225 nm far-UVC LEDs enabled directly on c-sapphire by tailored AlN molar fraction in n-AlGaN layers

This study investigates the direct epitaxial growth of far-ultraviolet-C (far-UVC) light-emitting diodes (LEDs) emitting at 225 nm on low-cost c-sapphire without the need of using cut-off filters, which are typically required for emissions above 230–234 nm due to human safety and performance concerns. The key challenge is balancing optical transparency and electrical conductivity simultaneously in the n-type AlGaN buffer layer grown on the metalorganic vapor phase epitaxy-grown AlN template, which is crucial for efficient far-UVC emissions. Herein, the aluminum (Al) composition in the Si-doped n-AlGaN buffer layer varies from 84% to 90%, observing a shift in the material's absorption edge from ∼229 to ∼223 nm. Electroluminescence measurements showed a significant increase in emission intensity with increasing Al content, attributed to reduced internal absorption and enhanced carrier confinement. The directly grown LED on c-sapphire achieved an external quantum efficiency of 0.12% and an optical power of 0.07 mW at 225 ± 1 nm under both pulse and continuous wave operation at room temperature. These results highlight the critical role of Al composition tuning in the n-AlGaN layer for developing efficient 225 nm far-UVC LEDs module suitable for microbial disinfection in human-occupied environments, including space and terrestrial applications.