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Nano-structured InGaN LED yields white light

By Julien Happich


Traditional approaches include colour down-conversions, combining high energy LEDs emitting in the blue or near ultra-violet band with a mix of phosphors that re-emit at different wavelengths. Generally, this approach emulates an incomplete white light spectrum at a lesser quantum efficiency than the original emitter (the LED covered in phosphor). The phosphors’ limited lifetime compared to that of the actual LED illuminating them can also negatively impact the overall longevity of the white light.

Other solutions combine multiple LED dies emitting at different peak wavelengths, but here again, white is a short-lived illusion, missing out on the natural continuum of true white light.

A team of researchers from the University of Hong Kong is confident broadband white light could be obtained from monolithic LED dies. In their recently published ACS Photonics paper “Monolithic Broadband InGaN Light-Emitting Diode”, the researchers disclose promising results using high indium content InGaN-GaN quantum well structures grown on a sapphire substrate.

The whole stack is then etched-through using a mix of silica nano-particles as a mask layer, leaving a mix of nano-pillar patterns throughout the LED die, ranging from nanotips about 150nm in diameter to microdisks about 7μm in diameter.

The nano-structuring process flow using dispersed silica beads (a, b) as nano-masks for a dry etch (c) yielding a combination of randomly distributed nanotips (d) which is then planarized.


Because the grown InGaN-GaN quantum well structures suffer from lattice mismatch induced strain, the whole idea is to leverage the difference in strain profiles across the nanotips (strain-relaxed) and the microdisks. A phenomenon known as the quantum-confined Stark effect (QSCE), peak wavelength is affected by a strain-induced piezoelectric field which reduces the effective bandgap energy, leading to a red-shift of the emission spectrum. This colour shift can be partially alleviated by releasing the strain through nanoscale structuring of the InGaN-GaN QWs stack, what the researchers did.

The nanotips emitted at wavelengths about 80nm shorter than the as-grown structures, while the larger 7μm microdisks emitted at the same wafer nominal wavelength of 575nm (as-grown).

By nano-patterning their monolithic LED die, the researchers mixed the long wavelength light from the strained InGaN-GaN QWs with the shorter wavelength light from the strain-relaxed Nano-tips.

(a) The monolithic phosphor-free white LED structure comprises arrays of nanostructures of different dimensions, (b) SEM images of the fabricated structure before and (c) after planarization.

The resulting die emitted concurrent blue, green and yellow light randomly distributed as per the nano-structuring process (using a random mix of silica spheres for the masks).

Close-up photograph of a nano-structured
monolithic LED, showing distinctive
blue-green-yellow emissions. The whole die is less
1x1mm square.
 

This is only a proof-of-concept, the researchers explain in their paper, though they hope to improve the uniformity of the light and colour distribution through the use of precise nano-patterning techniques such as electron beam or nanoimprint lithography. Emission is also tuneable along the colour gamut by adjusting the relative concentrations of the nanotips and microdisks, while more continuous emissions could be achieved through the use of multiple nanotip sizes each exhibiting a different degree of strain-relaxation (and emission shift versus the plain InGaN-GaN QWs structures).

Lead researcher Anthony H.W. Choi, Associate Professor from the Department of Electrical and Electronic Engineering at the University of Hong Kong, filed a US patent (US20130313517) back in 2013 as the main inventor. It was recently granted and since then has been extended to China, Europe and the World.

Visit the University of Hong Kong at www.eee.hku.hk

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