Technical Insight
GaN-on-Si wafers get ready for LED production
Will a new raft of GaN-on-Si materials systems speed the LED industry's switch to silicon, asks Compound Semiconductor.
Group of blue emitting led on a black background
While LED manufacturers usually buy wafers and then perform epitaxy in-house, an alternative manufacturing route is emerging. Recently, a handful of companies has unveiled GaN-on-Si templates that allow manufacturers to grow LED layers directly onto a silicon wafer.
Up and coming players include, Germany-based Azzurro Semiconductors, and US companies, Translucent and Kyma. But why the dash to produce engineered on-silicon templates?
Silicon is cheaper than the most commonly-used wafer, sapphire, but the substrate represents only 5% of the total LED package cost. Clearly no major cost savings here.
However, factor in the opportunity to manufacture on a silicon line, and this is what makes the switch to silicon wafers very worthwhile.
“Most players would be interested in using this technology as by using silicon they could access depreciated CMOS equipment,” explains Pars Mukish, technology analyst at Yole Développement, France. “This is where the real cost savings are.”
But despite the cheaper manufacturing costs, nearly 95% of commercial blue and white LEDs are still grown on sapphire, around 5% made on SiC, and less than 0.1% deposited on silicon wafers. Clearly, industry has yet to be convinced that silicon-based LEDs will make the grade.
Worries over lattice mismatches between GaN and Si layers leading to bowing and cracking of wafers at high epitaxy temperatures as well as poor manufacturing yields and LED performance run deep. But this could soon change.
Earlier this month, Azzurro Semiconductors, Germany, announced that Taiwan-based LED manufacturer, Epistar, had transferred existing GaN LED structures, previously grown on sapphire, to its 150mm GaN-on-Si materials system in just 16 weeks.
This template comprises a 150mm silicon wafer onto which an AlN buffer layer, stain-engineered buffer layer and highly doped n-GaN layers, have been deposited. The LED manufacturer then grows the additional GaN layers, specific to the LED architecture, onto the template.
“The issue is how to produce a buffer structure on the silicon template in a sensible time, and that's where our IP is coming into play,” says Erwin Ysewijn, vice president of marketing and sales asserts. “The template has a 3.5 micron thick buffer layer with an n-GaN termination, which is in effect the same as the LED leads. We call this plug and play; you take your standard crystal structure and plug it into another material.”
And success looks imminent. Both Azzurro and Epistar have confirmed the latest LED performance is on a par with the sapphire-based equivalent, with Ysewijn also highlighting Azzurro's intent to ship 200mm templates next year.
“Our 150mm templates are in production and we are shipping to market,” he says. “The 200mm templates will go into production in the second quarter of next year, and I would say we will be sampling these from the first quarter.”
Azzurro also reports that its strain engineered templates enable 150mm LED epiwafers with a bow value of less than 20 µm, while a 1.5mm by 1.5mm LED chip will have a 125Lm/W output. But what are the actual yields?
Ysewijn remains tight-lipped here, saying yields are 'industry standard'. Meanwhile Shao You Deng, associate vice president of product management at Epistar, categorically states more work needs to be done.
“Yes, the best record of the optical output power is very close to LEDs grown on a sapphire substrate, but we still have many production issues to overcome, including defects, voids, pits, uniformity bowing and cracking,” says Deng. “Yield and cost will trigger production... there is a long road to go before we release a product.”
So if LED makers are still grappling with these issues, will we see the template business model taking off soon? Or will manufacturers keep their production processes - including epitaxial growth of buffer and LED layers – in-house? Mukish from Yole Développement, admits the answer is not yet clear.
“The template business model will help manufacturers to invest a lot less in additional equipment, so it would be a lot more economical to grow [LED structures] on this technology,” he says. “If come the mid-term this technology is performing, then yes, they will buy the templates.”
