technology  LEDs


Pyramids pave the way to monolithic white LEDs


There is now little headroom left to improve the efficacy of the conventional white LED.But major gains in efficiency are possible by replacing the traditional combination of a blue chip and yellow phosphor with a monolithic,multiple wavelength source.One great way to do this is to work with nano-scale pyramids, say Samsung’s Taek Kim, Joosung Kim and Moonseung Yang.


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EDs will utterly dominate the display backlighting market in the next year or so. This has spurred LED chip manufacturers to hunt for new, lucrative applications, and by far the biggest of these is general illumination. Producers of LEDs are already starting to tap into this market, but they will only make significant inroads when they can substantially cut the cost per lumen of this emitter, a goal that can be fulfilled through increases in luminous efficacy.


This revolution in lighting will be driven by white LEDs, which combine a blue-emitting chip with a phosphor that is pumped with blue light and emits yellow. In this device, the luminous efficacy of the white emission resulting from colour mixing is limited – even if the efficiency of the blue LED and the phosphor are very high, there is still an unavoidable energy loss due to the difference between the energy of the photons used to pump the phosphor and the energy of those emitted. Using a


32 www.compoundsemiconductor.net November/December 2011


phosphor also has a practical downside – coating this material onto the LED chip adds to manufacturing costs.


The ultimate approach is a monolithic white LED that emits multiple spectra, such as blue and yellow or the three primary colours. However, fabricating such a device is challenging due to the plummeting efficiencies of InGaN blue LEDs at longer wavelengths and the rapid fall-off in the efficiency of red InGaP LEDs at shorter wavelengths. Acting together, these weaknesses lead to an absence of efficient green and yellow emitters – the so-called ‘green-gap’ – and they hamper the realisation of a monolithic white LED.


At Samsung Advanced Institute of Technology (SAIT) we have being developing a novel, alternative LED architecture for adressing the green gap and enabling the fabrication of monolithic white LEDs. Our technology is based on nano-scale pyramids.


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