news analysis
Soraa inches closer to affordable GaN crystals
AS MANUFACTURERS of LEDs strive to deliver cheaper yet more efficient lamps, many in the industry doubt whether structures grown on sapphire or silicon substrates will make the grade.
Complex epi-layer stacks with high defect densities, caused by crystal lattice mismatches between the substrate and GaN-based active layers, don’t exactly point the way to cheap and easy manufacturing. And this doesn’t even factor in the tricky issue of handling large diameter wafers bowed by thermal expansion mismatches.
So what to do? According to a niche, but growing, band of industry players, LED manufacturers should look beyond non- native substrates as well as the conventional vapour phase epitaxy processes used to grow LED layers on them.
Instead, high current density GaN structures could be cost effectively fabricated on quality GaN substrates, which, crucially, have been grown via a novel, but established process called ammonothermal crystal growth.
California-based Soraa is exploring this strategy and was recently selected by the US Advanced Research Projects Agency- Energy (ARPA-E) to lead a project to develop bulk GaN substrates. The LED manufacturer already sells LED-based lamps fabricated on the same HVPE- grown GaN substrates used in laser diodes for Blu-ray applications, but hopes to cut manufacturing costs and raise substrate quality with its ammonothermal growth technique.
The technique is based on an analogous method used to grow synthetic quartz crystals, in which seed crystals are crystallised within thick-walled steel autoclaves that withstand the high temperatures and pressures required for crystal growth.
As Soraa vice president of bulk technology, Mark D’Evelyn, says: “[This growth process] is highly scalable and inexpensive, bringing the promise of excellent manufacturability and crystal quality to ammothermal GaN crystal growth.”
However, growing GaN crystals in this way demands higher temperatures and pressures, stretching your standard steel pressure vessel beyond its limits. To counter this, other GaN crystal makers have constructed sophisticated autoclaves based on nickel super-alloys, which has come at a cost.
“These super-alloys have better high- temperature properties, including strength and creep resistance, but are much more expensive and difficult to scale up,” he says.
“In addition, the crystal growth rates [within these autoclaves] are quite low.” To tackle these problems, D’Evelyn and colleagues have developed a proprietary GaN crystal growth technique, called SCoRA - Scalable Compact Rapid Ammonothermal - that they believe will cut the costs associated with existing GaN ammonothermal crystal growth, and produce commercial-grade substrates. Instead of using a super-alloy autoclave, the team has constructed a robust steel reactor lined with an insulating ceramic layer to protect the steel from high, internal processing temperatures.
As D’Evelyn explains, the crystal growth components are placed within a compartment inside the two-layer pressure apparatus, which is filled with ammonia and then internally heated to kick-start crystal growth. “The outer metal is cooled externally, doesn’t get very hot, so steel works fine for us, thank you very much,” he adds.
But has Soraa successfully grown high quality GaN crystals? Yes. A laboratory- scale reactor, built prior to the ARPA-E funding of early 2011, demonstrated the technique’s feasibility. And with the government cash, the team has since built a pilot-scale reactor and grown crystals at, says D’Evelyn, higher growth rates than rival companies.
“[Competitors] have demonstrated two- inch crystals of extremely high quality,”
Right: Soraa’s MR16 flagship product, the GaN on GaN LED
October 2012
www.compoundsemiconductor.net 13
asserts D’Evelyn. “We have grown two- inch crystals. There is a range in the quality of our crystals, with the very best crystals tending to be smaller and the largest ones not yet ready for manufacturing, but we are developing the growth process for high yield and high growth rates, and expect to produce extremely high quality four inch crystals in the future.”
While both D’Evelyn and his colleague, Mike Krames, chief technology officer at Soraa, assert these four-inch crystals will have a lower defect density than any HVPE-grown equivalent, neither will say exactly when these will be produced.
“We have more work to do on ammonothermal growth and frankly, still have a lot more legs to go on HVPE substrates,” adds Krames. Still, the future looks very bright for Soraa. Recent reviews of the firm’s GaN on GaN LEDs cite a more vibrant and consistent beam compared to existing halogen lamps.
And excitingly, progress on their ammonothermal growth process will feed into the company’s laser diode division, which is developing devices for displays and hand-held projector markets. “Using our technique, we have already demonstrated high quality [substrates] from the m-plane non-polar, semi-polar and c-plane polar planes of GaN crystals,” says D’Evelyn.
“As our laser team explores a range of crystallographic planes we have told them that whatever they decide they need, we can make.”
© 2012 Angel Business Communications. Permission required.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131