Technical Insight
GaN growth: an extra inch
GaN crystal newcomer, Fairfield Crystal Technology, is ready to take substrates to greater lengths, reports Compound Semiconductor.
When it comes to growing high quality, decent-sized GaN crystals, US government is keen to fund companies that are making it happen.
In the last two years, vertically integrated developer of GaN-on-GaN LEDs, Soraa has, won more than $5 million dollars from DoE agency, Advanced Research Projects Agency-Energy (ARPA-E), to develop large area, low cost GaN substrates. Meanwhile GaN crystal developer, Kyma, recently bagged some $3 million to advance its technology for manufacturing substrates, shortly after releasing commercially available two inch GaN substrates.
But now, in the same wave of funding, a third company, Fairfield Crystal Technology, has also attracted $1.5 million to develop a new technique to speed up the growth of GaN single-crystal boules. Perhaps lesser known in the GaN crystal field, the Connecticut-based crystal developer has been manufacturing semiconductor and optical crystal materials since 2004 - key crystals include AlN, ZnS and ZnO - but started looking at GaN two years ago.
"A lot of organisations take one crystal and focus on that but we've looked at different crystals which has given us a good understanding of crystal growth with different techniques, using a range of furnaces," says chief executive, Andy Timmerman. "You know we've done a lot on ZnO have successfully launched AlN so we see this as the next stepping stone to advance our crystal growth technology and work it into commercialisation."
Naturally, government backing helps. ARPA-E sees the material as crucial to a new generation of power devices, hence the hefty weighting in its $27 million SWITCHES program, Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High Efficiency Systems.
And as Timmerman highlights only last month, the Department of Defense opened up solicitations for projects on GaN Technology for GPS L-band space power amplification, as part of its Small Business Innovation Research program. "GaN really has a good fit for our overall research and commercialisation of different products," he adds.
However, Fairfield Crystal is no stranger to US government interest in crystal growth. Preceding 2010, it had won just over $1 million from the National Science Foundation to develop ZnS, CdS, ZnSe and AlN crystals.
Then come 2010, it received $200,000 from the independent government agency to develop a novel approach, and demonstrate and sample one inch diameter free-standing GaN wafers for III-Nitride light emitters and detectors. Three years on, NSF funds run into the millions of dollars with the company working on pilot production of two inch GaN.
But what exactly is the novel approach? Timmerman remains tight-lipped; when asked if the process is based on either HVPE or ammonothermal methods, he confirms it is not based on the latter.
"It is the current state of the art. We know some folks are using ammonothermal, Kyma has been using an HVPE process, and others are looking at a combination of the two. But ours is different," he says. "It's using a combination of expertise from other crystal growth, be it AlN or ZnO, or even just some of the techniques we've gleaned from growing our optical crystals."
One such technique, as evident from a patent published just last year entails a method for growing a zinc oxide (ZnO) single crystal boule from ZnO seed within an iridium crucible placed inside a physical vapour transport (PVT) furnace system. According to the patent, this technique overcomes inadequacies in hydrothermal, CVT and melt growth techniques.
Timmerman declined to comment on the relevance of this to his company's GaN crystal growth, but he is clear that a pure source material is essential the process. "We have found that having a very very clean source material is a priority," he says. "And we've developed an in-house process with high quality source material.
With this in hand, the team now intends to drive GaN substrate costs down by growing longer boules and slicing more wafers per growth run. And of course, a crucial part of the ARPA-E grant, is to grow three inch GaN crystals, within the next three years.
"The time frames are very aggressive, especially since in a number of years other organisations have also been trying," admits Timmerman. "We're cautious but confident we can get there, although we are not dismissive of the challenges."
