Full text of DOI:10.1557/JMR.2000.0054

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Ordered nanostructure of single-crystalline GaN nanowires in
a honeycomb structure of anodic alumina
G.S. Cheng, L.D. Zhang, S.H. Chen, Y. Li, L. Li, X.G. Zhu, Y. Zhu, and G.T. Fei
Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129,
Hefei 230031, People’s Republic of China, and Institute of Advanced Study,
University of Science and Technology of China, Hefei 230026, People’s Republic of China
Y.Q. Mao
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences,
Nanjing 210008, People’s Republic of China
(Received 19 March 1999; accepted 25 October 1999)
Ordered nanostructure of single-crystalline GaN nanowires in a honeycomb structure of
anodic alumina was synthesized through a gas reaction of Ga2O vapor with a constant
ammonia atmosphere at 1273 K in the presence of nano-sized metallic indium
catalysis. Atomic force microscopy, x-ray diffraction, Raman backscattering
spectroscopy, scanning electron microscopy, and transmission electron microscopy
indicate that the ordered nanostructure consists of single-crystalline hexagonal wurtzite
GaN nanowires in the uniform pores of anodic alumina about 20 nm in diameter and
40–50 ␮m in length. The growth mechanism of the ordered nanostructure is discussed.
The photoluminescence spectrum of this nanostructure is also reported.
The alumina membrane was fabricated by means of
I. INTRODUCTION
anodization.11,12 Briefly, annealed in vacuum (773 K)
and degreased in acetone, highly pure Al plate (99.999%)
was anodized in 0.3 M sulfuric acid solution at a constant
potential of 26 V for 10 h. After removal of the anodic
oxide layer in a mixture of phosphoric acid and chromic
acid solution, the textured Al plate was anodized again
for 6 h under the same conditions as for the first anod-
izing. Then the Al layer of the specimen was removed
in a saturated HgCl2 solution. After the coating layer
was dissolved in acetone, the bottom part of the mem-
brane was removed in phosphoric acid to form a regular
membrane.
The metallic indium was evaporated in vacuum on
one side surface of the anodic alumina to form the nano-
sized indium particles on one bottom surface of the uni-
form pores.
The gas reaction of the Ga2O vapor with a constant
flowing ammonia atmosphere was carried out in a hori-
zontal quartz tube10 at a temperature as high as 1273 K.
The alumina membrane was first put on the top of mix-
tures of Ga and Ga2O3 with mole ratio 4:1 inside an
alumina crucible. The alumina crucible was placed in the
hot zone inside the quartz tube and held in a constant
flowing ammonia atmosphere (300 standard cubic centi-
meters per minute) at 1273 K for 2 h.
The fabrication of ordered nanostructures has attracted
considerable attention1–3 because of their great potential
for testing and understanding fundamental concepts
about the roles of orderliness, dimensionality, and size in
optical, electrical, magnetic, and mechanical properties
and for the applications in ultrathin colorful light emis-
sion display devices and colorful copier instruments. On
the other hand, GaN-based materials have been the sub-
ject of intensive research for blue and ultraviolet light
emission4,5 and high-temperature/high-power electronic
devices.6,7 Much effort has been made to synthesize GaN
membranes,8 nanoparticles,9 and nanorods or nano-
wires.10 To the best of our knowledge, few reports about
ordered array of the GaN nanowires were found to date.
Here, we demonstrate an interesting approach by which
the ordered nanostructure of the single-crystalline GaN
nanowires was fabricated in the honeycomb structure of
anodic alumina.
II. EXPERIMENTAL
The two-step fabrication method was employed to
synthesize the ordered nanostructure of the single-
crystalline GaN nanowires in the honeycomb structure of
anodic alumina. The first step was to synthesize the or-
dered nanostructure substance, and then the single-
crystalline GaN nanowires were assembled into each of
the highly ordered honeycomb pores of the anodic alu-
mina through a gas reaction.
After recovery to room temperature, the obtained
specimen was characterized by a D/Max-rA rotating-
anode x-ray diffractometer with Cu K␣ radiation (␭ ס
1.542 Å) and a Spex-1403 laser Raman scattering spec-
J. Mater. Res., Vol. 15, No. 2, Feb 2000
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G.S. Cheng et al.: Ordered nanostructure of single-crystalline GaN nanowires in a honeycomb structure of anodic alumina
trometer with 514.5 nm incident wavelength and
200 mW output power. Before the gas reaction was un-
dertaken, an Auto Probe CP atomic force microscope
(AFM) was employed to characterize the morphology of
the as-prepared anodic alumina substance. Scanning
electron microscopy (SEM) was performed on a JEOL
JSM-6300 scanning electron microscope. Transmission
electron microscopy (TEM) and selected-area electron
diffraction (SAED) of the specimen were performed on a
JEM-200CX transmission electron microscope. The pho-
toluminescence spectrum (PL) of the specimen was
measured in a Hitachi 850 fluorescence spectrophotom-
eter with a Xe lamp at room temperature under 250 nm
ultraviolet light excitation and 290 nm filter wavelength.
Before SEM observation, metallic Pt was evaporated
on the surface of the specimens with the evaporation
current of 7 mA for 45 min to form a conducting film to
avoid electrostatic charging. For TEM observation, the
specimen was pulverized in ethyl alcohol and then dis-
persed onto a carbon film covered on a copper grid.
GaN phase, and A(400) and A(440) correspond to the
cubic ␬–Al2O3, phase. In the Raman back-scattering
spectrum at room temperature (Fig. 3), the peaks 531,
560, 569, 733, and 745 cm−1 satisfactorily agree to pho-
III. RESULTS AND DISCUSSIONS
FIG. 2. X-ray diffraction spectrum of the ordered nanostructure of the
single GaN nanowires in the honeycomb structure of the alumina
membrane.
On the AFM image (Fig. 1), pores of the membrane in
the present work are 20 nm in diameter and 45 ␮m in
depth, with pore spacing of 60 nm, respectively. The
depth of the alumina pore is proportional to the anodi-
zation time.12 Longer anodization time favors not only
increasing the pore depth but also extending the unifor-
mity of each pore and the highly regular zone of the
anodic alumina.
In the x-ray diffraction spectrum (Fig. 2), the reflec-
tion peaks of (100), (002), (101), (321), (102), (110),
(103), and (112) correspond to the hexagonal wurtzite
FIG. 1. Atomic force microscopy image of the honeycomb structure
of the anodic alumina.
FIG. 3. Raman backscattering spectrum of the ordered nanostructure.
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G.S. Cheng et al.: Ordered nanostructure of single-crystalline GaN nanowires in a honeycomb structure of anodic alumina
FIG. 4. SEM images of the ordered nanostructure: (a) low-magnification image of cross section and surface of the ordered nanostructure, (b)
low-magnification image of the ordered nanostructure surface, (c) high-magnification image of the ordered nanostructure surface.
non vibration frequencies of A1(TO), E1(TO), E2,
A1(LO), and E1(LO) modes of crystalline wurtzite
GaN,13 respectively. The first peak at 420 cm−1 corre-
sponds to the phonon vibration frequency of the A1(g)
mode of Al2O3. The Raman backscattering spectrum
results are consistent with those of x-ray diffrac-
tion (XRD).
Figures 4(a)–4(c) present SEM morphologies of low-
magnification cross section and surface, low-
magnification surface, and high-magnification surface of
the ordered nanostructure, respectively. SEM results in-
dicate that the aligned nanowires grow out of the uniform
pores of the anodic alumina [Fig. 4(a)] and some of them
end somewhere above the anodic alumina substance
[Fig. 4(b)]. The high-magnification image of the ending
point [Fig. 4(c)] shows that a large number of the nano-
wires aggregate on their ends like tenuous cotton fibers
above the surface of the anodic membrane.
The high-magnification image of TEM (Fig. 5) shows
that the diameter size of the individual nanowire is in the
general vicinity of 20 nm, which is in concert with the
AFM results of the honeycomb structure of the anodic
alumina. The SAED pattern is consistent with the single-
crystalline nature. The patterns can be indexed to the
reflection of the wurtzite GaN [001] (the insert in Fig. 5).
The gas reaction11,14 taking place in the nanometer
reaction space of the uniform pores can be expressed as
(VLS) of quasi-one-dimensional materials,16,17 the coex-
isting phase of Ga and N atoms in the supersaturated In
nanoparticles results in nucleation and crystallization of
GaN. The Ga and N atoms go on solving into the liquid-
In nanoparticles, and then the supersaturated state ap-
pears again. The GaN grows and crystallizes as solid
nanowires along some prior growth orientation in the
alumina membrane nanochannels. The growth of GaN
nanowires terminates out of the surface of the alumina
membranes when they aggregate on one end of them.
In the VLS process, the liquid catalyst nanoparticle
acts as the energetically favored site for absorption of
gas-phase reactants and defines the size of the required
material. The In catalyst nanoparticle defines the diam-
eter size of the GaN nanowire. However, the size of the
In catalyst nanoparticle is determined by the size of the
alumina membrane nanochannel. So the nanochannel of
the alumina membrane with the greatest aspect ratio10
(depth/diameter) not only favors the gas reaction in the
uniform nanochannel to form GaN nanowires but also
constrains the GaN nanowire to preferentially grow
along the nanochannel axis and defines the diameter of
GaN nanowires. In summary, the high uniformity of the
4Ga(l) + Ga2O3(s) → 3Ga2O(v)
,
(1)
Ga2O(v) + 2NH3(g) → 2GaN(s) + H2O(g) + 2H2(g)
.
(2)
The pressure of Ga2O vapor generating from reaction (1)
at 1273 K is about 7.2 torr.15 It is expected that the GaN
formation originates from the gas reaction (2) of Ga2O
vapor with the flowing ammonia on the surface of the
liquid-In nanoparticle (In melting point is 430 K). The
continuous solving of Ga and N atoms into the liquid-In
nanoparticles leads to a supersaturated state formation.
According to the vapor–liquid–solid growth mechanism
FIG. 5. High-magnification TEM image of an individual single-
crystalline GaN nanowire. The insert in Fig. 4 is the pattern [001] of
selected area electron diffraction of an individual GaN nanowire.
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G.S. Cheng et al.: Ordered nanostructure of single-crystalline GaN nanowires in a honeycomb structure of anodic alumina
IV. CONCLUSION
alumina membrane nanochannels and the nucleation
and growth reactions in the presence of the liquid-In
catalyst nanoparticles may be responsible for the for-
mation of the ordered nanostructure of the single-
crystalline GaN nanowires in the honeycomb structure of
the anodic alumina.
The PL spectrum pattern (Fig. 6) consists of one
strong broad (440 nm) and one sharp (505) emission
peak. The PL investigation of the unassembled anodic
alumina shows that the sharp-emission peak originates
from the substance itself for the mechanism of singly
ionized oxygen vacancies.18 Therefore, it can be deduced
that the other broader peaks result from the light emission
of the ordered nanostructures of the single-crystalline
GaN nanowires, which apparently means that there is a
stronger light emission band in the visible light range and
the emission center is 440 nm.
The synthesis of the ordered nanostructure of the
single-crystalline GaN nanowires on anodic alumina is
achieved by means of the gas reaction of Ga2O vapor
with the constant flowing ammonia atmosphere. The ob-
servation of AFM, XRD, Raman backscattering spec-
trum, and TEM confirm that the single-crystalline GaN
nanowires are uniformly assembled into the ordered
pores of the anodic alumina. The strong photolumines-
cence efficiency in the visible light range is investigated
in the ordered nanostructure.
ACKNOWLEDGMENTS
Authors are grateful to Professor W.P. Cai and Profes-
sor G.W. Meng for valuable discussion and suggestions.
The financial support of this work by the National
Climbing Project of China “Nanostructured Materials
Science” is acknowledged.
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FIG. 6. Photoluminescence spectrum of the ordered nanostructure of
the single-crystalline GaN nanowires in the alumina membrane.
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