UV LED Created with Nanorod Heterostructures

Michael A. Greenwood

Vertically aligned nanorod arrays that exhibit strong near-ultraviolet emission have been created by researchers seeking to simplify the LED fabrication process.

Sung Jin An and Gyu-Chul Yi of the National CRI Center for Semiconductor Nanorods and Department of Materials Science and Engineering in Pohang, South Korea, report that they have created an LED composed of n-GaN/ZnO vertically aligned nanorod heterostructures grown on p-GaN substrates.

This schematic illustrates a GaN/ZnO coaxial nanorod heterostructure on a p-GaN substrate (left).The field-emission-gun scanning electron microscopy image shows the nanorod heterostructures (right). The inset shows a high-resolution transmission electron microscopy image of the nanorod heterostructures. Reprinted with permission of Applied Physics Letters.

This approach achieved strong near-ultraviolet emission at room temperature and has the potential to be employed in a variety of optoelectronic devices, including solar cells, the researchers said. The use of nanorod heterostructures in such applications has been limited, in part because they are difficult to fabricate.

The nanorods were created by using ZnO nanoneedles as the core material and adding layers of GaN through metallorganic vapor-phase epitaxy. Ammonia and trimethyl gallium were used as reactant sources, and the carrier gas was hydrogen. The growth temperature ranged from 500 to 650 °C.

After the GaN layers were added, the diameter of the nanorods increased from 40 to 52 nm. Field-emission-gun scanning electron microscopy revealed that the density of the structures was as high as 10¹⁰ cm^–2 and that they exhibited good vertical alignment.

During tests of the nanorod arrays, the investigators applied a current of 2 mA, and the nanorod heterostructures emitted blue and violet light, strong enough that it could be seen easily with the naked eye. The intensity of the light grew steadily as the current was increased to 5 mA. No degradation was observed in the emission over a period of several days.

The researchers said that the technique could simplify LED fabrication, which could lead to reduced costs.

Applied Physics Letters, Sept. 17, 2007, 123109.