Researchers at the National
Institute of Standards and Technology (NIST) have cultivated many thousands
of nanocrystals in what looks like a pinscreen or "pin art" on silicon,
a step toward reliable mass production of semiconductor nanowires for millionths-of-a-meter-scale
devices such as sensors and lasers.
NIST researchers grow nanowires made of semiconductors—gallium nitride
alloys—by depositing atoms layer-by-layer on a silicon crystal under high
vacuum. NIST has the unusual capability to produce these nanowires without using
metal catalysts, thereby enhancing luminescence and reducing defects. NIST nanowires
also have excellent mechanical quality factors.
The latest experiments, described in Advanced Functional Materials,* maintained
the purity and defect-free crystal structure of NIST nanowires while controlling
diameter and placement better than has been reported by other groups for catalyst-based
nanowires. Precise control of diameter and placement is essential before nanowires
can be widely used.
The key trick in the NIST technique is to grow the wires through precisely
defined holes in a stencil-like mask covering the silicon wafer. The NIST nanowires
were grown through openings in patterned silicon nitride masks. About 30,000
nanowires were grown per 76-millimeter-wide wafer. The technique controlled
nanowire location almost perfectly. Wires grew uniformly through most openings
and were absent on most of the mask surface.
Mask openings ranged from 300 to 1000 nanometers (nm) wide, in increments of
100 nm. In each opening of 300 nm or 400 nm, a single nanowire grew, with a
well-formed hexagonal shape and a symmetrical tip with six facets. Larger openings
produced more variable results. Openings of 400 nm to 900 nm yielded single-crystal
nanowires with multifaceted tops. Structures grown in 1,000-nm openings appeared
to be multiple wires stuck together. All nanowires grew to about 1,000 nm tall
over three days.
NIST researchers analyzed micrographs to verify the uniformity of nanowire
shape and size statistically. The analysis revealed nearly uniform areas of
wires of the same diameter as well as nearly perfect hexagonal shapes.
Growing nanowires on silicon is one approach NIST researchers are exploring
for making "nanowires on a chip" devices. Although the growth temperatures
are too high—over 800 degrees Celsius—for silicon circuitry to tolerate,
there may be ways to grow the nanowires first and then protect them during circuitry
fabrication, lead author Kris Bertness says. The research was partially supported
by the Defense Advanced Research Projects Agency (DARPA) Center on NanoscaleScience
and Technology for Integrated Micro/Nano-Electromechanical Transducers (iMINT)
at the University of Colorado at Boulder.
* K. A. Bertness, A. W. Sanders, D. M. Rourke, T. E. Harvey, A. Roshko, J.B.
Schlager and N. A. Sanford. Controlled nucleation of GaN nanowires grown with
molecular beam epitaxy. Advanced Functional Materials. Published online: July
13, 2010. DOI: 10.1002/adfm.201000381