Physica E 37 (2007) 153–157 Dimensional evolution of silicon nanowires synthesized by Au–Si island-catalyzed chemical vapor deposition D.W. Kwak, H.Y. Cho, W C. Yang à Department of Physics and Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul 100-715, South Korea Available online 11 September 2006 Abstract This study explores the nucleation and morphological evolution of silicon nanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemical vapor deposition. The Au–Si islands are formed by Au thin film (1.2–3.0 nm) deposition at room temperature followed by annealing at 700 1C, which are employed as a liquid-droplet catalysis during the growth of the Si-NWs. The Si-NWs are grown by exposing the substrates with Au–Si islands to a mixture of gasses SiH 4 and H 2 . The growth temperatures and the pressures are 500–600 1C and 0.1–1.0 Torr, respectively. We found a critical thickness of the Au film for Si-NWs nucleation at a given growth condition. Also, we observed that the dimensional evolution of the NWs significantly depends on the growth pressure and temperature. The resulting NWs are 30–100 nm in diameter and 0.4–12.0 mm in length. For Si (0 0 1) substrates 80% of the NWs are aligned along the / 111S direction which are 301 and 601 with respect to the substrate surface while for Si (1 1 1) most of the NWs are aligned vertically along the /111S direction. In particular, we observed that there appears to be two types of NWs; one with a straight and another with a tapered shape. The morphological and dimensional evolution of the Si-NWs is significantly related to atomic diffusion kinetics and energetics in the vapor–liquid–solid processes. r 2006 Elsevier B.V. All rights reserved. PACS: 66.30.h; 68.70.w; 81.15.Gh Keywords: Si nanowires (Si-NWs); Au–Si alloy droplets; VLS; Chemical vapor deposition; Diffusion kinetics; Morphological evolution 1. Introduction The ongoing reduction of electronic device size has led to a transition of technological approach from top–down to bottom–up due to current lithographic limitations. The controlled fabrication of the self-organized nanostructures as a building blo ck in the bottom–up approach has been significant for advanced technological applications. In particular, one dimensional silicon nanowires (Si-NWs) have recently become of interest for potential applications in various technologies such as optics, electronics, and chemical sensors. This is the Si-NW s can offer the possibility of integration with conventional Si integrated- circuit technology [1–3]. The quantum effects in the electronic and optical properties of the nanodevices are strongly related to nanostructure’s dimensions [2]. Therefore, good control of the dimensions and alignments of the NWs is required to employ them as elements of nanodevices. Vapor–liquid–solid (VLS) growth method has been widely employed for the NW growth of various materials [3]. In metal island-catalyzed growth of Si-NWs via the VLS processes, the diameter and alignment of the NWs can be readily controlled by the size of the catalytic metal islands and the substrate orientation [4,5]. However, the diameters of the reported NWs did not correspond to that of the metal islands but were extensively modified with variation in growth conditions [4,6]. Thus, detai led studies on the correlation of the growth parameters with morpho- logical evolution of Si-NWs in the VLS processes are still required for the well-controlled growth. In the VLS growth mechanism, the evolution of the NWs proceeds with three well-known stages: metal alloying process, crystal nuclea- tion, and axial growth [7]. These processes involve mass transport through metal alloying and energetics of the ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.07.017 à Corresponding author. E-mail address: wyang@dongguk.edu (W C. Yang). system. Thus, these factors will determine the dimension and orientation of the evolving Si-NWs. In this study, we investigate the morphological evolution of Si-NW s on Si substrates grown by nanoscale Au–Si island-catalyzed rapid thermal chemical vapor deposition (RTCVD). The initial nucleation of the NWs from the Au–Si islands is examined while varying island sizes. Also, we investigate the variation in the morphology and dimension of the NWs depending on the growth pressures, temperatures and times. In particular, preferential growth directions of the NWs are identified for Si (0 0 1) and (1 1 1) substrates. The results are discussed in terms of atomic mass transport and energetics at interfaces of vapor/liquid and liquid/solid in the NW growth via the VLS mechanism. 2. Experimental procedure P-type Si (0 0 1) and (1 1 1) wafers were employed as substrates. The wafers were cleaned ultrasonically with acetone and methanol for 10 min, and then rinsed under running de-ionized wat er. In order to remove native oxide, the wafers were dipped into 2% HF (HF:H 2 O ¼ 1:50) for 3 min and then flushed by dry nitrogen. The cleaned wafers were transferred into an e-beam evaporator chamber to deposit 1.2–3.0 nm thick Au films with a growth rate of 0.01 nm/s at room tempe rature. Then, the Au-deposited substrates were transferred to the RTCVD chamber, where they were annealed in hydrogen ambient at 0.5 Torr and 700 1C for 10 min to form Au–Si islands. After the island formation, the substrate temperature was reduced to Si-NW growth temperatures and the substrates were exposed to a mixture of SiH 4 (1–4 sccm) and H 2 (50 sccm) for 30–120 min. The growth temperatures and total chamber pressures are 500–600 1C and 0.1–1.0 Torr, respectively. The morpho logy of the grown Au–Si islands and the Si-NWs were characterized by using a field emission scanning electron microscopy (FESEM) for a 301 tilted view and cross-sectional geometries. The dimen- sions of the Au–Si isl ands and the Si-NWs were measured from the obtained SEM images. 3. Results and discussion To explore initial nucleation of Si-NWs depending on catalysis island size, we initiated the NW growth with varying Au film thicknesses (1.2, 2.0 and 3.0 nm). Fig. 1 shows the substrate morphology before and after exposing the annealed Au films to the silane (SiH 4 ) mixture gases. Upon annealing at above the Au–Si eutectic temperature (360 1C), the Au film reacts with Si substrate and dissolves Si to form Au–Si alloy liquid [8]. Further annealing leads to a transformation of the liquid into Au–Si alloy droplet structures, whose shape is determined by minimization of the surface and interface energy of the liquid/substrate. Also, the composition of the Au–Si liquid alloy droplets will follow the liquid at annealing tempera- tures [8]. Thus, the islands in Figs. 1(a) and (b) were formed from the Au–Si alloy droplets after cooling down to room temperature. For annealing temperature of 700 1C, the composition of the Au–Si alloy islands might be 9% Si, which can be estimated from Au–Si binary phase diagram [8]. The surface shape of the islands is smooth and circular even though the edge of the islands are irregular in Figs. 1(a) and (b), indicating that the islands were formed from the liquid droplets. As the Au film thickness was increa sed, the average size of the islands became larger while the size distribution was broader and the number density of the islands was reduced. For a 1.2 nm thick Au film, the average diameter of the islands was 8 nm and all islands were smaller than 20 nm (Fig. 1(a)) while for 2.0 nm Au film, the average diameter was 13 nm and the fraction of the islands smaller than 20 nm was about 85% (not shown in Fig. 1). For further increase in thickness of 3.0 nm, the average diameter was increased to 15 nm and the fraction of the islands smaller than 20 nm was decreased to 77% (Fig. 1(b)). These results indicate that for thicker films the initially nucleated Au–Si alloy droplets would tend to grow larger through droplet coarsening with neighboring droplets [9]. Thus, the diameter of the islands will be more uniform for Au films thinner than 1.2 nm at a given annealing temperature. After silane exposure of the Au–Si droplets on Si in Figs. 1(a) and (b), we observed the formation of nanostructures on the surface. Figs. 1 (c) and (d) display the resulting SEM images obtained with 301 tilt with respect to the horizon. For the 1.2 nm Au film, the nanostructures are observed to be pillar shaped over all surfaces (Fig. 1(c)). The nanopillars (NP) seem to be grown prior to the nucleation of the NWs [10]. For the 2.0 nm film, similar NP structures were observed. However, for the 3.0 nm film we observed the nucleation of the Si-NWs with average diameter of 60 nm and length of 350 nm (Fig. 1(d)). It may indicate that a critical thickness of Au films exists for the Si-NW nucleation in the given growth conditions. In other words, there exists a minimum size of Au–Si droplets to initiate Si-NW nucleation from the droplet catalysis. The bright tips of the NWs seem to be Au–Si droplets in Fig. 1(d). The diameters of the NWs are similar to those of the Au–Si droplets. The existence of a critical Au film thickness for initiation of NW nucleation may be related to competition of Si homogeneous nuclei formation in the Au–Si droplets with the adatom attach- ment at the Au–Si droplet-substrate interface [11]. To examine the effects of growth pressure on the Si-NW morphology and dimension, a series of samples of 3.0 nm of Au films were exposed to the sil ane mixture gases at total pressures ranging from 0.1 to 1.0 Torr. For 0.1 Torr, the surface morphology displays an early stage of the nucleation of the Si-NWs (Fig. 2(a)). Most of the nanostructures seem to be NP shaped and some are of NW shape. As the growth pressure increased to 0.5 Torr, more NWs nucleated and the NWs coexisted with the NPs (Fig. 2(b)). For 1.0 Torr, most of the NPs transformed to the NWs and the resulting surface shows randomly ARTICLE IN PRESS D.W. Kwak et al. / Physica E 37 (2007) 153–157154 oriented and dense Si-NWs distribution (Fig. 2(c)). The diameter of the NWs is essentially constant (60 nm) while the number density of the NWs increases rapidly with increasing pressure (Fig. 2(d)). Also, the length increases linearly from 0.8 to 4.0 m m (not shown in Fig. 2(d)). In the VLS processes, the nucleation and growth of the Si- NWs are strongly related to the degree of Si super- saturation in the Au–Si droplets because the difference of ARTICLE IN PRESS Fig. 1. SEM images of Au–Si islands grown on Si (0 0 1) substrates for (a) 1.2 nm and (b) 3.0 nm of Au deposited at room temperature and followed by annealing at 700 1C. (c) and (d) Tilted SEM images of the (a) and (b) substrates after exposure to a mixture of SiH 4 (2 sccm) and H 2 (50 sccm) for 60 min at 0.1 Torr and 550 1C, respectively. Fig. 2. Tilted SEM images of Si-NWs grown on Si (0 0 1) for 60 min at 550 1C and a total pressure of (a) 0.1, (b) 0.5, and (c) 1.0 Torr. The Au films (3 nm) annealed at 700 1C were exposed to a gas mixture of SiH 4 (4 sccm) and H 2 (50 sccm). The diagram (d) is number density and average diameter of the NWs as a function of growth pressure. D.W. Kwak et al. / Physica E 37 (2007) 153–157 155 Si concentration between vapor–liquid interface and liquid–solid interface will determine the growth rate of the NWs [10]. As the growth pressure increases, dissolution of more Si into Au–Si droplets will lead to increasing Si chemical potential in the liquid droplets with a relationship of Dm Si ¼ kT ln p, where Dm Si is Si chemical potential in the liquid droplets and p is a molecular Si overpressure [4]. Note that the growth rate of the NWs, R, is determined by the chemical potential difference at the interface of liquid/ solid by the relation ship, R expðm Si =kTÞ [4]. Correspond- ingly, the growth rate of the NWs is proportional to pressure. This enhanced growth rate with increasing pressure would result in more transition from the NPs to NWs and the corresponding rapid increase in the NW density and length, as shown in Fig. 2 . Also we explored the variation in the Si-NW morphol- ogy and dimension depending on growth temperature. In our growth temperature ranges, linear and randomly oriented Si-NWs were formed on the surfaces, which have similar morphologies as shown in Fig. 2(c). However, the dimensional variation was distinct. As the temperature increases from 500 to 600 1C, the average diameter of the NWs increases from 55 to 130 nm while the density of the NWs decreases rapidly (Fig. 3). In particular, the number density for 500 1C is lower than for 550 1C, which would result from the existence of more NPs at lower temperature due to lesser transition of the NWs from the NPs. As the temperature increases, more amount of Si can dissolve into the Au–Si droplets following the liquid of the Au–Si binary phase diagra m. The increased chemical potential can enhance the Si-NW growth rate at the interface between Au–Si droplet and Si-NW seeds as well as the transition of the NWs from the existing NPs. In contrast, the rapid increase in diameter at higher temperature can be explained by the NW coalescence with neighboring NWs during NW vertical growth, or possibly coalescence of the Au–Si droplet or the NPs at the initial nucleation stage before the NW nucleation. This explanation is consistent with decreasing number density due to the coalescence. To explore the axial growth direction of the NWs, we grew the NWs on Si (0 0 1) and (1 1 1) with optimized growth conditions obtained from the above studies. Fig. 4 displays cross-sectional SEM images of the Si (0 0 1) and (1 1 1) sample surfaces. The dimensions of the NW on both surfaces are similar. However, the growth direction was distinct. For Si (0 0 1), the axial directions of the NWs varied in the range 30–901 with respect to the substrate surface and approximately 80% of the NWs were aligned along the angles of 301 and 601 (Fig. 4(a)), which is a preferential /111S growth direction of the NWs [12].In contrast, for Si (1 1 1), most of the NWs are aligned along the vertical direction /111S (Fig. 4(b)). Our results are consistent with previous reports [12]. This preferential growth direction might be explained by energetics at the interface between Au–Si droplets and initial Si-NW seeds on the substrate. After the Si-NWs seeds are nucleated below the droplets, the free energy of the liquid–solid system is determined by the surface energy of the Si-NW seeds and the interface energy of the liquid droplet–solid NW seed. Note that the surface energy of crystal structures depends on the crystal orientation and that the interface energy of the NWs is proportional to the diameter of the NWs [12]. Thus, for the NWs with diameter larger than 60 nm grown with our growth conditions, the axial growth along the /111S direction will give rise to having a minimum surface and interface energies at the interface of Au–Si droplets and Si seeds. ARTICLE IN PRESS Fig. 3. Number density and average diameter of the NWs as a function of growth temperature for the samples grown on Si (0 0 1) for 60 min at 1.0 Torr and various temperatures. The Au films (3 nm in thickness) were exposed to a mixture of SiH 4 (4 sccm) and H 2 (50 sccm). Fig. 4. Cross-sectional SEM images of Si-NWs grown on (a) Si (0 0 1) and (b) Si (1 1 1) substrates for 60 min.at 550 1C and 1.0 Torr with SiH 4 (4 sccm) and H 2 (50 sccm), respectively. D.W. Kwak et al. / Physica E 37 (2007) 153–157156 In addition, we investigated the dimensional variation of the NWs with growth time from 150 to 120 min. For growth time shorter than 60 min, the average diameter of the NWs is 60 nm and the diameter of each NW is essentially constant along the length direction independent of the growth time while the length is proportional to the growth time. The average length growth rate was 0.08 mm/min. In contrast, longer growth time gave rise to distinct morphology of the NWs. For 120 min growth, the length of all the NWs increased to 12 mm while the diameters varied (Fig. 5). The wider NWs would be formed by NW coalescence. In particular, we found the existence of two types of NWs from the NWs with different diameters (inset of Fig. 5). Some of the narrower NWs have slightly tapered shape, the diameter decreasing with increasing distance from the Si substrate while most of the NWs have uniform diameter along their length. The formation of the tapered NWs might result from a slight loss of Au during NW growth in length [10]. The morphology of both types of the NWs indicates that the vertical growth rate by catalyzing Au–Si droplet is more dominant than lateral growth rate. 4. Conclusion We studied the morphological and dimensional evolu- tion of the Si-NWs on Si (0 0 1) and (1 1 1) surfaces grown by using Au–Si nanoisland-catalyzed RTCVD. For a given growth condition, a critical thickness of the Au film exists for the nucleation of the Si-NWs. The growth rate, dimension, and orientation of the NWs can be controlled by the growth pressure, temperature, time, and substrate orientation. The resulting NWs are 30–100 nm in diameter and 0.4–12 mm in length depending on the growth conditions. For both Si (0 0 1) and (1 1 1) sub- strates, most of the NWs were aligned along the /111S direction. We fabricated vertically well-aligned and rela- tively uniform sized NWs on Si (1 1 1) surfaces with optimized growth parameters. In addition, we observed two types of NWs, with straight and tapered shapes for the NWs grown with a longer growth time. In the Si-NW growth via the VLS processes, the morphological and dimensional evolution of the Si-NWs are considerably related to mass transport through the Au–Si liquid droplets and surface and interface energies at the interface of liquid–solid. Acknowledgments This work was supported by the Quantum-Functional Semiconductor Research Center in the Dongguk Univer- sity and by the National Program for Tera Level Nano Devices through MOST. References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [3] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [4] E.I. Givargizov, J. Cryst. Growth 31 (1975) 20. [5] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J. Wang, Appl. Phys. Lett. 78 (2001) 2214. [6] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci. Technol. B 15 (1997) 554. [7] Yiying Wu, Peidong Yang, J. Am. Chem. Soc. 123 (2001) 3166. [8] B. Ressel, K.C. Prince, S. Heun, Y. Homma, J. Appl. Phys. 93 (2003) 3886. [9] W C. Yang, M. Zeman, H. Ade, R.J. Nemanich, Phys. Rev. Lett. 90 (2003) 136102. [10] J.W. Dailey, J. Taraci, T. Clement, D.J. Smith, J. Drucker, S.T. Picraux, J. Appl. Phys. 96 (2004) 7556. [11] S. Sharma, T.I. Kamins, R.S. Williams, Appl. Phys. A 80 (2005) 1225. [12] V. Schmidt, S. Senz, U. Gosele, Nano Lett. 5 (2005) 931. ARTICLE IN PRESS Fig. 5. Cross-section SEM images of Si-NWs grown on Si (1 1 1) substrates for 120 min. The NWs were grown at the same condition as in Fig. 4 except for the growth time. Inset is a magnified SEM image of the end regions of the NWs. D.W. Kwak et al. / Physica E 37 (2007) 153–157 157 . Physica E 37 (2007) 153–157 Dimensional evolution of silicon nanowires synthesized by Au–Si island-catalyzed chemical vapor deposition D.W. Kwak, H.Y. Cho,. morphological evolution of silicon nanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemical