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Vapor–solid–solid growth of crystalline silicon nanowires using anodic aluminum oxide template
Thin Solid Films 519 (2011) 3603–3607
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Vapor–solid–solid growth of crystalline silicon nanowires using anodic aluminum oxide template C.Y. Kuo, C. Gau ⁎ Institute of Aeronautics and Astronautics, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 21 January 2011 Accepted 21 January 2011 Available online 31 January 2011 Keywords: Silicon nanowires Vapor–solid–solid growth Aluminum anodic oxide Transmission electron microscopy
a b s t r a c t Silicon nanowires (SiNWs) were grown at low temperatures close to metal silicon eutectic point on a silicon substrate using gold catalyst coupled with assistance of the aluminum anodic oxide template. Either a vapor– solid–solid (VSS) growth process below metal silicon eutectic temperature or a vapor–liquid–solid (VLS) process at slightly higher temperatures was observed. The transmission electron microscopy coupled with both the X-ray energy dispersive spectroscopy and the selected area electron diffraction was adopted to characterize the SiNWs. Although the mechanism triggering the VSS process is still not clear, both the geometric and morphological characteristics of the SiNWs grown by the VSS process are discussed and compared with the SiNWs grown by the VLS process. The VSS SiNWs have a much slower growth rate (less than 100 nm/h), a smaller and more uniform diameter (in the range of 15.22 nm) due to a much slower rate of silicon diffusion and much smaller amount of silicon (6.8 wt.%) dissolved in the solid nanocatalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon nanowires (SiNWs) are attractive and interesting as a potentially useful semiconductor material [1,2]. SiNWs can be implemented into various devices including nano-sensors used for bio- and chemical detections [3,4], vertical and lateral field effect transistors [5,6], logic gates [7], electronics [8], noise signal measurement [9] and solar energy devices [10,11]. The most frequently used method to synthesize SiNWs is by the vapor–liquid–solid (VLS) growth process since this process can readily make the nanowires connect with other components to form a useful device [12–14]. The growth mechanism of these VLS nanowires has been proposed very early by Wagner [15]. The size and density of the VLS SiNWs can be readily defined by the size and density of the nanosize metal catalysts [16]. However, the growth of VLS SiNWs must occur above the catalyst-Si eutectic temperature so that the nanocatalyst is a liquid state, which is beneficial for absorption and subsequent decomposition of silane (SiH4) gas. Decomposition of silane produces silicon that in turn can be dissolved into the liquid nanocatalyst, become supersaturated and precipitate at solid–liquid interface leading to the growth of a crystalline nanowire. Au is the frequently used catalyst to grow nanowire as it can be readily dissolved in the silicon substrate to form a eutectic at the Au–Si eutectic temperature of 363 °C [17]. Other metal catalysts have also been used to grow SiNWs. However, Wang et al. [18] identified a very interesting but different
⁎ Corresponding author. E-mail address: [email protected] (C. Gau). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.280
growth mechanism, i.e. the vapor–solid–solid (VSS) process, to grow single crystalline SiNWs below the eutectic temperature using aluminum nanoparticles as catalysts. It appears that the Al-catalyzed VSS SiNW growth is similar to the Al-mediated solid-phase epitaxy. Recently, Mn-mediated VSS growth of silicon and germanium nanowires has also been reported [19]. The VSS growth process has the advantages of growth at much lower temperature, which is beneficial when a low fabrication temperature is required for devices. In addition, the unintentional incorporation of impurities, particularly from the metal catalyst, may be reduced in the VSS nanowires due to the reduction in atom diffusivity and solid solubility associated with the lower temperatures. This has generated much interest in the growth of nanowires by the VSS process. For example, the VSS growth process has been reported for TiSi2-catalyzed SiNWs [20], Cu3Gecatalyzed GeNWs [21] and Au-catalyzed InAsNWs [22]. However, Aucatalyzed VSS growth of SiNWs has never been reported since not only the Au–Si system lacks a solid eutectic phase, but also the Au–Si eutectic temperature is relatively low as compared with other metal catalyst–silicon system. The growth of the VSS nanowires with Au as catalyst below Au–Si eutectic temperature may not be possible. Here we present growth of SiNWs by the VSS process using gold as nanocatalyst and anodic aluminum oxide (AAO) as a template. The use of AAO as template was originally designed to define the locations of nanowires as done by others [23–26]. However, it is found later that not only does AAO help to grow crystalline SiNWs at a much lower temperature, but also to grow crystalline nanowires at a temperature lower than the eutectic point of the Au–Si compound. Without use of the AAO template, the Au-mediated VSS growth of silicon nanowires is not possible.
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AAO templates have been used to grow carbon nanotubes (CNTs) [27], SiNWs [23–26] and TiO2 nanorods [28] in ordered arrays for field emission and hybrid organic solar cells [29,30]. It has been found that to grow CNTs within ordered pores in a chemical vapor deposition chamber requires no metal catalyst [31]. The exact mechanism to grow CNTs without using a catalyst is not clear. The same technique has also been used to grow SiNWs arrays without using a catalyst [32]. However, the growth temperature is relatively high, and is about 900 °C with SiH4 as a source gas diluted with H2. It has been inferred [33] that there are a large number of Lewis acid nature surface sites in amorphous and transition alumina and that these sites have the intrinsic catalytic activity of transition alumina in front of the decomposition of SiH4. This suggests that the internal pore surface within alumina has a catalytic behavior in addition to its templating effect. Therefore, the current work adopts the use of both the AAO template and the gold nanoparticles as catalysts to grow SiNWs, using the ordered pores of AAO to grow SiNWs in an ordered array. It is expected that the catalytic behavior of both the gold and the AAO can make the growth temperature of SiNWs much lower. 2. Experimental method In the current work, the AAO template is fabricated on a p-type (111) silicon wafer pre-deposited with 3-nm thick gold film. Then, fabrication of the AAO template is started after depositing a 200-nm thick Al film on top of the gold. The two-step anodization method [34,35] was used to produce a nanoporous AAO template to make the porous structures more regular and uniform. The anodization was carried out in a 0.3 M oxalic acid solution at 20 °C under a constant voltage at 30 V applied for 5 min. Then, the nanopores were widened
in a 5% H3PO4 solution for 10 min and the residual alumina barrier layer above the Au film was removed completely [23]. This procedure is required in order to proceed with the second anodization step (5 min, 30 V). The average diameter of the AAO pores made by this process is 50.4 nm. After preparation of AAO template was completed, the sample was put into a low pressure chemical vapor deposition (LPCVD) chamber (DF550-6, SEMCO) for growth of SiNWs. Before the growth, the substrate is annealed at the growth temperature for 20 min until the gold film breaks up to form Au–Si alloy nanoparticles. The SiNW arrays were synthesized for 1 or 2 h inside the AAO template with a SiH4 flow rate at 100 sccm and N2 flow rate at 100 sccm. The growth pressure is set at 45 Pa, and the temperatures can be 350 °C, 400 °C and 500 °C, respectively. The LPCVD chamber is relatively large and is for growth of 6 in. wafer. In order to ensure uniform temperature in the growth chamber, electric heaters are distributed uniformly around the entire wall of the chamber. A total of three thermocouples are inserted into the chamber for temperature measurements. One is near the entrance, one is in the central region, and the other one is near the exit of the chamber. The temperature difference among these three thermocouples is controlled within 3 °C. Each of the thermocouples has a measurement accuracy of ±1 °C. The temperature of the thermocouple in the central region is used as the growth temperature of the SiNWs. After growth, the SiNWs grown on the substrate is dispersed in a solution with 5 wt.% H3PO4 at 50 °C under sonication for 1 h. The SiNWs that can be attached to the transmission electron microscopy (TEM) grids (Lacey Formvar stabilized with carbon) when the grids is dipped into the solvent, are used for analysis under high-resolution transmission electron microscopy (HRTEM, JEM-2100F Electron Microscope). The TEM grids consist of both the copper rings and carbon nets that in
Fig. 1. The VSS growth of SiNWs in AAO template at 350 °C for 1 h: (a) schematic diagram of the VSS nanowires grown in the AAO template, (b) morphology of SiNWs in the AAO nanopores from the top view SEM image, (c) SiNW arrays after removal of the AAO template, (d, e) magnified view of the marked area, (f) failure of SiNWs growth without use of the AAO template.
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3. Results and discussion
Fig. 2. Schematic of the Au-rich region of the Au–Si binary phase diagram . VLS and VSS are indicated for arbitrarily chosen temperatures. Data from Ref. [17].
the X-ray energy dispersive spectroscopy (EDS) measurements of the SiNWs, large amount of Cu and C appears as shown in the later figures. These elements should not be accounted during analysis of different compositions in SiNWs.
A schematic of the synthesized SiNWs array is shown in Fig. 1(a). Scanning electron microscopy (SEM) image of the SiNWs grown for 1 h in the AAO nanopores is shown in Fig. 1(b). The conditions for the flow rate of the source gasses are very critical for growth of SiNWs, especially in a lower temperature process. In fact, the growth rate of the SiNW arrays at 350 °C is so low that the nanowires do not emerge from the pores of the AAO. The nanowire arrays can be clearly visualized when the AAO template is removed, as shown in Fig. 1(c) or in much greater magnifications in Fig. 1(d) or (e). The slow growth rate of the SiNWs is attributed to the low temperature (at 350 °C) which reduces the rate of decomposition of the silane and of precipitation of the silicon at the particle–vapor interface. The growth occurs below the Au–Si eutectic temperature, as shown in Fig. 2 for the Au–Si binary phase diagram. Therefore, the eutectic metal at this low temperature (350 °C) is a solid. It is expected that precipitation of silicon in the solid metal is much slower than in the liquid. For the VLS process, the growth temperature has to be higher than the eutectic temperature of 363 °C so that the Au–Si catalyst particle is a liquid. Due to the supersaturation of Si in the droplet, the position in the phase diagram will be on the right hand side of the liquidus line, as indicated in Fig. 2. For the VSS growth, the catalyst particle remains in
Fig. 3. TEM morphology of SiNWs grown by VSS process at 350 °C for 1 h with assistance of the AAO template: (a) straight single wire, the square indicates the location of the HRTEM image, (b) HRTEM image of the SiNW surface and (c) the corresponding SAED pattern focused on a single SiNW, (d) EDS spectrum and relative concentration data taken at the middle of the SiNW as shown in the inset, showing no trace of gold; and (e) EDS spectrum and relative concentration data taken at the SiNW tip as shown in the inset, indicate a large fraction of Au and a small fraction of Si.
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its solid phase. The complete TEM analysis of this kind of nanowire is shown in Fig. 3 with TEM operation voltage at 200 kV. TEM image of the nanowire with the Au catalyst on the tip, as shown in Fig. 3(a), does not indicate tapering of SiNWs as those found tapering of SiNWs grown by the VSS process with Al as catalyst [18]. It has been found [36,37] that tapering of nanowires is caused by uncatalyzed deposition of Si on the sides of the nanowires, causing a radial growth of the nanowires. Tapering of nanowire was also found in growth of SiNWs with Au as catalyst [13], and was attributed to the gradual consumption of the Au catalyst that has actually diffused along the nanowire surface. However, the current EDS measurement on the nanowire does not indicate any Au left on the nanowire surface, as shown in Fig. 3(d). It appears that in the growth of VSS SiNWs with Au as catalyst, the non-tapering of nanowire is a result of low temperature (350 °C) process that lateral deposition of silicon on the sides of the nanowire is much reduced and that the solid Au catalyst is not readily consumed or diffused along the nanowire surface. The crystalline structure of the nanowire can be realized by the high resolution of TEM, as shown in Fig. 3(b), and the corresponding selected-area electron diffraction (SAED) pattern focused on this location, as shown in Fig. 3(c). The SAED pattern indicates that the diffraction spots are symmetric and organized in a precise parallelogram. Therefore, it can be concluded that the diamond lattice structure of bulk silicon is also preserved in the SiNWs. The EDS analysis for the tip of the nanowire where the Au catalyst is located, as shown in Fig. 3(e), indicates that the major component in the tip is Au and the silicon is only a small fraction of the total amount. The small fraction of the silicon in the Au catalyst is a result of an order of magnitude lower solubility of the solid Au to the silicon, as compared with the solubility of the liquid Au to the silicon [17]. It is expected that the rate of precipitation of the supersaturated silicon from the solid Au is much lower than that from the liquid. This leads to a result that the growth rate of the nanowire by the VSS process is much slower. Even with 2 h growth, most of the nanowires are still in the pores of the AAO template. In order to verify that the use of the AAO template can effectively reduce the growth temperature to 350 °C, a silicon substrate, predeposited with gold nanoparticles without formation of AAO template, was used to grow silicon nanowires at the same growth condition. No any SiNW is found, as shown in Fig. 1(f) where only gold nanoparticles are observed. In fact, without the AAO template, the SiNWs cannot be even grown at 400 °C, which is above the Au–Si eutectic temperature. However, on the opposite, one has attempted to grow SiNWs on a silicon substrate with formation of AAO template but without using any Au nanoparticles. The growth of SiNWs at temperatures below 700 °C was never successful. It appears that in the low temperature regions below 700 °C, not only the AAO template but also the Au nanoparticles play a very important role to assist growth of the SiNWs either by the VSS or the VLS process. As a comparison, SiNWs growth at the VLS process is also performed. The SEM morphology of the SiNWs grown at different temperatures is shown in Figs. 4(a) and 5(a). Not only the growth rate increases with increasing the temperature, but also the diameter of the nanowire increases with increasing the temperature. Tapering of the nanowires is found for nanowires grown at 500 °C, as shown by the TEM image in Fig. 4(b). The slight tapering is a result of the diffusion of the Au along the nanowire surface which gradually reduces the volume of the Au catalyst during the growth and reduces the growth diameter of the nanowire. The EDS spectra of the nanowire, as shown in Fig. 4(c), do indicate evidence of the small amount of Au catalyst which diffuses along the silicon nanowire surface. This finding is very similar to the one of the SiNWs grown at a much higher temperature with Au as catalyst but without use of AAO template [13]. To grow SiNWs at 400 °C, non-tapering of the nanowire and the crystalline structure of the nanowire are obtained, as shown
Fig. 4. (a) SEM image for the SiNW arrays grown by VSL process at 500 °C, (b) TEM image for a single SiNW and (c) EDS spectrum measured at the middle of the SiNW.
in Fig. 5(b) for TEM image and the inset in Fig. 5(b) for the SAED pattern focused on the nanowire surface. The non-tapering of the nanowire is attributed to the low temperature (400 °C) growth process which makes diffusion of the Au catalyst along the nanowire surface more difficult. The EDS analysis on the silicon nanowire surface indicates that no trace of Au is found, as shown in Fig. 5(c). On the tip of the nanowire where the Au catalyst is located, the EDS analysis indicates only a very small fraction of Au as compared with a large fraction of silicon, as shown in Fig. 5(d) or in the inserted table. This indicates that in the VLS growth process the liquid Au catalyst can absorb a large amount of Si from the precursor and become supersaturated, as compared with the VSS growth process that the solid Au catalyst can absorb only a small amount of the silicon. The large amount of supersaturated silicon can be readily precipitated from the catalyst and lead a more rapid growth of the SiNWs. The rapid precipitation of the silicon from the metal catalyst also leads to a much greater diameter of the VLS nanowires. The average diameter of the VLS nanowires grown at 500 °C is 51.31 nm, the average diameter of the VLS nanowires grown at 400 °C is 28.61 nm, and the average diameter of the VSS nanowires grown at 350 °C is only 15.22 nm. The diameter of the VSS SiNW in the TEM image, as shown in Fig. 3a, is the largest SiNW one can only find due to difficulty in picking up the much smaller size nanowires in the solution. Typical size of the VSS SiNWs grown on the substrate can be measured and obtained in the SEM image, as shown in Fig. 1(e) where the nanowire has a diameter close to 10 nm. The results of much slower growth rate and much smaller diameter of the VSS SiNWs in the current work agree qualitatively with the findings in the growth of VSS Ge nanowires [38]. This is attributed to the lesser Si content in the Au catalyst due to the much lower solubility of the solid Au to the silicon, as compared with the solubility of the liquid Au to the silicon, as discussed previously. The current growth of VSS SiNWs below the Au–Si eutectic temperature can also be re-confirmed by the following arguments. Based on the special feature of the nanowire grown by the VSS process, the capillary effects represented by the Gibbs–Thomson increase in free energy may not be so significant to cause a significant reduction in the eutectic temperature of Au–Si nanoparticles, as discussed in the ref. [38] for growth of GeNWs. In addition, it has been
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the SiNWs arrays is attributed to the use of both the gold nanoparticles and the AAO template as catalysts. Without assistance of the AAO template, the gold mediated VSS growth of SiNWs is not possible. This may be attributed to both the lack of Au–Si eutectic solid phase and the extremely low Au–Si eutectic temperature. The VSS growth process can be identified by the growth temperature lower than the Au–Si eutectic temperature and the very small fraction of the Si in the Au catalyst due to the much lower solubility of the solid Au than the liquid Au. The small fraction of silicon dissolved in the Au catalyst results in a slower rate of precipitation which leads to a much lower growth rate and a much smaller diameter of the nanowires. The VSS SiNWs are also crystalline like the VLS SiNWs. However, they exhibit non-tapering. The nontapering of the VSS SiNWs is attributed to the facts that the solid Au catalyst is not readily consumed and diffused along the nanowire surface, and uncatalyzed deposition of Si on the sides of the nanowires is absent in this growth process.
References
Fig. 5. The SiNWs grown by VLS process at 400 °C for 1 h with AAO template: (a) SEM image of the SiWNs arrays, (b) TEM image of the wires with Au catalyst on the tip (the inset is the SAED pattern focused on a single SiNW), (c) EDS spectrum and concentration data, taken at the middle of the SiNW, does not indicate any Au content and (d) EDS spectrum and concentration data taken at the SiNW tip showing a large fraction of Si but a small fraction of Au.
shown [39] that GeNWs can be grown at either the VSS or the VLS process below the eutectic temperature for 265 to 355 °C depending upon the temperature history of the sample and the pressure of Ge2H6. An initial growth above the Au–Ge eutectic temperature leading to the VLS growth process can maintain the VLS growth process even though the growth temperature is suddenly reduced below the Au–Ge eutectic temperature as long as the catalyst can be maintained at a liquid state. However, when the metal catalyst is maintained at a solid state, the growth of nanowires at the VLS process is not possible. In the present work, however, both the annealing temperature for the nanocatalysts and the growth temperature (both kept at 350 °C) for the silicon nanowire has never been greater than the Au–Si eutectic temperature. The nanocatalysts in the current work could never become a liquid state, but a solid state. Thus, the SiNWs could never be possibly grown by the VLS process at 350 °C, but by the VSS process. 4. Conclusions A summary can be made for the growth of single crystalline SiNWs arrays below the Au–Si eutectic temperature. The growth of
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Vapor–solid–solid growth of crystalline silicon nanowires using anodic aluminum oxide template C.Y. Kuo, C. Gau ⁎ Institute of Aeronautics and Astronautics, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 21 January 2011 Accepted 21 January 2011 Available online 31 January 2011 Keywords: Silicon nanowires Vapor–solid–solid growth Aluminum anodic oxide Transmission electron microscopy
a b s t r a c t Silicon nanowires (SiNWs) were grown at low temperatures close to metal silicon eutectic point on a silicon substrate using gold catalyst coupled with assistance of the aluminum anodic oxide template. Either a vapor– solid–solid (VSS) growth process below metal silicon eutectic temperature or a vapor–liquid–solid (VLS) process at slightly higher temperatures was observed. The transmission electron microscopy coupled with both the X-ray energy dispersive spectroscopy and the selected area electron diffraction was adopted to characterize the SiNWs. Although the mechanism triggering the VSS process is still not clear, both the geometric and morphological characteristics of the SiNWs grown by the VSS process are discussed and compared with the SiNWs grown by the VLS process. The VSS SiNWs have a much slower growth rate (less than 100 nm/h), a smaller and more uniform diameter (in the range of 15.22 nm) due to a much slower rate of silicon diffusion and much smaller amount of silicon (6.8 wt.%) dissolved in the solid nanocatalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon nanowires (SiNWs) are attractive and interesting as a potentially useful semiconductor material [1,2]. SiNWs can be implemented into various devices including nano-sensors used for bio- and chemical detections [3,4], vertical and lateral field effect transistors [5,6], logic gates [7], electronics [8], noise signal measurement [9] and solar energy devices [10,11]. The most frequently used method to synthesize SiNWs is by the vapor–liquid–solid (VLS) growth process since this process can readily make the nanowires connect with other components to form a useful device [12–14]. The growth mechanism of these VLS nanowires has been proposed very early by Wagner [15]. The size and density of the VLS SiNWs can be readily defined by the size and density of the nanosize metal catalysts [16]. However, the growth of VLS SiNWs must occur above the catalyst-Si eutectic temperature so that the nanocatalyst is a liquid state, which is beneficial for absorption and subsequent decomposition of silane (SiH4) gas. Decomposition of silane produces silicon that in turn can be dissolved into the liquid nanocatalyst, become supersaturated and precipitate at solid–liquid interface leading to the growth of a crystalline nanowire. Au is the frequently used catalyst to grow nanowire as it can be readily dissolved in the silicon substrate to form a eutectic at the Au–Si eutectic temperature of 363 °C [17]. Other metal catalysts have also been used to grow SiNWs. However, Wang et al. [18] identified a very interesting but different
⁎ Corresponding author. E-mail address: [email protected] (C. Gau). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.280
growth mechanism, i.e. the vapor–solid–solid (VSS) process, to grow single crystalline SiNWs below the eutectic temperature using aluminum nanoparticles as catalysts. It appears that the Al-catalyzed VSS SiNW growth is similar to the Al-mediated solid-phase epitaxy. Recently, Mn-mediated VSS growth of silicon and germanium nanowires has also been reported [19]. The VSS growth process has the advantages of growth at much lower temperature, which is beneficial when a low fabrication temperature is required for devices. In addition, the unintentional incorporation of impurities, particularly from the metal catalyst, may be reduced in the VSS nanowires due to the reduction in atom diffusivity and solid solubility associated with the lower temperatures. This has generated much interest in the growth of nanowires by the VSS process. For example, the VSS growth process has been reported for TiSi2-catalyzed SiNWs [20], Cu3Gecatalyzed GeNWs [21] and Au-catalyzed InAsNWs [22]. However, Aucatalyzed VSS growth of SiNWs has never been reported since not only the Au–Si system lacks a solid eutectic phase, but also the Au–Si eutectic temperature is relatively low as compared with other metal catalyst–silicon system. The growth of the VSS nanowires with Au as catalyst below Au–Si eutectic temperature may not be possible. Here we present growth of SiNWs by the VSS process using gold as nanocatalyst and anodic aluminum oxide (AAO) as a template. The use of AAO as template was originally designed to define the locations of nanowires as done by others [23–26]. However, it is found later that not only does AAO help to grow crystalline SiNWs at a much lower temperature, but also to grow crystalline nanowires at a temperature lower than the eutectic point of the Au–Si compound. Without use of the AAO template, the Au-mediated VSS growth of silicon nanowires is not possible.
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AAO templates have been used to grow carbon nanotubes (CNTs) [27], SiNWs [23–26] and TiO2 nanorods [28] in ordered arrays for field emission and hybrid organic solar cells [29,30]. It has been found that to grow CNTs within ordered pores in a chemical vapor deposition chamber requires no metal catalyst [31]. The exact mechanism to grow CNTs without using a catalyst is not clear. The same technique has also been used to grow SiNWs arrays without using a catalyst [32]. However, the growth temperature is relatively high, and is about 900 °C with SiH4 as a source gas diluted with H2. It has been inferred [33] that there are a large number of Lewis acid nature surface sites in amorphous and transition alumina and that these sites have the intrinsic catalytic activity of transition alumina in front of the decomposition of SiH4. This suggests that the internal pore surface within alumina has a catalytic behavior in addition to its templating effect. Therefore, the current work adopts the use of both the AAO template and the gold nanoparticles as catalysts to grow SiNWs, using the ordered pores of AAO to grow SiNWs in an ordered array. It is expected that the catalytic behavior of both the gold and the AAO can make the growth temperature of SiNWs much lower. 2. Experimental method In the current work, the AAO template is fabricated on a p-type (111) silicon wafer pre-deposited with 3-nm thick gold film. Then, fabrication of the AAO template is started after depositing a 200-nm thick Al film on top of the gold. The two-step anodization method [34,35] was used to produce a nanoporous AAO template to make the porous structures more regular and uniform. The anodization was carried out in a 0.3 M oxalic acid solution at 20 °C under a constant voltage at 30 V applied for 5 min. Then, the nanopores were widened
in a 5% H3PO4 solution for 10 min and the residual alumina barrier layer above the Au film was removed completely [23]. This procedure is required in order to proceed with the second anodization step (5 min, 30 V). The average diameter of the AAO pores made by this process is 50.4 nm. After preparation of AAO template was completed, the sample was put into a low pressure chemical vapor deposition (LPCVD) chamber (DF550-6, SEMCO) for growth of SiNWs. Before the growth, the substrate is annealed at the growth temperature for 20 min until the gold film breaks up to form Au–Si alloy nanoparticles. The SiNW arrays were synthesized for 1 or 2 h inside the AAO template with a SiH4 flow rate at 100 sccm and N2 flow rate at 100 sccm. The growth pressure is set at 45 Pa, and the temperatures can be 350 °C, 400 °C and 500 °C, respectively. The LPCVD chamber is relatively large and is for growth of 6 in. wafer. In order to ensure uniform temperature in the growth chamber, electric heaters are distributed uniformly around the entire wall of the chamber. A total of three thermocouples are inserted into the chamber for temperature measurements. One is near the entrance, one is in the central region, and the other one is near the exit of the chamber. The temperature difference among these three thermocouples is controlled within 3 °C. Each of the thermocouples has a measurement accuracy of ±1 °C. The temperature of the thermocouple in the central region is used as the growth temperature of the SiNWs. After growth, the SiNWs grown on the substrate is dispersed in a solution with 5 wt.% H3PO4 at 50 °C under sonication for 1 h. The SiNWs that can be attached to the transmission electron microscopy (TEM) grids (Lacey Formvar stabilized with carbon) when the grids is dipped into the solvent, are used for analysis under high-resolution transmission electron microscopy (HRTEM, JEM-2100F Electron Microscope). The TEM grids consist of both the copper rings and carbon nets that in
Fig. 1. The VSS growth of SiNWs in AAO template at 350 °C for 1 h: (a) schematic diagram of the VSS nanowires grown in the AAO template, (b) morphology of SiNWs in the AAO nanopores from the top view SEM image, (c) SiNW arrays after removal of the AAO template, (d, e) magnified view of the marked area, (f) failure of SiNWs growth without use of the AAO template.
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3. Results and discussion
Fig. 2. Schematic of the Au-rich region of the Au–Si binary phase diagram . VLS and VSS are indicated for arbitrarily chosen temperatures. Data from Ref. [17].
the X-ray energy dispersive spectroscopy (EDS) measurements of the SiNWs, large amount of Cu and C appears as shown in the later figures. These elements should not be accounted during analysis of different compositions in SiNWs.
A schematic of the synthesized SiNWs array is shown in Fig. 1(a). Scanning electron microscopy (SEM) image of the SiNWs grown for 1 h in the AAO nanopores is shown in Fig. 1(b). The conditions for the flow rate of the source gasses are very critical for growth of SiNWs, especially in a lower temperature process. In fact, the growth rate of the SiNW arrays at 350 °C is so low that the nanowires do not emerge from the pores of the AAO. The nanowire arrays can be clearly visualized when the AAO template is removed, as shown in Fig. 1(c) or in much greater magnifications in Fig. 1(d) or (e). The slow growth rate of the SiNWs is attributed to the low temperature (at 350 °C) which reduces the rate of decomposition of the silane and of precipitation of the silicon at the particle–vapor interface. The growth occurs below the Au–Si eutectic temperature, as shown in Fig. 2 for the Au–Si binary phase diagram. Therefore, the eutectic metal at this low temperature (350 °C) is a solid. It is expected that precipitation of silicon in the solid metal is much slower than in the liquid. For the VLS process, the growth temperature has to be higher than the eutectic temperature of 363 °C so that the Au–Si catalyst particle is a liquid. Due to the supersaturation of Si in the droplet, the position in the phase diagram will be on the right hand side of the liquidus line, as indicated in Fig. 2. For the VSS growth, the catalyst particle remains in
Fig. 3. TEM morphology of SiNWs grown by VSS process at 350 °C for 1 h with assistance of the AAO template: (a) straight single wire, the square indicates the location of the HRTEM image, (b) HRTEM image of the SiNW surface and (c) the corresponding SAED pattern focused on a single SiNW, (d) EDS spectrum and relative concentration data taken at the middle of the SiNW as shown in the inset, showing no trace of gold; and (e) EDS spectrum and relative concentration data taken at the SiNW tip as shown in the inset, indicate a large fraction of Au and a small fraction of Si.
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its solid phase. The complete TEM analysis of this kind of nanowire is shown in Fig. 3 with TEM operation voltage at 200 kV. TEM image of the nanowire with the Au catalyst on the tip, as shown in Fig. 3(a), does not indicate tapering of SiNWs as those found tapering of SiNWs grown by the VSS process with Al as catalyst [18]. It has been found [36,37] that tapering of nanowires is caused by uncatalyzed deposition of Si on the sides of the nanowires, causing a radial growth of the nanowires. Tapering of nanowire was also found in growth of SiNWs with Au as catalyst [13], and was attributed to the gradual consumption of the Au catalyst that has actually diffused along the nanowire surface. However, the current EDS measurement on the nanowire does not indicate any Au left on the nanowire surface, as shown in Fig. 3(d). It appears that in the growth of VSS SiNWs with Au as catalyst, the non-tapering of nanowire is a result of low temperature (350 °C) process that lateral deposition of silicon on the sides of the nanowire is much reduced and that the solid Au catalyst is not readily consumed or diffused along the nanowire surface. The crystalline structure of the nanowire can be realized by the high resolution of TEM, as shown in Fig. 3(b), and the corresponding selected-area electron diffraction (SAED) pattern focused on this location, as shown in Fig. 3(c). The SAED pattern indicates that the diffraction spots are symmetric and organized in a precise parallelogram. Therefore, it can be concluded that the diamond lattice structure of bulk silicon is also preserved in the SiNWs. The EDS analysis for the tip of the nanowire where the Au catalyst is located, as shown in Fig. 3(e), indicates that the major component in the tip is Au and the silicon is only a small fraction of the total amount. The small fraction of the silicon in the Au catalyst is a result of an order of magnitude lower solubility of the solid Au to the silicon, as compared with the solubility of the liquid Au to the silicon [17]. It is expected that the rate of precipitation of the supersaturated silicon from the solid Au is much lower than that from the liquid. This leads to a result that the growth rate of the nanowire by the VSS process is much slower. Even with 2 h growth, most of the nanowires are still in the pores of the AAO template. In order to verify that the use of the AAO template can effectively reduce the growth temperature to 350 °C, a silicon substrate, predeposited with gold nanoparticles without formation of AAO template, was used to grow silicon nanowires at the same growth condition. No any SiNW is found, as shown in Fig. 1(f) where only gold nanoparticles are observed. In fact, without the AAO template, the SiNWs cannot be even grown at 400 °C, which is above the Au–Si eutectic temperature. However, on the opposite, one has attempted to grow SiNWs on a silicon substrate with formation of AAO template but without using any Au nanoparticles. The growth of SiNWs at temperatures below 700 °C was never successful. It appears that in the low temperature regions below 700 °C, not only the AAO template but also the Au nanoparticles play a very important role to assist growth of the SiNWs either by the VSS or the VLS process. As a comparison, SiNWs growth at the VLS process is also performed. The SEM morphology of the SiNWs grown at different temperatures is shown in Figs. 4(a) and 5(a). Not only the growth rate increases with increasing the temperature, but also the diameter of the nanowire increases with increasing the temperature. Tapering of the nanowires is found for nanowires grown at 500 °C, as shown by the TEM image in Fig. 4(b). The slight tapering is a result of the diffusion of the Au along the nanowire surface which gradually reduces the volume of the Au catalyst during the growth and reduces the growth diameter of the nanowire. The EDS spectra of the nanowire, as shown in Fig. 4(c), do indicate evidence of the small amount of Au catalyst which diffuses along the silicon nanowire surface. This finding is very similar to the one of the SiNWs grown at a much higher temperature with Au as catalyst but without use of AAO template [13]. To grow SiNWs at 400 °C, non-tapering of the nanowire and the crystalline structure of the nanowire are obtained, as shown
Fig. 4. (a) SEM image for the SiNW arrays grown by VSL process at 500 °C, (b) TEM image for a single SiNW and (c) EDS spectrum measured at the middle of the SiNW.
in Fig. 5(b) for TEM image and the inset in Fig. 5(b) for the SAED pattern focused on the nanowire surface. The non-tapering of the nanowire is attributed to the low temperature (400 °C) growth process which makes diffusion of the Au catalyst along the nanowire surface more difficult. The EDS analysis on the silicon nanowire surface indicates that no trace of Au is found, as shown in Fig. 5(c). On the tip of the nanowire where the Au catalyst is located, the EDS analysis indicates only a very small fraction of Au as compared with a large fraction of silicon, as shown in Fig. 5(d) or in the inserted table. This indicates that in the VLS growth process the liquid Au catalyst can absorb a large amount of Si from the precursor and become supersaturated, as compared with the VSS growth process that the solid Au catalyst can absorb only a small amount of the silicon. The large amount of supersaturated silicon can be readily precipitated from the catalyst and lead a more rapid growth of the SiNWs. The rapid precipitation of the silicon from the metal catalyst also leads to a much greater diameter of the VLS nanowires. The average diameter of the VLS nanowires grown at 500 °C is 51.31 nm, the average diameter of the VLS nanowires grown at 400 °C is 28.61 nm, and the average diameter of the VSS nanowires grown at 350 °C is only 15.22 nm. The diameter of the VSS SiNW in the TEM image, as shown in Fig. 3a, is the largest SiNW one can only find due to difficulty in picking up the much smaller size nanowires in the solution. Typical size of the VSS SiNWs grown on the substrate can be measured and obtained in the SEM image, as shown in Fig. 1(e) where the nanowire has a diameter close to 10 nm. The results of much slower growth rate and much smaller diameter of the VSS SiNWs in the current work agree qualitatively with the findings in the growth of VSS Ge nanowires [38]. This is attributed to the lesser Si content in the Au catalyst due to the much lower solubility of the solid Au to the silicon, as compared with the solubility of the liquid Au to the silicon, as discussed previously. The current growth of VSS SiNWs below the Au–Si eutectic temperature can also be re-confirmed by the following arguments. Based on the special feature of the nanowire grown by the VSS process, the capillary effects represented by the Gibbs–Thomson increase in free energy may not be so significant to cause a significant reduction in the eutectic temperature of Au–Si nanoparticles, as discussed in the ref. [38] for growth of GeNWs. In addition, it has been
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the SiNWs arrays is attributed to the use of both the gold nanoparticles and the AAO template as catalysts. Without assistance of the AAO template, the gold mediated VSS growth of SiNWs is not possible. This may be attributed to both the lack of Au–Si eutectic solid phase and the extremely low Au–Si eutectic temperature. The VSS growth process can be identified by the growth temperature lower than the Au–Si eutectic temperature and the very small fraction of the Si in the Au catalyst due to the much lower solubility of the solid Au than the liquid Au. The small fraction of silicon dissolved in the Au catalyst results in a slower rate of precipitation which leads to a much lower growth rate and a much smaller diameter of the nanowires. The VSS SiNWs are also crystalline like the VLS SiNWs. However, they exhibit non-tapering. The nontapering of the VSS SiNWs is attributed to the facts that the solid Au catalyst is not readily consumed and diffused along the nanowire surface, and uncatalyzed deposition of Si on the sides of the nanowires is absent in this growth process.
References
Fig. 5. The SiNWs grown by VLS process at 400 °C for 1 h with AAO template: (a) SEM image of the SiWNs arrays, (b) TEM image of the wires with Au catalyst on the tip (the inset is the SAED pattern focused on a single SiNW), (c) EDS spectrum and concentration data, taken at the middle of the SiNW, does not indicate any Au content and (d) EDS spectrum and concentration data taken at the SiNW tip showing a large fraction of Si but a small fraction of Au.
shown [39] that GeNWs can be grown at either the VSS or the VLS process below the eutectic temperature for 265 to 355 °C depending upon the temperature history of the sample and the pressure of Ge2H6. An initial growth above the Au–Ge eutectic temperature leading to the VLS growth process can maintain the VLS growth process even though the growth temperature is suddenly reduced below the Au–Ge eutectic temperature as long as the catalyst can be maintained at a liquid state. However, when the metal catalyst is maintained at a solid state, the growth of nanowires at the VLS process is not possible. In the present work, however, both the annealing temperature for the nanocatalysts and the growth temperature (both kept at 350 °C) for the silicon nanowire has never been greater than the Au–Si eutectic temperature. The nanocatalysts in the current work could never become a liquid state, but a solid state. Thus, the SiNWs could never be possibly grown by the VLS process at 350 °C, but by the VSS process. 4. Conclusions A summary can be made for the growth of single crystalline SiNWs arrays below the Au–Si eutectic temperature. The growth of
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