발광파장 590~630nm 영역 AlGaInP LED의 효율향상 연구 = Efficiency enhancement of LED having the wavelength from 590 to 630nm using AlGaInP

다국어 초록 (Multilingual Abstract)

III-V compound semiconductors are known as an attractive material for optical devices such as solar cells, laser diodes (LDs), and light-emitting diodes (LEDs). In particular, light-emitting diodes based on AlGaInP materials, with a wavelength ranging from 570 nm to 630 nm, have been developed for commercial applications such as illumination, traffic lights, vehicle lighting and indicators. Ever higher efficiency AlGaInP LEDs have been required for high power devices with a low operating voltage. The efficiency of AlGaInP LEDs has been significantly improved by either increasing the internal quantum efficiency by inserting multiple hetero-structures and multiple quantum barriers, or by increasing the light extraction efficiency by using a distributed Bragg reflector (DBR), surface texturing, bottom window, photonic crystal (nano-imprint).
Although numerous studies have been reported for AlGaInP LEDs with a high efficiency, certain intrinsic limitations still exist that considerably decrease their overall efficiency.
In this study, high efficiency AlGaInP LED (590~630nm) was fabricated on GaAs substrate using metal organic chemical vapor deposition (MOCVD) system. The structure of fabricated AlGaInP LED consisting of window layer / p-confinement / active region (MQW) / n-confinement / substrate is similar to that of an ordinary AlGaInP LED. For improving the efficiency of the AlGaInP LED, we investigated the improvement of extraction efficiency and internal quantum efficiency.
Photoluminescence (PL), X-ray diffractometer (XRD), Scanning electron microscopy (SEM), Atomic force microscopy (AFM), Electroluminescence (EL), Electrochemical capacitance-voltage (ECV) profiler, Hall effect measurement system, Light emitting diode electric-optical system (LEOS) were used to evaluate the surface morphological, structural, crystallographical, optical, electrical properties of AlGaInP LED.
Firstly, For improving the efficiency of the AlGaInP LED, we investigated the efficiency improvement of 590 nm AlGaInP light emitting diode with a reflective top electrode in chapter 3. The output power of the LEDs with a RTE depends strongly on the reflectivity of the DBR used as its reflector. At an injection current of 30 mA, the highest output power of 1mW was obtained from a 590 nm AlGaInP LED having the 12 pairs DBR with ~85% R, which showed a relative increase of around 18% over a conventional LED.
And, we investigated an air gap-induced hybrid DBR for 630nm AlGaInP LED in chapter 4. Typically, to improve the light extraction efficiency of AlGaInP-based LEDs, a distributed Bragg reflector (DBR) is inserted between the substrate and the n-cladding layer. By selectively etching the AlxGa1-xAs DBRs, the air gap could be inserted into the sides of DBR structures. With the AH-DBR structures, the output power of LEDs was enhanced by 15% in comparison to LEDs having conventional DBRs due to the effective reflection of obliquely incident light by the air gap structures.
In addition, we investigated the effects of the post-diffusion of Zn on the surface of the GaP window for the reduction of the amount of surface defects in chapter 5. The characteristics of 630 nm AlGaInP LED with GaP window layers treated with a post Zn-diffused and in situ Mg-doped layer were investigated. The optical characteristics of the AlGaInP LED chip with Zn diffusion layer showed the highest output power of 11 mW as compared with either the conventional AlGaInP LED chips or the AlGaInP LED chip with Mg-doped layer at an injection current of 40 mA. Therefore, the efficiency of 630 nm AlGaInP LED can be improved if the GaP window layer is treated with the post Zn-diffusion process.
In summary, the light output power of AlGaInP LED chip with RTE and AH-DBR shows relative increase of 18% and 15% respectively, as compared to conventional LED chip. The light output power of 630nm AlGaInP LED chip with Zn diffusion layer is about 1.47 times higher than that of conventional LED chip under an injection current of 40mA.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Overview of AlGaInP material 1
• 1.2. AlGaInP LEDs: researches, developments, and
• applications 4
• Reference (for Chapter 1) 11
• Chapter 2. Experimental equipment and techniques 14
• 2.1. MOCVD system 14
• 2.2. Analytical Equipment 28
• 2.2.1. High resolution X-ray diffraction (HRXRD) 28
• 2.2.2. Photoluminescence (PL) spectroscopy 31
• 2.2.3. Scanning electron microscopy (SEM) 37
• 2.2.4. Atomic force microscopy (AFM) 42
• References (for Chapter 2) 47
• Chapter 3. Efficiency Improvement of 590 nm AlGaInP Light Emitting Diode with a Reflective Top Electrode 48
• 3.1 Introduction 48
• 3.2 Experimental Procedure 49
• 3.3 Results and Discussion 52
• 3.4 Summary 58
• References (for Chapter 3) 59
• Chapter 4. Air-Hybrid Distributed Bragg Reflector Structure for Improving Light Output Power in AlGaInP-Based LEDs 62
• 4.1. Introduction 62
• 4.2. Experimental Procedure 63
• 4.3. Results and Discussion 64
• 4.4. Summary 75
• References (for Chapter 4) 76
• Chapter 5. Reduction of surface defects on the GaP window layer of 630 nm AlGaInP LED using post Zn -diffusion process 77
• 5.1. Introduction 77
• 5.2. Experimental Procedure 78
• 5.3. Results and Discussion 81
• 5.4. Summary 91
• References (for Chapter 5) 92
• Chapter 6. Conclusions 95
• Curriculum vitae 97
• Abstract (Korean) 100