technology LEDs
and light output. The only part of LED fabrication that cannot be performed by Wang’s team is the packaging of the chip. “Eventually we’ll have to do this, but at this moment we would like to make a collaboration with a company that does packaging.”
So today’s evaluations are based on bare chips. They have found that their treatment of 0.3 mm by 0.3 mm chips driven at 20 mA leads to a doubling of light output, while plotoluminescence intensity undergoes a massive hike, increasing by a factor of 6-10.
Seren Photonics has access to growth facilities at the EPSRC National Centre for III-V Technologies in Sheffield
Seren is currently led by managing director, Carl Griffiths, who has experience in the III- V industry, having previously worked for the Welsh instrumentation firm ORS. Griffiths has a very wide experience in marketing and management, according to Wang, plus good expertise in demonstrating the company’s LEDs to potential customers. Seren also has a chairman, Peter Grant, who is the director of Fusion IP, plus an employee in charge of financing.
Funding for the development of Seren Photonics’ technology partly comes from Fusion IP and another, undisclosed company. In addition, efforts can be supported by some of the research grants that Wang receives from the Engineering and Physical Sciences Research Council (EPSRC). It’s hard to put an exact figure on total funding, but Wang estimates that it is about £0.5 million, which is enough to support two post-docs that are developing Seren’s technology. More investment in the company could follow shortly – several companies have been in contact, and this may lead to further funding for Seren, or the sharing of its technology with other chipmakers.
Today the devices are fabricated at the EPSRC National Centre for III-V Technologies, and Seren just has to pay the costs of accessing these facilities. Measurements of the devices that are produced are performed in Wang’s lab, which is equipped with tools for measuring LED photoluminescence, electrical behavior,
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www.compoundsemiconductor.net April/May 2010
Wang claims that his treatment can not only boost LED output – it can also cut the cost- per-lumen of these emitters. He argues that his process is compatible with LED epiwafers of reasonable material quality. When treated with his process, he claims that they can outperform LEDs made with the very best material. “With Nichia, the crystal quality is high, but the cost is also high. Our idea is to use standard, commercially available epiwafers – not the best available.”
Obviously, lowering the cost-per-lumen by this approach is only possible if the treatment process per wafer is cheaper than the difference in price between the highest quality epiwafers and standard equivalents. Wang claims that this is the case. He says that his treatment technology requires just a few processing steps, and these can be performed with the existing equipment found in chipmakers’ manufacturing facilities, so there are no concerns over high capital expenditure.
LEDs deployed for general lighting applications are driven at high current densities, and their performance is hampered by droop. This phenomenon, which is attracting tremendous debate over its origin, causes a decline in external quantum efficiency as drive current increases. Wang says that his treated devices are less susceptible to droop, because they are able to deliver an equivalent light output at lower drive current.
One key question concerning Wang’s treatment is whether it could deliver a doubling of the output of all forms of chip. Cree recently reported a lab device
delivering over 200 lm/W, and other researchers have claimed that the theoretical efficiency limit for LEDs is just over 300 lm/W. The implication is that a doubling of light output from Cree’s chip is impossible. However, Wang argues that theoretical efficiency figures can be misleading. “The theoretical value for the limit depends on a lot of factors. I’ve heard a talk that says 480 lm/W. It’s unclear – nobody knows.”
Chipmakers are a secretive bunch, and it is possible that one of them may already be using a process like that developed by Wang to boost LED output. But Wang thinks that this is unlikely. He argues that his process is not related to improving epitaxial quality, which is the path of progress pursued by the leading chipmakers, such as Nichia and Cree. But to be on the safe side, Wang and his colleagues have employed experts to trawl through the patent literature on their behalf. This turned up nothing that encroaches on Seren’s technology.
Even if Seren’s IP is safe for year to come, it would be tough for this start-up to take on the big chipmakers. However, that is not the plan for the company – it wants to license its technology to these firms, and work with them. To this end, over the next twelve months the Sheffield spin-out is aiming to attract more funding from companies that are interested in sharing its technology. Armed with this cash, it will then develop better devices for sampling.
Seren’s technology is still it its infancy, and it will be interesting to see how much progress can be made over the next few years. “Currently we don’t know what is the limit that we can have – it could be ten times higher, it could be twenty times higher,” enthuses Wang. But even if it’s only half as good as that, then Seren is destined for success, because its technology will play a key part in the development of brighter LEDs for general lighting.
Tao Wang is a Reader in Semiconductor Materials and Devices and an EPSRC Advanced Research Fellow
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