industry LEDs
LEDs are now employed for a wide variety of applications,from solid-state lighting to projectors and green house illumination
infrared LEDs – have been overshadowed by their blue cousins in recent years, their development stretches back far further, to the 1960s. Efforts in this direction are tremendously important, because a vast number of applications require yellow, red or hyper-red illumination: Projectors; colour-mixing systems for warm white illumination; lighting sources for green houses that marry hyper-red and blue light; and closed circuit TV, adaptive cruise control for cars and light curtains in elevators that require high-brightness infrared illumination. Despite common perceptions to the contrary, it is clear that the solid-state lighting requirements of today far transcend ‘just white light’.
At Osram Opto Semiconductors, which is based in Regensburg, Germany, we have recently achieved a significant technological breakthrough that will help in this regard, and one that merits a ‘red alert’: The first red LED with an efficacy exceeding 200 lm/W. This new benchmark for the red LED is a fruit of 12 years of diligent effort associated with our development of thinfilm technology.
A little history
The most widely used material for developing long- wavelength visible LEDs is AlInGaP, which can span 560 nm to 660 nm. The first devices made with this quaternary were homojunction LEDs incorporating a simple pn junction. However, over time this device has evolved from a lab curiosity to a high-performance, commercial product (see Figure 1).
The introduction of mature processes has driven up yield; and efficiency has rocketed, thanks to the introduction of more sophisticated device architectures incorporating carrier capture in quantum wells.
Growth of LED epistructures by MOCVD on high-quality substrates can routinely produce longer-wavelength emitters with internal efficiencies exceeding 90 percent. But the high refractive index of the AlInGaP LED traps most of the light: Only 4 percent leaves the chip directly and can be used for illumination; with the remaining 96 percent either re-absorbed by the material, or reflected at the interface between chip and air, before eventually being absorbed by the device (see Figure 2).
These high levels of absorption held back the performance of red LEDs. Devices with a peak wavelength of 615 nm, for example, produce a peak efficiency of just 40 lm/W (see Figure 3).
To stop wasting so many photons, we have pioneered and developed ThinFilm technology (see Figure 2). With this approach, in contrast to a conventional LED, the
Figure 1: The performance of red,yellow,green and blue LEDs has come on in leaps and bounds over the last 50
year.Figure adpted from Semiconductors and Semimetals 48 48 (Publisher: Elsevier/Academic Press)
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www.compoundsemiconductor.net March 2012
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