讲座人：Hao-Chung Kuo

讲座时间：9月2日上午10点

地点：海韵行政楼C505

Abstract

GaN-based light-emitting-diodes (LEDs) have been developed in various applications due to its widely tunable wavelength from ultraviolet to blue/green. Nevertheless, the most expected application, solid-state lighting, is still developing, which means the state-of-the-art InGaN/GaN LEDs should be further improved. Although the light-extraction efficiency has been significantly improved by different techniques, the internal quantum efficiency (IQE) still suffers a major obstacle, i.e., the substantial decrease in efficiency with increasing injection current [1]. This efficiency droop behavior strongly limits the development of many specific applications which require the operation current of the InGaN/GaN LEDs under high injection levels.

Carrier overflow out of the active region as well as inefficient injection and transportation of holes have been identified to be the major reasons of efficiency droop. Conventional GaN-basedc-plane quantum wells suffer from the quantum-confinedStark effect (QCSE) as a result of a spontaneousand piezoelectric polarization field parallel to the [0001]direction. This strong polarization field was recognized to be responsible for electron overflow out of multiple quantum wells (MQWs) [2]. On the other hand, asymmetric mobility of electrons and holes is also a major issue in GaN-based materials. The low hole mobility causes their feebleness in transportation along active region, such as insufficient transportation across the electron blocking layer (EBL) and accumulation of holes at the quantum wells nearp-GaN [3, 4].

In this paper, we first introduce the physical mechanisms of efficiency droop in GaN-based LEDs and its origin. Then we propose several methods to reduce this effect, such as alternative substrates, semi-polar (1-101) MQWs, low temperature insertion layer between n-GaN and MQWs, graded-thickness MQWs (GQWs), and graded-composition EBL (GEBLs). The results show that by reducing the polarization field in MQWS, enhancing hole transportation in MQWs, and enhancing hole injection across EBL, the efficiency droop behavior could be successfully reduced. Finally high efficiency white LED with efficacy >180 lm/W at 3000K was demonstrated. 

Brief Biography

Hao-Chung Kuo received the B.S. degree in physics from National Taiwan University, Taiwan, R.O.C., the M.S. degree in electrical and computer engineering from Rutgers University, New Brunswick, NJ, in 1995, and the Ph.D. degree from Electrical and Computer Engineering Department, University of Illinois at Urbana Champaign, in 1999. He has an extensive professional career both in research and industrial research institutions that includes: MTS I in Lucent Technologies, Bell Laboratories (1993–1995); and a Senior R&D Engineer in Fiber-Optics Division at Agilent Technologies (1999–2001) and LuxNet Corporation (2001–2002). Since October 2002, he has been with the National Chiao TungUniversity as a Faculty Member of the Institute of Electro-Optical Engineering.  He is now the Associate Dean, Office of International Affair, NCTU. His current research interests include semiconductor lasers, VCSELs, blue and UV LED lasers, quantum-confined optoelectronic structures, optoelectronic materials, and Solar cell. He has authored and coauthored 300 internal journal papers, 2 invited book chapter, 10 granted and 20 pending patents. Prof. Kuo is Assoc. Editor of IEEE/OSA J. of Lightwave Technology and JSTQE-special issue Solid State Lighting (2009). National Science Council of  Taiwan- Dr. Ta-You Wu Award  吳大猷青年奬 (2007). The Optical Engineering Society of Taiwan - Yang facility Award 光學工程學會青年奬 (2007). 傑出人才基金會優秀學者獎助 (2010-) 國家能源計畫 奈米國家型計畫主持人 （2009-）經濟部技術處審查委員 (2010-) IEEE Photonic Society, Senior Member (2006)IET (UK), 會士.                                                                    

外事秘书在2013-08-20提交