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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
Silicon nanowires grown on Si(1 0 0) substrates via thermal reactions with carbon nanoparticles S. Botti a, * , R. Ciardi a , R. Larciprete b,c , A. Goldoni b , L. Gregoratti b , B. Kaulich b , M. Kiskinova b a ENEA, UTS Tecnologie Fisiche Avanzate, Via Enrico Fermi 45, 00044 Frascati, Italy b Sincrotrone Trieste, S.S. 14 Km 163,5 in Area Science Park, 34012 Basovizza (TS), Italy c CNR-IMIP, Zona Industriale – 85050 Tito Scalo (PZ), Italy Received 4 November 2002; in final form 16 December 2002 Abstract The effect of thermal processing at 1050 °C of a dispersed film of carbon nanoparticles deposited on a Si substrate with a native SiO 2 layer has been studied by scanning electron microscopy and scanning photoelectron spectromi- croscopy. It has been found that the thermal processing results in formation of pyramidal-shaped defects of 2–7 lm with strongly reduced SiO 2 content with silicon wires of diameter ranging between 30 and 50 nm decorating the pyramid walls. The nucleation of the Si nanowires occurs via reduction of the native oxide layer by the nanosized carbon particles, without the need of metal catalysts and at temperatures relatively lower than that used in similar techniques. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction Silicon nanowires with diameter of several tens of nanometers and length of tens of micrometers, exhibit unusual quantum confinement effects and interesting electrical and optical properties with promising technological impact in the microelec- tronic field [1–7]. Several techniques have already been used for synthesis of silicon nanowires, such as vapour–liquid–solid growth (VLS) catalysed by a gold layer on Si [1], laser ablation of metal containing targets [7] and, most recently, Ni-as- sisted solid–liquid–solid process [3] and carbo- thermal reduction of Fe-catalysed SiO 2 particles [4]. Some of the proposed growth models show that the formation of Si nanowires does not re- quire the presence of a metal, because the top SiO 2 layer can play the role of a catalyst in the wire nucleation [2–5]. We have already shown that Si nanowires can be synthesised without metal catalyst by simple car- bothermal reduction of the native oxide layer on a Si(1 0 0) substrate, assisted by C nanoparticles (CNPs) deposited by laser pyrolysis [8,9]. Anneal- ing of the Si substrate covered by a dispersed layer of CNPs produces square-like pyramidal voids which act as nucleation sites for growth of Si nanowires with diameter of 40–50 nm. The Si nanowire production was favoured at low CNPs Chemical Physics Letters 371 (2003) 394–400 www.elsevier.com/locate/cplett * Corresponding author. Fax: +39-06-9400-5312. E-mail address: botti@frascati.enea.it (S. Botti). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00090-3 coverage, when the carbothermal reduction oc- curring at the CNPs/substrate interface is termi- nated after complete consumption of CNPs in contact with Si. On the contrary, when the CNP density is high, the excess of nanoparticles reorga- nise into self-assembled carbon nanotubes [10–12]. Here we used two laterally resolved techniques, scanning electron microscopy (SEM) and syn- chrotron radiation scanning photoelectron mi- croscopy (SPEM), to obtain deeper insight on the morphology, composition and the lateral distri- bution of the chemical species on the surface obtained under conditions that we believe to fa- vour Si nanowire formation, i.e., annealing of the low density CNPs/Si(1 0 0) interface at 1050 °C. 2. Experimental The CNPs were prepared by CO 2 laser-induced decomposition of acetylene–ethylene mixtures in a flow reactor [8,9]. The CNPs were amorphous and largely hydrogenated with a mean size of 50 Æ 30 nm. After synthesis, the CNPs were sprayed on the Si(1 0 0) substrates and resistively heated up to 1050 °Cat10 À6 atm. The annealed samples were analysed ex situ by SEM (Jeol 5400, resolution of 3 nm with an accelerating voltage of 30 kV) and SPEM. The SPEM measurements were carried out with the experimental station of the ESCA Mi- croscopy beamline [13,14] at the synchrotron radiation facility ELETTRA (Trieste, Italy). The SPEM uses a Fresnel zone plate lens to focus the photon beam into a submicrometer spot while the emitted photoelectrons are collected by an hemispherical analyser mounted at a grazing angle with respect to the incident beam and the sample normal. The system can operate in a photoelectron spectroscopy mode from a microspot and in im- aging mode. The images are obtained by scanning the sample with respect to the focused beam col- lecting simultaneously photoelectrons emitted from a selected elemental core level. All experi- ments reported here were carried out with a pho- ton energy of 500 eV, lateral resolution of 0.12 lm and overall energy resolution of 0.4 eV. The SPEM measurements were performed on the Ôas receivedÕ sample and after annealing in situ at 450 ° C. 3. Results Figs. 1a and b shows SEM images obtained after annealing a Si substrate covered with a low density CNPs at 1050 °C. They reveal the pres- ence of nanostructured filaments and tubules (with diameter ranging between 30 and 50 nm) inside square-like features (ÔsquaresÕ). These ÔsquaresÕ are the basal plane of void defects, often observed during carbonisation of Si surfaces [15–17]. They are hollow inverted pyramids (base at the interface, vertex in the substrate) formed by {1 1 1} Si planes and usually have dimension be- tween 2 and 7 lm. In the SPEM Si 2p maps (see Figs. 1c and d), obtained by collecting the pho- toelectrons corresponding to non-oxidised Si, the ÔsquaresÕ appear as brighter areas (consisting of four triangles of different grey level), which indi- cates that locally the native SiO 2 layer has been partially reduced. Fig. 1c also reveals higher density of defects on the right side of the Si map where they even overlap. This distribution reflects the initial non-uniform distribution of the pristine CNPs onto the surface. Although the nanostruc- tures formed inside the defects cannot be resolved by SPEM, their presence and composition can be elucidated from the chemical maps and the pho- toelectron spectra measured inside and outside the ÔsquaresÕ. The three left panels in Fig. 2a show raw maps obtained collecting the C 1s photoelectrons and the Si 2p photoelectrons emitted from the Si and SiO 2 states, respectively. In these maps topo- graphic artefacts obscure the real ÔconcentrationÕ contrast: because of the photoelectron detection geometry the emission from defect sides facing (opposite to) the electron analyser is enhanced (reduced). In fact, the contrast variations of the SiO 2 and C maps is very similar to the topographic maps, obtained by collecting the secondary elec- trons with kinetic energies negligibly affected by the Si or C core level emission and used for re- moval of the topographic artefacts from the im- ages [18]. The true C, Si and SiO 2 concentration maps (right panels in Fig. 2), obtained by dividing the raw maps to the topographic map, clearly manifest the higher Si content inside the ÔsquaresÕ, where the relative amount of SiO 2 and C is S. Botti et al. / Chemical Physics Letters 371 (2003) 394–400 395 reduced. The higher elemental Si signal can be easily explained assuming formation of Si-based filaments formed via reduction of the oxide layer, which decorate the pyramid walls. This result is confirmed by the XPS spectra shown in Fig. 2b. The Si 2p spectra from both regions contain a SiO 2 component (the broad band at 103.6 eV, shifted by $4 eV with respect to Si 2p 3=2 (99.3 eV) and Si 2p 1=2 (99.9 eV) peaks), which derives from the native oxide layer terminating the Si substrate. The major difference is that inside the ÔsquaresÕ the SiO 2 =Si intensity ratio is $5 times smaller than that of the surrounding areas, where the SiO 2 component is dominant. The C 1s spectra taken inside and outside the pyramidal voids exhibit similar lineshapes peaked at 284.7 eV and a bit lower intensity inside (see the processed C map), whereas a much higher C concentration should be detected if the filaments are not Si-based but originate from the CNPs as residuals or by-prod- ucts. In fact, as already reported, much higher CNP density is required for self-organisation of these particles into carbon nanotubes [10–12]. The shape of the C 1s spectra change only slightly upon sample heating to 450 °C which in- dicates that the detected C cannot be related to contaminants adsorbed upon exposure to atmo- sphere, but is a constituent of the near-surface layer. The BE position of the C 1s peak covers the energy region corresponding to sp 3 and sp 2 hy- bridised C emitting at 285 and 284.3 eV [19], and the peaks measured both inside and outside the pyramids appear intermediate between sp 3 and sp 2 hybridisation, indicating the presence of poorly organised carbon species. This C 1s energy posi- tion excludes the formation of stoichiometric SiC, which has a component at a much lower BE (282.6 eV) [20]. This is in accordance with the Si 2p spectrum, where the SiC component should appear at 100.4 eV [20]. Fig. 1. Pyramidal-shaped voids formed at the CNPs/Si(1 0 0) interface annealed at 1050 °C. (a) SEM image of pyramidal void and (b) magnified view of silicon nanowires in the void. (c) (38:5 Â 38:5 lm 2 ) SPEM maps taken on the Si 2p peak and (d) a magnification of the same region (19 Â 19 lm 2 ) measured with higher lateral resolution. 396 S. Botti et al. / Chemical Physics Letters 371 (2003) 394–400 The Si 2p image in Fig. 3 reveals the growth of a relatively large structure, protruding out of the merged pyramidal voids. This image is taken in a sample region with a higher density of ÔsquaresÕ, which coalesce in disordered patterns, resulting in round voids and occasionally microstructures which stick out of the ÔsquaresÕ. The spectrum of this wire-like feature has a rather prominent Si component at 99.3 eV, whereas the C 1s intensity is further reduced compared to the spectrum (dashed line) taken inside the voids shown in Fig. 2. This confirms that the structures standing out of the walls are almost C-free and that the detected car- bon has mostly contamination origin. Fig. 4 shows the SEM image of a sample ob- tained by increasing the annealing time, showing a dense film of silicon nanowires. The wires have smooth surfaces and exhibit bends and kinks. The newly formed wires adhere to the ones previously formed and they mechanically twist or knit to- gether. 4. Discussion The SEM and SPEM analysis of the samples annealed for a short time (Figs. 1 and 2) demon- strates that the formation of the pyramidal-shaped defects plays a crucial role in nucleation of Si nanowires, which grow mostly along the pyramid walls. These defects are typical features appearing on the Si side of the SiC/Si interfaces [15–17] during Si carbonisation, their origin being attrib- uted to presence of oxygen and oxygen-related defects in bulk Si wafers [21,22]. However, microscopic imaging of the pyramidal defects at Fig. 2. (a) (12 Â 24 lm 2 ) SPEM maps taken collecting the Si 2p photoelectrons corresponding to elemental Si (Si) and SiO 2 (SiO2) components and C 1s photoelectrons (C). The left and right panels show the raw and processed images, respectively. (b) Si 2p and C 1s core level spectra taken inside (upper) and outside (lower) ÔsquaresÕ. S. Botti et al. / Chemical Physics Letters 371 (2003) 394–400 397 SiC/Si interfaces revealed mostly smooth walls without any evidence of nanosized structures protruding out of the surface [15,21,22]. Often the pyramidal defects are buried below a rather ho- mogeneous SiC layer which connects the voids and are considered as a source of Si atoms outdiffusing from the hole bottom to the surface. The results reported here do not show presence of any homogeneous film covering the pyramidal voids formed at the CNPs/Si(1 0 0) interface and the SPEM Si 2p spectra do not contain any spectroscopic feature related to SiC. Apparently under these reaction conditions (annealing at 1050 °C in low vacuum) the native SiO 2 layer cannot be completely removed. On one hand it acts as a barrier for the direct interaction between hydro- carbons and Si and formation of SiC, and on the other hand, being exposed to ÔactiveÕ C containing species, it is gradually reduced with production of Si-based nanostructures and volatile CO. This indicates that despite the similar dimensions (a few microns) and surface density ($10 6 cm À2 ) the defects observed in the present study should not be identical to that formed at Si/SiC interfaces [15–17]. In brief, the reaction mechanism can be de- scribed as follows. The first reaction step is partial reduction of the native silicon oxide layer accord- ing to the scheme, SiO 2 þ C ! SiO þ CO, where C stays for the highly active carbon nanoparticles. Due to the high reactivity of carbon particles, in this case the temperature required for carbother- Fig. 4. (a) SEM micrograph showing a high density of Si nanowires: (b) magnified view of the same sample. Fig. 3. Top: (19 Â 19 lm 2 ) Si 2p image of a region showing merged pyramidal voids and a spiral wire protruding out from them. Bottom: XPS spectra taken on the spiral wire and outside the pyramidal voids (see arrows). The dashed spectrum is taken inside one of the ÔsquaresÕ in Fig. 2 and is shown for the sake of comparison. 398 S. Botti et al. / Chemical Physics Letters 371 (2003) 394–400 mal reduction is lower than that usually required [4], leading to a sizeable SiO formation even at temperature as low as 1050 °C. The formation of voids seems to be triggered by the faster con- sumption of the oxide layer below the adsorbed nanoparticles. At the sites where severe carbo- thermal oxide reduction occurs, the developed surface stress might favour formation of cavities with lowest energy {1 1 1} symmetry planes. Ex- trusion of material during such collapse of the Si(1 0 0) surface lattice seems to be the process leading to nucleation of Si nanowires along the {1 1 1} plane walls. Disproportionation of Si monoxide seems to be the key reaction for the formation of Si nanowires by thermal evaporation of pure SiO [23] or mixed Si–SiO 2 [24] powders. Similarly, in our case a possible reaction route leading to the nucleation of the Si nanostructures is the decomposition of silicon monoxide 2SiO ! Si þ SiO 2 at 1050 °C followed by the precipitation of Si atoms which are expelled from the mixture as crystalline nanowires. A direct re- duction of the SiO 2 layer due to the reactive spe- cies, eventually present in the residual atmosphere of vacuum chamber, can be excluded as key step in the formation of Si nanowires since it would act homogeneously of the sample surface without originating the observed thinning of oxide layer in correspondence of the ÔsquaresÕ. The XPS spectra reported in Figs. 2 and 3 are a clear evidence that the Si nanowire are less oxidised than the regions surrounding the ÔsquaresÕ. After deconvolution of the Si 2p spectra we determined an oxide thickness of $6.5 and 20 AA for the ÔsquareÕ and outside areas, using the relationship d oxide ¼ k oxide cosðaÞ ln½1 þðI 4þ =I 0 Þ=c, where I 4þ and I 0 are the integrated intensities of the Si and Si oxide components, k is the Si 2p photoelectron mean-free path in SiO 2 (4.7 AA in our case), a is the acceptance angle of the ana- lyser with respect to the sample plane, and c ¼ 0:7 is the intensity ratio of pure SiO 2 and Si [25]. This result differs from the previously re- ported 3 nm thick SiO 2 layer surrounding the Si core of the Si nanowires grown with different techniques [5], which is assumed to have a crucial role in limiting the side growth of the nano- structure. 5. Conclusions Silicon nanowires can be successfully synthes- ised without metal catalysts by annealing the Si substrates at 1050 °C in the presence of active carbon nanoparticles. The Si nanowire formation is well described by the oxide-assisted growth model. The present results have proved that in the initial reaction stage the produced Si nanowires decorate the walls of pyramidal voids, formed during carbothermal reduction of the native SiO 2 layer covering Si substrates. Acknowledgements The authors wish to thank D. Lonza for his technical assistance and C. Cepek for fruitful dis- cussions. References [1] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1871. [2] Z. Zhang, X.H. Fan, L. Xu, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 337 (2001) 18. [3] H.F. Yan, Y.J. Xing, Q.L. Hang, D.P. Yu, Y.P. Wang, J. Xu, Z.H. Xi, S.Q. Feng, Chem. Phys. Lett. 323 (2000) 224. [4] X.C. Wu, W.H. Song, K.Y. Wang, T. Hu, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 336 (2001) 53. [5] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, I. Bello, S.T. Lee, Chem. Phys. Lett. 299 (1999) 237. [6] Y.F. Zhang, Y.H. Tang, H.Y. Peng, N. Wang, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 75 (1999) 1842. [7] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [8] S. Botti, R. Coppola, F. Gourbilleau, R. Rizk, J. Appl. Phys. 88 (2000) 3396. [9] S. Botti, A. Celeste, Appl. Phys. A 67 (1998) 421. [10] S. Botti, R. Ciardi, M.L. Terranova, V. Sessa, S. Piccirillo, M. Rossi, M. Vittori-Antisari, Appl. Phys. Lett. 80 (2002) 1441. [11] S. Botti, R. Ciardi, M.L. Terranova, V. Sessa, S. Piccirillo, M. Rossi, Chem. Phys. Lett. 355 (2002) 395. [12] R. Larciprete, S. Lizzit, S. Botti, C. Cepek, A. Goldoni, Phys. Rev. B 66 (2002) 121402R. [13] L. Casalis, W. Jark, M. Kiskinova, D. Lonza, P. Melpignano, D. Morris, R. Rosei, A. Savoia, A. Abrami, C. Fava, P. Furlan, R. Pugliese, D. Vivoda, G. Sandrin, S. Botti et al. / Chemical Physics Letters 371 (2003) 394–400 399 F.Q. Wei, S. Contarini, L. De Angelis, C. Gariazzo, P. Nataletti, G.R. Morrison, Rev. Sci. Instrum. 66 (1995) 4870. [14] L. Gregoratti, M. Marsi, M. Kiskinova, Synchrotron Rad. News 12 (1999) 40. [15] R. Scholz, U. G € oosele, E. Niemann, F. Wischmeyer, Appl. Phys. A 64 (1997) 115. [16] L O. Bjorketun, L. Hultman, I.P. Ivanov, Q. Wahab, J E. Sundgren, J. Cryst. Growth 182 (1997) 379. [17] J. Hofmann, S. Veprek, J. Heindl, J. Appl. Phys. 85 (1999) 2652. [18] S. Gunther, A. Kolmakov, J. Kovac, M. Kiskinova, Ultramicroscopy 75 (1998) 35. [19] J. D ııaz, G. Paolicelli, S. Ferrer, F. Comin, Phys. Rev. B 54 (1996) 8064. [20] P. Reinke, D. Rudmann, P. Oelhafen, Phys. Rev. B 61 (2000) 16967. [21] K. Chul, C. Park, J. Roh, K.S. Nahm, Y.H. Seo, J. Vac. Sci. Technol. A 19 (2001) 2636. [22] A. Leycuras, Appl. Phys. Lett. 70 (1997) 1533. [23] Z.W. Pan, Z.R. Dai, L. Xu, S.T. Lee, Z.L. Wang, J. Phys. Chem. B 105 (2001) 2507. [24] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev. B 58 (1988) R16024. [25] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J. Jarmoff, G. Hollinger, Phys. Rev. B 38 (1988) 6084. 400 S. Botti et al. / Chemical Physics Letters 371 (2003) 394–400 . Silicon nanowires grown on Si(1 0 0) substrates via thermal reactions with carbon nanoparticles S. Botti a, * , R. Ciardi a ,. - see front matter Ó 200 3 Elsevier Science B.V. All rights reserved. doi: 10. 101 6/S 000 9-2614 (03 )00 0 90- 3 coverage, when the carbothermal reduction oc- curring