Growth of amorphous silicon nanowires

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Growth of amorphous silicon nanowires Z.Q. Liu, W.Y. Zhou, L.F. Sun, D.S. Tang, X.P. Zou, Y.B. Li, C.Y. Wang, G. Wang, S.S. Xie * Group 412, Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-32#, Beijing 100080, People's Republic of China Received 16 February 2001; in ®nal form 4 April 2001 Abstract We have grown vertically aligned amorphous silicon nanowires on Au±Pd co-deposition silicon oxide substrate by thermal chemical vapor deposition using SiH 4 gas at 800°C. The diameter of silicon nanowires is in the range 10±50 nm and the length is about 1 lm. Transmission electron microscopy (TEM) observations show that the grown silicon nanowires are of an amorphous state and some of nanowires appear to bifurcate in the vertically growth process. The eect of H 2 gas etchings on the catalytic size and the eect of catalytic size on the formation of the vertical growth nanowires are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Nano-materials are attracting much attention because of the electronic, magnetic, optical, bio- logical, and chemical characteristics they have that are not obtained with conventional materials. Among these low-dimensional materials, one- dimensional materials, such as nanotubes [1±4], semiconductor nanowires [5±10] and metal nano- wires [11] have been of recent heightened interest because these materials oer fundamental scien- ti®c opportunities for investigating the in¯uence of size and shape with respect to optical, electronic, and mechanical properties. For silicon, it is promising to emit visible light by reducing its dimension, in which the motion of carriers is con®ned, causing a possible transformation of the electronic band structure from indirect band gap to direct band gap. This has stimulated intensive interest in preparing silicon nanowires. In addi- tion, if such wires can be ordered and assembled into appropriate architectural environment, then a host of nanoelectronic applications can be envi- sioned. Until now controlling the size and length of these synthesized nanomaterials have been practical problems, which seriously restrict the future applications. To date, silicon nanowires have been successfully prepared through dierent ways, such as excimer laser ablation [5,6], chemical vapor deposition (CVD) [12,13], stress limited oxidation [14,15]. However, all of these previous studies have got crystalline state silicon nanowires with a thin oxide outer layer. Amorphous state silicon nanowires have been reported very slightly. Recently, Yan et al. [16] have prepared amor- phous silicon nanowires via a solid±liquid-solid mechanism. Among the above-mentioned growth tech- niques, the CVD process may have its special bene®ts that one can more easily control the 29 June 2001 Chemical Physics Letters 341 (2001) 523±528 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +86-10-8264-9531. E-mail address: ssxie@aphy.iphy.ac.cn (S.S. Xie). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 513-9 nucleation site as well as dierentiate between growth and tail ends of the nanowires. In this Letter, we use chemical vapor deposition of silane to prepare silicon nanowires. Our research group had previously reported the growth amorphous silicon nanowires on AuPd/SiO 2 /Si substrates by using thermal chemical vapor deposition [17]. We extend the work of H 2 gas etchings to the nano- wires growth. The eect of H 2 gas etchings on the catalytic size and the eect of the catalytic particle size on the diameter and alignment of amorphous silicon nanowires are discussed. Bifurcate phe- nomena morphology was found in vertical growth process. 2. Experimental The substrates used in our experiments were 8 X cm n-type Si(1 0 0) wafers with an oxide layer about 60 nm in thickness. They were ultrasoni- cally stirred for 30 min in acetone solution to clean their surfaces. The cleaned substrates were then deposited with Au±Pd ®lm about 0.5 and 1 min under 10 À1 Torr at 100 V and 20 mA by using ion sputter ®lms deposition system (Hitachi, E-1010). The thickness of the Au±Pd alloy ®lm was estimated as approximately 5 nm and 10 nm, respectively. Put them into a tube furnace that has been described elsewhere [18]. Prior to deposition, the Au±Pd alloy-coated substrates were pretreated in ¯oating H 2 gas for approximately 180 min. The purpose of this pretreatment was to break the smooth Au±Pd alloy ®lms into discrete islands and then control the size of the catalytic size. After pretreatment, the silane gas was introduced into the reactor for the start of deposition stage. The pretreatment and deposition parameters are listed in Table 1. The as-grown silicon nanowires were examined by a ®eld-emission scanning electron microscope (SEM; S-4200, Hitachi), and energy-dispersive X-ray (EDX) spectra were recorded by SiLi detector attached to SEM. A transmission electron microscope (TEM; JEOL JEM-200 CX at 200 kV) was used to characterize the structures of silicon nanowires. 3. Results and discussio n Fig. 1a±d shows the SEM images of the silicon nanowires grown on a substrate, which was de- posited with AuPd alloy as catalyst for 10 nm. Fig. 1a shows that large amounts of nanowires are formed, which are of a uniform length up to 2 micrometers. The growth rate of the nanowires is estimated to be ca. 15 nm/min. The diameter of the nanowires is about 40 nm. A lot of nanowires are assembling along the same direction and are not parallel to the surface of the substrate (see Fig. 1a,b). All of the nanowires are terminated by the nanoparticles with the diameter about 60±80 nm at their tips (see Fig. 1c). Some of the nanowires become curved near their tops. The EDX spectra taken from these nanoparticles showed the pres- ence of gold, palladium, oxygen and silicon (0.6, 0.3, 26.6 and 72.5 at%, respectively). Compared with the nanoparticles, the nanowires are com- posed of silicon and a small amount of oxygen, indicating that no catalytic elements exist in the nanowires. Top view of the nanowires is shown in Fig. 1d. An interesting phenomenon is that bifur- cation of the nanowires was found at the tail of the nanowires (see arrowhead in Fig. 1b,d) High-resolution TEM was employed to detect the structure of the nanowires in detail. Using an ultrasonic treatment in alcohol solution for 30 min, Table 1 Pretreatment and depostion parameters Pretreatment Depostion Gas H 2 /He SiH 4 /He Pressure (Torr) 150 150 Flow rate (Sccm) 10/100 10/100 Temperature (°C) 800 Æ 10 800 Æ 10 Time (min) 180 60 524 Z.Q. Liu et al. / Chemical Physics Letters 341 (2001) 523±528 the nanowires were separated from the substrate. And then we dropped the solution to the copper grid for TEM observation. From Fig. 2a, we can see that the nanowires are of a uniform diameter about 40 nm and have smooth surfaces. Further- more, TEM reveals that the nanowires are in a perfect amorphous state. The selected-area elec- tron diraction of silicon nanowires shown in the inset of Fig. 2a con®rms this point. In our TEM observation, we also found a bi- furcation phenomenon of amorphous silicon nanowires. Fig. 2b±d shows the dierent bifurca- tion morphologies of the nanowires. `Y' shape (see Fig. 2b) and `T' shape (see Fig. 2c) junctions are found. The smooth curvatures associated with the junctions suggest that these structures were actu- ally formed during the growth process, rather than during TEM observations. More complex inter- connections containing metal catalyst are also found (see Fig. 2d). In order to see the eect of the catalytic particle size on the diameter and alignment of amorphous silicon nanowires, we also use the substrate, which has been sputtered with Au±Pd alloy thickness of about 5 nm. The growth process is the same as 10 nm Au±Pd deposited substrate. Fig. 3a±b shows the SEM images of the nanowires grown on 5 nm Au±Pd deposited substrate. Comparing with the Fig. 1. SEM images of the silicon nanowires grown on a substrate deposited with AuPd alloy as catalyst for 1 min. (a) Low-mag- ni®cation images of silicon nanowires. (b) and (c) A magni®ed view of (a). (d) Top view of the vertically aligned silicon nanowires. Z.Q. Liu et al. / Chemical Physics Letters 341 (2001) 523±528 525 thicker Au±Pd alloy ®lms, we can see that the di- ameter of the nanowires is 30 nm. The alignment of the nanowires is improved. Compared with our previously work [17], we got amorphous aligned silicon nanowires instead of the randomly distributed nanowires. The only dierence in our experiment is that the substrates were pretreated by H 2 etching before the nano- wires growth. For comparison, the substrate which only annealed in ¯uent He without H 2 etching was also studied. Fig. 4a show the SEM image of the unetching substrate with 5 nm Au±Pd ®lms. From Fig. 4 we can see that AuPd ®lms on the substrate have broken up to form a large amount of nano- particles. The nanoparticles size are uniform and are of about 30 nm diameter. However, when the substrate was pretreated with ¯uent H 2 under the same condition, the diameter of the nanoparticles (see Fig. 4b) become smaller than that of the nanoparticles shown in Fig. 4a. It con®rmed that H 2 etching really plays a positive role in control- ling and deducing the catalytic size. The decreasing size of the catalyst may result in the aligned growth of the nanowires. We think that vapor±liquid±solid (VLS) mech- anism accounts for the amorphous state silicon nanowires growth in our experiment. The mecha- nism has been put forward by Wagner and Ellis Fig. 3. SEM images of the nanowires grown on 5 nm Au±Pd deposited substrate. (a) Low-magni®cation images of silicon nanowires. (b) A magni®ed image of (a). Fig. 2. TEM images of the silicon nanowires dispersed on a carbon-coated copper microgrid. (a) A low-resolution TEM image of the nanowires, the inset is the selected-area electron diraction of the nanowires. (b)±(d) The bifurcation structure of the silicon nanowires. 526 Z.Q. Liu et al. / Chemical Physics Letters 341 (2001) 523±528 [19] It is well know that the impurity agent, which plays an important role in the formation of pref- erential growth ®ber, will be found at the tip of the obtained nanowires. The SEM images (shown in Fig. 1) con®rm this point. We propose a model to explain the bifurcation growth process under VLS growth mechanism (see diagram in Fig. 5). During the aligned nanowires growth process, some of the nanowires will form kinks due to the weight of the catalyst on their tip. It may be easy for the melting catalytic nanoparticles on its top to meet together and coalesce to become a larger catalyst. From Fig. 1, we found that the size of catalyst on the tip of the nanowires become larger than that of the particles shown in Fig. 4b. In the end, the bifur- cation growth is formed. 4. Conclusion Aligned amorphous silicon nanowires on a large scale of Au±Pd co-deposition silicon oxide substrate by thermal chemical vapor deposition were obtained. The catalytic particle size of Au±Pd catalysts decreases, the diameter of the nanowires decreases and the vertical alignment is enhanced. H 2 pretreatment before growth can deduce the catalytic nanoparticles sizes. There are bifurcation Fig. 5. Schematic diagrams of the bifurcation growth model. Fig. 4. SEM images of the annealing substrates with 5 nm Au±Pd ®lms at 800°C for 180 min. (a) without H 2 gas etching, (b) with H 2 gas etching. Z.Q. Liu et al. / Chemical Physics Letters 341 (2001) 523±528 527 growth phenomena in this kind of aligned growth process. Acknowledgements This work is supported in part by the National Natural Science Foundation of China. References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] O. Stephan, P.M. Ajayan, C. Colliex, P. Redich, J.M. Lambert, P. Bernier, P. Le®n, Science 266 (1994) 1683. 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In our TEM observation, we also found a bi- furcation phenomenon of amorphous silicon nanowires.