Tiny silicon nano wires synthesis on silicon wafers

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

Physica E 24 (2004) 328–332

Tiny silicon nano-wires synthesis on silicon wafers

Junjie Niua, Jian Shaa,b, Yujie Jia, Deren Yanga,

a State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China

b Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China

Received 20 April 2004; accepted 10 June 2004 Available online 11 August 2004

Abstract

Tiny silicon nano-wires (SiNWs) were synthesized on silicon wafers by the chemical vapor deposition (CVD) technique The morphology and structure of tiny SiNWs were analyzed by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively The results indicate that the tiny SiNWs were part-crystalline structure and were about 3 nm in minimal diameter Based on the line shift of Raman spectra, the structure transform of the tiny SiNWs was discussed The defect-inducing growth mechanism will probably provide a new method for the minimum of the one-dimensional nano-materials

r2004 Elsevier B.V All rights reserved

PACS: 71.55.Cn; 81.05.Ys

Keywords: Nano-wires; Synthesis; Silicon

1 Introduction

One dimension nano-materials have stimulated

much interest for their potential applications in

nano-electronics, optics, flat face display, etc

[1–6] Many techniques are employed to fabricate

one-dimension nano-materials, such as laser

abla-tion, chemical vapor deposition (CVD), physical

vapor deposition, aqua-solution method, sputter

deposition, etc [7–16] Especially, the research emphases have focused on the growth character-istics[17]diameter minimum[18,19] Recently Hu

et al fabricated the silicon nanowires with diameters of 10–30 nm sub-grow on the surface

of large amorphous SiO2nanowires with diameters

of 200–400 nm using floating-zone (FZ, 1233–1273 K) melt-vapor method [20] And the Silicon nano-wires (SiNWs) grown on silicon wafers have also attracted much attention because

of their potential compatibility in the miniaturiza-tion of integrated circuit (IC) [21–23] But the previous methods either cannot get the very thin

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doi:10.1016/j.physe.2004.06.041

Corresponding author Tel: 571-87951667; fax:

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E-mail address: mseyang@zju.edu.cn (D Yang).

wires or introduce the other substances such as

polymers Also the process is complex and

expensive

In this paper, we synthesized the very thin and

long large-scale part-crystalline SiNWs branch

with diameters of about 3–8 nm sub-grow on the

main well-crystalline SiNWs with diameters of

about 40–50 nm growing on silicon wafers by a

simple physical vapor deposition The

transmis-sion electron microscopy (TEM) and scanning

electron microscopy (SEM) images of the SiNWs

displayed the appearance and surface pattern The

X-ray diffraction (XRD) and selected area electric

diffraction (SAED) gave an obvious

well-crystal-line structure data of the SiNWs Also the Raman

scattering spectra of the SiNWs indicated the

optical properties related to temperature

depen-dence on laser heating and the small size effect

Furthermore, the growth mechanism was also

simply discussed

2 Experimental

Tiny SiNWs were prepared on a heavily doped

p-type (1 1 1) silicon wafer with a resistivity of

about 0:001 O cm as substrate by means of a CVD

process First, a magnetic sputtering technique was

used to deposit gold as a catalyst on the silicon

substrate (the thickness of Au film is 

100–200 nm) The substrate was then placed in a

quartz tube furnace, which was pumped down to

10 Pa When the temperature reached 630 1C, a

mixture gas of argon, hydrogen, and silane with

the ratio of 100:20:15 was allowed into the

chamber The pressure and temperature in the

chamber were kept at 1450 Pa and 630 1C during

the deposition After the deposition, the substrate

was removed from the furnace and was

investi-gated by means of XRD (Rigaku, D/MAX 2550

PC), a Raman scattering spectroscope (Nicolet

Almega) and a SEM (JEOL, JSM-5610LV),

respectively And then, the deposited matters on

the substrate were dissolved in an ethanol solution

Finally, the solution was placed dropwise on a

copper grid, which was covered with a very thin

carbon film, so that the deposited materials could

be analyzed with TEM (200 kV, Phillip CM200)

equipped with an energy-dispersive X-ray spectro-meter (EDX)

3 Results and discussion Top view SEM image of the SiNWs grew on the silicon substrate is shown in Fig 1 A large quantity of long and thick SiNWs (trunk SiNWs) were observed on the surface of the silicon wafer The TEM images (Fig 2) indicate a plenty of tiny SiNWs attached on the top of a thick SiNW They are about 3–8 and 40–50 nm in diameter, respec-tively And the holistic SiNWs have a well crystalline structure (the Si (1 1 1) direction is indicated in the upper left of theFig 2) The TEM image in Fig 3a shows a large scale tiny SiNWs And the chemical characterizations of the SiNWs using EDX show that they are composed of silicon with neglectable traces of oxygen (Fig 3b) It can

be seen that Si, Cu and O with smaller quantity were mainly detected Obviously the intensity of the Si is the strongest among these peaks Cu peak came from Cu grid for TEM analysis, and O peak mainly from the weak surface oxidation of nano-wire or adsorption of oxygen on the nano-nano-wire because of the impure fixed gases and low vacuum system This means that the very tiny wires were mainly composed of silicon core and a small quantity of silicon oxide sheath Detailed analysis

Fig 1 Top view SEM image of the SiNWs grew on the silicon substrate (the part of Au nanoparticles was pointed by white arrows).

on the lattice images of the tiny SiNWs gives an

inter-planar spacing of 0.314 nm completely

corresponding to that of Si (1 1 1) planes by the

HRTEM in Fig 3a It was seen that the tiny

SiNWs have a double-layer nature with a

crystal-line silicon core and an amorphous SiO2 outer

shell just like the Hu reported[20] And the XRD

data inFig 4showed that the Si peaks were very

strong with the Si (1 1 1), Si (2 2 0), and Si (3 1 1)

The intensity indicated the Si (1 1 1) is the possible leading growth direction of the SiNWs The Au peaks in the figure come from the catalyst film for the preparation of the sample (the images of Au tips attaching to the SiNWs are shown in Fig 1

and2) The remarkable crystalline structure of the SiNWs was confirmed clearly by above XRD data Here we briefly develop the defect-inducing growth mechanism of the silicon wires based on the previous reports [20,24] According the vapor–li-quid–solid (VLS) mechanism, the catalyst can induce the deposition atoms to form a droplet and grow nano-wires The mostimportant is the size and state of the catalyst which would play a key effect on the quantity and diameter size of the wires In our experiments, the size of most catalysts is very small and uniform This con-tributes the small catalyst particles to easily form the nucleating point with the deposited atoms and come into a wire Once the trunk SiNWs formed, high surface density of the SiNWs and the unstable growth surroundings lead to a plenty of faults and defects such as oxygen vacancies produce The properties of the silicon wafer also express some effects in the form of these defects in our experimentation The existence of these defects provides a good new nuclear center and will form a wire to continue to grow with the more silicon atoms joining in at low temperature The new nano-droplet composed of defects and silicon atoms is easier to reach supersaturation to grow

Fig 2 TEM image of the SiNWs The diameter of the trunk

SiNW is about 40 nm The diameters of plenty of tiny SiNWs,

which attach on the tip, is about 3 nm The upper left inset is the

SAED image of the same SiNWs.

Fig 3 HRTEM image of tiny SiNWs (the inset is the TEM

image of the large scale of tiny SiNWs) (a) and the EDX

spectrum of the SiNWs (b) In the EDX spectrum, the Si, Cu

and O with smaller quantity were detected Obviously the

intensity of the Si is the strongest among these peaks Cu peak

came from Cu grid, and O peak mainly from the possible weak

surface oxidation of SiNWs during the synthesis or the

adsorption of oxygen on the SiNWs due to the survived oxygen

in TEM system.

10 20 30 40 50 60

70

Au SiNWs

Au(220) Au(200)

Au(111)

Si(311) Si(220)

Si(111)

2 Theta(degree)

Fig 4 XRD data of the SiNWs.

SiNW because of the lower energy compared with

the Au–Si droplet Here the growth procedure can

be regarded as the developed VLSmodel with the

defects as catalyst but the metal As the

inter-planar spacing of Si (1 1 1) planes needs low-energy

to form the crystalline array at a relatively low

temperature, finally the Si (1 1 1) dominates the

main growth direction just as the SAED and XRD

data showed Here one case must be mentioned,

because the form of the tiny SiNWs was effected

by the properties of the defects, new nucleation

density, and other surrounding conditions, the

quantity of the tiny SiNWs is lesser compared with

the thick SiNWs

We carried out Raman spectrum at room

temperature at an excitation wavelength of

532 nm In Fig 5, the characteristic peaks of

SiNWs at 494 and 480 cm 1 with a clearly broad

downshift of nearly 35 cm 1 compared with the

bulk silicon of 523 cm 1 Such a feature could be

due to different temperature dependence on laser

heating and the small size effect of SiNWs[25,26]

The peak of 480 cm 1 and asymmetry of the peak

should contribute to the thin SiOx sheath out of

the silicon nano-wire and the defects which are

contained among the silicon nano-wire

4 Conclusions

In conclusion, we have synthesized large-scale, very tiny, and very long SiNWs on the substrate of p-Si (1 1 1) wafer using the simple approach of CVD method at 630 1C The part-crystalline tiny SiNWs with diameters of 3–8 nm sub-grow on the trunk crystalline SiNWs with diameters of 40–50 nm The defect-inducing growth mechanism explained the growth of the tiny SiNWs at low temperature The deeper understanding of the growth mechanism of the tiny SiNWs might contribute a good suggestion for the successful synthesis and device application of smaller one-dimensional quantum wires In the end, the Raman spectra of the SiNWs were discussed This also shows a potential application in optical waveguide of the nano-wires in the future

Acknowledgements This work was supported by the National Natural Science Foundation of China (Project

No 50272057) and the key project of Chinese Ministry of Education The authors would like to thank Mr X.R Huang and Z.C Chen, Zhejiang University, for their helps with Raman spectrum

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