INDUSTRY LEDs
employ lighting designers, who are more than sophisticated enough to appreciate the difference in the quality of light.”
To realise a full-colour spectrum, Soraa’s lamps pump red, blue and green phosphors with a violet-emitting chip. This is a markedly different approach from that used in most white-light sources, which employ a blue LED to excite a yellow phosphor, which is sometimes combined with a red variant.
“When you excite with a blue LED, it is not possible with a phosphor to tune the light down to violet, so typical competitor products are missing the violet all together,” explains Devine. This omission makes a massive difference to the appearance of anything containing whiteners, which are excited by violet light.
Customers investing in Sorra’s MR16 lamps are rewarded by getting light quality of a halogen source, but at a 75 percent gain in efficiency – despite the 10 percent reduction in efficiency for pumping with a violet, rather than blue. The high efficiency of the Soraa lamp stems from a novel chip design with very high extraction efficiency.
Triangular chips
50 mm diameter, free- standing SCoRA crystal
Insights into the design of this LED are provided in an Applied Physics Letters paper, which details a triangular chip (note that the geometry reported in that journal does not represent a specific commercial device). The LEDs reported in that paper are grown by MOCVD on GaN substrates, and have a triangular shape to ensure in-plane light is extracted from the chip after one or two bounces within it.
To optimise extraction, Soraa’s engineers model the proportion of light extracted from the LED at various chip heights. If it is just a few microns high, the device behaves like a thin-film chip with extraction resulting from just surface roughening. However, if the height of the chip increases to 50 µm or more, extraction is appreciably higher, thanks to light exiting the device via its sidewalls.
Triangular-shaped LEDs made at Soraa, which have a roughened top surface and sides with lengths of about 380 µm, produce a maximum external quantum efficiency of just over 73 percent. This high value is aided by the native substrate, which adds considerably to the device costs, but enables growth of an epistructure that is free from extended defects.
To prevent the LED costs within MR16s from becoming extortionate, devices are driven at incredibly high current densities, so very little chip real estate is required in each lamp. With a conventional LED – which is formed on sapphire, SiC or silicon – driving the device in this way leads to a significant reduction in efficiency, due to a controversial malady known as droop. This still affects Soraa’s LEDs, but the impact is far more modest, due to the combination of improved crystalline quality (due to native GaN substrate) and a light-generating active region that is far larger than a conventional device.
When the current is cranked up, the output produced by a single LED is sufficient for a halogen lamp (for the MR16 form factor, up to 75 W equivalence is achieved). Using a single source is beneficial, because it casts a single shadow. According to Soraa, rivals have multi- source lamps producing multiple shadows that fall far short of the 75 W equivalence mark.
This divergence from the norm, in terms of LED count, plus a difference in the operating temperature regime, mean that Soraa’s lamp was never going to be an off-the-shelf product. Instead, the lamp had to be designed specifically for a single GaN-on-GaN LED, making it better suited to a vertically integrated approach for the manufacture of MR16s.
But that is not to say every single manufacture steps takes place at Soraa. “We’re doing the design of the light chip and manufacturing the LED ourselves. Light chip manufacture and lamp [assembly] are done at sub-cons,” explains Devine. Sales of MR 16s then occur through web sites, and also via a network of distributors in Europe, Asia, Australia and New Zealand.
Making substrates To try to reduce the cost of these lamps, Soraa is developing a novel ammonothermal process to make its own substrates. “We have a team working on bulk GaN development, because
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www.compoundsemiconductor.net August / September 2013
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