Citation: Feng, Z.C.; Liu, Y.-L.; Yiin, J.; Chen, L.-C.; Chen, K.-H.; Klein, B.; Ferguson, I.T. Synthesis, Structural and Magnetic Properties of Cobalt-Doped GaN Nanowires on Si by Atmospheric Pressure Chemical Vapor Deposition. Materials 2023, 16, 97. https://doi.org/10.3390/ ma16010097 Academic Editor: Johann Bouclé Received: 18 November 2022 Revised: 12 December 2022 Accepted: 17 December 2022 Published: 22 December 2022 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/). materials Article Synthesis, Structural and Magnetic Properties of Cobalt-Doped GaN Nanowires on Si by Atmospheric Pressure Chemical Vapor Deposition Zhe Chuan Feng 1, *, Yu-Lun Liu 2 , Jeffrey Yiin 1 , Li-Chyong Chen 3 , Kuei-Hsien Chen 4 , Benjamin Klein 1 and Ian T. Ferguson 1 1 Southern Polytechnic College of Engineering and Engineering Technology, Kennesaw State University, Marietta, GA 30060, USA 2 Department of Electrical Engineering, Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan 3 National Taiwan University Center for Condensed Matter Sciences, Taipei 10617, Taiwan 4 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan * Correspondence: zcfeng@ntu.edu.tw or zfeng6@kennesaw.edu Abstract: GaN nanowires (NWs) grown on silicon via atmospheric pressure chemical vapor de- position were doped with Cobalt (Co) by ion implantation, with a high dose concentration of 4 × 10 16 cm −2 , corresponding to an average atomic percentage of ~3.85%, and annealed after the implantation. Co-doped GaN showed optimum structural properties when annealed at 700 ◦ C for 6 min in NH 3 ambience. From scanning electron microscopy, X-ray diffraction, high resolution trans- mission electron microscope, and energy dispersive X-ray spectroscopy measurements and analyses, the single crystalline nature of Co-GaN NWs was identified. Slight expansion in the lattice constant of Co-GaN NWs due to the implantation-induced stress effect was observed, which was recovered by thermal annealing. Co-GaN NWs exhibited ferromagnetism as per the superconducting quantum interference device (SQUID) measurement. Hysteretic curves with Hc (coercivity) of 502.5 Oe at 5 K and 201.3 Oe at 300 K were obtained. Applied with a magnetic field of 100 Oe, the transition point between paramagnetic property and ferromagnetic property was determined at 332 K. Interesting structural and conducive magnetic properties show the potential of Co-doped GaN nanowires for the next optoelectronic, electronic, spintronic, sensing, optical, and related applications. Keywords: cobalt-doped GaN nanowires; atmospheric pressure chemical vapor deposition; scanning and transmission electron microscopy; X-ray diffraction; energy dispersive X-ray spectroscopy; superconducting quantum interference device 1. Introduction Great breakthroughs on gallium nitride (GaN)-based material and devices as well as industrial applications have been of recent interest [1,2]. GaN-based one dimensional (1D) nanostructure, such as GaN nanowire, etc., has attracted much research attention [3–22]. These types of GaN-based nanowires (NWs) possess unique electronic, optical, magnetic, and catalytic features, and exhibit numerous novel and interesting properties [3–29]. Com- pared to conventional planar films, NWs, with characteristics of a high surface-to-volume ratio and uniaxial charge transport path, provide a better choice for multiple nanoscale ap- plications. GaN NWs possess a semi-discrete density of states with a continuous transport path for carriers, due to its 1D configuration [3]. GaN NWs are good candidates for photocatalytic applications. Their energy positions and band edges are aligned with redox levels in electrolytes, leading to an improved and controllable photocatalytic activity [7]. The large surface-to-volume ratio of GaN NWs allows a high integration density on devices and systems. An enhanced light extraction Materials 2023, 16, 97. https://doi.org/10.3390/ma16010097 https://www.mdpi.com/journal/materials