Betting on GaN-on-GaN for efficient white light
The luminous efficacy of radiation (LER) in lumens per watt is defined as the ratio of the photometrical flux (luminous flux of visible light) and the radiometrical flux of the light source (radiant power of the total emitted spectrum).
The LER indicates how much of the light output we humans perceive (as white light or its visible components) versus the total energy put into emitting the light. Pier gave some examples of light sources, a tungsten light bulb typically delivers 15lm/W, while the sun spectrum including the non-visible bands is only 93lm/W, but the LER of the sun spectrum truncated to its 400-700nm visible bands is at 251lm/W for natural white light at a 5800K colour temperature.
Now, according to Pier, a combination of two or more individual spectral lines should yield optimum efficacy. Nowadays, most “white LEDs” are in fact Blue LEDs covered with a mixture of phosphors. The blue light (of higher energy) pumps or excites the phosphors that re-emit light at different set spectrum bands, depending on their chemistry.
“The LER depends very much of the spectrum bands being used and is relatively independent from the drive current or the temperature” explained Pier, taking as an example the Luxeon M white LED, a 5700K device capable of delivering 316lm/W. Another device put forward as being the state of the art was Philips Lumiled’s Luxeon Altilon core, combining four dies into a single 3.5×3.76 chip package and exhibiting a luminance of 75Mcd/m2, a flux of 1000lm and a LER of over 200lm/W. This is to compare with halogen lamps with a 30Mcd/m2 luminance, high intensity discharge (HID) lamps at 60Mcd/m2 and the sun 1600Mcd/m2. The colour rendering index (CRI) achieved by this product is considered very good at 93.
GaN-on-GaN
Today, about 95 percent of GaN LEDs are manufactured on sapphire wafers but the scope of using cheaper and more widely available Si wafers to scale up the production of GaN-on-Si devices is an attractive one.
While a recent forecast from market analysis firm IHS Inc expects GaN-on-Silicon LEDs to increase their market share from 1% today to 40 percent by 2020 (mostly taking market share from both sapphire and silicon carbide wafers), Californian startup company Soraa is betting on GaN-on-GaN for high efficiency white LEDs.
Looking at system efficiency, Soraa’s Principal Scientist Aurélien David started with LED efficiency metrics to highlight the inherent limitations of conventional LEDs. Since overall system efficiency results from the combination of internal quantum efficiency (IQE), extraction efficiency (Cex) and package efficiency (PE), one should look at improving all three.
“Growing GaN or InGaN on foreign substrates such as sapphire, SiC or silicon yields epitaxial defects, dislocations which are all detrimental to the internal quantum efficiency (IQE)” noted David, adding that efficiency droop is also a fundamental physical limitation of power LEDs on foreign substrates. Running at high efficiency requires a low current density (under 100A.cm-2), which translates into larger devices.
Comparing crystal growth quality: GaN-on-GaN versus GaN-on-foreign substrates
Then, not all white lights are equal, he explained the audience, partly because of the chosen phosphor mix, missing out on some of the red bands, but also because using a blue LED to pump these phosphors means the overall light output will typically lack the violet and the cyan bands.
“Even being efficient and cheap, Cathode Fluorescent Lamps (CFLs) failed to convince consumers because of their poor light quality”, David remarked before coming up with GaN-on-GaN LEDs as the ultimate solution.
First because growing GaN LEDs on a bulk GaN substrate yields a higher crystal quality (with a dislocation density up to 1000x lower compared to GaN on foreign substrates) which reduces epitaxial constraints and translates into a higher IQE and less droop.
The manufacturing process is also much simpler than for GaN on foreign substrates, skipping the carrier and lift-off steps typically required. Growing devices on bulk GaN also gives Soraa access to all crystal planes for the design of volumetric chips. According to Soraa’s physical models, at around 90%, the extraction efficiency of specially “roughened surface” volumetric chips is well beyond the thin-film limit currently in the range of 82 to 84%.
Soraa’s single volumetric GaN-on-GaN LED, and a fully packaged LED assembly.
What’s more, the devices exhibit excellent power density uniformity, with no current crowding even when driven at ten times the current density of traditional LEDs. This means that GaN-on-GaN LED dies about 15 to 25 times smaller than typical power LED dies can be driven at high power and yet retain a high external quantum efficiency. Soraa stress-tested its LEDs for several thousands of hours at junction temperatures up to 140ºC without noticeable performance shifts. Using the TM-21 convention, the company predicts a lifetime of over 30,000 hours for its products under standard conditions.
Using proprietary Si-based wafer level packaging, Soraa packed up to 36 dies under a mix of red, green and blue phosphors to produce high brightness compact LEDs.
When asked if Soraa could be cost competitive despite its reliance on expensive GaN substrates, David replied with a resounding yes! “The substrate is expensive, but we can drive the LEDs from 15 to 25 times harder, so we get much more light output while using much less substrate”. “The first generation of LED technology (on foreign substrates) is maturing slowly, but GaN-on-GaN LEDs offer a breakthrough in output power per wafer, and we are only at the beginning” he added.
Then back on the white light quality issue, David only had praise for the new devices, with a measured CRI of 95. Because Soraa’s chips emit violet light to pump a proprietary mix of phosphors, the company is able to generate cyan light, as opposed to the cyan gap of common blue-pumped LEDs.
“But the CRI is only part of the story when it comes to quality of light” emphasized David, “It is only a metric for so-called colour fidelity, and a debatable one according to ongoing academic research”.
For better whiteness rendering, a lot of everyday objects (including white shirts, paper, plastic, and even your teeth) contain optical brightening agents that absorb UV and violet light and emit blue, making white look brighter. By emitting violet light, Soraa’s LEDs are able to properly activate the whitening agents and render white objects, which regular white LEDs can’t do.
Comparing light spectrums for different "white lights"
David would not be too specific about the GaN substrates the company uses, except that they are commercially available Hydride Vapor Phase Epitaxy (HVPE) GaN substrates. But he acknowledged that the company is also doing research on the growth of bulk GaN, experimenting with bulk GaN wafers (so fare too small and expensive to be commercially viable).
As for the future of GaN-on-GaN LEDs, David is very optimistic. “Currently, the LED market is still vertical, manufacturers want to do it all by themselves. But substrate price erosion alone will drive further adoption of this technology”.
“Then if in a few years we can make cheap GaN substrates, the benefits will be so compelling for the whole LED industry, that you will ask, why bother do anything else?”
Helping GaN-on-GaN LEDs take the lighting world by storm as David would bet, Soraa is commercializing a slick MR16 Lamp which received the 2013 Red Dot Award for all its innovative features.
Later after his keynote, David was kind enough to demonstrate the lamp against the nearest competing device, and the colour rendering difference was striking. Exit the yellowish tones! Welcome the bright whites.
David is cautious to express the luminous output of Soraa’s products as the centre beam candlepower (CBCP), arguing that communicating in lumens can be deceiving, because the lumens need to be in the centre beam (not outside a specific beam angle) to be considered as useful.
The company says it achieves a high CBCP by combining a high lumens output from a small source size together with good optical design. Here again, centre beam candlepower comparisons should only be made among lamps designed with the same beam angle. With this in mind, David gave us some elements of comparison.
“For instance, you will find that for a 3000K 24 degree lamp, Philips’ state of the art MR16 lamp outputs just about 2000 cd of CBCP at 10 watts (40 watts equivalent, CRI80) while a Soraa product with a CRI of 95 can deliver a CBCP of 2750 cd (11.5 watts, 65 watts equivalent)” David noted in complementary email exchange.
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