They've been a long time coming, but early next year high brightness nanowire LEDs, manufactured by Glo, should be commercially available. Following nearly a decade of post-research development - the Sweden-based Lund University spin-out was founded in 2005 - Glo's 3D LEDs look ready to challenge existing planar LEDs.
"We already have products in customers' hands and expect to start shipping in the next couple of quarters," says Nathan Gardner, chief technology officer of Glo-USA. "We expect to offer better green LEDs than planar devices as well as a higher performing blue LED. Red nanowire devices will follow so we will have an RGB solution all based on the nitride semiconductor system."
But why the focus on nanowires? Compared with conventional planar LED heterostructures, these vertical 3D structures promise several advantages. For starters, growing vertical nanowires onto a substrate prevents the accumulation of wafer stresses that takes place when planar layers are deposited instead. Here, differences in each material's coefficient of expansion lead to lattice mismatches during fabrication and a wafer wrought with defects that degrade device yields and performance.
What's more, the vertical structures boost light output efficiency, thanks to the large surface-to-volume ratios, and are compatible with low-cost large-area silicon substrates.
As Gardner explains, Glo's LEDs comprise vertically aligned GaN nanowire arrays, grown via MOCVD, over a selective-area growth mask deposited on a GaN/sapphire template. Holes in the mask, imprinted via lithography and dry-etched, guide nanowire growth.
The actual diode structure consists of an InGaN active region, deposited on the m-plane sidewalls of the n-type GaN nanowire core. This active region is followed by a p-type AlGaN layer, p-type GaN and a heavily doped p-type GaN contact layer.
"TEM analysis shows the dislocations existing in the n-type GaN template rarely propagate into the nanowire," he says. "[And so] the active region is deposited on a defect-free m-plane template."
SEM of Glo nanowire chip showing individual nanowires (zoom) [Glo]
Glo hasn't publicly released technical specifications - this information is only provided under a non-disclosure agreement to customers that are sampling devices - but Gardner claims overall performance of green and blue devices is on par with today's commercial planar devices.
"The peak quantum efficiencies of our green devices are within 10% of the commercially available green peak efficiencies while the internal quantum efficiency of our blue LED is equivalent to the state-of-the-art planar commercial blue [device]," he says. "And the reliability data from the devices show they meet normal customer requirements."
To date, Glo has focused on fabricating LEDs on sapphire substrates. And while Gardner asserts the nanowire LED manufacture is compatible with large area silicon substrates, the company's efforts will remain focused on sapphire for the foreseeable future.
"The nanowire device structure in general is quite amenable to being fabricated on large area silicon substrates, and compared to planar structures, we do not have the complexity and cost of growing a buffer layer," he says. "But the companies manufacturing planar LEDs don't perceive silicon as being advantageous from a cost perspective until 8 inch diameters are reached. So once volumes increase and should the demand come, then we will look at silicon."
Quality or cost?
In March this year, the US-based nanowire LED developer, Aledia, and Glo's closest rival, unveiled prototype microwire GaN on silicon LEDs fabricated on 8 inch silicon wafers. Customer samples are expected early 2014.
At the time, chief executive, Giorgio Anania, told Compound Semiconductor, the LEDs could be grown three times faster than planar GaN LEDs, would probably cost around four times less than conventional devices, but his company would not be targeting high performance markets.
"Initially we will not offer the best Lumens per Watt, but many applications do not need this, what they really need is best lumens per dollar," he said.
Clearly Glo is taking a different tack. Gardner states his company's nanowire device manufacturing speeds are comparable with those of planar LEDs, and for the time being, so is the cost.
"Eventually, devices could be cheaper than planar but we don't think we will offer such a solution in the next year to year and a half," he says. "Our tactic has always been to use the nanowire topology for LEDs as a platform to generate breakthrough performance across all colours of the visible spectrum. Our real aspiration is to exceed the performance of planar."