Si nanowires synthesized with cu catalyst

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Si nanowires synthesized with Cu catalyst Y. Yao ⁎ , S. Fan Tsinghua-Foxconn Nanotechnology Research Center, Department of Physics, Tsinghua University, Beijing 100084, PR China Received 15 March 2006; accepted 4 April 2006 Available online 6 May 2006 Abstract The metal copper which is a newly developed interconnecting material for integrated circuit (IC) has been used as the catalyst to catalyze the formation of the Si nanowires in high temperature tube furnace. The growth direction of the straight Si nanowires is b111N and the polyhedron η″- Cu 3 Si alloy is on the tip of the Si nanowires. The synthesis temperature of the Si nanowires is 500 °C. Such a low temperature implies that the vapor–solid (VS) should be the growth method. The cheap Cu catalyst is favorable for the mass synthesis of Si nanowires. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Deposition; Catalysts 1. Introduction Because of the importance of silicon in the microelectronic industry, 1D silicon nanostructure, Si nanowires, has attracted many research interests. The p–n junction devices have been fabricated based on the p-doping and n-doping Si nanowires and Si nanowire filed effect transistors (FET) have showed better performance than the planar metal-oxide-semiconductor FET (MOSFET) [1,2]. Nanosensors based on the Si nanowires FET structure have been fabricated [3,4]. Because of the limited dimension, the quantum confinement effect of the Si nanowires has been observed in the photoluminescence (PL) meas urement [5]. The polarization of the PL spectrum for Si nanowires has also been reported [6,7]. Vapor–liquid–solid (VLS) [8,9] is an important way to syn- thesize Si nanowires. It has been reported that Au catalyst particles can limit the diameter of the Si nanowires and usually induct the aligned Si nanowires arrays on the silicon substrate [10–13]. However, it is not economical to synthesize the mass of Si nanowires with Au catalyst because of the expensive value of Au particles or Au-gel. Some other cheaper metals, such as Fe [14] and Ni [15], has been selected to catalyze the Si nanowires growth, but it is not favorable to introduce such metals into the IC processing since they are “toxic” for the semiconductor device. Cu is a newly developed interconnection material for silicon chip because of its better performance than aluminum in the lower resistivity which means the little time delay and the better reliability against the degradation by the metal migration at high current [16]. According the phase diagram of CuSi alloy [17] (Fig. 1), it is reasonable to expect Cu as an appropriate catalyst for the growth of Si nanowire. To date there are no reports about the copper catalyzing Si nanowires. In this paper, the growth condition and the morphology of Si nanowires catalyzed by copper particles are described. 2. Experimental Nanocluster deposition system (ND 60, Oxford Applied Re- search) has been used to prepare the Cu catalyst on the Si wafer. After being sputtered from the Cu target, only the Cu nano- particles with selected diameter could pass through the quad- rupole mass spectrometry in the ND 60 and deposit on the b111N Si wafer. The sputtering power was about 120 W. The selected mass was 147,074 a.m.u., corresponding diameter was about 4 nm. Deposition time was 20 min. The Si wafer covered with Cu nanoparticles was transferred into the alumina tube and heated in the horizontal furnace. The temperature increased from room temperature to 500 °C in 20 min with 100 sccm Ar flow and the pressure was about 8 Torr. Then the pressure and temperature were kept for 30 min with the introduction of 20 sccm SiH 4 flow for the Si nanowires growth. The products Materials Letters 61 (2007) 177 – 181 www.elsevier.com/locate/matlet ⁎ Corresponding author. E-mail address: y-yao@mail.tsinghua.edu.cn (Y. Yao). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.045 were characterized with EF-SEM (Sirion 200, FEI) and high resolution TEM (Tecnai G2 F20, FEI). 3. Results and discussion Fig. 2a shows the SEM image of the Cu clusters deposited on the Si wafer. The diameter of most Cu clusters is about 5 nm (Fig. 2b), almost the same as the expected 4 nm. The size distribution is relatively uni- form due to the mass filter in the ND 60. To investigate the diameter variation of the catalyst during heating, the Si wafer covered with Cu clusters has been heated on 500 °C without any gas feeding. Fig. 2c depicts the diameter distribution of the after-heated Cu catalyst. The Cu clusters have aggregated into the larger nanoparticles and the diameter distribution becomes wider. After being heated in the high temperature tube furnace with SiH 4 feeding, the surface of the Si wafer changed to light yellow. SEM image (Fig. 2d) of the after-grown Si wafer demon- strates that there are many thin and straight nanowires covering the surface of the Si wafer. The nanowires prolong several micrometers randomly. In the high magnification image (Fig. 2e), a small spot can be observed on tip of the straight nanowire and the diameter of the tip is as the same as the nanowires. The Si nanowires have been scraped from the Si wafer and moved to Cu grid for TEM characterization. Fig. 3a displays the low mag- nification TEM image of the Si nanowires. Some silicon particles are mixed with the Si nanowires. A straight nanowire 30 nm wide, a typical diameter for the thin Si nanowires is shown in Fig. 3b. There is a 4 nm thick amorphous layer covering the nanowire. The high resolution TEM image (down image inserted in Fig. 3b) reveals that the straight Si nanowire is well crystalline. Diffractogram patterns, a fast Fourier transformation (FFT) from the high resolution image, indicate that the growth direction of the Si nanowires is along b111N (up image inserted in Fig. 3b). The lattice distance along the growth direction of the nanowires is about 3.14 Å, which well agrees with the distance between Si {111} facets. However, the contrast variation on the nanowires in the low magnification TEM image (Fig. 3b) indicates that there should be some defects in the Si nanowires. As disclosed in Fig. 3c, the {111} stack faults and micro-twin boundaries are the dominated defects in the Si nanowires. There is a polyhedron dark tip with the flat facets on the straight Si nanowire. Fig. 3d shows the clear lattices contrast of both Si nanowire and the dark tip. The size of the tip is as large as the diameter of the Si nanowires. It is different with the preview reports about the Au catalyzing Si nanowires, in which the tips are ball-like Au particles [1– 3]. Fig. 3e is the diffractogram patterns of the interface between the tip and the nanowire. Two groups of patterns can be distinguished: one should be indexed as the diffraction patterns of Si (0 1 ¯ 1 ¯ )* reversal plane, another could be ascribed to orthorhombic η″-Cu 3 Si (1 ¯ , 19, 0)* reversal plane [18]. The diffractogram spots are indexed in Fig. 3e with solid and dashed lines. The {111} facet of Si nanowires is almost parallel to the {003} facet of the η″-Cu 3 Si. The diffractogram patterns of the alloy tip are also displayed in Fig. 3f and the sharp spots prove that the tip is the single crystal η″-Cu 3 Si alloy. This result confirms that the copper-rich η″-CuSi alloy should be formed when the alloy liquid is cooled down to the room temperature (Fig. 1). An interesting result that should be emphasized is that the growth temperature of the Si nanowires is 500 °C, much lower than the eutectic temperature 802 °C. It means that the VLS mechanism should not occur during the growth. The formation of Si nanowires may be ascribed to the vapor–solid (VS) growth mechanism. Under the frame- work of VS mechanism, the decomposed Si from SiH 4 deposits on the surface of Cu nanoparticles and forms solid η′-CuSi alloy (between 467 °C and 558 °C). The Si diffuses in the solid–solution alloy and will Fig. 1. Cu–Si phase diagram [17]. 178 Y. Yao, S. Fan / Materials Letters 61 (2007) 177–181 separate from the solid alloy to form the Si nanowires when the concentration is supersaturated. The diameter of the Si nanowires is similar to the size of the CuSi alloy nanoparticles. So the tip of the Si nanowires is polyhedron, not the ball-like tip. 4. Conclusion The Si nanowires could be grown with Cu catalyst. The diameter of the Cu nanoparticles could affect the size and the quantity of the nanowires. The growth temperature is 500°C and the growth direction of the Si nanowires is b111N. TEM images indicate the well crystalline of the thin and straight Si nanowires with η″-Cu 3 Si alloy tips. VS growth mechanism should be responsible for the formation of Si nanowires. The cheap Cu catalyst and the low growth temperature are favorable to the mass synthesis of the Si nanowires. Acknowledgements Financial support from the National Natural Science Founda- tion of China (NNSFC 10334060) and National Basic Research Program of China (973 Program 2005CB623606) is gratefully acknowledged. The authors also thank Mr. Liguo Xu for his helpful assistant work. Fig. 2. The SEM images of a) the Cu nanoparticles on the surface of Si wafer (scale bar is 1 μm). b) The diameter distribution of the Cu nanoparticles. c) The diameter distribution of the after-heated Cu nanoparticles. d) The Si nanowires grown on the Si wafer (scale bar is 5 μm). e) The large magnification SEM images of a Si nanowire (scale bar is 1 μm). 179Y. Yao, S. Fan / Materials Letters 61 (2007) 177–181 Fig. 3. a) The low magnification TEM image of the Si nanowires. b) The TEM image of a straight Si nanowire. (The down inserted image is the high resolution image of the nanowires and the up inserted image is the corresponding diffractogram patterns.) c) The {111} stack faults and micro-twins in the straight Si nanowire. d) High resolution image of the tip. e) The diffractogram patterns of the interface between the tip and the nanowire. (The solid and dashed lines indicate the patterns from Si and η″-Cu 3 Si alloy, respectively.) f) The diffractogram patterns of the dark tip. 180 Y. Yao, S. Fan / Materials Letters 61 (2007) 177–181 References [1] Y. Cui, X. Duan, J. Hu, C.M. Lieber, J. Phys. Chem., B 104 (2000) 5213. [2] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [3] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [4] X.T. Zhou, J.Q. Hu, C.P. Li, D.D.D. Ma, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 369 (2003) 220. [5] S.Q. 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Massalski, Binary Alloy Phase Diagram, Second editionASM International, 1990. [18] J.K. Solberg, Acta Crystallogr., A 34 (1978) 684. 181Y. Yao, S. Fan / Materials Letters 61 (2007) 177–181 . Si nanowires synthesized with Cu catalyst Y. Yao ⁎ , S. Fan Tsinghua-Foxconn Nanotechnology Research Center, Department of Physics, Tsinghua University,. the straight Si nanowires is b111N and the polyhedron η″- Cu 3 Si alloy is on the tip of the Si nanowires. The synthesis temperature of the Si nanowires is