Journal of Crystal Growth 381 (2013) 87–92

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Journal of Crystal Growth

0022-02http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/jcrysgro

Wire or no wire—Depends on the catalyst layer thickness

Feng Ji Li a, Sam Zhang a,n, Jyh-Wei Lee b,c, Dongliang Zhao d

a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporeb Department of Materials Engineering, Ming Chi University of Technology, Taipei 24301, Taiwan, Chinac Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei 24301, Taiwan, Chinad Central Iron and Steel Research Institute, 76 South Xueyuanlu Rd, Haidan District, Beijing 100081, China

a r t i c l e i n f o

Article history:Received 13 May 2013Received in revised form18 June 2013Accepted 4 July 2013

Communicated by K. Deppert

the catalyst layer thickness and formation of the nanowire. Results show that in the case of thin layer of

Available online 13 July 2013

Keywords:ThicknessCrystalline siliconCoaxial nanowireCore–shell particleSupersaturationOxidation

48/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jcrysgro.2013.07.010

esponding author. Tel.: +65 67904400.ail addresses: MSYZhang@ntu.edu.sg, msyzhan

a b s t r a c t

Crystalline silicon (Si) nanowire could be directly grown from Si wafer upon thermal annealing in thepresence of catalyst such as gold (Au). However, the role of the catalyst layer thickness is yet elucidated.In this work, 10 nm, 20 nm, and 40 nm Au layers were respectively sputtered on Si wafer substrates,followed by 2 min thermal annealing at 1000 1C under Ar atmosphere, to find the relationship between

catalyst, crystalline-Si/amorphous-SiOx coaxial nanowires grew. But with thicker layers of catalyst,no wires were found but crystalline Au particles capsulated with amorphous SiOx. The catalyst andnanowire morphologies and structures were carefully examined through a scanning electron microscope,X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and selectedarea diffraction. A model is developed to explain the formation mechanism of the Si/SiOx and Au/SiOx

core–shell nanostructures.& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Crystalline silicon (Si) nanowires are important building blocks insemiconductor devices [1–4], Crystalline-Si/amorphous-SiOx, or c-Si/a-SiOx, coaxial nanowires are reported to be grown by laser ablation [5],chemical vapor deposition [6,7]; thermal degradation of diphenylsi-lane in a supercritical hexane fluid [8], thermal evaporation [9], orthermal annealing of metal-covered Si wafers [10–13]. Among them,the thermal annealing method follows a solid–liquid–solid (SLS)growth mechanism [14,15]. The SLS growth of c-Si/a-SiOx and a-SiOx

nanowires grown from different metallic catalyst layers has beenreported in a few papers. For instance, c-Si/a-SiOx nanowires aregrown via thermal annealing of Si wafers covered by a 1-nm Fe film[11], 10-nm Au film [13,16], 40-nm [17], or 50-nm Ni film [18].Amorphous SiOx (i.e., the oxidized Si) nanowires are grown viathermal annealing of Si wafers covered by 2–10 nm Au film [19],5-nm Pt film [15], 15-nm Pt/Au film [20], 30-nm [21], or 40-nmNi film[14]. Till now, however, how the metallic catalyst layer thicknessaffects the growth of c-Si/a-SiOx or a-SiOx nanowires is yet elucidated.In this work, thermal annealing was carried out on Si wafers coated by10-nm, 20-nm, and 40-nm Au layers to study if they all grownanowires. The results show that the thin layers do but thick layersdo not. Scanning electron microscope, X-ray diffraction, transmission

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g@live.com (S. Zhang).

electron microscopy, energy dispersive X-ray spectroscopy andselected area diffraction patterns were employed to examine theresultant morphology and structure. A solid–liquid–solid model wasdeveloped to explain the formation mechanisms.

2. Experimental details

2.1. Deposition of Au layers

Au catalyst layers were sputtered on Si wafers in an auto finecoater (JFC-1600, JEOL, Japan), consisting of a basic unit and a rotarypump. The cathode contains a permanent magnet to create anefficient glow discharge for sputtering. N-type Si (100) wafer wasused as the substrate (10 mm�10 mm in area, 475 μm in thicknessand 0.5 nm in room mean square surface roughness). Before loadinginto the sputtering chamber, the substrate was ultrasonically cleanedin acetone for 20 min, followed by 10 min in alcohol. Once thechamber pressure reached around 10 Pa, 0.5, 1, and 2 min sputteringof an Au target (purity, 99.99%) at room temperature deposited about10-nm, 20-nm, and 40-nm-thick Au catalyst layers on the Si wafersubstrates.

2.2. Growth of Si nanowires

The as-sputtered Au layers underwent thermal annealing in anadvanced rapid thermal system (ARTS150, Premtek international

F.J. Li et al. / Journal of Crystal Growth 381 (2013) 87–9288

Inc., Taiwan, China) in an Ar ambient at 1000 1C for 2 min. Theannealing chamber was purged with Ar (purity, 99.999%) at 3000standard cubic centimetres per minute (sccm) for 15 min beforeramping up to 1000 1C at 50 1C s�1. And then, the chamber washeld at 1000 1C for 2 min. After the annealing, the chamber wasnaturally cooled down to room temperature for about 60 min.During the whole annealing process (ramping, dwelling, andcooling), the inflow of the Ar gas was maintained at 3000 sccm.

2.3. Characterization

Field emission scanning electron microscopy (JEOL, JSM-6701F,JEOL Ltd., Japan, 10-kV operating voltage) was employed to observethe cross section and the surface morphology of the Au catalyst layersbefore and after annealing. The polycrystalline structure of the as-sputtered Au film was examined by glancing incidence X-ray diffrac-tion (PANalytical B.V., Almelo, Netherlands). High resolution transmis-sion electron microscope (TEM, JEOL 2010, 2100, 200-kV operatingvoltage, Japan) was employed to reveal the morphology and structureof the grown nanowires and particles. Energy dispersive X-rayspectroscopy (EDX, EDAX Inc., US) is applied to characterize thecomposition of the nanowire and particles. Selected area electrondiffraction (SAED) patterns were carried out to characterize the growthorientations of the grown nanowires and particles. In determining thegrowth orientations, the SAED patterns were generated by carefullyadjusting the tilt angle along the x- and y- axis of the sample holderto match the standard database that is cited from software JEMS(cf., Supporting database).

3. Results and discussion

3.1. Morphology and structure of the Au catalyst layers

3.1.1. As-sputtered Au layersThe cross section of the as-sputtered Au layers with thickness

of 10 nm, 20 nm, and 40 nm is clearly shown in Fig. 1(a)–(c).

Fig. 1. Morphology and structure of the as-sputtered Au catalyst layers: (a) 10 nm, (bdiffraction patterns generated from the 40 nm Au layer.

The rougher surface of the 10-nm thick Au layer may be resultedfrom the attachment of the small broken Si pieces during cuttingthe sample. Fig. 1d shows that the 40 nm Au layer surface isuniformly covered with small Au nanoparticles with diameter ofaround 10 nm. The XRD spectrum generated from the 40-nm Aulayer is shown in Fig. 1e, where six characteristic peaks: (111)centered at 38.31, (200) centered at 44.61, (220) centered at 64.71,(311) centered at 77.61, (222) centered at 82.41, and (400) centeredat 98.11 are detected, exhibiting the typical face-centered cubic (FCC)crystalline structure of Au (cf., PDF♯ 00-001-1172, Supporting data 1).

3.1.2. Annealed Au layer surfacesUpon 2-min thermal annealing, nanowires are found lying on

the surface of the Si wafer coated with 10-nm Au layer (cf., Fig. 2a).The wires entangling with each other are of around 70 nm indiameter, several microns in length. Small Au nanoparticles withdiameter of around 70 nm are also clearly seen on the Si waferseeding the growth of the nanowires (cf., Fig. 2a, dark arrow).However, no nanowires are found on the Si wafer covered with20-nm Au layer, except for the micron-sized Au particles withdiameter in the range of 150–524 nm (cf., Fig. 2b). A 68-nm-diameter particle is occasionally observed (cf., Fig. 2b, dark arrow).The formation of the particles is attributed to the effect of surfacetension at high temperature. As the Au layer thickness increased to40-nm, bigger particles with diameter ranging from 281 nm to722 nm are found. Small particles with diameter of around 104-nm are occasionally observed (cf., Fig. 2c, dark arrows). Smallamount of nanowires with diameter of 28–48 nm, and length ofaround 3 μm, are occasionally found on the surface. They are notdirectly seeded by the big Au particles, but by the small ones thatare collapsed from the big ones (cf., Fig. 2d, dark arrows). It is alsonoted that all the big particles are shielded by a layer of whitematter (cf., Fig. 2d, white arrows). It is known that the Au particlescollapsed from the Au layers could easily melt with Si waferbecause the eutectic point of the Au–Si alloy is only 363 1C [22].Since the commercial argon gas usually contains a small amount of

) 20 nm, and (c) 40 nm. (d) Surface morphology of the 40 nm Au layer. (e) X-ray

Fig. 2. Surface morphology of the annealed Au catalyst layers with thickness of (a) 10 nm, (b) 20 nm, and (c)–(d) 40 nm.

F.J. Li et al. / Journal of Crystal Growth 381 (2013) 87–92 89

oxygen, and the annealing chamber is not under high vacuumenvironment, the Au–Si droplet is easily oxidized into Au–Si–Odroplet. The droplet could serve as a nucleation medium fornanowire growth at temperature of 1000 1C. Supersaturation isdirectly related to the diameter of the Au–Si–O droplet, thusdetermines growth of nanowires. Upon supersaturation of Si orSi-oxide in the droplet, Si/Si-oxide coaxial nanowire could pre-cipitate from the partially oxidized droplet, while the completelyoxidized Au–Si–O droplet could precipitate the Si-oxide nanowire.But if Si in the droplet never reaches supersaturation, the dropletwould solidify into an Au–Si–O particle at the end of the process.For the annealed 40-nm Au layer, smaller Au particles areoccasionally broken from the big ones. Thus it is possible to growinto nanowires as long as Si and SiOx are supersaturated in thesmall Au–Si–O droplets (cf., Fig. 2d).

3.2. Structure of the nanowires

Fig. 3a shows that a nanowire of around 40 nm diameter isconstructed with a 10 nm dark core surrounded by a 16 nm graysheath. The SAED patterns in the inset are generated from thenanowire. A unit cell of cubic structure (lattice parameter¼5.43 Å)was consistent with the diffraction patterns. The SAED patterns areindexed for the [101] zone axis of single-crystalline Si (cf., Supportingdata 2). This suggests that the nanowire is crystalline Si growing along½111� orientation surrounded by amorphous SiOx sheath (i.e. c-Si/a-SiOx). Fig. 3b shows the enlarged rectangle area revealing the crystal-line Si core and the amorphous SiOx sheath. The lattice spacing of3.14 Å agrees closely with that of Si ð111Þ. There is a dihedral angle of70.51 between ð111Þ and ð111Þ (lattice spacing, 3.14 Å) in the atom-resolved TEM image, perfect matches with the theoretical dihedralangle calculated through α¼ cos �1ðð111Þ∧ð111ÞÞ. The two-dimen-sional Fast Fourier Transform (FFT) patterns of the lattice resolved coreimage are shown in inset of Fig. 3b. It is generated from the Si [101]zone axis. The FFT patterns in conjunction with the lattice-resolvedcore image double confirm the growth is along the Si ½111� orienta-tion. As shown in Fig. 3c, a nanowire is seeded by a 64-nm-diameterdark Au particle at the end of the nanowire. The EDX spectrum

generated from the ending nanocluster confirms that the dark particleis Au (cf., Fig. 3d). The EDX elements are C, Au, Si, O and Cu, where Cand Cu come from the supporting Cu grid, and Si and O are from theconnected nanowire or the surrounding shell. The SAED spotsgenerated from the bottom dark particle could not be indexed sincethey are not from a specific zone axis (cf., Fig. 3e). However, they stillcould indicate the single crystal structure of the ending Au particle. Itis noted that the weight concentration of Si in Au is calculated to bearound 21.4 wt% (i.e., 9.37/43.89) (cf. Fig. 3d), over the solubility18.6 wt% in the Au–Si phase diagram [22], no wonder Si precipitatesout of the droplet. The transition of the polycrystalline face-centered-cubic structure of the sputtered Au layer to the single crystalline ball-shaped Au nanoparticle reveals that the formation of the liquid Au–Siphase does take place during the growth of nanowires [23,24]. It isnoted that another smaller Au particle is also entrapped in thenanowire close to the bigger one. It is known that supersaturation ofSi in the liquid Au–Si droplet is the key for precipitation of Si nanowire[23,24]. It is not difficult to infer that any element (e.g., Si or Au) orcompound (e.g., Si-oxide) precipitated from the liquid Au–Si–O dropletinto the solid form either in nanoparticle or core must reach super-saturation. Therefore, whether or not the nanowire is entrapped withAu catalyst depends on the supersaturated content in the Au–Si–Odroplet. Since Au could also dissolve from the underneath Au layer orsurrounding Au particles like the continuously dissolution of Si fromthe underneath wafer, the entrapping of the smaller Au nanoparticlein Fig. 3c is resulted from the supersaturation of Au and SiOx, ratherthan Si and SiOx.

3.3. Structure of the particles

A core–shell particle that is generated from the 40-nm Au layeris shown in Fig. 4a. The dark core is of a diameter in range of 477–683 nm. It is embedded in a gray shell with thickness ranging from22 nm to 73 nm (cf., Fig. 4b and c). The EDX spectrum generatedfrom the particle confirms that it is composed with Au, Si and O(cf., Fig. 4d). The EDX elements are C, Au, Si, O and Cu, where C andCu come from the supporting Cu grid, and Au, Si and O are fromthe core–shell particle. Since the particle is too thick to obtain a

Fig. 3. Structure of c-Si/a-SiOx nanowires. (a) A coaxial nanowire, inset shows the SAED patterns. (b) Enlarged rectangle area in (a), inset shows the FFT patterns of the atom-resolved core. (c) A nanowire is seeded by an Au particle, (d) EDX spectrum of the bottom Au particle in (c), inset shows the elemental composition, and (e) SAED spotsgenerated from the bottom Au particle in (c).

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high resolution transmission electron microscopy image, onlyselected area diffraction (SAED) is carried out on the core–shellparticle to get the crystalline information (cf., Fig. 4e). A unit cell ofcubic structure (lattice parameter¼4.07 Å) is consistent with theSAED patterns. The SAED patterns are indexed for the [111] zoneaxis of the single crystalline Au, revealing that the dark core is Au(cf., Supporting data 3). Though liquid AuSi alloy phase exists inAu–Si–O droplet at 1000 1C, crystallization of Au atoms takes placeto form the Au core in the cooling stage of the annealing process(i.e., at the end of the annealing process). It is noted that theweight concentration of Si in Au particle is calculated to be onlyaround 0.6 wt% (i.e., 0.27/44.89) (cf., Fig. 4d). That is far less thanthe upper limit of the weight concentration, no wonder it will notprecipitate. In conjunction with the EDX spectrum, the gray shell isconfirmed to be amorphous SiOx (i.e., c-Au/a-SiOx). It is also notedthat the amount of oxygen in Fig. 3d (18.62 at%) is much higherthan that in Fig. 4d (3.09 at%). This is also consistent with theamount of Si in Fig. 3d (18.62 at%)) and Fig. 4d (1.01 at%), becauseSi is easier than Au to be oxidized.

3.4. Growth mechanism

Taking into account all the evidence gathered above, we nowpeep into what could have happened in the whole process asfollows (cf., Fig. 5). Upon annealing of the sputtered layer of Au onSi wafer at 1000 1C (cf., Fig. 5a), the Au layers melt and, due tosurface tension as a result of the different thermal expansion

coefficient between Au and Si (cf., at 20 1C, Au is of a linercoefficient of 14.2�10�6/1C; while Si is only of a liner coefficientof 3.0�10�6/1C [25]), collapse into particles with diameters abouthundreds of nanometers. The size of the particles increases withthe layer thickness (cf., Fig. 5b). Since the eutectic point of Au–Si isonly 363 1C [22], at least 700 1C lower than the melting point ofAu, Au particles would easily melt on Si wafer beneath (the“solid”) to form eutectic droplet (the “liquid”) at the annealtemperature 1000 1C (cf., Fig. 5c). Si atoms diffuse through thesolid (wafer)/liquid (droplet) interface into the liquid droplet (cf.,Fig. 5c). On the one hand, the Au–Si droplets are surrounded bythe residual oxygen and the Ar gas flow in the annealing chamber[15]. Thus the surface of the Au–Si droplet is oxidized into Au–Si–O. On the other hand, supersaturation of Si in the liquid metal-Sidroplet is known as the key for precipitation of Si nanowire[23,24]. Therefore, the precipitation of SiOx from the liquid Au–Si–O droplet into the solid must satisfy supersaturation. In con-junction with the coaxial Si/SiOx nanowire, it is speculated that theAu–Si and Au–Si–O phases are coaxially distributed in the Au–Si–Odroplet. A second liquid–solid interface forms when the liquidphase becomes supersaturated with Si and Si-oxide, resulting inthe growth of Si/SiOx coaxial nanowire constructed with crystal-line Si core surrounded by amorphous SiOx sheath (the “solid”again) (cf., Fig. 5d). The nanowire always grows out from the liquiddroplet while the droplet always stays on the wafer (thus thecatalyst droplet is always found at the “root” of the nanowireinstead of “atop” the nanowires) (cf., Fig. 3c). As the nanowire

Fig. 4. Structure of Au/SiOx core–shell particles (a) an Au core is surrounded by a layer of SiOx shell. (b)–(c) Enlarged rectangle area A and B in (a). (d) EDX spectrum of thecore–shell particle, inset shows the elemental composition. (e) SAED patterns of the core–shell particle.

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freezes out from the liquid “foundation”, it cannot stand thus lieson the surface of the wafer. The quantitative value of residualoxygen determines the structure of the grown nanowires thatcould be of a coaxial or all-SiOx structure. However, this studyfocused on the catalyst layer thickness effect on the growth ofnanowires. Therefore, the exact quantitative value, such as theconcentration or partial pressure was not measured. In case of thebigger droplet, such as the 20 nm and 40 nm layers in this study,as the solubility of the Si in Au is not reached, no supersaturationthus no precipitation of Si atoms therefore no Si nanowire growth.In the end, the droplet becomes a c-Au/SiOx core–shell particle.The formation of the particle is suggested to be resulted from therearrangement of the atoms in the Au–Si–O droplet in the coolingstage of the annealing process (i.e., at the end of the annealingprocess), where Au atoms crystallize into a single-crystal core,while Si and O atoms form the amorphous shell (cf., Fig. 5d).

The driving force for such a continuous diffusion of Si atoms fromthe Si wafer into the liquid Au–Si–O eutectic droplet, and thenthrough the droplet into a solid nanowire could be attributed tothe aspects of the droplet itself and the growth atmosphere. (a) In thedroplet, the concentration gradient and supersaturation of Si arethe driving force for the growth. The droplet directs the growthorientation and defines the diameter of the nanowire. Ultimately,the nanowire growth terminates when the temperature is below

the eutectic point of the Au–Si–O droplet or the amount of Si in thedroplet could not reach supersaturation anymore. (b) From theaspect of the growth atmosphere, the carrier gas (Ar, in this work)surrounds the semi-spherical shape Au–Si–O droplet and exchangeenergy and momentum with the atoms in the droplet, resulting inovercooling to the droplet. Such an overcooling is critical to initiatethe preferential unidirectional nanowire growth.

4. Conclusions

Whether or not silicon (Si) nanowires form depends on whetheror not the catalyst liquid droplets are supersaturated with Si, that inturn, is determined by the thickness of the catalyst layer (thus thesize of the droplet) at given annealing temperature and time. Whenthe catalyst layer is too thick thus the droplet size is too big for agiven temperature and time (as in the case of the 20 nm and 40 nmthick gold (Au) layers), there is not enough time for the diffusion ofSi into the liquid Au–Si–O droplet to realize supersaturation, nonanowires form. In this case, the unsaturated Au–Si–O dropletbecomes an Au/SiOx core–shell particle as a result of the rearrange-ment of the Au, Si and O atoms in the cooling stage of the annealingprocess.

Fig. 5. Schematic diagram of the solid–liquid–solid growth of Si/SiOx coaxial nanowires and the formation of the Au/SiOx core–shell particles. (a) Au catalyst layers aresputtered on Si (100) wafers. (b) Au layers are collapsed into Au particles at high temperatures. (c) Eutectic melting of the Au particles with the beneath Si wafer substrate atthe interface, followed by the simultaneous oxidation. (d) (I) Precipitation of Si/SiOx coaxial nanowires from the Si and Si-oxide supersaturated Au–Si–O droplets as Sicontinuously diffuses from the beneath wafer; or (II) formation of Au/SiOx core–shell particles as Si in the big Au–Si–O droplets could never reach supersaturation within thelimited annealing time.

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Acknowledgments

This work was supported by the Singapore Ministry of Educa-tion′s Research Grant T208A1218 ARC4/08. This work is alsosupported by CISRI Grant: 11020990A.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jcrysgro.2013.07.010.

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