TECHNOLOGY CONFERENCE REPORT
at Osram Opto Semiconductors and the MPIE, Germany.
Albercht unveiled details of performance of this team’s latest LEDs. They require just 2.91 V to operate at 350 mA, and when combined with phosphor technology, they produce 104 lm/W of warm-white light with a colour-rendering index of 83. The efficacy increase over last year’s devices is mainly due to a 6 percent fall in output voltage, which has stemmed from improvements to the epitaxial layers and the quality of the quantum wells.
Tak, in contrast, did not disclose the latest results from his group. But he did reveal that the performance of these LEDs, which are grown on
200 mm silicon, is much higher than that of the devices announced at the 2011 ICNS meeting. Back then, Samsung’s researchers extracted 580 mW from a 1 mm by 1mm LED grown on 4-inch silicon. The device was driven at 350 mA under a forward voltage of 3.2 V.
Highlights of Tak’s talk included insights in the difficulties of growing GaN-on- silicon LEDs, and how to overcome them. One of the biggest challenges stems from the significant lattice and thermal mismatches between the substrate and epitaxial layers. These differences can cause the wafer to bow, or even crack.
A common approach to addressing this is to turn to a buffer structure that fine-tunes the stresses and strains in the epistructure. According to Tak, with
this approach Samsung’s engineers can realise a bow of less than 30 µm. They have found that in order to realise a very low bow, it is critical to start with a silicon substrate with a low degree of warp. One downside of the buffer structure is that in order for it to be effective, it can have to be quite thick, and that adds substantially to the time and cost of producing an LED epiwafer. But Samsung’s engineers are addressing this: Their latest LED epistructures are about 5 µm-thick, compared with 8 µm of growth for the previous generation of devices. “They have a similar processing time to sapphire,” said Tak.
Enhancing colour quality It is possible to build white light sources with a high colour quality by switching from a phosphor-based approach to the mixing of the output of red, green and blue sources. Unfortunately, this form of lighting system, which can be used to make projection sources, is currently held back by the efficacy of the green LED, which is far behind that of its blue and red counterparts.
This weakness is known as the green gap. While GaN LEDs can be very efficient in the blue, and GaAs-based devices can produce high efficiencies in the red, neither maintains their performance when the composition in the quantum wells is adjusted so that it emits in the green (or the yellow). The arsenide material system will never emit efficiently in the green, because carrier confinement is so weak in this spectral range, but it is possible that nitride LEDs
could emit more efficiently at longer wavelengths.
One of the weaknesses of GaN-based devices is that they are plagued by strong internal electric fields, but this can be mitigated by turning to thinner quantum wells that increase electron-hole overlap and therefore enhance carrier recombination. The performance of these LEDs is also hampered by deterioration to material quality that occurs when indium content is increased in the well so that its emission is pushed to longer wavelengths.
Encouraging news coming out of ICNS is that a novel active region can combat the green gap. Speaking on behalf of Toshiba’s Corporate R&D Centre, Rei Hashimoto revealed that a modified active region – it contains 3 nm-thick InGaN quantum wells capped with a 1 nm-thick AlGaN barrier, prior to the deposition of a 10 nm-thick InGaN barrier – enables the fabrication of very efficient yellow-emitting LEDs. Driven at 20 mA, these devices produce a peak emission at 570 nm, and emit 8.4 mW at an external quantum efficiency of 19.3 percent. Indium content in the well is estimated to be about 25 percent, and less than 1 percent in the barrier.
Hashimoto argued that one of the key benefits of the new design of active region is an improved surface flatness. He and his co-workers optimised the growth conditions for the quantum wells and barriers, and by selecting the ideal growth temperature, they eliminated indium-rich clusters at the surface.
An alternative approach to forming a solid-state, green-emitting source was outlined by Thomas Lehnhardt from Osram. He revealed the results of a project using a blue LED to pump a green-emitting active region made from InGaN quantum wells. This approach culminated in the construction of a 1 mm by 1 mm device emitting at 535 nm with a wall-plug efficiency of 22 percent. Efficacy for this device is 127 lm/W, and could rise to 138 lm/W by inserting Osram’s latest LEDs, which have a lower forward voltage for the same drive current.
ICNS featured three posters sessions, which were all well attended
Lehnhardt made a strong case for using this form of complex, green-emitting structure in preference to a direct- emitting green LED. He pointed out that using a blue-pump source makes
October 2013
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