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6 January 1999 Ž. Chemical Physics Letters 299 1999 237–242 Si nanowires grown from silicon oxide N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, I. Bello, S.T. Lee ) Center of Super Diamond and AdÕanced Films, Department of Physics and Materials Science, The City UniÕersity of Hong Kong, Hong Kong, China Received 10 August 1998 Abstract Bulk-quantity Si nanowires have been synthesized by thermal evaporation of a powder mixture of silicon and SiO . 2 Transmission electron microscopy showed that, at the initial nucleation stage, silicon monoxide vapor was generated from the powder mixture and condensed on the substrate. Si nanoparticles were precipitated and surrounded by shells of silicon oxide. The Si nanowire nucleus consisted of a polycrystalline Si core with a high density of defects and a silicon oxide shell. The growth mechanism was proposed to be closely related to the defect structure and silicon monoxide. q 1999 Elsevier Science B.V. All rights reserved. Nanometer-wide silicon wires have attracted much attention in recent years because of their potential for applications in the field of microelectronics. One of the challenging issues has been the synthesis of this one-dimensional form of nanowires on large scales. Since the successful growth of Si whiskers by the Ž. wx vapor–liquid–solid VLS method 1,2 , many ef- forts have been made to improve the synthesis of Si nanowires by employing different techniques, such as the photolithography technique combined with wx etching 3–5 and scanning tunneling microscopy wx 6,7 . For the VLS method, Au had to be used and this caused contamination. The diameters of Si whiskers obtained from VLS were determined by the size of Au particles. Other techniques were compli- cated and could not produce bulk quantities of Si nanowires. ) Corresponding author. E-mail: apannale@cityu.edu.hk Recently, Si nanowires have been successfully synthesized by a novel method of laser ablation of wx metal-containing Si targets 8–11 . Previous investi- wx gations 8,9 have shown that metal or metal-silicide nanoparticles acted as the critical catalyst during the deposition assisted by laser ablation. For example, Fe could form Fe-silicides at high temperatures of 12008C. A growth mechanism of Si wires has been wx ascribed to the VLS reaction 8,9 . However, a dif- ferent model has been proposed which is supported by the experiment which showed that metal catalyst were not observed in Si nanowires even when metals wx were mixed in the target 10 . Moreover, it was discovered that metal was not necessary for Si nanowire synthesis by laser ablation. Instead, SiO 2 was the special and effective catalyst which largely wx enhanced Si nanowire growth 12 . High-resolution Ž. transmission electron microscopy HRTEM investi- gations have shown that high-density defects and silicon oxide outer layers play important roles for wx nanowire growth 12 . In this Letter, we report that 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž. PII: S0009-2614 98 01228-7 () N. Wang et al.rChemical Physics Letters 299 1999 237–242238 bulk-quantity Si nanowires were synthesized by ther- mal evaporation of a highly pure Si powder mixed with SiO . Observations of Si nanowire nucleation 2 and growth morphology by transmission electron Ž. microscopy TEM are documented. By combining these observations with the results of a Raman study, we discuss the growth mechanisms. Si nanowires can be synthesized by laser ablation of a powder mixture of silicon and SiO in an 2 Ž evacuated quartz tube in an Ar atmosphere 500 . wx Torr 12 . However, in the present work, without the assistance of laser ablation, Si nanowires were syn- thesized by simple thermal evaporation at 12008C. The solid source was highly pure Si powder mixed Ž with about 70 wt% SiO all materials were from 2 . Goodfellow, purity 99.99% . The temperature around the quartz tube where the nanowire grew was about 9308C. After 12 h of thermal evaporation, Si Ž. Ž. Ž. Fig. 1. a TEM image showing the morphology of Si nanowires synthesized by the evaporation method. b – d Nucleation stage of the Si nanowires. () N. Wang et al.rChemical Physics Letters 299 1999 237–242 239 Ž. Fig. 1 continued . Ž. nanowire product sponge-like, dark red in color formed on the inside wall of the quartz tube. To collect Si nanowire nuclei, a Mo grid was placed in the region of the quartz tube where the nanowires grew. Some Si nanowires nucleated and grew on the grid. The Mo grid was directly observed in Philips CM200FEG transmission electron microscope work- ing under 200 kV. Raman measurements were car- ried out using with a Renishaw 2000 micro-Raman system. Fig. 1a shows the typical morphology of as-grown Ž Si nanowires. The nanowires major component in .Ž. the product are extremely long ) 10 mm with uniform diameters and smooth surfaces. Si nanopar- ticles are found to coexist with the nanowires. A striking feature is that Si nanoparticles appear in the form of chain. Si nanowire nucleation on the Mo grid is shown in Fig. 1b. In initial stage, Si nanopar- ticles were formed as identified by electron diffrac- tion. Most nanoparticles piled up on the substrate. Ž Notably, some favorable particles nuclei of . nanowires stood alone and underwent faster growth since their preferable growth direction was normal to Ž. the surface of the substrate see Fig. 1b–d . There was no detectable metal catalyst or impurity formed on the tips of the nanowire nuclei. Each nucleus simply consisted of a crystalline Si core and an amorphous outer layer. The chemical composition of the nuclei was determined by electron energy disper- Ž. sive spectroscopy EDS . Only silicon and oxygen were detected which indicated that the amorphous outer layer should have been silicon oxide. The Si () N. Wang et al.rChemical Physics Letters 299 1999 237–242240 crystalline core contained a high density of defects. Most of the defects showed their contrast along the growth axis of the nucleus. These defects were quite Ž similar to the planar defects stacking faults and ²:. micro-twins along the axis of Si nanowire in 112 wx observed in Si nanowires in our previous work 10 . It is believed that silicon oxide plays an important role in nanowire growth. We investigated the native silicon oxide on single Si crystal surfaces. The oxide thickness was only 2–3 monolayers. However, the oxide shells of nanowires were quite thick. We Ž. observed that the shell thickness up to 3 nm gener- wx ally depended on the diameter of the nanowire 10 . In the present experiment, the vapor materials gener- ated from the mixture of silicon and SiO at 12008C 2 consisted mainly of SiO, with little silicon. This was supported by the observation that the material con- densed on the water-cooled Cu finger was Si O xy Ž. xs0.51, ys 0.49 as determined by EDS. This chemical composition was reliable since the vapor phase was quenched on the cool finger. Silicon Ž. monoxide SiO is an amorphous semiconductor of high resistivity which can easily be generated from Ž. powder mixtures especially in equimolar mixtures wx of silicon and SiO by heating 13–15 . TEM inves- 2 tigations confirmed the amorphous structure of the SiO deposited on the Cu finger surface. By heating the SiO sample in TEM, silicon precipitation was Ž. observed see Fig. 2a . Such precipitation of Si nanoparticles from annealed SiO is quite well known wx 15 . According to the above observations, we propose that the growth mechanism is silicon oxide assisted. Ž. The vapor phase of Si O x) 1 generated by ther- x mal evaporation is the key factor. The nucleation of nanoparticles is assumed to occur at the substrate by different decompositions of silicon oxide at the rela- tively low temperature of 9308C as shown below. Si O™ Si qSiO x)1 Ž. xxy1 and 2SiO™ SiqSiO . 2 These decompositions result in the precipitation of silicon nanoparticles, i.e. the nuclei of Si nanowires, clad by shells of silicon oxide as observed in Fig. 1b. The growth process may involve the following factors. The relatively thick Si O on nanowire tips x wx 12 acts as a catalyst. The SiO component of the 2 shell, which could be formed during decomposition of SiO in nanowire growth, retards the sideways growth of the nanowire. Defects, such as stacking faults in the nucleus tips, enhance the one-dimen- Ä4 sional growth. The 111 surface, which has the lowest surface energy among the surfaces in silicon, plays an important role during nanowire growth. Since surface energy is more important when the crystal size is reduced to the nanometer scale, the Ä4 appearance of 111 surfaces of the Si crystals paral- lel to the axes of the nanowires reduces the system energy. Combined, these factors determine the ²: growth direction of Si nanowires to be 112 . This proposed growth mechanism is supported by the results of Raman study as shown in Fig. 3a. The peak at 521 cm y1 is broad and strongly asymmetric compared to that from a single Si crystal. Such a feature could be due to the small size effect of Si wx nanocrystals or defects 11,16 since there were many nanoparticles in the product, as well as Si nanowires wx containing a high-density of defects 10,11 . In addi- tion, the presence of SiO shells also contributes to the asymmetry of the Raman peak. As shown in Fig. Ž 3a, the spectrum taken from SiO deposited on the . Cu finger contains a broad peak located at about 480 cm y1 . For comparison, Si nanowires which Ž were fully oxidized by annealing in the air white in . color were studied. No Raman scattering was de- Ž. tected see Fig. 3a . According to EDS measurement, the fully oxidized nanowires consisted mainly of SiO . 2 Ž. Fig. 3b shows strong photoluminescence PL of SiO at about 740 nm. The fully oxidized nanowire gives a weak PL peak at about 600 nm. The PL from Si nanowire product is weak and complicated. A typical PL spectrum from Si nanowires covers the range of 600–800 nm range. Clearly, the SiO and SiO components of the nanowires are the main 2 contributors to this spectrum. The proposed mechanism for nucleation and growth can predict some of the morphology of nanowires. For example, during the evaporation, Si O vapor was continually generated and nucleation x could occur with different crystalline orientation ei- ther on the side surfaces or tips of the nanowires. The former resulted in the forking of the nanowires Ž. observed frequently and the latter caused re-nuclea- () N. Wang et al.rChemical Physics Letters 299 1999 237–242 241 Ž. Ž. Fig. 2. a Nanoparticles precipitated by heating the SiO thin film. b HRTEM image of the Si nanoparticle chain. () N. Wang et al.rChemical Physics Letters 299 1999 237–242242 Ž. Fig. 3. a Raman spectra taken from the as-grown Si nanowires, Ž. SiO and fully oxidized Si nanowires. b PL spectra taken from the as-grown Si nanowires, SiO and fully oxidized Si nanowires. tion. The nuclei formed on the tips in an unfavorable growth direction could not grow fast and re-nuclea- tion occurred again. Such re-nucleation resulted in Ž. the formation of nanoparticle chains see Fig. 1 . HRTEM image taken from one of the chains pro- vided proof for this growth mechanism. As shown in Fig. 2b, the silicon particles in the chain have differ- ent orientations and most of the particles are not wx aligned with their 112 orientations parallel to the growth direction. In conclusion, bulk-quantity Si nanowires have been synthesized by thermal evaporation of mixture of silicon and SiO powder. Si oxide vapor gener- 2 ated from the powder mixture condensed on the substrate and then decomposed, forming Si nanopar- Ž. ticles nuclei of nanowires . A Si nanowire nucleus consisted of a polycrystalline Si core with a high density of defects and a silicon oxide shell. The growth mechanism was proposed to be closely re- lated to the defect structure of Si crystal cores and SiO. Acknowledgements This work was financially supported in part by the Research Grants Council of Hong Kong and the Strategic Research Grants of the City University of Hong Kong. References wx Ž. 1 R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 1964 89. wx Ž. 2 E.I. Givargizov, J. Cryst. 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Setiowati, S. Kimura, J. Am. Ceramic Soc. 80 1997 757. wx Ž. 15 G. Hass, C.D. Salzberg, J. Opt. Soc. Am. 44 1954 181. wx Ž. 16 G. Nolsson, G. Nelin, Phys. Rev. B 6 1972 3777. . taken from the as -grown Si nanowires, Ž. SiO and fully oxidized Si nanowires. b PL spectra taken from the as -grown Si nanowires, SiO and fully oxidized Si nanowires. tion decompositions of silicon oxide at the rela- tively low temperature of 9308C as shown below. Si O™ Si qSiO x)1 Ž. xxy1 and 2SiO™ SiqSiO . 2 These decompositions