Once intent on lighting up the LED industry with its novel process to fabricate nano-rod LEDs, Seren Photonics is now focusing on large semi-polar GaN templates for a brighter light. Compound Semiconductor reports.

While around half of start-up businesses will have shut up shop within four years of launch, UK-based, Seren Photonics, remains safe and sound.

Established in late 2009, the high brightness LED developer has since won nearly £2.5 million in equity and research funds, and has recently entered a second funding round. But change is afoot.

Known for its process to fabricate high efficiency InGaN/GaN nano-rod LEDs - pioneered by founder and chief technical advisor Professor Tao Wang, Sheffield University - Seren is now touting a second technology; two inch non-polar and semi-polar GaN templates. Claimed to enhance light output and reduce droop, enabling high efficiency green LEDs and laser diodes, the templates boast lower defect densities than equivalent products as well as low wafer curvature.

As Seren chief executive, Dr Carl Griffiths, explains, the template process is derived from the same basic physics as the original nano-rod LED technology. To date, only relatively small size (10mm x 10mm) semi-polar GaN substrates have been available as manufacturing has been via sawing from GaN boules grown by HVPE or Ammon-thermal techniques.

However, the company has now used its growth-on-nanorod and AlN buffer layer approach to grow two inch non-polar and semi-polar substrates. Here, nano-rod structures are created using the company’s self-organised nickel nano-mask technique followed by a plasma etch. The purpose of the nano-rods is to rapidly block defects thus creating a low defect density layer in less than 5µm of GaN.


The TEM cross-section shows the lower nano-rod layer and a low defect density over-layer.

"We arrive at a very low defect density in a very very thin GaN layer so the amount of wafer bow on our product is negligible," explains Griffiths. "This is key at larger diameter substrates."

And while the process is similar to other nano-column based concepts - IQE acquired UK start-up NanoGaN in 2009 to exploit its proprietary nano-column process for producing GaN substrates - the results are different.

As chief development officer, Dr Bedwyr Humphreys, highlights: "Our process is similar to other concepts using nano-columns but there are fundamental differences in the way re-growth takes place, which reduces defect density on our templates."

So where next for the company? As Griffiths asserts, his company has always been about creating high value IP and licensing that IP to manufacturers, but its next generation templates are a little different.

"We need to go further along the supply chain with this," he says. "We intend to do some small-scale production first and then look for a large manufacturing partner to engage with or license the technology to."

And so templates have already been passed onto Tier One companies - Seren awaits feedback - and the company also hopes to join forces with a large-scale epitaxy manufacturer to transfer the technology and ramp up volumes.

"We see the future more lying in line with the semi and non-polar templates so we want to raise capital to facilitate small-scale manufacturing before engaging fully with volume manufacturers," explains Griffiths. "We anticipate ramping up volumes in 18 to 24 months’ time as well as proceeding from two inch to four and six inch substrates."

But the plans don't stop at sapphire. With manufacturers now eyeing silicon as a potential cheap substrate on which to grow LED layers, research is currently underway at Seren to deposit its template layers on silicon. "Moving onto this second platform would certainly be beneficial in opening up the power semiconductor market for us, so silicon is definitely on our roadmap," says Humphreys.

The end of nano-rod LEDs?

But what about the company's original aim to 'unlock the full potential of nitride-based,

ultra high-efficient, white LEDs' with its nano-rod based process? Griffiths asserts work is ongoing, and we may see more yet.

"There is such an incredible overlap in the fundamental technology of our two [technologies] that we certainly won't be stopping [the nano-rod work]," he says. "Professor Wang is currently working on two or three new technologies that rely on the capability to make the nano-column structure."

Griffiths will not be drawn on details, only to say his researchers "have spent a lot of time in the last six months understanding the different process steps, such as the exact nano-column size you need."

At the same time, the company has been busy fostering relationships with manufacturers and government in China. Griffiths believes that at this point in time, the template technology is more appealing to companies in the West as Asia-based counterparts focus on driving down LED manufacturing costs in response to the recent deluge of LEDs on the market. But he expects this will change.

"Right now the emphasis for us is to work with the Tier One players that have a long reach into research as these are the guys what will bring the technology forward and make it applicable," he says. "But I think in two to three years’ time as the technology matures, these templates will migrate over to China where Asia-based manufacturers can then drive the costs down."

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