IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 7, APRIL 1, 2009 477 Improvement on Optical Properties of GaN Light-Emitting Diode With Mesh-Textured Sapphire Back Delineated by Laser Scriber Ko-Tao Lee, Yeeu-Chang Lee, Jenq-Yang Chang, and Jeng Gong Abstract—The optical properties of gallium nitride light-emitting diode with mesh-textured sapphire back delineated by laser scriber and subsequently coated with silver–copper layers were investigated. This new structure improves the optical characteristics. The electroluminescence and luminous intensities are 30% and 20% stronger than that without mesh-textured trench and silver–copper at 20 mA, respectively. The maximum luminous intensity has 1.6 times enhancement, which is mainly from higher light extraction by the mesh-textured trench. Index Terms—Electroluminescence (EL), laser scribe, light-emitting diode (LED), mesh-texture. I. INTRODUCTION G ALLIUM nitride (GaN) blue light-emitting diodes (LEDs) [1] have been widely used as traffic signals, backlights of liquid crystal displays, head lamps of automo- biles, etc. The development of high-efficiency GaN LEDs for blue and ultraviolet light sources has attracted intense research over the past several years. In order to improve the light ex- traction, a lot of the previous works such as p-GaN surface roughness [2], transparency electrode [3], patterned sapphire substrate [4]–[6], and photonic crystals [7] were developed. These methods were used to improve the light extraction from GaN side and they are often exclusionary. No method was developed to improve the light extraction from the reduction of sapphire absorption. Usually, the thickness of sapphire has to be kept at about 100 m for the mechanical strength. In order to utilize the light emitted backward, a silver (Ag) reflector was used, but the re- flected light will travel through sapphire twice and the absorp- tion is also doubled. The laser lift-off technique [8], [9] can remove the sapphire absorption. However, the process is too complicated and it will also damage GaN epilayers during laser Manuscript received September 28, 2008; revised January 11, 2009. First published February 03, 2009; current version published March 18, 2009. This work was supported in part by the National Science Council, R.O.C., under con- tract NSC 95-2221-E-007-259-MY2. K.-T. Lee and J. Gong are with the Institute of Electronics Engineering, Na- tional Tsing Hua University, Hsinchu 300, Taiwan (e-mail: d9663829@oz.nthu. edu.tw; jgong@ee.nthu.edu.tw). Y.-C. Lee is with the Department of Mechanical Engineering, Chung Yuan Christian University, Jhongli 320, Taiwan (e-mail: yclee@cycu.edu.tw). J.-Y. Chang is with the Department of Optics and Photonics, National Central University, Jhongli 320, Taiwan (e-mail: jychang@cc.ncu.edu.tw). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2013726 Fig. 1. Fabrication process of GaN LED with mesh-textured trench on the back side. lift-off process [10]. If a mesh-textured trench is scribed into the sapphire backside, the travel of backward and reflected light can be shorter and the mechanical strength can be preserved. In ad- dition, the textured surface of the scribed mesh-textured trench can provide Rayleigh scattering for the blue light and enhance the light extraction. In this letter, we investigated the optical properties of GaN LED with mesh-textured sapphire back delineated by laser scriber and subsequently coated with silver–copper (Ag–Cu) layers. The new method proposed can successfully lower the absorption in sapphire and hence enhance the light extraction. We will also propose that it has potential to combine any other methods to further improve the light extraction. II. EXPERIMENTS GaN epilayers were grown on sapphire substrate by metal–organic chemical vapor deposition. They consist of 4- m-thick Si-doped n-type GaN, seven periods of InGaN–GaN (5 nm/9 nm) multiple quantum-wells (MQWs), 50-nm Mg-doped p-type Al Ga N cladding layer, and 0.25- m-thick Mg-doped p-type GaN. There are three types of LED structures as shown below. The fabrication of LED chip was accomplished by a standard p-GaN etching and metallization process. A transparent in- dium–tin–oxide electrode was used on p-GaN to enhance the light extraction. Titanium–gold and nickel–gold metals were used for p-GaN and n-GaN ohmic contacts, respectively. The sapphire substrate was lapped to 100 m and then diced to 300 300 m chips (LED-1). The fabrication process of GaN LED with mesh-textured trench and Ag–Cu layers on the back side is shown in Fig. 1. The mesh-textured trench was etched on sapphire backside by a high-power 1.2-W 355-nm ultraviolet (UV) laser scriber with 1041-1135/$25.00 © 2009 IEEE