Cost has been a major barrier in keeping people from buying energy-efficient LED lightbulbs. While they last much longer than $1 incandescent bulbs or $4 compact fluorescents, their sticker-price is daunting: a 60-watt equivalent LED bulb costs $15 to $25.
Now one of the world’s largest LED makers, Osram Opto Semiconductors, says it has perfected a technique that could significantly cut the production cost of LEDs.
White LEDs are typically made by coating blue gallium-nitride LEDs with yellow phosphors. Manufacturers normally grow the gallium-nitride in thin layers on top of costly sapphire substrates. Osram is making the devices on silicon substrates instead. Silicon substrates cost a third as much as sapphire and could get even cheaper, since they’re made in larger pieces.
The company says its silicon-based white LEDs produce 127 lumens for each watt of power, with a power efficiency of 58 percent, comparable to state-of-the-art commercial LEDs grown on sapphire. Peter Stauss, a project manager at Osram, says researchers are now testing and optimizing the devices, and expect to start selling them in the next two to three years.
Osram joins a few other companies in the race to bring cheaper gallium-nitride-on-silicon LEDs to market. China’s Lattice Power claims to have already started commercial production using the same technique. Last month, California startup Bridgelux announced that it has teamed up with Toshiba to make LEDs on silicon. Meanwhile, U.K.-based Plessey Semiconductors plans to produce gallium-nitride-on-silicon LEDs by the end of this year using technology acquired from Cambridge University spinoff CamGaN. Philips and Samsung are also said to be pursuing the silicon approach.
Osram has not said how much cheaper its new LEDs will be, but Bridgelux and Plessey both claim that the silicon approach could cut LED production costs by 75 percent or more. Bridgelux also predicts that its process could bring the cost of a 75-watt equivalent LED lightbulb, which now costs $40, down to under $5.
Compared to sapphire, it’s easier and cheaper to make large-diameter silicon wafers, which cuts cost. Another appealing reason to switch to silicon is to take advantage of existing machines that are used to fabricate silicon computer chips. Bridgelux has already demonstrated devices grown on 20-centimeter-wide silicon wafers.
The challenge with growing gallium-nitride on silicon is that the two materials expand and contract at different rates. LEDs are made at high temperatures, and when the materials are cooled, the light-emitting gallium-nitride layer cracks because of tension with the silicon underneath. To get around this problem, Osram and others grow additional thin films of materials such as aluminum gallium-nitride around the gallium-nitride, balancing out the tension produced during cooling.
While many LED makers are opting for silicon, California startup Soraa is using gallium-nitride substrates. This also reduces the mismatch in the crystal structure between the two layers, says Soraa CTO Michael Krames, resulting in a material with 1,000 times fewer defects than that grown on silicon. The resulting LEDs are more expensive than silicon-backed ones, but they are also 10 times brighter, which helps make up for the extra cost. “It’s really the performance of the LED that drives the overall cost of the [lightbulb],” Krames says. “Also, the substrate is a small part of the overall product cost.”
Right now though, Stauss says, the cost advantage of silicon substrates is more favorable to LED manufacturers. “Gallium-nitride substrates might be a choice when they are much cheaper, but today there’s no real opportunity in using them,” he says.
Steve Denbaars, a professor of materials and electrical and computer engineering at the University of California, Santa Barbara, and a cofounder of Soraa, believes that both gallium-nitride and silicon substrates have a fair shot at replacing sapphire. While gallium-nitride might have a performance edge as well, he says, “there is a chance silicon will beat sapphire solely because you have a lot of silicon [fabrication facilities] sitting around the world.”
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