Characterization of tin catalyzed silicon nanowires synthesized by the hydrogen radical assisted deposition method

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Characterization of Tin-catalyzed silicon nanowires synthesized by the hydrogen radical-assisted deposition method

Minsung Jeon ⁎ , Hisashi Uchiyama, Koichi Kamisako

Department of Electronic and Information Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 11 September 2008

Accepted 2 October 2008

Available online 9 October 2008

Keywords:

Tin catalyst

Silicon nanowires

Hydrogen radicals

VLS mechanism

Phase diagram

Tin-catalyzed silicon nanowires (SiNWs) were synthesized at various hydrogen gasflow rates using the hydrogen radical-assisted deposition method Large quantities of SiNWs with various crystal phases were synthesized and their characteristics were estimated Tin-capped SiNWs were straightly grown and their structures were changed with increasing hydrogen gasflow rates Their diameters on the bottom side were increased ranging from approximately 50 to 200 nm and their lengths extended up to ~ 2 µm with increasing hydrogen gasflow rates

© 2008 Elsevier B.V All rights reserved

1 Introduction

Since the synthesis of carbon nanotubes [1] , much attempt has

been devoted to synthesizing one-dimensional nanostructure

materi-als, such as nanowires, nanorods, nanotubes and nanoribbons [2]

These nanomaterials provide a good system to research the

depen-dence of electrical, optical and magnetic properties [3 –8] They are

also expected to play an important role as both interconnections and

functional units in fabrication of electronic, optoelectronic and

electrochemical devices with low-dimensional structures In recent

years, silicon nanowires (SiNWs) as one-dimensional structure have

attracted due to their unique mechanical and semiconducting

properties SiNWs have been synthesized by using various methods,

such as chemical vapor deposition (CVD) [9] , oxide-assisted [10] ,

template-assisted [11] and laser ablation method [12] via well-known

vapor –liquid–solid (VLS) mechanism [9,13] Moreover, various metal

nanoparticles, such as Au, Al, Ga, Ti and Sn [9,14 –17] , have been

studied for synthesizing SiNWs Among these, tin (Sn) appears to be

the favorable catalyst for low temperature synthesis from its phase

diagram, because the Sn –Si alloy has relatively low eutectic

tempera-ture as 232 °C [18] The low melting point materials form eutectic with

silicon at low temperature and with extremely low content of the

elemental semiconductors We have also reported a simple way to

synthesize SiNWs using the low-melting-point metal catalysts, such

as In and Ga, by the hydrogen radical-assisted deposition method

[19,20] In particular, the synthesis of SiNWs with Sn catalyst, which

has relatively low eutectic temperature with Si, has been reported by a

few researchers [17,21] Moreover, their properties are not well-known yet In this study, therefore, the SiNWs are synthesized using

Sn nanoparticles as catalyst at various growth conditions and their characteristics are investigated.

2 Experimental Tin (Sn) metal thin film as catalyst is evaporated on Corning #1737 glass substrates Before metal film evaporation, the glass substrates are cleaned in a bath containing acetone, ethanol and deionized water with ultrasonic agitation for 5 min The substrates are located in vacuum chamber and Sn metal film approximately 100 nm is deposited The Sn-coated glass substrates are set and heated at 400 °C in the experimental vacuum chamber with a pressure of 2 × 10− 5Torr Hydrogen (H2) gas is introduced into the 1/2-inch diameter trumpet-like quartz tube, which

is surrounded by microwave cavity Then, the hydrogen radicals generated by 2.45 GHz microwave are irradiated for 1 min onto the samples to fabricate nanosize metal particles To fabricate nanoparticles,

a H2gas flow, microwave power and working pressure are selected to

100 sccm, 40 W and 0.5 Torr, respectively For synthesizing silicon nanowires (SiNWs), silane (SiH4) gas as Si source is introduced into the experimental chamber from a ring-type copper tube that has many ori fices, and it is reacted with hydrogen radicals generated by microwave in the quartz discharge tube In order to investigate the effect of synthesis conditions, SiNWs are synthesized using the hydrogen radical-assisted deposition method [19] at various hydrogen gas flow rates ranging from 130 to 180 sccm Detailed other synthesis conditions are summarized in Table 1

SiNWs are synthesized for 10 min, and their characteristics are estimated by Field Emission Scanning Electron Microscopy (FE-SEM) and X-ray diffractometer (XRD) For further investigation, the synthesized

Materials Letters 63 (2009) 246–248

⁎ Corresponding author Tel./fax: +81 42 388 7446

E-mail address:joseph@cc.tuat.ac.jp(M Jeon)

0167-577X/$– see front matter © 2008 Elsevier B.V All rights reserved

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j o u r n a l h o m e p a g e : w w w e l s ev i e r c o m / l o c a t e / m a t l e t

SiNWs are removed onto a carbon copper grid The detailed characteristics

of as-synthesized SiNWs are analyzed by a scanning transmission electron

microscopy (S-TEM) and an energy dispersive X-ray spectrometer (EDS).

3 Results and discussion

Before synthesis of silicon nanowires (SiNWs), hydrogen radical treatment is performed

on the Sn-coated substrate to fabricate Sn nanoparticles The hydrogen radical treatment is

effective to obtain voluminous SiNWs Subsequently, the SiNWs were synthesized at various

hydrogen (H2) gasflow rates In order to investigate the morphological property of

as-synthesized SiNWs at varied conditions, a FE-SEM observation was performed after

synthesis of SiNWs for 10 min at 400 °C

Fig 1(a)–(c) shows the low-magnification FE-SEM images of the as-synthesized SiNWs

at various H2gasflow rates: (a) 130 sccm, (b) 150 sccm and (c) 180 sccm Voluminous SiNWs

were whisker-likely synthesized at all growth conditions Their structures were gradually

changed with increasing H2gasflow rate When the SiNWs were synthesized at H2gasflow

of 130 sccm, the SiNWs were smoothly curved as shown in inset ofFig 1(a) On the other

hand, when the SiNWs were synthesized at above H2flow rate of 130 sccm, the SiNWs were

straightly grown as shown in insets ofFig 1(b) and (c) However, their sizes became thicker

with increasing H2gasflow The diameters of SiNWs on the bottom side were gradually

increased ranging from approximately 50 to 200 nm with increasing H2gasflow rates

Moreover, their lengths extended up to ~2 µm It indicates that the H2gasflow affect the

growth of SiNWs Moreover, the high-magnification FE-SEM images explained that the

SiNWs were tapered and that Sn catalysts remained on the tip of SiNWs (see red circles in

inset ofFig 1(a)) It means that the SiNWs are synthesized via vapor–liquid–solid (VLS)

mechanism[9,13] The detailed explanation of the VLS mechanism will be followed later

Above mentioned samples were also examined by XRD measurement.Fig 1(d) shows the

XRD patterns of SiNWs synthesized at varied H2gasflow rates The XRD patterns indicate that

the synthesized SiNWs are highly crystallized silicon with diffraction peaks of (111), (220) and (311) Moreover, the Sn metal peaks of (200) and (101) were also weakly detected at 30.66 and 32.044 deg., respectively (see black arrows inFig 1(d)) These peaks were revealed because the Sn catalysts were located on the top of SiNWs as shown in inset ofFig 1(a) Such SiNWs grew randomly with different crystal orientations

Fig 2shows a schematic of the Sn–Si alloy binary phase diagram[18] SiNWs are typically synthesized via the VLS growth mechanism at temperatures higher than the Sn–

Si eutectic temperature This VLS mechanism has been introduced by Wagner et al to growth single crystalline silicon[9] A typical VLS growth starts with the dissolution of gaseous reactants into a nano-size metal liquid droplet, followed by nucleation and growth

Table 1

Synthesis conditions for silicon nanowires (SiNWs)

H2flow [sccm] SiH4flow [sccm] M.W.P [W] Press [Torr] Temp [°C] Time [min]

Fig 1 FE-SEM images of the as-synthesized SiNWs at hydrogen gasflow of (a) 130 sccm, (b) 150 sccm and (c) 180 sccm All the scale bars represent 2.5 µm (d) XRD patterns of the

as-flow rates

Fig 2 Schematic of the Sn–Si alloy binary phase diagram

247

M Jeon et al / Materials Letters 63 (2009) 246–248

of crystalline wires Here, the Sn nanoparticles and decomposed SiH4are typical metal

catalyst and silicon source used to synthesize SiNWs The fabricated Sn nanoparticles after

hydrogen radical treatment provide energetically favored Sn–Si sites for the adsorption of

incoming vapor silicon sources The adsorbed Si atoms diffused into the liquid Sn

nanoparticles, which results in the formation of a Sn–Si alloy The continued adsorption of the Si sources into the Sn–Si alloy liquid droplets leads to the supersaturation of the Sn–Si eutectic in a broad region above 230 °C, which results in the growth of a solid Si nucleus The Sn nanoparticles are pushed away from the substrate by continuous Si source feeding

in the liquid–solid interface and it is lifted upward by the growing SiNWs As a result, large quantities of SiNWs are synthesized

The detailed compositions of as-synthesized SiNWs via VLS mechanism were examined by S-TEM and EDS measurement.Fig 3(a)–(c) shows a high-magnification SEM image of as-synthesized Si nanowire and corresponding EDS analysis The S-TEM micrograph in inset ofFig 3(a) represents the Si nanowire capped by a catalyst nanoparticle The EDS measurement was operated during the S-TEM observation The EDS analysis carried out on the nanowire stem and metal nanoparticle As can be seen in

Fig 3(b), the Si nanowire comprises only Si element The other peaks, such as C and Cu, were also detected It is attributed to the effect of the TEM grid.Fig 3(c) shows the EDS spectra taken for the nanoparticle shown in inset ofFig 3(a) It indicates that the nanoparticle comprises Sn and Si elements The Sn nanoparticle located on the top of Si nanowire implies that a Sn catalyst assisted VLS mechanism is typically related in the growth of SiNWs These results denote that the as-synthesized SiNWs by our experimental method are pure crystalline silicon without oxygen and other impurities

4 Conclusion Tin-catalyzed silicon nanowires (SiNWs) were synthesized at various hydrogen gas flow rates using the hydrogen radical-assisted deposition method Voluminous SiNWs, which have various crystal phases, were whisker-likely synthesized at all growth condition Their structures were gradually changed with increasing hydrogen gas flow rate The diameters of SiNWs on the bottom side were increased ranging from approximately 50 to 200 nm and their lengths extended up to ~2 µm It indicates that the SiNWs can be controlled by the introduced hydrogen gas flow rates.

References

[1] Iijima S Nature 1991;354:56

[2] Wang ZL Nanowires and Nanobelts: Materials, Properties and Devices Kluwer Academic Publishers; 2003

[3] Westwater J, Gosain DP, Usui S Jpn J Appl Phys 1997;36:6204

[4] Alivisatos AP Science 1996;271:933

[5] Kang SH, Kim JY, Kim HS, Sung YE J Ind Eng Chem 2008;14:52

[6] Cui Y, Lieber CM Science 2001;291:851

[7] Chung SW, Yu JY, Heath JR Appl Phys Lett 2000;76:2068

[8] Duan X, Niu C, Sahi V, Chen J, Parce JW, Empedocles S, et al Nature 2003;425:274 [9] Wagner RS, Ellis WC Appl Phys Lett 1964;4:89

[10] Wang N, Tang YH, Zhang YH, Lee CS, Bello I, Lee ST Chem Phys Lett 1999;299:237 [11] Zhang XY, Zhang L, Meng GW, Li GH, Phillipp NY, Phillipp F Adv Mater 2001;13:1238 [12] Morals AM, Lieber CM Science 1998;279:208

[13] Givargizov EI J Cryst Growth 1975;31:20

[14] Wang Y, Schmidt V, Senz S, Gosele U Nat Nanotechnol 2006;1:186

[15] Sunkara MK, Sharma S, Miranda R, Lian G, Dickey EC Appl Phys Lett 2001;79:1546 [16] Kamins TI, Williams RS, Chen Y, Chang YL, Chang YA Appl Phys Lett 2000;76:562 [17] Chen ZH, Jie JS, Luo LB, Wang H, Lee CS, Lee ST Nanotechnology 2007;18:345502 [18] Olesinski RW, Abbaschian GJ Bull Alloy Phase Diagr 1984;5:273

[19] Jeon MS, Kamisako K Mater Lett 2008;62:3903

[20] Jeon MS, Tomitsuka Y, Kamisako K J Ind Eng Chem 2008.doi:10.1016/j.jiec.2008.06.004 [21] Shao M, Hui H, Li M, Ban H, Wang M, Jiang J Chem Commun 2007;8:793

Fig 3 (a) S-TEM micrograph of the silicon nanowire capped by a Sn catalyst

nanoparticle (b) and (c) represent the corresponding EDS spectra taken from the

nanowire stem and the catalyst nanoparticle, respectively