Article pubs.acs.org/Langmuir
Annealed Au-Assisted Epitaxial Growth of Si Nanowires: Control of Alignment and Density Yi-Seul Park, Da Hee Jung, Hyun Ji Kim, and Jin Seok Lee* Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, South Korea S Supporting Information *
ABSTRACT: The epitaxial growth of 1D nanostructures is of particular interest for future nanoelectronic devices such as vertical field-effect transistors because it directly influences transistor densities and 3D logic or memory architectures. Silicon nanowires (SiNWs) are a particularly important 1D nanomaterial because they possess excellent electronic and optical properties. What is more, the scalable fabrication of vertically aligned SiNW arrays presents an opportunity for improved device applications if suitable properties can be achieved through controlling the alignment and density of SiNWs, yet this is something that has not been reported in the case of SiNWs synthesized from Au films. This work therefore explores the controllable synthesis of vertically aligned SiNWs through the introduction of an annealing process prior to growth via a Au-catalyzed vapor−liquid−solid mechanism. The epitaxial growth of SiNWs was demonstrated to be achievable using SiCl4 as the Si precursor in chemical vapor deposition, whereas the alignment and density of the SiNWs could be controlled by manipulating the annealing time during the formation of Au nanoparticles (AuNPs) from Au films. During the annealing process, gold silicide was observed to form on the interface of the liquid-phase AuNPs, depending on the size of the AuNPs and the annealing time. This work therefore makes a valuable contribution to improving nanowire-based engineering by controlling its alignment and density as well as providing greater insight into the epitaxial growth of 1D nanostructures.
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INTRODUCTION One-dimensional nanostructures, such as semiconductor nanowires (NWs), have long been known to possess fascinating physical properties that are not observed in their bulk counterparts, as well as being capable of quantum confinement effects that are found in low-dimensional nanostructures.1 Of these various nanomaterials, Si nanowires (SiNWs) have proven particularly attractive for use as building blocks in the development of novel nanodevices, including applications such as electronic and photonic devices,2−5 bio and chemical sensors,6 and solar cells.7 In particular, vertically aligned SiNWs may prove to be an excellent candidate for array devices such as vertical field-effect transistors (VFET), thus offering higher transistor densities and novel 3D logic or memory architectures.8 Recently, it has even been proven possible to employ vertically aligned SiNWs for biotechnological applications such as drug delivery9 thanks to the noncytotoxic nature of silicon.10 In principle, a prerequisite for the use of SiNWs in these aforementioned applications is that its electrical and biological properties are readily understood © XXXX American Chemical Society
and controllable, which in turn is strongly dependent on the diameter,11,12 length,13 density,14 and growth direction15 of the SiNWs. Well-defined and vertically aligned SiNW arrays can be fabricated by means of a top-down approach using lithographic patterning, followed by anisotropic etching.16 However, the resulting low quality of its physical properties, such as the rough surface of the SiNW sidewalls that is caused by the etching process,17 renders them unsuitable for device applications. In contrast, high-quality single-crystalline SiNWs have been grown by a bottom-up approach using the vapor−liquid−solid (VLS) process,18 which also allows control over key parameters such as diameter,19 length,20 density,21 growth direction,22 and shape.23 Other techniques have also been used for the growth of SiNWs, including laser ablation,24,25 supercritical fluid Received: August 28, 2014 Revised: October 3, 2014
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Figure 1. Schematic diagrams of (a) the furnace used for the synthesis of vertically aligned SiNWs and (b) vertically aligned SiNW growth on a Si(111) substrate covered with a Au film.
Figure 2. SEM images of AuNPs with different annealing times at a given annealing temperature on a Si(111) substrate covered with (a−d) 1- and (e−h) 10-nm-thick Au films. AuNPs were formed at 860 °C for (a, e) 0, (b, f) 10, (c, g) 30, and (d, h) 60 min. Scale bars are (a−d) 200 nm and (e− h) 1 μm. Plot of AuNP diameter as a function of annealing time for (i) 1- and (j) 10-nm-thick Au films. The uncertainty has been calculated from the standard deviation of the measured values.
solution phase,26 molecular beam epitaxy,27 and chemical vapor deposition (CVD).21 In the case of single-crystalline SiNW synthesis by CVD, growth follows the VLS mechanism when assisted by a metal catalyst.28 Because the size and position of this metal catalyst directly influence the diameter and position of the SiNWs, respectively,29 vertical growth of SiNWs can be achieved by maintaining epitaxial interfaces between the SiNWs and Si substrate. Typically, Au nanoparticles (AuNPs) are the most commonly used metal catalyst, resulting in a narrow diameter distribution of SiNWs despite the fact that the AuNPs themselves agglomerate into larger particles by strong van der Waals attractive forces and Ostwald ripening at high temper-
ature. The density of the SiNWs can also be controlled to a limited extent by adjusting the concentration of AuNPs dispersed in solution. In the preparation of a substrate covered with AuNPs, the AuNPs are first electrostatically attached using a polymer and then immobilized on the substrate. However, it is difficult to grow SiNWs epitaxially using AuNPs owing to the fact that the lattice matching between the crystal structures of the AuNPs and Si(111) substrate is interrupted by the intercalated polymer layer used as a binder. Consequently, the vertical alignment of the SiNWs is reduced. A lack of reproducibility when using AuNPs presents another clear problem that is yet to be solved.30 B
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Figure 3. Tilt-view (20°) SEM images of SiNWs after VLS growth on AuNPs formed after different annealing times at a given growth temperature on a Si(111) substrate covered with a (a−d) 1- or (e−h) 10-nm-thick Au film. SiNWs were synthesized from AuNPs formed at 860 °C for (a, e) 0, (b, f) 10, (c, g) 30, and (d, h) 60 min. Scale bars are (a−d) 1 and (e−h) 2 μm. Plot of the diameter and density of vertically aligned SiNWs as a function of annealing time for (i) 1- and (j) 10-nm-thick Au films after VLS growth. The uncertainty has been calculated as the standard deviation of the measured values.
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RESULTS AND DISCUSSION The growth process for vertically aligned SiNWs, using a Au thin film as a metal catalyst, is described in Figure 1.10 When the temperature of the substrates is increased, the Au thin film evaporated on the Si(111) wafer starts to crack randomly and then migrate onto the wafer, thus resulting in liquid-phase Au nanodroplets of various sizes (Figure 1b). This cracking allows the Au to be randomly deposited over the Si(111) wafer, with it then diffusing over the surface by two well-known mechanisms for Au migration to form Au nanoparticles (AuNPs).32 The first of these migration mechanisms is coalescence, which occurs when AuNPs in contact merge to form a single, larger AuNP in order to reduce the total interfacial energy of the system. In contrast, the subsequent Ostwald ripening process involves the removal and transfer of Au atoms from one AuNP to another by evaporation or surface diffusion, resulting in AuNPs with irregular diameters. Upon reaching the reaction temperature, vapor-phase Si precursors are introduced into the liquid-phase AuNPs by 10% H2 in Ar carrier gas, thereby forming single crystals of Si by precipitation from a Au−Si alloy upon reaching supersaturation in what is known as a vapor−liquid−solid (VLS) mechanism.21 If the lattice of the precipitated Si crystal matches that of the Si(111) substrate, then the epitaxial growth of SiNWs occurs in the [111] direction. However, the irregular diameter of the AuNPs means that the SiNW diameter is also not uniform. It is also impossible to control the SiNW density, owing to the fact that AuNPs are naturally produced upon reaching the growth temperature. We adopted an annealing stage between the temperature ramp up and growth periods in order to address such
The epitaxial growth of SiNWs can be more easily conducted using a Au film directly deposited by Au evaporation as a metal catalyst for SiNW growth.10,31 The use of a Au film rather than AuNPs promotes the vertical growth of the SiNWs30 as the lattice matching between the Au film and Si(111) substrate is maintained. It is, however, difficult to control the density of SiNWs in this manner because it tends to result in quite dense AuNPs being formed randomly upon reaching the high temperatures required for growth. This ultimately results in a broad diameter distribution of SiNWs, thus making the use of a Au thin film as a catalyst for SiNW growth superior to AuNPs with regard to epitaxial growth but much less effective when it comes to controlling the diameter and density of SiNWs.10 In this study, we report on a Au-assisted epitaxial growth method for the synthesis of SiNW arrays with controlled alignment and density. On the basis of the fact that Au can migrate and agglomerate on a substrate at high temperature because of the high mobility of Au atoms, we controlled the size and density of the as-produced AuNPs by regulating the time of the annealing process used for the formation of AuNPs from the Au film. Furthermore, the properties of the AuNPs formed, and therefore the SiNWs, could also be varied by changing the thickness of the Au film. This means that although the annealing process has an effect on the alignment and density of vertically aligned SiNWs, it is dependent on the initial thickness of the Au film. The mechanisms of this annealed Au-assisted epitaxial growth were studied in detail and are presented in detail herein. C
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Figure 4. XRD pattern showing the alignment of SiNWs synthesized from (a) 1- and (b) 10-nm-thick Au films annealed at 860 °C for different times. The XRD peak marked with an asterisk is from Au present at the tip of the SiNWs.
diameter as well as increasing the standard deviation by broadening the divide between the smaller and larger AuNPs.36 Figure 3 shows 20° tilt-view SEM images of SiNWs that were synthesized from AuNPs formed at 860 °C by 0, 10, 30, and 60 min of annealing on a Si(111) wafer covered with a 1- or 10nm-thick Au film. In the case of a 1-nm-thick Au film, the number of vertically aligned SiNWs is reduced with increased annealing time, with a subsequent increase in misaligned SiNWs. To understand more clearly the characteristics of SiNWs produced from a 1-nm-thick Au film, the top diameter and density of vertically aligned SiNWs were measured and plotted as a function of annealing time, as shown in Figure 3i. This shows that the diameters of the SiNWs are 71.76 ± 8.62, 72.17 ± 15.59, 75.74 ± 17.10, and 90.56 ± 19.76 nm with densities of 2.57 ± 0.32, 1.90 ± 0.35, 1.31 ± 0.34, and 0.57 ± 0.23 NWs/μm2 for annealing times of 0, 10, 30, and 60 min, respectively. The increasing diameter of the SiNWs synthesized from the 1-nm-thick Au film corresponds to the results for increased annealing time,32 as shown in Figure 2a−d, thus indicating Ostwald ripening. Also, the vertically aligned SiNWs (Figure 3a−d) show a tendency to reduce their density by distorting their alignment. For the 10-nm-thick Au film, a plot of the diameter and density of the vertically aligned SiNWs as a function of annealing time is shown in Figure 3j. From this, the diameters of the SiNWs are found to be 217.02 ± 51.52, 230.35 ± 54.08, 305.49 ± 63.79, and 327.50 ± 67.97 nm with densities of 2.00 ± 0.28, 1.93 ± 0.41, 1.37 ± 0.92, and 0.69 ± 0.20 NWs/μm2 for annealing times of 0, 10, 30, and 60 min, respectively. As with the 1-nm-thick Au film, the diameter of SiNWs can be seen to increase with annealing time for the same reasons mentioned above. The density of the vertically aligned SiNWs (Figure 3e−h) also decreases with increasing annealing time. Furthermore, the top-view SEM images of the SiNWs synthesized with different annealing times, in which vertically aligned SiNWs appear as dots, further reveal that their density is reduced to 0.0336 ± 0.017 NWs/μm2 by increasing the annealing time up to 120 min (Supporting Information, Figure S2). Interestingly, close scrutiny of the SEM images of SiNWs synthesized with a 10-nm-thick Au film reveals that the fraction of AuNPs not involved in the growth of SiNW is gradually increased with increasing annealing times. We believe that this phenomenon adequately explains the difference in density of SiNWs compared to the density of the 1-nm-thick Au film. This means that with a 10-nm-thick Au film, the alignment of SiNWs
conventional problems as the limitation in controlling the density of vertically aligned SiNWs generated from a Au film (Supporting Information, Figure S1). The formation of AuNPs from 1- and 10-nm-thick Au films by annealing was confirmed prior to SiNW growth, with Figure 2 showing SEM images of AuNPs formed after 0, 10, 30, and 60 min of annealing at 860 °C on a Si(111) substrate covered with a 1- or 10-nm-thick Au film. Distinctly different features were observed between the AuNPs obtained from the two different film thicknesses. This can be attributed to the fact that Au films are known to melt into various circular or triangular shapes around intact AuNPs,33 which in this study were observed only in the case of AuNPs formed from a 10-nm-thick Au film. With short annealing times, the AuNPs begin to form a continuous Au film by cracking and therefore do not have a specific morphology, instead being spread out on the substrate. However, with a longer annealing time, the AuNPs continuously migrate and aggregate on the surface of the substrate, eventually developing a crystalline morphology from irregularly to hexagonally shaped.34 To monitor the progress of Au migration on the Si(111) substrate quantitatively as a function of annealing time, the diameters of the AuNPs produced from 1- and 10-nm-thick Au films were also measured and plotted (Figure 2i,j). In the case of the 1-nm-thick Au film, an increase in the annealing time from 0 to 60 min resulted in a simultaneous increase in the average AuNP diameter and the standard deviation of this average. The measured diameters of the AuNPs are 13.18 ± 7.56, 18.56 ± 8.08, 25.03 ± 8.82, and 30.59 ± 12.07 nm for 0, 10, 30, and 60 min of annealing, respectively (Figure 2i). With an increase in the annealing time, the average diameter and standard deviation of the AuNPs from the 10-nm-thick Au film are also increased. The diameters of these AuNPs were found to be 241.44 ± 39.82, 242.86 ± 53.11, 281.86 ± 53.42, and 293.51 ± 67.09 nm for 0, 10, 30, and 60 min of annealing, respectively (Figure 2j). This phenomenon can be explained by Au migration on the substrate under the high-temperature conditions of annealing.32 With an increase in the annealing time, AuNP formation is therefore dependent on Ostwald ripening because the NPs are kept at a high temperature for a long period of time. Consequently, after 60 min of annealing, there occurs is a little evaporation of Au atoms,35 and most Au atoms migrate from small to large AuNPs at the annealing temperature of 860 °C based on Ostwald ripening. Therefore, the small AuNPs become smaller and the large AuNPs become larger. This, in turn, caused an increase in the average AuNP D
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Figure 5. (a) Low-magnification TEM image of a SiNW. The scale bar is 500 nm. Inset of (a): SAED pattern indexed for single-crystalline Si. (b) High-resolution TEM image taken from the area designated in (a). The scale bar is 2 nm. (c) Tilt-view (20°) SEM image of SiNWs at the bottom of a substrate synthesized from a 10-nm-thick Au film annealed at 860 °C for 60 min. The scale bar is 1 μm. (d) EDX data for the remaining AuNPs that did not synthesize SiNW after VLS growth. The inset table shows the existence of an oxide layer on the AuNPs.
Figure 6. XRD patterns of AuNPs annealed at 860 °C for different times from (a) 1- and (b) 10-nm-thick Au films.
remains consistent with increasing annealing time, with only the density of the vertically aligned SiNWs being reduced. The X-ray diffraction (XRD) patterns of the SiNWs synthesized from a 1-nm-thick Au film exhibit a major (111) peak at 28.58° with all annealing times, as shown in Figure 4a, with the peak marked by an asterisk being from Au at the tips of SiNWs. XRD peaks at 47.56 and 56.40° correspond to the (220) and (311) directions. The relative intensities of the (220) peaks are 0.0071, 0.0928, 2.4396, and 9.4151% for annealing times of 0 to 60 min, and the relative intensities of the (311) peaks are 0.0056, 0.0618, 1.0716, and 4.5649%. These XRD results support the SEM images shown in Figure 3a−d, with the consistent (111) peak at 28.58° for all annealing times in the XRD pattern of SiNWs synthesized from a 10-nm-thick Au film (Figure 4b) corresponding to the SEM images seen in Figure 3e−h. However, unlike Figure 4a, no peaks are discernible at 47.56 or 56.40°, which means that most of the SiNWs are restricted to growth in only the [111] direction. The relative intensity of this (111) peak at 28.587° is directly proportional to the number of vertically aligned SiNWs and thus is reduced by the decreasing SiNW density. A surprising observation is
that almost-grown SiNWs did not fall on the substrate, with the alignment of the SiNWs instead being nearly perfect. The morphology and crystallinity of the SiNWs were also characterized by TEM analysis. Figure 5a shows a representative low-magnification TEM image of a SiNW synthesized from a 10-nm-thick Au film annealed at 860 °C for 0 min, wherein the diameter and length are 217 nm and 3 μm, respectively. The selected area electron diffraction (SAED) pattern (Figure 5a) and high-resolution TEM (HRTEM) image (Figure 5b), taken from the area designated in Figure 5a, confirm that the SiNW is single-crystalline in nature and grows along the [111] direction with 0.334 nm spacing. Figure 5c shows the bottom area of a Si(111) substrate after VLS growth with a 10-nm-thick Au film annealed for 60 min. In this, we see that the majority of synthesized SiNWs are aligned in a single direction that is perpendicular to the Si(111) substrate, which means that they have maintained epitaxial growth. Energy-dispersive X-ray spectroscopy (EDX) analysis of the elemental composition confirms the existence of an oxide layer on AuNPs remaining after VLS growth (Figure 5d). We believe that the presence of this oxide layer on AuNPs melted onto the E
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Figure 7. (a) High-magnification SEM image of AuNPs annealed from a 10-nm-thick Au film at 860 °C for 60 min. (b) EDX elemental mapping of the same area in (a). Scale bars in (a) and (b) are 500 nm. EDX data for (c) AuNPs and (d) melted AuNPs in a Si(111) substrate annealed at 860 °C for 60 min. The inset table shows the quantitative atomic ratio.
SEM images of AuNPs annealed at 860 °C for 60 min clearly show numerous AuNPs melted onto the Si(111) substrate, as indicated by white arrows in Figure 7a. These melted AuNPs, which were characterized as gold silicide by XRD analysis in Figure 6b, are seen as foggy regions surrounding the original AuNPs, which are indicative of spreading and diffusion into the Si(111) substrate.40 Figure 7b shows the EDX elemental mapping of the same location, with the quantitative atomic ratios of AuNPs and melted AuNPs shown in Figure 7c,d. An oxide layer is observable only in the melted AuNPs,41 as indicated by white arrows. Figure S3 (Supporting Information) confirms that there are no melted AuNPs after 0 min of annealing, with AuNPs clearly discernible from the Si(111) substrate in SEM images. Furthermore, EDX elemental mapping and the quantitative atomic ratio analysis of AuNPs in Figure S3 show that there is no detectable oxide layer on the AuNPs. Because the reactor used is not an ultrahigh vacuum (UHV) system, oxygen is present during the annealing period and results in localized oxidation of the substrate42 and gold silicide.39 The observed decrease in the density of vertically aligned SiNWs, as shown in Figure 3, is explained by the formation of an oxide layer with increased annealing time. In the case of the 1-nm-thick Au film, only limited gold silicide is formed when annealed at 860 °C, so oxidation occurs only on the surface of the Si(111) wafer. This oxide layer begins to form from the outside of the AuNPs, though it is also possible for it to form underneath them if their diameter is less than 30.59 ± 12.07 nm (Figure 2i). Because the epitaxial growth of SiNWs is not possible on AuNPs underneath an existing oxide layer, its presence disrupts the direct contact between AuNPs and the Si(111) substrate.43,44 In the case of a 10-nm-thick Au film, gold silicide is actively formed during annealing and then readily covered with an oxide layer.41 Therefore, although the reason may be different from that of the 1-nm-thick Au film, the number of vertically aligned SiNWs is nonetheless reduced by an increase in the annealing time because gold silicide is readily transformed by oxidation into a different material. The oxidation of gold silicide blocks the supplied Si precursor, so no SiNWs are synthesized. Furthermore, although an oxide layer is also formed on the surface of Si, no oxide layer is
Si(111) substrate is the reason that SiNWs are not synthesized after 60 min of annealing with a 10-nm-thick Au film. Instead, the alignment of the SiNWs remains consistent with increasing annealing time, and only the density of vertically aligned SiNWs decreases. To confirm the changes in the Au film during the annealing process, XRD analysis was conducted on Au films deposited by evaporation and annealed for different times,37 ranging from 10 to 60 min (Figure 6). In the case of a 1-nm-thick Au film, no Si(111) peak is observed in the as-deposited Au film, but this begins to appear after annealing at 860 °C (Figure 6a). When the annealing time is increased, the AuNPs agglomerate into one large AuNP through coalescence and Ostwald ripening. Consequently, the relative intensity of the Si(111) peak is increased by increased exposure of the Si(111) substrate, as shown in Figure 2d. Furthermore, the intensities of Au(111) and Au(200) remain consistent because of the small size of the AuNPs. Peaks corresponding to Au3Si(121), Au3Si(014) (PDF 00-024-0463), and Au5Si2(130) (PDF 00-036-0938) are also observed at 34.68, 35.66, and 39.52°, even though the Au film after deposition is not annealed. There is no change seen in these gold silicide peaks in any of these films. In the case of a 10-nm-thick Au film, Si(111) peaks are observed that increase in relative intensity with annealing time, as in the case of a 1-nm-thick Au film (Figure 6b). The relative intensity of the Au(111) and Au(200) peaks also increases with annealing time because of the crystallization of the AuNPs; however, there is a difference in that the intensity of the gold silicide peaks increases with annealing time, indicating the formation of gold silicide during annealing.38,39 When AuNPs exist at high temperature with a Si(111) wafer, the Si atoms in the wafer are transported into liquid-phase AuNPs at the Au−Si interface to form gold silicide. In the case of those AuNPs obtained from a 1-nm-thick Au film, the contact area of this interface is insufficient to transport the Si atoms required for the formation of gold silicide because the size of the AuNPs is on the order of several tens of nanometers (Figure 2i). In contrast, the contact area of AuNPs from a 10-nm-thick Au film is large enough to accept the transported Si atoms from the Si(111) wafer because their size is on the order of several hundred nanometers (Figure 2j). F
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Figure 8. Illustration depicting SiNW growth from AuNPs formed at 860 °C after 60 min (annealing time) on a Si(111) substrate covered with (a) 1- and (b) 10-nm-thick Au films. AuNPs, AuSiNPs, and SiNWs are represented by yellow, orange, and red, respectively, and the oxide layer is indicated in blue.
synthesize density-controlled SiNWs when a Au film is used as a catalyst. The results obtained with both thin and thick Au films exhibited differences with regard to the formation of gold silicide and the contact area between AuNPs and the Si(111) substrate, but an overall tendency toward reduction in vertically aligned SiNW density with annealing time is the same regardless of Au thickness. When the thin Au films are annealed, an oxide layer can be formed underneath the AuNPs, which affects the growth of vertically aligned SiNWs by interrupting the contact between AuNP and the Si(111) substrate. As a result, the degree of alignment of SiNWs can also be controlled by regulating the annealing time, although controlling the overall quantity of SiNWs from an Au film is still regarded as impossible. Nevertheless, we believe that these results can help to improve nanowire-based engineering by controlling the alignment and density and also providing greater insight into the epitaxial growth of 1D nanostructures.
formed underneath the AuNPs formed from a 10-nm-thick Au film. This means that contact between them and the Si(111) wafer is not interrupted; consequently, vertically aligned SiNWs are more readily synthesized by maintaining a connection with the matching crystal lattice of the substrate. The growth mechanism for vertically aligned SiNWs on a Si(111) substrate is illustrated in Figure 8, with (a) a thin Au film and (b) a thick Au film used as a catalyst. In this process, the formation of gold silicide is vital because it influences the alignment and density of vertically aligned SiNWs. To obtain this gold silicide, two processes are required: first, Si atoms must be transported at the Au−Si interface;45 second, sufficient heating must be applied to the Au film during the annealing process.38,39 With a thin Au film (1 nm thickness), the contact between AuNPs and the Si(111) substrate is insufficient for the transportation of Si atoms because of the small contact area, so the formation of gold silicide does not occur. Furthermore, an oxide layer is formed on the Si(111) substrate during the annealing period following the formation of the AuNPs.41 The small size means that an oxide layer can potentially also be formed underneath the AuNPs, which disrupts contact with the substrate, inhibits epitaxial growth, and results in the synthesized SiNWs having a distorted alignment. In the case of a thick Au film (10 nm thickness), the contact area between AuNPs and the Si(111) substrate is sufficiently greater that the transportation of Si atoms can accommodate, so gold silicide is formed while the high temperature is maintained. When the annealing time is increased, a larger amount of gold silicide is formed and is readily oxidized after annealing in a non-UHV system.42 However, the AuNPs themselves cannot be oxidized under these conditions. Although an oxide layer is formed on the surface of the Si(111) substrate, the AuNPs maintain contact because of their large size. Vertically aligned SiNWs are therefore synthesized on the remaining AuNPs that interact with the Si(111) substrate but not on the oxidized gold silicide NPs. The density of vertically aligned SiNWs can therefore be controlled by regulating the annealing time, which in turn affects the amount of gold silicide formed.
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EXPERIMENTAL SECTION
Chemicals. Silicon tetrachloride (SiCl4, 99.998%) and ammonium fluoride (NH4F, ∼40% in water) were purchased from Aldrich. Hydrofluoric acid (HF, 32−52% solution in water) was purchased from Acros. Acetone and isopropyl alcohol were purchased from Ducksan. Finally, Si(111) wafers were purchased from LG Siltron. Preparation of a Au-Covered Substrate. The Si(111) wafer substrate was first cleaned with acetone and isopropyl alcohol and then dried using nitrogen gas. This was subsequently dipped for 4 min into a buffered HF solution (9% HF solution (48−52%) in water, 32% NH4F solution (∼40%) in water) to remove the native oxide layer and then rinsed with water. The as-prepared Si(111) wafer was then covered with an Au film by e-beam evaporation. Next, the substrate was diced into small pieces with dimensions of 1 cm (width) × 1 cm (length), with each resulting Au-covered substrate being used for SiNW growth. Synthesis of SiNWs. Vertically aligned SiNWs were synthesized by CVD in a 12 in. horizontal tube furnace (Lindberg/Blue M) equipped with a 1-in.-diameter quartz tube, as shown in Figure 1a. The prepared substrate was then placed in the tube furnace in the center of the heating zone. Prior to the experiment, the quartz tube was evacuated and flushed repeatedly with 10% H2 gas (high purity, 99.999%) in order to minimize oxygen contamination. The reaction diagram used for the controllable growth of vertically aligned SiNWs using a Au film as a catalyst is shown in Figure S1 (Supporting Information). When the Au-covered Si(111) substrate was loaded into the center of the tube furnace, the temperature was increased to a growth temperature of 860 °C at a rate of 20 °C/min during the ramping period. Once the growth temperature was reached, the Aucovered Si(111) substrate was held at this temperature for different
CONCLUSIONS
Density-controlled, vertically aligned SiNWs were synthesized by the annealing of Au films between the ramp up and growth periods. This achievement has great significance for controlling the density of SiNWs because it is ordinarily impossible to G
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annealing times in order to determine the effect of this on SiNW growth. During the ramping and annealing period, the Au film is transformed into AuNPs by cracking at high temperature. The 10% H2 in Ar gas mixture used in the reaction plays two roles. One is as a carrier gas (C), which passes through a bubbler containing a SiCl4 solution to provide a Si precursor and is then carried into the reactor. The other is as a dilution gas (D), which directly enters the reactor and adjusts the concentration of the Si precursor. During the ramping period, only the dilution gas is introduced at 100 sccm, whereas both the carrier gas and dilution gas are added during the growth period at flow rates of 100 and 750 sccm, respectively. After 10 min of growth, the reactor was cooled to room temperature. Characterization. The surface morphologies of the synthesized, vertically aligned SiNWs were characterized using a field-emission scanning electron microscope (FE-SEM) (JEOL JSM-7600F) at an acceleration voltage of 15 kV with energy-dispersive X-ray spectroscopy (EDX) for chemical analysis, which was performed on the assynthesized product on the substrates. We obtained X-ray diffraction (XRD) patterns of the SiNWs using a Rigaku diffractometer (D/MAX1C) with a monochromatic beam of Cu Kα radiation. Then, the singlecrystalline structure of SiNWs was analyzed with a JEOL 2010 transmission electron microscope (TEM). The samples for TEM imaging were made by depositing an ethanol solution of SiNWs prepared by sonication of the as-synthesized substrate in ethanol onto holey carbon 300 mesh copper grids (Structure Probe, Inc.).
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ASSOCIATED CONTENT
S Supporting Information *
Reaction diagram and additional SEM and EDX data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Nano·Material Technology Development Program (2012M3A7B4034986) funded by the National Research Foundation and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2012-0009562). Additionally, it was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2022597).
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ABBREVIATIONS AuNPs, gold nanoparticles; SiNWs, silicon nanowires; CVD, chemical vapor deposition; VLS, vapor−liquid−solid; AuSiNP, gold silicide nanoparticle
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REFERENCES
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