Growth of amorphous silicon nanowires

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

Growth of amorphous silicon nanowires

Z.Q Liu, W.Y Zhou, L.F Sun, D.S Tang, X.P Zou, Y.B Li,

Group 412, Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,

P.O Box 603-32#, Beijing 100080, People's Republic of China Received 16 February 2001; in ®nal form 4 April 2001

Abstract

We have grown vertically aligned amorphous silicon nanowires on Au±Pd co-deposition silicon oxide substrate by thermal chemical vapor deposition using SiH4gas at 800°C The diameter of silicon nanowires is in the range 10±50 nm and the length is about 1 lm Transmission electron microscopy (TEM) observations show that the grown silicon nanowires are of an amorphous state and some of nanowires appear to bifurcate in the vertically growth process The e€ect of H2 gas etchings on the catalytic size and the e€ect of catalytic size on the formation of the vertical growth nanowires are discussed Ó 2001 Elsevier Science B.V All rights reserved

1 Introduction

Nano-materials are attracting much attention

because of the electronic, magnetic, optical,

bio-logical, and chemical characteristics they have that

are not obtained with conventional materials

Among these low-dimensional materials,

one-dimensional materials, such as nanotubes [1±4],

semiconductor nanowires [5±10] and metal

nano-wires [11] have been of recent heightened interest

because these materials o€er fundamental

scien-ti®c opportunities for investigating the in¯uence of

size and shape with respect to optical, electronic,

and mechanical properties For silicon, it is

promising to emit visible light by reducing its

dimension, in which the motion of carriers is

con®ned, causing a possible transformation of the

electronic band structure from indirect band gap

to direct band gap This has stimulated intensive interest in preparing silicon nanowires In addi-tion, if such wires can be ordered and assembled into appropriate architectural environment, then a host of nanoelectronic applications can be envi-sioned Until now controlling the size and length

of these synthesized nanomaterials have been practical problems, which seriously restrict the future applications To date, silicon nanowires have been successfully prepared through di€erent ways, such as excimer laser ablation [5,6], chemical vapor deposition (CVD) [12,13], stress limited oxidation [14,15] However, all of these previous studies have got crystalline state silicon nanowires with a thin oxide outer layer Amorphous state silicon nanowires have been reported very slightly Recently, Yan et al [16] have prepared amor-phous silicon nanowires via a solid±liquid-solid mechanism

Among the above-mentioned growth tech-niques, the CVD process may have its special bene®ts that one can more easily control the

Chemical Physics Letters 341 (2001) 523±528

www.elsevier.nl/locate/cplett

* Corresponding author Fax: +86-10-8264-9531.

E-mail address: ssxie@aphy.iphy.ac.cn (S.S Xie).

0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V All rights reserved.

PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 5 1 3 - 9

nucleation site as well as di€erentiate between

growth and tail ends of the nanowires In this

Letter, we use chemical vapor deposition of silane

to prepare silicon nanowires Our research group

had previously reported the growth amorphous

using thermal chemical vapor deposition [17] We

catalytic size and the e€ect of the catalytic particle

size on the diameter and alignment of amorphous

silicon nanowires are discussed Bifurcate

phe-nomena morphology was found in vertical growth

process

2 Experimental

The substrates used in our experiments were

8 X cm n-type Si(1 0 0) wafers with an oxide layer

about 60 nm in thickness They were

ultrasoni-cally stirred for 30 min in acetone solution to

clean their surfaces The cleaned substrates were

then deposited with Au±Pd ®lm about 0.5 and

using ion sputter ®lms deposition system (Hitachi,

E-1010) The thickness of the Au±Pd alloy ®lm

was estimated as approximately 5 nm and 10 nm,

respectively Put them into a tube furnace that has

been described elsewhere [18] Prior to deposition,

the Au±Pd alloy-coated substrates were pretreated

purpose of this pretreatment was to break the

smooth Au±Pd alloy ®lms into discrete islands

and then control the size of the catalytic size

After pretreatment, the silane gas was introduced

into the reactor for the start of deposition stage

The pretreatment and deposition parameters are

listed in Table 1

The as-grown silicon nanowires were examined

by a ®eld-emission scanning electron microscope (SEM; S-4200, Hitachi), and energy-dispersive X-ray (EDX) spectra were recorded by SiLi detector attached to SEM A transmission electron microscope (TEM; JEOL JEM-200 CX at 200 kV) was used to characterize the structures of silicon nanowires

3 Results and discussion Fig 1a±d shows the SEM images of the silicon nanowires grown on a substrate, which was de-posited with AuPd alloy as catalyst for 10 nm Fig 1a shows that large amounts of nanowires are formed, which are of a uniform length up to

2 micrometers The growth rate of the nanowires is estimated to be ca 15 nm/min The diameter of the nanowires is about 40 nm A lot of nanowires are assembling along the same direction and are not parallel to the surface of the substrate (see Fig 1a,b) All of the nanowires are terminated by the nanoparticles with the diameter about 60±80 nm at their tips (see Fig 1c) Some of the nanowires become curved near their tops The EDX spectra taken from these nanoparticles showed the pres-ence of gold, palladium, oxygen and silicon (0.6, 0.3, 26.6 and 72.5 at%, respectively) Compared with the nanoparticles, the nanowires are com-posed of silicon and a small amount of oxygen, indicating that no catalytic elements exist in the nanowires Top view of the nanowires is shown in Fig 1d An interesting phenomenon is that bifur-cation of the nanowires was found at the tail of the nanowires (see arrowhead in Fig 1b,d)

High-resolution TEM was employed to detect the structure of the nanowires in detail Using an ultrasonic treatment in alcohol solution for 30 min,

Table 1

Pretreatment and depostion parameters

Pretreatment Depostion

524 Z.Q Liu et al / Chemical Physics Letters 341 (2001) 523±528

the nanowires were separated from the substrate.

And then we dropped the solution to the copper

grid for TEM observation From Fig 2a, we can

see that the nanowires are of a uniform diameter

about 40 nm and have smooth surfaces

Further-more, TEM reveals that the nanowires are in a

perfect amorphous state The selected-area

elec-tron di€raction of silicon nanowires shown in the

inset of Fig 2a con®rms this point

In our TEM observation, we also found a

bi-furcation phenomenon of amorphous silicon

nanowires Fig 2b±d shows the di€erent

bifurca-tion morphologies of the nanowires `Y' shape (see

Fig 2b) and `T' shape (see Fig 2c) junctions are

found The smooth curvatures associated with the junctions suggest that these structures were actu-ally formed during the growth process, rather than during TEM observations More complex inter-connections containing metal catalyst are also found (see Fig 2d)

In order to see the e€ect of the catalytic particle size on the diameter and alignment of amorphous silicon nanowires, we also use the substrate, which has been sputtered with Au±Pd alloy thickness of about 5 nm The growth process is the same as 10

nm Au±Pd deposited substrate Fig 3a±b shows the SEM images of the nanowires grown on 5 nm Au±Pd deposited substrate Comparing with the

Fig 1 SEM images of the silicon nanowires grown on a substrate deposited with AuPd alloy as catalyst for 1 min (a) Low-mag-ni®cation images of silicon nanowires (b) and (c) A magni®ed view of (a) (d) Top view of the vertically aligned silicon nanowires.

thicker Au±Pd alloy ®lms, we can see that the

di-ameter of the nanowires is 30 nm The alignment

of the nanowires is improved

Compared with our previously work [17], we

got amorphous aligned silicon nanowires instead

of the randomly distributed nanowires The only

di€erence in our experiment is that the substrates

nano-wires growth For comparison, the substrate which

also studied Fig 4a show the SEM image of the

unetching substrate with 5 nm Au±Pd ®lms From

Fig 4 we can see that AuPd ®lms on the substrate

have broken up to form a large amount of

nano-particles The nanoparticles size are uniform and

are of about 30 nm diameter However, when the

same condition, the diameter of the nanoparticles (see Fig 4b) become smaller than that of the nanoparticles shown in Fig 4a It con®rmed that

H2 etching really plays a positive role in control-ling and deducing the catalytic size The decreasing size of the catalyst may result in the aligned growth

of the nanowires

We think that vapor±liquid±solid (VLS) mech-anism accounts for the amorphous state silicon nanowires growth in our experiment The mecha-nism has been put forward by Wagner and Ellis

Fig 3 SEM images of the nanowires grown on 5 nm Au±Pd deposited substrate (a) Low-magni®cation images of silicon nanowires (b) A magni®ed image of (a).

Fig 2 TEM images of the silicon nanowires dispersed on a

carbon-coated copper microgrid (a) A low-resolution TEM

image of the nanowires, the inset is the selected-area electron

di€raction of the nanowires (b)±(d) The bifurcation structure

of the silicon nanowires.

526 Z.Q Liu et al / Chemical Physics Letters 341 (2001) 523±528

[19] It is well know that the impurity agent, which

plays an important role in the formation of

pref-erential growth ®ber, will be found at the tip of the

obtained nanowires The SEM images (shown in

Fig 1) con®rm this point We propose a model to

explain the bifurcation growth process under VLS

growth mechanism (see diagram in Fig 5) During

the aligned nanowires growth process, some of the

nanowires will form kinks due to the weight of the

catalyst on their tip It may be easy for the melting

catalytic nanoparticles on its top to meet together

and coalesce to become a larger catalyst From

Fig 1, we found that the size of catalyst on the tip

of the nanowires become larger than that of the

particles shown in Fig 4b In the end, the

bifur-cation growth is formed

4 Conclusion Aligned amorphous silicon nanowires on a large scale of Au±Pd co-deposition silicon oxide substrate by thermal chemical vapor deposition were obtained The catalytic particle size of Au±Pd catalysts decreases, the diameter of the nanowires decreases and the vertical alignment is enhanced

catalytic nanoparticles sizes There are bifurcation

Fig 5 Schematic diagrams of the bifurcation growth model.

Fig 4 SEM images of the annealing substrates with 5 nm

Au±Pd ®lms at 800°C for 180 min (a) without H 2 gas etching,

(b) with H 2 gas etching.

growth phenomena in this kind of aligned growth

process

Acknowledgements

This work is supported in part by the National

Natural Science Foundation of China

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