A New Advance
in Gallium Nitride Nanowires
Yarris, (510)486-5375, email@example.com
grown on a substrate of lithium aluminum oxide, gallium
nitride nanowires are triangular in cross section (top).
cross section of gallium nitride nanowires grown on a magnesium
oxide substrate is hexagonal. Although compositionally identical,
the electronic properties of nanowires differ with different
the first time ever, the researchers have been able control the
direction in which a gallium nitride nanowire grows. Growth direction
is critical to determining the wire's electrical and thermal conductivity
and other important properties.
"Our results will come as a surprise to those who have said
that growth direction can't be controlled, that you get what you
get when you grow semiconductor nanowires," says Peidong
Yang, a chemist with Berkeley Lab's Materials Sciences Division
and a professor with UC Berkeley's Chemistry Department, who led
report discussing these research results first appeared in the
online edition of the journal Nature Materials on July 25. In
addition to Yang, co-authors of the report, "Crystallographic
alignment of high-density gallium nitride nanowire arrays,"
were Yanfeng Zhang, Donald Sirbuly, and Jonathan Denlinger
of Berkeley Lab, and Tevye Kuykendall, Peter Pauzauskie,
and Joshua Goldberger of UC Berkeley.
are eager to tap into the enormous potential of gallium nitride
for use in high-power, high-performance optoelectronic devices.
Already, single-crystalline gallium nitride nanowires and nanotubes
have shown promise in blue light emitting diodes, short-wavelength
ultraviolet nanolasers, and nanofluidic biochemical sensors.
over nanowire growth direction is extremely desirable, in that
anisotropic parameters such as thermal and electrical conductivity,
index of refraction, piezoelectric polarization, and band gap
may be used to tune the physical properties of nanowires made
from a given material," Yang says.
and his research group have been pioneers in the fabrication of
semiconductor nanowires, especially gallium nitride, zinc oxide,
and silicon/germanium. The wires they've produced measure only
a few nanometers in diameter but stretch out to several microns
in length. For this experimental work, they grew single-crystal
gallium nitride nanowires using a metalorganic chemical
vapor deposition (MOCVD) technique that was similar to an earlier
technique they used to produce nanowire lasers.
their earlier work, Yang and his group demonstrated the ability
to control the size, aspect ratio, position, and composition of
their nanowires. Now they've added the ability to control crystallographic
growth direction. The key to this new capability is the selection
of a choice substrate.
Yang, "In nanowires made from the exact same gallium nitride
material but grown on different substrates, the light-emission
of these wires was blue-shifted by 100 meV (milli-electron volts).
We believe the emission difference is a clear manifestation of
the different crystal growth directions."
this study, Yang and his group used substrates of lithium aluminum
oxide and magnesium oxide. The crystals of both materials are
geometrically compatible with gallium nitride crystals, but the
lithium aluminum oxide features a two-fold symmetry that matches
the symmetry along one plane of the gallium nitride crystals,
whereas the magnesium oxide has a three-fold symmetry that matches
gallium nitride symmetry along a different plane.
a result, when a vapor of gallium nitride condenses on either
of these substrates, the resulting nanowires grow perpendicular
to the substrate but aligned in a direction unique to each substrate.
Because of the different growth directions, cross sections of
the gallium nitride nanowires grown on lithium aluminum oxide
form an isosceles triangle, while the cross sections of those
grown on magnesium oxide are hexagonal.
goal is to put together a generic scheme for controlling the directional
growth of all semiconductor nanowires," says Yang. "When
we can do this, we will be able to answer some important fundamental
questions, such as how would the carrier mobility, light emission,
and thermoconductivity differ along different crystallographic
directions for nanowires with the same compositions and crystal
structures. The use of MOCVD for gallium nitride nanowire growth
will also allow us to integrate nanowires and thin films of various
compositions so we can start making real devices."
believes that he and his group are within a few months of being
able to produce a light-emission diode, a transistor, or a hybrid,
nanowire-thin film laser.
research effort was funded by the U.S. Department of Energy's
Office of Science, the Camille and Henry Dreyfus Foundation, the
Beckman Foundation, and the National Science Foundation.