Iron catalytic growth of prism shaped single crystal silicon nanowires by chemical vapor deposition of silane

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

Iron-catalytic growth of prism-shaped single-crystal silicon

nanowires by chemical vapor deposition of silane

Chen Li a,b,1, Chi Gu a,b,1, Zengtao Liu a,b, Jinxiao Mi c, Yong Yang a,b,*

a State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, Fujian 361005, China

b Department of Chemistry, Xiamen University, Xiamen, Fujian 361005, China

c Department of Material Science and Engineering, Xiamen University, Xiamen, Fujian 361005, China

Received 26 April 2005; in final form 9 May 2005

Available online 29 June 2005

Abstract

Single-crystal silicon nanowires with the prism structures were synthesized by chemical vapor deposition of SiH4gas at 450°C

Fe particles which were located at the tip of the CNTs were employed as a catalyst for the growth of silicon nanowires (SiNWs) Transmission electron microscopy studies of the materials showed that the nanowires have a diameter of 50–70 nm and a length of several micrometers High-resolution transmission electron microscopy demonstrated that the nanowires have excellent single-crys-tal characteristics Both the CNTs and Fe play a key role in the growth process of the SiNWs A growth mechanism was proposed for the growth of silicon nanowires under our experimental conditions

Ó 2005 Elsevier B.V All rights reserved

1 Introduction

One-dimensional nanostructures are significant for

nanoscale devices based on their distinctive structures

and properties Since silicon has long been the dominant

material in semiconductor and microelectronics

indus-tries, thus the study of synthesis and properties of

nanostructured silicon material is one of the most

important subjects in the research of nanostructured

material field Nanostructured silicon such as wires[1],

ribbons [2]and tubes [3]have been widely studied for

the applications in nanoscale electronics and photonics

[4], field-effect transistors (FETs) [5]and so on Many

efforts have been made to prepare silicon nanowires

(SiNWs) by different methods, such as laser ablation

[6,7], thermal evaporation deposition[8], chemical vapor

deposition (CVD)[9–11], and supercritical fluid–liquid–

solid (SFLS) synthesis [12] The most popular growth mechanism for SiNWs is vapor–liquid–solid (VLS) growth mechanism [13,14] In VLS process, a metal, such as gold or iron that forms a low temperature eutec-tic phase with silicon is used as the catalyst that has been heated to a temperature greater than the eutectic tem-perature (363°C for Au–Si, 1207 °C for Fe–Si)[15] In

has also been developed to produce large quantities of SiNWs There, an excimer laser is used to ablate a solid composite target of highly Si powder mixed metals (Fe,

Ni or Co) The target temperature was 1100–1400°C and the nanowires growth temperature was selected as 900–1100°C In general, when using pure Fe metal as

a catalyst for the growth of silicon nanowires, almost all methods in the literatures [6,16] require the growth temperature higher than 1000°C It is quite difficult to grow single-crystal SiNWs without Au catalyst assis-tance at the lower temperature

In this work, the SiNWs were grown at 450°C by using common catalyst of iron (Fe), which was previously used as a catalyst for growing CNTs More

0009-2614/$ - see front matter Ó 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.cplett.2005.05.117

* Corresponding author Fax: +86 592 2185753.

E-mail addresses: yyang@xmu.edu.cn (Y Yang), chenli@xmu.

edu.cn (C Li).

1 These authors contributed equally to this work.

www.elsevier.com/locate/cplett

importantly, it should be noted that the growth of the

SiNWs here is different from the conventional VLS

mechanism, revealing the crystalline structures of the

crystal may have influence on the structure of the grown

one-dimensional nanostructures The interface between

Si and Fe prefers to take the least lattice mismatch

2 Experimental

The anodic aluminum oxide (AAO) templates were

formed by using a two-step anodization method as

amount of iron was deposited into the bottom of

the AAO pores using an alternating current (AC)

elec-trodeposition method Elecelec-trodeposition was carried

out in a ammonium iron (II) sulfate hexahydrate

(NH4)2Fe(SO4)2Æ6H2O solution (30 g/L, pH 5.0) at

50 Hz After that, the CNTs were formed in the pores

of AAO template by pyrolyzing a flow of mixture of

acetylene (10 sccm) and nitrogen (50 sccm) at 650°C

for 1 h In order to expose CNTs tips containing iron

catalyst, we remove the AAO template partly before

deposition of silicon Prior to the pyrolysis of silane,

the quartz boat tube furnace was purged with pure

While the flow of pure nitrogen gas (50 sccm) remains

as a carrier gas, a flow of 5% silane (diluted in high

purity argon) with a flow rate of 5 sccm (Si partial

pressure of 4 Torr) was pyrolyzed to grow silicon

nanostructure under Fe catalysis at the tip of the

CNTs Several sets of pyrolysis times were

imple-mented in our experiments to form the SiNWs with

different lengths The sample was investigated in

scan-ning electron microscopy (SEM) (Oxford Company,

LEO-1530) Furthermore, AAO templates together

1 h, then in a 1 M NaOH to remove the residue

alu-minum and AAO template completely The samples

were prepared for high-resolution transmission

micros-copy (HRTEM) (FEI Company, TECNAI-F30)

obser-vation and energy dispersive X-ray (EDX) spectra

examination

3 Results and discussion

Fig 1is an SEM image for the cross-sectional view of

the SiNWs on CNTs array after partly dissolving the

AAO template It can be seen that one-dimensional

sil-icon nanowires were exactly located at the end of the

carbon nanotubes with high density These SiNWs

tended to
stick to each other The diameter of the

SiNWs is about 50–70 nm, which is well consistent with

the diameter of the CNTs and AAO pores It also can be

seen that most SiNWs have a length of several microns

Figs 2a,b are the TEM images and HRTEM image which provide an insight into the structure of the SiNWs, respectively These SiNWs were broken down from the CNTs during ultrasonically cleaning for TEM observation It can be found from Fig 2a that the as-grown SiNWs are actually the prism-shaped nanostructures Fig 2b displays a typical HRTEM im-age taken from the edge area of the SiNWs (indicated

by the box area in Fig 2a) Detailed analysis on lattice images give interplanar spacing of 0.31 nm, closely cor-responding to that of Si(1 1 1) plane which reveals the growth direction [1 1 1] of these SiNWs A selected-area electron diffraction (SAED) pattern (seeFig 2a, left in-set) taken from top face (Fig 2a, indicated by red arrow)

of the tilted image was taken along [1 1 1] zone axis of single crystal Si Another SAED pattern was taken along [2 1 1] zone axis of single crystal Si (see Fig 2b, left inset) From the data above, we can confirm the as-grown prism of single crystal SiNWs (Fig 1a) is a

(2 1 1) and ( 2 1 1), (0 1 1) and (0 1 1) three pairs

of parallel planes It belongs to cubic silicon structure with a = 0.5341 nm, space group Fd3m Fig 2c gives

an EDX spectra taken from an individual SiNW, which further confirmed that the nanowires are compose of silicon with a minimal content of oxygen

In order to further investigate the growth process of the SiNWs, different pyrolysis time of silane were carried out in this work It can be found fromFigs 3a–d that there are different morphologies of the SiNWs at differ-ent growth stages With the increasing of pyrolysis time

of silane, a triangle-shaped silicon tip at initial stage

Fig 1 The SEM image of the SiNWs grown on the CNTs after removing the AAO template partly.

transforms gradually to a square, ultimately to SiNWs.

It also can be revealed that in growth process of SiNWs,

the CNTs have a size-confined effect on the growth of

SiNWs through confining the size of metal catalyst In

with Fe catalysis This implied that the CNTs play an

important role on growth of the SiNWs It has been

re-ported that carbothermal reduction of the CNTs can

help silicon powder to form nanostructures [18,19]

Thus, it is believed that the CNTs facilitate the pyrolysis

of silane and growth of the SiNWs That is, the CNTs,

as an assistance of the Fe catalyst, help to greatly

de-crease the reaction temperature During the whole

CVD process, the CNTs always keep Fe particles active state and maintain highly reductive atmosphere around them when silane diffuses into them

It can be clearly seen from Fig 4that Fe particle is located between C and Si and joints them for each other (see Fig 4b), which is different from the conven-tional vapor–liquid–solid (VLS) growth mechanism re-ported before [9] In addition, the temperature of growing silicon is just 450°C which is far below the eutectic temperature of Fe–Si alloy (>1200°C) The

450°C just reach the temperature where SiH4 start a decomposing reaction The normal VLS mechanism

is not applicable to explain our results Fig 4a shows

Fig 2 (a) The TEM image of the SiNWs with a SAED image (upper left inset) taken from the face indicated by the red arrow and a schematic representation of crystal faces grown (lower left inset) (b) The HRTEM images of the enlarged box area in (a) with its SAED image (left inset) (c) EDX spectrum taken from an individual SiNWs (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)

a HRTEM image of triangle-shaped area of the

CNTs–Fe–Si junction which provides the information

of the starting nucleation The reciprocal lattice peaks,

which were obtained from fast fourier transforms

(FFT) of the lattice-resolved images of outer layer

area (Fig 4a, left inset) shows the interplanar spacing

values of about 0.38, 0.22 and 0.31 nm which are

in-dexed to the (1 1 0), (2 1 1) and (1 1 1) silicon planes,

respectively Fourier transforms image of an inner

cat-alyst area (Fig 4, right inset) solely exhibits Fe(1 1 0)

of an identified spacing value of 0.20 nm It should be

noted that the spacing values of Si(2 2 0) (d2 2 0=

1.9201 A˚ ) and Si(2 1 1) (d2 1 1= 2.2172 A˚ ), which help

to form the prism shaped SiNWs, are very close to

one of Fe(1 1 0) plane (d1 1 0= 2.0268 A˚ )

It has been reported that the crystallographic lattice

structure at the interface is important in defining the

structural characteristics of the grown nanowires The

interface prefers to take the least lattice mismatch[20]

Thus, we think the preferred growth direction and the

3D-prism shape should be determined by the Fe(1 1 0)

as is discussed above It is confirmed by an elemental

mapping (Fig 4b) of another developing junction

specimen at early stage The Si(2 2 0) and Si(2 1 1) which are the side planes of the prism shaped SiNWs, are determined by the Fe(1 1 0) as these planes are well matched to Fe(1 1 0) Thus, the specific plane of Fe particles play an important role in initiating nucleation and growth of silicon nanostructures, resulting in mor-phology and orientation control The interface between

Si and Fe prefers to take the least lattice mismatch Besides, the Si{1 1 1} has the lowest surface energy among the Si simple index surfaces In a word, with the assistance of the CNTs and the catalysis of Fe par-ticles, preference of taking the least lattice mismatch be-tween Si and Fe leads to the growth of the single-crystal SiNWs with well-defined structures at the lower temperature

4 Summary

In conclusion, by chemical vapor deposition of silane, the large-scale SiNWs were grown under the catalysis of

Fe particles at the lower temperature – 450°C These nanowires have a diameter of 50–70 nm and a length

Fig 3 TEM images of silicon nanostructures growth by different pyrolysis time of silane (a) a triangle-shaped silicon tip grown on a CNT by 30-min pyrolysis time (b) A rectangle silicon grown on a CNT by 40-min pyrolysis time and white arrow indicates transformation direction from triangle-shaped silicon to rectangle silicon (c) 150 min (d) 240 min.

of several microns Through the HRTEM and SAED

observation, the nanowires exhibit the excellent single

crystal nature At the whole growth process, the CNTs

not only have a size-confined effect on the growth of SiNWs, but also facilitate the pyrolysis and deposition

of silane on iron and growth of the SiNWs because of their carbothermal reduction The growth process pro-posed here which is different from the conventional VLS process should be ascribe to preference of taking the least lattice mismatch between Si and Fe The pref-erence leads to the control of morphology and crystal-line orientation of the SiNWs

Acknowledgments

We are grateful for financial support from National Natural Science Foundation of China (Grant Nos

29925310 and 20021002) and Ministry of Science and Technology of China (Grant No 2001CB10506)

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Fig 4 (a) TEM image of a triangle shaped silicon, the upper-left inset

is the two-dimensional fourier transform of the outer layer of the

triangle (indicated by arrow) depicting lattice spacing of Si(1 1 1) of

0.31 nm, Si(2 1 1) of 0.22 nm and Si(2 2 0) of 0.38 nm, respectively, the

bottom-right inset is the two-dimensional fourier transform of central

part of the triangle image (also indicated by arrow) depicting lattice

spacing of 0.207 nm, which is indexed to Fe(1 1 0) (b) TEM image and

its element-mapping of triangle-shaped region The insets are the

distribution of C, Fe and Si, respectively.