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Zinc Oxide Nanorods Produce Laser Light

3/6/2008

Zinc oxide (ZnO) is a wide bandgap semiconductor that emits laser light in the ultraviolet range up to room temperature. Because ZnO has a strong tendency for self-organized growth, research has investigated the growth of whisker-like ZnO nanorods with diameters in the range of a few tens to a few hundred nanometers and lengths of several micrometers. It has long been thought that such 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.

A team of researchers at Germany’s University of Karlsruhe 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/alumina substrate covered with a gold catalyst. 

The substrate is heated until the gold melts and forma a droplet. The zinc vapor forms an alloy with the gold. If this alloy is supersaturated, ZnO nanorods start to grow in the presence of oxygen at the positions of the gold droplets. Growth continues as long as zinc and oxygen are available and the temperature of the alloy stays above its eutectic temperature. The diameter of the growing nanorods is a function of the gold droplet diameter. The main keys to successful growth are the easy-to-handle patterning of the ordered nanoparticle arrays, together with the stable growth conditions.

The ZnO nanorods are [0001] oriented single crystals with a diameter of 200 nm and a length of 4.7 microns, with a rod-to-rod spacing of 500 nm. The rod arrays show strong UV emission and single nanorods demonstrate lasing behavior with multiple UV lasing modes. The lasing emission peaks are sharp, with a linewidth about 0.1 nm and a fast decay time of ~8 ps. The nanorods have similar quality to high quality bulk ZnO single crystals.

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. They 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.

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. For further information, contact Huijuan Zhou at huijuan.zhou@physik.uni-karlsruhe.de.