Optoelectronic Properties of GaN-Based Light-Emitting Diodes with Different Mesa Structures

Xueyun Song, Xianghua Zeng, Junbin Zhang, Yuzhe Jin, Xiangdong Meng

College of Physics & Technology, Yangzhou University, Yangzhou, China.

DOI: 10.4236/msa.2012.312122                

Abstract

GaN/InGaN lighting-emitting diodes with different mesa structures are studied. The current-voltage characteristics, light output power, luminous efficiency, and peak wavelength of the GaN/InGaN lighting-emitting diodes with different mesa patterns are compared. It shows that the current-voltage characteristics of the chips with more mesa areas are im- proved greatly by reducing the current crowding. With higher injection current the light output powers of GaN-based LED with more mesa areas are enhanced. And the chips with more P-electrode area have a smaller red shift and a little bit blue shift due to the reduction of the current crowding.

Keywords

GaN-Based Leds; Mesa Structure; Optoelectronic Properties

1. Introduction

With small volume, light weight and long lifetime advantages, GaN-based LED has a wide range of applications, such as automobiles, backlighting, traffic lights, etc. It can be fabricated to white LED by many approaches, and expected to become the new fourth-generation solidstate lighting [1-5]. But most GaN-based LEDs have an insulating sapphire substrate, so their P and N electrodes are on one side, which inevitably produce current crowding effect [6-9] and result in an uneven distribution of current，serious heating effect, lower light extraction efficiency and output power, and severe red shifted peak of emission wavelength [10-13].

There are many methods to improve the output efficiency [14-17], but fabricating GaN-based LEDs with less current crowding has been an objective of researchers engaged in the filed [6,13]. And it is a good method to reduce the current crowding by optimizing the chip’s electrode which will make the current distribute uniformly. In Ref [11], they reported on the efficiency of GaN-based LED with different mesa patterns, but they only designed the finger interdigitated patterns. This paper reports on the efficiency of GaN-Based LEDs with different mesa structures, here the mesa patterns have a better symmetry and some of them are more complicated (as shown in Figure 1), therefore more results are presented. By comparing the photoelectric parameters of these samples, such as current-voltage (V-I) characteristics, output power, luminous efficiency, peak and dominant wavelength, we obtain that the mesa area or the Pelectrode area is directly related to the performance of the chips. On the one hand, the chips with more mesa area have a better V-I characteristics, on the other hand, as the mesa area will prevent the light emission, the lightemitting efficiency will decrease a little bit smaller, the output power and luminous efficiency are either. Therefore, it needs to compromise between optical and electrical properties in order to obtain more favorable optoelectronic performance of LEDs.

(a)(b)

Figure 1. Micrograph of InGaN/GaN LEDs’ with different electrodes. (a) #1; (b) #2; (c) #3; (d) #4; (e) #5; (f) #6.

2. Heterostructure and Chip Design

GaN Based LEDs, which are grown by metal organic chemical vapor (MOCVD) on c-plane (0001)-oriented 2 inch sapphire substrate, consist of a thick undoped GaN buffer layer, an highly conductive n-GaN lower cladding layer, an InGaN/GaN multiple quantum well (MQW) active region, a p-GaN upper cladding and highly doped contact layer. Top-emitting LEDs with size of 356 μm × 356 μm were fabricated using standard photolithography and Cl₂/SiCl₄/Ar inductively coupled plasma etching for current isolation purposes. Firstly, the p-GaN and active region were partially etching by an ICP etcher to expose an n-GaN lower cladding layer for electrode formation. Subsequently, a 350 nm thick ITO was deposited on the p-GaN top layer as transparent conductive layer (TCL). And Cr-Au metals were deposited using the electronbeam evaporation system for the P and N electrodes. Finally, SiO₂ was deposited as a passive layer to protect chips by plasma enhanced chemical vapor deposition (PECVD).

The masks make up of the basic unit, and each unit contains six different electrode shapes, each shape with 13 dies. The epitaxial wafer within basic unit size should have a good consistency, so comparing performance of chips with different electrodes are more reasonable within the basic unit. For simplicity, the LEDs corresponding to the six mesa patterns are called the samples #1 - #6 in sequence, their corresponding parameters, such as Mesa area, P-electrode area, Light-emitting area, and P-N electrodes separation for six samples are presented in Table 1, where the P-N electrodes separation is a typical length between P and N electrode as the labeled number one in Figure 1. The light output power of packaged LEDs was measured in an integrating sphere at constant temperature of T = 298 K in a dc current mode.

3. Results

Figure 2 shows the measured voltage-current (V-I) char-

Conflicts of Interest

The authors declare no conflicts of interest.

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