Diluted potassium hydroxide is all what it takes to boost AlGaN nanowire UV LEDs
Although they represent an improvement over planar AlGaN-based UVLEDs, AlGaN nanowires ultraviolet light emitting diodes are known to suffer from low efficiency partially because of the strong surface recombination caused by surface states such as an oxidized surface. Doing away with complex surface passivation methods found in literature, often requiring the use of toxic chemicals, the researchers at KAUST achieved an effective passivation of the AlGaN nanowires surface through a simple immersion in a diluted potassium hydroxide (KOH) solution for up to 40 seconds.
Inspection via transmission electron microscopy shows the nanowire structures remained almost intact after the passivation process, their surface being slightly flattened, while ultraviolet light output from devices built using such passivated AlGaN nanowires increased their light output by nearly 50% (the paper reports 49.7%), at a wavelength of 338nm.
For the experiment, the AlGaN nanowires were grown on titanium-coated silicon substrates using an ebeam evaporator, and top/bottom metal contacts were applied after the passivation process using a standard UV contact lithography process to create a 300×300μm2 UV LED.
The authors attribute the light output enhancement to the removal of the surface dangling bonds and oxidized nitrides (Ga−O or Al−O bonds) at the surface, with nanowires having relatively fewer defect states after passivation than the as-grown ones. The researchers also emphasized that output power continuously increased as the current density increased up to 120A/cm2, thanks to the absence of efficiency droop as the nanowires were grown on metal substrates which have better heat dissipation.
On the basis of these results, the researchers suggest the UV LED’s performance could be further optimized by using AlGaN/AlGaN multiple quantum well structures in the active region instead of using an AlGaN double heterostructure. Another improvement could come from using different metal substrates, for example aluminium which has a much higher reflectivity than titanium in the UV spectral range, to improve the light extraction efficiency. Minimizing UV absorption by the bottom n-GaN layer and the top p-GaN layer could further enhance light output.
KAUST – www.kaust.edu.sa
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AlN nanowall-based UV LEDs boast 60% internal quantum efficiency
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