Enhancement of the external quatum efficiency of GaN-based light emitting diode by various wet etching techniques
- 주제(키워드) LED , light emitting diode , wet etching
- 발행기관 고려대학교 대학원
- 지도교수 성태연
- 발행년도 2013
- 학위수여년월 2013. 2
- 학위구분 석사
- 학과 일반대학원 신소재공학과
- 원문페이지 60 p
- 실제URI http://www.dcollection.net/handler/korea/000000039113
- 본문언어 영어
- 제출원본 000045745924
초록/요약
One of the most important problems regarding the fabrication of high-efficiency GaN-based light-emitting diodes is the existence of trapped light within a high refractive-index GaN. According to the Snell’s law, the light generated at an active region cannot escape from the LEDs, when emitted outside the escape cone defined by the narrow critical angle, indicating that a large fraction of the light beyond critical angle would experience total internal reflection along the lateral direction and hence, internal loss. Several approaches have been proposed to enhance the light extraction efficiency, such as surface roughening, patterned sapphire substrate and photonic crystal structures. However, from the standpoint of device fabrication, a simple and low cost process is essential. In this work, in an attempt to enhance the extraction efficiency, we introduced sidewall texturing by using wet etching techniques. KOH and AZ400K etching techniques have several advantages, such as low cost, simplicity and low damage. A KOH solution (concentration: 3~8M, etching time: 1~3hr) and an AZ400K developer (time: 1~3hr) is used for etching in this experiment.. Wet chemical etching was carried out under the photo-electrochemical condition. The sidewalls and surface morphologies were examined using a scanning electron microscope. It is shown that the sidewall surface along the (11-20) direction is smooth and flat, but pyramidal shapes are formed along the (1-100) direction because of the GaN structural characteristics. A wet etching process is defined as 2GaN+3H2O à Ga2O3+2NH3. On the (1-100) direction, OH ions can attack Ga atoms easily. However, on the (11-20) direction, they cannot attack Ga atoms due to the negatively charged nitrogen. In addition, we use the FDTD simulation methodology to predict and optimizing the light extraction efficiency of sidewall textured LED. The effect of the sidewall texturing was analyzed by the light output power and I-V characteristics. The effects of the optimal layer thickness and optimal sidewall wet etching conditions on the LED performance is described and discussed.
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Table of Contents
1. Introduction ----------------------------------------------------------------------------------- 1
2. GaN based semiconductor for illumination ------------------------------------------------2
2.1. Introduction ------------------------------------------------------------------------- 2
2.2. Basic principle of Light emitting diodes (LEDs)
2.2.1. Basic structure of LED ----------------------------------------------------------2
2.2.2. Carrier recombination and p-n junction ---------------------------------------3
2.2.3. Efficiency of light emitting diodes (LEDs)-----------------------------------7
2.3. General structures of LEDs ---------------------------------------------------- ---9
2.3.1. Mesa-structure LEDs ---------------------------------------------------------9
2.3.2. Flip-chip LEDs (FCLEDs) -------------------------------------------------10
2.3.3. Vertical LEDs (VLEDs) ----------------------------------------------------10
2.4. Several technologies for high performance LEDs------------------------------15
2.4.1 Thin-film flip-chip LED structure (TFFLEDs)-------------------------------15
2.4.2. Photonic crystal LED (PXLEDs)----------------------------------------------16
2.2.6. Surface Plasmon (SP) enhanced LEDs---------------------------------------16
3. Optical Simulation Method
3.1. FDTD (Finite Difference Time Domain)Method ------------------------------20
3.1.1. One Dimensional FDTD in free space ----------------------------------------20
3.1.2. Two Dimensional FDTD simulation ------------------------------------------23
4. Experimental procedures ------------------------------------------------------------------ 24
4.1. Experimental procedures -------------------------------------------------------- 27
5. Result and discussion ---------------------------------------------------------------------- 29
5.1. Scanning electron microscope(SEM) image data ----------------------------- 29
5.2. Optical simulation analysis data (FDTD, Berreman matrix)----------------- 32
5.3. Forward voltage characteristics ------------------------------------------------ 42
5.4. Light output – current (L-I) characteristics ------------------------------------45
6. Summary ------------------------------------------------------------------------------------- 46
