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Lighting up a greener future

The world spends 20% of the electricity produced today on lighting. If we all lit up our homes and offices using light-emitting diodes (LEDs) instead of the conventional incandescent and fluorescent lighting, our electricity consumption will be significantly reduced. At present, efficient LEDs emitting blue and red coloured light are available. However, to achieve white and colour-tuneable lighting, light from red, blue and green LEDs need to be mixed, and the best LEDs emitting green light are currently only a third as efficient as the red and blue ones.


My project focuses on developing cubic GaN LEDs that emit green light efficiently. GaN is a material commonly used to make LEDs that emit blue light, and it can exists in two different forms with different crystal structures: hexagonal wurtzite, and cubic zincblende. Commercial GaN-based LEDs commonly have the hexagonal crystal structure, but it is believed that the electric fields inherent to this crystal structure results in the low efficiency in green LEDs. By using the cubic crystal structure, such fields are eliminated and the efficiency of green LEDs can be improved. However, cubic GaN is not as stable as the hexagonal GaN, so more care has to be taken when growing the GaN crystal to ensure that only the cubic phase is formed.

LEDs emit light based on a concept called the multiple quantum well structure. The light emitting region of an LED consists of very thin (< 10 nm) layers of alternating GaN and InGaN alloy to create the multiple quantum well structure. The quantum wells spatially confine electrons and holes that are injected into the active region, thus increase the chances of an electron and a hole recombining to give off a photon of light. The nanometre layer thicknesses (or rather thinness!) require a growth process that allows very precise control over the amount of material deposited, and a technique called metalorganic vapour phase epitaxy (MOVPE) is often used. During MOVPE, different elements are transported into a reaction chamber by a carrier gas onto a heated substrate, where the elements react and nucleate on. The choice of material for the substrate is important, with factors such as cost and compatibility with cubic GaN to consider.

In my work, I use cubic silicon carbide on silicon wafers to template cubic GaN LED growth. So far, cubic GaN has been successfully grown on large 4-inch and 6-inch wafers. This is sufficient to make hundreds of LEDs, but to make enough LEDs that can all of us can use, the lab-based growth process has to be translated to large-scale production in industry, which we are doing in collaboration with Plessey Semiconductors.

 Lok Yi Lee

NanoDTC PhD Associate 2016

Centre for Gallium Nitride, Department of Materials Science & Metallurgy

(All images are Lok Yi Lee's)


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Call for Mini Project proposals

Jun 19, 2019

The NanoDTC invites Mini Project proposals from Cambridge Academics for its incoming c2019 cohort. Submission deadline is 11th Oct 2019.

Kevin Lim's paper chosen as the Editor's Pick in APL Materials

Mar 15, 2019

Kevin Lim (c2017) is first author of a paper chosen as an Editor's Pick in APL Materials. The work was done as part of his mini project.

40 new EPSRC studentships for NanoDTC

Feb 04, 2019

We are pleased to announce that EPSRC have awarded a new Nano CDT grant of 40 studentships for training the next generation of interdisciplinary innovative nanoscientists

Midi+PhD Project Proposals from Cambridge Academics

Dec 19, 2018

We are now accepting project proposals for Midi (May-Jul 2019) + PhD projects (starting Oct 2019) for our c2018 students. Deadline 18 Feb.