Si nanowires synthesized with cu catalyst

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Si nanowires synthesized with Cu catalyst

Y Yao ⁎ , S Fan Tsinghua-Foxconn Nanotechnology Research Center, Department of Physics, Tsinghua University, Beijing 100084, PR China

Received 15 March 2006; accepted 4 April 2006 Available online 6 May 2006

Abstract

The metal copper which is a newly developed interconnecting material for integrated circuit (IC) has been used as the catalyst to catalyze the formation of the Si nanowires in high temperature tube furnace The growth direction of the straight Si nanowires isb111N and the polyhedron

η″-Cu3Si alloy is on the tip of the Si nanowires The synthesis temperature of the Si nanowires is 500 °C Such a low temperature implies that the vapor–solid (VS) should be the growth method The cheap Cu catalyst is favorable for the mass synthesis of Si nanowires

© 2006 Elsevier B.V All rights reserved

Keywords: Nanomaterials; Deposition; Catalysts

1 Introduction

Because of the importance of silicon in the microelectronic

industry, 1D silicon nanostructure, Si nanowires, has attracted

many research interests The p–n junction devices have been

fabricated based on the p-doping and n-doping Si nanowires

and Si nanowire filed effect transistors (FET) have showed

better performance than the planar metal-oxide-semiconductor

FET (MOSFET)[1,2] Nanosensors based on the Si nanowires

FET structure have been fabricated[3,4] Because of the limited

dimension, the quantum confinement effect of the Si nanowires

has been observed in the photoluminescence (PL) measurement

[5] The polarization of the PL spectrum for Si nanowires has

also been reported[6,7]

Vapor–liquid–solid (VLS)[8,9]is an important way to

syn-thesize Si nanowires It has been reported that Au catalyst

particles can limit the diameter of the Si nanowires and usually

induct the aligned Si nanowires arrays on the silicon substrate

[10–13] However, it is not economical to synthesize the mass of

Si nanowires with Au catalyst because of the expensive value of

Au particles or Au-gel Some other cheaper metals, such as Fe

[14]and Ni[15], has been selected to catalyze the Si nanowires

growth, but it is not favorable to introduce such metals into the IC

processing since they are“toxic” for the semiconductor device

Cu is a newly developed interconnection material for silicon chip because of its better performance than aluminum in the lower resistivity which means the little time delay and the better reliability against the degradation by the metal migration at high current[16] According the phase diagram of CuSi alloy[17]

(Fig 1), it is reasonable to expect Cu as an appropriate catalyst for the growth of Si nanowire To date there are no reports about the copper catalyzing Si nanowires In this paper, the growth condition and the morphology of Si nanowires catalyzed by copper particles are described

2 Experimental

Nanocluster deposition system (ND 60, Oxford Applied Re-search) has been used to prepare the Cu catalyst on the Si wafer After being sputtered from the Cu target, only the Cu nano-particles with selected diameter could pass through the quad-rupole mass spectrometry in the ND 60 and deposit on the b111N Si wafer The sputtering power was about 120 W The selected mass was 147,074 a.m.u., corresponding diameter was about 4 nm Deposition time was 20 min The Si wafer covered with Cu nanoparticles was transferred into the alumina tube and heated in the horizontal furnace The temperature increased from room temperature to 500 °C in 20 min with 100 sccm Ar flow and the pressure was about 8 Torr Then the pressure and temperature were kept for 30 min with the introduction of

20 sccm SiH4flow for the Si nanowires growth The products

Materials Letters 61 (2007) 177 –181

www.elsevier.com/locate/matlet

⁎ Corresponding author.

E-mail address: y-yao@mail.tsinghua.edu.cn (Y Yao).

0167-577X/$ - see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.matlet.2006.04.045

were characterized with EF-SEM (Sirion 200, FEI) and high

resolution TEM (Tecnai G2 F20, FEI)

3 Results and discussion

Fig 2a shows the SEM image of the Cu clusters deposited on the Si

wafer The diameter of most Cu clusters is about 5 nm (Fig 2b), almost

the same as the expected 4 nm The size distribution is relatively

uni-form due to the mass filter in the ND 60 To investigate the diameter

variation of the catalyst during heating, the Si wafer covered with Cu

clusters has been heated on 500 °C without any gas feeding.Fig 2c

depicts the diameter distribution of the after-heated Cu catalyst The Cu

clusters have aggregated into the larger nanoparticles and the diameter

distribution becomes wider After being heated in the high temperature

tube furnace with SiH4feeding, the surface of the Si wafer changed to

light yellow SEM image (Fig 2d) of the after-grown Si wafer

demon-strates that there are many thin and straight nanowires covering the

surface of the Si wafer The nanowires prolong several micrometers

randomly In the high magnification image (Fig 2e), a small spot can

be observed on tip of the straight nanowire and the diameter of the tip is

as the same as the nanowires

The Si nanowires have been scraped from the Si wafer and moved

to Cu grid for TEM characterization.Fig 3a displays the low

mag-nification TEM image of the Si nanowires Some silicon particles are

mixed with the Si nanowires A straight nanowire 30 nm wide, a typical

diameter for the thin Si nanowires is shown inFig 3b There is a 4 nm

thick amorphous layer covering the nanowire The high resolution

TEM image (down image inserted inFig 3b) reveals that the straight

Si nanowire is well crystalline Diffractogram patterns, a fast Fourier

transformation (FFT) from the high resolution image, indicate that the

growth direction of the Si nanowires is alongb111N (up image inserted

in Fig 3b) The lattice distance along the growth direction of the nanowires is about 3.14 Å, which well agrees with the distance between Si {111} facets However, the contrast variation on the nanowires in the low magnification TEM image (Fig 3b) indicates that there should be some defects in the Si nanowires As disclosed in

Fig 3c, the {111} stack faults and micro-twin boundaries are the dominated defects in the Si nanowires

There is a polyhedron dark tip with the flat facets on the straight Si nanowire.Fig 3d shows the clear lattices contrast of both Si nanowire and the dark tip The size of the tip is as large as the diameter of the Si nanowires It is different with the preview reports about the Au catalyzing Si nanowires, in which the tips are ball-like Au particles[1– 3].Fig 3e is the diffractogram patterns of the interface between the tip and the nanowire Two groups of patterns can be distinguished: one should be indexed as the diffraction patterns of Si (0 1¯ 1¯ )* reversal plane, another could be ascribed to orthorhombicη″-Cu3Si ( 1¯, 19, 0)* reversal plane[18] The diffractogram spots are indexed inFig 3e with solid and dashed lines The {111} facet of Si nanowires is almost parallel to the {003} facet of theη″-Cu3Si The diffractogram patterns

of the alloy tip are also displayed inFig 3f and the sharp spots prove that the tip is the single crystalη″-Cu3Si alloy This result confirms that the copper-richη″-CuSi alloy should be formed when the alloy liquid is cooled down to the room temperature (Fig 1)

An interesting result that should be emphasized is that the growth temperature of the Si nanowires is 500 °C, much lower than the eutectic temperature 802 °C It means that the VLS mechanism should not occur during the growth The formation of Si nanowires may be ascribed to the vapor–solid (VS) growth mechanism Under the frame-work of VS mechanism, the decomposed Si from SiH4deposits on the surface of Cu nanoparticles and forms solidη′-CuSi alloy (between

467 °C and 558 °C) The Si diffuses in the solid–solution alloy and will Fig 1 Cu –Si phase diagram [17]

separate from the solid alloy to form the Si nanowires when the

concentration is supersaturated The diameter of the Si nanowires is

similar to the size of the CuSi alloy nanoparticles So the tip of the Si

nanowires is polyhedron, not the ball-like tip

4 Conclusion

The Si nanowires could be grown with Cu catalyst The

diameter of the Cu nanoparticles could affect the size and the

quantity of the nanowires The growth temperature is 500°C and

the growth direction of the Si nanowires isb111N TEM images

indicate the well crystalline of the thin and straight Si nanowires

with η″-Cu3Si alloy tips VS growth mechanism should be responsible for the formation of Si nanowires The cheap Cu catalyst and the low growth temperature are favorable to the mass synthesis of the Si nanowires

Acknowledgements

Financial support from the National Natural Science Founda-tion of China (NNSFC 10334060) and NaFounda-tional Basic Research Program of China (973 Program 2005CB623606) is gratefully acknowledged The authors also thank Mr Liguo Xu for his helpful assistant work

Fig 2 The SEM images of a) the Cu nanoparticles on the surface of Si wafer (scale bar is 1 μm) b) The diameter distribution of the Cu nanoparticles c) The diameter distribution of the after-heated Cu nanoparticles d) The Si nanowires grown on the Si wafer (scale bar is 5 μm) e) The large magnification SEM images of a Si nanowire (scale bar is 1 μm).

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Y Yao, S Fan / Materials Letters 61 (2007) 177 –181

Fig 3 a) The low magnification TEM image of the Si nanowires b) The TEM image of a straight Si nanowire (The down inserted image is the high resolution image

of the nanowires and the up inserted image is the corresponding diffractogram patterns.) c) The {111} stack faults and micro-twins in the straight Si nanowire d) High resolution image of the tip e) The diffractogram patterns of the interface between the tip and the nanowire (The solid and dashed lines indicate the patterns from Si and η″-Cu 3 Si alloy, respectively.) f) The diffractogram patterns of the dark tip.

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