Morphology and growth mechanism study of self assembled silicon nanowires synthesized by thermal evaporation

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CHEMICAL PHYSICS LETTERS

www.elsevier.nl/locate/cplett

Morphology and growth mechanism study of self-assembled silicon nanowires synthesized by thermal evaporation

Z Zhang ', X.H Fan, L Xu, C.S Lee, S.T Lee ”

Center of Super-Diamond and Advanced Films & Department of Physics and Materials Science, City University of Hong Kong,

83 Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China

Received 18 December 2000; in final form 8 January 2001

Abstract

Silicon nanowires (SiNWs) grown from ‘sunflower-seed’- and “‘mushroom’-shaped particles have been observed by electron microscopies The SiNWs were synthesized by thermal evaporation of S10 powders without any metal cata- lysts The SiNWs grown on the sunflower-seed-shaped particles had sub-branches of SiNWs terminated by Si bulbs The SiNWs on the mushroom-shaped particles were densely and uniformly distributed on the surface of the mushroom cone The growth history suggests that these SINWs were formed by nucleation which originated from the surface of amorphous S10 particle matrixes via phase separation and precipitation followed by growth through oxide-assisted vapor-—soild reaction © 2001 Elsevier Science B.V All rights reserved

1 Introduction

The electronic, magnetic, optical and chemical

properties of nano-materials can be very different

from their bulk counterparts and depend sensi-

tively on their size, shape and composition For

example, bulk silicon is very good in electronic but

poor in light emission properties at room temper-

ature, because of its indirect band gap of ~1.1 eV

and a small exciton binding energy (~15 eV) In

contrast, silicon nanowires (SiNWs) of a few na-

nometers in diameter have shown unusual photo-

luminescence and Raman spectra [1-4], implying a

* Corresponding author Fax: +852-2784-4696

E-mail address: apannale@cityu.edu.hk (S.T Lee)

' On leave from Beijing Laboratory of Electron Microscopy,

Center of Condensed Matter Physics, Institute of Physics,

Chinese Academy of Sciences, P.O Box 2724,100080, Beijing,

China

strong quantum size confinement effect which re- laxes the k-selection rule to overcome the indirect nature of optical transition In addition, lithium doping of SINWSs [5] has a promising application

in energy storage as advanced battery cell materi- als

The size, shape and structure of SINWs depend sensitively on their composition, as well as the temperature and other parameters of the synthetic process The experimental results of thermal evaporation synthesis have shown that SiNWs are rich in morphology in different deposition tem- perature regions under the same process condition The variation of morphology not only exists in the diameter distribution from 10—100 nm, but also in the diversity of shapes from octopus-like, chain- like, spring-like, tadpole-like, to single wires with a constant diameter [6-9] Though the formation and structure characterization of those SiNWs

of different morphologies have been extensively

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

PII: S0009-2614(01)00183-X

investigated recently by transmission electron mi-

croscopy (TEM), the growth mechanism related to

various morphologies remains an open question

One of the reasons is that since SINW specimens

for TEM examination are normally chosen and

removed from specific regions of a substrate where

they have been deposited, the growth history re-

lated to the as-deposited position cannot be traced

to the whole deposition-collecting-substrate re-

gions under different temperatures Accordingly,

we apply scanning electron microscopy (SEM) to

investigate the morphology variation of SiNWs

directly at their original deposited sites and carried

out chemical analysis by X-ray energy dispersion

spectroscopy (XEDS) Although SEM cannot

provide the high spatial resolution images of the

SiNW structure achievable by TEM, it neverthe-

less can allow a systematic study of the growth

process in different temperature zones directly at

the original deposition locations

Thus far, oven-laser ablation [10-13] and ther-

mal evaporation [14-17] are two methods nor-

mally used for the synthesis of SiNWs The key

point in these two methods is the formation of a

sufficient amount of silicon atoms and/or silicon

oxide clusters in gas phase from the target powders

of silicon or silicon oxide by laser ablation or high

temperature evaporation A growth mechanism

for these two cases is the vapor—liquid—solid (VLS)

model, in which a metal catalyst (Ni or Fe) liquid

droplet plays the central role in dissolving/ab-

sorbing the vapor-phase silicon atoms and/or sili-

con oxide clusters When the Ni(Fe)Si, droplet

reaches supersaturation after dissolving sufficient

silicon atoms from the gas phase, precipitation of

silicon nanowires from the droplet can be induced

Based on a systematic analysis of the growth

mechanism of semi-conductor nanowires, Lee et al

[18] proposed an oxide-assisted model by which

the SiNWs were formed in two steps Firstly,

amorphous nanoparticles of SiO are nucleated on

the surface of SiO matrix particles, followed by

phase separation and successive silicon precipita-

tion due to the disproportionation or oxidation—

reduction reaction of amorphous SiO in the tem-

perature range 950—-1250°C Secondly, the amor-

phous nanoparticles of SiO at the tips of SiNWs

continuously absorb Si-O clusters from the gas

phase, which ensures the continual phase separa- tion and precipitation and results in the formation

of SiNWs made of a Si core and a SiO) sheath The one-dimensional growth of SiNWs is facili- tated by the SiO, sheath that confines the lateral growth of SiNWs, and the high absorptivity for Si-O clusters of the molten SiO nanoparticle at the

tip as a growth front [18] The semi-liquid nature

of the SiO tip is due to melting point lowering

induced by the nanosize effect associated with the SiO nanoparticle Actually, phase separation and/

or precipitation of both silicon and SiO, crystal-

lites in nanometer size have been well documented

in the literature [19-21]

In the present Letter we report a systematic

SEM study of SiNW growth history related to the

morphology and growth temperature We show

that the formation of SiNWs is indeed closely re- lated to the amorphous SiO particles, and the growth and self-assembling process of SINWs on the surface of SiO particles strongly support the oxide-assisted growth model of SiNWs with vari- ous morphologies [18]

2 Experimental

To study the growth history and related mech- anism, silicon substrate wafers used as SiNWs

collectors were arranged in the temperature re- gions ranging 600-1350°C within the furnace The relationship of SiNW morphology to different temperature zones on the substrate was analyzed with SEM (Philips SEM FEG Model XL30) The

possible impact of chemical composition on the morphology of SiINWs was investigated by using X-ray energy dispersion spectroscopy (XEDS) at-

tached to the SEM The thermal evaporation

condition for SiNWs synthesis was the same as that reported previously [8] but with a growth time

of 10 min The short synthesis time was used so as

to study the initial stage of SINW growth

3 Results and discussions

Fig 1 shows the SEM images of SiNWs and particles on the Si substrate at a temperature of

tes :

Acc * Spot Magn Det WD ——————————Ì Imm

500kV 30 28x SE 120 ZZSINW-19-7 ý

Fig 1 SEM images of SiNWs deposited on Si at about 1000°C

(a) SINWs are connected and grown radially outward from the

surface of the particle: and (b) some SiNWs show preferred

growth directions from different sized particles

around 1000°C The composition of the particles

was confirmed by XEDS to be Si,,O, with a m/n

ratio close to 1:1 Careful microstructure exam-

ination shows that the Si substrate serves only as

a collector for SINW deposition having no other

relationship to SiINWs A characteristic feature of

Fig 1 is that the SiNWs are grown either radially

outward from the particle (Fig la) or in some

preferred directions (Fig 1b) The SEM images

reveal a close relationship between the SiNWs

and SiO particles in that all visible SiO particles

are always connected to SiNWs, as evident from

Fig 1b Another noticeable feature is that many

more SiNWs appear on the surface of larger

particles, as is clear from Fig la at higher mag-

nification

The relationship between the SiNWs and SiO

particles can be seen more clearly from Fig 2a,

AccV SpotMagn “Dồi WD2E———— 5m,

500kV 30 6937x SE 128 ZZ-SINW-2

Acc.V Spot Magn Det WD 600kV30 19874x SE 128 ZZ-SINW-2

Acc.V Spot Magn Det AWD § = 27T, ve

B500kV30 6937x SE 129 Siig

Fig 2 (a) and (b) An initial growth stage of SiNWs can be

found from fresh SiO particles and (c) ‘C’ indicates a piece unfolded from the left side of the figure

which shows that SiNWs are grown directly from

the surface of a SiO particle In addition, each

SiNW has a nanoparticle at its tip while its root is connected to a hole on the particle surface Fig 2b

is an SEM image from the same region in Fig la but with a high magnification, which shows clearly

the initial growth stage of SiNWs from the surface

of SiO particles The initial growth stage of SINWs

can be further visualized from Fig 2c The clean

surface (marked by letter C) was produced by

unfolding from a grown surface layer as shown in

Fig 2a,b, thus it remains relatively clean as it re-

ceived little exposure to the growth ambient On

the other hand, the covered layer is full of SINWs,

while the curved edge of the unfolded layer (de-

noted by an arrowhead in Fig 2c) is also relatively

free of SiNWs

In accordance with previous TEM studies

[12,18,22], the above SEM results provide further

experimental evidence that the nucleation sites of

SiNWs are on the surface of a SiO matrix, where

the amorphous SiO nanoparticles are formed, and

successive phase separation (or disproportiona-

tion) and precipitation occur under suitable tem-

perature and chemical composition

Due to the morphological sensitivity of SINWs

to the growth temperature and composition, many

types of SiNWs can be formed, rendering the de-

termination of the growth mechanism of SiNWs more difficult Fig 3a is a low-magnification SEM

image of SiNWs deposited on the Si substrate at a

temperature about 1200°C A characteristic fea-

ture of this image is that there are some sunflower- seed-shaped particles, with the size of about

0.2 x 0.5 mm” In the vicinity of this type of par- ticle, there are also some small particles with ir- regular shapes These particles are connected by

many relatively straight SINWs of uniform diam- eter as strands, as shown in Fig 3b Images at

higher magnification in Fig 3c show that the surface of the particles is fully covered with self- assembled SiNWs A careful examination of the image at still higher magnification (Fig 3d) shows

Fig 3 (a) Self-assembled SiNWs are grown on the surface of sunflower-seed-shaped SiO particles; (b) straight SiNWs are found

connecting these particles; (c) SINWs have characteristic sub-branches of SiNWs; and (d) silicon bulbs are found at the tips of the

SiNWs sub-branches.

the self-assembled SiNWs to have many sub-

branches of SiNWs grown out from their surface

The sub-branches of SiNWs are all terminated by

Si bulbs, making them different from those SiNWs

reported before [5,6] The average diameter of the

main branch of SiNWs is in the range 40-60 nm,

and that of the sub-branch of SiNWs is 30-40 nm

The bulb on the tip of the sub-branch is less than

100 nm in diameter The sub-branch SiNW is thin

and normally curved at the region close to its

connecting point with the main branch of SiNWs,

and its diameter increases gradually to form the

bulb at its tip HREM results reveal that the main-

and sub-branches of SiNWs are clothed with a

SiO, out-layer of a few nm in thickness HREM

and electron diffraction results show that the bulbs

also have a silicon core and an amorphous SiO;

outer layer A systematic XEDS analysis shows

that the sunflower-seed-shaped particles are com-

posed of silicon and oxide only, while the oxygen

content varies from the top to bottom of the par-

ticles The oxygen content decreases from 54 at.%

at the head of the seed particle (denoted by letter H), to 45 at.% at the middle (noted as M), and

down to 34 at.% around the bottom of the parti-

cles (marked as B) The oxygen content of the

particles implies that the SINWs were nucleated and grown from the sunflower-seed-shaped SiO, particles This again supports the oxide-assisted growth model of SiNWs

The dependence of SiNW morphology on

temperature can also be extracted from the fea- tures revealed in the temperature region of 1180°C, where particles with the mushroom shape

are observed on top of silicon substrates, as shown in Fig 4a SEM image at higher magnifi-

Pn eC

| ey

Fig 4 (a) SINWs covered mushroom-shaped SiO particles deposited at 1180°C, (b) a mushroom-shaped particle, (c) SINWs on the smooth cone surface of a mushroom-shaped particle, and (d) many straight and parallel SINWs connecting a mushroom-shaped particle and another particle nearby (possibly a piece broken off from the former).

cation (Fig 4b) shows that these particles have a

cone shape pussy surface and the top surface of

the cone is rather rough Fig 4c is the SEM im-

age of the mushroom-shaped particles at still

higher magnification, which shows clearly that the

surface is full of self-assembled SiNWs 30-40 nm

in diameter SiNWs at the top surface of the

mushroom particles are smaller in diameter Al-

though HREM studies confirm these curved

nanowires to be SiNWs, XEDS analysis reveals

an obvious change in oxygen content from 54

at.% at the bottom to 27 at.% at the top surface

of the mushroom particles That is the Si/O ratio

changes from about 1:1 to 3:1 There are two

possibilities for the higher oxygen signals by

XEDS at the bottom (denoted by letter B) and

middle (marked as H) than that at the top (T)

parts shown in Fig 4b Firstly, though the outer

shell of SiNWs on the cone surface at regions B

and H has the same chemical content as that in

the region T, the inner part underneath the outer

shell of the former has a higher oxygen content,

thus giving rise to an XEDS signal rich in oxygen

A second possibility is that the SiNWs on the

surface of the mushroom particles indeed have a

different oxygen content

Fig 4d shows that a mushroom-shaped particle

and a piece nearby (apparently separated from the

former) are connected by many straight SiNWs

The string-like SINWs connecting the two particles

may be understood as follows While the SiNWs

grown directly and self-assembled on the surface

of the particles are highly curved, those SiNWs

connecting two particles are constrained to be-

come straight and aligned in parallel

From the above experimental results, we

conclude that SiNWs attached to the surface of

two particles are not formed individually in the

gas phase first and then deposited on the surface

of the particles Instead, they are nucleated di-

rectly from the surface of the SiO particles or

matrix, and grown continuously by phase sepa-

ration (or disproportionation) and precipitation

from the nano-sized amorphous particles on the

tip, which serves as a growth front The pre-

cipitation-induced SiO, outer layer prevents fur-

ther lateral growth, thus favoring the growth of

SiNWs along one dimension All these results

agree well with the oxide-assisted growth model

of SiNWs [18]

Acknowledgements

The work described in this Letter was partially

supported by a grant from the Research Grants

Council of the Hong Kong Special Administration

Region, China (Project No 9040459)

References

[1] R.P Wang, G.W Zhou, Y.L Liu, 8.H Pan, H.Z Zhang, D.P Yu, Z Zhang, Phys Rev B 61 (2000) 16827 [2] $.Q Feng, D.P Yu, H.Z Zhang, Z.G Bai, Y Ding,

J Cryst Growth 209 (2000) 513

[3] J.D Holmes, K.P Johnston, R.C Doty, B.A Korgel,

Science 287 (2000) 1471

[4] Y.F Zhang, Y.H Tang, H.Y Peng, N Wang, C.S Lee,

I Bello, $.T Lee, Appl Phys Lett 75 (1999) 1842

[5] G.W Zhou, H Li, H.P Sun, D.P Yu, Y.Q Wang,

X.J Huang, L.Q Chen, Z Zhang, Appl Phys Lett 75

(1999) 2447

[6] Y.Q Zhu, W.K Hsu, N Grobert, M Terrones, H Terrones, H.W Kroto, D.R.M Walton, B.Q Wei, Chem

Phys Lett 26 (2000) 312

[7] Y.Q Zhu, W.K Hsu, M Terrones, N Grobert, W.B Hu, J.P Hare, H.W Kroto, D.R.M Walton, Chem Mater 11

(1999) 2709

[8] H.Y Peng, Z.W Pan, L Xu, X.H Fan, N Wang, C.S Lee, §.T Lee, Adv Mat 2000

[9] Y.H Tang, Y.F Zhang, N Wang, C.S Lee, X.F Duan,

I Bello, $.T Lee, J Appl Phys 85 (1999) 7981 [10] A.M Morales, C.M Liber, Science 279 (1998) 208 [11] D.P Yu, C.S Lee, I Bello, X.S Sun, Y.T Tang, G.W Zhou, Z.G Bai, Z Zhang, S.Q Feng, Solid State

Commun 106 (1998) 403

[12] N Wang, Y.T Tang, Y.F Zhang, C.S Lee, S.T Lee,

Phys Rev B 58 (1998) 16024

[13] Y.F Zhang, Y.T Tang, N Wang, D.P Yu, C.S Lee,

I Bello, $.T Lee, Appl Phys Lett 72 (1998) 1835 [14] D.P Yu, Z.G Bai, Y Ding, Q.L Hang, H.Z Zhang, J.J Wang, Y.H Zou, W Qian, G.C Xiong, H.T Zhou, S.Q Feng, Appl Phys Lett 72 (1998) 3458

[15] N Wang, Y.H Tang, Y.F Zhang, C.S Lee, I Bello, S.T

Lee, Chem Phys Lett 299 (1999) 237

[16] J.L Gole, J.D Stout, W.L Rauch, Z.L Wang, Appl

Phys Lett 76 (2000) 2348

[17] Y.F Zhang, Y.H Tang, C Lam, N Wang, C.S Lee,

I Bello, $.T Lee, J Cryst Growth 212 (2000) 115 [18] S.T Lee, Y.F Zhang, N Wang, Y.H Tang, I Bello,

C.S Lee, J Mat Res 14 (2000) 4503

[19] G Hollinger, Y Jugnet, T.M Duc, Solid State Commun

22 (1977) 277.

[20] E Fogarassy, J.L Regolini, C Fuchs, A Grob, In: G.G [21] M Nagamori, J.A Boivin, A Claveau, J Non-Cryst Sol

Technology and Devices, Editions de Physique, Les Ulps [22] S.T Lee, N Wang, Y.F Zhang, Y.H Tang, MRS France, 1986, p 255 Bulletin, August 1999.