KAUST Team Grows Droop-free UVLEDs On Metal/Silicon Substrate
AlGaN-based quantum-disks-in-nanowires could provide more compact, environmentally-friendly and reliable UV sources
Despite demand for more compact, environmentally-friendly and reliable UV sources, the performance of UVLEDs has remained limited. This is due to factors such as the presence of a high density of defects and dislocation; inefficient p-type doping and TM (transverse magnetic) polarized emission of Al-rich AlGaN quantum well structures; and a lack of efficient thermal dissipation channels under current device schemes.
To date, the best-attained external quantum efficiency (EQE) for UVLEDs with operational wavelength (270-280)nm is below 10 percent at 20mA, and the efficiency values decrease drastically with further decreasing wavelengths. Additionally, output power drops significantly as a larger current is injected.
These problems are mainly caused by the highly resistive p-AlGaN layers which not only lead to large electrical loss but also generate severe heat, resulting in further efficiency degradation at high injection levels. As a result, conventional planar UVLED structures face tremendous challenges.
Recently, a new UVLED device approach has been proposed by a team from King Abdullah University of Science and Technology (KAUST), led by Bial Janjua and Haiding Sun et al. The work was published in Optics Express Vol. 25, Issue. 2, under the title 'Droop-free AlxGa1-xN/AlyGa1-yN quantum-disks-in-nanowires ultraviolet LED emitting at 337nm on metal/silicon substrates.'
By adopting AlGaN/AlGaN quantum-disks-in-nanowires grown on a thick titanium-coated silicon substrate, as shown in Figure 1 (above), the Kaust researchers demonstrated a droop-free UVLED with a turn-on voltage of ~5.5 V and a narrow line width of 11.7 nm of electroluminescence spectrum emitting at 337 nm. They were able to inject current as high as 300mA in 0.5 x 0.5 mm2 device, in another word, the current density is up to 120A/cm2 without showing any power saturation.
Normally, efficiency droop is commonly observed in both visible and UVLEDs at high injection levels, which is mainly attributed to Auger recombination. By growing UVLEDs either on metal or metal-on-silicon substrate, the team introduced a better heat dissipation channel as well as excellent current injection channel as a result of metal, which eventually achieves better device performance.
Another important fact, says the team, is that because of the effective lateral stress relaxation, nearly defect-free AlGaN nanowires can be formed directly on silicon or other foreign substrates (such as metal, graphene, diamond, etc) guaranteeing the success of their devices.
Recent studies have further shown, both theoretically and experimentally, that significantly enhanced magnesium-dopant incorporation and large hole concentration can be achieved in low dimensional Al-rich AlGaN nanowires, compared to their planar counterparts due to the reduced magnesium-dopant formation energy and the suppression of defect/ impurity incorporation.
This is just the first demonstration of UVLEDs under a new platform. More efforts can be made to further improve device╒s optical performance by (1) removing the silicon substrate (2) finding alternative metal substrate, such as bulk Aluminum which is more UV-reflective (3) growth optimisation for better uniformity and nanowire density, etc. Nevertheless, this droop-free characteristic of UV Q-disk-in-NW LED is the first step towards an alternative eco-friendly, and cost-saving solution for replacing mercury-based lamp for a plethora of applications.