Potential for high-resolution, high-contrast white light sources

Phosphor-filled silicon structures allow for much greater spatial resolution and better contrast behavior than conventionally used phosphors. These innovative structures, which are being developed at the Fraunhofer Application Center for Inorganic Phosphors in Soest in collaboration with the Fraunhofer ISIT, also allow for active air or water cooling of the phosphor so the heat produced during light conversion can be removed more efficiently. Thus they are a promising alternative to LED matrix systems and liquid crystal displays (LCDs), particularly for very high-resolution applications.

Leuchtstoff Siliziumstruktur
© Fraunhofer AWZ Soest
Laser scanning microscopy image of a single pixel. The height is coded using the color scale, which ranges from +2 µm (red) to -50 µm (blue).

Light emitting diodes (LEDs) have become an essential part of modern lighting technology. In particular, lighting systems with a large light output and spatial resolution are becoming more and more important. They are used in projection systems and televisions as well as in cars, where high-resolution, adaptive front lighting systems improve light quality and safety compared to conventional headlights thanks to their adjustable, non-glare light.

LED matrix systems and liquid crystal displays (LCDs) are state of the art. White light is created using a blue-emitting LED chip or laser, combined with a yellow-emitting, pixelated phosphor. The phosphors are pixelated through either laser structuring or etching. The resolution of these systems depends on how well the individual phosphor pixels are visually separated from one another.

Phosphor-filled silicon structures are thus a promising alternative, especially for very high-resolution applications. “The desired structures are etched into a silicon wafer and then filled with phosphor powder. These pixelated phosphors allow for much higher spatial resolution, since we can create very small pixel structures with dimensions of just a few micrometers,” explains Dr. Franziska Steudel, the “Phosphor Design” team leader at the Fraunhofer Application Center for Inorganic Phosphors (AWZ) in Soest. The center recently presented the new research results jointly with the Fraunhofer Institute for Silicon Technology (ISIT), in an article published in the journal “elektronik industrie.” When the phosphor is activated by a blue laser beam, the structures produce outstanding contrast resolution as well as Lambertian luminous intensity distribution.

“Because of their high thermal conductivity, the silicon structures not only improve visual resolution, but also significantly help with heat management,” says Dr. Peter Nolte, who heads the “Reliability of Phosphors” team at the Fraunhofer AWZ in Soest. The smaller the pixels, the less the phosphor is heated, since the surrounding silicon walls efficiently remove heat. The porous phosphor layer even allows the phosphor to be cooled with air or water in applications with a very high power density.