Wavelength Stability Enhancement in GaN- based Vertical Nanorod Light Emitting Diode Arrays Minjeong Kim

다국어 초록 (Multilingual Abstract)

Wavelength Stability Enhancement in GaN-Based Vertical Nanorod Light Emitting Diode Arrays Minjeong Kim Dept. of Photonics and Nanoelectronics The Graduate School Hanyang University This study presents the fabrication of a vertical nanorod LED array that minimizes the effects of lattice mismatch between InGaN and GaN, leading to less variation in emission wavelength compared to conventional planar LEDs. Both the vertical array and planar LEDs were fabricated to the same size of 200 x 200 μm² (Figure 1). The device fabrication status was confirmed through scanning electron microscopy, and photoluminescence measurements were conducted to detect any defects in the epitaxial layer. Considering the 600 nm diameter of each nanorod LED, precise measurements were carried out at 50 nm intervals using a confocal microscope, revealing no significant defects in the epitaxy. Subsequent analysis of the emission wavelengths under varying light energy and current intensity indicated a wavelength shift of 0.5 nm for the vertical nanorod array and 2.3 nm for the planar LED in photoluminescence. This suggests that the nanorod structure alleviates the stress caused by lattice mismatch, thereby improving the quantum-confined Stark effect. In electroluminescence, a minimal wavelength shift of 0.5 nm for the vertical array and 0.8 nm for the planar LED was observed. Overall, the vertical nanorod LED array showed less wavelength variation in response to injected energy, enhancing the color purity of the fabricated LEDs. Figure 1. Schematic image of (a) the vertical nanorod LED array and (b) conventional planar LED

목차 (Table of Contents)

• Table of Content . i
• List of Figure . iii
• List of table . vi
• Abstract vii
• Chapter1. Introduction 1
• 1.1 Evolution of Contemporary Display Technologies 1
• 1.2 Challenges in GaN-Based LEDs 4
• Chapter 2. Background 6
• 2-1. Quantum-confined stark effect . 6
• 2-2. Band filling effect and carrier screening . 7
• Chapter 3. Experiment Section 10
• 3-1. Substrate Preparation 10
• 3-2. Fabrication of the Vertical Nanorod Array and Conventional Planar
• LED 11
• 3-3. Device characterizations 15
• Chapter 4. Result and Discussion . 16
• 4-1. Fabricated two types of LEDs. 16
• 4-2. Comparison of the Vertical LED Arrays and Planar LED 20
• 4.2.1 Electrical Characteristics . 20
• 4.2.2 Photoluminescence 22
• 4.2.3 Electroluminescence 26
• Chapter 5. Conclusion . 30
• Reference 31
• 국문요지 34
• Declaration . 35
• 연구 윤리 서약서 . 36
• List of Figure
• Figure 1. Schematic image of (a) the vertical nanorod LED array and (b)
• conventional planar LED ix
• Figure 2. Schematic representation of the Quantum-confined Stark effect;(a)
• uniform bandgap alignment in the absence of an internal electric field
• and (b) bent bandgap under the influence of an internal electric field. 9
• Figure 3. Schematic image of (a) screening effect and (b) band filling effect
• [1] 9
• Figure 4. Fabrication process for the vertical nanorod LED array and
• conventional planar LED: (a) substrate with nanoimprinted resin pattern
• and deposited Cr layer, (b) Cr and SiO2 etching to define the nanorod
• pattern, (c) RIE process to etch down to the ITO layer revealing the
• nanorod structure, (d) deposition of SU-8 photoresist to define the
• geometry of the LED array, (e) further etching to form the mesa
• structure and expose the n-GaN layer, and (f) final deposition of Ti/Au
• to establish the contact electrodes 13
• Figure 5. Scanning electron microscope image of each LED after the process;
• (a) Vertical nanorod LED array, (b) Conventional planar LED 14
• Figure 6. Illustrates the electroluminescence emission from a 200 x 200 μm²
• area of the vertical nanorod LED arrays under various injection current
• levels. The images show the progression of light emission intensity as
• the current increases from 0.4 µA to 8 µA. 18
• Figure 7. SEM Images of LED Structures; (a) Nanorod LED array with
• broken ITO layer intended for connecting the p-GaN, (b) A planar LED
• featuring multiple layers within the active region. 19
• Figure 8. Light output power-Current-Voltage curve; (a) vertical nanorod
• LED array and (b) conventional planar LED . 21
• Figure 9. Photoluminescence Mapping Images over 10x10 µm² area using a
• Confocal Imaging System at 100 µW laser power.; (a) PL intensity of
• the nanorod LED array, (b) PL wavelength of the nanorod LED array,
• (c) PL intensity of the planar LED, (d) PL wavelength of the planar LED.
• 24
• Figure 10. PL spectrum of (a) nanorod LED array and (b) planar LED 25
• Figure 11. Electroluminescence (EL) mapping obtained for a 200 x 200 μm²
• area with a 1 μm² resolution at 2.54 V; (a) EL intensity of the nanorod
• LED array, (b) the EL wavelength of the nanorod LED array, (c) EL
• intensity of the planar LED, (d) EL wavelength of the planar LED. 28
• Figure 12. EL spectrum of (a) nanorod LED array and (b) planar LED 29
• List of table
• Table 1. Comparison between microLED, OLED, and LCD.[6] . 3