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EETE JAN 2014

Forum led europe Betting on GaN-on-GaN for efficient white light By Julien Happich “How eficient is white light?” asked Dr Harald Pier, team leader for EMEA technical solutions at Philips Lumileds, during a round table on white LEDs held at Forum LED Europe in Paris. 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 nonvisible 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 3.5x3.76 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 GaNon 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. 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 Comparing crystal growth quality: GaN-on-GaN versus GaNon foreign substrates, as seen by Soraa. 16 Electronic Engineering Times Europe January 2014 www.electronics-eetimes.com


EETE JAN 2014
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