Silicon nanowires grown on si(1 0 0) substrates via thermal reactions with carbon nanoparticles

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Silicon nanowires grown on Si(1 0 0) substrates via

thermal reactions with carbon nanoparticles

S Botti a,*, R Ciardi a, R Larciprete b,c, A Goldoni b, L Gregoratti b,

a

ENEA, UTS Tecnologie Fisiche Avanzate, Via Enrico Fermi 45, 00044 Frascati, Italy

b

Sincrotrone Trieste, S.S 14 Km 163,5 in Area Science Park, 34012 Basovizza (TS), Italy

c

CNR-IMIP, Zona Industriale – 85050 Tito Scalo (PZ), Italy Received 4 November 2002; in final form 16 December 2002

Abstract

The effect of thermal processing at 1050°C of a dispersed film of carbon nanoparticles deposited on a Si substrate with a native SiO2 layer has been studied by scanning electron microscopy and scanning photoelectron spectromi-croscopy It has been found that the thermal processing results in formation of pyramidal-shaped defects of 2–7 lm with strongly reduced SiO2content with silicon wires of diameter ranging between 30 and 50 nm decorating the pyramid walls The nucleation of the Si nanowires occurs via reduction of the native oxide layer by the nanosized carbon particles, without the need of metal catalysts and at temperatures relatively lower than that used in similar techniques

Ó 2003 Elsevier Science B.V All rights reserved

1 Introduction

Silicon nanowires with diameter of several tens

of nanometers and length of tens of micrometers,

exhibit unusual quantum confinement effects and

interesting electrical and optical properties with

promising technological impact in the

microelec-tronic field [1–7] Several techniques have already

been used for synthesis of silicon nanowires, such

as vapour–liquid–solid growth (VLS) catalysed by

a gold layer on Si [1], laser ablation of metal

containing targets [7] and, most recently,

Ni-as-sisted solid–liquid–solid process [3] and

carbo-thermal reduction of Fe-catalysed SiO2 particles [4] Some of the proposed growth models show that the formation of Si nanowires does not re-quire the presence of a metal, because the top SiO2

layer can play the role of a catalyst in the wire nucleation [2–5]

We have already shown that Si nanowires can be synthesised without metal catalyst by simple car-bothermal reduction of the native oxide layer on a Si(1 0 0) substrate, assisted by C nanoparticles (CNPs) deposited by laser pyrolysis [8,9] Anneal-ing of the Si substrate covered by a dispersed layer

of CNPs produces square-like pyramidal voids which act as nucleation sites for growth of Si nanowires with diameter of 40–50 nm The Si nanowire production was favoured at low CNPs

www.elsevier.com/locate/cplett

*

Corresponding author Fax: +39-06-9400-5312.

E-mail address: botti@frascati.enea.it (S Botti).

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

doi:10.1016/S0009-2614(03)00090-3

coverage, when the carbothermal reduction

oc-curring at the CNPs/substrate interface is

termi-nated after complete consumption of CNPs in

contact with Si On the contrary, when the CNP

density is high, the excess of nanoparticles

reorga-nise into self-assembled carbon nanotubes [10–12]

Here we used two laterally resolved techniques,

scanning electron microscopy (SEM) and

syn-chrotron radiation scanning photoelectron

mi-croscopy (SPEM), to obtain deeper insight on the

morphology, composition and the lateral

distri-bution of the chemical species on the surface

obtained under conditions that we believe to

fa-vour Si nanowire formation, i.e., annealing of the

low density CNPs/Si(1 0 0) interface at 1050°C

2 Experimental

The CNPs were prepared by CO2laser-induced

decomposition of acetylene–ethylene mixtures in a

flow reactor [8,9] The CNPs were amorphous and

largely hydrogenated with a mean size of 50
30

nm After synthesis, the CNPs were sprayed on the

Si(1 0 0) substrates and resistively heated up to

1050°C at 106 atm The annealed samples were

analysed ex situ by SEM (Jeol 5400, resolution of

3 nm with an accelerating voltage of 30 kV) and

SPEM The SPEM measurements were carried out

with the experimental station of the

ESCAMi-croscopy beamline [13,14] at the synchrotron

radiation facility ELETTRA(Trieste, Italy) The

SPEM uses a Fresnel zone plate lens to focus

the photon beam into a submicrometer spot while

the emitted photoelectrons are collected by an

hemispherical analyser mounted at a grazing angle

with respect to the incident beam and the sample

normal The system can operate in a photoelectron

spectroscopy mode from a microspot and in

im-aging mode The images are obtained by scanning

the sample with respect to the focused beam

col-lecting simultaneously photoelectrons emitted

from a selected elemental core level All

experi-ments reported here were carried out with a

pho-ton energy of 500 eV, lateral resolution of 0.12 lm

and overall energy resolution of 0.4 eV The SPEM

measurements were performed on the Ôas receivedÕ

sample and after annealing in situ at 450°C

3 Results Figs 1a and b shows SEM images obtained after annealing a Si substrate covered with a low density CNPs at 1050 °C They reveal the pres-ence of nanostructured filaments and tubules (with diameter ranging between 30 and 50 nm) inside square-like features (ÔsquaresÕ) These ÔsquaresÕ are the basal plane of void defects, often observed during carbonisation of Si surfaces [15–17] They are hollow inverted pyramids (base

at the interface, vertex in the substrate) formed by {1 1 1} Si planes and usually have dimension be-tween 2 and 7 lm In the SPEM Si 2p maps (see Figs 1c and d), obtained by collecting the pho-toelectrons corresponding to non-oxidised Si, the ÔsquaresÕ appear as brighter areas (consisting of four triangles of different grey level), which indi-cates that locally the native SiO2 layer has been partially reduced Fig 1c also reveals higher density of defects on the right side of the Si map where they even overlap This distribution reflects the initial non-uniform distribution of the pristine CNPs onto the surface Although the nanostruc-tures formed inside the defects cannot be resolved

by SPEM, their presence and composition can be elucidated from the chemical maps and the pho-toelectron spectra measured inside and outside the ÔsquaresÕ

The three left panels in Fig 2a show raw maps obtained collecting the C 1s photoelectrons and the Si 2p photoelectrons emitted from the Si and SiO2 states, respectively In these maps topo-graphic artefacts obscure the real ÔconcentrationÕ contrast: because of the photoelectron detection geometry the emission from defect sides facing (opposite to) the electron analyser is enhanced (reduced) In fact, the contrast variations of the SiO2and C maps is very similar to the topographic maps, obtained by collecting the secondary elec-trons with kinetic energies negligibly affected by the Si or C core level emission and used for re-moval of the topographic artefacts from the im-ages [18] The true C, Si and SiO2 concentration maps (right panels in Fig 2), obtained by dividing the raw maps to the topographic map, clearly manifest the higher Si content inside the ÔsquaresÕ, where the relative amount of SiO and C is

S Botti et al / Chemical Physics Letters 371 (2003) 394–400 395

reduced The higher elemental Si signal can be

easily explained assuming formation of Si-based

filaments formed via reduction of the oxide layer,

which decorate the pyramid walls This result is

confirmed by the XPS spectra shown in Fig 2b

The Si 2p spectra from both regions contain a SiO2

component (the broad band at 103.6 eV, shifted by

4 eV with respect to Si 2p3=2 (99.3 eV) and

Si 2p1=2 (99.9 eV) peaks), which derives from

the native oxide layer terminating the Si substrate

The major difference is that inside the ÔsquaresÕ the

SiO2=Si intensity ratio is 5 times smaller than

that of the surrounding areas, where the SiO2

component is dominant The C 1s spectra taken

inside and outside the pyramidal voids exhibit

similar lineshapes peaked at 284.7 eV and a bit

lower intensity inside (see the processed C map),

whereas a much higher C concentration should be

detected if the filaments are not Si-based but

originate from the CNPs as residuals or

by-prod-ucts In fact, as already reported, much higher CNP density is required for self-organisation of these particles into carbon nanotubes [10–12] The shape of the C 1s spectra change only slightly upon sample heating to 450 °C which in-dicates that the detected C cannot be related to contaminants adsorbed upon exposure to atmo-sphere, but is a constituent of the near-surface layer The BE position of the C 1s peak covers the energy region corresponding to sp3 and sp2 hy-bridised C emitting at 285 and 284.3 eV [19], and the peaks measured both inside and outside the pyramids appear intermediate between sp3and sp2

hybridisation, indicating the presence of poorly organised carbon species This C 1s energy posi-tion excludes the formaposi-tion of stoichiometric SiC, which has a component at a much lower BE (282.6 eV) [20] This is in accordance with the

Si 2p spectrum, where the SiC component should appear at 100.4 eV [20]

Fig 1 Pyramidal-shaped voids formed at the CNPs/Si(1 0 0) interface annealed at 1050 °C (a) SEM image of pyramidal void and (b) magnified view of silicon nanowires in the void (c) (38:5  38:5 lm 2 ) SPEM maps taken on the Si 2p peak and (d) a magnification of the same region (19  19 lm 2 ) measured with higher lateral resolution.

The Si 2p image in Fig 3 reveals the growth of a

relatively large structure, protruding out of the

merged pyramidal voids This image is taken in a

sample region with a higher density of ÔsquaresÕ,

which coalesce in disordered patterns, resulting in

round voids and occasionally microstructures

which stick out of the ÔsquaresÕ The spectrum of

this wire-like feature has a rather prominent Si

component at 99.3 eV, whereas the C 1s intensity is

further reduced compared to the spectrum (dashed

line) taken inside the voids shown in Fig 2 This

confirms that the structures standing out of the

walls are almost C-free and that the detected

car-bon has mostly contamination origin

Fig 4 shows the SEM image of a sample

ob-tained by increasing the annealing time, showing a

dense film of silicon nanowires The wires have

smooth surfaces and exhibit bends and kinks The

newly formed wires adhere to the ones previously formed and they mechanically twist or knit to-gether

4 Discussion The SEM and SPEM analysis of the samples annealed for a short time (Figs 1 and 2) demon-strates that the formation of the pyramidal-shaped defects plays a crucial role in nucleation of Si nanowires, which grow mostly along the pyramid walls These defects are typical features appearing

on the Si side of the SiC/Si interfaces [15–17] during Si carbonisation, their origin being attrib-uted to presence of oxygen and oxygen-related defects in bulk Si wafers [21,22] However, microscopic imaging of the pyramidal defects at

Fig 2 (a) (12  24 lm 2 ) SPEM maps taken collecting the Si 2p photoelectrons corresponding to elemental Si (Si) and SiO 2 (SiO2) components and C 1s photoelectrons (C) The left and right panels show the raw and processed images, respectively (b) Si 2p and C 1s core level spectra taken inside (upper) and outside (lower) ÔsquaresÕ.

S Botti et al / Chemical Physics Letters 371 (2003) 394–400 397

SiC/Si interfaces revealed mostly smooth walls

without any evidence of nanosized structures

protruding out of the surface [15,21,22] Often the

pyramidal defects are buried below a rather

ho-mogeneous SiC layer which connects the voids and

are considered as a source of Si atoms outdiffusing

from the hole bottom to the surface

The results reported here do not show presence

of any homogeneous film covering the pyramidal

voids formed at the CNPs/Si(1 0 0) interface and

the SPEM Si 2p spectra do not contain any

spectroscopic feature related to SiC Apparently

under these reaction conditions (annealing at 1050

°C in low vacuum) the native SiO layer cannot be

completely removed On one hand it acts as a barrier for the direct interaction between hydro-carbons and Si and formation of SiC, and on the other hand, being exposed to ÔactiveÕ C containing species, it is gradually reduced with production of Si-based nanostructures and volatile CO This indicates that despite the similar dimensions (a few microns) and surface density (106 cm2) the defects observed in the present study should not

be identical to that formed at Si/SiC interfaces [15–17]

In brief, the reaction mechanism can be de-scribed as follows The first reaction step is partial reduction of the native silicon oxide layer accord-ing to the scheme, SiO2þ C ! SiO þ CO, where C stays for the highly active carbon nanoparticles Due to the high reactivity of carbon particles, in this case the temperature required for

carbother-Fig 4 (a) SEM micrograph showing a high density of Si nanowires: (b) magnified view of the same sample.

Fig 3 Top: (19  19 lm 2 ) Si 2p image of a region showing

merged pyramidal voids and a spiral wire protruding out from

them Bottom: XPS spectra taken on the spiral wire and outside

the pyramidal voids (see arrows) The dashed spectrum is taken

inside one of the ÔsquaresÕ in Fig 2 and is shown for the sake of

comparison.

mal reduction is lower than that usually required

[4], leading to a sizeable SiO formation even at

temperature as low as 1050°C The formation of

voids seems to be triggered by the faster

con-sumption of the oxide layer below the adsorbed

nanoparticles At the sites where severe

carbo-thermal oxide reduction occurs, the developed

surface stress might favour formation of cavities

with lowest energy {1 1 1} symmetry planes

Ex-trusion of material during such collapse of the

Si(1 0 0) surface lattice seems to be the process

leading to nucleation of Si nanowires along the

{1 1 1} plane walls Disproportionation of Si

monoxide seems to be the key reaction for the

formation of Si nanowires by thermal evaporation

of pure SiO [23] or mixed Si–SiO2 [24] powders

Similarly, in our case a possible reaction route

leading to the nucleation of the Si nanostructures

is the decomposition of silicon monoxide

2SiO! Si þ SiO2 at 1050 °C followed by the

precipitation of Si atoms which are expelled from

the mixture as crystalline nanowires Adirect

re-duction of the SiO2 layer due to the reactive

spe-cies, eventually present in the residual atmosphere

of vacuum chamber, can be excluded as key step in

the formation of Si nanowires since it would act

homogeneously of the sample surface without

originating the observed thinning of oxide layer in

correspondence of the ÔsquaresÕ

The XPS spectra reported in Figs 2 and 3 are

a clear evidence that the Si nanowire are less

oxidised than the regions surrounding the

ÔsquaresÕ After deconvolution of the Si 2p spectra

we determined an oxide thickness of 6.5 and

20
AAfor the ÔsquareÕ and outside areas, using the

relationship doxide¼ koxidecosðaÞ ln½1 þ ðI4þ=I0Þ=c,

where I4þ and I0 are the integrated intensities of

the Si and Si oxide components, k is the Si 2p

photoelectron mean-free path in SiO2 (4.7
AAin

our case), a is the acceptance angle of the

ana-lyser with respect to the sample plane, and

c¼ 0:7 is the intensity ratio of pure SiO2 and Si

[25] This result differs from the previously

re-ported 3 nm thick SiO2 layer surrounding the Si

core of the Si nanowires grown with different

techniques [5], which is assumed to have a crucial

role in limiting the side growth of the

nano-structure

5 Conclusions Silicon nanowires can be successfully synthes-ised without metal catalysts by annealing the Si substrates at 1050 °C in the presence of active carbon nanoparticles The Si nanowire formation

is well described by the oxide-assisted growth model The present results have proved that in the initial reaction stage the produced Si nanowires decorate the walls of pyramidal voids, formed during carbothermal reduction of the native SiO2

layer covering Si substrates

Acknowledgements The authors wish to thank D Lonza for his technical assistance and C Cepek for fruitful dis-cussions

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