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

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 (<0.8nm). As a result, we find that the most appropriate Ni thickness for the Ni/Al electrode is 0.9 nm. We fabricate 279nm 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.