Improving the Efficiency of AlGaN Deep‐UV LEDs by Using Highly Reflective Ni/Al p‐Type Electrodes

• Published in: Physica status solidi. A, Applications and materials science Vol. 215; no. 8
• Main Authors: Maeda, Noritoshi, Jo, Masafumi, Hirayama, Hideki
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 21.04.2018
• Subjects: AlGaN; Aluminum gallium nitrides; deep‐UV LEDs; Efficiency; Electrodes; external quantum efficiency; Injection current; Light emitting diodes; Nickel; Organic light emitting diodes; Quantum efficiency; Quantum wells; reflective electrodes; Thickness; Ultraviolet radiation
• ISSN: 1862-6300; 1862-6319
• DOI: 10.1002/pssa.201700435

AlGaN‐based deep‐ultraviolet light‐emitting diodes (DUV LEDs) have a wide range of applications and a large market is expected. However, the efficiency of DUV LEDs is still much lower than that of blue LEDs due to the quite low light‐extraction efficiency (LEE). We are developing high LEE DUV LEDs by introducing a transparent contact layer and a highly reflective Ni/Al p‐type electrode. In this work, we investigate optimization of the Ni layer thickness of the highly reflective Ni/Al p‐type electrode for AlGaN DUV LEDs. We find that the reflectivity in the UV region becomes higher as the thickness of the Ni layer becomes smaller; however, the external quantum efficiency (EQE) is reduced if the Ni layer is too thin (<0.8 nm). As a result, we find that the most appropriate Ni thickness for the Ni/Al electrode is 0.9 nm. We fabricate 279 nm AlGaN quantum well (QW) DUV LEDs with the optimized electrodes and demonstrate an enhancement in EQE by a factor of 1.8 and a maximum EQE of 9% under bare wafer measurement conditions. We also demonstrate a high output power of 33 mW under an injection current of 100 mA. Currently, an increase in the efficiency of deep‐ultraviolet light‐emitting diodes (DUV LEDs) is one of the major subjects toward the expected large market in sterilization. Maeda et al. improve the light‐extraction efficiency (LEE) 1.8 times by using a highly reflective Ni/Al p‐electrode that is finely adjusted for the Ni thickness. As a result, for the 280 nm band DUV LEDs a maximum external quantum efficiency (EQE) of 9% and an output‐power of more than 33 mW are obtained.