICP) and 2-step mesa etching on n-GaN. An optimal n-electrodes design to strengthen the uniformity of current distribution and LOP of high power VLEDs with a chip size of 1×1 mm2 is demonstrated. The present design allows the patterned n-electrodes having their distances to the active region of VLED decreases with increasing their lateral distances to the contact pad, which could balance the difference in voltage drops on the n-GaN layer as encountered in conventional VLEDs. Consequently, as compared to conventional VLED with flat n-GaN structure, the proposed VLEDs shows the highest increase in LOP by 33.3% (38.4%) at 350 mA.
    Second, GaN-based VLEDs with trench etching and arrayed p-electrodes in improving current spreading and the efficiency of light extraction were fabricated and investigated. For a 2 × 2 array VLED with a die size of 1020 × 1020 μm2, enhancements in LOP by 0.38% and WPE by 2.79% at 364.4 mA/mm2 as compared with that of regular VLED were achieved experimentally, which are attributed to improved current spreading from the arrayed p-electrode and trench designs as well as enhanced light emission from the trench region.
    In addition, an efficient surface texturing technique that uses patterned trench etching and selective formation of GaN nanostructures on the trench bottoms to improve the light extraction of VLEDs is proposed and demonstrated. Compared with conventional VLEDs, significant improvements in LOP and WPE at 350 mA of about 37.6% and 5.1%, respectively, were obtained. It is noted that the effective lateral light emission harvested by patterned trenches and the strongly enhanced angular randomization of photons that minimizes the total internal reflection at the GaN/air interface are responsible for the LOP and WPE improvements.
    Finally, to further enhance the light extraction efficiency of GaN-based TFFC-LEDs, a surface roughening technique using KrF excimer laser ablation and chemical wet etching is also demonstrated. With the proposed twofold surface texturing scheme with circular protrusions superimposed by hexagonal cones, the angular randomization of photons at the emission surface was maximized, enhancements in LOP of 13.08% and WPE of 2.87% at 350 mA compared to those of a TFFC-LED without surface texturing were obtained.
    It is highly expected that VLEDs and TFFC-LEDs in associating with the efficient surface roughening scheme could be very potential for the promotion of SSL in the near future.

    Contents Abstract (in Chinese) I Abstract (in English) IV Acknowledgements VII Contents VIII Table captions XI Figure captions XII Chapter 1 Introduction 1-1 Overview of GaN-based high power LEDs 1 1-2 Issues of external quantum efficiency in GaN-based LEDs 4 1-3 Substrate transfer engineering of VLEDs and TFFC-LEDs 10 1-3.1 Metal electroplating 12 1-3.2 Wafer bonding 13 1-3.3 Laser lift-off 14 1-3.4 Surface roughening techniques 17 1-4 Motivations and thesis organization 18 Chapter 2 Use of a two-step etching on n-GaN surface and an optimal dendritic-shaped n-electrode design to improve current spreading in vertical-structured GaN-based LEDs 2-1 Introduction 21 2-2 Device fabrication 22 2-3 Results and discussion 26 2-3.1 Current density distribution simulation 26 2-3.2 Electrical and optical characteristics 29 2-4 Summary 33 Chapter 3 Enhanced light emission in vertical-structured GaN-based LEDs with trench etching and arrayed p-electrodes 3-1 Introduction 35 3-2 Device fabrication 36 3-3 Results and discussion 39 3-3.1 Surface morphology 39 3-3.2 Ray-tracing simulation and current density distribution simulation 40 3-3.3 Electrical and optical characteristics 44 3-4 Summary 48 Chapter 4 Enhanced light extraction of GaN-based vertical LEDs with patterned trenches and nanostructures 4-1 Introduction 49 4-2 Device fabrication 51 4-3 Results and discussion 54 4-3.1 Surface morphology 54 4-3.2 Ray-tracing simulation 56 4-3.3 Electrical and optical characteristics 57 4-4 Summary 61 Chapter 5 Improving the performance of power GaN-based thin-film flip-chip LEDs through a twofold roughened surface 5-1 Introduction 62 5-2 Device fabrication 63 5-3 Results and discussion 67 5-3.1 Surface morphology 67 5-3.2 EDS analysis 70 5-3.3 Ray-tracing simulation 70 5-3.4 Electrical and optical characteristics 73 5-4 Summary 76 Chapter 6 Conclusions and suggestions for future study 6-1 Conclusions 77 6-2 Suggestions for future study 81 References 84 Table captions Table 1-1 SSL-LEDs Metric Roadmap by U. S. department of energy (DOE) [1.27] 3 Table 2-1 Shading ratio of fence-shaped and dendritic-shaped n-electrodes 26 Table 2-2 Structural parameters used in the Crosslight APSYS simulation 29 Table 2-3 The measured optical characteristics of fabricated VLEDs. 33 Table 3-1 Structural parameters used in the Crosslight APSYS simulation 43 Table 3-2 Ray-tracing calculated results obtained from a TracePro simulation 43 Table 6-1 Conclusions of all technologies in this dissertation 81 Figure captions Fig. 1-1 Bandgap energies of InN, GaN, AlN, and the ternary alloys InxGa1−xN and AlyGa1−yN with a rainbow depiction of the visible spectrum. [1.23] 2 Fig. 1-2 The evolution and anticipated performance of LEDs and other light sources versus development time [1.28] 4 Fig. 1-3 EQE and peak wavelength of high-power LEDs based on the InGaN light emitting layer and InGaAlP light emitting layer material systems [1.31] 6 Fig. 1-4 Plot of IQE, measured using PL, versus wavelength of III-nitride quantum-well structures [1.32] 7 Fig. 1-5 Refraction of light at the interface between air and GaN, including total internal reflection 8 Fig. 1-6 Fresnel power transmittance from GaN to air 9 Fig. 1-7 Schematic diagram of nickel electroplating scheme used in this dissertation 13 Fig. 1-8 Schematic diagram of wafer boning scheme used in this dissertation 14 Fig. 1-9 Schematic diagram of KrF excimer laser system of KrF excimer laser beam source and BDS for excimer laser 15 Fig. 1-10 Laser lift-off process 16 Fig. 1-11 Temporal and spatial variation of the temperature at the sapphire/GaN interface (depth zero) during and after a KrF excimer laser pulse [1.60] 17 Fig. 2-1 Key fabrication processes of VLEDs with a two-step etching on n-GaN surface or an optimal dendritic-shaped n-electrode design 23 Fig. 2-2 Schematic device structures of VLEDs. (a) VLED-A with a ladder-type n-GaN and fence-shaped n-electrode. (b) VLED-B with a ladder-shaped n-GaN and a dendritic-shaped n-electrode. (c) VLED-C with a fence-shaped n-electrode and a ladder morphology of n-GaN right under patterned n-electrode 25 Fig. 2-3 Schematic fence-shaped and dendritic-shaped n-electrode of VLEDs. 26 Fig. 2-4 (a) Schematic current path of VLEDs with intact flat and ladder-shaped n-GaN. (b) The calculated current density distribution across the active region of VLEDs by Crosslight simulation 28 Fig. 2-5 Near-field emission intensity images of fabricated VLEDs measured beam profile at 350 and 700 mA 31 Fig. 2-6 Comparison of V-I and LOP-I characteristics among various fabricated VLEDs 32 Fig. 2-7 Comparison of reverse I-V characteristics among various fabricated VLEDs 32 Fig. 2-8 Comparison of WPE-I characteristics among various fabricated VLEDs. 32 Fig. 3-1 Key fabrication processes of VLEDs in this work 38 Fig. 3-2 Exploded view drawings, OM, and SEM images of the proposed VLEDs. (a) Regular VLED, (b) VLED-A, and (c) VLED-B. The cross-sectional structure of the fabricated sample is also shown in (a). The active region of VLEDs comprises a five-period GaN/InGaN MQWs structure. 40 Fig. 3-3 (a) Ray-tracing calculated results obtained from a TracePro simulation. (b) Current density distribution at the bottom layer of the active region and (c) LOP-J/LOP-I characteristics of the three VLEDs obtained from Crosslight simulation 43 Fig. 3-4 (a) OM images of light emission at different current densities measured using a beam profile. (b) Distribution of light intensity along broken line AA’ shown in Fig. 4-1(a) 45 Fig. 3-5 Forward I-V characteristics of the fabricated VLEDs. Inset shows reverse I-V curves 47 Fig. 3-6 LOP-J/LOP-I characteristics of the fabricated VLEDs. 47 Fig. 3-7 WPE-J/WPE-I characteristics of the fabricated VLED 47 Fig. 4-1 Key fabrication processes of VLEDs with patterned trenches and selective formation of GaN nanostructures on trench bottoms 53 Fig. 4-2 SEM images of n-GaN surface of fabricated VLEDs. 55 Fig. 4-3 Schematics of the simulated VLEDs used in the ray-tracing simulation. The calculated near-field emission intensity images obtained from ray-tracing simulations 57 Fig. 4-4 Light emission from the fabricated VLEDs at 100 mA. The (a) Images of near-field emission intensity measured by a beam profiler and (b) mappings of light emission intensity along path a-b 59 Fig. 4-5 Forward I-V characteristics of the fabricated VLEDs. Inset shows reverse I-V curves 60 Fig. 4-6 LOP-I curves of fabricated VLEDs 60 Fig. 4-7 WPE-I curves of fabricated VLEDs 60 Fig. 5-1 Key fabrication processes of TFFC-LEDs with u-GaN surface roughening achieved by KrF excimer laser irradiation and chemical wet etching 66 Fig. 5-2 SEM images of u-GaN surface of TFFC-LEDs: (a) without etching process (LED-A), (b) after etched by KOH solution (LED-B), (c-g) after etched by KrF laser with 0, 60, 90, 120, 150 and 180 pulses at energy density 800 mJ/cm2, and (h-l) after etched by KrF laser with different pulses, and KOH solution (LED-C) 69 Fig. 5-3 The SEM images of the u-GaN surface of TFFC-LEDs with 120 pulses of laser irradiation and EDS profiles 70 Fig. 5-4 Schematic devices and calculation results obtained from ray-tracing simulations showing effectiveness of surface texturing scheme 72 Fig. 5-5 Comparison of EL spectra of fabricated TFFC-LEDs. 74 Fig. 5-6 (a) The measured near-field emission intensity images and (b) the distribution of light intensity along the path a-b of the fabricated TFFC-LEDs at 50 mA 74 Fig. 5-7 Comparison of I-V characteristics of fabricated TFFC-LEDs. Inset shows the reverse I–V characteristics 75 Fig. 5-8 Comparison of LOP-I characteristics of fabricated TFFC-LEDs. 75 Fig. 5-9 Comparison of WPE-I characteristics of fabricated TFFC-LEDs 75 Fig. 6-1 (a) VLED-B with a ladder-shaped n-GaN and a dendritic-shaped n-electrode, (b) VLED-C with a two-step etching right under the patterned electrodes and a fence- fence-shaped n-electrode, and (c) VLED with a dendritic-shaped n-electrode and a ladder morphology of n-GaN right under patterned n-electrode 82 Fig6-2 Using textured phosphor layers through nanoimprint lithography to improve the LEE and color rendering index of white LEDs 83

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