3772 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 24, DECEMBER 15, 2011 Interactions of Diffraction Modes Contributed From Surface Photonic Crystals and Nanoholes in a GaN-Based Light-Emitting Diode Szu-Chieh Wang, Yun-Wei Cheng, Yu-Feng Yin, Liang-Yi Chen, Liang-Yu Su, Yen-Jen Hung, and Jian Jang Huang, Senior Member, IEEE Abstract—Photonic crystals (PhCs) were typically fabricated on the mesa surface of an LED to improve light extraction, which is regarded as the weak coupling between the laterally propagated light in the epilayers and the surface nanostructure. Here, we re- port GaN-based LEDs with the PhC structure on the mesa sur- face and nanohole reflectors surrounding the light-emitting mesa. The output power of the new LED structure is higher than that of the device with only surface PhCs due to the enhanced diffrac- tion of low-order modes propagated in the lateral direction, in ad- dition to the higher order mode light diffraction from the surface PhCs. From the relative angular spectra, the interaction of in-plane optical wave with the nanoholes (which are etched through mul- tiple quantum wells) is much stronger than that with surface PhCs, suggesting an efficient light diffraction to the surface normal by nanoholes. Index Terms—Light-emitting diode (LED), nanohole arrays, photonic crystals (PhCs). I. INTRODUCTION I N the past couple of years, photonic crystals (PhCs) have been widely explored to improve light extraction and to modify radiation profiles of LEDs [1]–[6]. A general thought on the functions of PhCs is that they help inhibit the forma- tion of laterally guided modes or convert the guided modes to radiation energy [7]–[11]. Most reports in the related field uti- lized shallow PhC structure on the device surface in order not to damage the multiple quantum wells (MQWs). As a result, the shallow patterns are only effective on higher order modes while a large portion of the optical energy of low-order modes is poorly extracted due to less overlap with the PhCs [12], [13]. In the past, we reported that nanorods at the periphery of the light-emitting mesa can help lateral light diffraction [15], [16] Manuscript received July 18, 2011; revised September 24, 2011; accepted Oc- tober 25, 2011. Date of publication November 01, 2011; date of current version December 14, 2011. This work was supported in part by the National Science Council, Taiwan, under Grant NSC 97-2221-E-002-054-MY3 and Grant NSC 100-2628-E-002-030-MY3. S.-C. Wang is with the Novatek Corporation, Hsinchu 300, Taiwan. Y.-W. Cheng, Y.-F. Yin, L.-Y. Chen, and L.-Y. Su are with the Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan. Y.-J. Hung is with the Neo Solar Power Corporation, Hsinchu 30078, Taiwan. J. J. Huang is with the Department of Electrical Engineering, Institute of Pho- tonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan (e-mail: jjhuang@cc.ee.ntu.edu.tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2011.2174412 and later suggested that a nanohole array at the mesa edge can improve total light output emission by 19% due to a more effi- cient conversion of laterally propagated modes to leaky modes [17]. The previous study has enabled us to investigate a more comprehensive device structure which employs surface PhCs for surface light extraction and nanohole arrays for lateral light diffraction to the surface normal. The combined structure pro- vides additional design flexibilities for device performance such as the radiation profiles. In this paper, LEDs with the PhCs at the p-type GaN sur- face and nanoholes at periphery of the mesa were fabricated and characterized. Their optical properties were compared to LEDs with only surface PhCs and with typical planar surface. The light output enhancement as well as the radiation profiles of the devices were benchmarked. And light diffraction behav- iors are further characterized by the angular spectra to reveal the interaction between the optical modes and the nanostructures. Finally, optical field distributions were simulated based on the 3-D finite-difference time domain (FDTD) method to verify the experimental results. II. DEVICE FABRICATION The LED samples were grown on a sapphire substrate by metal–organic chemical vapor deposition with the epilayers consisting of a GaN buffer layer, a 2 m Si-doped n-type GaN layer, five pairs of 17 nm thick InGaN/GaN MQWs, and a 160 nm Mg-doped p-type GaN layer. As for the device fabrication, a SiO thin film was deposited by plasma-enhanced chemical vapor deposition as the etching hard mask. The PhC patterns defined by e-beam lithography on the SiO film were then transferred to GaN by the ion inductively coupled plasma dry etching process. For the PhC structure on the mesa surface (called “surface PhC”), the etching depth is 70 nm, around half the thickness of the p-GaN. The device with the surface PhCs on the entire mesa is defined as “SLED” [see Fig. 1(a)]. To realize nanoholes at the periphery of the mesa, the center 200 m 200 m area was covered by photoresist, and then the PhCs at the edge 40 m is further etched (with the original SiO thin layer as the etch mask). The overall etch depth of nanoholes is around 400 nm. The device is called “SHLED” and is shown in Fig. 1(b). In the next step, light-emitting mesa was defined and the contact pads were evaporated. The transparent conducting layer is composed of Ni/Au (5 nm/5 nm) and both the p- and n-type contact electrodes are Ti–Au (15 nm/200 nm). For comparison purpose, the planar LED is 0733-8724/$26.00 © 2011 IEEE