Silicon quantum wires arrays synthesized by chemical vapor deposition and its micro structural properties

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

Silicon quantum-wires arrays synthesized by chemical vapor deposition and its micro-structural properties

M Lu a, M.K Li a, L.B Kong a, X.Y Guo b, H.L Li a,*

a Department of Chemistry, Lanzhou University, Lanzhou 730000, China

b Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, China

Received 16 July 2002; in final form 1 May 2003

Abstract

Well-aligned arrays of silicon nanowires (SiNWs) have been synthesized by a chemical vapor deposition (CVD) template method without catalyst The micro-structures of the SiNWs were studied by high-resolution transmission electron microscopy (HRTEM) Selected-area electron diffraction (SAED) and X-ray diffraction (XRD) indicate that each nanowire is essentially a single crystal and has a different orientation in an array According to VLS mechanism, the growth of SiNWs without catalyst is related to the structure of template The superior field emission behavior is believed to result from the oriented growth and the sharp tips of SiNWs

Ó 2003 Elsevier Science B.V All rights reserved

1 Introduction

Quantum wires of silicon as a special form of

crystalline silicon have stimulated much interest

because of their low dimension and

quantum-confinement effect [1–4] It has been suggested that

they may be used for developing one-dimensional

(1D) quantum wires, high-speed field effect

tran-sistors and light-emitting devices with extremely

low power consumption In order to be capable of

being incorporated effectively into devices, these

applications usually require controlled orientation

and size of the grown nanostructure To date,

sil-icon nanowires (SiNWs) have been successfully

synthesized by different methods, such as laser ablation, thermal evaporation and lithography [2–8] However, SiNWs produced by most of these methods are of random orientation, self-gathering and twisting each other, which restrict their ap-plications in nanoelectronic Recently, a-SiNWs were controlled grown directly on a Si substrate via a solid–liquid–solid mechanism [9] Liu et al [10] have prepared vertically aligned a-SiNWs on a large scale on silicon oxide substrate by thermal chemical vapor deposition (CVD) Metal catalyst

is an essential element in these methods, which is required for the nucleation and growth of SiNWs

In this Letter, for the first time, we employed alumina template to prepare well-aligned SiNWs arrays by CVD without catalyst The size and shape of SiNWs can be readily controlled by the template and may vary over a wide range

www.elsevier.com/locate/cplett

*

Corresponding author Fax: +86-931-891-2582.

E-mail address: lihl@lzu.edu.cn (H.L Li).

0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V All rights reserved.

doi:10.1016/S0009-2614(03)00747-4

according to the template used [11–14] More

im-portantly, there are three advantages First,

tem-plate-confined growth demonstrated to be an

efficient approach to the production of highly

or-dered and isolated nanowires arrays over large

areas Second, it becomes possible to extend the

traditional ideas of using catalyst for the growth of

SiNWs Third, compared with a high density of

defects near the tip of SiNWs prepared by other

methods, our present SiNWs with sharp tips and

perfect lattices might be promising materials for

future nanoprobes and superior field emitters

2 Experimental

Alumina template was prepared by anodic

ox-idation of electropolished aluminum plate at a cell

voltage of 20 V in 0.5 M phosphoric acid at 25°C

for 1.5 h [14,15] After anodization, the alumina

membrane was separated from aluminum

sub-strate using the voltage-decreasing method [16]

Finally, the membrane was rinsed thoroughly with

distilled water and dried in a pure nitrogen flow

Subsequently, the membrane was placed in a

quartz boat and then inserted into the center of a

4-cm-long quartz tube reactor heated by tungsten

filament Atmosphere in the reactor was pumped

with a mechanical vacuum pump A flow of H2

(10 ml/min) and Ar (30 ml/min) was purged for

0.5 h before the reactor was heated to reaction

temperature, 900°C Then a flow of SiH4was

in-troduced at the same rate with H2(10 ml/min) for

1 h After deposition, the SiH4and H2flows were

turned off, and the sample was cooled to room

temperature in an Ar atmosphere

Before TEM characterization, the deposits on

one side of the alumina membrane were removed

by polishing with alumina power The SiNWs were

released from the template in 6 M NaOH for 24 h,

and then thoroughly washed with distilled water

Conventional TEM analysis and high-resolution

transmission electron microscope (HRTEM) were

both performed on a JEOL-2010 microscope at

200 kV equipped with link-ISIS energy dispersive

spectroscopy (EDS) elemental composition

ana-lyzer SEM image of SiNWs was obtained as

fol-lowers: the sample was glued (using epoxy) to a

metallic support with the cross-section up and then immersed into 6 M NaOH solution for 20 min in order to dissolve alumina The sample was then sputtered with
10 nm of Au prior to imaging (JSM-5600LV electron microscope) For lower angle X-ray diffraction (XRD) study, the silicon films on both surfaces of the alumina membrane were removed The sample was transferred on to the standard silicon supporter and the spectrum was obtained using a D/MAX-2400 X-ray dif-fractometer

The field emission measurements were carried out in a vacuum chamber at a pressure of about

107 Torr at room temperature The sample was used as the cathode, while a copper sheet polished served as an anode The distance between the an-ode and the sample (cathan-ode) surface was con-trolled by the thickness of a mica spacer containing a hole (
1 mm2) in the center Voltages

up to 3 kV were applied to the anode and the emission current was detected with a micro-amp-erometer

3 Results and discussion Fig 1a shows the cross-sectional SEM image of the SiNWs array after dissolving alumina One can see that the nanowires produced are very straight and form a well-aligned array, indicating there are few defects and little growth stress As would be expected, the nanowires are of a uniform diameter

of about 50 nm, which is consistent with the pore diameter of the alumina template A sharp tip is found at the end of each nanowire, which could be useful to a field emitter Meanwhile, there is a silicon surface film at the bottom of the SiNWs, which is always presented in other nanomaterials synthesized by template method [14–18] Fig 1b shows the SiNWs are still parallel with each other when alumina is partially dissolved, maintaining their orientation within the template The EDS spectra collected from the middle part of the SiNWs arrays (Fig 1c) show the presence of sili-con in addition to Al and O C and Cu are at-tributed to the copper micro-grid with carbon film used to support a sample in TEM measurement The presence of trace of P confirms our previous

finding that anion ions of anodizing electrolytes

are incorporated in the formation of alumina

template during the anodization procedure [19]

A single SiNW is shown in Fig 2a, in which

alumina is dissolved completely The selected-area

electron diffraction (SAED) pattern taken from

this SiNW is show in Fig 2b From this photo, it

can be seen that the diffraction spots are clear and

organized in an almost precise hexagon or paral-lelogram, indicating that the diamond lattice structure of bulk Si is also preserved in the SiNWs According to the geometry analyses of electron diffraction, the cubic indices of the diffraction spots in the electron diffraction pattern are de-marcated The similar results were obtained on the other SiNWs, indicating that each single SiNW is a single crystal

In order to reveal the micro-structure of SiNWs

in detail, the HRTEM was employed to investigate them at atomic scale The incident electron beam is parallel to the [1 1 0] zone axis The HRTEM image

in Fig 3a shows the representative micro-structural characteristics of individual SiNWs It is clear that the straight SiNWs has smooth surface and the

Fig 1 (a) SEM image of SiNWs arrays, (b) TEM image of

SiNWs arrays, (c) EDS spectrum of SiNWs arrays.

Fig 2 (a) TEM image of a single SiNW, (b) SAED pattern of the single SiNW, the inset data are the cubic indices of the diffractive spots.

change in diameter along its length is seldom

ob-served The SiNWs are virtually defect-free and

show no kink, dislocation and small angle

bound-aries because of the periodic change growth

direc-tion along the length of the SiNWs The growth

plane is one of the (1 1 1) planes and the fast growth

direction is along the [)2 1 1] axis of the SiNWs

The (1 1 1) plane family, which is the densest

packed plane with the lowest surface energy in

silicon structure, hence is important for SiNWs

nucleation and growth When the (1 1 1) planes are

parallel to the axes of the nanowires, the system

energy is reduced significantly In this case, the

(1 1 1) planes aligned parallel to the growth axes

Fig 3a also shows that the tip of the SiNWs is

sharp and has a perfect lattice structure This is in

contrast with the previous SiNWs that are round

and contain a high density of stacking faults and

micro-twins Obviously, the sharp tip suggests a

distinctly different formation mechanism

com-pared with the previous works

Fig 3b shows the image of the silicon film

de-posited on the surface of the template, which has

rough surface The interplanar spacing between visible fringer is 0.28 nm, corresponding to the (0 0 2) plane of silicon It is visible that the (0 0 2) lattice fringes are not continuous due to the dis-location, micro-twins and stacking faults indicated

by the arrow The film can be modeled as con-sisting of segments with different orientation but with a similar structure as the SiNWs We also notice a thin film amorphous layer exists around the Si film, which is identified to be amorphous silicon oxide resulting from surface oxidation The XRD spectrum of the SiNWs arrays in Fig 4 contains seven peaks, which are identified to match well with the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2) and (5 1 1) diffraction peaks of the diamond lattice structure of bulk silicon through the PCPDFWIN software The other unmarked peaks are assigned to the alumina Calculated from the interplanar spacing of the most intense (1 1 1) peak (d ¼ 0:3147 nm), the lattice parameter

of the SiNWs can be obtained as aSiNWs¼ 0.5451

nm, which is 0.387% larger than the standard value

aSi¼ 0.5430 nm for bulk silicon, revealing there is

a slight lattice expansion and distortion in the SiNWs structure From the XRD results, the array

of SiNWs appears to be polycrystalline structure, which seems contradicting to the above results of SAED This can be understood that the statistical results obtained by the XRD pattern and the dif-fraction patterns of different whiskers indicate different orientations

The growth of SiNWs catalyzed by metal parti-cles is usually considered to be a vapor–liquid–solid

Fig 3 Typical HRTEM images of SiNWs (a) and Si film

de-posited on the surface of SiNWs arrays Fig 4 XRD spectrum of SiNWs arrays.

mechanism However, it is clear that the

conven-tional VLS mechanism based on the role of

cata-lyst [20–22] could not explain the growth of

SiNWs, because we did not use any catalyst in the

deposition We propose the following mechanism

During the early stage of the CVD process, most

of the silicon atoms were originated from the

dis-sociation of silane at high temperature There are a

large of Lewis acid nature of surface sites in

amorphous and transition alumina and these sites

have the intrinsic catalytic activity of transition

alumina in front of the decomposition of silane

[23] Therefore it is possible that the internal

channel surface within alumina has a catalytic

behavior in addition to its template effect Most

silicon atoms in vapor phase will be deposited on

the walls of alumina nanochannels The high

density of dangling bonds at the surface of atomic

Si will lead to the bonding each other between

silicon atoms and a continuous diffusion into the

channels of alumina Further condensation will

then produce SiNWs in the channels of alumina

Furthermore, the carrier gas Ar will collide with

the pore surface and the atomic Si has absorbed

and exchange energy and momentum with the

atom, causing overcooling at the surface Because

the precipitation, nucleation and growth of SiNWs

always occurred at the area near the cold fringer,

such an overcooling is important for providing

temperature gradient used as an external driving

force for nanowire growth However the reason

for the formation of sharp tip on the SiNWs is not

clear yet, further work is required to understand

this phenomenon

ZhangÕs has demonstrated SiNWs emitters

dis-play attractive field emission properties, which

may be exploited for practical applications [24]

Fig 5 illustrates the curve of current versus voltage

(I–V curve) for SiNWs arrays, revealing the

ro-bustness of the emission process from the emitter

The turn-on field for electron emission, defined as

the macroscopic fields needed to produce a current

density of 0.01 mA/cm2, is
14 V/lm The I–V

data analyzed by the Fowler–Nordheim theory

[25] is presented in the inset of Fig 5 As can be

seen, almost straight line is obtained, indicating

that the field emission from SiNWs is a barrier

tunneling quantum mechanical process The

superior field emission behavior is believed to originate from the sharp tips and oriented growth

of SiNWs

4 Conclusion

In conclusion, the morphologies and micro-structures of well-aligned SiNWs arrays synthe-sized by CVD template method were investigated

by electron microscopy It is found from the re-sults of SAED that each nanowire is a single crystal, while XRD suggests that SiNWs have different orientation in the array The growth mechanism of SiNWs without catalyst was dis-cussed based on the function of the alumina tem-plate In addition, field emission measurements show that ordered SiNWs arrays with perfect lat-tice would be very useful for field emission and other nanoelectronic device applications

Acknowledgements This work is supported by the National Natu-ral Science Foundation of China (Grant No 60171004)

Fig 5 Current–voltage characteristics of SiNWs arrays (inset: Fowler–Nordheim plot of SiNWs arrays).

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