Enhancement of light extraction efficiency of 280-nm UV LEDs using SiO2 microsphere and microlens arrays

Abstract

Ultraviolet (UV) light-emitting diodes (LEDs) are useful in applications such as water/air purification, sterilization, and biosensing. However, due to the low external quantum efficiencies (ηEQE) of III-Nitride semiconductor UV LEDs, the technology has struggled to achieve penetration into many of these potential applications. While the active regions of UV LEDs have been well optimized, allowing for internal quantum efficiencies of greater than 60%, light extraction efficiency (ηEXT) remains a significant obstacle, and is limited to less than 10% in conventional UV LEDs, limiting their ηEQEs to around 1% for wavelengths below 300 nm. Surface texturing of the p-GaN or p-AlGaN layer in top-emitting UV LEDs has allowed for improvements in ηEQE at the expense of hole injection efficiency. Etching of the sapphire or AlN substrates to form lenses avoids this tradeoff in bottom-emitting LEDs, but is exceptionally time and resource intensive. Here, we investigated a novel method of enhancing ηEXT of AlGaN multiple quantum well UV LEDs at 280 nm using self-aligned monolayers of SiO2 microspheres and microlenses. Finite-difference time-domain simulations were utilized to investigate the effects of these nanostructure monolayers on the ηEXT of DUV LEDs emitting at 280 nm, and predicted up to 2.31x times enhancement of ηEXT. Electroluminescence (EL) measurements were performed in tandem with our simulations of UV LEDs. At normal incidence, up to 6.1% and 12.7% EL intensity enhancements were observed using 700 nm SiO2 microspheres and microlenses, respectively. These promising enhancements in output power may allow for high ηEQE in UV LEDs.