nanomaterials Review In(Ga)N Nanostructures and Devices Grown by Molecular Beam Epitaxy and Metal-Assisted Photochemical Etching Abdul Kareem K. Soopy 1,† , Zhaonan Li 2,† , Tianyi Tang 3,4 , Jiaqian Sun 3,4 , Bo Xu 3,4 , Chao Zhao 3,4, * and Adel Najar 1, *   Citation: Soopy, A.K.K.; Li, Z.; Tang, T.; Sun, J.; Xu, B.; Zhao, C.; Najar, A. In(Ga)N Nanostructures and Devices Grown by Molecular Beam Epitaxy and Metal-Assisted Photochemical Etching. Nanomaterials 2021, 11, 126. https://doi.org/10.3390/nano11010126 Received: 11 December 2020 Accepted: 31 December 2020 Published: 7 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Physics, College of Science, United Arab Emirates University, Al Ain 15551, UAE; abdulkareemks@gmail.com 2 Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China; lizhaonan@hust.edu.cn 3 Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences & Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Beijing 100083, China; tangtianyi@semi.ac.cn (T.T.); sunjiaqian@semi.ac.cn (J.S.); srex@semi.ac.cn (B.X.) 4 College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Science, Beijing 101804, China * Correspondence: zhaochao@semi.ac.cn (C.Z.); adel.najar@uaeu.ac.ae (A.N.); Tel.: +971-50-817-3348 (A.N.) † These authors contributed equally to this work. Abstract: This review summarizes the recent research on nitride nanostructures and their applications. We cover recent advances in the synthesis and growth of porous structures and low-dimensional nitride nanostructures via metal-assisted photochemical etching and molecular beam epitaxy. The growth of nitride materials on various substrates, which improves their crystal quality, doping efficiency, and flexibility of tuning performance, is discussed in detail. Furthermore, the recent development of In(Ga)N nanostructure applications (light-emitting diodes, lasers, and gas sensors) is presented. Finally, the challenges and directions in this field are addressed. Keywords: GaN; InGaN; photochemical etch; molecular beam epitaxy 1. Introduction Group III–V semiconductors, in particular, III-nitrides and their compounds, have recently received growing interest from the scientific community owing to their fascinating fundamental properties and novel applications. The outstanding properties of these mate- rials, including wide direct bandgap, chemical and thermal stability, piezo/pyroelectricity, high conductivity/mobility, and biocompatibility, enable reliable operation of devices together with the awareness of different environmental and ecological challenges [1–4]. Nanostructured GaN and InGaN possessing the above properties have become revolution- ary materials. The flexible bandgap range from 0.64 for InN to 3.45 eV for GaN, covering a uniquely ultra-wide spectrum [1], is ideal for broader applications. Advances in the synthesis of these materials have led to major developments in practical nanoscale smart objects, such as light-emitting diodes (LEDs), probes, photovoltaics, and lasers [5–9]. These materials are also used as efficient photoanodes in solar-powered water splitting, which turns an alternative energy source into fuel, albeit releasing CO 2 in the process [10]. Nanostructures are usually prepared using bottom–up [11–13] or top–down [14] approaches. Well-aligned GaN and InGaN nanostructures have been obtained by metal– organic chemical vapor deposition (MOCVD) [15–17], molecular beam epitaxy growth (MBE) [18], the vapor–liquid–solid (VLS) mechanism [19], laser-assisted catalytic growth (LCG) [20], and the ion-etching reaction [21,22]. In this review, we focus on MBE and metal-assisted photochemical etching (MacEtch), which can be used to grow high-quality nitride nanostructures. MBE is a popular bottom– up method for growing arsenide materials and devices [23]. In recent years, MBE has Nanomaterials 2021, 11, 126. https://doi.org/10.3390/nano11010126 https://www.mdpi.com/journal/nanomaterials