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 1Physica E 24 (2004) 278–281
Sulfide-assisted growth of silicon nano-wires by
thermal evaporation of sulfur powders Junjie Niua, Jian Shaa,b, Deren Yanga,*
a
State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China
b Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China
Received 23 March 2004; accepted 11 May 2004 Available online 10 July 2004
Abstract
Silicon nanowires (SiNWs) with a diameter of B20 nm were synthesized by the thermal evaporation of sulfur powders on silicon wafers The source of the SiNWs came from the silicon substrates It is considered that the generated SiS compound assisted the formation of SiNWs Finally, the Raman shift of SiNWs was discussed
r2004 Elsevier B.V All rights reserved
PACS: 71.55.Cn; 81.05.Ys
Keywords: Silicon; Nanowires; Sulfide assisted
1 Introduction
Recently the one-dimensional nano-materials,
especially silicon nanowires (SiNWs), have
stimu-lated much interest because of their different
properties in comparison with the corresponding
bulk materials [1–6] Several synthesized methods
for SiNWs have been reported, including laser
ablation[7], chemical-vapor-deposition (CVD) via
vapor–liquid–solid (VLS) mechanism[8–13],
ther-mal evaporation via oxygen-assisted [14–17] and
solid–liquid–solid (SLS) mechanisms [18–20], and
electronic-chemical method[21]
In this paper, the thermal evaporation of sulfur powders on silicon wafers used to grow SiNWs is reported Compared with the other approach, this process was simple and the source of SiNWs was from the silicon wafer substrates but not from the silane gas [11]
and silicon oxide [14,17] It is also found that sulfide played an important role in the formation
of SiNWs, therefore, a sulfide-assisted growth mechanism was suggested In the experiments, the samples were checked by filed emission scanning electron microscopy (FESEM), transmis-sion electron microscopy (TEM), and X-ray diffraction (XRD), respectively Finally, the Ra-man spectroscopy was also used to investigate the SiNWs
*Corresponding author Tel./fax: +86-571-879-523-22.
E-mail address: mseyang@zju.edu.cn (D Yang).
1386-9477/$ - see front matter r 2004 Elsevier B.V All rights reserved.
doi:10.1016/j.physe.2004.05.002
Trang 22 Experimental
SiNWs were produced on p-type (1 1 1) silicon
wafers with a resistivity of about 0.001 O cm by
means of a low-vacuum CVD system First,
several pieces of silicon wafers and plenty of sulfur
powders were placed in a semi-sealed alumina boat
which was put at the center of a horizontal quartz
tube furnace Then the furnace was evacuated to
reach 30 Pa by a mechanical pump The
tempera-ture of the system was then raised to 900C at a
heating rate of 25C min1and continually up to
1250C at a heating rate of 10C min1, and held
at 1250C for 30 min at a constant pressure of
30 Pa After reaction, the weak black and yellow
substrates with the as-grown materials were
removed from the furnace and characterized by
FESEM (FEI, Sirion), TEM (JEOL, JEM200CX),
XRD (Rigaku, D/MAX-rA), and Raman
scatter-ing spectroscopy (Nicolet Almega), respectively
The possible chemical composition of the
as-grown materials on the wafers was investigated
by using energy-dispersive X-ray spectroscopy
(EDX) attached to the FESEM
3 Results and discussion
The FESEM morphology of the SiNWs with a
diameter ofB20 nm (that of a very few SiNWs is
over 50 nm) is shown inFig 1 The EDX (the inset
ofFig 1) taken from the corresponding nanowires
indicates that the nanowires were silicon
How-ever, oxygen could also be detected The oxygen is
considered to have come mainly from the surface
oxidation of nanowires This suggests that the
nanowires were composed of silicon and silicon
oxide as sheath Actually, some SiNWs could be
oxidized to be silicon oxide nanowires because of
the low-vacuum system and high temperature, as
found in our previous work[6] The TEM image of
a number of curved SiNWs is shown in Fig 2 It
can be seen that the average diameter of the
SiNWs wasB20 nm, while the minimum one was
less than 10 nm The structure of the SiNWs is
indicated by the selected area electric diffraction
(SAED) image in the top right ofFig 2 The weak
electric diffraction spots proved that the SiNWs
did not crystallize well This might be due to the fast growth rate of the SiNWs The thorough analysis of crystal nature is indicated by the XRD data as shown in Fig 3 The sharp peaks of Si (1 1 1), Si (3 1 1), Si (4 0 0), and Si (3 3 1) indicates that the SiNWs were crystalline Some crystal Al peaks and low-intensity peaks of SiS2 (3 0 1) and SiS (2 1 3) are simultaneously observed Al peaks
Fig 1 FESEM image of the SiNWs on a silicon wafer The lower left inset is the EDX taken from the corresponding sample.
Fig 2 TEM image of the SiNWs The top right inset is the SAED taken from one of the SiNWs On top left is the magnified TEM image of a SiNW with tip.
Trang 3come from the sample preparation process for
the XRD analysis, while SiS2comes mainly from
the decomposition of SiS The discussion of the
growth mechanism is displayed below
As the function of silicon oxide in the
oxygen-assisted mechanism [15,16], in our experiments,
the silicon sulfide (SiS) also played a key role
in assisting the growth of SiNWs Therefore, a
sulfide-assisted model for SiNW growth is
sug-gested here
The reaction procedure mainly took two steps
One was that the sulfur reacted with the silicon
substrate and generated SiS compound at the
lower temperature (B900C) The next step was
that the SiS decomposed to be Si and SiS2 at
higher temperature (B1000C) During the
de-composition, SiNWs grew up from the generated
Si as source The reaction equations are shown
below
S þ O2 ¼ SO2m ð1Þ
S þ Si ¼ SiS ðB900CÞ ð2Þ
2SiSm ¼ Si þ SiS2 ðB1000
and
SiS2þ 2H2O ¼ SiO2þ2H2Sm: ð4Þ
When the temperature reached B900C, Eq (2)
happened and the SiS film was produced
Con-tinually, the SiS is decomposed into Si and SiS at
higher temperature (B1000C), which could be confirmed by the XRD spectrum in Fig 3 Therefore, it is believed that the SiNWs generated from the SiS acted as nucleation centers which were located at the tip of the SiNWs, as shown in the top left ofFig 2 Thus, the tip should contain SiS2 But there was no S signal in the EDX spectrum (the inset ofFig 1) and only weak SiS2
peaks are displayed in XRD data (Fig 3) This is because the SiS2 could sublimate and disappear when the temperature was higher than 1090C Furthermore, SiS2 was also easy to react with
H2O, which exists in air, as illustrated in Eq (4) Surely, not only SiS but also other sulfides such
as zinc sulfide, ferric sulfide, etc, which form silicon sulfides by the reaction with silicon, can also be used to assist the growth of SiNWs In principle, all of silicon compounds, such as silicon sulfide shown in this paper and silicon oxide [17], can assist the formation of SiNWs It is called the silicon compound-assisted mechanism for SiNWs growth
The SiNWs and bulk silicon in comparison were also checked using Raman spectroscopy (Fig 4)
It is clear that the 510.5 cm1 peak of the SiNWs (Fig 4(a)) shows a B10 cm1 downshift compared with the 520.3 cm1peak of bulk silicon (Fig 4(b)) Usually, the peak of 510.5 cm1 was regarded to be the first-order transverse optical phonon mode (TO) The downshift might be
0
500
1000
1500
2000
Al(111) Al(200) SiS2(213) SiS2(301)
Al(220)
Al(311) Si(331) Si(400)
Si(311) Si(111)
2 θ (degrees)
Fig 3 XRD spectrum of the as-grown SiNWs.
0 1000 2000 3000 4000
b
a 510.5cm -1
520.3cm -1
Raman Shift (cm-1) Fig 4 Raman spectra of SiNWs (a) and bulk silicon (b).
Trang 4associated with the quantum confinement effect
and laser heating effect [22,23]
4 Conclusion
In summary, silicon nanowires (SiNWs) with a
diameter ofB20 nm were successfully synthesized
on silicon wafers by thermal evaporation of sulfur
powders It is considered that the decomposition
of SiS resulted in the formation of SiNWs
Furthermore, a sulfide-assisted growth model of
SiNWs was suggested At last, the Raman shift of
SiNWs was also discussed
Acknowledgements
This work was supported by the National
Natural Science Foundation of China (Nos
50272057 and 60225010) and the Key Project
of Chinese Ministry of Education The authors
would like to thank Prof Youwen Wang and
Mr Z.C Chen for their great help in the
measurements of TEM and Raman spectroscopy
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