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Pushing white light efficiency through GaN-on-GaN

Posted: 23 Dec 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Philips Lumileds? GaN-on-GaN? white LED? Soraa? luminous efficacy?

At a recent round table on white LEDs held at Forum LED Europe in Paris, Harald Pier, a team leader for EMEA technical solutions at Philips Lumileds posed a question that will have many in the industry pondering for solutions. He asked: "How efficient is 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 color 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 modern was Philips Lumiled's Luxeon Altilon core, combining four dies into a single 3.5 x 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 color rendering index (CRI) achieved by this product is considered very good at 93.

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 expects GaN-on-Silicon LEDs to increase their market share from one percent 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.

GaN-on-GaN versus GaN-on-foreign substrates

Figure 1: Comparing crystal growth quality: GaN-on-GaN versus GaN-on-foreign substrates.

Looking at system efficiency, Soraa's principal scientist Aurelien 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.cm2), which translates into larger devices.

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