Characteristics of InGaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates with Various Pattern Heights

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2012, Article ID 346915, 6 pages doi:10.1155/2012/346915 Research Article Characteristics of InGaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates with Various Pattern Heights Sheng-Fu Yu,1 Sheng-Po Chang,1 Shoou-Jinn Chang,1 Ray-Ming Lin,2 Hsin-Hung Wu,2 and Wen-Ching Hsu3 1 Department of Electrical Engineering, Institute of Microelectronics, Advanced Optoelectronic Technology Center, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan 2 Department of Electronic Engineering, Green Technology Research Center, Chang-Gung University, Taoyuan 333, Taiwan 3 Sino-American Silicon Products Incorporated, Hsinchu 300, Taiwan Correspondence should be addressed to Sheng-Po Chang, Received 5 March 2012; Revised 26 May 2012; Accepted 8 June 2012 Academic Editor: Sheng-Rui Jian Copyright © 2012 Sheng-Fu Yu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The optical and electrical characteristics of InGaN-based blue light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSSs) with different pattern heights and on planar sapphire by atmospheric-pressure metal-organic chemical vapor deposition were investigated. Compared with planar sapphire, it was found that the LED electroluminescence intensity is significantly enhanced on PSSs with pattern heights of 0.5 (21%), 1.1 (57%), 1.5 (81%), and 1.9 (91%) µm at an injected current of 20 mA. The increased light intensity exhibits the same trend in a TracePro simulation. In addition, it was also found that the level of leakage current depends on the density of V-shape defects, which were measured by scanning electron microscopy. 1. Introduction InGaN-based light-emitting diodes (LEDs) are useful for a wide range of visiblelight applications. They are commonly used in traffic signals, liquid crystal display backlights, microprojectors, car headlights, and fullcolor displays, among other applications. White LEDs have significant potential for becoming a popular lighting choice because of their advantages in terms of energy consumption, device lifetime, durability, and safety, along with their ecofriendliness. In general, white LEDs are fabricated using a blue LED with yellow phosphors. Because of their high external quantum efficiency (EQE), blue LEDs have attracted considerable attention. According to an analysis of the lighting market by the U.S. Department of Energy, the blue-based white LED will be increasingly popular in the coming decades. Although white LEDs have become very efficient [1–3], further improvements are still needed to enable them to replace traditional candescent and fluorescent lamps for commercial applications. EQE is affected by both internal quantum efficiency (IQE) and the light extraction rate. A high IQE value over 90% has been anticipated in recent years, but the light extraction rate is still extremely low because the refractive index of GaN (n = 2.3) is higher than air (n = 1). The critical angle is roughly 24.6◦ , which indicates that less light is extracted from the surface [4]. For this reason, several alternative approaches have been introduced to improve light extraction efficiency, including approaches that make use of p-GaN roughness [5], indium tin oxide (ITO) mesh [6], a laser liftoff process [7], and a patterned sapphire substrate (PSS) [8–11]. In particular, use of a PSS not only enhances the light extraction rate but also decreases threading dislocation defects because the growth mechanism is similar to epitaxial lateral overgrowth (ELOG) [12]. Moreover, a PSS with an uninterrupted single growth process has higher production yields. However, very rough substrates can cause epitaxial growth problems, such as generation of V-pit defects and staking fault formation. Related defects can degrade aspects of the PSS-LED’s performance, such as its electrostatic 2 Journal of Nanomaterials ×10−6 1.4 1.2 EL intensity (a.u.) 1 0.8 0.6 0.4 0.2 0 Figure 1: SEM image of patterned substrate with pattern diameter of roughly 3 µm and pattern distance of roughly 2 µm. discharge (ESD) capabilities, device lifetime, leakage current, and quantum efficiency of radiative recombination. Defects that are related to the use of different pattern heights in the sapphire substrate, such as degraded electrical performance, have rarely been examined. In this study, we grew the standard LED structure on patterned sapphire substrates with varying pattern heights, and on a planar sapphire substrate. The light extraction efficiency was evaluated for the different pattern heights and compared to results obtained using TracePro optical simulation software. In addition, we also evaluated the leakage current and used SEM measurements to characterize its relationship to V-pit defects. 2. Experiment The patterned height sapphire substrate was etched using inductively coupled plasma (ICP) reactive ion etching. After dry etching, the pattern diameter and pattern distance were roughly 3 µm and 2 µm, respectively. The etching times were varied to generate pattern heights of 0 (planar sapphire), 0.5, 1.1, 1.5, and 1.9 µm, which were then verified by confocal microscopy. A scanning electron microscopy (SEM) image of one of the cone-shaped patterns is shown in Figure 1. After fabrication, blue InGaN/GaN LEDs were grown on the substrates using atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) with an SR2000 system. Trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn), and ammonia (NH3 ) were used as precursors. Silane (SiH4 ) and bis(cyclopentadienyl)magnesium (Cp2 Mg) were used as the n-dopant and p-dopant sources. The LED structure consisted of a 25 nm thick GaN nucleation layer, a 2.5 µm thick undoped GaN layer, a 2 µm thick highly doped n-type GaN layer, five pairs of InGaN (2.5 nm)/GaN (12.5 nm) MQWs, a 25-nm-thick p-type AlGaN electron blocking layer, and a 100 nm thick Mg-doped GaN layer. The u-GaN growth temperature was increased to 1180◦ C to achieve better coalescence on the patterned sapphire substrate (relative to using the typical temperature of roughly 0 20 40 60 Current (mA) Planar sapphire Pattern height: 0.5 µm Pattern height: 1.1 µm 80 100 Pattern height: 1.5 µm Pattern height: 1.9 µm Figure 2: EL intensity for different pattern heights as a function of injected current. 1130◦ C). After the LEDs were grown on the substrates, the samples were then treated in a quartz furnace under an N2 atmosphere to activate the Mg-doped GaN. The annealing temperature and elapsed time were 700◦ C and 30 minutes, respectively. LEDs with 300 × 300 µm2 sizes were formed by conventional photolithography followed by chlorine-based inductively coupled plasma etching. Both p- and n-contacts were located (...truncated)