  Citation: Kim, T.K.; Islam, A.B.M.H.; Cha, Y.-J.; Oh, S.H.; Kwak, J.S. Demonstration of Efficient Ultrathin Side-Emitting InGaN/GaN Flip-Chip Light-Emitting Diodes by Double Side Reflectors. Nanomaterials 2022, 12, 1342. https://doi.org/10.3390/ nano12081342 Academic Editor: Antonio Di Bartolomeo Received: 22 March 2022 Accepted: 11 April 2022 Published: 13 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Demonstration of Efficient Ultrathin Side-Emitting InGaN/GaN Flip-Chip Light-Emitting Diodes by Double Side Reflectors Tae Kyoung Kim 1,† , Abu Bashar Mohammad Hamidul Islam 1,† , Yu-Jung Cha 1 , Seung Hyun Oh 2 and Joon Seop Kwak 1, * 1 Department of Energy Engineering, Korea Institute of Energy Technology, Naju-si 58330, Jeollanam-do, Korea; tkkim@kentech.ac.kr (T.K.K.); abmhis@kth.se (A.B.M.H.I.); yjcha@kentech.ac.kr (Y.-J.C.) 2 Lumens Co Ltd., Yongin 446901, Gyeonggi-do, Korea; oh.dave@lumens.co.kr * Correspondence: jskwak@kentech.ac.kr † These authors contributed equally to this work. Abstract: This work proposes an InGaN/GaN multiple-quantum-well flip-chip blue ultrathin side- emitting (USE) light-emitting diode (LED) and describes the sidewall light emission characteristics for the application of backlight units in display technology. The USE-LEDs are fabricated with top (ITO/distributed Bragg reflector) and bottom (Ag) mirrors that cause light emission from the four sidewalls in a lateral direction. The effect of light output power (LOP) on lateral direction is consistently investigated for improving the optoelectronic performances of USE-LEDs. Initially, the reference USE-LED suffers from very low LOP because of poor light extraction efficiency (LEE). Therefore, the LEE is improved by fabricating ZnO nanorods at each sidewall through hydrothermal method. The effects of ZnO nanorod lengths and diameters on LOP are systematically investigated for optimizing the dimensions of ZnO nanorods. The optimized ZnO nanorods improve the LEE of USE-LED, which thus results in increasing the LOP > 80% compared to the reference LED. In addition, the light-tools simulator is also used for elucidating the increase in LEE of ZnO nanorods USE-LED. Keywords: light-emitting diodes; ultrathin side-emitting; ZnO; nanorods; light output power; light extraction efficiency 1. Introduction Nitride-based multiple-quantum-well (MQW) light-emitting diodes (LEDs) have been applied for different kinds of commercial applications such as full-color displays, traffic displays, indoor and outdoor lighting, automobiles, and backlight units of liquid crystal displays (LCDs) because they are more reliable, efficient, cost effective, and have longer life spans [1–5]. These applications require high light output power (LOP). Generally, LOP decreases on account of low internal quantum efficiency (IQE) and light extraction efficiency (LEE) caused by the high defect density and low light escaping probability since 80% of the emitted photons in a conventional lateral LED structure are totally reflected from the interface of GaN-based epitaxial layer (n = 2.5) and air (n = 1), respectively. Various research has been proposed for improving the device IQE [6–8]. On the other hand, the patterned sapphire substrate (PSS) not only enhances the light-escaping probability but also improves the crystal quality by reducing the threading dislocations [9]. The LEE of GaN- based LEDs can also increase by designing bottom emission LEDs, especially vertical [10] and flip-chip [11–13] LEDs because of the smaller difference in refractive indexes between sapphire (n = 1.78) and air (n = 1). Despite the limitation of LOP, the GaN-based LEDs are widely used in LCD technology where a single LED works like a pointed light source that is then converted into a sheet light source for an LCD backlight unit [14]. In addition, the solid-state based LCD technology has gradually become mature because of extensive research in material, device innovation, Nanomaterials 2022, 12, 1342. https://doi.org/10.3390/nano12081342 https://www.mdpi.com/journal/nanomaterials