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Fabrication of GaN nanowires by ammoniating Ga2O3/Al2O3 thin films deposited on Si(111)

with radio frequency magnetron sputtering

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2004 Nanotechnology 15 724

(http://iopscience.iop.org/0957-4484/15/7/002)

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INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY

Nanotechnology 15 (2004) 724–726 PII: S0957-4484(04)71828-0

Fabrication of GaN nanowires byammoniating Ga2O3/Al2O3 thin filmsdeposited on Si(111) with radio frequencymagnetron sputteringChengshan Xue, Qinqin Wei, Zhencui Sun, Zhihua Dong,Haibo Sun and Liwei Shi

Institute of Semiconductors, Shandong Normal University, Jinan 250014,People’s Republic of China

Received 13 November 2003Published 8 April 2004Online at stacks.iop.org/Nano/15/724DOI: 10.1088/0957-4484/15/7/002

AbstractGaN nanowires were synthesized by ammoniating Ga2O3/Al2O3 thin filmsdeposited on Si(111) with radio frequency magnetron sputtering. Thecylindrical structures were as long as several micrometres, with diametersranging between 5 and 40 nm. X-ray diffraction (XRD, Rigaku D/max-rBCu Kα), scanning electronic microscope (SEM, HitachiH-8010) andhigh-resolution TEM (HRTEM) results show that most of the GaNnanowires have a single-crystal hexagonal wurtzite structure with major axis[001] alignment. A minority of them are polycrystalline, composed ofmicrograins with different growth orientations.

1. Introduction

Gallium nitride (GaN), which has a direct and wide bandgapof 3.4 eV at room temperature, has been considered asa promising material for photoelectronics, high-temperatureand high-power electronic devices [1]. Due to thecontinual demand for reduction in device size, and thenano-materials’ electronic, magnetic, optical, biologicaland chemical characteristics that are not obtained withconventional materials, it naturally attracted much attentionfor the preparation of low-dimensional materials. In recentyears, much effort has been made in the synthesis of GaNnanowires. Cheng et al [2] formed GaN nanowires in anodicalumina membranes (AAMS) through a gas-phase reaction ofGa2O vapour with flowing NH3. Chen and Yeh [3] producedGaN nanowires on a silicon wafer or quartz plate usingpolycrystalline indium powder as a catalyst. He et al [4]grew GaN nanowires and nanotubes by direct reaction of Gavapour with flowing NH3. Yang et al [5] grew GaN nanowiresby ammoniating a Ga2O3 thin film deposited on quartz withradio frequency magnetron sputtering. Goldberger et al [6]synthesized GaN nanotubes using ZnO nanowires as templates.

In this system, we chose α-Al2O3 material on Si toreplace the sapphire substrates for growth of GaN because

almost all high-quality GaN films are grown on sapphire andα-Al2O3 may serve as an insulator layer between the GaN andthe Si substrate. In particular, epitaxial growth of insulatorlayers on Si is of great importance in achieving silicon-on-insulator (SOI) structures and for the long-range goal of three-dimensional integrated circuits. If GaN can be prepared onα-Al2O3/Si, it offers a very attractive potential to harmonicallyincorporate GaN optoelectronic devices into silicon-based verylarge scale integrated circuits. In this paper, we report thesynthesis of GaN nanowires by ammoniating Ga2O3/Al2O3

thin films with radio frequency magnetron sputtering.

2. Experimental details

Si(111) substrates were cleaned by standard cleaningprocedures. Ar, N2 and NH3 gases of 99.999% purity wereused as either the sputtering gas or reactive gas. The sampleswere prepared by a two-step method. The first step was togrow Ga2O3/Al2O3 films on Si(111). This was prepared ina JCK-500A rf magnetron sputtering system. Si substrateswere held on the water-cooled upper electrode to maintain thesubstrate’s temperature close to room temperature. The Al2O3

films were grown by sputtering a sapphire target with rf powerof 150 W for 10 min, with 1.2 Pa sputtering pressure (pure

0957-4484/04/070724+03$30.00 © 2004 IOP Publishing Ltd Printed in the UK 724

Fabrication of GaN nanowires by ammoniating Ga2O3/Al2O3 thin films deposited on Si(111) with radio frequency magnetron sputtering

30 35 40 45 50

Inte

nsi

ty(a

.u)

2 theta(degree)

(002

)(1

01)

Figure 1. X-ray diffraction pattern of the sample.

Figure 2. SEM image of the GaN nanowires.

argon). Subsequently, the Ga2O3 films were grown on themby sputtering a sinter Ga2O3 target with rf power of 150 W for90 min and 1.2 Pa sputtering pressure (10% nitrogen in argon).The distance between the target and the substrate was 8 cm.

Subsequently, the samples were ammoniated in an opentube furnace inside a horizontal quartz tube. When the furnacereached the equilibrium temperature of 950 ◦C, the samplesplaced on a quartz boat were pushed into the centre of thefurnace. Flowing nitrogen with a flow rate of 1500 ml min−1

was first introduced into the tube for 10 min to flush out theresidual air. Then ammonia was flowed into the tube at a flowrate of 1000 ml min−1 while the N2 was switched off. Theammoniating time was 15 min. At the end of the ammoniatingtime, the NH3 was flushed out by N2 before the boat wasremoved from the furnace.

The samples were characterized by x-ray diffraction(XRD, Rigaku D/max-rB Cu Kα), scanning electronicmicroscopy (SEM, HitachiH-8010) and high-resolutionTEM (HRTEM) images which were obtained with a PhilipsTecnai-20U-TWIN electron microscopy system operated at200 keV.

3. Results and discussions

Figure 1 shows the x-ray diffraction pattern of the sample.There are two diffraction peaks located at 2θ = 34.5◦ and

Figure 3. (a) A HRTEM image of a straight GaN nanowire. (b) AHRTEM image of an eburnean chain GaN nanowire from the samesample.

Figure 4. (a) The HRTEM image with higher magnification and theSAED pattern of a straight nanowire. (b) The HRTEM image withhigher magnification and the SAED pattern of an eburnean chainGaN nanowire.

36.8◦.The reflection peaks of (002) and (101) correspond tothe hexagonal GaN wurtzite structure with lattice constantsof a = 0.318 nm and c = 0.518 nm. These values arein good agreement with the published values [4] for GaNnanowires and indicate that the material is predominantlypolycrystalline hexagonal wurtzite GaN. The high intensityof the (002) diffraction peak indicates that the growth of thesample shows a preference for the [111] orientation.

Figure 2 shows the SEM image of the sample. Thenanowires cross each other and wind like some tenuous cottonfibres, which are randomly distributed on the surface of theGaN films. The cylindrical structures were as long as severalmicrometres with diameters ranging between 5 and 40 nm.

Figure 3(a) shows the HRTEM morphology of anindividual straight nanowire with diameter of about 30 nm.It is straight and the diameter along the growth direction isuniform. Figure 3(b) shows a nanowire, which twists and isassembled by lots of micrograins and looks like an eburneanchain.

Figure 4(a) shows the typical lattice-resolved HRTEMimage of the same straight nanowire as in figure 3(a) withhigher magnification and its corresponding SAED pattern. Thegrowth direction is along [001] and the distance between two(001) planes is ∼0.310 nm, which is approximately the same asthat of the bulk wurtzite GaN crystal. The clear lattice fringesconfirm that the synthesized nanowires are single-crystal GaN.The inset shows a selected-area electron diffraction (SAED)pattern of the nanowires that can be indexed to the reflection ofhexagonal GaN crystals along the [001] direction. Figure 4(b)shows the HRTEM image of the same eburnean chain GaNnanowire as in figure 3(b) with higher magnification and also

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the SAED pattern. From figure 4(b) we can see that the growthdirections of each micrograin are different from each other.The SAED pattern confirms this nanowire is polycrystal GaN.

The detailed growth mechanism is still under study.However, we can briefly describe the stages as follows. It iswell known that at 850 ◦C or higher NH3 decomposes stepwiseto NH2, NH and N, and Ga2O3 also begins to decomposeto Ga2O and Ga. A GaN molecule can be yielded throughthe reaction of Ga2O and NH3 and the reaction of N and Ga.These GaN molecules agglomerate into micrograins, which arenecessary for this form of nanostructure. Then the micrograinsheap up and form the eburnean chain nanowires. When thegrowth directions of the micrograins of the eburnean chainnanowires orientate in the same direction, the single-crystalGaN nanowires are formed.

The role of the Al2O3 films during the growth of GaNnanowires is still under study.

4. Conclusion

In summary, GaN nanowires were synthesized by ammoniat-ing Ga2O3/Al2O3 thin films deposited on Si(111) with radiofrequency magnetron sputtering. In our growth system thereis no assistance from a catalyst or geometric confinement. Thecylindrical structures were as long as several micrometres withdiameters ranging between 5 and 40 nm. Most of the nanowires

are single-crystal hexagonal wurtzite GaN nanowires. A feware polycrystalline, being assembled by some micrograins withdifferent growth directions, which give the fibres an eburneanchain outline. These eburnean chain nanowires may be theintergradation from GaN micrograins to single-crystal GaNnanowires. This is significant for us in understanding thegrowth mechanism of GaN nanowires in this system.

Acknowledgments

The authors were grateful to Professor JingPing Zhang in ourinstitute for valuable discussions and suggestions. This projectis supported by the Key Research Program of National NaturalScience Foundation of China (no. 90201025) and the NationalNatural Science Foundation of China (no. 90301002).

References

[1] Nakamura S and Tietian J 1994 Appl. Phys. Lett. 74 1687[2] Cheng G S, Zhang L D, Zhu Y, Fei G T and Li L 1999

Phys. Lett. 75 2455[3] Chen C and Yeh C 2000 Adv. Mater. 12 738[4] He M O, Minus I, Zhou P Z, Mohammed S N, Halpern J B,

Jacobs R, Sarney W L, Salamanca-Riba L and Vispute RD 2000 Appl. Phys. Lett. 77 3731

[5] Yang L, Xue C, Wang C and Li H 2003 Nanotechnology 1450–2

[6] Goldberger J, He R, Zhang Y, Lee S, Yan H, Choi H-J andYang P 2003 Nature 422 599–602

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