Page 1 of 7 2013-ILDC-424 GaN-Based Multiple Quantum Well Light-Emitting-Diodes Employing Nanotechnology for Photon Management Yu-Hsuan Hsiao, Meng-Lin Tsai, and Jr-Hau He* Abstract-Nanostructures have been proved to be an efcient way of modifying/improving the performance of GaN-based light-emitting-diodes (LEDs). The achievements in photon management include strain relaxation, light extraction enhancement, radiation pattern control, and white light devices. In this review we discuss the impact and the underlying physics of applying nanotechnology on LEDs. A variety of nanostructures are introduced as well as the fabrication techniques. Inde Terms-Light emitting diode, GaN, multiple quantum well, nanostructure I. INTRODUCTION The InxGa'_xN material system has dominated the light-emitting-diode (LED) market fom near-UV to green light since Shuji Nakamura developed the fust practical bright-light blue LED in the 1990s_ With the increase of x (i.e., higher In concentration), the emission wavelength shifs fom UV to green. However, it is difcult to grow thick InGaN layers due to lattice mismatch and In segregation caused especially with high In content. The commercialized nitride LEDs have utilized the multiple quantum well (MQW) structure to relieve the problem. The MQW confguration is formed by stacking InxGa'_xN and GaN layer alternately, with the thickness of each layer being several nanometers. In addition of LEDs, the InGaN/GaN MQW structure has been widely utilized as lasers [1]-[3] and solar cells [4]-[10]. On the other hand, the fabrication of nanostructures has become a popular research topic in recent years. Various techniques have been proposed to fabricate nanostructures, including cost-effective methods such as aqueous solution method or the precise control of structures by e-beam lithography. In this review, we will focus on the GaN-based LEDs and discuss the employment of nanostructure on such devices. The effect of photon management by the nanostructures will be This work was supported by National Science Council of Taiwan (l02-2628-M-002-006-MY3 and 101-2221-E-002-1I5-MY2) and National Taiwan University (IOR70823). Yu-Hsuan Hsiao, Meng-Lin Tsai, and Jr-Hau He are with the Institute of Photonics and Optoelectronics & Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan (e-mail: jhhe@cc.ee.ntu.edu.tw). discussed in Section II, and a miscellaneous collection of fabrication techniques will be given in Section III. In characterizing the performance of an LED, one will frst refer to its effciency. The exteral quantum effciency (EQE) of an LED is defned by the number of photons emitted divided by the number of electrons injected, and can be written as: 1EQE = 1IQE '1exlr ' where II Q E and Iexlr represent the interal quantum effciency (IQE) and the light extraction effciency of the LED, respectively. The IQE is the ability to produce photon under carrier injection in the active region, while the extraction effciency refers to the ability to get the produced photon out to the surrounding (thus the light can be measured or observed). In this report, we reviewed various techniques of employing nanotechnology to improve the performance of the InGaN MQW LEDs, including IQE and extraction efciency. II. PHOTON MANAGEMENT Due to the lattice mismatch in the InGaN/GaN heterostructure, strain and polarization felds are present in the MQW. Additionally, the mismatch between GaN and substrates results in poor crystal quality in the active region. These facts are responsible to the low IQE. On the other hand, because of the large refactive index difference between GaN (n � 2.43) and air, considerable Fresnel loss and total interal refection (TIR) loss are unavoidable at the interface, leading to a low extraction efciency. These problems can be resolved by utilizing nanostructure-based photon management techniques. We summarize the methods of photon management in the following categories: 2.1 Strain Relaation Conventional InGaN MQW LEDs are fabricated by utilizing metal-organic chemical vapor deposition (MOCVD) to grow epitaxial GaN and InGaN layer on sapphire. However, due to the lattice mismatch between sapphire substrates and GaN, threading dislocation is induced and results in poor crystal quality. Using a patterned sapphire substrate (PSS) or shaping the LED into nanorods (NRs) can relieve the strain and thus achieve higher IQE. The details of these two methods will be discussed in Section 3.1 and 3.7. 978-1-4673-5202-4/12/$31.00 © 2013 IEEE