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 ∼275 nm exhibit a maximum output power
of ∼25 mW at 150 mA, a peak external quantum efficiency of
∼4.7%, and a wall plug efficiency of ∼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