Progress and Outlook of 10% Efficient AlGaN‐Based (290‐310 nm)‐Band UVB LEDs

Abstract

Eco‐friendly aluminum gallium nitride (AlGaN) materials for the epitaxial growth of ultraviolet‐B (UVB) light emitting diodes (LEDs) on c‐Sapphire have attracted much attention due to their low fabrication cost and possibility of high external‐quantum efficiency (EQE) and light power. However, the long‐term low wall plugs efficiency (WPE) and light power remain a significant bottleneck impeding the commercialization of UVB LED modules. Our special growth techniques for 50% relaxed and 4µm‐thick AlGaN buffer layer (BL) are available to achieve a maximum internal‐quantum efficiency (IQE) surpassing 57% in 300 nm‐Band UVB LEDs. In this review paper, we describe the impact of special growth of thick AlGaN BL as well as n‐AlGaN electron injection layer (EIL) and their crystal quality on the (290‐310nm)‐Band LED’s performances, compared to the conventional n‐AlGaN/p‐GaN based UVB LEDs. The last 6 years have seen large improvements in the AlGaN‐based UVB LED, with efficiencies improved by over 10% at Riken. The main drivers have been improved electrical, optical design, better carriers confinement in the multi‐quantum wells (MQWs) of the UVB LEDs and the generation as well as transportation of hole toward the MQWs. Also, the influence of a thin “Valley” layer in p‐type multi‐quantum barrier electron‐blocking‐layer (p‐MQB EBL) on 2D hole generation and hole injection via intra‐band tunneling (HIT) was attempted. Briefly, these improvements at Riken in the development of high‐efficiency transparent UVB LEDs are including the modulation of Al compositions in the p‐AlGaN hole injection layer (HIL), fabrication methods in MOCVD, and p‐side contact materials including highly reflective p‐electrodes. Notably, we highlight the effects of undoped (ud)‐AlGaN final barrier (FB) of quantum‐well strategies against the electron blocking and promoting the hole tunneling and incorporation of the soft polarized Mg‐doped p‐type AlGaN HIL for better hole injection toward the MQWs to improve the performance of UVB LEDs. As a result, high hole concentration ∽ 2 × 1016 cm−3 in the Al‐graded p‐AlGaN HIL, where the hole‐trap level appeared to have been effectively suppressed by excimer laser annealing (ELA) treatment, and quite low resistivity of 24 Ω‐cm at room temperature (RT) was achieved. Optically, enhanced reflection from Ni/Mg or Ni/Al p‐electrode and improved light extraction within the transparent p‐AlGaN LED have had a large impact. In particular, the transparent AlGaN‐based 290‐310 nm UVB LED has improved significantly in recent years and is now almost approaching that of UVC LED (EQE ∽ 10%). Such new features have increased the efficiencies of transparent 310 nm and 290‐304 nm UVB LEDs, respectively, to a recently confirmed 4.7% and 9.6% with light powers of 29 mW and 40 mW on wafer, world record values of a mere 2 year ago. Finally, we give our perspectives to combine photonic, crystals and electrical constraints into practical architectures for AlGaN‐based UVB LEDs and also identify future research directions as well as potential applications of UVB LED modules technologies, such as those in immunotherapy (psoriasis, vitiligo), vulgaris treatment (310 nm), plant growth under UVB lightning (310 nm), the production of vitamin D3 in the human body (293‐304 nm), the production of phytochemicals in green leaves of vegetables (310 nm) and inactivation of SARS‐CoV‐2.This article is protected by copyright. All rights reserved.