나노스피어 리소그라피를 이용한 수직형 발광 다이오드의 광추출효율 향상에 관한 연구 = The study on enhancement of light extraction efficieney in vertical light emitting diode using nanosphere lithography

다국어 초록 (Multilingual Abstract)

GaN-based Light emitting diodes (LEDs) have attracted considerable interest in variety of applications such as traffic light, full color display and general lighting. However, the luminous efficiency of GaN-based white LEDs is still lower than that of fluorescent light lamps. The light extraction efficiency of most conventional LEDs is limited by the total internal reflection (TIR) and Fresnel reflection those occur at the semiconductor/air interface. Due to the large difference in the refractive index between the semiconductor and air, only a small fraction of light can escape to the surrounding air. Therefore, there is still a great need for improvement of the light extraction efficiency, as well as the internal quantum efficiency.
In order to avoid total internal reflection, various attempts, including surface texturing method, device fabrication on pre-patterned substrate, and insertion of two-dimensional 2D photonic crystals, have been studied. These structures were fabricated by laser holographic lithography, e-beam lithography, etc. However, each lithographic technique has its own drawback to apply in large area economically. Therefore, the nanosphere lithography (NSL) was proposed as a simple technique for the fabrication of nano-scale patterns. This technique is an easy and economic process using a self-assembled single layer of spheres.
In this study, we report on the structural, optical, and electrical properties of n-GaN patterned Vertical LEDs by using a NSL technique. To enhance the light extraction efficiency, a NSL technique was applied to pattern the n-GaN surface of Vertical LED devices. Single layer of hexagonal close packed (HCP) polystyrene (PS) spheres of uniform size (diameter 500 nm, 3 mm) was coated onto the n-GaN layer by a simple spin-coating method. To reduce the size of the spheres, a HCP array of PS spheres was then etched using reactive ion etching (RIE). The PS sphere arrays act as an etching mask to produce an ordered pillar-shape nano-patterns onto the n-GaN layer. For making hole-shape patterns onto the n-GaN layer, nickel metal deposited on the PS shpere forms the etching mask after eliminating the PS sphere. Structural properties of pillar and hole patterned n-GaN surfaces were investigated by Scanning Electron Microscope (SEM).
We also investigated the enhancement of light extraction in n-GaN patterned Vertical LEDs. The electroluminescence intensity and light output power of the Vertical LED with pillar and hole patterned n-GaN layer shows much higher than those of the conventional Vertical LED, at an injection current of 350 mA. It was also found that the optical power of the Vertical LED with pillar and hole patterned n-GaN layer was enhanced by 3.4~4.9 times, compared with that of the conventional Vertical LED, due to a reduced total internal reflection at the n-GaN/air interface and increased light scattering at the patterned surface.

목차 (Table of Contents)

• 영 문 요 약
• .................................................................................................
• i
• 그 림 목 차
• .................................................................................................
• ⅵ
• 표 목 차
• .................................................................................................
• x
• 제 1 장 서 론
• 1
• 제 2 장 이론적 고찰
• 5
• 2.1 LED (Light Emitting Diode) 개요 .......................................................
• 5
• 2.2 GaN LED의 광학적 특성 .......................................................................
• 9
• 2.2.1 광추출 효율 (Light Extraction Efficiency) ...................................
• 9
• 2.2.2 탈출원뿔 (Light escape cone) ......................................................
• 11
• 2.2.3 Lambertian Emission Pattern .......................................................
• 14
• 2.3 LED 광추출효율 개선 방안 ...................................................................
• 17
• 2.3.1 Surface texturing ..........................................................................
• 17
• 2.3.2 2차원 광결정 (2 Dimensional Photonic Crystal) .........................
• 18
• 2.4 나노스피어 리소그라피 (Nanosphere lithography) .............................
• 21
• 2.4.1 Nanosphere lithography의 개요 ...................................................
• 21
• 2.4.2 Self-assembled Polystyrene nanosphere lithography array .....
• 22
• 2.4.3 Polystyrene nanosphere lithography 방법 ..................................
• 23
• 제 3 장 연구 내용 및 결과
• 25
• 3.1 Polystyrene sphere를 이용한 Nanosphere lithography ....................
• 25
• 3.1.1 실험개요 ..........................................................................................
• 25
• 3.1.2 실험방법 ..........................................................................................
• 26
• 3.1.3 실험결과 ..........................................................................................
• 27
• 3.2 Nanosphere lithography를 적용한 n-GaN 표면 패터닝 연구 ............
• 35
• 3.2.1 실험개요 ..........................................................................................
• 35
• 3.2.2 실험방법 ..........................................................................................
• 35
• 3.2.3 실험결과 ..........................................................................................
• 38
• 3.3 Nanosphere lithography가 적용된 n-GaN 표면 패터닝을 통한
• Vertical LED의 광추출효율 향상 연구 ..................................................
• 43
• 3.3.1 실험개요 ..........................................................................................
• 43
• 3.3.2 실험방법 ..........................................................................................
• 43
• 3.3.3 실험결과 ..........................................................................................
• 45
• 제 4 장 결 론
• 56
• 참고 문헌 .......................................................................................................
• 58
• 감사의 글 .......................................................................................................
• 60