Nanowires function as tiny light sources

Sarah L. Stern

Researchers from the National Institute of Standards and Technology in Boulder, Colo., have produced nanowires in gallium nitride and related alloys that may potentially find use as nanoscale lasers and LEDs. The nanowires are hexagonal rods that are 30 to 100 nm in diameter and are typically grown to 10 μm in length. Thus, the emission aperture of a nanowire-based laser or LED would be ~50 times narrower than that of an edge-emitting device fabricated by planar growth and processing techniques.

Traditionally grown thin epitaxial films of III-nitride alloys — such as those incorporating gallium nitride — suffer from high densities of defects. By contrast, the investigators found that their nanowires were essentially free of defects. According to team member Norman A. Sanford, their results corroborated recent reports from researchers in Europe.

The scientists grew the nanowires on silicon substrates by means of nitrogen-plasma-assisted molecular beam epitaxy and grew a 50-nm buffer layer of aluminum nitride on the substrate prior to switching to a GaN nanowire growth mode. The long axis of the wires is parallel to the crystallographic c-axis of the GaN crystal lattice.

A field of nanowires created by nitrogen-plasma-assisted molecular beam epitaxy on a silicon substrate can be used as lasers and LEDs. Color was added for contrast. Courtesy of Lorelle Mansfield/NIST.

Typically, the wires are separated by about 50 to 200 nm. Their density can be partially controlled by varying the growth conditions, and their length appears to be limited only by the time of growth. The researchers chose the ~10-μm length for convenience. Experiments are under way to explore length-dependent properties in the nanowires.

The nanowires were removed from their growth substrates by ultrasonic agitation in an organic solvent and then dispersed onto sapphire substrates for spectroscopic experiments. Photoluminescence experiments were performed over a temperature range from 2.8 to 296 K. Excitation was performed with a helium cadmium laser operating at 325 nm. The photoluminescence peak emission intensity at room temperature was observed to be 10 percent of that recorded at 4 K.

After their experiments, the researchers concluded that nanowires could be used for a wide variety of applications. Reprinted with permission of Applied Physics Letters.

Team member John B. Schlager found that the photoluminescence intensity increased with excitation intensity; however, extended exposure of a nanowire to the excitation beam eventually could reduce emission intensity.

Moreover, the axial nanowire morphology permitted Schlager to distinquish the photoluminescence that was polarized parallel to the crystallographic c-axis from the photoluminescence that was polarized perpendicular to the same axis. Distinguishing the polarizations of the photoluminescence is important for the characterization of GaN but is almost impossible to accomplish using the traditional planar thin-film morphology.

Depending on the alloy composition used, the wires can emit in the ultraviolet or visible wavelength ranges. For the GaN wires, peak emissions occurred near the bandgap, which was 365 nm at room temperature. When indium was added to produce InGaN, the wavelength emission became longer and produced blue, green or red light. If AlGaN alloys were used instead, emission would tend toward the UV range.

Nanowires functioning as nanoscale light sources are expected to have numerous applications, including nanoelectromechanical system lab-on-chip devices and specialized sensors.

The researchers plan to conduct further studies to determine the effects of excitation intensity and sample preparation on photoluminescence behavior. Also, their long-term aim is to grow more complex and intricate nanowire heterostructures to realize nanowire LEDs, lasers and photodetectors.

Applied Physics Letters, May 23, 2006, 213106.