MOCVD 선택적 영역 에피택시를 이용한 GaN 기반 나노와이어의 성장 제어 및 광학적 특성 = Growth Control and Optical Properties of GaN Based Nanowires Grown by Selective Area Epitaxy Using MOCVD

다국어 초록 (Multilingual Abstract)

Recently, AR/VR technologies have emerged as next-generation displays. Consequently, there is a rapidly increasing demand for Micro-LEDs as light sources for ultra-high-resolution display platforms. Micro-LEDs possess high brightness, high resolution, fast response speeds, and excellent stability. Among various materials, GaN, a III-nitride semiconductor, is garnering significant attention as a light source material for Micro-LEDs. GaN-based materials exhibit superior physical and chemical stability along with outstanding optoelectronic properties. Furthermore, their bandgap can be engineered from the ultraviolet to the infrared spectrum by adjusting Al and In compositions, enabling the realization of various wavelengths. However, challenges such as process limitations in individual pixel transfer for full-color implementation and the "Green Gap" phenomenon in InGaN-based LEDs remain to be addressed. In this study, we aim to overcome the disadvantages of conventional GaN-based LEDs by maximizing the structural advantages of nanowires through the Selective Area Growth (SAG) method via Metal-Organic Chemical Vapor Deposition (MOCVD) and implementing multi-color emission within a single substrate and a single nanowire.
To analyze the physical and chemical properties of the GaN nanowire structures fabricated in this study, the growth behavior and verticality of the nanowires were analyzed using field-emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM). The elemental distribution and surface chemical states at the GaN nanowire and seed stages were investigated through energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses. Furthermore, the crystal structure was confirmed via X-ray diffraticon (XRD), and finally, the optical properties of localized regions of the GaN nanowires and nanowire arrays were precisely measured using a Micro-PL system.
In the first part of this research, a photolithography-based Selective Area Growth (SAG) technique was introduced. Among GaN nanowire growth methods, the SAG approach can secure higher verticality and higher-quality uniform nanowires compared to the Vapor-Liquid-Solid (VLS) growth method, facilitating the formation of Quantum Wells. By utilizing this, patterns of different diameters were formed on a single substrate and grown simultaneously, inducing differences in Indium (In) diffusion lengths according to the nanowire diameter. As a result, horizontal multi-color implementation from 460 nm (Blue) to 518 nm (Green) was successfully achieved within a single substrate without additional process changes.
In the second part of this research, a new SAG pattern formation technology using metal droplets was developed to overcome the low efficiency of conventional lithography processes. Through the Au droplet-based patterning process, process time and costs were drastically reduced, enabling the growth of high-verticality long nanowires of approximately 20 μm. As a result of precisely analyzing the indium composition gradient according to the height of the long nanowires, vertical multi-color emission was successfully implemented, emitting different colors from 540 nm (Green) to 460 nm (Blue) from the top to the bottom within a single nanowire.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1. 1. Micro-Light Emitting Diode (Micro-LED) 1
• 1. 2. Optical and Structural Characteristics of III-Nitride Semiconductors 4
• 1. 3. Compairison of Nanowire structure 7
• 1. 3. 1. VLS Growth of GaN Nanowire 10
• 1. 3. 2. Selective Area Growth of GaN Nanowire 13
• 1. 4. Multi-Color of GaN Nanowire LED 18
• 1. 4. 1. Horizontal Multi-Color Emission via Photolithography SAG Patterning 19
• 1. 4. 2. Vertical Multi-color Emission via Metal Droplet Assisted SAG Patterning 21
• 1. 5. Experimental equipment 23
• 1. 5. 1. MOCVD (Metal-organic chemical vapor deposition) 23
• 1. 5. 2. Micro-PL (Photoluminescence) 27
• 1. 5. 3. FE-SEM (Field Emison Scanning Electron Microscope) 30
• Chapter 2. Result & Discussion 33
• 2. 1. Horizontal Multi-color Nanowire LEDs Fabricated on Photolithography SAG Patterns 33
• 2. 1. 1. Fabrication of Mask Patterns using Photolithography Processes 33
• 2. 1. 2. Analysis of n-GaN Seed and Nanowire Growth Behavior 36
• 2. 1. 3. Growth of InGaN/GaN MQW Active Layers and Diameter-Dependent Optical Properties 44
• 2. 2. Vertical Multi-color Nanowire LEDs Fabricated on Metal-Droplet Assisted SAG Patterns 49
• 2. 2. 1. Fabrication Method and Analysis of SAG Patterned Substrates using Metal Droplets 49
• 2. 2. 2. Selective Growth Characteristics of GaN Seeds Based on Pattern Size 59
• 2. 2. 3. Structural Analysis of High Aspect Ratio n-GaN Nanowires 63
• 2. 2. 4. Growth of InGaN/GaN MQW Active Layers and Length Dependent Optical Properties 67
• Chapter 3. Conclusion 78
• References 80
• Abstract (Korean) 87