ZnO Nanowire LEDs Have UV Output

Daniel S. Burgess

In a continuation of their work with LEDs based on ZnO nanostructures embedded in polymer, scientists at Portland State University in Oregon have obtained emission centered at 393 nm from the devices. Such emitters may enable the development of large-area lighting on flexible substrates.

Rolf Könenkamp, a physics professor at the university and leader of the research team, said that the ZnO nanowires are unique in that they can be produced at temperatures as low as 85 °C.

“These temperatures and the fabrication process itself are compatible with glass and even with flexible polymer substrates,” he said.

Crystalline ZnO nanowires embedded in polystyrene form the basis of LEDs. Annealing the nanostructures at 300 °C increases the emission and yields a narrow line in the near-ultraviolet. Courtesy of Rolf Könenkamp, Portland State University.

Earlier, the investigators found that the embedded nanowires produced broadband output centered at 620 nm and extending from the near-UV into the near-IR (see “ZnO Nanowire LEDs Emit White Light,” Photonics Spectra, March 2005, page 116). They now have discovered that an additional annealing step at 300 °C changes the electroluminescence spectrum such that a narrow emission line appears at 393 nm.

The researchers fabricate the LEDs by low-temperature electrodeposition in an aqueous solution to yield vertically oriented, 100- to 200-nm-diameter ZnO nanowires up to 2 μm in length on fluorine-doped SnO₂-coated glass. Using a spin-coating process, they embed the nanowires in polystyrene, which acts as an insulator, thinning the coverage at the tips of the nanowires to 50 nm or less using plasma etching. They then overlay the structure with a 0.5-μm-thick layer of the P-doped polymer.

The scientists are working to demonstrate that the approach is compatible with silicon substrates. Such emitters could be useful for on-chip optical interconnects, Könenkamp suggested.

Nano Letters, October 2005, pp. 2005-2008.