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• Chinese Optics Letters
• Vol. 19, Issue 12, 121404 (2021)
Lingrong Jiang1、2、3, Jianping Liu1、2、3、*, Lei Hu1、2、3, Liqun Zhang1、3, Aiqin Tian1、3, Wei Xiong1、3、4, Xiaoyu Ren1、3, Siyi Huang1、2、3, Wei Zhou1、3, Masao Ikeda1、3, and Hui Yang1、2、3
Author Affiliations
• 1Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China
• 2School of Nano-tech and Nano-bionics, University of Science and Technology of China, Hefei 230026, China
• 3Key Laboratory of Nanodevices and Applications, Chinese Academy of Sciences, Suzhou 215123, China
• 4Nano Science and Technology Institute, University of Science and Technology of China, Hefei 230026, China
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Abstract

Absorption induced by activated magnesium (Mg) in a p-type layer contributes considerable optical internal loss in GaN-based laser diodes (LDs). An LD structure with a distributed polarization doping (DPD) p-cladding layer (CL) without intentional Mg doping was designed and fabricated. The influence of the anti-waveguide structure on optical confinement was studied by optical simulation. The threshold current density, slope efficiency of LDs with DPD p-CL, and Mg-doped CL, respectively, were compared. It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency, which were caused by decreasing internal loss and hole injection, respectively.

1. Introduction

Visible laser diodes (LDs) based on group III nitride materials have been employed as light sources in many fields such as full-color laser projection, laser lighting, under water communication, and material processing[15]. Despite their considerable commercial success in some areas, great efforts to further decrease threshold current density and to increase slope efficiency of LDs are needed to meet wider application requirements[6]. One of the obstacles hindering progress is the large optical internal loss, including absorption in $p$-doped layers, absorption by the passive regions, re-absorption of quantum wells (QWs), and absorption and scattering related to chip processing. It is believed that optical absorption loss caused by impurity doping, especially magnesium (Mg) doping, is the main source of internal loss[79]. Kioupakis et al. reported that acceptor-bound hole absorption was the dominant mechanism by theoretical calculation[8]. For the conventional structure of GaN-based LDs, Mg-doped AlGaN is applied as the electron blocking layer (EBL) and $p$-cladding layer (p-CL). These heavily doped layers have a large overlap with the modal optical field. Optical loss caused by Mg doping in visible LDs is approximately $10−15 cm−1$ based on the absorption coefficient data given by Sizov et al.[9], while the typical internal loss of the LDs is about $15−30 cm−1$, which means a majority of internal loss originates from the Mg doping layer. Thus, the performance improvements of visible LDs are limited by Mg doping induced internal loss. Decreasing the internal loss can be achieved by reducing the overlap of the waveguide mode with a Mg-doped layer, specifically, shifting the optical field away from the doped layer or decreasing the doping concentration. The movement of the optical field to the $n$-side may have detrimental impacts on the optical confinement factor of QWs and mode confinement, therefore leading to lower mode gain and a stronger substrate mode[10], respectively. Decreasing the Mg doping concentration directly will increase electrical resistance of the $p$-type layer because of the low ionization rate of Mg in nitride materials, which can increase forward voltage and decrease injection efficiency.

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Lingrong Jiang, Jianping Liu, Lei Hu, Liqun Zhang, Aiqin Tian, Wei Xiong, Xiaoyu Ren, Siyi Huang, Wei Zhou, Masao Ikeda, Hui Yang. Reduced threshold current density of GaN-based green laser diode by applying polarization doping p-cladding layer[J]. Chinese Optics Letters, 2021, 19(12): 121404