Breaking the Transverse Magnetic-Polarized Light Extraction Bottleneck of Ultraviolet-C Light-Emitting Diodes Using Nanopatterned Substrates and an Inclined Reflector

AlGaN-based light-emitting diodes (LEDs) operating in the deep-ultraviolet (UV-C) spectral range (210-280 nm) exhibit extremely low external quantum efficiency, primarily due to the presence of large threading dislocations and extremely low transverse magnetic (TM) light extraction efficiency. Here, we have demonstrated that such critical issues can be potentially addressed by using AlGaN quantum-well heterostructures grown on a hexagonal nanopatterned sapphire substrate (NPSS) and a flip-chip-bonded inclined Al mirror. Our finite-difference time domain-based numerical analysis confirms that the maximum achievable efficiency is limited by the poor light extraction efficiency due to the extremely low TM-polarized emission. In our experiment, with the usage of a meticulously designed hexagonal NPSS and an inclined Al side wall mirror (> 90% reflective in the UV-C wavelength), the AlGaN quantum-well UV-C LEDs showed nearly 20% improvement in the light output power and efficiency compared to the conventional flat flip-chip LEDs. The UV-C LEDs operating at similar to 275 nm exhibit a maximum output power of similar to 25 mW at 150 mA, a peak external quantum efficiency of similar to 4.7%, and a wall plug efficiency of similar to 3.25% at 15 mA under continuous wave (CW) conditions. The presented approach opens up new opportunities to increase the extraction of UV light in the challenging spectral range by using properly designed patterned substrates and an engineered Al reflector.

Automated summary

This study demonstrates that the use of hexagonal nanopatterned sapphire substrates and inclined Al reflectors can improve the external quantum efficiency and light extraction efficiency of AlGaN-based deep-ultraviolet LEDs, leading to significant enhancements in light output power and efficiency.