Research and development efforts around the globe seek to boost the light output of solid-state sources in the visible and UV spectrums, attempting to broaden the application space.
While great gains have been made in the light output of both LEDs and OLEDs, further improvements in light output could widen the adoption in ways ranging from lower-cost general lighting products to new applications in the UV spectrum. BluGlass hopes to improve visible-light LEDs with a new method to grow the top epitaxial layers. Sensor Electronic Technology Inc (SETi) is partnering on a research team working in the ultraviolet (UV) LED space and could enable new sanitation applications. Meanwhile, Pixelligent has announced advancements in a surface treatment technology that can be applied to LED and OLED optics to boost light output.
RPCVD research makes 10x leap
Australia-based BluGlass Limited is focused on changes in the typical high-brightness LED manufacturing process that can yield lower-cost components with light output that matches or exceeds the performance of today's LEDs. Most LEDs today are manufactured using metal-organic chemical-vapor deposition (MOCVD) reactors to grow all of the epitaxial layers. BluGlass is using a remote-plasma chemical-vapor deposition (RPCVD) process to grow the final p-GaN (p-type gallium nitride) layer of an LED after the prior layers were grown in a typical MOCVD reactor.
The nearby image illustrates the technology with a depiction of the layers that comprise an LED. By using RPCVD for the p-GaN layer, BluGlass said it can deliver lower-cost components, more light out of the p-GaN layer, and support a more environmentally-friendly manufacturing scheme. The primary advantage of RPCVD is a simpler tool that operates at low temperature, and frees the MOCVD reactor sooner to process the next batch of LED wafers.
In its latest laboratory tests, BluGlass said it manufactured a 473-nm blue LED with light output of 1615 µW when driven at 10 mA. While that performance trails standard LEDs, it is a ten-fold improvement from what BluGlass publicly revealed in December 2012. The company believes that it is on the path toward greater gains. And the company said its latest BLG-3300 RPCVD tool is slated to begin growing GaN later in the month.
Of course, there are many approaches being explored to lower-cost LEDs. One such option is the use of silicon substrate. In a separate story we cover startup Lattice Power and the company's most recent funding round devoted to so-called GaN-on-silicon (Gan-on-Si) technology.
UV-C applications in sanitation
Moving to the UV space, the biggest potential growth area is in the UV-C band or what SETi calls deep UV (DUV). We covered the UV applications space in an article on presentations at the February Strategies in Light conference. Moreover, Crystal IS recently announced advancements in efficiency of UV-C LEDs.
Still, there is a lot of further room for improvement and efficient UV-C LEDs could lead toward products such as small solar-powered water purification systems. UV LED manufacturer SETi is chasing such goals working with the US Army Research Laboratory and the Rensselaer Polytechnic Institute.
Unlike the Crystal IS approach to raise efficiency through the use of native substrates, SETi is focused on using the lower-cost sapphire substrates that are prevalently used for visible-light LEDs. But the latest research uses migration-enhanced metal-organic chemical-vapor deposition (MEMOCVD) technology to reduce the dislocation or defect density that hampers light extraction. The researchers claim to have achieved external quantum efficiency in the 10% range, much higher than sapphire-based LEDs on the market. SETi revealed the breakthrough in the IOP Science research journal.
Pixelligent PixClear technology
Targeting the treatment of optics, Pixelligent is working on what it calls a nanocrystalline synthesis and surface-modification technology that can be applied in the back end of OLED or LED manufacturing processes and indeed has supplied the product commercially. The platform called PixClear has the potential to greatly increase the external quantum efficiency of solid-state lighting (SSL) sources.
In OLEDs, for example, the mismatch in the index of refraction between the planar materials and air is significant. The result is very high efficiency inside the OLED stack, near 100%, but only 20–40% external efficiency. Indeed, total internal reflection (TIR) in the layers results in many of the photons produced being converted into heat.
Our sister publication Laser Focus World has a deeper article on the efficiency challenge and the PixClear approach. The technology can also improve thermal properties and the optical reliability of SSL systems.