News Article
New highs for the LED
Soraa claims that its LEDs set new benchmarks for generating and extracting light
A scanning electron microscope image of the triangular volumetric flip-chip device produced by Soraa. This LED has an extraction efficiency of 90 percent and a peak internal quantum efficiency of 95 percent.
Soraa claims to have broken the record for LED wall-plug efficiency for high current densities and temperatures found within a lighting fixture.
The latest device has the "˜triangular volumetric' design of its predecessors, but with the notable refinement of a flip-chip architecture that features both contacts on the same side of the device.
"We show that our improved design is superior across the line "“ extraction, epitaxy, electrical efficiency "“ and thus better demonstrates the extremely high potential of GaN-on-GaN technology," remarks Soraa's Christophe Hurni.
The West-coast outfit uses HVPE-grown GaN substrates as a foundation for its LEDs. According to Hurni, this platform provides many advantages over sapphire, silicon and SiC, including: better material quality, thanks to low-dislocation-density substrates; and better light extraction for high-power-density LEDs.
The latest triangular, volumetric LEDs emit at around 415 nm, have 400 µm-long sides, and have n- and p-type contacts on the bottom of the structure. Wall plug efficiency peaks at 84 percent at 25 °C, and is 70 percent at 100 A cm-2 "“ and this level of performance is maintained at 85 °C.
Delivering high performance at this elevated temperature is crucial, argues Hurni. "Real-world lighting systems heat up during operation, and even with good heat-sinking, a junction temperature of 85 °C or above is common."
The team have evaluated all the factors that influence the wall-plug efficiency "“ it is the product of the package efficiency, extraction efficiency and internal quantum efficiency.
The packaged efficiency, which is the ratio of photons emitted by the chip to those escaping the test package, is 94 percent, according to ray-tracing software. The package has not been optimised for the die, but there is actually little benefit in doing so, according to Hurni: "In practice, package efficiency only matters for phosphor-converted white light."
Efforts at Soraa have shown that extraction efficiency "“ the ratio of photons escaping the device to those radiated by the active region "“ is limited by thin-film LED architectures. A combination of surface roughness and chip shaping is able to increase the extraction efficiency of light of all trajectories, with advanced modelling by the team indicating an improvement of 10 percent over thin-film device structures. Comparisons of values from the light extraction model and experimental results on a series of chips suggest that the latest device has an extraction efficiency of 90 percent.
The third factor governing the wall-plug efficiency is the internal quantum efficiency, and this is influenced by defects, active region design, current-density-induced droop and thermal effects. Using a native substrate helps to reduce some of these loss mechanisms, because the epilayers have fewer defects and there is greater freedom in the design of the structure.
Measurements indicate that the internal quantum efficiency peaks at 95 percent at 25 °C and 92 percent at 85 °C. Droop is very low, with the internal quantum efficiency still 85 percent at 85 °C and a current density of 100 A cm-2, a value that is representative of realistic operating conditions.
The team has applied the well-known ABC model to plots of internal quantum efficiency as a function of current density. These simulations, which include phase-space filling effects, confirm the view held by those at Soraa that Auger scattering is the most plausible cause of droop.
