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How many people do you need to change a light bulb?

Aug 01, 2013
None - it's an LED bulb



Gallium nitride-on-silicon is a hot topic for LEDs and power devices.

But LEDs could be the game changer - in the immediate future at least - as LED bulbs can last a lifetime. With new government regulations to make our world more environmentally friendly, and the demise of incandescent bulbs, LEDs are on the rise.

It was in the late 1800s when Thomas Edison invented the first commercially practical incandescent light that some of us are still using.

But it's time for us to move on.

In the last ten years, there have been a cluster of companies targeting the massive lighting market. The question is who will be the quickest to come up with the goods? We want the brightest, eco-friendliest most versatile LED bulb at the lowest cost which is also easy on the eye.

It's a tall order.

Most white to blue semiconductor LEDs incorporate the wide bandgap compound semiconductor gallium nitride (GaN). And you may be spoilt for choice with the different substrates that can be used in making the device - these include sapphire, SiC, silicon and GaN itself.

Many of these substrates are costly though. The exception is silicon, which comes in large substrate sizes such as 6" and 8".

So what are the challenges of growing GaN on silicon?

Firstly the difference in lattice parameter between the most prevalent silicon (111) and GaN is large (around 17 percent). One of the key aspects to counter this is growing a layer (often AlN) - this layer prevents chemical reactions between gallium and silicon and makes the wafer behave like a substrate having lattice parameters closer to that of GaN. Then the buffer layer (mainly AlGaN) is grown to further reduce the mismatch.

There is also a 52 percent thermal mismatch between GaN and silicon. This means that once the wafer is cooled down after growth, the GaN will contract at a different rate to the silicon substrate.

Having said that, the plus side would work two-fold. Silicon substrates are cheap and also, growing on silicon would enable fabs to use their existing tools without having to invest in costly alternative equipment.

Current players in the GaN-on-Si market include Toshiba and Plessey. Both companies have recently launched LEDs for the lighting market. But efficiencies are still relatively low.

Using technology initiated by Colin Humphreys' group at Cambridge, Plessey and Aixtron are working together to optimise a process using a close coupled showerhead (CCS) reactor capable of growing 6 inch GaN-on-Si.

Apart from Toshiba, competitors include Lattice Power, Transphorm, Azzurro, Infineon, Philips Lumileds, Samsung and imec (and partners). Soitec, Azzurro, Kyma, Translucent and EpiGaN provide GaN-on-Si templates.

So only time and investment will tell who comes out on top.

NEWS
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