IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 5, MARCH 1, 2011 287 Enhanced Performance of Nitride-Based Blue LED With Step-Stage MQW Structure Hsiao-Chiu Hsu, Yan-Kuin Su, Fellow, IEEE, Shyh-Jer Huang, Chi-Yao Tseng, Chiao-Yang Cheng, and Kuan-Chun Chen Abstract—A step-stage InGaN/GaN multiquantum-well (MQW) structure can enhance the efficiency of GaN-based light-emitting diodes (LEDs). Compared to dual-stage MQW LEDs, the step-stage MQW LEDs have lower forward voltage and higher light output. The measured light output power of step-stage LEDs operating at 350 mA shows an increase of approximately 23% with an external quantum efficiency (EQE) increase of 6.6%, when compared to dual-stage LEDs. Index Terms—Electron-injection layer (EIL), GaN-based light-emitting diodes (LEDs), multiple quantum-well (MQW), step-stage. I. INTRODUCTION T HE III-nitride semiconductors have shown great potential for applications in optoelectronic devices, such as high brightness blue/green LEDs and laser diodes (LDs) [1]. To achieve highly efficient LEDs, various research groups have provided numerous plausible methods, such as the improve- ment of carrier injection [2], [3], the decrement of leakage current [4] or the reduction of nonradiative recombination [5], [6], all of which are designed to enhance carrier recombination probability in the MQW active region of the optical devices. For the improvement of the carrier injection in the active region, Rebane et al. reported that the insertion of charged asymmetric resonant tunneling (CART) structure as the EIL could increase the electron injection rate in the active region by electron tunneling effect [2]. However, adding a thick CART layer tends to degrade the crystalline quality of overlying layers, especially in the active region which inevitably leads to the serious efficiency droop in the optical devices because of the enhancement of the nonradiative recombination [7]. To resolve this problem, our research group hereby presents a dual-stage LED, in which the EIL of the CART structure is Manuscript received October 05, 2010; revised December 02, 2010; accepted December 18, 2010. Date of publication December 23, 2010; date of current version February 16, 2011. This work was supported by Advanced Optoelec- tronic Technology Center, LED Lighting and Research Center, National Cheng Kung University, National Science Council, the TDPA, the Bureau of Energy, and Ministry of Economic Affairs of Republic of China (R.O.C.) in Taiwan under Contracts (TDPA97-EC-17-A-07-S1-105, NSC 97-2623-E-168-001-IT, and No. 98-D0204-6). H.-C. Hsu, Y.-K. Su, S.-J. Huang, C.-Y. Cheng, and K.-C. Chen are with the Institute of Microelectronics and Department of Electrical Engineering, Ad- vanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan (e-mail: yksu@mail.ncku.edu.tw). C.-Y. Tseng is with the Department of RDII, Huga Optotech Inc., Taichung 407, Taiwan. 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.2010.2102348 Fig. 1. Illustration of TMIn flow rate for controlling the indium composition during the growth of dual-stage and step-stage MQW structure. replaced by one set of InGaN/GaN MQW with constant indium composition [3]. The EIL of dual-stage LED could also reduce the internal piezoelectric field by acting as a prestrain layer for the reduction of the strain in the overlying wells [8]. In this work, the previously mentioned constant-indium-doped EIL is further replaced by a step-stage MQW structure to improve the performance of InGaN/GaN LEDs. In step-stage MQWs, one set of indium-stepwise-doped MQW structure is used. This modification scheme succeeds in improving the crystal quality of the active region, which in turn helps to enhance the carrier injection. To ensure the improvements of the electrical and optical properties of step-stage LEDs, a dual-stage LED is also fabricated for comparison. II. EXPERIMENT All samples were grown on -face 2-inch sapphire sub- strates by metal organic chemical vapor deposition (MOCVD). Trimethyl-gallium (TMGa), trimethy-lindium (TMIn), am- monia (NH ), biscyclopentadienil (Cp Mg) and disilane (Si H ) were used as the precursors and dopant sources. The growth procedure for two different types of GaN-based LEDs is described as follows: First, a GaN nucleation layer is grown on the substrate before depositing 1- m-thick undoped GaN layer and 1- m-thick n-type GaN layer. Afterward, a set of InGaN/GaN EIL along with 5 pairs of 3-nm-thick In Ga N well layers and 7-nm-thick Si-doped GaN barrier layers are then grown. Next, 6 pairs of In Ga N/GaN MQW as an active region are deposited. To fabricate the dual-stage LED and step-stage LED, constant- and stepwise-indium-doped MQW structures are grown by controlling the flow rate of TMIn during the growth of EIL, as shown in Fig. 1. Note that the TMIn flow increases in a stepwise fashion when depositing the step-stage MQW EIL. The moles of indium injection for dual- and step-stage MQW EIL are controlled to be equal to one another during the epitaxial process. Finally, the prior epitaxial layers are capped with a 0.2- m-thick -type GaN 1041-1135/$26.00 © 2010 IEEE