TECHNOLOGY LEDs
indium richness is to lower the growth temperature, but this leads to a hike in defect density. Although a more elevated temperature reduces the defect density, thermal instability of InN makes it more challenging to form InGaN films with a higher indium composition.
To try and combine high growth temperatures with high indium-content, we investigated and optimised the growth rate of InGaN. This approach paid dividends, as we found that by increasing the growth rate by a factor of ten, the same InGaN composition could be obtained at a higher growth temperature. This allowed us to grow high-indium- content InGaN quantum wells with a high crystal quality at the desirable, higher growth temperature.
Another tweak that we made to the conventional active region was to replace the GaN barriers with those made from InGaN. This switch decreased the defect density in the active region, thanks to a reduction in lattice mismatch between the well and barrier. By improving crystal quality − while not changing the QCSE and having little, if any, impact on the internal electric field − we were able to increase photoluminescence decay time and grow an active region with five quantum wells that did not exhibit a decline in photoluminescence intensity [4].
The result of all of our refinements to the active region is a tremendous increase in the efficiency of GaN-based LEDs emitting in the green-yellow, yellow and amber.
Driven at 20 mA, our 570 nm LED that emits in the yellow delivers 8.4 mW at an external quantum efficiency of 19.3 percent [5]. Meanwhile, at the same drive current, our 560 nm green-yellow LED and 584 nm amber LED deliver
Figure 4. Increasing the growth rate holds the key to realising a higher indium composition at a high growth temperature
11.2 mW and 4.9 mW at external quantum efficiencies of 25.5 percent and 11.4 percent, respectively [6].
These results show that our approach goes a long way to addressing the ‘green gap’ and the issues associated with the construction of full-LED lighting systems. But we still have much more work to do.
Another of our goals is to improve the performance of our devices as the current through them is cranked up from 20 mA to the levels required in lighting systems. At present, our long-wavelength LEDs suffer from severe droop and a large blue-shift in emission wavelength with current. These weaknesses will not need to be addressed to reach an external quantum efficiency of 50 percent – that should be possible by improving InGaN crystal quality – but the QCSE and droop will have to be addressed if these
long-wavelength LEDs are to catch the level of performance of their blue-emitting cousins.
If full-colour lighting systems are to appeal to the public, they will also have to be affordable. To help to trim costs, we can switch the substrate used to make our LEDs from sapphire to silicon. This move will be relatively easy to implement, because today we have replaced the sapphire substrate with 200 mm silicon for the mass-production of blue LEDs.
This switch, plus our approach to addressing the green gap, could usher in an era of solid-state lighting that is not only highly efficient and long-lasting, but capable of delivering high-quality, adjustable white-light.
© 2014 Angel Business Communications. Permission required.
References 1. U.S. Department of Energy, “Multi-Year Program Plan” April (2013) 2. D.Z. Feezell J. Disp.Technol. 9 190 (2013) 3. T. Shioda
et.al. Phy. Status Solidi A. 209, 473 (2012). 4. J. Hwang
et.al. Proceedings of SPIE - The International Society for Optical Engineering. 8625, 86251G (2013). 5. R. Hashimoto
et.al. Proceedings for ICNS-2013 to be published. 6. S. Saito
et.al. Appl. Phys. Express 6 (2013) 111004
March 2014
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