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
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
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.
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