Rational growth of highly oriented amorphous silicon nanowire films

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Rational growth of highly oriented amorphous

silicon nanowire films

Department of Physics, School of Physics, State Key Laboratory for Mesoscopic Physics, Electron Microscopy Laboratory,

Peking University, Room 211, Building N, Beijing 100871, China Received 10 April 2003; in final form 7 May 2003

Abstract

Amorphous silicon nanowire films were rationally synthesized using a simple approach The films consist of highly oriented nanowires of 30 lm in length and 20–80 nm in diameter The morphology, microstructure features, and chemical composition of the nanowires were analyzed using electron microscopy and Raman spectroscopy A novel model concerning solid–liquid–solid phases was proposed to explain the growth mechanism of the nanowires This approach should be very useful to direct the controlled growth of nanomaterials

Ó 2003 Published by Elsevier Science B.V

1 Introduction

One-dimensional nanomaterials have been a

focused research field since the first pioneering

work of the discovery of carbon nanotubes [1] and

nanowires [2–6] A diverse variety of

semiconduc-tor nanowires, such as silicon, GaAs, GaN, and

ZnO nanowires, were synthesized using different

approaches Of those nanowire materials, silicon

nanowires (SiNWs) have great scientific and

technological importance, and have attracted

much research interest [7,8] For example, the

sil-icon nanowires have been used as the building

blocks to build nano-scale logic and

computa-tional circuits [9], nanodiodes, and random access memory [10] Controlled growth of the nanowires

in their morphology, orientation, for example, is the key to success both for characterization of physical properties and exploration of device ap-plication of the nanowires; however, it is extremely difficult to realize In this Letter, we will report the rational synthesis of highly oriented amorphous SiNW films on centimetric substrates The growth mechanism was explained under a novel frame-work of the solid–liquid–solid (SLS) mechanism

2 Experimental

A conventional hot-filament CVD system was modified to grow the silicon nanowires A thin layer of nickel 5 nm in thickness was deposited by thermal evaporation on a 5 mm
5 mm p-type Si

www.elsevier.com/locate/cplett

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Corresponding author Fax: +8601062759474.

E-mail address: yudp@pku.edu.cn (D Yu).

1 Authors contribute equally to the work.

0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V.

doi:10.1016/S0009-2614(03)00781-4

(1 1 1) wafer Such a substrate was heated to about

900°C for 10min in the sample stage of the

HF-CVD system, while a H2 gas flow was introduced

into the chamber at 20sccm during the growth to

keep an ambient pressure about 1.5 kPa The

original shiny surface of the substrate became gray

after cooling down to room temperature The

morphology of the as-grown product was analyzed

using an scanning electron microscope (SEM,

DB235 FIB,FEI) A Hitachi-9000 NAR

high-res-olution transmission electron microscope (TEM)

equipped with nano-beam energy dispersive

spec-troscopy (EDS) was used to characterize the

mi-crostructure and chemical composition of the

nanowires Raman spectrum was measured using

a Renishaw 2000 system with a laser source of

514.5 nm

3 Results and discussions

In a brief view of SEM analysis, the whole

substrate was found to be covered with a thick

layer of wool-like product, as is shown in Fig 1

The wool-like layer with homogeneous thickness

can be easily scratched from the substrate, which is

marked with arrow in the SEM image In the magnified SEM image in the left inset, one piece of the wool-like carpet was scratched from the sub-strate and was folded on top of the film The right inset shows the details of the edge of the layer, and reveals that the film consists of fine free-standing wires of very high density, and has a thickness of about 30 lm (also the length of the nanowires) The growth rate of nanowires is faster than 100 nm/s The catalyst nanoparticles were found at the bottom of the a-SiNWs, which is marked with arrow in the right inset, revealing a base growth of the nanowires Whether a base or a top growth depend on the wetting condition between the cat-alyst droplets and the substrate If the wetting is very good, the interaction force between the cat-alyst droplets and the substrate can be very strong and the nanoparticles will stay at the substrate as a base growth; otherwise if the wetting is bad, a top growth will work In the present case, the wetting between Si and Si2Ni should be good because the silicon has a very large solubility in Si2Ni, so the catalyst droplets should stay at the bottom of the nanowires We repeated the procedure several times under a similar growth conditions and found that the wool-like films were completely repro-ducible This demonstrates that our method is controllable and easy to scale up.The SEM images

in Fig 2 reveal that the nanowires are highly ori-ented perpendicular to the substrate As is seen from the cross-sectional view along the edge of the scratched film in Fig 2a, the orientation of the nanowires is widespread over the whole substrate Detailed SEM view in Fig 2b shows that the nanowires appear straight and parallel to each other

Part of the product was scratched off and used

to measure the Raman spectrum in a micro-beam mode from different places of the sample Two peaks around 300 and 516 cm1 were observed, as

is shown in Fig 3a It is well known that those two Raman peaks are characteristic of a silicon struc-ture, corresponding to the second-order transverse acoustic phonon mode (2TA), and the first-order transverse optical phonon mode (TO) of silicon, respectively The Raman result confirmed that the nanowire film is composed of silicon The possi-bility of the formation amorphous silicon oxide

Fig 1 SEM image revealing a wool-like film on large area The

SEM image in the left inset reveals that one sheet of the film was

scratched off the substrate The film has a thickness about 30

micrometers, and consists of pure nanowires, as shown in the

right inset The bright contrast indicated by an arrow shows the

catalyst layer between the substract and nanowire film, which

provides evidence for a base growth.

nanowires can be excluded by the following

dis-cussions Because the growth was conducted in a

steel CVD chamber, it guarantees a very good

vacuum status, and the hydrogen gas keeps a

reduction atmosphere On the other hand, the substrate we used is the commercial microelec-tronic wafers having very thin native oxide layer (usually <1 nm), which is not thick enough to grow oxide nanowire layer as thick as 30 lm The microstructure and chemical composition

of the nanowires were also analyzed using TEM Fig 3b shows a TEM image of the silicon nano-wires The diameter of the nanowires ranges from 20to 80nm Selected-area electron diffraction (SAED) reveals that the nanowires are amorphous The inset in the left shows the corresponding EDS spectrum, which reveals that the nanowires are mainly composed of silicon The remaining oxygen peak comes from the surface oxidation of the nanowires Nanosized particles larger than the nanowire diameter were found attached to the end

of the nanowires, which is marked with an arrow in the right inset EDS analysis shows that the nanoparticles were composed of both silicon and nickel Those nanoparticles can provide evidence for the growth mechanism of the nanowires

It was found that the growth mechanism of the amorphous silicon nanowires (a-SiNWs) is differ-ent from the convdiffer-entional vapor–liquid–solid (VLS) model [11,12] The growth circumstances in the present case are completely different from that

in laser ablation [2,3], or in the CVD method [8] in which the silicon source is supplied directly from the vapor phase When the vapor phase plays an important role, the growth of the SiNWs is mostly controlled by the well-known VLS mechanism In the present circumstance, however, no Si vapor phase is introduced into the system as in CVD growth of SiNWs Though the eutectic point of

Fig 2 SEM images showing the high orientation of the nanowire films (a) Edge of the film showing a wide-spread orientation of the nanowires (b) Details of the highly oriented nanowire film.

Fig 3 (a) Raman spectrum of the nanowire film scratched off

from the substrate Two peaks at 300 and 516 cm 1 , were

ob-served which correspond to 2TA, and TO modes of silicon,

respectively (b) TEM image showing the morphology of the

silicon nanowires The inset in the left shows the EDS spectrum

and Si and O are visible, where oxygen comes from the surface

oxidation of the nanowires The inset in the right shows a Si–Ni

nanoparticle capped at the end of the nanowire.

Si2Ni is 993°C, the small-size-melting effect makes

it possible for the deposited Ni nanoparticles to

react with the Si substrate at temperature above

900°C, forming Si2Ni eutectic liquid droplets So

the source materials comes from directly the

sub-strate instead from the vapor phase in the present

case Therefore, we proposed a novel model to

explain the growth of the a-SiNWs, which involves

the SLS phases in the growth process In the SLS

growth, the catalyst Ni dissolves directly the

sili-con substrate to form Si2Ni eutectic liquid phase in

the following reaction:

where s represents the solid phase and l the liquid

phase

Because the silicon has a large solubility in the

Si2Ni liquid phase, more Si atoms will be dissolved

continuously into the liquid droplets When the

liquid phase becomes supersaturated, the a-SiNWs

will grow out off the liquid droplets, which can be

represented as follows:

SiðsÞ þ NiSi2ðlÞ ! SisuperNi ðlÞ

! Si2NiðlÞ þ Siw ðsÞ ð2Þ (super represents silicon supersaturation and w

represents the finally solidified SiNWs)

Since the silicon substrate was placed face up

directly on the hot filament to be heated from the

back, so the temperature gradient in the substrate

must be considerable The temperature gradient

should be the driving force for the silicon substrate

to be dissolved to form low temperature Ni–Si

eutectic liquid phase, forming silicon nanowires at

the cooler side

The SLS mechanism is in some extent an

anal-ogy to the VLS model In the SLS controlled

growth of nanowires, the eutectic Ni–Si liquid

droplets have to stay at the surface of the silicon

substrate in order to grow continuously, and the

solidified nanoparticles shall remain at the bottom

of the nanowires, as is shown in the right inset in

the SEM image in Fig 1 In this image, there exists

a bright-contrasted layer between the substrate

and the nanowires film (marked with an arrow),

and EDS analysis proved that this

bright-con-trasted layer consists of Si and Ni

Two more questions shall be addressed here First, it is not well understood why the final nanowires are in amorphous state instead of a crystalline phase The most possible explanation is the unusual high growth rate The estimated growth rate is about 100 nm/s Such a high growth rate may explain why the resulting nanowires are amorphous instead of crystalline, because the growth is so rapid that the atoms have no time to stack themselves into crystalline order Second, we think that the crowding effect between the very dense nanowires plays an important role to keep the nanowires staying oriented From the SEM images in the previous sections, it is also evident that the density of the a-SiNWs is very high, so the Van der Waals force between nanowires should be large This interaction force is another important factor to keep the nanowires grow upward and to

be oriented towards each other

The a-SiNWs grown on the substrate have re-markable surface/volume ratio, and can show physical/chemical properties completely different from the bulk In fact, it was recently revealed that the lithium battery using SiNWs as electrode ma-terials showed a capacity as high as eight times that of the ordinary one [13] Therefore, the a-SiNWs may have potential applications such as rechargeable battery of high capacity with por-table size The a-SiNWs can also be useful in chemistry, biology, electronics and other fields on consideration of their huge specific surface Fur-thermore, it is believed that the present rational synthesis method can be used to direct the con-trolled growth of other nanowire structures

4 Conclusion Highly oriented amorphous silicon nanowire films have been synthesized rationally on large-area silicon substrate via heat treatment of the nickel-coated silicon substrate The morphology, microstructure, and chemical composition of the nanowires were characterized using electron microscopy and Raman spectroscopy The growth

of the a-SiNWs cannot be explained by the con-ventional VLS mechanism, and a novel SLS model was proposed to explain reasonably the growth of

the amorphous silicon nanowires The as-grown

a-SiNWs and the proposed growth model should

be useful to direct the controllable growth of other

nanowire structures

Acknowledgements

This project was financially supported by

na-tional Natural Science Foundation of China

(NSFC, No 50025206, 20151002), and by the

Research Fund for the Doctoral Program of

Higher Education (RFDP), China

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