Scripta Materialia 57 (2007) 281–284

www.elsevier.com/locate/scriptamat

Facile fabrication and structural studies of filteredGe nanowires from aged Al–Ge alloy

Keisuke Sato,a Kenji Kaneko,a,* Tomoharu Tokunaga,a

Yasuhiko Hayashib and Zenji Horitaa

aDepartment of Materials Science and Engineering, Faculty of Engineering, Kyushu University,

744, Motooka, Nishi-ku, Fukuoka 819-0395, JapanbDepartment of Environmental Technology and Urban Planning, Nagoya Institute of Technology,

Gokiso, Showa, Nagoya 466-8555, Japan

Received 26 February 2007; revised 19 March 2007; accepted 20 March 2007Available online 1 May 2007

Single crystalline Ge nanowires have been fabricated by filtration of rod-shaped Ge precipitates from aged Al–Ge alloy usingHCl solution. The diameter of the Ge nanowires ranged from 30 to 100 nm when the aging conditions of the Al–Ge alloy wasaltered. The Ge nanowires grow preferentially in the [110] direction, which is the same as observed earlier for rod-shaped Geprecipitates in the aged Al–Ge alloy.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Aluminum alloys; XRD; TEM

Recently, it has become the trend to reduce the sizeof electronic device structures and circuits, and this hasmotivated the investigation of nanoscale semiconductormaterials. In particular, fundamental research into one-dimensional materials, nanowires and nanotubes, hasbecome very important because of their interestinggeometries, properties and potential applications [1].Namely, they offer the prospect of reaching higherdevice densities than conventional semiconductortechnology.

Since semiconductor Ge has long been an importantmaterial in high-speed microelectronics and infraredoptical devices, the fabrication of Ge nanowires hasreceived much attention. Because of its small dimension,semiconductor Ge nanowire holds considerable promisein improving the indirect band gap structure and opticalproperties [2–4]. It should also be noted that the effectiveBohr radius of the excitons in Ge is reported to be24.3 nm [5], so that if Ge nanowires with diameters ofless than 24.3 nm can be fabricated, quantum size effectswill take place. Furthermore, a quantum confinement-induced direct band gap, of �0.8 eV, is expected to

1359-6462/$ - see front matter � 2007 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2007.03.054

* Corresponding author. Tel./fax: +81 92 802 2959; e-mail: kaneko@zaiko.kyushu-u.ac.jp

appear for Ge nanowires growing in the [110] direction,with Ge nanowires growing in the [100] and [111] direc-tions being characterized by an indirect band gap of0.66 eV [6].

The growth of Ge nanowires was first reported byHeath and LeGoues in 1993, who synthesized themusing a solvothermal approach in which GeCl4 and phe-nyl–GeCl3 were reduced by sodium metal in an alkanesolvent at elevated temperature and pressure [7]. Sincethen, various methods of synthesis have been used, suchas chemical vapor deposition [8], the vapor–liquid–solidprocess [9,10], ion beam synthesis [11], the solution–liquid–solid mechanism [12], thermal evaporationsynthesis [3] and the laser ablation method combinedwith vapor–liquid–solid growth [1]. These processesusually require multi-step reactions system and expen-sive equipment, so that they are generally cost-ineffectivefor industrial purposes. It is therefore desirable to devel-op a simple and relatively cheap method to synthesizeGe nanowires.

The solid solubility of Al in Ge is negligible, whereasGe has a maximum solid solubility in Al of �2.0 at.% atthe eutectic temperature (693 K), which decreasesrapidly with decreasing temperature [13]. Previously,Hugo and Muddle observed the presence of rod-shapedGe precipitates 20–30 nm in diameter along the h110iAl

directions in an Al–1.5 at.% Ge alloy aged at 473 K [14].

sevier Ltd. All rights reserved.

Figure 1. Typical bright-field (a) and dark-field (b) TEM images usingthe diffraction spot indicated by the white arrow in (b).

Figure 2. The XRD pattern (a) and a typical SEM image (b) of thefiltered Ge powders.

282 K. Sato et al. / Scripta Materialia 57 (2007) 281–284

Recently, Deaf also observed rod-shaped Ge precipi-tates in an Al–1.0 at.% Ge alloy aged at 523 K [15].

Herein, we report a simple process for the fabricationof Ge nanowires by filtering rod-shape Ge precipitatesfrom an aged Al–Ge alloy using HCl solution. We alsodemonstrate that it is possible to control the dimensionsof nanowires by varying the aging conditions.

An Al–1.6 at.% Ge alloy was prepared from a melt of99.99% purity Al and 99.999% purity Ge using an elec-tric furnace in an argon atmosphere, and cast to producean ingot with air cooling. The ingot was homogenized at698 K for 24 h using the electric furnace, then a cylindri-cal-shaped specimen was prepared from the ingot by alathe and swaging machine. The cylindrical specimenunderwent solution treatment at 698 K for 24 h usingthe electric furnace in an argon atmosphere, andquenched in water to room temperature. The alloy wasthen aged at 453 K for 3–15 h using the electric furnacein an argon atmosphere, and quenched once again inwater.

The aged alloy was cut into thin disks of 0.5 mmthickness which were then dissolved in 8.4 mol l�1 HClsolution with supersonic vibration to enhance the disso-lution of Al and to disperse the Ge nanowires. Themixed solution was filtered using a hydrophilic polytet-rafluoroethylene (PTFE) filter with 200 nm diameterholes. The HCl solution with the same concentrationwas added again to remove the remaining contaminantscompletely, then washed several times with distilledwater. The filtered materials, in the form of black pow-der, were collected and dried in air. Bulk Al–Ge alloyspecimens aged at 453 K were prepared for transmissionelectron microscopy (TEM) by twin-jet electropolishingwith a solution of 10% perchloric acid–20% glycerine–70% ethanol at a temperature of 293 K and at an appliedvoltage of 12 V (0.2 A).

X-ray diffraction (XRD) patterns (25–120�) wereobtained using a RINT-2100 X-ray diffractometer(Rigaku, Japan) equipped with a diffracted beammonochromator (Co Ka source, k = 1.7889 A). Themorphologies of the powders were characterized by ascanning electron microscope (SEM) S-4300SE (HIT-ACHI, Japan) operated at 10 keV. Then both structuraland analytical characterizations were carried outusing TECNAI-20 (FEI, The Netherlands) and JEM-2010FEF (JEOL, Japan) transmission electron micro-scopes, both operated at 200 keV and a high-resolutiontransmission electron microscope, JEM-4000EX (JEOL,Japan), operated at 400 keV. High-angle annular dark-field imaging and energy-dispersive X-ray spectroscopy(EDS) using a scanning method were also applied toobtain elemental EDS maps from the Ge nanowires.

Figure 1a shows a typical bright-field TEM image ofGe precipitates in the Al–Ge alloy, in which a largenumber of triangular plates and rod-shaped Ge precipi-tates are distributed within the field of view. A typicaldark-field TEM image, Figure 1b was also obtainedfrom the same region as Figure 1a using the diffractionspot indicated by a white arrow in the selected-area dif-fraction pattern (inset). Several spots are seen within theselected-area diffraction pattern, the intense spots corre-sponding to diffracted beams from the Al matrix and theweaker spots corresponding to those from the diamond

structure of the Ge precipitates. The presence of otherweaker spots is due to double diffraction. THe dark-fieldTEM image shows that there is a cube–cube orientationrelationship between the Ge precipitates and the Almatrix.

The purity and crystallinity of filtered powders wereexamined using XRD from the filtered Ge powders agedat 453 K for 3 h. Figure 2a shows the XRD pattern, inwhich three characteristic low index peaks appear at2h = 31.94, 53.44 and 63.65�, which can be readilyindexed as (111), (220) and (311) of the crystallinediamond structure of Ge, respectively. The latticeconstant was calculated as 5.63 A using the Braggformula.

Figure 2b presents a typical SEM image of the filteredGe powders aged at 453 K for 3 h, showing the presenceof a nanowire with a diameter of 17 nm and a length of270 nm. Large precipitates with other morphologies,

K. Sato et al. / Scripta Materialia 57 (2007) 281–284 283

triangular plate and other precipitates, are rarely seendue to the filtration process.

The sizes of the Ge nanowires are found to bestrongly dependent on the aging period, as their maxi-mum dimensions gradually increase, as seen in Figure3a and b, which shows their diameters and lengths,respectively. These phenomena are very similar to theaging characteristics on an Al–Ge alloy [16]. Kanekoet al. reported that the nucleation and growth of rod-shaped Ge precipitates are more advanced after longeraging periods [16], so that if the Al–Ge alloy is agedfor a longer period, then Ge nanowires with largerdimensions can be expected to be fabricated. Some Genanowires reached 30 nm in diameter and 600 nm inlength after 3 h of aging, and some reached 100 nm indiameter and 3 lm in length after 15 h of aging. Genanowires with smaller diameters and lengths can befabricated after shorter aging periods, as can be seenfrom the average diameter and length of Ge nanowires.

Figure 4a is a low-magnification TEM image andFigure 4b is a high-resolution TEM image of a Ge nano-wire fabricated by the precipitation–filtration process.The fast Fourier transform (FFT) diffraction pattern,top inset in Figure 4b, obtained from the high-resolutionTEM image of a Ge nanowire, is consistent with theh111i zone axis of single-crystal-structured Ge. Further-more, indexing the FFT diffraction pattern indicatesthat the Ge nanowires are grown along the h110i direc-

Figure 3. Diameter and length of Ge nanowires filtered from Al–Gealloy with respect to the aging period.

Figure 4. A low-magnification of a nanowire (a) and a high-resolutionTEM image (b) of the boxed region of the nanowire in (a). Inset is theFFT diffraction pattern obtained from the region in (b).

tions, as in the case of rod-shaped Ge precipitates withinthe aged Al–Ge alloy [14–16]. The spacing between adja-cent planes is measured as 3.98 A, which yields a latticeconstant of 5.63 A, a figure which is slightly smaller thanthe known diamond crystal structure of Ge, 5.66 A. Thesame value is also derived from the XRD pattern, asshown in Figure 2a, using the Bragg formula. These re-sults strongly indicate that the negligible amount ofimpurity, which is probably Al incorporated substitu-tionally from the matrix, within the nanowire causedthe decrease in the lattice constant.

In addition, it is clearly seen from the HRTEM imagein Figure 4b that the nanowire is highly crystalline andcovered by a thin amorphous layer of less than 1 nmthickness.

To investigate the effect of Al matrix on the composi-tion of Ge nanowires, EDS analysis was carried out.Major peaks of Ge–L and Ge–K are easily recognizable,as shown in Figure 5.

In conclusion, we have developed a simple precipita-tion–filtration process to fabricate single-crystalline Genanowires from aged Al–Ge alloy using HCl solution.The diameter of the Ge nanowires ranges from 20 to100 nm and their length ranges from 500 to 3000 nm,dependent on the aging conditions. It was possible tofabricate Ge nanowires of less than 24.3 nm diameteralong the [11 0] growth direction to achieve a quantumsize effect so that the charge carriers would be free tomove only along the wire. Devices consisting of Genanowires with smaller diameter (namely less than24.3 nm) would be expected to show less power con-sumption than conventional devices. Furthermore, Ros-setti et al. suggested that an indirect gap semiconductormaterial should begin to resemble a direct gap materialas the microcrystal size decreases [17].

Judging from electron diffraction patterns and fromlattice resolved images, it is also possible to determinethat the nanowires grow preferentially along the [110]direction, which is the same direction as the rod-shapedGe precipitates in the aged Al–Ge alloy. We elucidatedthe composition of the nanowire using energy-dispersiveX-ray analysis of individual nanowires, which demon-strated that the crystalline nanowire consists mainly ofGe with a negligible amount of substitutionally incorpo-rated Al atoms as impurity, and is also surrounded byvery thin amorphous oxide layers. The Ge nanowires

0

1000

2000

3000

4000

5000

6000

7000

0 5 10 15 20 25Energy [keV]

coun

ts

GeKα

GeKβ

CuKβ

CuKα

GeLa

Figure 5. EDS signal recorded from a Ge nanowire.

284 K. Sato et al. / Scripta Materialia 57 (2007) 281–284

will probably be protected from further oxidation bythese thin amorphous oxide layers.

In future, Ge nanowire will need to be purified fur-ther to high purity, so that the Ge nanowires fabricatedby this process can be employed for electronic devicestructures and circuits.

This work was supported in part by ‘Grant-in-Aidfor the 21st Century COE Program (Functional Innova-tion of Molecular Informatics)’ of the Ministry ofEducation, Culture, Sports, Science and Technology ofJapan.

[1] A.M. Morales, C.M. Lieber, Science 279 (1998) 208.[2] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello,

S.T. Lee, Phys. Rev. B 61 (2000) 4518.[3] B. Yu, X.H. Sun, G.A. Calebotta, G.R. Dholakia, M.

Meyyappan, J. Clust. Sci. 17 (2006) 579.[4] C. Harris, E.P. O’Reilly, Physica E 32 (2006) 341.[5] Y. Maeda, N. Tsukamoto, Y. Yazawa, Y. Kanemitsu, Y.

Masumoto, Appl. Phys. Lett. 59 (1991) 3168.

[6] A.N. Kholod, V.L. Shaposhnikov, N. Sobolev, V.E.Borisenko, F.A. D’Avitaya, S. Ossicini, Phys. Rev. B 70(2004) 035317-1.

[7] J.R. Heath, F.K. LeGoues, Chem. Phys. Lett. 208 (1993)263.

[8] D. Wang, H. Dai, Angew. Chem., Int. Ed. 41 (2002)4783.

[9] Z.W. Pan, S. Dai, D.H. Lowndes, Solid State Commun.134 (2005) 251.

[10] Y. Wu, P. Yang, Chem. Mater. 12 (2000) 605.[11] T. Muller, K.-H. Heinig, B. Schmidt, Nucl. Instr. and

Meth. B 175–177 (2001) 468.[12] T. Hanrath, B.A. Korgel, J. Am. Chem. Soc. 124 (2002)

1424.[13] F.A. Shunk (Ed.), Constitution of Binary Alloys, 2nd

Supplement, McGraw-Hill, New York, 1969.[14] G.R. Hugo, B.C. Muddle, Acta Metall. Mater. 38 (1990)

351.[15] G.H. Deaf, Physica B 348 (2004) 115.[16] K. Kaneko, K. Inoke, K. Sato, K. Kitawaki, Z. Horita, I.

Arslan, P.A. Midgley, Ultramicroscopy, accepted forpublication.

[17] R. Rossetti, R. Hull, J.M. Gibson, L.E. Brus, J. Chem.Phys. 83 (1985) 1406.