Raman spectroscopy and field electron emission properties of aligned silicon nanowire arrays

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

Physica E 30 (2005) 169–173 Raman spectroscopy and field electron emission properties of aligned silicon nanowire arrays Chun Li a , Guojia Fang a,à , Su Sheng a , Zhiqiang Chen a , Jianbo Wang b , Shuang Ma a , Xingzhong Zhao a 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/cm 2 . These highly densified and ordered SiNW arrays can be expected to have favorable applications in vacuum electronic or optoelectronic devices. r 2005 Elsevier B.V. All rights reserved. PACS: 71.55.Cn; 85.45.Db; 78.30.Àj 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 emis sion [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 sp 3 -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. ARTICLE IN PRESS 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 à Corresponding author. Tel.: +86 27 87642784; fax: +86 27 68752569. E-mail address: gjfang@whu.edu.cn (G. Fang). 2. 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 au toclave 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% HNO 3 aqueous solution for 60 s. Finally, the as- prepared sampl es 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 wer e prepared by ultrasonicating the nanowi res 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 cm À1 . Field emission behavior was investigated using a diode structure with an anode–cathode spacing of 50 mm in a test chamber maintained at 10 À4 Pa. 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 an d SiNW arrays after being treated with an HNO 3 solution. The branched dendritic structure of silver on the top of SiNWs is shown in Fig. 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 HNO 3 compared 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 struc tural characterization of the SiNWs was performed with TEM and HRTEM. Fig. 3 shows a typical individual nanowire ARTICLE IN PRESS 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. C. Li et al. / Physica E 30 (2005) 169–173170 40 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 amorphou s layer sheathing the crystalline core of the SiNW (bottom right inset in Fig. 3), which is identified to be amorphous silicon oxide (SiO x ) 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 AgNO 3 –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 cm À1 with the full-width at half-maximum (FWHM) of 4.6 cm À1 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 Ram an peak of SiNWs is at 516.2 cm À1 with an FWHM of 14.2 cm À1 (a downshift by 4 cm À1 ). 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 ARTICLE IN PRESS 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. C. Li et al. / Physica E 30 (2005) 169–173 171 Raman-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: IðoÞ¼ Z d 3 qCð0; qÞ     2 ½o À oðqÞ 2 þ G 0 2 ÀÁ 2 , (1) 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. G 0 is 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; qÞj 2 ¼ expðÀq 2 L 2 =4p 2 Þ 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 of Fig. 4. Two effe cts 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. 5 shows the results of current density versus anode–cathode voltage (I–V) with the inset being the corresponding Fowler–Nordheim (FN) plot [ln ðJ=E 2 Þ versus 1=E]. The FN plot shows that the measured data fit well to the linear relationship given by the following equation: lnðJ=E 2 l Þ¼lnðA=fÞÀBf 3=2 =E l , (2) with two distinct regions , where A ¼ 1:54  10 À6 AeVV À2 , B ¼ 6:83  10 3 eV À3=2 Vmm À1 , J is the current density, b is the field enhancement factor, and F is the work function of emitter materials. The local electric field (E l ) can be related to b and the macroscopic field (E m )byE l ¼ bE m ¼ bV =d, where V is the applied voltage, and d is the distance between the cathode and the anode. The field enhancement factor can be de termined 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-field region indicates that the local 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 cond ucting the same electric field sweeping at least 10 times, no considerable change was found. The turn-on field, which we de fine as the electric field required to detect a current density of 0.01 mA/cm 2 ,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 ARTICLE IN PRESS 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). C. Li et al. / Physica E 30 (2005) 169–173172 (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. 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