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
Oriented silicon nanowires on silicon substrates from oxide-assisted growth and gold catalysts Yuan Yao a , Fanghua Li b , Shuit-Tong Lee a, * a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China b Institute of Physics and Center for Condensed Matter Physics, CAS, Beijing, China Received 17 February 2005; in final form 4 March 2005 Available online 25 March 2005 Abstract High-density, oriented silicon nanowires (SiNWs) array were fabricated on (0 0 1) silicon substrates by the oxide-assisted growth method assisted with Au catalyst in a hot filament chemical vapor deposition system. The yield of SiNWs was different with the synthesis temperature. Au particles were present at the tips of the SiNWs and limited the wire diameter. High resolution transmis- sion electron microscopy revealed the epitaxial SiNWs on the Si substrate. Ó 2005 Elsevier B.V. All rights reserved. One-dimensional (1D) nanomaterials have attracted intense interest because of their many unique properties not found in the bulk materials. Silicon nanowires (SiNWs) are a particularly important 1D nanomaterial because silicon is most widely used in electronic indus- try. Besides excellent electronic property, nanoscale sili- con materials also possess interesting optical [1] and field-emission [2] properties, which may be exploited for optoelectronic applications. There are various ways to synthesize SiNWs, includ- ing the classic metal-catalyti c vapor–liquid–solid (MC - VLS) [3] method, the simple thermal-evaporation oxide-assisted growth (OAG) method [4], and the solu- tion-grown method [5]. MC-VLS and OAG are widely used to fabricate SiNWs and they have their respective merits and shortcomings. The MC-VLS method offers better control of diameter [6] and patterning [7] of SiNWs but relatively low yield of production; whereas the OAG method can produce SiNWs in large quantities and without metal contamination, but with less control in wire pattern and diameter. Recently, SiNWs have been grown by combining the OAG method and metal catalysts [8]. Here, we report the successful fabrication of well-oriented array of SiNWs with controlled diame- ter using a similar approach. SiNWs were fabricated in a hot-filament chemical va- por deposition (CVD) system. (0 0 1) silicon substrates were etched in 5% hydrofluoric (HF) acid, sonicated in acetone, and resistively coated (thermal evaporation) with 1 nm Au film at a base pressure of 5 · 10 À6 mbar. A molybdenum foil was used to heat the silicon sub- strate, while another boat-like molybdenum foil con- taining SiO powder worked as a source heater. The base pressure of the CVD chamber was 10 À6 Torr. Dur- ing growth, the system was closed with no pumping and no carrier gas. The temperature of the SiO source was kept at 1300 °C, while that of the substrate varied from 600 to 1000 °C for different experiments. Thermocouples (mounted to the source and substrate) supplemented by infrared (IR) pyrometer were employ ed to measure the temperature. The pressure in the chamber increased to $10 À2 Torr after 30 min of growth. The processing details are shown in Fig. 1. 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.027 * Corresponding author. Fax: +852 2784 4696. E-mail address: apannale@cityu.edu.hk (S T. Lee). www.elsevier.com/locate/cplett Chemical Physics Letters 406 (2005) 381–385 After growth, the Si substrate surface showed differ- ent colors dependent on the temperature to which it was heated; varying from light yellow at 900 °C to gray at 600 °C. Figs. 2a,b are the scanning electron micros- copy (SEM) images of the silicon substrate surface after growth at 800 and 700 °C, respectively. It is clear that the surface was covered with high density, oriented array of aligned nanowires. The average length of the nanowires was less than 5 lm, while the density of the wires varied with the substrate or deposition tempera- ture. The largest density ($400/lm 2 ) of the nanowires was obtained at 700 °C, while the wire orientation ap- pears to be insensitive to the deposition temperature. No nanowires were observed if the deposition tempera- ture was above 800 °C or below 700 °C. The yellow color above 800 °C came from the roughened surface of the substrate. To investigate the microstructure of SiNWs, the as-grown nanowires were scratched from the substrate and dispersed in alcohol. A few drops were put on the holey carbon copper grids for transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) examination. TEM image (Fig. 2c) shows a dark particle capping the nanowire. Silicon element mapping image (Fig. 2d) indicates that Si was absent in the dark cap. Figs. 2e,f display, respectively, the EDX spectrum of the nanowire and its particle cap, which reveals the gold signal only from the cap. It is thus established that oriented SiNWs with the gold particle cap were formed on the silicon substrate. The diameter variation of the nanowires deposited at different temperature is illustrated in Fig. 3a. The chart shows the average wire diameter increased, while the wire density decreased with increasing deposition tem- perature. Fig. 3b depicts the diameter distribution of the SiNWs and Au particle tips grown at 700 °C. The bars indicate that the diameters of the SiNWs range mostly between 10 and 30 nm, whereas the diameters of the Au particle tips primarily fall between 20 and 40 nm. Consequently, the diameter of the nanowires is well limited by that of the Au caps. HRTEM image (Fig. 4a) shows the as-grown SiNWs consist of a crystalline core and an amorphous silicon oxide sheath, similar to the SiNWs grown by the OAG method without Au catalyst. As illustrated in Fig. 3c, although some nanowires grew along Æ111æ direction, the dominant growth direction of SiNWs was Æ112æ with Æ110æ being the second dominant direction, the same as the SiNWs grown by the OAG method. The Æ112æ direction is different from the common Æ111æ growth direction for SiNWs fabri cated by the Au- catalytic VLS method. To investigate the initial wire growth, the cross-section of the sample grown for 10 min was examined by HRTEM. Fig. 4b shows a typical SiNW of 20 nm in diameter grew epitaxially on the (0 0 1) silicon substrate with a 40 nm Au cap. Signif- icantly, the interface between the wire and substrate is essentially free of defects. The high-quality epitaxy at the interface is expected to subsequently guide the oriented growth of SiNWs on the Si substrate. The growth process of SiNWs can be described as follows [8]: First, when the substrate is heated to high temperature (700–800 °C), the 1 nm gold film would break up to form 20–40 nm Au particles. The Au parti- cles will dissolve Si from the Si substrate to form the eu- tectic Au–Si alloy. As the SiO x vapor arrives at the Au particle, SiO x will disproportionate at the particle sur- face into Si and SiO 2 . Silicon will dissolve in the Au par- ticle, while the silicon oxide will remain at the particle surface. When the Si concentration in the Au particle reaches super-saturation, Si will separate out at the interface and grow epitaxially on the Si substrate. Sili- con oxide will flow over the Au particle surface and form a layer sheathing the SiNW. The diameter of the SiNWs is thus limited by the size of the Au particles. Fig. 1. Flowchart of the fabrication process of oriented silicon nanowires. 382 Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385 It is well recognized that the formation of metal–Si eutectic alloy is the key point of the MC-VLS growth of SiNWs and the growth temperature is dependent on the metal catalyst. As Au and Si can form an eutectic al- loy as low as 363 °C [9], consequently SiNWs have been synthesized under 500 °C using Au as the catalyst and SiH 4 as the source [10]. On the other hand, we have shown that SiNWs are produced by the catalyst-free OAG method only at temperatures above 900 °C [4]. The present results show that the production of SiNWs Fig. 2. (a) SEM image of oriented SiNWs arrays synthesized at 800 °C and (b) 700 °C. The scale bar is 2 lm. (c) TEM image of a SiNW with an Au particle tip and (d) corresponding silicon elemental mapping. The scale bar is 50 nm. (e) EDX spectrum of the nanowire and (f) its Au particle tip. Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385 383 is limited to the temperature range of 700–800 °C using the OAG and Au catalyst. The reduced growth temper- ature relative to that of metal-free OAG may be attrib- uted to the Au catalytic effect in lowering SiO decomposition temperature. Close TEM observation shows there is a thin silicon oxide layer covering the Au particle tips. During growth, the arriving silicon atoms have to diffuse through the thin oxide layer to reach the Au particle. Thus, the growth temperature should be sufficiently high to allow silicon penetrating through the oxide layer to form the eutectic alloy so as to sustain the growth of SiNWs. It is likely that the oxide layer stops silicon diffusion into the Au particle at temperatures less than 700 °C, thus hindering the growth of SiNWs at lower temperatures. Moreover, it was found the Si substrate was covered by a thin poly-crystal silicon and amorphous silica film if the growth temperate was higher than 800 °C. The growth direction of SiNWs from the MC-VLS method with Au catalyst is predominantly along the Æ111æ direction [3]. In the present method, the SiNWs primarily grew along the Æ112æ and Æ110æ directions, similar to SiNWs grown by the OAG method without any catalysts. As shown in Fig. 4a, the side surfaces of the SiNW are made of the {1 1 1} and {1 1 0} facets for the nanowire grown along [1 1 2] direction. The pres- ence of those crystal facets could minimize the total en- ergy of the nanowire because the surface energy of the {1 1 1} facets is the lowest and the energy of the side sur- faces dictates the total surface energy of a SiNW [11]. Recently, it was reported that the small-diameter SiNWs Fig. 3. (a) Variation of wire density and diameter as a function of deposition temperature. (b) Histogram of diameter distribution of SiNWs and Au particle tips. (c) Growth direction of SiNWs deposited at 700 °C. Fig. 4. (a) HRTEM image of a SiNW. The growth direction is along Æ112æ orientation. (b) The initial growth of SiNWs with an Au cap showing the epitaxial relation to the Si substrate. 384 Y. Yao et al. / Chemical Physics Letters 406 (2005) 381–385 grown from the MC-VLS method also adopted the Æ110æ and Æ112æ orientations [12]. In the present work, the initial growth of the SiNWs is along Æ001æ direction as a result of epitaxy to the substrate, as depicted in Fig. 4b. Therefore, the present results suggest that the initial Æ 001æ orientation of SiNWs may change to the more popular Æ110æ and Æ112æ directions during subse- quent growth of SiNWs as described elsewhere [13]. In summary, high-density, oriented SiNWs array were grown on (0 0 1) silicon substrates by the OAG method assisted by Au catalyst. The yield of SiNWs reached the largest at 700–800 °C. 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