industry LEDs
growth substrate is removed and the active epilayers are bonded to a carrier. Absorption of light in the substrate is negated by inserting, via deposition, a highly reflecting mirror between the epilayer and carrier. And in addition, the ThinFilm LED features: A thinner active region that reduces absorption; superior light extraction, thanks to surface roughening or the introduction of microprisms; and no shadowing, because the current is directed away from the bondpad.
Our development of ThinFilm LEDs, which began in 1998, has had a tremendous impact on the efficacy of red emitters. Although the technology took some time to master, by 2010 we were able to produce 140 lm/W LEDs.
The high efficiency of our ThinFilm LEDs goes hand-in- hand with other strengths. They are also scalable. This means that, in theory, all chip sizes can be made by similar methods, possess similar characteristics, and deliver a level of performance that just depends on the dimensions of the device.
In addition, these surface-emitting chips deliver a highly desirable Lambertian beam pattern, and can be manufactured in high volumes using cost-efficient design and manufacturing processes.
These results were obtained some time ago, and we believe that our latest generation of ThinFilm die can increase the output power of bare and packaged die by 30-50 percent and 10-30 percent, respectively.
Towards new highs
During the last few years we have made further strides in terms of efficiency, cost and reliability. This has been accomplished by carrying out a detailed investigation of loss mechanisms and introducing new designs to combat these losses.
One aspect of the LED that has been improved is a reduction in its internal absorption. This pays dividends even if light has to travel ten times through the die before escaping the semiconductor. Absorption has been trimmed by increasing the bandgap and adjusting the doping in some layers.
These steps could also drive up operating voltage and ohmic resistance, so to address this we have improved the conductivity in certain layers.
Another of our improvements is optimisation of contact design, which helps to lower the operating voltage. With more current paths spaced closer together, the electrical current can be evenly distributed across the entire chip. Images of 1mm2
die reveal the dense
arrangement of these metals’ current paths, and strong electrical performance despite adjusted doping levels.
Figure 2: A substantial proportion of the light generated in a conventional LED,which is built on an absorbing substrate,is trapped within the chip (top).Light extraction improves dramatically with a ThinFilm LED architecture incorporating a metal mirror between the active epilayers and carrier (bottom).In this superior device,the current path, depicted by the blue line,is directed away from the bondpad to avoid
shadowing.The light path (shown in red) is initially toward the substrate,but changes direction thanks to reflection by the underlying
mirror.Light is redirected toward the chip surface,where it can leave the semiconductor die
Figure 3: The historic efficiency of red LED brightness.Blue dots indicate volume emitters on an absorbing GaAs substrate and orange triangles
indicate the performance of Osram’s ThinFilm LED since 1999
March 2012
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