Synthesis of Si Nanowires by Using Atmospheric Pressure Chemical Vapor Deposition with Sicl4

Journal of the Korean Physical Society, Vol. 59, No. 2, August 2011, pp. 485∼488

Synthesis of Si Nanowires by Using Atmospheric Pressure Chemical Vapor Deposition with SiCl4

Taejin Choi and Hyungjun Kim∗ School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea

Doyoung Kim School of Electricity & Electronics, Ulsan College, Ulsan 680-749, Korea

Taehoon Cheon and Soohyun Kim School of Materials Science and Engineering, Yeungnam University, Daegu 705-717, Korea

(Received 28 April 2011)

Silicon nanowires were synthesized by using a vapor-liquid-solid method and atmospheric pressure chemical vapor deposition with silicon tetrachloride as the Si source. A thin Au film (1-nm thick) deposited on a Si substrate was used as a catalyst, and Au nanodots were formed subsequently by using a high-temperature chemical vapor deposition process. Small-diameter Si nanowires (∼15 nm) with a thin amorphous oxide sheath (2 ∼ 3 nm) were densely grown under the condition of abundant H2 gas and proper N2 carrier gas flow into the hot-wall chamber. The high crystallinity of the Si nanowires was verified by using high-resolution transmission electron microscopy, selected area electron diffraction, and Raman spectroscopy.

PACS numbers: 81.07.-b, 81.07.Bc, 81.15.Gh Keywords: Silicon nanowire, Silicon tetrachloride, Atmospheric pressure chemical vapor deposition, Vapor- liquid-solid, High-resolution transmission electron microscopy, Single-crystalline DOI: 10.3938/jkps.59.485

I. INTRODUCTION Si in catalyst. Among several different metal catalysts, including Fe [12], Ti [13], Ga [14], Al [15], and Au [16], the Au catalyst has been the primary choice due to its One-dimensional nanostructures such as nanowires needing a relatively low eutectic temperature to form a have a wide range of applications such as field-effect tran- Si-Au alloy (363 ◦C at about 19 wt% Si) compared to sistors [1,2], nano electronic devices, including biochem- other metal catalysts. For a CVD-based VLS process, ical sensors [3], photovoltaic cells [4], and even battery hydride gas, such as silane [17] or disilane [16], has been anodes [5]. Silicon nanowires (Si NWs) have attracted mostly used as the Si source. Especially, the synthesis great attention as a promising building block for emerg- of Si NWs under ambient pressure, such as in the atmo- ing device fabrication due to the potential use of quan- spheric pressure chemical vapor deposition (APCVD), tum confinement effects [6], resulting in novel electrical has benefits in terms of process cost and productivity. and optical properties, including a direct band gap [7]. One of the most widely employed Si precursors for the Various synthesis methods, such as laser ablation [8], synthesis of Si NWs is SiH4, which enables a low syn- chemical vapor deposition (CVD) [9], chemical etching thesis temperature compared to halide precursors, such [10], and thermal evaporation [11], have been used to as SiCl4. However, SiCl4 has benefits compared to SiH4 produce Si NWs. The CVD-based Si NW is mainly syn- in terms of safety and cost. Moreover, well-aligned Si thesized through the vapor-liquid-solid (VLS) process, NWs with a clean surface were reported due to the “self- which is a typical bottom-up fabrication process [9]. etching” effect of gaseous HCl byproduct during growth Si NW formation during the VLS process occurs as the using SiCl4 [18]. Earlier studies on Si NW synthesis us- following sequences; the diffusion of silicon atoms decom- ing SiCl4 and Au catalyst reported that the synthesis of posed from Si compounds into catalytic nanodots, forma- thin (<50 nm) Si NWs was very difficult. Zhang et al. re- tion of metal-Si eutectic liquid alloys, and precipitation ported the synthesis of thin (<20 nm) Si NWs by using of single-crystalline Si wires under the supersaturation of APCVD-based VLS with a Ni catalyst on Al2O3 substrates [19]. Especially, the SiCl4/H2 molar ratio turned ∗E-mail: [email protected] out to be critical for controlling the diameter of the Si -485- -486- Journal of the Korean Physical Society, Vol. 59, No. 2, August 2011

Fig. 1. Schematic of the APCVD experimental set-up for Si NW synthesis. Fig. 2. FE-SEM image of Au nanodots formed on a Si substrate at an annealing temperature of 900 ◦C under an ambient H2 atmosphere (the scale bar is 100 nm). NWs, which was also reported recently [20, 21]. How- ever, the thin Si NWs, based APCVD using SiCl4 and Au catalyst, has rarely been reported. In this study, we investigated the synthesis of thin (<15 nm) and single-crystalline Si NWs by using a Au catalyst. We prepared a uniform-sized sub-20-nm Au catalyst by using a very thin (<1 nm) Au film followed by annealing. The Si NW synthesis was investigated for various synthesis temperatures and gas flow ratios. The morphology and the microstructure of the synthesized Si NWs were analyzed by using high resolution transmis- Fig. 3. FE-SEM images of Si NWs at reactor temperatures of (a) 900 ◦C (the scale bar is 300 nm) and (b) 1000 ◦C (the sion electron microscopy (HR-TEM), scanning electron scale bar is 1 µm) for no N2 carrier gas flow and a hydrogen microscopy (SEM), and Raman spectroscopy. flow rate of 1000 sccm.

II. EXPERIMENT III. RESULTS AND DISCUSSION

The APCVD system is schematically shown in Fig. 1. Generally, the size of the metal catalyst is widely Si NWs were synthesized for 30 min at reactor tempera- known to affect greatly the diameter of the resulting tures of 900 ∼ 1000 ◦C. A thin layer of gold (∼1 nm) on Si NWs. Thus, we tried to obtain a small size of Au an n-type silicon (1 0 0) wafer was prepared by evapora- dots with a narrow distribution in size. For this, a very tion and then annealed to form gold nanodots under a H2 thin Au film (<1 nm) was evaporated, and dense gold atmosphere at the center of a hot quartz tube. As shown dots were obtained by subsequent annealing. Under the in Fig. 1, high-purity hydrogen gas (99.9999%) was sup- proper annealing conditions, which were described in the plied directly to the CVD reaction tube at a flow rate previous section, a relatively uniform, small-size Au nan- of 1000 standard cubic centimeters per minutes (sccm) odot array was obtained, as shown in Fig. 2. The average during the entire synthesis process to make the H2 atmo- diameter was 15 nm, with small distribution (standard sphere. In the initial experiments, the growth was car- deviation: ∼4 nm). ried out under a SiCl4 (Aldrich, 99.998%) vapor pressure Then, the Si NWs were synthesized using the Au dot with no carrier gas. At the later stage of experiments, array as a catalyst. First, the effect of the growth tem- however, a nitrogen gas (99.999%) was used as a carrier perature was investigated. Figure 3 shows FE-SEM im- gas at flow rates of 5, 10, 25, and 45 sccm to enhance the ages of Si NWs deposited at different reactor tempera- transport for longer and denser NWs. tures. For this, SiCl4 was flowed into the system with- After the reaction, the Au/Si sample was found to be out a carrier gas. At a reactor temperature of 900 ◦C, coated with dense Si NWs. The samples were charac- Si NWs with lengths of ∼1 µm and diameters of 10 ∼ terized with field-emission scanning electron microscopy 20 nm were produced on the Si substrate, as shown in (FE-SEM, JSM-6700F). Si NWs were separated from the Fig. 3(a). When the reactor temperature was increased substrate immersed in distilled water for 2 hours by us- to 1000 ◦C, relatively thick (∼80 nm), a few micrometer ing ultra-sonification. Then, the water containing the long Si NWs were obtained, as shown in Fig. 3(b). The NWs was dispersed on a glass substrate, dried, and ex- increase in diameter was attributed to the lateral growth amined via Raman spectroscopy (LabRam HR, Ar-ion on the sides and the bottoms of Si NWs at elevated tem- laser, 514 nm) at room temperature. Also, the NW so- peratures. Thus, the reactor temperature of 900 ◦C was lution was dispersed on a grid for the observation with chosen for the growth of dense and thin Si NWs. HR-TEM (Tecnai G2 F20 S-TWIN) and selective area Then, Si NWs were formed on a silicon wafer coated electron diffraction (SAED). with gold by changing the flow rates of N2 carrier gas to Synthesis of Si Nanowires by Using Atmospheric Pressure Chemical Vapor Deposition with SiCl4 – Taejin Choi et al. -487-

Fig. 4. FE-SEM images of Si NWs at N2 carrier gas flow rates of (a) 5 and (b) 45 sccm (under a H2 flow rate of 1000 sccm) at a reactor temperature of 900 ◦C (the scale bar is 1 µm). observe the relationship between the morphology of the Si NWs and the quantity of nitrogen carrier gas. FE- SEM images of representative morphologies of Si NWs are shown in Fig. 4. The Si NWs (Fig. 4(a)) deposited Fig. 5. HR-TEM image of single Si NW dispersed on a under the condition of 5 sccm of carrier gas were rela- HR-TEM grid. Inset (a) is the 2D Fourier transform image tively dense and long compared with the Si NWs grown of this HR-TEM figure, and inset (b) is an image of the SAED with no carrier gas flow (Fig. 3(a)). The average diam- for the same NW. eter of the Si NWs in Fig. 4(a) was about 20 nm. On the other hand, few NWs were observed on a substrate with a rough surface when the carrier gas flow rate was increased to 45 sccm, as shown in Fig. 4(b). This was attributed to the Si substrates being etched instead of Si NWs being synthesized under this condition. It should be noted that there have only been a few re- ports on the effect of the carrier gas during Si NW synthesis using a SiCl4 precursor and an Au catalyst under APCVD conditions. The results shown above indicate that the control of the carrier gas flow is very impor- tant in Si NWs synthesis using SiCl4. In other words, the flow of the carrier gas should be optimized to obtain highly dense NWs with well-controlled diameter. The use of a carrier gas is thought to enhance the transport efficiency of SiCl4 vapor, resulting in a dense array of Si NWs under the appropriate conditions. However, when Fig. 6. (Color online) Raman spectrum of the Si NWs the carrier gas flow is very high, etching rather than the transferred onto a glass substrate. The insert is a picture of formation of Si NWs is dominant during APCVD. The the Si NWs transferred onto a glass substrate. formation of HCl byproduct can be expected from the CVD process, resulting in severe etching of Si substrates and Si NWs. The two-dimensional Fourier transform image of Fig. 5 In order to observe the microstructures of individual shows that the highlighted spots are due to lattice fringes NWs for various thicknesses of the inner silicon core and of the (1 1¯ 1), (2 2 0), and (3 1 1) directions, as shown in the outer SiO2 layer, we performed HR-TEM observa- inset (a) of Fig. 5. The ring shape of the center is due to tions. Figure 5 shows a HR-TEM image of Si NWs sep- the amorphous oxide sheaths and the rough background. arated from the Fig. 4(a) sample. The image shows that The highlighted spots in the SAED patterns (inset (b) of the Si NW consists of a 10-nm-diameter crystalline sili- Fig. 5) reveal that the NW has a crystal structure, and con core and an outer oxide sheath of 2 ∼ 3 nm in thick- the main crystallographic growth direction of this NW ness. This is a typical structure for an APCVD-grown appears to be the (1 1¯ 2)¯ zone axis of Si. The triangular- Si NW [21]. The oxide sheath formation is due to oxy- shaped shadow is due to the tip of HR-TEM equipment. gen atoms binding with Si dangling bonds under high The Raman spectrum of the Si NWs revealed a large temperature and atmospheric pressure. The relatively peak at 520.1 cm−1, as shown in Fig. 6. The prominent thin oxide sheath is due to the etching caused by HCl, Raman peak centered at 520.1 cm−1 with a shoulder at as reported previously [18]. 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