INDUSTRY LEDs
what we’d ideally like to buy is not available,” admits Devine. “We’ve developed a type of reactor, called a SCoRA (scalable, compact, rapid ammonothermal), that is making very good progress.”
The traditional ammonothermal method, which has been pioneered by Ammono of Warsaw, Poland, produces GaN of incredibly high quality, but growth rates can be as low as just a few microns per hour. In comparison, Soraa’s approach is capable of growth at greater than 40 µm/hr, and 10-30 µm/hr in all directions.
One of the key features of the Soraa reactor is its internal heating. This circumvents the material property limitations of a conventional ammonothermal chamber. The Soraa reactor is also claimed to be cheaper and easier to scale than traditional autoclaves, which are fabricated from nickel-based superalloys. The capsule that contains the raw materials – seed crystals, a polycrystalline GaN nutrient, a mineraliser and ammonia – is surrounded by a heater, followed by a ceramic shell providing structural support and thermal isolation, and finally an externally-cooled outer shell providing mechanical confinement.
Thanks to low conductivity of the ceramic, the steel shell can remain below 200 °C at an operating temperature of 750 °C, allowing it to maintain high creep resistance.
GaN crystals grown in this reactor are transparent but yellow, due to residual impurities, and have a dislocation density below 104
cm-2 . These crystals
have been sliced into wafers that have been used as the foundation for the growth of InGaN/GaN heterostructures. The intensity and full-width half maximum of the photoluminescence produced by this sample is virtually identical to that emitted
by an identical structure formed on HVPE-grown GaN. However, electroluminescence emitted by the former structure is complicated by a yellow luminescence emanating from the substrate.
Although engineers are now working to try and improve the transparency and luminescence properties of their boules, it will be several years before Soraa is making its own substrates for device manufacture. “We’re not able to call the exact date,” explains Devine. “It will depend on the application. The specifications for lasers are not as stringent as those for LEDs. We have a laser business, and we could begin to use our own substrates earlier in our laser manufacturing, than for LEDs.”
Solid-state lighting, however, will continue to provide the majority of the company’s revenue. During the next 12 months Soraa will ramp its production of the MR 16 lamp family, while moving into expanded flood lamp configurations.
“As you go into 2015 and 2016, the possibilities broaden,” explains Devine. “For what the next area after the flood lamps will be, we need to do some more market research, as well as go through market acceptance of our flood lamp products.”
One option for broadening the portfolio is to launch a replacement for the 60 W incandescent. This is not a trivial move, though, according to Devine, because it would mean a transition to a market where the price of light is more highly valued than its quality. “A market like that, which is a pure commodity, is a couple of years away from when it’s going to be the right time for us to approach it.”
© 2013 Angel Business Communications. Permission required.
Skin tones under typical 80 CRI blue-based LED, left, compared to 95 CRI 95 R9 Soraa Vivid LED, right
Further reading D. Ehrentraut et. al. Jpn. J. Appl. Phys. 52 08JA01 (2013)
M. Cich et. al. Appl. Phys. Lett. 101 223509 (2012)
August / September 2013
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