氮化銦鎵綠光發光二極體之研製｜國立中央大學博碩士論文系統

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研究生：	林宏誠 Hung-Cheng Lin
論文名稱：	氮化銦鎵綠光發光二極體之研製 Investigation of InGaN Green Light-Emitting Diodes
指導教授：	綦振瀛 Jen-Inn Chyi
口試委員:
學位類別：	博士 Doctor
系所名稱：	資訊電機學院 - 電機工程學系 Department of Electrical Engineering
畢業學年度：	97
語文別：	英文
論文頁數：	129
中文關鍵詞：	氮化鎵、氮化銦鎵、發光二極體
外文關鍵詞：	GaN, InGaN, LED
相關次數：	點閱：9 下載：0
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本論文主旨為開發525 nm 氮化銦鎵多重量子井綠光發光二極體磊晶及製程關鍵技術。首先我們提出兩種有效提升綠光發光二極體效率的磊晶技術，其一是『三甲基銦處理量子井磊晶技術』，其二是『氮化銦鎵/氮化鎵超晶格結構與調變摻雜鎂磊晶技術』，分別研究其材料特性及探討發光效率提升之原理。隨後將氮化鎵及綠光發光二極體結構成長於不同幾何外型之微透鏡陣列圖案化藍寶石基板上方，研究其成長模式與材料特性，並分析其對於提升綠光發光二極體發光效率之影響。最後，我們提出兩種有效提升光萃取效率的製程技術，分別為『自然形成遮罩法濕式蝕刻藍寶石基板技術』與『利用切割道圖案化提升光萃取效率技術』，並探討不同蝕刻深度對於元件光電特性及萃取效率之影響。
在探討三甲基銦處理量子井提升綠光發光二極體發光效率的研究上，藉由原子力顯微鏡與穿遂式電子顯微鏡觀察分析，我們發現三甲基銦處理量子井技術，可以有效降低量子井內部的V型缺陷密度，並改善量子井介面間的平坦度。藉由此技術，綠光發光二極體操作於20 mA下，其發光效率有43%的提升。在『氮化銦鎵/氮化鎵超晶格結構與調變摻雜鎂磊晶技術』方面，我們利用氮化銦鎵/氮化鎵超晶格結構的極化場效應，與調變摻雜技術，可以有效提升鎂的活化率，並可降低材料串連阻值，藉由此技術應用於綠光發光二極體，可以有效提升約兩倍之發光效率。在電性方面，使用此成長技術之綠光發光二極體具有很低的逆向漏電流之優點。
在成長氮化鎵與綠光發光二極體結構於不同幾何外型之微透鏡陣列圖案化藍寶石基板上方之研究，藉由掃瞄式電子顯微鏡、穿遂式電子顯微鏡、電子束激發光譜法觀察分析氮化鎵成長模式與材料特性，我們發現抑制氮化鎵之晶核層成長於微透鏡幾何外型之凸部，可以提供較多側向成長氮化鎵區域，減少缺陷密度，進而提升氮化鎵之材料品質。接著我們將525 nm 綠光發光二極體結構成長於不同幾何外型之微透鏡圖案藍寶石基板，我們發現其有不同之發光強度。經由拉慢光譜及X光繞射研究殘餘應力顯示，底層氮化鎵殘餘應力越小，綠光發光二極體之發光效率越高。
在『自然形成遮罩法應用於濕蝕刻藍寶石基板技術』的研究上，藉由高溫硫酸蝕刻藍寶石基板表面，形成Al(SO4)及Al(SO4)3•17H2O之多晶硫酸鹽鋁化合物做為自然遮罩，並同時蝕刻藍寶石基板，再配合高溫磷酸兩段式蝕刻法，去除自然遮罩，形成微角錐幾何外型之圖案化藍寶石基板。此技術應用於氮化銦鎵發光二極體上，約有20%效率提升，此種技術由於不需要額外黃光微影製程技術，具有簡單且低成本之優點。在『利用切割道圖案化提升發光效率之製程技術』的研究上，我們提出一種提高光萃取效率之方法，此技術對於操作電壓的影響相當少（約0.04V），這是因為將電流路徑與高光萃取率區域分離，可將表面態或缺陷對於操作電壓之影響降到最低。在蝕刻深度的研究上，我們發現蝕刻深度對於光萃取效率有很大影響，當在蝕刻深度為1.5微米時，具有最高之光萃取效率，這是因為當蝕刻至藍寶石基板時，破壞了光波導區域，主動層所發出光無法順利傳導致高光萃取效率區域時，對於光萃取效率反而減少之效果。

This dissertation describes the growth and characterization of InGaN/GaN multiple-quantum-well (MQW) green light-emitting diode structures grown by low-pressure metal-organic vapor phase epitaxy (MOVPE). The content of this thesis is divided into the following three parts.
First, we propose two methods for enhancing the luminescence efficiency of MQW. They are trimethylindium (TMIn) treatment process and Mg modulation-doped InGaN/GaN superlattices (MD-SLS) structure. Green light-emitting diodes prepared by the TMIn treatment method exhibit higher output power than the control device. The external quantum efficiency of the LEDs is increased by 43%. Both atomic force microscopy and transmission electron microscopy images indicate that TMIn treatment process produces InGaN/GaN MQW with smooth interface and low V-shape defect density, which are essential for high efficiency InGaN/GaN LEDs in the green and longer wavelength region. In addition, we demonstrate that low-temperature grown Mg modulation-doped InGaN/GaN superlattices structure is well suited for the p-contact layer of InGaN-based green LEDs. The light output of LEDs with this p-contact layer is increased approximately twofold as compared with that of conventional LEDs. The observed increase in quantum efficiency is attributed to the enhanced activation of Mg through the piezoelectric field in MD-SLS.
Second, we prepare a series of patterned sapphire substrates with micro-lens of different shapes. Their effects on growth mode, material quality, residual strain, as well as the optical properties of the GaN epilayers and 525 nm green LEDs are systematically investigated. Growth mode analysis shows that micro-lens with a sharp tip prohibits the nucleation and growth of GaN on its top and leads to a wider lateral growth region with low dislocation density. Green light-emitting diodes grown on these PSSs also exhibit different external quantum efficiency. Experimental results show that the output power of 525 nm green LEDs is higher when grown on GaN with lower residual strain. The spectral blue shift with injection current is also less. This phenomenon might be attributed to the lower internal field in the quantum wells grown on GaN buffer layer with lower residual strain.
Finally, we present two novel patterning techniques to improve the external quantum efficiency of InGaN/GaN quantum well light-emitting diodes. They are naturally etched sapphire substrates (NESSs) and dicing streets technology. Simply leaving a sapphire substrate in hot sulfuric acid, faceted islands are formed on the substrate due the spontaneous formation of an insoluble mixture of polycrystalline aluminum sulfates as a natural mask. It is shown that the light output power of InGaN LEDs grown on the substrate is enhanced by nearly 20% despite the fact that the LEDs already have an indium-tin oxide transparent contact layer and a roughened surface. The uniformity of luminescence intensity and wavelength of the LEDs grown on two inch NESSs is also better than that of the LEDs grown on flat sapphire substrates. Furthermore, patterning dicing streets is also found effective in enhancing the light extraction efficiency of InGaN/GaN multiple-quantum-well light-emitting diodes. The external quantum efficiency is increased by 9.4% for the LEDs with a partially etched pattern while its forward voltage is increased only 0.04 V. This result demonstrates that the proposed approach, which separates the current flow from the light extraction region, gives less adverse effect often encountered in conventional patterning and roughening processes.

CONTENTS
中文論文提要....................................................................................................i
Dissertation Abstract…………………………………………….....………..iii
Contents……………………………………………………………………....vii
Table Captions………..……………………………………………………….x
Figure Captions……..………………………………………………………...xi
Chapter 1 Introduction
1.1	Development of Wide Band-gap Nitride-based
Light-emitting Diodes……………………………………………………..01
1.2	Development of InGaN Green Light-emitting Diodes………………….. 03
1.2.1  Polarization Effect………………………………………………..05
1.2.2  V-shape Defect……………………………………………………06
1.2.3  Carrier Leakage Effect……………………………………………07
1.2.4  Light Extraction…………………………………………………..07
1.3	Brief Introduction of This Dissertation…………………………………...09
Chapter 2 Luminescence Efficiency Investigation of InGaN
Multiple-quantum-well Green Light-emitting Diodes
2.1	Trimethylindium (TMIn) Treatment Surface
Smoothing Process………………………………………………………11
2.1.1  Introduction of TMIn Treatment Surface
Smoothing Process………………………………………………..11
2.1.2  Experimental Details……………………………………………..12
2.1.3  Effect of TMIn Treatment Time…………………………………13
2.1.4  Effect of TMIn Treatment Location……………………………..18
2.1.5  Effect of TMIn Treatment Temperature…………………………19
2.1.6  Mechanism of TMIn Treatment…………………………………21
2.1.7  LED Device Results………………………………………………23
2.2	Mg-modulation-doped InGaN/GaN Superlattices………………………..25
2.2.1	Introduction of Mg-modulation-doped InGaN/GaN
Superlattices……………………………………………………...25
2.2.2	Experiment Details………………………………………………25
2.2.3	Effect of Mg-modulation-doped InGaN/GaN
Superlattices…………………………………………………….26
2.2.4  Material Quality Modification…………………………………...29
2.2.5	LED Device Results……………………………………………..33
2.3 Summary…………………………………………………………………..39
Chapter 3 Growth and Characterization of Epitaxial GaN
Material and Light-emitting Diodes on Lens
Shape Patterned Sapphire Substrates
3.1 Introduction of Lens Shape Patterned Sapphire Substrates……………….41
3.2 Experiment Details………………………………………………………..42
3.3 Results and Dissuasion……………………………………………………43
3.3.1 Growth Mode Analysis…………………………………………….43
3.3.2 Material Characterization of GaN Grown on Micro-lens PSS….…49
3.3.3 Electrical and Optical Properties of LEDs on the PSS…………….52
3.3.4 Residual Stress of GaN Grown on Micro-lens PSS………………..54
3.4 Summary…………………………………………………………………..58
Chapter 4 Novel Pattern Technologic for InGaN Light-emitting Diodes
4.1	Masklessly Wet-etched Sapphire Substrate………………………………59
4.1.1  Introduction of Masklessly Wet-etched Sapphire Substrate……...60
4.1.2  Experiment Details……………………………………………..…61
4.1.3  Etched Face Analysis……………………………………………..61
4.1.4  Effect of Etching Time……………………………………………62
4.1.5  Electrical and Optical Properties of LEDs on the NESSs………...67
4.2	Patterning the Dicing Streets……………………………………………...70
4.2.1  Introduction of Patterning the Dicing Streets…………………….70
4.2.2  Experiment Details……………………………………………….72
4.2.3  Effect of Etching Depth..…………………………………………73
4.2.4  Light Extraction Analysis………………………………………...77
4.2.5  The Reliability Characteristics……………………………………79
4.3  Summary……………………………………………………………...…80
Chapter 5 Conclusions and Future Work………………………………….81
Appendixes…………………………………………………………………...85
Reference……………………………………………………………………..90
Publication List……………………………………………………………..107
Table Captions
Table 2.1: Optical properties of the InGaN/GaN MQWs grown with TMIn treatment for 60, 120, and 180 s.
Table 2.2: Optical properties of the InGaN/GaN MQWs grown with different TMIn treatment location.
Table 2.3: Optical properties of the InGaN/GaN MQWs grown with different TMIn treatment temperature.
Table 2.4: Summary of structural and Hall measurements.
Table 2.5: Results of root-mean-square surface roughness and etch-pit-density (EPD) of the LED with different p type growth condition.
Table 3.1: Results of X-ray diffraction (XRD), photoluminescence (PL), etch pit density (EPD) and CL pit density (CL-PD) measurement on 5-μm-thick GaN epilayers grown on type I, type II, and type Ш micro-lens PSS.
Table 3.2: Summary of InGaN LEDs grown on the three types patterned sapphire substrates.
Table 3.3: Results of μ-Raman and XRD measurements on 5-μm-thick GaN epilayers grown on type I, type II, and type Ш micro-lens PSS.
Table 4.1: Summary of the average etching depth, mesa size, density, and root-mean-square (RMS) roughness of GaN epilayers with different etching time.
Table 4.2: Summary of structural and optical characteristics of GaN and InGaN LEDs grown on NESSs prepared with different etching time.
Table 4.3: Overview of GaN LEDs on patterned sapphire substrate.
Table 4.4: Results of the wafer mapping of forward voltage (Vf) at 20 mA of the LEDs at various etching depth.
Table 4.5: Overview of the GaN LEDs with pattern technology.
Table 5.1: Various models for the efficiency droop.
Figure Captions
Fig. 1.1: Evolution of nitride semiconductor.
Fig. 1.2: Chronological change of EQE of InGaN-GaN green LEDs.
Fig. 1.3: State-of-art EQE for visible-spectrum LEDs.
Fig. 1.4: Evolution of extraction efficiencies for InGaN-GaN LEDs.
Fig. 2.1: RT PL spectra and XRD spectra of InGaN/GaN MQWs with and without TMIn treatment.
Fig. 2.2: Temperature-dependent integrated PL intensity of InGaN/GaN MQW with and without TMIn treatment.
Fig. 2.3: Atomic force microscopic images (3×3 μm2) of the surface morphology of [(a) and (b)] SQW and [(c) and  (d)] MQW structures without and with TMIn treatment, respectively.
Fig. 2.4: TEM images of the InGaN/GaN MQW with (a) and without (b) TMIn treatment.
Fig. 2.5: RT PL spectra of InGaN/GaN MQWs with different TMIn treatment location.
Fig. 2.6: RT PL spectra of InGaN/GaN MQWs with different TMIn treatment temperature.
Fig. 2.7: A simple schematic diagram to illustrate the growth mechanism: (a) As grown InGaN/GaN, (b) TMIn treatment and inter diffusion, (c) Indium cluster remove, and (d) Indium as surfactant for the grow GaN barrier layer.
Fig. 2.8: Light output-current characteristics of the LEDs grown with and without TMIn treatment.
Fig. 2.9: The band diagram for the Mg-doped In0.07Ga0.93N/GaN MD-SLS.
Fig. 2.10: Temperature dependences of the hole concentration in sample A and C.
Fig. 2.11: XRD curve of the p type InGaN/GaN MD SLS with different growth temperature.
Fig. 2.12: Measured hole concentration (Pc), mobility (μM), resistivity (ρR), indium composite, and RMS surface roughness (RMSR) of p-InGaN MD-SLS samples growth from 840 to 920 oC.
Fig. 2.13: Measured hole concentration, mobility, resistivity and RMS roughness of p-InGaN MD-SLS samples growth with various Mg flow from 50 to 150 sccm at 900 oC.
Fig. 2.14: Electroluminescence intensity of LED chips with the three p-type doping schemes. At an injection current of 20 mA, the spectral width of the emission for sample A’ to C’ is 155.1 meV, 152.2 meV, and 146.7 meV, respectively.
Fig. 2.15: SIMS profiles of Mg for HT p-GaN and LT p-InGaN/GaN MD-SLS.
Fig. 2.16: The band diagram for the p-GaN LED and p-InGaN/GaN MD-SLS LED.
Fig. 2.17: I-V characteristic of LED chips with three p-type doping schemes.
Fig. 2.18: Atomic force microscopic images (10×10 μm2) of the surface morphology of sample A’ to C’ LED structures with different p type growth condition.
Fig. 3.1: Atomic force microscopic images (10×10 μm2) of the surface morphology of (a) type I micro-lens, (b) type II micro-lens, (c) type Ш micro-lens. Their diameter/spacing/depth are about 3/1.5/1.5, 3/1.5/1.6, and 3/2/1.7 µm, respectively.
Fig. 3.2: Scanning electron microscopy (SEM) images of GaN epilaers grown on three types of micro-lens PSS for growth time (1a)/(2a)/(3a) 20 min, (1b)/(2b)/(3b) 60 min, and (1c)/(2c)/(3c) 180 min.
Fig. 3.3: (a) Cross-sectional and (b) top view SEM images of a GaN layer grown on types I micro-lens PSS for growth time 20 min.
Fig. 3.4: A stick and ball diagram of a hexagonal structure. The density of dangling bond for (a) {1-101} N-polarity (b) {11-22} N-polarity is 16 nm-2 and 18 nm-2, respectively. (Lc=5.185Å, La=3.189Å)
Fig. 3.5: (a)/(b)/(c) Cross-sectional SEM images and (d)/(e)/(f) schematic growth modes of GaN grown on type I, type II, and type Ш micro-lens PSSs.
Fig. 3.6: (a)/(b)/(c) Plan view CL mapping and (d)/(e)/(f) cross-sectional TEM images of a 5-μm-thick GaN layer grown on three types PSS.
Fig. 3.7: Typical peak energy, light output power and external quantum efficiency for the InGaN LEDs grown on the three types patterned sapphire substrates.
Fig. 3.8: E2-high phonon peak of 250 μm-thick GaN substrate, 5 μm-thick GaN grown on type I, type II, and type Ш micro-lens PSSs. Higher wave-number means higher compressive strain.
Fig. 4.1: Nomarski optical images of a wet-etched sapphire sample after: (a) etching in sulfuric acid (96%) at 275 oC for 5 min, and (b) etching in a phosphoric acid (86%)-based solution at 275 oC for 2 min. (c) Facets that identified by atomic force microscopy. The insets of (a) and (b) are the SEM images of the sapphire surface, respectively.
Fig. 4.2: Atomic force microscopic images (50×50 μm2) of the surface morphology of sapphire with different etching time from 2.5 to 20 minutes.
Fig. 4.3: SEM images of GaN grown on patterned sapphire substrates right after the vertical growth stage: (a) a cross-sectional view and (b) a 45 view, and right after the lateral growth stage: (c) a cross-sectional view and (d) a 45 view.
Fig. 4.4: Typical light output power, external quantum efficiency, and forward voltage versus forward current for the InGaN LEDs grown on the patterned and plane sapphire substrates.
Fig. 4.5: Wafer mapping results on the (a) luminescence intensity, (b) forward voltage (Vf), and (c) dominant wavelength of the LEDs (~500 dies) grown on conventional sapphire and NESS substrates.
Fig. 4.6: (a)/(b) Schematics of  LED with an etched patterned dicing street.
Fig. 4.7: (a) Top view SEM image for the etched patterned dicing street LED. (b) SEM image of area A shown in (a).
Fig. 4.8: Wafer mapping of (a)/(b)/(c)/(d)/(e) forward voltage (Vf) of the sample A to E.
Fig. 4.9: Light output power of the LEDs at various etching depth. Plan view photomicrograph of the device in operation is shown in the inset.
Fig. 4.10: Luminescence intensity imaged by a charge-coupled device camera for the LED chips (a) without pattern, (b) with a partially etched pattern, and (c) with a fully etched pattern driven at  25 mA, respectively. The images were taken under a 20-dB attenuation filter.
Fig. 4.11: Typical output beam pattern for the different dicing streets patterned LED.
Fig. 4.12: Plots at drop rate of output power in the control LED and dicing street LED.
Fig. A.1: Illustration of the coupling of light from an LED light source into an optical fiber.

                                

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