Nanoscale LEDs Emit UV Light

Nanowires have been used to create tiny, highly efficient ultraviolet LEDs that could help build the next generation of biosensors and optical communications devices.

Researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg developed the assembly technique, which they said can be readily adapted for commercial production, in collaboration with scientists from the University of Maryland in College Park and Howard University in Washington.

While the creation of nanoscale LEDs is not new, what is notable is that the NIST LEDs emit ultraviolet (UV) light. Light-based nanoscale devices, such as LEDs, could be important building blocks for a new generation of ultracompact, inexpensive technologies, including sensors and optical communications devices. UV LEDs are particularly important for data storage and biological sensing devices, such as detectors for airborne pathogens.

[Top] Micrograph of a complete nanowire LED with the end contact. The long nanowire (A) is about 110-µm long and a shorter nanowire (B) crosses it. The bright circular section is the metal post from which the nanowires are aligned. [Bottom] Optical image of the same nanowire in action. Most of the light emitted from the device is in the ultraviolet portion of the spectrum, but enough visible light is generated to see it glowing. (Images courtesy NIST)

Nanowires made of a particular class of semiconductors that includes aluminum nitride, gallium nitride and indium nitride are the most promising candidates for nanoscale LEDs. But, said NIST researcher Abhishek Motayed, "The current nanowire LEDs are created using tedious nanowire manipulation methods and one-by-one fabrication techniques, which makes them unsuitable for commercial realization."

The NIST team used batch fabrication techniques, such as photolithography (printing a pattern into a material using light, similar to photography), wet etching and metal deposition. They aligned the nanowires using an electric field, eliminating the delicate and time-consuming task of placing each nanowire separately.

A key feature of the new nanowire LEDs,the researchers said, is that they are made from a single compound, gallium nitride (GaN). Each LED consists of an "n-type" GaN nanowire placed on the surface of a "p-type" GaN thin film. "N-type" and "p-type" refer to semiconductors with, respectively, an abundance of electrons and an abundance of positively charged electron vacancies called holes. P-n junctions made from the same basic compound yield more efficient LEDs than those made with different compounds, and can operate at lower power.

When the proper voltage is applied to the junction, it emits light with a peak wavelength of 365 nm, which falls squarely in the UV range. The group produced and tested more than 40 of these LEDs; all showed very similar emission properties. They also displayed excellent thermal stability -- withstanding temperatures up to 750 °C -- and operational stability, showing no signs of deterioration even after two continuous hours of operation at room temperature. These properties indicate that this LED production method yields reliable, stable devices.

The researchers said their method could be used to fabricate other nanowire structures as well as applications requiring a large area of nanoscale light sources. Their work is described in a recent issue of Applied Physics Letters.

For more information, visit: www.nist.gov