Photonics: Bright white light

09/27/2010

Making way for a new generation of light-emitting diodes based on zinc oxide

Fig. 1: Schematic illustration of a ZnO-based UV-emitting LED with a green phosphor, and a photograph of the operating LED.
Fig. 1: Schematic illustration of a ZnO-based UV-emitting LED with a green phosphor, and a photograph of the operating LED.

Inset © 2010 AIP

White light-emitting diodes (LEDs) are replacing incandescent light bulbs in a variety of applications, including car headlights. An issue with the current generation of white LEDs, however, is that their color balance tends toward a bluish hue. Researchers from the WPI-AIMR at Tohoku University in collaboration with other institutions in Japan have now demonstrated that LEDs constructed from zinc oxide (ZnO) compounds could solve this problem while also being relatively easy to fabricate1.

“The solid-state lighting market has been expanding rapidly, and now exceeds several billion dollars. I think there is room for ZnO LEDs to penetrate this market, particularly because of its potentially very cheap fabrication process,” comments Masashi Kawasaki, the WPI-AIMR researcher who lead the team.

The best color balance is achieved in white LEDs when the light-converting phosphors inside the diode are excited with ultraviolet (UV) light, rather than the blue light used in the LEDs available on the market today. Shifting the blue emission of gallium nitride (GaN), used in conventional blue LEDs, to UV is difficult to achieve because of a lack of suitable substrates that could be fabricated at practical cost. Zinc oxide, on the other hand, can be grown easily on commercially available, UV-compatible ZnO substrates, but preparing high-quality light-emitting films using standard industrial growth processes has been challenging. For example, a laser deposition technique developed previously for the fabrication of ZnO-based LEDs requires extreme processing conditions and large variations in temperature, making the approach unsuitable for large-scale production.

Instead, Kawasaki’s team used a ‘molecular-beam epitaxy’ process — a widely used technique that grows uniform atomic layers by exposing a substrate to ultrapure atomic clouds of certain elements. The researchers deposited thin films of ZnO with magnesium to shift the native blue emission of ZnO to UV, and by optimizing the growth process and structural details of the device, they were able demonstrate very efficient UV emission.

For commercial use, the efficiency of the devices still needs to be improved by a factor of about 100. While this may seem substantial, the present device is already 10,000 more efficient than earlier ZnO-based LEDs. “The remaining efficiency gap compared with GaN LEDs could easily be closed with improvements such as optimizing the electric contacts to the LEDs,” says Kawasaki. Once that happens, he is convinced that ZnO will not only challenge the dominance of GaN but also lead to the development of efficient UV lasers.

References

  1. Nakahara, K., Akasaka, S., Yuji, H., Tamura, K., Fujii, T., Nishimoto, Y., Takamizu, D., Sasaki, A., Tanabe, T., Takasu, H., Amaike, H., Onuma, T., Chichibu, S. F., Tsukazaki, A., Ohtomo, A. & Kawasaki, M. Nitrogen doped MgxZn1−xO/ZnO single heterostructure ultraviolet light-emitting diodes on ZnO substrates. Applied Physics Letters 97, 013501 (2010). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.