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News

25 September 2007

 

LED makers report progress at ICNS

Last week’s 7th International Congress of Nitride Semiconductors (ICNS-7) in Las Vegas, NV, USA attracted over 930 delegates (a big increase on the 700 at 2005’s ICNS-6 in Bremen, Germany). Apart from the opening plenary talk by Klaus Ploog (director of the Paul Drude Institute for Solid State Electronics in Berlin until retiring in 2006), due to the more than 220 presentations, one of the few sessions not held in parallel was a special LED session ‘High Efficiency Solid State Lighting: Solutions for Global Warming’ featuring speakers from LED manufacturers Philips Lumileds of San Jose, CA, USA, Cree Inc of Durham, NC, USA, Korea’s Seoul Semiconductor Co Ltd and Japan’s Stanley Electric Company Ltd, as well as Rensselaer Polytechnic Institute Lighting Research Center in Troy, NY, USA.

Lumileds’ executive VP of R&D Frank Steranka reviewed how InGaN-based white LED technology development in 2007 is currently ‘on track’ according to the US Department of Energy’s 2002 Solid-State Lighting LED roadmap. This projected the development of luminous efficacy (in lumens per Watt) from 25 in 2002 to 75 in 2007, 150 in 2012 and 200 in 2020, along with lifetime developing from 20,000 hours in 2002 and 2007 to 100,000 hours in 2012 and 2020.

Steranka highlighted that, in 2007, several companies have products currently on the market at the requisite 75lm/W. A 1000lm source can hence be made without active cooling (indeed, in early September Cree demonstrated 1050lm from a single-die cool-white R&D LED with a luminous efficacy of 72lm/W driven at a current of 4A). However, large heat-sinks are still needed for passive cooling above 75lm/W, Steranka adds.

This January, Lumileds reported record luminous efficacy of 115lm/W for a single 1mm x 1mm cool-white chip in R&D driven at a current of 350mA (together with 131.5lm/W for a 2mm x 2mm chip, 138.6lm/W for a 3mm x 3mm chip, and 142.4lm/W for a 4mm x 4mm chip). Most recently, in mid-September Cree reported 129lm/W for packaged cool-white LEDs in R&D driven at 350mA with a light output of 135.7lm.

For the 2012 target of doubling efficacy of production LEDs from 75lm/W to 150lm/W, a challenge is the need to boost the drive current from 350mA to 2A (i.e. for a luminous flux of 2000lm). However, Steranka says that it will also be necessary to double the LED’s internal quantum efficiency at a current density of 250A/cm² from today’s 45% to about 90%.

A major problem in white power LEDs is the ‘droop’ in efficacy by at least 50% as the drive current density is increased from 100mA to 1000mA. However, in February Lumileds claimed that it has overcome the problem, although it did not give details of the enabling epitaxial technology at the time. But in a ‘late news’ paper in a separate session at ICNS, Nathan Gardner, section manager III-Nitride Epitaxy at Lumileds, previewed work to be published in Applied Physics Letters in the next few weeks on droop-free c-plane GaN LEDs grown by MOCVD.

Gardner attributes the normal droop in external quantum efficiency with increasing drive current in multi-quantum-well (MQW)-based LEDs to be due to non-radiative Auger recombination. This is high in InGaN LEDs, which have carrier densities of over 10^18cm-3, Gardner adds.

A possible solution is to use a larger active region. However, Garner points out that adding quantum wells does not improve the internal quantum efficiency, as holes get stuck at the p-doped side of the QW stack, so only a few quantum wells emit photons.

Lumileds’ solution is to instead use a double heterostructure (DH), which can have an electron-hole recombination region of over 6nm, compared to just 1nm for an MQW-based LED. The result is that, although quantum efficiency is lower, its peak is shifted to higher current density. So, for example for an encapsulated flip-chip LED with a 1mm x 1mm chip emitting at a wavelength of 444nm, a 9nm DH LED has higher quantum efficiency above a current density of 100A/cm² than an MQW LED with two 2.5nm quantum wells.

Meanwhile, following Steranka in the special LED session, Cree’s co-founder and director of advanced optoelectronics John Edmond gave an overview of the firm’s progress through its 1mm x 1mm Gen 2 chip (which has a luminous efficacy of 104lm/W at 350mA) to its bigger 2mm x 2mm Gen 3 chip (which has luminous efficacy of 133lm/W for cool-white LEDs at 350mA). Cree’s current production LEDs range in luminous flux up to 100-120lm (with a correlated color temperature of 6000K, a forward voltage of 3.25V and a drive current of 350mA). Edmonds added that, to achieve 150lm/W, it would be necessary to reduce the forward voltage to 3V in order to raise external quantum efficiency.

Seoul Semiconductor’s president/CEO Chung H Lee outlined the firm’s cool-white LED roadmap, including its progress from typical (rather than maximum) efficacy of 20lm/W in 2005 through 40lm/W in 2006 to 48lm/W in Q2/2007, and projections of 80lm/W in Q4/2007, 100lm/W in Q2/2008 and 120lm/W in Q4/2008. He added that, further to its existing product with a luminous flux of 200lm, the firm aims to announce a 240lm product this year. Most recently, in mid-September, the firm achieved a luminous flux of 420lm from a single chip in R&D driven at 600mA.

In an invited talk in a separate session on ‘Solid State Lighting and LEDs’, Nichia's Yukio Narukawa highlighted the firm’s NS6W083A product for lighting applications. Its typical luminous flux at a drive current of 300mA is 80lm. However, Narukawa says that this is ‘not enough’ for general lighting, and that this must be improved to more than 100lm. Nevertheless, Narukawa adds that, in R&D, the firm’s ‘HP-White’ high-power white LED technology produces a luminous flux of 145lm with an efficacy of 134lm/W at 350mA, so the production target of 100lm would appear to be within reach.

See related items:

Cree reports record efficacy for power LEDs

Cree achieves 1000 lumens from single-die LED

Analysts forecast seven-year wait before LEDs penetrate home lighting market

Osram sampling 1000lm OSTAR LED for general lighting

Lumileds solves LED efficacy ‘droop’, supporting adoption of 1000mA drive current

Philips Lumileds claims white LED record of 115lm/Watt at 350mA

Nichia develops record 150lm/W white LED

Seoul Semiconductor claims record with 100 lm/W @ 350 mA P4 LED

Search: InGaN White LED

Visit Cree: www.cree.com

Visit Lumileds: www.philipslumileds.com

Visit Nichia: www.nichia.com

Visit Seoul Semiconductor: www.seoulsemicon.com