Oriented silicon nanowires on silicon substrates from oxide assisted growth and gold catalysts

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

Oriented silicon nanowires on silicon substrates from

oxide-assisted growth and gold catalysts

a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong,

Tat Chee Avenue, Kowloon, Hong Kong SAR, China

b Institute of Physics and Center for Condensed Matter Physics, CAS, Beijing, China

Received 17 February 2005; in final form 4 March 2005

Available online 25 March 2005

Abstract

High-density, oriented silicon nanowires (SiNWs) array were fabricated on (0 0 1) silicon substrates by the oxide-assisted growth method assisted with Au catalyst in a hot filament chemical vapor deposition system The yield of SiNWs was different with the synthesis temperature Au particles were present at the tips of the SiNWs and limited the wire diameter High resolution transmis-sion electron microscopy revealed the epitaxial SiNWs on the Si substrate

2005 Elsevier B.V All rights reserved

One-dimensional (1D) nanomaterials have attracted

intense interest because of their many unique properties

not found in the bulk materials Silicon nanowires

(SiNWs) are a particularly important 1D nanomaterial

because silicon is most widely used in electronic

indus-try Besides excellent electronic property, nanoscale

sili-con materials also possess interesting optical [1] and

field-emission [2] properties, which may be exploited

for optoelectronic applications

There are various ways to synthesize SiNWs,

includ-ing the classic metal-catalytic vapor–liquid–solid

(MC-VLS) [3] method, the simple thermal-evaporation

oxide-assisted growth (OAG) method[4], and the

solu-tion-grown method [5] MC-VLS and OAG are widely

used to fabricate SiNWs and they have their respective

merits and shortcomings The MC-VLS method offers

better control of diameter [6] and patterning [7] of

SiNWs but relatively low yield of production; whereas

the OAG method can produce SiNWs in large quantities

and without metal contamination, but with less control

in wire pattern and diameter Recently, SiNWs have been grown by combining the OAG method and metal catalysts [8] Here, we report the successful fabrication

of well-oriented array of SiNWs with controlled diame-ter using a similar approach

SiNWs were fabricated in a hot-filament chemical va-por deposition (CVD) system (0 0 1) silicon substrates were etched in 5% hydrofluoric (HF) acid, sonicated in acetone, and resistively coated (thermal evaporation) with 1 nm Au film at a base pressure of 5· 10 6

mbar

A molybdenum foil was used to heat the silicon sub-strate, while another boat-like molybdenum foil con-taining SiO powder worked as a source heater The base pressure of the CVD chamber was 10 6Torr Dur-ing growth, the system was closed with no pumpDur-ing and

no carrier gas The temperature of the SiO source was kept at 1300C, while that of the substrate varied from

600 to 1000C for different experiments Thermocouples (mounted to the source and substrate) supplemented by infrared (IR) pyrometer were employed to measure the temperature The pressure in the chamber increased to

10 2 Torr after 30 min of growth The processing details are shown inFig 1

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

doi:10.1016/j.cplett.2005.03.027

* Corresponding author Fax: +852 2784 4696.

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

www.elsevier.com/locate/cplett Chemical Physics Letters 406 (2005) 381–385

After growth, the Si substrate surface showed

differ-ent colors dependdiffer-ent on the temperature to which it

was heated; varying from light yellow at 900C to gray

at 600C Figs 2a,b are the scanning electron

micros-copy (SEM) images of the silicon substrate surface after

growth at 800 and 700C, respectively It is clear that

the surface was covered with high density, oriented

array of aligned nanowires The average length of the

nanowires was less than 5 lm, while the density of the

wires varied with the substrate or deposition

tempera-ture The largest density (400/lm2) of the nanowires

was obtained at 700C, while the wire orientation

ap-pears to be insensitive to the deposition temperature

No nanowires were observed if the deposition

tempera-ture was above 800C or below 700 C The yellow

color above 800C came from the roughened surface

of the substrate To investigate the microstructure of

SiNWs, the as-grown nanowires were scratched from

the substrate and dispersed in alcohol A few drops were

put on the holey carbon copper grids for transmission

electron microscopy (TEM) and high-resolution TEM

(HRTEM) examination TEM image (Fig 2c) shows a

dark particle capping the nanowire Silicon element

mapping image (Fig 2d) indicates that Si was absent

in the dark cap Figs 2e,f display, respectively, the

EDX spectrum of the nanowire and its particle cap,

which reveals the gold signal only from the cap It is thus

established that oriented SiNWs with the gold particle

cap were formed on the silicon substrate

The diameter variation of the nanowires deposited at

different temperature is illustrated inFig 3a The chart

shows the average wire diameter increased, while the

wire density decreased with increasing deposition

tem-perature Fig 3b depicts the diameter distribution of

the SiNWs and Au particle tips grown at 700C The

bars indicate that the diameters of the SiNWs range

mostly between 10 and 30 nm, whereas the diameters

of the Au particle tips primarily fall between 20 and

40 nm Consequently, the diameter of the nanowires is well limited by that of the Au caps

HRTEM image (Fig 4a) shows the as-grown SiNWs consist of a crystalline core and an amorphous silicon oxide sheath, similar to the SiNWs grown by the OAG method without Au catalyst As illustrated in Fig 3c, although some nanowires grew along Æ1 1 1æ direction, the dominant growth direction of SiNWs was Æ1 1 2æ with Æ1 1 0æ being the second dominant direction, the same as the SiNWs grown by the OAG method The Æ1 1 2æ direction is different from the common Æ1 1 1æ growth direction for SiNWs fabricated by the Au-catalytic VLS method To investigate the initial wire growth, the cross-section of the sample grown for

10 min was examined by HRTEM Fig 4b shows a typical SiNW of 20 nm in diameter grew epitaxially on the (0 0 1) silicon substrate with a 40 nm Au cap Signif-icantly, the interface between the wire and substrate is essentially free of defects The high-quality epitaxy

at the interface is expected to subsequently guide the oriented growth of SiNWs on the Si substrate

The growth process of SiNWs can be described as follows [8]: First, when the substrate is heated to high temperature (700–800C), the 1 nm gold film would break up to form 20–40 nm Au particles The Au parti-cles will dissolve Si from the Si substrate to form the eu-tectic Au–Si alloy As the SiOx vapor arrives at the Au particle, SiOx will disproportionate at the particle sur-face into Si and SiO2 Silicon will dissolve in the Au par-ticle, while the silicon oxide will remain at the particle surface When the Si concentration in the Au particle reaches super-saturation, Si will separate out at the interface and grow epitaxially on the Si substrate Sili-con oxide will flow over the Au particle surface and form a layer sheathing the SiNW The diameter of the SiNWs is thus limited by the size of the Au particles

Fig 1 Flowchart of the fabrication process of oriented silicon nanowires.

It is well recognized that the formation of metal–Si

eutectic alloy is the key point of the MC-VLS growth

of SiNWs and the growth temperature is dependent on

the metal catalyst As Au and Si can form an eutectic

al-loy as low as 363C[9], consequently SiNWs have been

synthesized under 500C using Au as the catalyst and SiH4 as the source [10] On the other hand, we have shown that SiNWs are produced by the catalyst-free OAG method only at temperatures above 900C [4] The present results show that the production of SiNWs

Fig 2 (a) SEM image of oriented SiNWs arrays synthesized at 800 C and (b) 700 C The scale bar is 2 lm (c) TEM image of a SiNW with an Au particle tip and (d) corresponding silicon elemental mapping The scale bar is 50 nm (e) EDX spectrum of the nanowire and (f) its Au particle tip.

Y Yao et al / Chemical Physics Letters 406 (2005) 381–385 383

is limited to the temperature range of 700–800C using

the OAG and Au catalyst The reduced growth

temper-ature relative to that of metal-free OAG may be

attrib-uted to the Au catalytic effect in lowering SiO

decomposition temperature Close TEM observation

shows there is a thin silicon oxide layer covering the

Au particle tips During growth, the arriving silicon

atoms have to diffuse through the thin oxide layer to reach the Au particle Thus, the growth temperature should be sufficiently high to allow silicon penetrating through the oxide layer to form the eutectic alloy so

as to sustain the growth of SiNWs It is likely that the oxide layer stops silicon diffusion into the Au particle

at temperatures less than 700C, thus hindering the growth of SiNWs at lower temperatures Moreover, it was found the Si substrate was covered by a thin poly-crystal silicon and amorphous silica film if the growth temperate was higher than 800C

The growth direction of SiNWs from the MC-VLS method with Au catalyst is predominantly along the Æ1 1 1æ direction [3] In the present method, the SiNWs primarily grew along the Æ1 1 2æ and Æ1 1 0æ directions, similar to SiNWs grown by the OAG method without any catalysts As shown in Fig 4a, the side surfaces of the SiNW are made of the {1 1 1} and {1 1 0} facets for the nanowire grown along [1 1 2] direction The pres-ence of those crystal facets could minimize the total en-ergy of the nanowire because the surface enen-ergy of the {1 1 1} facets is the lowest and the energy of the side sur-faces dictates the total surface energy of a SiNW [11] Recently, it was reported that the small-diameter SiNWs

Fig 3 (a) Variation of wire density and diameter as a function of

deposition temperature (b) Histogram of diameter distribution of

SiNWs and Au particle tips (c) Growth direction of SiNWs deposited

at 700 C.

Fig 4 (a) HRTEM image of a SiNW The growth direction is along Æ1 1 2æ orientation (b) The initial growth of SiNWs with an Au cap showing the epitaxial relation to the Si substrate.

grown from the MC-VLS method also adopted the

Æ1 1 0æ and Æ1 1 2æ orientations[12] In the present work,

the initial growth of the SiNWs is alongÆ0 0 1æ direction

as a result of epitaxy to the substrate, as depicted in

Fig 4b Therefore, the present results suggest that the

initial Æ0 0 1æ orientation of SiNWs may change to the

more popularÆ1 1 0æ and Æ1 1 2æ directions during

subse-quent growth of SiNWs as described elsewhere[13]

In summary, high-density, oriented SiNWs array were

grown on (0 0 1) silicon substrates by the OAG method

assisted by Au catalyst The yield of SiNWs reached the

largest at 700–800C Au particles were present at

the tips of the SiNWs and limited the wire diameter The

present SiO–Au approach offers certain advantages over

the common metal-catalytic VLS method and the OAG

method, such as the absence of toxic and flammable gases

and the control of size and epitaxial growth of SiNWs

Acknowledgment

This work is supported by a Central Allocation

Pro-ject (No CityU 3/04C) of the Research Grants Council

of Hong Kong SAR

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