Technical Insight
GaN-on-silicon LEDs are catching incumbents
Engineers at Samsung and Osram claim that LEDs grown on silicon can deliver a performance that is very close to that of today's emitters grown on sapphire
Blue-emitting LEDs grown on silicon are now delivering efficiencies that are almost equal to that of the incumbent devices, which are formed on sapphire.
“The performance gap is receding fast, and is less than 10 percent,” claimed Youngjo Tak from Samsung Advanced Institute of Technology, Korea, in a talk to delegates at the International Conference on Nitride Semiconductors (ICNS) in Washington.
A similar view was offered by Martin Albrecht from the Leibniz Institute for Crystal Growth, who has been working with engineers at Osram Opto Semiconductors and the MPIE, Germany: “GaN-on-silicon LEDs have comparable efficiency to GaN-on-sapphire.”
Speaking in a session devoted to GaN-on-silicon LEDs on Monday 26 August, Tak refused to disclose the results of Samsung's best devices, which are grown on 200 mm silicon. But he did reveal that the performance of these LEDs 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 and driven at 350 mA under a forward voltage of 3.2 V.
Albercht was more forthcoming, providing details of performance of this team's latest LEDs. They requires just 2.91 V to operate at 350 mA, and when combined with phosphor technology 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.
GaN-on-silicon LEDs are attracting tremendous attention within the compound semiconductor industry, due to their promise to slash the cost of solid-state lighting.
“To replace the fluorescent lamp, we must reduce the cost [of LEDs] more and more,” proclaimed Tak, who argued there are two ways to do this: trim manufacturing costs or improve performance, such as by increasing efficiency.
Substrates have a big role to play in both approaches. Tak said that although the cost-per-lumen for LEDs grown on sapphire is falling, it will plateau in 2015, and alternative platforms are needed to make light-emitting chips more affordable.
Switching from 4-inch or 6-inch sapphire to 200 mm silicon can cut costs by 31 percent and 48 percent, respectively. That's partly because the sapphire substrates are not that cheap – according to Tak, it's typically $500 for 6-inch sapphire and $1500 for an 8-inch version – and also because LEDs grown on silicon can be processed in depreciated silicon fabs.
Tak dismissed the use of GaN substrates for cutting LED costs. Although Soraa has shown that a native substrate can yield very efficient LEDs operating at incredibly high current densities, so far fewer chips are needed to generate an equivalent power output; Tak argued that the cost of the GaN platform is too expensive. It has fallen over the last few years, but it is still around $1300 for a 2-inch substrate.
Growing an LED on silicon is not easy, because there are lattice and thermal mismatches between the substrate and epitaxial layers that can cause the wafer to bow, or even crack. It is also not possible to grow GaN directly onto silicon, due to meltback, so AlN must be deposited first.
By controlling the shape of the wafer with an AlGaN-based buffer structure, Samsung's engineers can realise a bow of less than 30 microns. 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.
The thicknesses of Samsung’s latest LED epistructures are about 5 microns, compared with 8 microns for the previous generation of devices. “They have a similar processing time to sapphire,” said Tak.
With a growth time now comparable to that for the incumbents, a performing that is just a fraction behind them, and lower manufacturing costs, GaN-on-silicon LEDs look to have a bright future.
