LAUSR

We report on the demonstration of high-performance tunnel junction deep ultraviolet (UV) light-emitting diodes (LEDs) by using plasmaassisted molecular beam epitaxy. The device heterostructure was grown under slightly Ga-rich conditions to promote the formation of nanoscale clusters in the active region. The device operates at 255 nm with a maximum external quantum efficiency of 7.2% and wall-plug of 4%, which are nearly one to two orders of magnitude higher than those of previously reported tunnel junction devices operating at this wavelength. The devices exhibit highly stable emission, with a nearly constant emission peak with increasing current, due to the strong charge carrier confinement related to the presence of Ga-rich nanoclusters. Efficiency droop, however, is observed at relatively low current densities. Detailed temperature-dependent measurements suggest that the presence of efficiency droop of deep UV LEDs is largely due to electron overflow. Published under license by AIP Publishing. https://doi.org/10.1063/5.0036286 Light within the UV-C wavelength range (<280nm) inactivates pathogens and can be used for the prevention of spoilage of food as well as for healthcare applications. Such critical applications have taken on an even more pressing need as UV-C light can effectively contain the spread of infectious diseases, making it a vital tool against the next global pandemic. Currently, mercury and xenon lamps are primarily used for these applications. AlGaN-based deep UV optoelectronic devices, however, are revolutionizing the industry, enabling much broader applications due to the absence of toxic materials, tunable emission wavelengths, significantly reduced power consumption, and relative ease of installation and use. To date, emission in the 260–280nm range is commonly used for this purpose. Recent work has shown that emission at even shorter wavelengths (higher energies), from 255nm to 220nm, can be more effective at sterilization, while the reduced photon penetration depth in skin can avoid the deleterious effects of human exposure to UV light. At present, external quantum efficiency (EQE) over 20% has been measured for UV lightemitting diodes (LEDs) with emission at 275nm and 10% for LEDs at 265nm. For LEDs emitting at 255 nm, EQE in the range of 1%–3% has been commonly reported. With proper device packaging, the highest reported EQE is only around 4.5%, with very limited wall-plug efficiency (WPE) less than 4%, primarily due to the difficulty in p-type doping and the result of poor hole transport. III-nitrides have highly asymmetric doping: the hole mobilities and concentrations of AlGaN are typically over one to several orders of magnitude lower than those for electrons. The vast imbalance in the electron and hole injection to the active region has several detrimental impacts on device performance, including significantly reduced carrier injection efficiency, severe electron overflow, and parasitic recombination outside the active region. Recent studies further suggest that electron overflow, among other factors such as Joule heating, Auger recombination and carrier delocalization, is a primary cause for the efficiency droop observed in UV LEDs. Tunnel junction structures have been investigated as an alternative to resistive p-AlGaN contact layers and absorptive p-GaN contact layers in UV LEDs. The reduced resistivity of the n-AlGaN contact layer helps to increase carrier injection to the active region and improve current spreading, leading to UV-C LEDs operating at 265 nm with EQE >10%. Previous work has investigated the critical effect of the thickness of the tunnel junction and doping of the p-AlGaN layers on device characteristics. Earlier studies on nitride tunnel junction structures have also shown that the transport of carriers across the tunnel junction is primarily determined by trapassisted tunneling, indicating the crucial role of incorporating Appl. Phys. Lett. 117, 241101 (2020); doi: 10.1063/5.0036286 117, 241101-1 Published under license by AIP Publishing Applied Physics Letters ARTICLE scitation.org/journal/apl