crystals Article Investigation of the Optimum Mg Doping Concentration in p-Type-Doped Layers of InGaN Blue Laser Diode Structures Chibuzo Onwukaeme and Han-Youl Ryu *   Citation: Onwukaeme, C.; Ryu, H.-Y. Investigation of the Optimum Mg Doping Concentration in p-Type- Doped Layers of InGaN Blue Laser Diode Structures. Crystals 2021, 11, 1335. https://doi.org/10.3390/ cryst11111335 Academic Editors: Degang Zhao and Baoping Zhang Received: 18 October 2021 Accepted: 30 October 2021 Published: 1 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). Department of Physics, Inha University, 100 Inha-ro, Michuhol-gu, Incheon 22212, Korea; hyginusonwuka@gmail.com * Correspondence: hanryu@inha.ac.kr; Tel.: +82-32-860-9200 Abstract: In GaN-based laser diode (LD) structures, Mg doping in p-type-doped layers has a sig- nificant influence on the device performance. As the doping concentration increases, the operation voltage decreases, whereas the output power decreases as a result of increased optical absorption, im- plying that optimization of the Mg doping concentration is required. In this study, we systematically investigated the effect of the Mg doping concentration in the AlGaN electron-blocking layer (EBL) and the AlGaN p-cladding layer on the output power, forward voltage, and wall-plug efficiency (WPE) of InGaN blue LD structures using numerical simulations. In the optimization of the EBL, an Al composition of 20% and an Mg doping concentration of 3 × 10 19 cm −3 exhibited the best performance, with negligible electron leakage and a high WPE. The optimum Mg concentration of the p-AlGaN cladding layer was found to be ~1.5 × 10 19 cm −3 , where the maximum WPE of 38.6% was obtained for a blue LD with a threshold current density of 1 kA/cm 2 and a slope efficiency of 2.1 W/A. Keywords: laser diode; blue laser; high-power laser; InGaN; nitride semiconductor 1. Introduction After the first development of nitride laser diodes (LDs) in 1996 [1] and first demon- stration of blue LDs in 2001 [2] by Nichia, the performance of blue LDs has improved considerably. To date, InGaN/GaN-based blue LDs have attracted significant attention for use in light sources for laser displays [3–5], laser-based white lighting [6–8], free-space or underwater communications [9–11], and laser-based materials processing [12–14]. In recent years, high-power operation of InGaN blue LDs with >5 W output power in a single chip has been demonstrated, mainly by company research groups such as Nichia, Osram, and Sony [3,4,12]. They also reported the low-threshold and high-efficiency operation of blue LDs with a threshold current density (J th ) < 1 kA/cm 2 , slope efficiency (SE) > 2 W/A, and wall-plug efficiency (WPE) > 40% [3,12]. Very recently, a university research group at Xiamen university also reported comparable performance of a blue LD with a J th of ~1 kA/cm 2 and an SE of 1.8 W/A [15]. Despite the remarkable progress in the development of high-power and high-efficiency blue LDs, the performance of InGaN blue LDs is still inferior to that of GaAs-based infrared LDs, which demonstrated a WPE of even more than 70% [16]. Mg doping in p-type- doped layers of GaN-based LDs is one of the main factors limiting the WPE. Owing to the high acceptor activation energy of Mg in (Al)GaN and the low mobility of hole carriers, the resistivity of p-type-doped layers in GaN-based LD structures is considerably larger than that of n-type layers, which results in a high operation voltage. To improve the conductivity of the p-AlGaN cladding layer and prevent electron leakage over the AlGaN electron-blocking layer (EBL), high-concentration Mg doping is required to increase the hole concentration. The low conductivity of the p-type layers is one of the main reasons limiting the WPE of GaN-based LDs [17,18]. Crystals 2021, 11, 1335. https://doi.org/10.3390/cryst11111335 https://www.mdpi.com/journal/crystals