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
Trang 1A study in the growth mechanism of silicon nanowires
with or without metal catalyst Jun-Jie Niu ⁎ , Jian-Nong Wang School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200030, PR China
Received 11 April 2007; accepted 22 June 2007 Available online 28 June 2007
Abstract
The growth mechanism of silicon nanowires synthesized with or without a metal catalyst via chemical-vapor-deposition (CVD) is discussed by using a developed vapor–liquid–solid and novel sulfide-assisted growth models, respectively The metal catalyst plays an important role on the catalytic growth However, the growth of silicon nanowires with sulfide is chiefly affected by the compound decomposition, gas stream, and temperature difference Silicon nanowires fabricated with metal can be self-organized while a large scale of samples can be achieved with metal-free catalyst The growth mechanism comparison between metal- and non-metal assisted methods for synthesizing silicon nanowires will supply a beneficial help in deepening the understanding of crystal procedure and improving the sample quality
© 2007 Elsevier B.V All rights reserved
Keywords: Nanomaterials; Crystal growth
1 Introduction
Silicon nanowires (SiNWs), as a candidate material for
nano-electronic devices, are being intensively studied[1,2] This is
because of the feasibility of integrating SiNWs as functional
building blocks into the existing CMOS technology [3]
Specifically, SiNWs can be applied in the fields including ultra
sensitive bio-sensor, field effect transistors (FETs), and
single-electron detector[4]
The fabrication of SiNWs involves assisted or
metal-free growth The metal-assisted growth mainly follows the
vapor–liquid–solid (VLS) mechanism, which was first
de-scribed by Wagner and Ellis in 1964 and developed by
Givargizov in 1975[5,6] The metal catalyst used was usually
Au although other metals were also used[7,8] The synthesis
methods mainly include laser ablation [9],
chemical-vapor-deposition (CVD) [10,11], thermal evaporation [12,13], and
growth from organic solution [14] Amongst these methods,
laser ablation is widely adopted However, this method does not
allow the localized growth on a patterned substrate for further
processing [8] Recently, a solid–liquid–solid (SLS) process
derived from the VLS mode was also reported for obtaining SiNWs [15] The growth based on an oxide, which was proposed by Lee and coworkers [16], is a typical sample of synthesis of SiNWs without a metal catalyst[17] In this case, the SiOx vapor decomposes into Si atoms, which act as the nuclear of SiNWs covered by shells of silicon oxide
Metal-assisted growth by CVD technique is a simple and efficient route for synthesizing controllable and even self-organized SiNWs at a low temperature [11,18] However, the metal-free growth based on sulfide is believed to have a potential for large-scale production The growth mechanisms, however, are still poorly understood under both circumstances This study thus attempts to improve the understanding on the basis of new experimental observations
2 Experimental
A CVD system was used for the fabrication of SiNWs (Fig 1) For metal-assisted growth, the transition metal such as
Au or Ni was used as a catalyst The substrate was put in a quartz tube furnace which was then pumped down to ∼20 Pa and heated to 600–900 °C The mixed gases of argon, hydrogen, and silane with a desired flow ratio were flowed For sulfide-assisted growth, silicon wafer was used both as the silicon source and
Materials Letters 62 (2008) 767 –771
www.elsevier.com/locate/matlet
⁎ Corresponding author Tel.: +86 21 62932050.
E-mail address: jjniu@sjtu.edu.cn (J.-J Niu).
0167-577X/$ - see front matter © 2007 Elsevier B.V All rights reserved.
doi: 10.1016/j.matlet.2007.06.056
Trang 2sample collector The ZnS or S powder was put in one side and
would be flowed to the wafer The reaction temperature was set
at about 1100 °C The samples were analyzed by scanning
electron microscopy (SEM, JSM-5610LV) and transmission
electron microscopy (TEM, JEM2100F)
3 Results and discussion
3.1 Metal-assisted growth of SiNWs
A basic aspect of the VLS mechanism is the metal particle acting as
a catalyst for the anisotropic growth of SiNW with a crystalline
structure A catalyst particle provides a site for absorption of
vapor-phase silicon atoms The continuous absorption induces the
supersat-uration of the formed liquid alloy with silicon atoms, which leads to nucleation and growth of a SiNW[19] Therefore, the diameter and location of the SiNW are determined by the features of the catalyst particle [20] Thus, it is important to control the catalyst size distribution and the delicate catalyst positioning The metal catalyst can be generated by thermal evaporation, sputtering, or electrochemical methods The particle size can be modified by varying the reaction parameters In particular, the catalyst distribution can be well-organized
by using a nano-channel-alumina (NCA) technique[11] Otherwise, the pressure of reaction channel and temperature are also important for the growth of SiNWs In the present CVD system (Fig 1), the pressure can be adjusted by a gas-controlled valve Normally, a high pressure can lead to a high yield of the sample If a very low pressure was used, plenty of Si atoms will be flowed away and have no time to contact
Fig 1 Diagrammatic sketch of the CVD system.
Fig 2 A) Binary phase diagram of Au and Si B) A mode of VLS mechanism.
Trang 3with the catalyst This results in formation of only a few of SiNWs The
reaction temperature is usually decided with the consideration of the
catalyst type According to the binary phase diagram of a metal and Si,
the temperature for SiNW formation must be above the eutectic point
as liquid droplets can form under this condition Here we give an
example of using Au as the catalyst As seen fromFig 2A, if the
reaction temperature is increased to be above the eutectic point of
363 °C (such as T1), a liquid alloy can form and will assist the growth
Considering the production output, a temperature of 620 °C was used
for the case of Au and 900 °C for the case of Ni to fabricate SiNWs
[11,21]
In general, the chemical reaction equations can be written as the
following:
Siþ Ox¼ SiOxðxb2Þ ð2Þ
(3)
Si is proffered by the decomposition of the precursor of SiH4
(Eq (1)) When dropping Si atoms come to contact with the metal
particle located on the substrate (Fig 2B a), a liquid metal–Si alloy will
be formed With more and more Si atoms added, the Si content in the
droplet will reach a saturated value (in the Au–Si system, this value is
∼25% at a point inFig 2A) If the supply of Si atoms is continued, the liquid alloy will be supersaturated with Si atoms and excessive Si atoms will then precipitate The precipitated atoms will grow freely with a crystalline structure (Fig 2B b) Since the orientation ofb111N
in Si lattice has the lowest energy, this orientation dominates the growth direction and the final extending direction of SiNW (Fig 3A) In particular, when the nanowire becomes longer, the droplet at the tip will
be pushed randomly and thus sometimes two or more will combine to form a bigger one, leading to an intercrossing structure.Fig 3B clearly shows the original growth stage of SiNWs As can be seen from the figure, if Si atoms cannot be added continuously into the droplet, the growth will be terminated and a short SiNW is obtained On the con-trary, if enough Si atoms are provided, a long SiNW with random direction will be observed (Fig 3C) In this case, an excellent crystalline nature of Si (111) is observed as shown inFig 3A In addition, the as-formed SiNW is easily oxidized with an oxide shell of 1–3 nm according to the Eq (2) (Fig 3A)
3.2 Sulfide-assisted growth of SiNWs
If the growth with a metal catalyst can cause contamination, the synthesis without metal will be useful for obtaining clean samples with high quality The oxide-assisted growth is an effective approach for large-scale production of high-quality SiNWs without metal [16,17] During the reaction, SiO decomposes into Si and SiO2 The pre-cipitated Si atoms under the assistance of outer SiO2shell will form a nuclear and thus grow up to a wire It is clear that silicon oxide plays a key role on the formation of SiNW Similar with this mode, we have
Fig 3 High resolution TEM image (A), SEM image of original growth stage (B), and final stage (C) of SiNWs synthesized via VLS mechanism.
Trang 4developed a novel sulfide-assisted mechanism to synthesize SiNWs in
large quantity[22] Relative to SiO decomposition, compound SiS also
plays a significant role on the growth of SiNW Furthermore, the gas
flow and temperature difference have simultaneously important effects
We now analyze the detailed growth process in the following sections
Firstly, the whole reaction equations can be summarized to:
2ZnSþ O2¼ 2ZnO þ 2S↑ðN∼200-CÞ; ð4Þ
Sþ Si ¼ SiSðN∼900-CÞ; ð5Þ
2SiS↑ ¼ Si þ SiS2ð∼950-C–1080-CÞ; ð6Þ
and
SiS2þ 2H2O¼ SiO2þ 2H2S↑: ð7Þ
The S source can be originated from ZnS or direct S powders[22] As
presented in Fig 4A, the S vapor formed at low temperature zone
(b∼900 °C) will be carried to a higher temperature zone (∼900 °C
bTb∼950 °C) In this region, the S vapor will encounter silicon wafer
which is used as a substrate and form plenty of SiS particles relative to the
Eq (5) When the temperature is continually increased to∼950–1080 °C,
the new-formed SiS compound will sublimate and decompose into Si and
SiS2(see Eq (6) and zone A inFig 4A) At the beginning, the Si/SiS2is
present as a phase of quasi-liquid droplet Thus, a large quantity of Si and
SiS2atoms are quickly flowed away by the protected gas to an area with
lower temperature (B inFig 4A) When temperature decreases, the Si
atoms are easier to reach saturation in quasi-liquid Si/SiS and precipitate
along the droplet to form a nuclear As a result, the precipitated Si atoms develop to be a SiNW with crystal directions along the energy lowest theory (B inFig 4A) Amongst this method, the reactions Eqs (5) and (6) are very fast and thus induce a quick growth of numbers of SiNWs with
a badly crystalline structure (Fig 4B) Otherwise, the Si is easy to be oxidized in low-vacuum system and the generated SiS2is very feasible to react with H2O (Eq (7)) The above factors easily cause a low-crystal quality of as-received SiNWs even amorphous SiO2nanowires 3.3 Advantage and disadvantage of with and without metal growth The mentioned metal-assisted and free-metal methods are both simple and convenient to synthesize SiNWs The VLS mechanism with metal catalyst has disadvantages of contamination, low quantity, and complicated process Furthermore, if the Si source is used by silane, the vacuum requirement for equipment is high However, this process can be reacted at
a relatively low temperature And growth rate can be controlled even can be self-organized with the assistance of template Thus it provides an operable route to combine with present nano-techniques for real applications The fabrication with sulfide-assisted mechanism cannot be easily controlled and the structure of SiNWs is badly crystalline even amorphous Further-more, the growth temperature is relatively higher However, by using this quick-growth technique, we can receive large scales of sample with a very simple device If we can control the growth velocity and improve the crystal nature, the process may have more applications in the future
4 Conclusions The catalyst features take important actions on metal-assisted growth Otherwise, the compound decomposition, gas stream, and temperature difference play key roles on sulfide-assisted synthesis The growth with metal catalyst can be controlled while the sulfide-assisted growth can obtain large scales of samples Acknowledgements
This work was supported by the Shanghai-Applied Materials Research and Development Fund (No 06SA06) and Youth Teacher Fund of Shanghai Jiaotong University (A2306B) We would like to thank Instrumental Analysis Center of Shanghai Jiaotong University, for their great helps in measurements
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