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 30 (2005) 169–173
Raman spectroscopy and field electron emission properties of aligned
silicon nanowire arrays Chun Lia, Guojia Fanga, , Su Shenga, Zhiqiang Chena, Jianbo Wangb,
Shuang Maa, Xingzhong Zhaoa
a Department of Physics and Center of Nanoscience and Nanotechnology Research, Wuhan University, Wuhan, 430072, PR China
b Center for Electron Microscopy, Wuhan University, Wuhan, 430072, PR China Received 20 March 2005; received in revised form 22 August 2005; accepted 30 August 2005
Available online 7 October 2005
Abstract
Arrays of aligned silicon nanowire (SiNW) were synthesized on a silicon (1 0 0) substrate by self-assembling electroless nanoelectrochemistry Compared with that of bulk crystal silicon, the first-order Raman peak of the silver cap-removed SiNW arrays shows a downshift and asymmetric broadening due to the phonon quantum confinement effects, and intensity enhancement Field electron emission from the SiNWs was also investigated The turn-on field was found to be about 12 V/mm at a current density of 0.01 mA/cm2 These highly densified and ordered SiNW arrays can be expected to have favorable applications in vacuum electronic or optoelectronic devices
r2005 Elsevier B.V All rights reserved
PACS: 71.55.Cn; 85.45.Db; 78.30.j
Keywords: Silicon nanowire; Electroless metal deposition; Field emission; Raman spectra
1 Introduction
One-dimensional nanostructure materials are expected
to play an important role as both interconnects and
functional units in fabricating electronic and optoelectronic
devices with nanoscale dimensions [1] SiNWs have
attracted increasing attention due to their novel
funda-mental physical properties such as light emission[2], field
electron emission[3], and quantum confinement effects[4]
Applications based on SiNW have been demonstrated in
field-effect transistors [5], logic circuits [6], chemical and
biological sensors[7], and thin film transistors[8] To date,
SiNWs have been prepared by chemical vapor deposition
[9,10], laser ablation [3,11], thermal evaporation [12],
template-assisted growth [13], oxide-assisted growth
(OAG)[14] and other methods[15] The SiNWs exhibit a
unique sp3-bonded crystal structure and a low work
function Field electron emission properties of taperlike SiNWs[14], sponge-like SiNW-induced films [3], and well-aligned SiNWs [16] have been reported However, these growth mechanisms have some limitations including high temperature or vacuum conditions, special templates and complex equipments A simple and efficient way to fabricate large-scale, highly oriented, and length-control-lable SiNWs at a lower temperature is an important and challenging issue Recently, electroless metal deposition (EMD) method was developed to prepare lager-area oriented SiNWs arrays on silicon substrates close to room temperature [17,18] However, the metal is always capped
on SiNWs during the preparation procedure[19]and little information has been reported on the physical properties of such aligned single-crystal SiNW arrays
In this work, SiNW arrays have been synthesized by self-assembly of EMD nanoelectrochemistry The silver nano-caps were removed by post-deposition treatment with nitric acid solution Afterwards, Raman spectroscopy and field electron emission properties were studied
www.elsevier.com/locate/physe
1386-9477/$ - see front matter r 2005 Elsevier B.V All rights reserved.
doi:10.1016/j.physe.2005.08.005
Corresponding author Tel.: +86 27 87642784; fax: +86 27 68752569.
E-mail address: gjfang@whu.edu.cn (G Fang).
Trang 22 Experimental details
The synthesis of aligned SiNWs array was carried out in
a Telfon-lined stainless-steel autoclave The n-type,
Sb-doped silicon (1 0 0) (resistivity 0.02 Ocm) wafer was
cleaned ultrasonically in acetone and ethanol for 20 min
each The cleaned silicon wafer was immersed in a mixture
of 4.6 mol/L HF aqueous solution and 0.02 mol/L silver
nitrate with equal volume Then, the autoclave was sealed
and transferred to a lab oven immediately After etching
for 60 min at 50 1C, the silicon wafer wrapped with a thick
silver film was taken out from the autoclave To remove the
capped silver, the as-prepared samples were dipped in
30 wt% HNO3 aqueous solution for 60 s Finally, the
as-prepared samples and the treated ones were rinsed with
de-ionized water, blown dry in air and subjected to further
analysis
Sirion FEG (Philips XL30) scanning electron
micro-scopy (SEM) attached with an energy-dispersive X-ray
spectrometer (EDXS, Genesis7000 EDAX) was used to
study the morphology and chemical composition of the
samples The transmission electron microscopy (TEM) and
the high-resolution transmission electron microscopy
(HRTEM) images were obtained with a JEOL JEM-2010
(HT) and a JEOL 2010F microscope, respectively The
samples were prepared by ultrasonicating the nanowires in
ethanol and placing a drop of the suspension on a TEM
carbon support film Raman scattering measurements were
performed using a Renishaw (RMRe1000) micro-Raman
spectrometer at room temperature Raman scattering modes were excited by means of the 514 nm line of an
Ar+ laser, and the Raman signals were measured in a backscattering geometry with a spectral resolution of 1.0 cm1 Field emission behavior was investigated using
a diode structure with an anode–cathode spacing of 50 mm
in a test chamber maintained at 104Pa A spherical-shaped stainless-steel probe with a tip diameter of 1 mm was used as an anode
3 Results and discussion Fig 1 shows typical SEM images of the as-prepared samples and SiNW arrays after being treated with an HNO3solution The branched dendritic structure of silver
on the top of SiNWs is shown inFig 1a and b The tilted
301 view image shows that the SiNWs are closely interconnected and held together in bundles (Fig 1c) This phenomenon is usual for nanosize materials caused by Van der Waals interaction As can be seen from the side view (Fig 1d), the nanowires are all straight, uniform and relatively vertical to the silicon substrate Their length can
be determined to be about 10 mm The EDX spectrum of the SiNWs after treatment with HNO3compared with that
of as-prepared samples reveals only one strong peak corresponding to silicon, which indicates that silver was removed completely (Fig 2a and b) Further structural characterization of the SiNWs was performed with TEM and HRTEM.Fig 3shows a typical individual nanowire
Fig 1 SEM images of as-prepared and treated SiNWs (a), (b) As-prepared SiNWs capped with silver (the dendritic structure of a silver cap); (c), (d) tilted
301 view and side view of post-deposition-treated SiNWs, respectively.
Trang 340 nm wide and 10 mm in length No metal particles were
observed at the side, top or bottom of the wires The top
left inset in Fig 3 shows the selected-area electron
diffraction pattern (SAED) of the corresponding nanowire with the electron beam parallel to the [310] zone axis, which proves its silicon crystalline nature SAED and HRTEM confirm that the length direction of the nanowire is along [0 0 1] The interplanar spacing between the visible fringer is 0.28 nm, corresponding to the (0 0 2) plane of silicon There
is a thin amorphous layer sheathing the crystalline core of the SiNW (bottom right inset inFig 3), which is identified
to be amorphous silicon oxide (SiOx) resulting from surface oxidation
The formation mechanism of aligned SiNWs arrays can
be understood as being a self-assembly metal nanoden-drite-assisted etching process with a localized microscopic nanoelectrochemical cell model [20] The deposited silver nanoclusters are uniformly distributed throughout the surface of the silicon wafer at the initial stage Self-assembly of silver nanoclusters to the dendrite structure and lack of coalescence to a compact grain film continue to cause etching of the silicon wafer along one direction in the AgNO3–HF solution At the end of the etching process, the wire structure is formed
The Raman spectra of bulk single-crystal silicon (c-Si) and SiNWs are shown in Fig 4 A Raman peak at 520.2 cm1with the full-width at half-maximum (FWHM)
of 4.6 cm1 can be seen in the Raman spectrum of c-Si, which can be attributed to the scattering of the first-order optical phonon (TO) of c-Si[21] In comparison, the first-order Raman peak of SiNWs is at 516.2 cm1 with an FWHM of 14.2 cm1(a downshift by 4 cm1) Its linewidth
is broadened and the line shape becomes increasingly asymmetric with an extended tail at low frequencies (Fig 4) Qualitatively, when the crystalline size decreases, momentum conservation will be relaxed ðqa0Þ and
Fig 2 EDXS of SiNW arrays (a) As-prepared sample; (b) sample after
treatment with HNO 3
Fig 3 TEM image of an individual SiNW with the corresponding SAED pattern (top left) in the inset The bottom right inset shows an HRTEM image
of a SiNW.
Trang 4Raman-active modes will not be limited to being at the
center of the Brillouin zone (G point) The smaller the
crystalline grain, the larger the frequency shifts and the
more asymmetric and the broader the peak becomes This
feature has been confirmed by experiments on
nano-crystalline silicon [22]and porous silicon [23] and SiNWs
fabricated by thermal evaporation [12] According to the
theoretical model proposed by Richter et al [24] and
Campbell et al.[25], the first-order Raman spectrum can be
described by the following equation:
I ðoÞ ¼
Z d3q Cð0; qÞ 2
½o oðqÞ2þ G0
2
where the phonon wave vector q is expressed in units of
2p=a, the crystalline grain size L is in units of a, with a
being the lattice constant of silicon and o(q) represents the
phonon dispersion curve G0is the geometrical sum of the
inverse lifetime of zone center phonon and there is an
increase of the linewidth by phonon dispersion C is the
weighting function in reciprocal space, which can be chosen
by physical arguments only [4] Using this model, the
experiment data were fitted well by choosing it as
jCð0; qÞj2¼expðq2L2=4p2Þ with L ¼ 10 nm as shown in
Fig 4 In addition, about 10-times enhancement of
first-order Raman spectrum compared with that of c-Si was
observed, which is shown in the inset ofFig 4 Two effects
could lead to the enhancement of Raman intensity Firstly,
the transmitted excitation intensity into the material should
increase according to decreasing area fraction of the
remaining silicon after etching, ignoring losses to diffuse
scattering Secondly, the Raman backscatter traveling
toward the surface may have encountered the
nano-interstice surface between the SiNWs, suggesting another
enhancement factor for the light excitation[26]
It is well known that cold cathode field electron emission
is one of the most important and promising applications of
nanoscale tubes or wires with sharp tip arrays For field
electron emission devices, the desired tip diameter of the
SiNWs must be less than 100 nm, and the aspect ratio must
be higher than 10[16] In this work, SiNWs with an aspect ratio higher than 200 have been successfully synthesized The EMD method enables one to control the size and aspect ratio of SiNWs through deposition parameters [17–20] Fig 5shows the results of current density versus anode–cathode voltage (I–V) with the inset being the corresponding Fowler–Nordheim (FN) plot [ln ðJ=E2Þ versus 1=E] The FN plot shows that the measured data fit well to the linear relationship given by the following equation:
with two distinct regions, where A ¼ 1:54 106A eV V2,
B ¼ 6:83 103eV3=2Vmm1, J is the current density, b is the field enhancement factor, and F is the work function of emitter materials The local electric field (El) can be related
to b and the macroscopic field (Em) by El¼bEm¼bV =d, where V is the applied voltage, and d is the distance between the cathode and the anode The field enhancement factor can be determined from the slope of the FN plot, if the work function of the emitter is known Assuming that
F equals 4.15 eV for Si[27], b was calculated to be 1270 and
616 from the slope of the fitted lines (a) and (b), respectively Clearly, the field emission from SiNWs underlies a barrier tunneling quantum mechanical mechan-ism The change of the slope of the FN plot from the low-field to the high-low-field region indicates that the local low-field conditions at the top of the nanowire may not always be linearly dependent on the applied voltage during the whole voltage sweeping [28] After conducting the same electric field sweeping at least 10 times, no considerable change was found The turn-on field, which we define as the electric field required to detect a current density of 0.01 mA/cm2, is estimated to be about 12 V/mm This value is comparable with those for other field emitters including carbon nanotubes [29], diamonds [30] and SiNWs fabricated by the laser-ablation method [3], chemical vapor deposition [9], oxide-assisted growth [14], and vapor–liquid–solid
Fig 4 Raman spectra of signals normalized to the same peak height to
illustrate shift and asymmetric broadening The inset shows the signals
from bulk c-Si and SiNWs.
Fig 5 Emission current–voltage characteristics of SiNWs The inset shows Fowler–Nordheim plots of ln(J/E 2 ) versus (1/E).
Trang 5(VLS) method [31] As this rapid synthesis method is not
inherently area limited, possesses lower growth
tempera-ture and can be scaled up with the reaction vessel size and
may also be compatible with standard lithographic
processes, this kind of SiNW arrays have potential
application in field emission microelectronic devices
4 Conclusions
A relatively rapid (a growth rate greater than 3 nm/s)
and inexpensive method of fabricating large-area silver
cap-removed SiNWs has been demonstrated on the basis of
an electroless metal deposition technique The
single-crystal SiNWs show a high aspect ratio greater than 200
with an average diameter of 40 nm The phonon quantum
confinement effects of the SiNW arrays were confirmed
through the downshifting, broadening and asymmetry of
the first-order Raman peak A good field electron emission
for SiNWs was observed with a turn-on field of 12 V/mm at
a current density of 0.01 mA/cm2 This kind of SiNW
directly formed on a silicon substrate would be of
particular interest for integration in the current
silicon-technology-based vacuum electronic and optoelectronic
devices
Acknowledgements
This work was supported by the National Natural
Science Foundation of China under Grant No 60244003
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