Displays Doped with Lanthanides
Brent D. Johnson
Emissive display technologies are benefiting from the development of new materials, notably organic LEDs. These electroluminescent devices do not require a front or back light, and they offer a wider angle of view than liquid crystal displays. Nevertheless, efforts toward developing full-color versions are complicated by the need for a material to serve as blue pixels.
Researchers at the University of Cincinnati's Nanoelectronics Laboratory, headed by Andrew Steckl, hope to resolve this issue by doping nitride semiconductors with lanthanides. The inorganic material combines the benefits of electroluminescence and high brightness in the blue range.
Adding the rare-earth elements to the semiconductors enables the hosts to produce visible wavelengths that would otherwise not be expected from their bandgaps. The lanthanides emit light from inner-shell states rather than from the valence shell, and their filled outer orbitals shield them from the effects of the host material, resulting in a constant wavelength of emission. Interaction with the host determines the intensity of the emitted radiation. In thulium-doped AlGaN, for example, this results in strong, blue emissions at high temperatures.
Steckl's laboratory created a company, Extreme Photonix LLC, in a university-supported incubator to develop this technology into a commercial product. The company is using europium as a dopant for red, thulium for blue and erbium for green. Other combinations of wide-bandgap semiconductors and lumophors also are being studied, such as manganese-doped zinc sulfide for yellow emission.
The researchers have found the emission intensity to be fairly constant from –50 to 100 °C. Another advantage of the materials is that they are robust and should enable devices more rugged than liquid crystal, plasma, fluorescent or organic LED displays. The lifetime is expected to exceed 50,000 hours.
Initially, Extreme Photonix is focusing on the semimobile market, for applications such as navigation systems in automobiles and vibration- and shockproof displays for the industrial factory floor. It has developed a 160 X 80-pixel prototype display and is working on QVGA (320 X 240-pixel) displays. "The brass ring would be high-definition television," Steckl said, but, for now, the inventor is taking a conservative approach to introducing this technology.
- The nonthermal conversion of electrical energy into light in a liquid or solid substance. The photon emission resulting from electron-hole recombination in a PN junction is one example. This is the mechanism employed by the injection laser.
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