α-Al2O3 나노구조물을 이용한 GaN 층 이종성장에 대한 연구

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 8. 윤의준.GaN 성장은 경제적, 기술적인 이유로 동종 기판을 사용한 에피성장이 불가능하기 때문에 주로 이종기판을 사용하여 성장이 진행된다. 이종 기판으로 가장 많이 사용되는 기판은 사파이어 기판이지만 격자상수 차이와 열팽창계수 차이로 인해여 많은 문제점들이 발생하게 된다. 가장 큰 문제점은 격자상수 차이로 인하여 소자에 효율과 신뢰성에 악영향을 미치는 수많은 결정 결함이 GaN 내부에 발생하게 되고 또한 열팽창계수 차이로 인해 성장 후 냉각 과정에서 GaN 층에 압축응력이 작용하게 되며 이에 wafer의 휨 현상이 발생한다. Wafer의 휨 현상은 사파이어 기판의 대면적화를 어렵게 하며, 휨을 줄이기 위해 두꺼운 기판을 사용하게 되면 원가비용 상승의 문제점이 발생하게 된다. 이러한 문제점을 해결하고자 본 연구에서는 cavity engineered sapphire substrate (CES)를 제작하였고 그 위에 유기금속화학증착장비를 사용하여 GaN을 성장시키고 분석을 진행하였다. 일반적인photolithography (PR) 기법과 원자층증착장비을 활용하여 CES 구조를 제작하였으며, 이때 PR은 희생층 역할과 CES 구조의 기본 형태를 잡아주게 된다. 비정질 알루미나는 PR 패턴 위에 증착하며 이후 중공구조물의 껍질을 형성하게 되고 이후 열처리를 통해 사파이어 기판과 같은 결정 구조를 갖는 알파상으로 결정화가 일어나게 된다. 이렇게 제작된 CES 위에 GaN 성장을 진행하였고 레이저 스캔 기법으로 기판의 휨을 측정하여 GaN 내부의 응력을 계산하였다. 그 결과 CES 구조를 도입하여 GaN 을 성장시켰을 때 기존 방법 대비 약 35%의 응력을 감소시키는 결과를 얻을 수 있었다. 본 기술을 응용하여 compliant 기판을 제작하고 결함 감소의 연구도 진행하였다. Compliant 기판이란 박막보다 얇은 기판을 이용하여 박막의 결정 결함을 줄이기 위해 사용되며, 본 연구에서는 원자층증착장비의 증착 횟수를 조절하여 26 nm 두께의 사파이어 멤브레인을 제작하였다. 그 위에 GaN 를 성장하고 결정 결함 분석을 진행하였다. 고분해능 투과전자현미경 분석을 통해 계면에서의 misfit dislocation을 관찰하였으며, 그 결과 GaN과 사파이어 멤브레인 계면에서의 misfit dislocation의 밀도가 사파이어 기판을 사용하였을 경우보다 28 % 감소하였고, 또한 GaN 표면으로 올라오는 threading dislocation의 밀도 또한 25 % 감소하였다. 또한 매우 얇은 멤브레인을 기판으로 사용함으로써 GaN 내부응력이 매우 낮아진 것을 Raman 과 PL 분석을 통해 확인할 수 있었다. 추가적으로 사파이어 멤브레인의 스트라이프 패턴을 이용하여 c면 InGaN/GaN MQW에서의 편광 발광을 구현하였다. 빛의 편광성은 LCD의 백라이트나 통신 등에 응용될 수 있다. 본 연구에서는 스트라이프 패턴을 이용함으로써 GaN 평면 내 이방성의 응력이 발생되며, 이는 가전자대의 분리를 야기시킨다. 이로 인해 그 위에 성장된 InGaN/GaN MQW 활성층에서 편광 발광이 관측되게 되며 편광률은 0.75 로 측정되었다.In this work, we focus on the growth of GaN and related III-nitride alloys using cavity engineered sapphire substrate (CES). Large mismatches in lattice constant and thermal expansion coefficient between GaN layer and sapphire substrate causes severe problems in the fabrication of high efficiency optoelectronic devices such as light emitting diodes (LEDs) or laser diodes (LDs). Wafer bow is one of issues in LED industry which hampers the mass production of LEDs using large-size sapphire wafers. Moreover, it also causes the crack in GaN layer during laser lift-off process for the fabrication of vertical LEDs. Wafer bow occurs after high temperature deposition, since the thermal expansion coefficient of the sapphire substrate is much larger than that of GaN so that biaxial compressive stress is generated in GaN layer. To overcome this problem, we intentionally inserted the cavities within the GaN layer to relax the biaxial stress. Cavity structure was fabricated using a conventional photolithography process and atomic layer deposition (ALD). Photoresist (PR) pattern acts as the basic frame of cavity structure as well as a sacrificial layer. An amorphous Al2O3 layer deposited on PR pattern by ALD acts as the shell of cavity. The amorphous Al2O3 layer was crystallized to α-phase Al2O3 which is the same crystal structure with sapphire substrate, allowing epitaxial growth of GaN. Then, GaN layer was successfully grown on CES. The reduction of compressive stress of GaN layer was examined by laser scanning technique and the stress decreases about 35% compared to the GaN layer on conventional sapphire substrate. CES fabrication method can be also applied to realize the compliant substrate. By controlling the number of ALD cycles, thickness of Al2O3 layer can be easily adjusted. We fabricated an ultra-thin (26 nm) Al2O3 membrane used as a compliant substrate for the growth of high quality GaN. The density of misfit dislocations per unit length at the interface between the GaN layer and the sapphire membrane was reduced by 28 % compared to GaN on the conventional sapphire substrate. Threading dislocation density in GaN on the sapphire membrane was measured to be 2.4 x 10^8/cm^2, which is lower than that for GaN on the conventional sapphire substrate (3.2 x 10^8/cm^2). XRD and micro-Raman results verifed that the residual stress in GaN on the sapphire membrane was as low as 0.02 GPa due to stress absorption by the ultra-thin compliant sapphire membrane. Finally, we utilized the stripe pattern of Al2O3 membrane to realize linearly polarized emission from MQWs grown on c-plane GaN template. The in-plane stress anisotropy in GaN layer by using FSM stress measurement tool. This anisotropic stress in GaN layer results in the in-plane polarization anisotropy in photoluminescence emission for MQWs on the stripe sapphire membrane. The result of in-plane polarization anisotropy can be explained in terms of the valence band structure modification. The degree of polarization is determined and is as high as 0.75.Chapter 1. Introduction 1 1.1 History of III-nitride materials 1 1.2 General properties of III-nitride materials 2 1.3 Compliant substrates for Heteroepitaxial Growth 6 1.4 Epitaxial lateral overgrowth (ELO) 12 1.5 Formation of misfit dislocation in compliant substrate 17 1.6 References 24 Chapter 2. Growth and analysis tools 26 2.1 Growth equipments 26 2.1.1 Atomic layer deposition (ALD) 26 2.1.2 Metalorganic chemical vapor deposition (MOCVD) 27 2.2 Analysis tools 30 2.2.1 Field emission scanning electron microscopy (FE-SEM) 30 2.2.2 Atomic forme microscopy (AFM) 30 2.2.3 Transmission electron microscopy (TEM) 30 2.2.4 Photoluminescence (PL) 31 2.2.5 X-ray diffraction (XRD) 31 2.2.6 Cathodoluminescence (CL) 32 2.2.7 Raman spectrascopy 32 2.2.8 Stress Measurement System 33 2.3 References 35 Chapter 3. Fabrication of cavity engineered sapphire substrate and its effect on stress reduction in GaN layer 36 3.1 Introduction 36 3.2 Experimental procedure 38 3.3 Results and discussion 42 3.3.1 Formation of photoresist Pattern 42 3.3.2 Amorphous Al2O3 deposition and thermal treatment 46 3.3.3 Growth of GaN on CES 54 3.3.4 Stress reduction in GaN layer on various CES 62 3.4 Summary 75 3.5 References 76 Chapter 4. Ultra-thin compliant sapphire membrane for the growth of GaN 79 4.1 Introduction 79 4.2 Experimental procedure 84 4.3 Results and discussion 87 4.3.1 Fabrication of sapphire membrane 87 4.3.2 Growth evolution of GaN 90 4.3.3 Structural properties of GaN and sapphire membrane 92 4.3.4 Analysis of dislocations 95 4.3.5 Reduction of strain in GaN 101 4.4 Summary 107 4.5 References 108 Chapter 5. Linearly polarized emission in c-plane InGaN/GaN LED 110 5.1 Introduction 110 5.2 Experimental procedure 113 5.3 Results and discussion 117 5.3.1 Peeling off GaN layer 117 5.3.2 Coalesced GaN layer and InGaN/GaN MQW on stripe pattern 120 5.3.3 Charateristic of InGaN/GaN MQW structure 124 5.3.4 Stress analysis 130 5.3.5 Linearly polarized emission of InGaN/GaN MQW structure 136 5.4 Summary 141 5.5 References 142 Chapter 6. Conclusion 145 국문 초록 147 Publication list 150Docto

코엔자임B12생산 개선을 위한 auxotrophic selection 시스템 개발

MasterCoenzyme B12 is an important cofactor involved in various metabolism and widely used in food, additives, and pharmaceuticals. Demand for coenzyme B12 steadily grows, but its chemical synthesis is highly complicated because of its complex structure. Microbial production of coenzyme B12 has recently attracted attention as an alternative. Especially using well-characterized microorganisms such as Escherichia coli as a host could be an efficient strategy for further engineering. In this study, we designed a novel strategy to improve the production of coenzyme B12 in previously engineered coenzyme B12 producing E. coli. First, the synthetic coenzyme B12 auxotrophic system was developed by using coenzyme B12–dependent methionine synthases with deletion of coenzyme B12-independent methionine synthase in E. coli. The synthetic auxotroph system showed coenzyme B12-dependent auxotrophic growth. Then it was applied to coenzyme B12 producing strain, resulting in improvement of the specific production of coenzyme B12 by modulating plasmid copy numbers. Although the auxotroph system has severely reduced cell biomass due to insufficient methionine synthesis, additional engineering of the auxotroph system by the varied expression level of metH could significantly improve cell biomass and coenzyme B12 production. Consequently, we demonstrated that our novel strategy can be effectively used to improve coenzyme B12 production, and it could be further applied to other coenzyme B12 producing strains

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Engineering E. coli to produce n-butanol from galactose

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