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TECHNOLOGY LEDs


Increase the aluminium content in this quaternary and emission can reach the yellow or green, but this increase in emission wavelength comes at the expense of efficacy, which plummets due to a tremendous hike in the density of non-radiative centres.


Moving to shorter wavelengths also reduces the confinement of electrons and holes in the quantum wells, which in turn limits efficiency and increases the decline in device performance with rising temperature.


The alternative approach is to stretch the emission of a GaN-based blue LED to longer wavelengths. The starting point is very promising: A blue LED combines an external quantum efficiency of over 75 percent with excellent thermal stability, a narrow full-width half maximum and a lifetime in excess of 40,000 hours. These attributes are not just advantageous for a full-LED, white- lighting system – they are also valued for full-colour projection.


One attractive attribute of InGaN LEDs is that they can, in theory, produce all the colours needed for a high-efficiency, full-LED white-lighting system. All that is needed to reach longer wavelengths is to increase the indium content. If a white lighting system were made in that way, it would be easy to construct, because all the coloured LEDs would exhibit similar levels of reliability, thermal stability and operating voltage.


These benefits have helped to spur the development of longer-wavelength, high-efficiency LEDs, such as those emitting in the green, yellow and amber. Progress has not been easy, however, due to two obstacles: the quantum- confined Stark effect (QCSE) and crystal degradation.


The QCSE, which is induced by a strong internal piezoelectric field in highly


strained InGaN quantum wells, leads to a pulling apart of electrons and holes in the quantum well. This separation of opposing charge carriers is more severe at higher indium content, and accounts for a decline in radiative efficiency with increasing wavelength.


It is possible to eliminate these internal fields by growing an LED on a non-polar substrate, while a semi-polar platform


Figure 1. The ‘green-gap’ problem is a decline in LED efficiency towards green wavelengths. Toshiba has improved efficiency in this spectral range with a novel active region formed with a faster growth rate


March 2014 www.compoundsemiconductor.net 45


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