Technical Insight
UV LEDs: will substrates slow growth?
As long wavelength ultraviolet LEDs penetrate commercial curing markets, shorter wavelength counterparts prepare to take on water sanitation, wireless handset disinfection and more. But will substrate and epitaxy issues stymie progress, asks Compound Semiconductor.
In March this year, France-based analyst firm Yole Développement forecast the ultraviolet LED market would mushroom from today's $45m to nearly $270m in the next four years.
Growth stems from UV curing system makers replacing incumbent mercury vapour lamps with smaller and longer-lasting long-wavelength UV-A LEDs. But throw in wild-card applications using shorter wavelength UV-B and UV-C LEDs - nail gel curing, miniature counterfeit money detectors and more - and the UV LED market could stretch further.
“We estimate that if new UV LED applications continue emerging, the associated business could represent nearly $30m by 2017, increasing the overall UV LED market size to nearly $300m,” says Mukish.
And right now, the industry looks buoyant. For the UV-A end of the market, new competition from China, Japan and Taiwan is stirring the likes of Nichia, Japan, SemiLeds, US, and Seoul Semiconductor, Korea, prompting lower prices and a wider adoption of the technology.
Crucially, the performance of these UV-A LEDs, emitting at 400 to 315nm, is now sufficient to rival incumbent technologies, largely thanks to relatively straight-forward manufacturing. These devices are fabricated by growing InGaN-based epilayers on sapphire substrates, with the same MOCVD techniques used in visible LED markets.
“We now think several players from the visible LED industry could move into this UV market, as over-capacity has impacted their margins,” adds Mukish. “This is not a huge market but the margins are higher.”
However, the shorter wavelength market for UV-B LEDs emitting at 315 to 280nm and UV-C LEDs emitting at 280 to 100nm is a different breed of animal. Fewer market players exist; US-based Sensor Electronic Technology Inc (SETi) leads the way with Crystal IS and HexaTech, both based in the US, emerging from development phases and eager to commercialise.
SETi opened a high volume manufacturing plant, last year and recently unveiled UV-B and UV-C LEDs targeting forensic analysis, water disinfection and other applications. Meanwhile, Crystal-IS - acquired by Japanese industrial giant Asahi Kasei last year – and HexaTech have both claimed record performances for UV-C LEDs targeting water and surface disinfection applications.
But despite the progress, crystal growth and epitaxy issues still need to be ironed out before shorter wavelength UV LEDs can achieve true commercial success. Devices can be manufactured by either depositing AlN layers onto a sapphire substrate or bulk AlN wafer. And herein lie the problems.
For starters, growing bulk AlN crystals isn't easy. Crystal IS, HexaTech, Nitride Solutions, US, and Germany-based company CrystAL-N have made great strides in growing the substrates, yielding high performance LEDs, but still two-inch wafers are not widely available and remain much more expensive than sapphire substrates.
“The performance of LEDs grown on a AlN substrate is better but the supply chain for bulk AlN substrates is not so evolved. Only two to three companies can provide these substrates on the open market and so costs haven't decreased,” says Mukish.
“This was also the case for GaN bulk substrates used in visible LEDs, and in the end LED manufacturers chose sapphire substrates,” he adds. “[Future UV market developments] depend on how suppliers of AlN substrates develop the size of substrates and also reduce costs.”
Substrate issues aside, these UV LED players must also solve epitaxy problems. Issues relating to, for example, strain and doping need to be fully tackled, and as UV-A LED manufacturers race ahead with well-used MOCVD techniques, deeper UV LED players are left to develop new methods or grapple with less established deposition technologies.
As Mukish points out: “Many LED manufacturers use HVPE to fabricate shorter wavelength chips, and this [deposition method] is not as advanced.”
Still progress has been made. HexaTech joined forces with Japan-based HVPE developer, Tokuyama, last year, producing UV-C LEDs described as having record-setting reliability and lifetimes. And SETi has unveiled deeper AlN-on sapphire UV LEDs grown via migration enhanced MOCVD and migration enhanced lateral epitaxial overgrowth.
“The UV LED market is not so big, which means there are not so many players to collaborate and really develop UV-B and UV-C chips,” says Mukish. “So we have seen Crystal IS and Hexa-Tech vertically integrate the supply chain to provide their own device. These companies are busy working on these issues.”
