by Dr. George Wang
Research on semiconductor nanowires has grown exponentially over the last
decade, with much attention focusing on their synthesis, fundamental properties,
and potential applications. Nanowires are high aspect ratio, wire-like
structures with diameters typically ranging from a few nanometers to a few
hundred nanometers. Nanowires comprised of virtually every semiconductor system,
including Si/Ge, II-VIs, and III-Vs, have been synthesized to date and exhibit a
variety of interesting morphologies including hexagonal, rectangular,
triangular, cylindrical, and even branched.
Interest in semiconductor nanowires is due in large part to their unique
thermal, mechanical, optical, chemical, and electrical properties, a result of
their high surface-to-volume ratio and their size, which intersects a number of
physical characteristic length scales, such as the exciton diffusion length and
Bohr radius, UV-visible wavelengths, phonon mean free path, and critical size of
magnetic domains. These novel properties have led to a number of intriguing
demonstrations of semiconductor nanowires as individual or integrated nanoscale
elements in a variety of applications ranging from thermoelectrics,
nanophotonics, sensing, piezoelectrics, energy harvesting and storage, and
The III-nitrides (AlGaInN) are technologically important direct band-gap
semiconductors which absorb and emit over a very broad and attractive energy
range from the UV to visible to infrared wavelengths, and are the basis for
commercial products like visible light-emitting diodes (LEDs) and blue laser
diodes (e.g. BluRay). As such, nanowires based on III-nitride semiconductors are
being explored for potential use in LEDs, lasers, photovoltaics, water
splitting, high speed/power electronics, and other applications.
However, before such nanowire-based applications can be practically realized,
several challenges exist in the areas of controlled and ordered nanowire
synthesis, fabrication of advanced nanowire heterostructures, and understanding
and controlling the nanowire thermal, electrical, mechanical, and optical
properties. At Sandia National
Laboratories, under the Solid-State
Lighting Science Energy Frontier Research Center and other programs, Dr. George T.
Wang and colleagues are investigating the synthesis and properties of
III-nitride based nanowires with the goal of addressing these many
Semiconductor nanowires can be fabricated by a variety of techniques,
including bottom-up approaches often involving a nanoscale metal catalyst
particle to direct 1D growth via the vapor-liquid-solid (VLS) mechanism, to
top-down lithographic approaches. While both of these synthetic approaches are
being explored at Sandia, the primary focus has been on VLS-based growth of GaN
and III-nitride core-shell nanowires using metal-organic chemical vapor
deposition (MOCVD). Figure 1 shows the template-free, aligned growth of GaN
nanowires on a sapphire substrate via this method.
The high nanowire density and degree of vertical alignment, which is
desirable for vertical device integration, is achieved by the proper selection
of the substrate crystal orientation and careful control of the metal catalyst
as well as the growth conditions.2-4 The nanowires
are single crystals, with triangular cross-sections (Figure 1b), and are free of
the device-detrimental defects known as dislocations which are common in
III-nitride films. This high crystalline quality of the nanowires, along with
the ability to make doped and alloy heterostructures over a broad, tunable
bandgap range, makes them attractive candidates for energy efficient
Figure 1. (a) Scanning
electron microscope (SEM) image of aligned GaN nanowire growth on sapphire; (b)
transmission electron microscope (TEM) image of a GaN nanowire with AlGaN shell
layer showing its triangular cross-section.
A variety of nanocharacterization techniques are also being employed by Dr. Wang
and his colleagues in order to understand and ultimately improve the nanowire
properties. For example, spatially-resolved cathodoluminescence experiments are
being used to map the frequencies and intensities of light emission from these
nanowires with nanoscale resolution,5 as shown in
This and other optical techniques that have been adapted to studying these
nanostructures, including near-field scanning microscopy6 and ultrafast7 and
deep-level optical spectroscopies8, have revealed
details such as the origin and concentration of impurities and other point
defects in the nanowires, with the goal of reducing them and their impact on
nanowire-based devices. Powerful 3D9 and in-situ
electron microscopy techniques10 have enabled, for
example, observing the physical breakdown of a nanowire device under high
electrical power in real-time at atomic-scale resolutions.11
Figure 2. (a)
Cathodoluminescence (CL) image showing blue light emission from GaN/InGaN
core-shell nanowires; (b) CL image showing defect-related yellow luminescence
from the surface region of a GaN nanowire.
In addition to single nanowire devices, ensembles of nanowires can also be
leveraged in interesting and advantageous ways. At Sandia, Dr. Wang and
colleagues have developed a technique that uses vertically aligned GaN nanowire
arrays as a high quality template for the growth of high quality GaN films on
inexpensive, lattice-mismatched substrates, as shown in Figure 3.12 The nanowires serve as strain compliant "bridges"
between the coalesced GaN film and the underlying, lattice-mismatched sapphire
substrate, which helps to minimize defect formation in the GaN film and hence
improve device performance.
Figure 3. (a) Artist's
rendering of nanowire-templated growth of a GaN film; (b) cross-section SEM
image showing demonstration of nanowire-templated GaN growth.
In summary, III-nitride semiconductor nanowires are intriguing new structures
that show great promise as efficient, nanoscale building blocks for applications
ranging from solid-state lighting and displays to photovoltaics. Many efforts
around the world are currently underway to better understand their synthesis and
properties in order to realize their full potential.
Funding from DOE Basic Energy Sciences (BES) DMSE, DOE EERE National Energy
Technology Laboratory, Sandia's LDRD program, and Sandia's Solid-State Lighting
Science Energy Frontier Research Center (DOE BES). Sandia National Laboratories
is a multi-program laboratory operated by Sandia Corporation, a wholly owned
subsidiary of Lockheed Martin company, for the U.S. Department of Energy's
National Nuclear Security Administration under contract DE-AC04-94AL85000.
1. A. I. Hochbaum, P. D. Yang, "Semiconductor Nanowires for
Energy Conversion", Chemical Reviews, 110, 527 2010.
2. Q. Li, G. T. Wang, "The Role of Collisions in the Aligned Growth
of Vertical Nanowires", J. Cryst. Growth (Netherlands), 310, 3706 2008.
3. Q. Li, G. T. Wang, "Improvement in Aligned GaN
Nanowire Growth using Submonolayer Ni Catalyst Films", Appl. Phys. Lett., 93,
4. G. T. Wang, A. A. Talin, D. J.
Werder, J. R. Creighton, E. Lai, R. J. Anderson, I. Arslan, "Highly aligned,
template-free growth and characterization of vertical GaN nanowires on sapphire
by metal-organic chemical vapour deposition", Nanotechnology, 17, 5773
5. Q. M. Li, G. T. Wang, "Spatial Distribution of Defect
Luminescence in GaN Nanowires", Nano Lett., 10, 1554 2010.
L. Baird, G. H. Ang, C. H. Low, N. M. Haegel, A. A. Talin, Q. M. Li, G. T. Wang,
"Imaging minority carrier diffusion in GaN nanowires using near field optical
microscopy", Physica B, 404, 4933 2009.
7. P. C. Upadhya, Q. M.
Li, G. T. Wang, A. J. Fischer, A. J. Taylor, R. P. Prasankumar, "The influence
of defect states on non-equilibrium carrier dynamics in GaN nanowires",
Semiconductor Science and Technology, 25, 2010.
Armstrong, Q. Li, Y. Lin, A. A. Talin, G. T. Wang, "GaN nanowire surface state
observed using deep level optical spectroscopy", Appl. Phys. Lett., 96,
9. I. Arslan, A. A. Talin, G. T. Wang, "Three-Dimensional
Visualization of Surface Defects in Core-Shell Nanowires", Journal of Physical
Chemistry C, 112, 11093 2008.
10. Y. Lin, Q. Li, A. Armstrong,
G. T. Wang, "In situ scanning electron microscope electrical characterization of
GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes", Solid
State Commun. (USA), 149, 1608 2009.
11. T. Westover, R.
Jones, J. Y. Huang, G. Wang, E. Lai, A. A. Talin, "Photoluminescence, Thermal
Transport, and Breakdown in Joule-Heated GaN Nanowires", Nano Lett., 9, 257
12. Q. Li, Y. Lin, J. R. Creighton, J. J. Figiel, G. T.
Wang, "Nanowire-templated lateral epitaxial growth of low-dislocation density
nonpolar a-plane GaN on r-plane sapphire", Adv. Mater., 21, 2416 2009.
Copyright AZoNano.com, Dr. George T. Wang (Sandia National