Startups Propose Disruptive Technologies to Improve HB LED Performance at Lower Cost
By Paula Doe, SEMI Emerging Markets
The major technical challenges involved in producing high-brightness LEDs with high enough light quality and low enough cost for a real volume market in solid state lighting creates a prime opportunity for disruptive solutions — and for the startups thinking “outside the box” with some of the most interesting nanotechnology solutions. One prime area for innovation is materials, where quantum dots and controlled crystal structures in substrates may have potential for better efficiency at less cost. Of course, other more established companies have potentially disruptive approaches as well, like testing die for radiometric emitted wavelength instead of the usual perceived color, to get better feedback for controlling yield.
One alternative for getting more of the ideal warm white light out of HB LEDs may involve the color conversion with quantum dots instead of the usual red phosphors. QD Vision (Watertown, MA) says its semiconductor nanocrystals can be tuned by particle size and material to specific, narrow-emission color profiles, to convert LED light to the desired color with 30 percent more lumens per watt than comparable phosphors alone. Lamp makers use a thin film of the quantum dot ink on the diffusive cover of their LEDs lamps to fine tune the color. QD vision is currently shipping product in volume to lamp makers.
The company also now is getting significant traction in the LED backlight market, with products planned for next year, reports CTO and co-founder Seth Coe-Sullivan. For backlights, a strip coated with the nanocrystal ink can be used over the blue LEDs directly, replacing the phosphor. The narrow band RGB emitters are tuned to best match the transmission of the color filter, for wider color gamet to improve the look of the display, while also letting more light through to improve power efficiency by a third.
Coe-Sullivan says the company offers a complete range of quantum dots containing no cadmium, and products that show no change after 9000 hours exposure to constant light flux at 20-30 mW/cm2.
Chinese Startup Lattice Power Produces Commercial Blue HB LEDs on Silicon
Chinese startup Lattice Power Corp. (Nanchang, China) is producing HB LEDs on silicon, with small blue and green devices for display applications in volume production, devices for LCD backlights ramping, and introduction of larger chips for general illumination coming soon.
The company’s unique technology to manage the thermal and lattice mismatch that usually cause stress and cracking in GaN layers deposited on silicon instead of sapphire or SiC is based on work by professor Feng-Yi Jiang and his research group of Nanchang University. With some recent patents granted, Lattice Power researchers have talked about results at some technical conferences, reporting at SPIE in California and at the Nitride Semiconductor conference in Korea, on a device with 100 lumen cool white (5,200K) output at 350mA from a 1mm x 1mm blue chip, made with InGaN/GaN on a (111) 2-inch silicon substrate, according to the abstract. Researchers reported that accelerated life tests showed reliability similar to LEDs on sapphire. Lattice Power researchers are also working on larger diameter silicon technology which will further provide scalability and a cost-effective solution.
Current production is of small 200 x 200µm blue and green chips for large signage applications, and the company is introducing larger chips for high-brightness applications. “We’re currently limited by our capacity, and are adding MOCVD capacity dramatically,” says Dr. Bo Lu, EVP of Lattice Power. The company has ordered multiple MOCVD reactors from Aixtron for delivery through 2010, for what Aixtron said was one of its largest orders to date from China. With the capacity expansion, Lattice Power plans to sell the blue chips to packaging houses for LCD backlighting and general lighting applications.
Four-year-old Lattice Power is funded by some $52 million from an international group of venture capital firms including GSR Ventures, Mayfield Fund, AsiaVest Partners, Temasek Holdings, and Keytone Ventures.
Inlustra Technologies Readies Non-Polar GaN Substrates
Aiming to shake up the laser and light emitting diode substrate sector in quite a different way is the UC Santa Barbara spinout Inlustra Technologies, Inc., which is growing non-polar and semi-polar GaN substrates directly, using a modified HVPE chemistry. CEO Benjamin Haskell says the company has demonstrated 2-inch and 3-inch product. It is sampling now and aims to start volume production early in 2011.
The first application is likely to be green and blue laser diodes, which are hard to fabricate in the usual orientation, and are less price sensitive than lighting. But as increasing volumes for these initial applications bring down prices, Haskell expects the technology to reduce costs for solid state lighting as well. Though the company cuts the cost of making GaN substrates with simpler equipment and low materials loss, Haskell argues that end-device costs will come down much more because making LEDs on GaN makes the whole manufacturing process easier, from simpler and shorter epi deposition to less complicated device processing. With less efficiency droop compared with conventional polar LEDs and higher light output, LED die can be smaller, and fewer die are needed to generate comparable light output to today's lighting solutions.
Order-of magnitude improvements in performance are possible with non-polar GaN devices and substrates, argues Haskell, because changing the orientation of the crystal eliminates the impact of polarization fields inherent to vertical devices grown in the conventional c-direction (as today's LEDs and LDs are). Devices made on the usual c-plane surface have to overcome these built-in electric fields — which may be on the order of megavolts per centimeter — to turn the devices on; loss of efficiency and color shifting are only two of the negative consequences of these fields. But if the nitride crystal is turned on its side and the device built on a non-polar face, this polarization remains within the growth plane and does not negatively impact performance. “These fields only affect devices along the polar c-axis,” says Haskell. “On the non-polar planes many of the challenges of GaN device design and implementation go away.”
Op-Test Looking for a Better Way to Test and Track Device Performance
Op-Test (Redding, CA) has been making HB LED production test tools for more than a decade, but more recently decided that there had to be a better way to test and track device performance than the traditional photometric constructions of the color the average human eye sees at a particular angle in a particular light. Instead, Op-Test uses a controlled energy test that provides a direct measurement of the electrical energy going into the die, and the corresponding optical energy coming out. “We also don’t transform the optical data, but stay with the radiometric wavelength information,” notes president Dan Morrow. “Radiometric data allows designers to better predict how the die will behave so they can design around it.” He also argues that the radiometric test data provides better feedback for controlling production uniformity and yield. The system is currently in beta test at a customer site.
These companies will discussing these developments in manufacturing technology with potential to advance the HB LED sector towards more lumens per dollar — at the Extreme Electronics session on HB LED manufacturing technology at SEMICON West on July 14. Also on the agenda are leading market researchers Strategies in Light, Yole Developpement and Canaccord Adams; LED makers Philips Lumileds and SemiLEDs, major equipment suppliers Applied Materials, KLA-Tencor, Ultratech and EVGroup; and innovative packaging startup Articulated Technologies.
To join one of the SEMI “Extreme Electronics” online communities (including Extreme LED), visit http://www.semi.org/en/EventsTradeshows/CTR_029333
SEMI Global Update
July 6, 2010