news analysis Seren prepares to launch novel LEDs
Can a UK start-up unlock the potential of nitride LEDs? Bagging a hefty £1.8m in equity and an extra £230,000 from the UK-based Technology Strategy Board last year, the future looks promising for Seren Photonics. Launched as a University of Sheffield start-up in 2010, equity researchers have tipped the high brightness LED developer for exit come 2015. And with the company now busy forging manufacturing partnerships with chip makers around the world, the analysts’ predictions look set to come true. So where next?
Since its inception, Seren Photonics has promised to deliver ultra- high efficiency LEDs to the lighting industry by applying novel post-processing techniques to bare chips. Technology pioneer and company chief technology officer Professor Tao Wang has always kept quiet about how his methods work, and while still fairly tight-lipped due to pending patents, details are emerging. In a bid to boost brightness, Seren has focused on developing InGaN/GaN nano-rod light emitting diodes. Nanorod LEDs are not a new concept – myriad researchers have already used lithography and dry-etching techniques to fabricate these devices – but Wang claims his LEDs perform better, saying: “Our studies indicate these InGaN-based nanorod LEDs can now be truly employed for practical applications for the first time.”
Wang’s process starts with depositing a 200 nm SiO2 film onto
any standard III-nitride epiwafer, with a 10 nm nickel layer then deposited onto this. The epiwafer is annealed and the nickel layer self-organises into nano-scale islands used to form silicon oxide nanorod arrays via standard reactive ion etching.
This silicon dioxide nanorod mask is then used to plasma etch an LED epiwafer down to its n-type layer, but this dry-etching process inevitably damages the epiwafer. However, Wang’s patent-pending process “heals” etching-induced damaged prior to final device fabrication, giving InGaN-based nanorod LEDs with an upright sidewall. His team uses surface plasmon coupling to enhance the quantum efficiencies of the InGaN/GaN quantum wells. Thanks to the nano-rod architecture, a film of silver or gold nano-particles can be deposited around the nano-rod quantum well structures, optimising the surface plasmon coupling effect, known to boost light emission.
“Normally you [deposit] nano-particles of silver or gold close to the InGaN quantum wells... But in a standard epi-wafer you deposit silver particles on top of the p-layer, and they are at least 200 nm from the quantum wells, so the surface plasmon coupling effect is almost zero,” explains Wang. “If you use a nano-structure you can put the silver particles around [this], so the distance between the quantum well and silver approaches zero, massively improving the surface plasmon coupling effect.”
Having successfully fabricated devices – with internal quantum efficiency increased by a factor of 8 in green nano-rods – Wang says Seren is ramping up efforts to grow structures on sapphire
Scanning electron microscopy image of a) nickel masks b)GaN nanorods c)GaN nanorods after Wang’s patent- pending healing process,and spin-coated ready for LED fabrication d) The nanorods after back-etching to expose the top p-type layer
substrates. “Most [devices] are grown on gallium nitride but the price of free-standing GaN is extremely high, so it is impossible to use this as a substrate for green LEDs,” he says. But as Wang highlights, his post-processing method can also be applied to epitaxy growth “We can improve the quality of the GaN on sapphire with our technology,” he says. “We put nanostructures into the intermediate layers, which is cost-effective and we also get a high recombination efficiency.”
But despite success, Seren’s LEDs are not widely available yet. As Wang says, the company has produced LEDs, but we will have to wait two to three years to see them on the market. In the meantime Seren is busy forming joint ventures with other companies to develop its technology. The company has confirmed a manufacturing agreement with an “Indian manufacturer” while according to Wang, talks are also underway with other companies in Germany, China, Taiwan and Korea. Indeed, other media reports suggest China- and Taiwan-based LED chip manufacturers are testing Seren’s LEDs with pilot production line samples showing improved brightness.
And as the company edges closer to commercialisation, Wang is pushing forward with his research. His group has recently fabricated InGaN/GaN green multi-quantum well structures based on nano-disks, rather than nanorods, that are said to boost internal quantum efficiency by a factor of up to 88. Wang is excited.
“We’ve just got the data and the increase in IQE is incredible,” he says. “We now plan to submit our results to a high profile journal, Science or Nature.”
March 2013
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