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Zinc oxide nanolasers stand out

07 Feb 2008

Researchers in Germany have fabricated nanorods made from zinc oxide that have been shown to emit laser light.

Arrays of well ordered, vertically aligned zinc oxide nanorods have been shown by scientists at the University of Karlsruhe to emit laser light. By focusing the laser excitation down to a spot size smaller than 1 µm, the researchers found that even single nanorods standing on a substrate could lase. They say that the nanorod arrays could be used to make UV nanolaser devices and light-source displays.

Zinc oxide (ZnO) is a wide bandgap semiconductor that emits laser light in the ultraviolet range up to room temperature. It has long been thought that ZnO nanorods could be used as building blocks for nanolasers, but it has proved difficult to produce arrays of 1D ZnO nanostructures that are well ordered, uniformly sized and aligned – which are essential for practical applications.

Huijuan Zhou and colleagues have now succeeded in growing such ordered structures with the right geometry for lasing. The uniform size of the nanorods is crucial for nanolaser applications, since the ability of the rods to lase depends strongly on the rod geometry.

The researchers produced their arrays using a vapour phase transport method. They used highly pure ZnO powder, which they reduced to release zinc vapour. This was then transported by a carrier gas to a GaN substrate covered with a gold catalyst.

At this point, the zinc vapour oxidizes into ZnO and expitaxially grows into nanorods at the gold sites. "The main keys to the successful growth are the easy-to-handle patterning of the ordered nanoparticle arrays, together with the stable growth conditions," Zhou told nanotechweb.org.

All of the rods in the array are identical, both in the growth direction and in size, she added.

To make nanolaser devices, however, it must be possible to fabricate pn junctions based on ZnO nanorods. But the researchers still face some challenges to obtain stable and reproducible n-type doped ZnO. "We believe that heterojunctions composed of n-ZnO nanorods on a p-type GaN substrate are a good option for nanolaser devices because ZnO has a stronger tendency for self-organized growth than other semiconductors like the III-Vs," explained Zhou.

Such electrically driven ZnO nanolasers could serve as highly-efficient miniaturized light sources in applications such as optical data storage, microanalysis and in combination with phosphor as bright white-light-source displays.

The team is also busy working on making ordered, uniformly sized n-ZnO nanorods/p-GaN heterostructures for light-emitting devices and laser diodes.

The researchers reported their results in Appl. Phys. Lett.

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