TECHNOLOGY CONFERENCE REPORT
The Gaylord Conventional Centre, situated on the bank of the Potomac river, hosted the tenth International Conference on Nitride Semiconductors. This meeting, which was held from 25th to 29th August, attracted about 900 delegates
to replace LEDs and make solid-state lighting more efficient; and advances in the efficiency of blue and green lasers.
Cutting costs on SiC One company that is already making lighting products with its own LEDs is the US firm Cree. At ICNS, one of Cree’s senior scientists, Hua-Shuang Kong, detailed improvements in the efficacy of the company’s LEDs. This firm is also trimming production costs, which will help to make a more compelling case for solid-state lighting. The fall in the cost per lumen realised by this vertically integrated firm – Kong described Cree’s activities as spanning “from dust to device to market” – was 45 percent in 2011, 40 percent in 2012, and should be 43 percent this year.
This increase in the bang-per-buck has been assisted by a continual improvement in the efficiency of the LED. Back in February, Cree claimed that it had broken the efficacy record for an LED driven at 350 mA with a device emitting 276 lm/W at a colour temperature of 4401K. And the performance of LEDs coming off of the production line is not that far behind, with the MK-R series that was launched in late 2012 delivering 200 lm/W.
In Kong’s opinion, the high level of performance stems from an excellent chip architecture and a high-quality SiC substrate, which forms a better platform than the more widely used material, sapphire. At Cree, LEDs are grown on single crystal 4H SiC that combines a high thermal conductivity – it is 4.2 W cm-1
K-1 in the a-direction and 3.7 W cm-1 K-1 in the c-direction – with
excellent transparency in the visible. Thanks to an absorption coefficient of less than 1 cm-1
, these substrates enable
the fabrication of an LED with a very high extraction efficiency. To increase light extraction, Cree’s engineers use a ray- tracing model to optimise chip geometry. By considering various aspects of chip design, including bevel cuts, internal mirrors, flip-chip geometries and surface texturing, engineers have increased the light extraction from a 1 mm2
chip to
88 percent. According to Kang, state-of- the-art room-temperature performance of blue LEDs emitting at 447 nm is now an output of 782 mW and an efficiency of 79 percent at a 350 mA drive current.
Cree’s blue LEDs are combined with yellow phosphors to make cool-white emitters, while warm-white variants use this chip to pump yellow and red phosphors. Both forms of white emitter are inserted into lighting products, where efficacy is compromised by thermal, optical and driver-related losses, according to Kong. He revealed that a chip emitting 115 lm/W at a colour temperature of 2700K delivers just 75 lm/W in a fixture, while a 6000K LED with a figure of 160 lm/W on the data sheet creates a fixture producing only 104 lm/W.
Fortunately fixture efficacies, like those of the LEDs, are improving, while purchase prices are falling. Back in 2008, Cree’s LR24 Troffer retailed for $400, had a colour-rendering index (CRI) of 90, and produced 3200 lm at 65 lm/W. Fast forward to today and the same light output is possible at 100 lm/W, while the purchase price has plummeted to $159.
Increases in the efficacy in lighting fixtures will continue, driven by gains in LED efficiency. By 2015, Kong expects LEDs with colour temperatures of 6000K and 2700K to produce 210 lm/W and 151 lm/W, respectively, while fixtures
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www.compoundsemiconductor.net October 2013
based on them will deliver 174 lm/W at a CRI of 75 and 125 lm/W at a CRI of 83, respectively.
Silicon-based LEDs
One promising option for accelerating the cost reduction in LEDs, and ultimately speeding the introduction of far more affordable solid-state lighting, is to switch to growing these devices on a silicon substrate.
“To replace the fluorescent lamp, we must reduce the cost [of LEDs] more and more,” proclaimed Youngjo Tak from Samsung Advanced Institute of Technology, Korea. In his talk, he argued that that although the cost-per-lumen for LEDs grown on sapphire is falling, it will plateau in 2015. So, in the long-term, alternative platforms are needed to make light-emitting chips more affordable.
Switching from 4-inch or 6-inch sapphire to 200 mm silicon is an attractive option, because it can cut costs by 31 percent and 48 percent, respectively. That’s partly because the sapphire substrates are not that cheap – according to Tak, it is typically $500 for 6-inch sapphire and $1500 for an 8-inch equivalent – and also because LEDs grown on silicon can be processed in depreciated silicon fabs.
The development of GaN-on-silicon LEDs is no longer in its infancy. Initially, the efficiency of these devices fell a long way short of the incumbents, but the difference is closing fast. “[It’s now] less than 10 percent,” claimed Youngjo Tak.
He is not the only researcher to claim that this gap is minimal: “GaN-on-silicon LEDs have comparable efficiency to GaN-on- sapphire,” revealed Martin Albrecht from the Leibniz Institute for Crystal Growth, who has been working with engineers
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