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9 January 2007


Cermet licenses phosphor-free GaN-on-ZnO white LED technology from Georgia Tech

Wide-bandgap semiconductor substrate and device developer Cermet Inc of Atlanta, GA, USA has negotiated an exclusive, royalty-based license agreement to use phosphor-free white LED technology, developed with the
Georgia Institute of Technology, that uses nitride emitter structures in combination with zinc oxide (ZnO) semiconductors, without the need for wavelength conversion by a phosphor.

Unlike current GaN-based white LEDs, which need a phosphor coating on the chip to convert its blue or near-ultraviolet light emission into white light, the GaN-on-ZnO white LEDs are phosphor-free and rely only on the
interaction between the two semiconductor materials gallium nitride and zinc oxide.

Cermet was the investigating organization (with Georgia Tech as subcontractor) for a three-year $3.8m US Department of Energy Solid State Lighting project Phosphor-Free Solid State Lighting Sources' from October 2003 and November 2006.

The firm's work focuses on growing conventional materials on novel substrates that possess unique physical properties with less internal strain. This process has the potential to increase efficiency; have emissions that can be adjusted by carefully applying potentials across the
substrate, and can be made to behave like a phosphor, absorbing photons of one color and emitting new photons of a different color.

Cermet's goal was to implement large-area ZnO fluorescent substrate technology and lattice-matched nitride epitaxy technology to address substrate, epitaxy, and device limitations in solid-state lighting.

Targeted innovations included:

  • MBE and MOCVD growth of truly lattice matched, low-defect density (104 cm2) nitride 330-420nm-wavelength LED structures on a ZnO substrate, resulting in reduced non-radiative recombination centers.
  • Optical pumping by the integrated nitride emitter of the ZnO substrate, doped to self-fluoresce in the visible spectrum, yielding white light emission.
  • Adjusting the color content of the white light by adjusting the substrate doping concentration.

In the first year of the program, Cermet demonstrated large-diameter doped bulk ZnO crystals, with greater than 80% transmission over the visible spectrum for a wafer thickness of 300 microns (and structural properties and
defect densities unchanged by the addition of up to 1019 cm-3 dopants), while InGaN epilayers with excellent structural properties and very low etch pit densities were grown by MBE and MOCVD.

Low-intensity phosphor-free white LEDs have since been fabricated. Cermet now aims to improve the electro-optic efficiency and to develop a range of white color temperatures.