pubs.acs.org/NanoLett
Wafer-Scale Synthesis of Single-Crystal Zigzag Silicon Nanowire Arrays with Controlled Turning Angles Huan Chen,† Hui Wang,† Xiao-Hong Zhang,*,† Chun-Sing Lee,‡ and Shuit-Tong Lee*,‡ †
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and ‡ Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China ABSTRACT Silicon nanowires (SiNWs) having curved structures may have unique advantages in device fabrication. However, no methods are available to prepare curved SiNWs controllably. In this work, we report the preparation of three types of single-crystal SiNWs with various turning angles via metal-assisted chemical etching using (111)-oriented silicon wafers near room temperature. The zigzag SiNWs are single crystals and can be p- or n-doped using corresponding Si wafer as substrate. The controlled growth direction is attributed to the preferred movement of Ag nanoparticles along 〈001〉 and other directions in Si wafer. Our results demonstrate that metal-assisted chemical etching may be a viable approach to fabricate SiNWs with desired turning angles by utilizing the various crystalline directions in a Si wafer. KEYWORDS Zigzag silicon nanowire, controlled synthesis, semiconductor nanowires
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about curved SiNWs have been reported.22-25 Therefore, it is of interest to develop simple and cost-effective processes to prepare zigzag SiNWs. A metal-assisted chemical etching approach has attracted wide attention recently because of its capability to make arrays of well-aligned and uniformly distributed SiNWs with controlled optoelectronic properties.26-28 In this approach, silver nanoparticles (AgNPs) are used as a catalytic “drilling bit” to etch tunnels into Si wafer in an AgNO3-HF solution. Due to the high density of AgNPs, neighboring tunnels might merge together leaving SiNWs in between.29,30 This formation process implies that kinetically controlled motility of the AgNPs could determine the shape of the resulting Si nanostructures, thus suggesting a possibility to synthesize curved SiNWs via a simple change of the migration direction of the AgNPs. To our knowledge, there is no published report on the synthesis of curved SiNWs with diverse turning angles. In this work, we report the wafer-scale synthesis of singlecrystal zigzag SiNWs with 150°, 125°, or 90° turning angle by controlling the crystallographic orientation of the Si wafer, reaction temperature, and etchant concentration. The experiments were carried out in a sealed reaction vessel26 using either n- or p-type (111)-oriented silicon wafers. Zigzag SiNWs were made by immersing cleaned silicon wafers in aqueous HF solution (4.6 M) with either 0.01 or 0.04 M sliver nitrate (hereafter, referred as Ag1 and Ag4 solutions, respectively) for 40 min at different temperatures. Figure 1 shows the scanning electron microscopy (SEM) images of a series of SiNW arrays that were synthesized on a (111)-oriented Si wafer by adjusting the etching conditions.
ne-dimensional (1D) semiconductor nanowires and nanotubes show outstanding electronic and optical properties, and they play a significant role in the advancement of nanotechnology.1-4 Among various 1D nanostructures, silicon nanowires (SiNWs) have attracted much attention due to the importance of silicon in the modern electronics industry. SiNWs are thus considered as a promising building block for assembling nanoelectronic and nanophotonic devices.5-8 Extensive efforts have been devoted to the synthesis of straight SiNWs and their fabrication into field-effect transistors,9-14 logic gates,15 resonators,16 and physical/biosensors.17-20 SiNWs have also been used as components for integrated macroscale devices with special functions. To realize the full potentials of SiNWs, new SiNWs building blocks with curved shapes are desirable, because they may be fabricated into novel devices with fewer welding joints and improved electric connection. Moreover, Ma et al.21 have shown that curved SiNWs, present in a very low percentage in SiNWs made by oxideassisted growth, can have novel electronic properties. However, the formation mechanisms of the curved SiNWs via gas phase approaches are unknown and the common approaches for SiNWs synthesis, like vapor-liquid-solid and oxide-assisted process, are inherently aimed to fabricate straight SiNWs. The scarcity of the curved SiNWs prevents the systematic studies of their properties; thus few reports
* To whom correspondence should be addressed,
[email protected] and
[email protected]. Received for review: 10/11/2009 Published on Web: 01/27/2010 © 2010 American Chemical Society
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FIGURE 1. Cross section SEM images of as-synthesized SiNWs on (111)-oriented silicon wafers after etching in solutions of (a) Ag1, 15 °C, (b) Ag4, 15 °C, (c) Ag1, 55 °C, (d) Ag4, 55 °C, and (e) Ag4, 75 °C, respectively. FIGURE 2. (a, b, and c) Cross-section SEM images of the three kinds of zigzag SiNW arrays on Si wafer, which was etched at 45-55 °C in Ag4. (d, e, and f) TEM (d) and SEM images of single zigzag nanowires. Inset: TEM images and SEAD patterns of the turning point of zigzag nanowires showing their growth directions alternating between (d) 〈111〉 and 〈113〉 periodically, (e) 〈111〉 and 〈100〉, and (f) two different 〈100〉 directions, respectively.
At lower temperatures (below 25 °C), AgNPs kept moving together along one direction and created arrays of straight SiNWs (Figure 1a,b). In sharp contrast, at the etching temperature of 55 °C, the movement directions of AgNPs may change and curved SiNWs can be found on the Si surface etched with Ag1 solution (Figure 1c). These novel Si nanostructures were more easily formed when etched in Ag4 solution (Figure 1d). Etching at 75 °C with Ag4 solution yielded nanowires dominantly in the zigzag structure (Figure 1e). To further investigate the formation of the zigzag structure, additional etching experiments were performed at 45-55 °C in Ag4 solution. Three types of curved SiNWs with different turning angles were observed. Parts a and d of Figure 2 show cross section SEM and transmission electron microscopy (TEM) images of the first type of zigzag SiNWs with a characteristic turning angle of 150°. The SiNWs have a smooth surface through their entire length and have diameters of about 100 nm. Most of the zigzag segments have repeated length between 0.5 and 1 µm. Selected area electron diffraction (SAED) and TEM analysis show that the longer and shorter segments have longitudinal orientations of 〈111〉 and 〈113〉, respectively (inset of Figure 2d and Figure S1a (in Supporting Information)). Parts b and e of Figure 2 show images of the second type of zigzag SiNWs. In this case, the SiNWs were found to transform from the straight line shape to zigzag structure at about 10 µm below the original wafer surface. Furthermore, TEM and SAED (Figure S1b (in Supporting Information) and Figure 2e) analyses revealed that the wire directions alternated between 〈111〉 and 〈100〉 with a characteristic turning angle of © 2010 American Chemical Society
125°. The third type of zigzag SiNWs has a characteristic turning angle of 90° (Figure 2, panels c and f). On comparison with other types of SiNWs, these SiNWs have a much larger repeating lengths of >10 µm. TEM (Figure S1c in Supporting Information) and SAED analysis show that these zigzag segments have longitudinal orientations that alternate between two orthogonal 〈100〉 directions (Figure 2f). The moving tracks of AgNPs were further investigated to investigate the formation mechanisms of the zigzag SiNWs. It can be seen from Figure 3 that direction alternation of the movement tracks of AgNPs corresponds well with the respective shapes of the formed SiNWs. Moreover, it can also be observed that the moving tracks of AgNPs in the first type of zigzag SiNWs are on the same plane, and they are smooth and continuous (Figure 3a). This observation shows that zigzag tracks are formed by periodic switches in the moving directions of the AgNPs. In contrast, the movement tracks of AgNPs in the second and third types of zigzag SiNWs are on different planes. This phenomenon suggests that the zigzag tracks in these cases are formed by interception of AgNPs movement in different directions, resulting in the observed “networks” of tunnels (Figure 3, panels b and c). The relative energies involved in AgNP movements along various crystallographic orientations in Si wafer play a significant role in the formation of the curved SiNWs. The 865
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FIGURE 4. The proposed growth or etching process of the zigzag Si nanowires. Detailed discussions are shown the text.
process, the AgNPs periodically switch their moving directions to etch out zigzag SiNW arrays as shown in Figure 4C-I. (2) Multiparticle etching mechanism: At higher etching temperatures/silver ion concentrations, the etching activity is enhanced, and more AgNPs can overcome the potential barrier to change their moving directions from the original 〈111〉 direction to the lowest energy 〈100〉 direction and steadily move along the most preferred directions. Different AgNPs at different locations might collectively move along one of the three different 〈100〉 directions. The merge of the moving tracks along the 〈100〉 and the 〈111〉 directions would result in SiNWs with a 125° turning angle (Figure 4C-II). Similarly, the SiNWs with 90° bends are formed by the merge of AgNPs moving along different 〈100〉 directions (Figure 4C-III). These results suggest a convenient approach for the wafer-scale synthesis of three types of single-crystal zigzag SiNW arrays near room temperature. We also found that straight and zigzag nanowires can be simultaneously formed at controlled positions on the same silicon wafer. Figure 5a shows cross-section images of a (111) Si wafer after etching in Ag4 at 45 °C. The as-received Si wafer before etching has a polished surface and a nonpolished surface. Interestingly, straight and zigzag SiNWs were formed on the originally polished and nonpolished surfaces, respectively. This result suggests that it is possible to control the etching morphology via the surface condition of the wafer. To verify this conclusion, we scratched part of a silicon wafer’s polished surface before etching. Upon etching in Ag4 at 15 °C, the scratched region produced zigzag SiNWs (right-hand side of Figure 5b) while straight SiNWs were developed on the unscratched, polished region. This controlled fabrication provides a practical approach to fabricate SiNWs with different shapes on selected regions. As an example, Figure 6 shows that Si wafers with different reflectivity/absorbance can be made by controlling the SiNW shapes.
FIGURE 3. SEM images of AgNPs movement tracks formed during chemical etching process, (a) continuous tunneling in the same plane with periodic direction alternation about 150° corresponding to the first kind zigzag SiNW. (b and c) AgNP movement tracks in different planes, going along with various crystallographic directions, and the crossed angle are 125° and 90° in (b) and (c), respectively.
most preferred migration direction of AgNPs in the Si wafer is the 〈100〉 direction.31,32 Indeed, when (100)-orientated Si wafer was used as substrate, the products were mostly straight SiNWs even at relatively high etching temperatures and etchant concentrations. However, when a non-(100) surface, in particular the more inert (111) surface (as we used in this experiment), was used as the starting surface, the 〈100〉 etching orientations were no longer the only etching direction because of the large angle between the 〈100〉 and the 〈111〉 directions. Generally, the AgNPs would first start moving perpendicular to the wafer along the 〈111〉 directions (Figure 4A), and then they either may keep moving along the 〈111〉 directions or may overcome higher potential barriers to move along other directions such as 〈100〉 or 〈113〉 (Figure 4B). On the basis of the above analysis, two possible mechanisms for the formation of the novel zigzag structures are proposed below and schematically illustrated in Figure 4C. (1) Single-particle etching mechanism: Due to perturbations at the reaction sites such as hydrogen bubbles, etc., individual AgNPs moving initially along the 〈111〉 direction might switch to another nearby direction such as 〈113〉. However, since the 〈113〉 orientation is not the most energetically favored orientation, as etching time increases, the AgNPs may switch back to 〈111〉. Repeating this perturbation © 2010 American Chemical Society
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FIGURE 6. (A) Photographs of Si wafers with different surface SiNW morphologies under sunlight. Sample (a) is a polished commercial Si wafer, and the top view and cross-sectional (inset) SEM images of the surfaces of samples b, c, and d are shown in (B), (C), and (D), respectively.
FIGURE 5. (a) Cross-section SEM image of a single-side polished (111)-oriented Si wafer etched in Ag4 at 45 °C. It can be seen that zigzag and straight SiNW arrays were obtained from the nonpolished and polished surfaces, respectively. (b) Top-view SEM image shows zigzag and straight SiNW arrays obtained from scratched (right) and unscratched (left) regions of a Si wafer in Ag4 at 15 °C.
etching process (Figure S2), the perturbations at the reaction sites, some possible approaches toward zigzag nanowires geometry control, and optical reflectance spectra for samples shown in Figure 6 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.
In summary, three types of single-crystal zigzag SiNWs with different turning angles have been prepared in wafer scale via a metal-assisted chemical etching approach. The zigzag structures are formed by changes in the moving direction of the AgNPs during etching. Shapes of the zigzag SiNWs can be controlled via the crystallographic orientation of Si wafer, reaction temperature, and etchant concentration. Significantly, SiNWs with different shapes on a wafer can be prepared by conditioning the local surface structure on the Si wafer.
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Acknowledgment. The work was partially supported by the National Basic Research Program of China (973 Program) (Grant No. 2006CB933000, 2010CB934500), National Natural Science Foundation of China (Grant No. 50825304, 50972190), and Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KGCX2-YW-391-2).
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Supporting Information Available. High-resolution TEM images of the three types of zigzag SiNWs with different turning angles (Figure S1), more evidence that AgNPs move in different crystallographic directions during the chemical © 2010 American Chemical Society
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