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

Abstract

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.

N-AlGaN Free Deep-Ultraviolet Light-Emitting Diode with Transverse Electron Injection

“…Short-wavelength 200–280 nm deep-ultraviolet light-emitting diodes (DUV-LEDs) play a promising role in sterilization, water purification, UV communication, and in situ sensing. , AlGaN based semiconductors are one of the most established materials for fabricating DUV-LEDs because they are environmentally friendly and have a direct wide bandgap ranging from 3.4 to 6.2 eV. However, although much progress has been achieved, the external quantum efficiencies (EQEs) of state-of-the-art AlGaN-based DUV-LEDs are generally below 10%. − One of the root causes is the poor quality of the high aluminum component material on the sapphire substrate. , The multiple quantum wells (MQWs) of conventional structure DUV-LEDs are grown on n-AlGaN; thus, the reduction of threading dislocation density (TDD) in the AlN and n-AlGaN on sapphire can be beneficial to increase the internal quantum efficiency (IQE) of the active layer significantly. − Several approaches to reducing TDD have been explored previously, − and the TDD of AlN as low as 4 × 10 7 cm –2 has been reported earlier in epitaxially laterally overgrown (ELO) AlN by Imura et al However, the TDD of n-AlGaN is difficult to reach the level of the AlN template due to various factors such as the lattice mismatch with AlN and the discrepancy of the growth model, − thereby deteriorating the quality of the upper layer material. Besides, the limited effective doping concentration and surface etching damage in Al-rich n-AlGaN have posed significant challenges to implementing ohmic contact and low resistance even through high-temperature annealing. , Furthermore, the upper-bound growth rate for the two-dimensional growth of Al-rich n-AlGaN is experimentally determined to be around 0.65 μmh –1 at 1100 °C .…”