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Available online at www.sciencedirect.com

Materials Chemistry and Physics 109 (2008) 249–252

Synthesis and characterization of GaN nanowireswith Tantalum catalyst

Hong Li, Chengshan Xue ∗, Huizhao Zhuang, Jinhua Chen,Zhaozhu Yang, Lixia Qin, Yinglong Huang, Dongdong Zhang

Institute of Semiconductor, Shandong Normal University, Jinan 250014, PR China

Received 25 July 2007; received in revised form 7 November 2007; accepted 13 November 2007

bstract

Single-crystalline wurtzite GaN nanowires have been synthesized through ammoniating Ga2O3/Ta films by RF magnetron sputtering. The prod-cts have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopyHRTEM), selected area electron diffraction (SAED), X-ray photoelectron microscopy (XPS) and photoluminescence (PL). The results showhat the nanowires have a hexagonal wurtzite structure with diameters ranging from 10 nm to 30 nm and lengths typically up to several tens of

icrometers. The representative photoluminescence spectrum at room temperature exhibits a strong UV light emission band centered at 364 nm.he growth mechanism of the crystalline GaN nanowires is discussed briefly. 2007 Elsevier B.V. All rights reserved.

ACS: 68.65.−k; 81.15.Cd; 78.30.Fs

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eywords: Nanowires; GaN

. Introduction

Gallium nitride (GaN), which has a direct and wide bandgapf 3.4 eV at room temperature, has been considered as an idealaterial for the fabrication of blue/green light emitting diodes

LDS), laser diodes (LDS) and high power integrated circuits1,2]. In the past decade, nanomaterials have received extensiveesearch interest for their great prospects in the fabrication ofanoelectronic devices and development of new nanotechnolo-ies [3]. So far, GaN materials have been synthesized by manyifferent techniques such as carbon nano-tube-confined reaction4], sublimation method [5], direct reaction of metal Ga and NH36] and metal-catalyzed growth based on the vapor–liquid–solidVLS) mechanism [7,8]. In this paper, GaN nanowires haveeen synthesized on Si substrates by RF magnetron sputteringsing tantalum as catalyst, which perhaps open a new field for

echnological application in nanodevices. This growth methodllows a continuous synthesis and produces a large quantity ofingle-crystal GaN nanowires at relatively low cost (only a small

∗ Corresponding author. Tel.: +86 53186182624; fax: +86 5316180017.E-mail address: xuechengshan@sdnu.edu.cn (C. Xue).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.11.021

uantity of tantalum is used). So, it would be of interest forommercial-scale production.

. Experimental details

We synthesized GaN nanowires from a self-assembled Ga2O3 film by reac-ion with NH3. There were two steps in the whole process. The first step washat the Ta films and Ga2O3 films were deposited in turn on Si (1 1 1) substraten a JCK-500A magnetron sputtering system. The targets for depositing Ta filmsnd Ga2O3 films were Ta with purity of 99.9% and sintered Ga2O3 with purityf 99.9%, respectively. The conditions of sputtering were as follows: the baseressure before sputtering was about 7.8 × 10−4 Pa. The working gas was purergon (≥99.99%) and the working pressure was 2 Pa. The radio frequency sput-ering power was adjust to 150 W. The sputtering time was about 5 min for Tand 90 min for Ga2O3.

The second step was that the Ga2O3/Ta films were ammoniated for 15 min at50 ◦C under flowing ammonia atmosphere in a horizontal tube furnace. Whenhe tube furnace reached the temperature of 850 ◦C steadily, the samples werelaced into the constant temperature zone of the furnace. Flowing nitrogen withflux of 500 ml/min was first introduced into the system for 5 min to expel air.hen ammonia with a flux of 500 ml/min flowed into the system for 15 min.inally, the NH3 was expelled by N2 before the samples were removed from

he furnace. We can find the color of samples turned to be light yellow aftermmoniating.

A Rigaku D/max-rB X-ray diffraction (XRD) meter with a Cu K� line, aitachi S-570 scanning electron microscope (SEM), a Philips TECNAI-20 high-

esolution transmission electron microscope (HRTEM), a MICROLAB MKIIX-

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Fig. 1. XRD pattern of the synthesized GaN nanowires.

ay photoelectron spectroscope (XPS) and an FLS920 fluorescence spectro-hotometer at room temperature were applied to examine the structure, surfaceorphology, composition and optical properties of the GaN nanowires.

. Results and discussions

Fig. 1 shows the typical XRD pattern of the nanowires weroduced. Three high peaks are found at 2θ = 32.4◦, 34.5◦, and6.7◦, respectively. All these peaks could be indexed to a hexag-nal wurtzite GaN phase with lattice constants of a = 0.3186 nmnd c = 0.5178 nm, which are the same as the reported valueor bulk GaN [9]. No diffraction peaks of Ga2O3 or othermpurities are found in any of our samples, suggesting thathe surface of products is predominantly hexagonal wurtziteaN.Fig. 2 shows the typical SEM images of the as-synthesized

anowires ammoniated at 850 ◦C at different magnifications.

t low magnification, it can be clearly seen from Fig. 2(a) that

he products consist of lots of high-density long nanowires inter-aced with each other. Liquid droplets, which are the remarkableign of the vapor–liquid–solid (VLS) mechanism, are found on

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Fig. 2. Typical SEM images of the synthesized G

Physics 109 (2008) 249–252

he tips of some nanowires at high magnification, as shown inhe inset of Fig. 2(b). And further observation demonstrates thathe nanowires possess a relatively smooth surface and a straight

orphology with diameters ranging from 10 nm to 30 nm andengths typically up to several tens of micrometers, indicating aonderful aspect ratio.The HRTEM images of a single nanowire were given in

ig. 3. Fig. 3(a) is the image tested with lower magnification. Iteveals that the GaN nanowire with a uniform diameter of about5 nm has clean and very smooth surface morphology. The insethows the selected area electron diffraction (SAED) pattern ofhe single nanowire, where (0 1̄ 1 1), (1 1̄ 0 1), (1 0 1̄ 0) diffractionpots present, which can be index to the reflection of hexagonalurtzite GaN single crystal. Fig. 3(b) is higher-magnification

mage of the single nanowire. The clear lattice fringes confirmhat the nanowire is high quality hexagonal single-crystallineaN. The distance between the two fringes is 0.276 nm, which

s corresponding to the plane distance of GaN (1 0 0), indicatinghe growth direction of the nanowires is perpendicular to theringes of (1 0 0) plane.

Fig. 4 shows the XPS for the as-synthesized GaN nanowires.ig. 4(a) shows a general scan in the binding energy rangingrom 0 eV to 600 eV. The peaks of the core level from Ga 3d,a 3p, Ga LMM Auger peak, C 1s, N 1s and O 1s are shown.

t is observed (Fig. 4(b)) that the peak of Ga 3d is located at theinding energy of 19.8 eV, indicating that the Ga atoms are inhe compound state (GaN) [10]. No satellite peak correspondingo Ga2O3 as reported by Pal and Sugino are found in Fig. 4(b)11]. As shown in Fig. 4(c), the binding energy of Ga 2p3/2nd Ga 2p1/2 are located at 1144.56 eV and 1117.88 eV, whilehe binding energy of the elemental Ga are at 1116.6 eV [12],118.5 eV [13], 1119.2 eV [14], which confirms the bondingetween Ga and N. As shown in Fig. 4(d), the binding energy of1s is located at 397.25 eV and the full-width at half maximum

s about 2.3 eV. The width and slight asymmetry of the N 1seak could be due to the chemisorbed elemental N in the as-ynthesized GaN [12]. The binding energy of the O 1s is locatedt 530.65 eV. Because the binding energy of chemisorbed O2

aN nanowires at different magnifications.

H. Li et al. / Materials Chemistry and Physics 109 (2008) 249–252 251

Fig. 3. HRTEM images of a single GaN nanowire.

Fig. 4. XPS spectra obtained from the GaN nanowires.

252 H. Li et al. / Materials Chemistry and

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Fig. 5. PL spectrum of the synthesized GaN nanowires.

n the surface ranged from 530.0 eV to 530.9 eV, the O 1s peakay be due to chemisorbed O2.The measurement of PL spectrum was performed at room

emperature and the data is shown in Fig. 5. A strong UV lightmission peak at 364 nm and a very weak light emission bandentered at 388 nm can be observed. The UV light emission hassmall blue shift compared with bulk GaN [15]. The weak emis-ion band centered at 388 nm might be due to the excitons boundo surface or other structure defects [16]. However, further works needed to study the PL mechanism of the GaN nanowires.

Although the detailed growth mechanism of the GaNanowires is still not fully understood, we might briefly explainhe process based on the above observations. Ta is very impor-ant for the fabrication of such GaN nanostructure. To text this,e also ammoniated Ga2O3 films under the same condition, ando such nanowires were found. As the fluidization temperaturef nanosized catalytic metal particles is lower than the melt-ng point of bulk metal [17], the Ta film broke up, and thenhe liquid Ta nanodroplets which act as energetically favorableites for absorption of gas-phase reactants are formed on the Siurface at high temperature. It is well known that Ga2O3 canecompose to gaseous Ga2O or Ga at temperature above 800 ◦C18] and NH3 decomposes stepwise to NH2, NH, H2 and N at50 ◦C [19]. Then the gaseous Ga2O, the atomic Ga, the atomicand NH3 could be absorbed in above-mentioned sites to form

a–Ga–N transition alloys. When the concentration of Ga–Nxceeds a saturation point in the liquid phase Ta–Ga–N alloyroplet, GaN begins to grow from liquid phase and depositso form nanowires. In the process, some nanodroplets could be

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Physics 109 (2008) 249–252

aken away by the flowing NH3, and then the regularly shapedips might be formed. However, the growth mechanism needs toe further studied.

. Conclusions

In summary, GaN nanowires were successfully preparedhrough ammoniating Ga2O3/Ta films on Si substrate at 850 ◦Cor 15 min. The morphology, structure and optical propertiesf the synthesized GaN nanowires were investigated by SEM,RD, HRTEM, XPS and PL. The nanowires were single-

rystalline wurtzite GaN with diameters ranging from 10 nm to0 nm and lengths typically up to several tens of micrometers.he PL spectrum exhibited a strong emission peak at 364 nm.he as-prepared nanowires might be used for technologicalpplications in future nanodevices.

cknowledgements

This project is supported by the Key Research Program of theational Natural Science Foundation of China (No. 90201025)

nd the National Natural Science Foundation of China (No.0301002).

eferences

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