Abstract
Straight GaN nanowires with a wurtzite structure were grown at 
by chemical vapor deposition with a 
system without adding carbon powder or gallium metal as a reducer. The self-catalysis Ga metal was generated by reacting the 
source and hydrogen decomposed from 
. Although no nanoparticles on the tips have been observed, the tip-growth mode via the vapor-liquid-solid growth mechanism is proposed to support the evidences of the straightness and long length of GaN nanowires. Under this mechanism, Ga catalysts provide a flow path for GaN molecules, which are obtained by the nitridation of adsorbed 
on Ga catalysts, to precipitate GaN nanowires in a fast rate.
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GaN with a wide direct bandgap of 
is one of the hottest semiconducting materials used as blue light-emitting devices, blue laser diode, and power electronic devices for flat panel displays, blue laser, and optical communication.1–4 Nanoscale one-dimensional (1D) materials, such as nanowires (NWs), nanorods, nanobelts, and nanotubes, have become of great interest due to their importance in basic scientific research and potential technology applications.5 They are expected to play an important role for electronic, optoelectronic, electrochemical, and electromechanical nano devices. Therefore, the investigation of GaN nanomaterials is important.
The vapor-liquid-solid (VLS) process was developed by Wagner and has become a widely used method for generating 1D nanostructures from a rich variety of inorganic materials. On the basis of this mechanism, the produced nanowires are expected to grow faster and to be smooth and straight instead of being tortuous with ramifications and having a bamboo-like microstructure. GaN nanorods, first reported by Han et al. , were prepared through a carbon nanotube-confined reaction with a 
source.6 To grow the GaN NWs, other chosen gallium sources have been the metallic Ga,7–9 
,10, 11 trimethyl gallium,12, 13 
,14 and 
.15 Many Ga-containing precursors for chemical vapor deposition (CVD) are expansive. Ga metal is the cheaper and easy-handle source for 1D CVD-GaN, therefore most 1D GaN were obtained with the 
system. For using 
as the Ga source for 1D CVD GaN, the 
-containing systems have high growth temperatures in order to decompose the stable oxide. Therefore, carbon or metallic Ga has been added to the 
-related system in order to lower reaction temperature and to form the gaseous 
, which reacts with 
to obtain 1D GaN. Han et al. synthesized GaN NWs with the 
-related system at lower a temperature of 
due to the nanotube-confined reaction with a short transport path between vapor source and carbon nanotubes. In order to have higher Ga-containing vapor pressure, 
-related system had the growth temperature above 
or operated at lower chamber pressure. As for the catalysts, the most popular catalyst for GaN NWs is nickel. Some successful examples of 1D GaN growth have been performed by self-catalyzing or using a gold catalyst.
In this study, we report the growth of GaN nanowires on 
substrate by CVD with a 
system at 
for 
without adding carbon powder or gallium metal. The growth, crystal structure, and microstructure of GaN NWs were investigated. The formation of GaN NWs via the VLS growth mechanism was evaluated. The direct synthesis of CVD-GaN nanowires with the 
system without incorporating carbon or gallium has not been reported in the literature to date.
Experimental
1D GaN NWs were prepared in an atmospheric hot-wall CVD reaction chamber. The growth of 1D GaN was conducted at a growth temperature of 
for 
by using a 
system. 
at a flow rate of 
was used as the reaction gas and carrier gas. The substrate was located at the center of the furnace with a 
-loaded crucible set next to it. In each experiment, 
powder of 
was loaded. The (100)-oriented Si substrates coated with sputtered Au were used for the NW growth. The phase formation of CVD-GaN was analyzed by X-ray diffraction [(XRD), Rigaku D/Max-2500, Japan]. A field-emission scanning electron microscope [(FESEM), JEOL JSM 6500F, Japan] equipped with energy-dispersive spectroscopy was used to observe growth morphology and analyze composition. Phase identification and microstructural characterization of nanowires were also conducted by a transmission electron microscope [(TEM), JEOL 3010, Japan].
Results and Discussion
Figure 1 shows FESEM images of GaN nanowires synthesized by CVD at 
on 
substrates for 
with a 
system. An enlarged FESEM image of the 
-grown GaN NWs is displayed in Fig. 1b. The GaN NWs had a long wire length of 
or with a fast growth rate over 
. The actual length was difficult to measure due to the abundance of the nanosized nanowires. The diameters of GaN NWs covered on the outer surface were mainly in the range of 
. Only a few works have reported the 1D GaN synthesized by the 
-containing system. Most of the reported results did not produce 1D GaN in abundant quantity nor was the 1D GaN straight with a smooth surface. Even the confined reaction produced the wavy morphology. Qiu et al. synthesized tapered GaN rods with a straight edge by using a 
system.10 Hu et al. produced the straight and hollow GaN nanotubes by conversion from 
nanotubes.11 They used the 
system to produce 
nanotubes by reacting 
with a graphite crucible at 
under 
, followed by confined nitridation conversion at 
to form GaN.
Figure 1. (a) FESEM images of GaN nanowires synthesized by CVD at 
on 
substrates for 
with a 
system and (b) enlarged image of (a).
Figure 2 shows an XRD pattern of GaN NWs deposited at 
by CVD on 
substrates with a 
system for 
. The XRD diffraction pattern was identified to be the hexagonal wurtzite structure for GaN NWs. The synthesized product was single-phase GaN. No other crystalline impurities were detected within the detection limit. Strong diffractions were contributed from (101), (002), and (100), in order of intensity. The composition analysis of the energy-dispersive-spectroscopy (EDS) spectrum by FESEM obtained 16.6, 14.4, 1, and 68% for Ga, N, O, and Si, respectively. The atomic ratio of 
was close to 1:1 for the stoichiometric GaN within experimental error.
Figure 2. XRD pattern of GaN NWs deposited at 
by CVD on 
substrates with a 
system for 
.
Figure 3 displays (a) a TEM image, (b) [-1-12-3] zone diffraction pattern, (c) [0001] zone diffraction pattern, and (d) the select-area composition analysis of GaN NWs synthesized by CVD at 
on 
substrates with a 
system for 
. The bright-field image in Fig. 3a shows a clear image at the tip end of a GaN nanowire. Obviously, there were no nanoparticles attached to the tips as evidence for the VLS growth mechanism. This nanowire had a diameter of 
. The edge of this GaN nanowire had a smooth, not wavy, surface. GaN NWs produced by CVD at 
using a 
-carbon powder-
system were wavy due to self-assembly growth behavior.16 Figures 3b and 3c show the diffraction patterns of the nanowire in Fig. 3a. These two diffraction patterns at different zone axes were helpful to identify the crystal structure of the grown nanowires. From the electron diffraction patterns, the hexagonal wurtzite structure was confirmed with [-1-12-3] and [0001] zone diffractions in Fig. 3b and 3c, respectively. The observed d(hkl) spacing distances were 2.75, 1.59, and 
for different (hkl) planes of (100), (110), and (200), respectively, which were consistent with the calculated values for pure GaN crystals. The composition analysis of the EDS spectrum by TEM (Fig. 3d) obtained 11.4% Ga and 11.0% N, except for the background signals from Si, O, C, and Cu. The microstructural analyses by TEM were consistent with those obtained from FESEM analyses.
Figure 3. (a) TEM image, (b) [-1-12-3] zone diffraction pattern, (c) [0001] zone diffraction pattern, and (d) the select-area composition analysis of GaN NWs synthesized by CVD at 
on 
substrates for 
with a 
system.
At the growth of GaN NWs with the 
system, no Au nanoparticles were observed at the tips of 
-grown GaN NWs (Fig. 3a). The growth of 1D nanomaterials without catalysts at the tips usually involves the vapor-solid mechanism. However, our 
-grown GaN NWs were straight without tortuous or bamboo-like microstructure. This information indicates that the vapor-solid mechanism might not be suitable. The VLS growth mechanism should be a better choice. This process was developed by Wagner and has become a widely used method for generating 1D nanostructures from a rich variety of inorganic materials.17 A bottom-growth mode via the VLS mechanism is hard to push the grown nanowires outward without making NWs distorted. A tip-growth mode can easily extend the nanowire length and support the evidence of the straightness of long nanowires. In our proposed VLS mechanism, based on the fact of no catalyst tips, the gold catalyst should not provide the path for oversaturated precipitation of GaN NWs. A self-catalysis VLS mechanism with a gallium melt on tips is favored. The Ga melt nanoparticles are produced by the reduction reaction of 
by the hydrogen decomposed from 
at high temperature of 
. Considering the insufficient reaction temperature of 
and the limited contact area, the carbothermal reaction between 
powder and a graphite crucible at 
is not favored. The decomposition of 
is expressed as
The generated hydrogen reduces 
to 
vapor then further to Ga vapor. The reactions are proposed as
The subscript ad refers to the atoms adsorbed on the substrate. The Ga vapor preferably adsorbs on Au nanoparticles to form adatoms, dissolutes, and dissolves into Au catalysts, then results in oversaturated precipitation to form the Ga melt nanoparticles for the self-catalyzed growth of 1D GaN. Once Ga precipitates, the vapor-solid mechanism can also occur to have Ga metal adatoms migrate and aggregate to form Ga catalysts. The reaction steps are listed as
represents Ga dissolved in Au catalyst. However, the amount of decomposed Ga is limited and cannot initiate the Au tip-mode growth by continuing dissolution into Au catalysts and precipitation, but a substitute of the Ga tip-mode growth. 
vapor is the major Ga-containing source in our 
system, and its adsorbates cannot dissolve into Au. On the basis of the thermodynamic considerations, 
nitridation is highly preferred.18, 19 The 
vapor is preferentially adsorbed on Ga and is nitridized by reacting with the adsorbed 
or gaseous 
to form GaN nuclei. The precipitated Ga melt provides a flow path for nitridized GaN nuclei. The selectivity of the Ga melt for 
instead of other or N-containing species involves a complex dissolution problem. A continuous flow of adsorbed GaN nuclei through the Ga melt path to form GaN nanowires leads to the fast growth of long and straight GaN NWs in 
. The Ga melt as a flow path will be also nitridized by 
above 
. The nitridation reactions on the surface of melt Ga are listed as follows
The kinetic reactions with adsorbed atoms or molecules were based on surface reactions, which have a larger opportunity for the molecule-collision-type reactions to occur. The possibility for the occurrence of the gas-phase reactions is relatively lower. Once the experiment is stopped, the Ga melts on the tips with dissoluted GaN nuclei will be nitridized, self-consumed, or vaporized. Therefore, no melt round tips have been observed for our GaN NWs prepared from a 
source system. Schematic presentation to explain the growth mechanism of the 1D GaN nanowires prepared by CVD with a 
system is shown in Fig. 4.
Figure 4. Schematic presentation of growth mechanism of the 1D GaN nanowires prepared by CVD with a 
system.
Conclusions
In summary, the self-catalyzed growth of GaN nanowires has been demonstrated by CVD at 
with 
as the Ga source and 
as the N source. The formations of the 
and Ga from 
for the CVD gaseous precursor and for the self-catalysis, respectively, were attributed to the reduction reactions of hydrogen decomposed from 
. Although there were no nanoparticles observed on the tips of nanowires, a tip-growth mode via the VLS growth mechanism remains the favorite after judging from its long length and straightness of GaN NWs. The self-catalysis Ga metal behaves as a flow channel for nitridized GaN molecules to precipitate GaN nanowires in a fast rate.
Acknowledgment
Funding for this study was provided by the National Science Council of the Republic of China under grant no. NSC 95-2221-E-011-220-MY2 .
National Taiwan University of Science and Technology assisted in meeting the publication costs of this article.