J. Phys. Chem. C 2008, 112, 17121–17126
17121
Initial Stage of Vapor-Liquid-Solid Growth of Si Nanowires Takahiro Kawashima,*,†,‡ Tatsunori Mizutani,§ Hiroyuki Masuda,⊥ Tohru Saitoh,⊥ and Minoru Fujii‡ Matsushita Electric Industrial Co., Ltd., 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan, and Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe UniVersity, Rokkodai, Nada, Kobe 657-8501, Japan ReceiVed: July 7, 2008; ReVised Manuscript ReceiVed: September 1, 2008
The initial stage of gold (Au)-catalyzed vapor-liquid-solid (VLS) growth of Si nanowires (SiNWs) was studied by transmission electron microscope (TEM) and atomic force microscope (AFM). Analysis and classification of the shape of SiNWs in TEM images revealed that there is strong correlation between the shape and crystallinity of Au catalysts and those of SiNWs, and the morphology of SiNWs is determined at a very early stage of the growth. The transition from initial horizontal growth of SiNWs on SiO2 substrates to vertical growth was studied by AFM, and it was demonstrated that SiNWs change growth direction abruptly by the formation of kinks. 1. Introduction Semiconductor nanowires (NWs) have recently attracted considerable research attention due to their unique physical properties1,2 and potential for device applications as transistors,3,4 chemical sensors,5,6 and light-emitting devices.7,8 Among different kinds of semiconductor NWs, silicon nanowires (SiNWs) have been most extensively studied because of their high compatibility with the standard complementary metal-oxidesemiconductor (CMOS) technology and the possible integration to future electronic devices. SiNWs have been synthesized by various methods,9-13 and a wide range of electronic devices have been demonstrated.3,5,14 Among them, the most widely used method is the vapor-liquid-solid (VLS) growth;9,10 the vapor precursors of silicon decompose at a metal catalyst droplet, diffuse through the catalyst, and are then solidified at the catalyst-SiNW interface. To realize SiNW-based electronic devices, it is important to gain precise control of the length, diameter, shape, and defects of SiNWs.11-17 Cui et al. found that the diameter of SiNWs depends strongly on that of a metal catalyst.11 Westwater et al. reported that the shape of SiNWs is related to the growth temperature and partial pressure of Si source gas.12 The initial stage of the growth has also been studied by some groups.15,16 Kalache et al. evaluated an incubation time, that is, the time required for the starting of nanowire growth, from a theoretical model and experimental results.15 Wu et al. confirmed the validity of the VLS growth mechanism by in situ transmission electron microscopy (TEM).16 These previous works considerably deepen our understanding of the mechanism of the VLS growth. However, the VLS process is still not perfectly controlled, and there still remain unknown factors that affect the structure of SiNWs. For example, small amounts of curved, * To whom correspondence should be addressed. E-mail address:
[email protected]. † Advanced Devices Development Center, Matsushita Electric Industrial Co., Ltd. ‡ Kobe University. § Material Science and Analysis Technology Center, Matsushita Electric Industrial Co., Ltd. ⊥ Image Devices Development Center, Matsushita Electric Industrial Co., Ltd.
branched, and particle-like SiNWs are usually included if we closely look at SiNW samples grown on SiO2 substrates.17 Although there may be a relation between the morphologies of SiNWs and catalysts, no detailed experimental work has been performed on the relation. One of the purposes of this work is to clarify the factors that control the shape of SiNWs and to reveal the mechanism of the formation of a variety of shapes. To achieve this, we first prepare Au catalysts with a variety of shapes and structures by acid treatment of colloidal Au particles. We then perform TEM observations of Au catalysts prior to SiNW growth and those after very short time growth of SiNWs. By analyzing the relation between the shape of the Au catalysts and the SiNWs at the very early stage of growth for a large number of samples, we will discuss the relation between the morphologies of Au catalysts and SiNWs. Another purpose of this work is to study the transition from the initial stage of growth where SiNW shape is significantly affected by the morphology of Au catalysts to the stable growth where most of the wires grow straight vertically to SiO2 substrates. We show from scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies that SiNWs first grow horizontally on SiO2 and at a certain stage of growth the growth direction is changed to vertical by the formation of kinks. 2. Experimental Section SiNWs were synthesized via VLS growth using a cold-wall infrared (IR) lamp-heated chemical vapor deposition (CVD) apparatus. Thermally oxidized Si(100) wafers (SiO2 thickness of 200 nm) were first treated with oxygen plasma, dipped into a 1 wt % solution of 3,5-diaminopentyltrimethoxysilane, and then dipped into a Au colloid solution (20 nm in diameter) for 5-30 min for the attachment of Au particles. Au particles attached in this process are negatively charged due to the adsorption of anions (citric acid). On the other hand, amino groups on the silane-coupling agent on the silanized silica surface are positively charged. Electrostatic interaction between them immobilizes Au particles on the wafer.11,23-25 By use of the wafer, high-quality SiNWs have been grown.11 In this work, in order to prepare Au catalysts with variety of shapes and structures, we added an acid treatment process. After
10.1021/jp8059568 CCC: $40.75 2008 American Chemical Society Published on Web 10/09/2008
17122 J. Phys. Chem. C, Vol. 112, No. 44, 2008
Kawashima et al.
Figure 1. Plan-view SEM images of SiNWs grown for 60 s. To form various shapes of Au catalysts, they are treated with the mixture of H2SO4 and H2O2 prior to the SiNW growth. Squares show SiNWs with different shapes: (A) straight; (B) curved; (C) particle-like; (D) branched with two Au catalysts; (E) branched with a single Au catalyst.
drying, wafers with Au particles attached were dipped into a 1:1:20 solution of sulfuric acid (H2SO4), hydrogen peroxide (H2O2), and water at 130 °C for 10 min (H2SO4/H2O2 treatment). H2SO4/H2O2 attacks organic materials and a part of the protectant and silane-coupling agent is etched. This weakens the electrostatic interaction between Au particles and substrates, allowing Au particles migrate over the substrate. Furthermore, etching of the protectant weakens repulsive forces between Au particles. These effects result in agglomeration of Au particles and formation of a variety of shapes (see the Supporting Information, Figure S1, which shows AFM images of particles before and after the treatment). After the acid treatment, wafers were loaded into the CVD chamber and then annealed in ultrahigh vacuum (UHV) by lamp heating at 350 °C for 5 min to remove organic moistures from the surface. For the growth of SiNWs, the flow rate of Si2H6 precursor and H2 were 100 and 800 sccm (cubic centimeters per minute at STP), respectively. The growth temperature and total pressure were fixed at 450 °C and 0.3 Torr, respectively. The growth duration was changed from 0 to 600 s. For TEM observations (Hitachi, HF-2200 operated at 200 kV) of SiNWs grown on thermally oxidized Si wafers, the wafers were thinned from the backside by ion milling. The morphology and shape of SiNWs were also studied by SEM (Hitachi, S-4000) and AFM (SII, SPA400) operated in a tapping mode. 3. Results and Discussion 3.1. Initial Stage of Growth. Figure 1 shows a plan-view SEM image of SiNWs grown for 60 s. We can see SiNWs with a variety of shapes. The bright hemispheres on top of the SiNWs are Au catalysts. The diameters of the wires are 20-100 nm and are almost the same as those of the Au catalysts. In Figure 1, we can classify SiNWs into five categories from the shape, that is, rod-like straight SiNWs (A), curved SiNWs (B), particlelike SiNWs (C), and branched SiNWs with more than two catalyst particles (D) and one catalyst particle (E). The large variation of the shape indicates that the shape is determined at a very early stage of the growth. In order to study the relation between the shape of Au particles and that of SiNWs, we performed plan-view TEM observations. Figure 2a shows a plan-view TEM image of the sample just before SiNW growth, that is, the wafer is annealed at 350 °C for 5 min in the growth chamber to remove organic moisture from the surface. Au catalysts with variety of shapes can be seen. Here, we will
Figure 2. Plan-view TEM images of (a) Au catalysts prior to SiNW growth, (b) SiNWs grown for 15 s, and (c) those grown for 60 s.
classify SiNWs from the shape of Au catalysts. The shape can roughly be classified into three categories as shown in Figure 3a-c. The first and second ones are particles with the aspect ratio of smaller than 1.75. The aspect ratio is defined as the ratio of long to short axis of Au catalysts. Within them, we define particles larger than 70 nm in diameter to type 1 and those smaller than 70 nm to type 2. The third one (type 3) is particles with the aspect ratio of larger than 1.75. Electron diffraction patterns reveal that type 1 particles are mainly polycrystalline, although single-crystal particles are partly involved. On the other hand, type 2 particles are almost always single-crystal. Type 3 particles are always polycrystalline (see the Supporting Information, Figure S2, which shows TEM images and electron diffraction patterns of type 1-3 particles). Type 1 and type 3 particles are considered to be formed by agglomeration and coalescent growth of small particles by the H2SO4/H2O2 treatment and annealing at 350 °C.
Initial Stage of Si Nanowires
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17123
Figure 3. Plan-view high-magnification TEM images of Au catalysts and SiNWs. The definition of types 1-4 is described in the main text. Panels a-c, d-f, and g-j are obtained from Figure 2, panels a, b, and c, respectively.
Figure 2b shows a plan-view TEM image of SiNWs grown for 15 s. We find SiNWs for all Au catalysts, that is, the catalyst activity is 100%. This high activity ratio is quite different from that of carbon nanotube (CNT), in which the ratio is reported to be a few percent in normal CVD except for supergrowth.18,19 The low ratio is generally due to inactivation of catalysts by covering the surface with amorphous carbon in the initial stage of the growth. The 100% growth of SiNWs just for 15 s is also significantly different from a previous report that discusses the incubation time of SiNW growth.15 In the paper, the incubation time for the growth of SiNWs at 450 °C with Au catalysts is reported to be 180 ( 20 s.15 If the same incubation time were required in the present system, no SiNWs could be grown in 15 s. A possible explanation for the difference is different source gases between ref 15 (SiH4) and the present work (Si2H6). The lower thermal stability of Si2H6 results in lower activation energy, which may significantly shorten the incubation time. Another possible reason is different morphology of catalysts. In this work, Au particles are formed before starting source gas flow, and thus the time necessary to form Au particles is zero. Attachment of Au catalysts on SiO2 surface is also a reason for the zero incubation time. If Au colloids are immobilized directly on a Si wafer, Si can diffuse through Au nanoparticles even at room temperature and form an oxide layer on the surface of Au particles. The oxide layer prevents SiNW growth. This effect is not expected on SiO2 substrates, and the surface of the Au particles is kept fresh until the supply of source gases starts. Figure 2c shows a plan-view TEM image after 60 s growth of SiNWs. Similar to the SEM image in Figure 1, SiNWs with a variety of shapes are observed. The next step is to reveal the correlation between the shapes of Au particles and SiNWs. However, since we do not trace the growth of SiNWs in situ, a one-to-one relation cannot be obtained. Furthermore, since
the shape of Au catalysts changes during growth due to the formation of a eutectic mixture, classification based on the catalyst shape is meaningless after SiNW growth. Therefore, statistical analysis of many particles is necessary to obtain information about the relation. Figure 3 summarizes the results of classification of Au catalysts and SiNWs in Figure 2 based on the shapes. The validity of this classification will be shown afterward by statistical analysis of the ratio of each type at different stages of the growth. The definition of types 1-3 for Au catalysts before SiNW growth (Figure 2a) is described above. After 15 s of growth (Figure 2b), the definitions of types 1-3 are as follows (Figure 3d-f): type 1, SiNWs with diameters larger than 70 nm and hemispherical Au catalysts; type 2, SiNWs with diameters smaller than 70 nm and hemispherical Au catalysts; type 3, SiNWs with diameters larger than 70 nm and nonhemispherical Au catalysts. After 60 s of growth (Figure 2c), we classify SiNWs into four categories (Figure 3g-j): type 1, rodlike SiNWs with diameters larger than 70 nm; type 2, rod-like SiNWs with diameters smaller than 70 nm; type 3, branched SiNWs with a single catalyst particle; type 4, branched SiNWs with more than two catalyst particles. In Figure 4, fractions of types 1-4 obtained for over 200 catalysts and SiNWs are summarized. We find that fractions of type 1 and type 2 are almost constant till the growth duration of 60 s. Furthermore, the fraction of type 3 is almost constant till the growth duration of 15 s and is almost the same as the sum of fractions of type 3 and type 4 SiNWs grown for 60 s. The almost constant ratio of the types indicates that the classification is valid, and there is strong correlation between the shape of catalysts and that of SiNWs. For example, rodlike SiNWs are grown from Au catalysts with relatively small aspect ratio (
