LEDs technology
G
eneral illumination offers by far the most lucrative market
for the white LED. But if this nitride-based device is to grab significant market share from incandescents and compact fluorescent lamps, then its output will have to increase and production costs fall.
One company that claims to have a technology that can do just this is a little known UK start-up called Seren Photonics. This University of Sheffield spin-off, which takes its name from the Welsh word for star, has an intriguing, low-cost device treatment that is claimed to boost light output by increasing extraction efficiency and internal quantum efficiency.
“We combine fundamental physics and device technology to achieve a new type of device,” says Tao Wang, the technical director and the current driving force behind the company’s technology. Exactly how the company increases its LED output is a bit of a mystery, because Wang is not prepared to go into specifics at this stage. That’s a shame for everyone with an interest in the inner workings of LED chips, but Wang’s tight-lipped approach is understandable: although Seren has filed for a key patent, it is yet to be granted. But when it is, Wang assures us that he will lift the veil and publish a series of papers detailing the technology.
Nitride veteran
Wang’s interest in the nitrides goes back a long way, and before he came to the UK he spent five years working at the Nitride Semiconductor Organization in Tokushima, Japan. During his time at this company, which is spin-off of the local university, he focused on epitaxial growth technology. While he was there he yearned for the freedom associated with an academic career, and in 2002 he took a lectureship at the University of Sheffield. “Sheffield has good facilities, and allows me to have my own group,” explains Wang.
When he arrived at Sheffield he began by focusing on improving the epitaxial quality of ultraviolet LED structures. “The UV LED is much more sensitive to dislocation density than the blue or the green. We wanted to use a different, novel technology to improve
the crystal quality by reducing the dislocation density.”
The standard approach to forming these structures involves a process known as two step growth: deposition of a low- temperature GaN nucleation layer; followed by growth of a thick GaN layer and then the main structure at a higher temperature. “There are a number of issues relating to that two step growth, particularly for the UV LED,” explains Wang. “For example, there are strong external absorption issues, a cracking issue, and there’s another major point – [Shuji] Nakumura’s patent. You can’t get around that.”
Wang and his team explored a different route to nitride growth based on a high- temperature, AlN buffer. Although this technology was developed for UV LEDs, it can be used to improve the crystal quality of any form of nitride device.
According to Wang, there are several advantages associated with the high- temperature AlN buffer approach, including freedom regarding the thickness of this AlN layer – it just needs to be smooth. Further gains for LED production including a massive reduction in the density of dislocations, particularly the screw type. Evidence for this is provided by X-ray rocking-curve measurements, and transmission electron microscopy.
While Wang was developing his process for improving UV LED material quality, he started to think about a new way to process devices. The technology that underpins Seren Photonics was underway. “The idea started in 2004, and we did some preliminary work in July 2007, which is directly related to the IP technology that Seren has,” explains Wang.
Getting going
When researchers at most universities think about starting a company, they have to go out and try and win funding themselves. At Sheffield, though, things are a quite different indeed. In 2005, this university gave the rights for all of its university-owned research to Fusion IP, which has been supporting Seren Photonics since its inception in December 2009.
Templates for Blue & UV LEDs
GaN, AlN, AlGaN, InN, InGaN
World leaders in development of Hydride Vapour Phase Epitaxy (HVPE) processes and techniques for the production of novel compound semiconductors
• Templates
• Wafer size: 50mm-150mm • Research grade InGaN wafers • Custom design epitaxy • Contract development
• Small and large batch quantities available
Wide range of materials (GaN, AlN, AlGaN, InN and InGaN) on different sizes and types of substrates (sapphire or SiC)
Contact us now!
Email:
plasma@oxinst.com Technologies and Devices International Tel: +1 301 572 7834
www.oxford-instruments.com/tdi1
April/May 2010
www.compoundsemiconductor.net 43
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