Power Enhancement of GaN-Based Flip-Chip Light-Emitting Diodes with Triple Roughened Surfaces Bo-Siao Cheng, Chia-En Lee, Hao-Chung Kuo  , Tien-Chang Lu, and Shing-Chung Wang Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan 300, R.O.C. Received September 22, 2008; accepted November 13, 2008; published online April 20, 2009 The flip-chip light emitting diodes (FC-LEDs) with triple roughened surfaces were fabricated comprising top surface sapphire textured layer, interface patterned sapphire layer, and bottom naturally textured p-GaN layer. Light extraction efficiency was enhanced by such triple textured layers. The light output power of FC-LEDs was increased 60% (at 350 mA current injection) compared to that of conventional FC- LEDs by implementing the triple roughened surfaces. The enhancement efficiency can be simulated and the simulated results showed the same trend as the results of experiment. # 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.04C115 1. Introduction Wide bandgap light-emitting diodes (LEDs) that are III- nitride, ranging from ultraviolet to the short-wavelength part of the visible spectrum have been intensely developed in the past ten years. 1) Recently, as the brightness of GaN-based LEDs has increased, applications such as traffic signals, backlight for cell phone, and liquid-crystal display television (LCD-TV) have become possible. 2) It has many advantages, such as energy-saving, long lifetime, environment friendly and stable. So it has a promising future to become a next generation light source. However, the external quantum efficiency of a GaN-based LED still requires improvement. This comes from the fact that the refractive index of the GaN (n GaN ¼ 2:45) differs greatly from that of the air (n air ¼ 1). The critical angle at the GaN–air interface determined by Snell’s law is about 24  [ c ¼ sin 1 ðn air =n GaN Þ], which limits the light output efficiency to 8.7% [decided by (1  cos  c )]. 3) A large fraction of light generated in the active region of the LED is absorbed by the GaN material and the metal pad at the GaN surface. Some efforts have been made to increase the light extraction efficiency of an LED by making the GaN surface rough. 4–7) All these methods have one thing in common, which is that photons generated within the LEDs can experience multiple oppor- tunities to find the escape cone. As a result, the light extraction efficiency and the LED output intensity could both be enhanced significantly. In this letter, GaN-based flip-chip LEDs (FC-LEDs) with triple roughened surfaces were fabricated by a combination of epi-growth naturally textured surface, 8) epi-growth on patterned sapphire substrate (PSS), and micro-pillar array sapphire surface techniques. The detail of device fabrica- tions and characteristics will be discussed. The ray-tracing simulation of FC-LEDs with textured surfaces were dis- cussed in order to investigate the fundamental of light output enhancement. 2. Experiment The LED samples were grown on a c-plane PSS by means of a metal-organic chemical vapor deposition (MOCVD) with a rotating-disc reactor (Emcore D75TM). The detail patterned sapphire substrate process could be described elsewhere. 9) The fabrication flowchart of GaN-based FC-LEDs with triple roughened surfaces was shown in Fig. 1. The LED structure with a size of 1000  1000 mm 2 consisted of a 30- nm-thick GaN nucleation layer on patterned sapphire, a 4- mm-thick Si-doped n-GaN layer, a 0.2-mm-thick InGaN/ GaN multiple quantum well (MQW) active layers, a 50-nm- thick Mg-doped p-AlGaN electron blocking layer, a 0.2-mm- thick Mg-doped p-GaN cladding layer, and n- InGaN/GaN short period super-lattice (SPS) tunneling contact layers for indium–tin-oxide (ITO). The p-GaN and active layers were partially etched by an inductively coupled plasma (ICP) etcher to expose an n-GaN layer for electrode formation. An ITO film (250 nm) was deposited on p-GaN layer as the transparent conductive layer. The Cr/Pt/Au (50 nm/50 nm/ 2500 nm) metals were deposited for the p- and n-contact pads. After completing the conventional face-up LED structure, the Ni (500 nm) metal was deposited onto the bottom side of sapphire substrate as the mask layer and then the sample was subjected to the ICP process to form the pineapple like pillar-array surface for light extraction purpose. 9) The processed LED wafer was then subjected to the laser scribe and broken into 1000  1000 mm 2 chips. Finally, the LED chips were flip-chip bonded on reflector coated silicon sub-mount using Panasonic ultra sonic flip chip bonder for electrical and optical measurement. Fig. 1. (Color online) The fabrication flowchart of GaN-based FC- LEDs with triple roughened surfaces.  E-mail address: hckuo@faculty.nctu.edu.tw Japanese Journal of Applied Physics 48 (2009) 04C115 REGULAR PAPER 04C115-1 # 2009 The Japan Society of Applied Physics