IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 22, NOVEMBER 15, 2009 1683 Polarization-Dependent Sidewall Light Diffraction of LEDs Surrounded by Nanorod Arrays Kun-Mao Pan, Yun-Wei Cheng, Liang-Yi Chen, Ying-Yuan Huang, Min-Yung Ke, Cheng-Pin Chen, Yuh-Renn Wu, and JianJang Huang, Senior Member, IEEE Abstract—The polarization behavior of the light-emitting diodes (LEDs) with nanorods surrounding the p-mesa is investigated. The nanorods were fabricated using a natural nanosphere lithography and are intended to diffract laterally propagated light. In the hor- izontal direction, s-polarized light is dominated since the injected carriers choose to fill up the lowest energy state in a direction par- allel to the quantum-well layers. The p/s-polarized ratio starts to increase with the increase of radiated angles and eventually satu- rates. Since the Bragg diffraction of laterally propagated p-polar- ized mode by nanorods is more efficient than the s-polarized light, the p/s-ratio of the device with nanorods is higher than that without rods. The p/s-ratio of the LED with nanorods is 1.96 at 90 , and is 1.52 when the integrating intensity between 0 and 90 is consid- ered. Index Terms—Diffraction, light-emitting diodes (LEDs), nanorods, polarization. I. INTRODUCTION A S GaN-based light-emitting diodes (LEDs) have become widely used in flat-panel displays, one emerging task in reducing the cost of the panels is by adopting a polarized light source so that the polarizer between the backlight module and liquid crystal is not needed. Therefore, the study of polarized LEDs has attracted much interest. A straight-forward approach to achieve the polarized light is to define nano-scale patterns on the chip surface, thus filtering out the undesired polarization or converting it to a certain polarization direction. Partially polar- ized output of green LEDs on a c-plane substrate was demon- strated by coupling surface plasmons in the one-dimensional Ag grating structure [1]. Also, polarization behaviors have been observed from photonic crystal LEDs in different lattice di- rections [2]. Furthermore, polarized results were shown from LEDs packaged with backside reflectors that collect sidewall emission [3]. There are also reports that focus on the epitaxial properties, GaN-based material in particular, that lead to po- larized light emission [4]–[6]. Generally, a quantum-confined Stark effect is observed for InGaN grown on the c-plane sap- phire substrate. It alters the valence band structures and thus re- Manuscript received May 26, 2009; revised August 18, 2009. First published September 09, 2009; current version published October 28, 2009. This work was supported by the National Science Council of Taiwan under Grants NSC 97-2221-E-002-054-MY3. The authors are with the Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei, 106, Taiwan (e-mail: jjhuang@cc.ee.ntu.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.2009.2031682 Fig. 1. Illustration of the device profile of an LED encompassed with nanorod arrays. sults in a nonpolar light emission. On the other hand, nonpolar- and semipolar-oriented nitride materials have enabled partially polarized light output. Therefore, polarized LEDs have been demonstrated using m-plane or a-plane sapphire substrates [7], [8]. In this work, we investigated the polarization behaviors of lat- erally propagated light diffraction from nanorods surrounding the light-emitting mesa. We used a nanorod fabrication tech- nology similar to our previous reported work on the enhance- ment of light output from LEDs encompassed with nanorods [9]. With a proper design of the light-emitting pattern, a polarized light output is demonstrated. The nanorod acts as the grating structure in the vertical direction, which possesses the property of polarization selectivity. II. DEVICES FABRICATION The sample was grown on a sapphire substrate by metal–organic chemical vapor deposition (MOCVD) with an epi-structure consisting of a 3- m n-type GaN layer, 12 periods of InGaN–GaN multiple quantum-well (MQW) layers, and a 0.16- m Mg-doped p-type GaN layer. The device fabrication starts from depositing a p-type Ni–Au (12 nm/150 nm) contact, which also functions as an etch mask for the subsequent induc- tively coupled plasma reactive ion etching (ICP-RIE) etching for p-mesa definition. As illustrated in Fig. 1, the probe contact pad on p-type is m with five m fingers (10 m away from each other) extended in both directions. Such a device structure blocks light emission from the surface area and is aiming on obtaining more accurate angular profiles in both s-polarized [transverse electric (TE)] and p-polarized [transverse magnetic (TM)] directions of sidewall emission from the longer mesa edges, thus mitigating interference and 1041-1135/$26.00 © 2009 IEEE Authorized licensed use limited to: National Taiwan University. Downloaded on December 2, 2009 at 05:34 from IEEE Xplore. Restrictions apply.