Zinc oxide (ZnO) is a wide bandgap semiconductor that emits laser light in the ultraviolet range up to room temperature. ZnO nanorods could be used as building blocks for nanolasers. However, it is difficult to produce well-ordered, uniformly sized and aligned arrays of 1D ZnO nanostructures that are needed for practical applications.

Huijuan Zhou of the University of Karlsruhe 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 by 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 gold catalyst-covered GaN substrate.

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, ZnO nanorod-based pn junctions are needed. The researchers still face some challenges here because they need 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. For example, for 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 results were reported in Appl. Phys. Lett.