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Physica E 40 (2008) 2462–2467 Investigation of Au and In as solvents for the growth of silicon nanowires on Si(1 1 1) Andrea Kramer à , Torsten Boeck, Peter Schramm, Roberto Fornari Institute for Crystal Growth, Berlin 12489, Germany Available online 14 February 2008 Abstract This paper reports on the bahavior of Au and In as solvents for the growth of silicon nanowires on a Si(1 1 1) substrate via vapor–liquid–solid (VLS) mechanism. Gold is the mostly used solvent for growing silicon nanowires but in the present work indium was also applied, as it may bring some advantages for later electronic application of the wires. The main focus of this work is the behavior of gold and indium on a silicon substrate but also the different morphologies and distributions of the grown wires are compared. Individual metal droplets have been located in pre-structured nanopores to serve as starting points for wire growth. The method used to exactly position the metal droplets and thus obtain a regular arrangement of nanowires is also presented. r 2008 Elsevier B.V. All rights reserved. PACS: 62.23.Hj; 68.03.Cd; 68.08.Bc; 81.16.Rf Keywords: Nanostructures; Silicon; Physical vapor deposition; Vapor–liquid–solid mechanism; Gold; Indium; Surface tension; Surface energy; Solubility; Focused ion beam structuring 1. Introduction Nanowire-based devices are of great interest in diverse areas ranging from electronics, optoelectronics and sensor components to biotechnology [1–3]. Among different fabrication methods for nanowi res, chemical vapor deposi- tion (CVD) and physical vapor deposition (PVD) are the most wid ely applied. The experi mental conditions depend not only on the growth method but also on the chosen nanowire material [4–6]. Common aim of all approaches is a perfect control of wire growth by experimental para- meters and a possibility to position the nanowires which is essential for most of the applications. In this work, the investigation of Au and In as solvents for the growth of silicon nanowires on Si(1 1 1) via PVD by means of the well-known vapor–liquid–solid (VLS) me- chanism will be presented. Silicon nanowires are mostly grown from gold droplets. It is still a controversial issue how gold is incorporated into the wire and thus how it influences the electronic properties of the wire. Gold is a de ep-level defect in bulk silicon and if this is also true for nanowires grown from gold droplets, an alternative metal for the growth would be necessary. For this reason, apart from gold we also tried indium as solvent for the growth. 2. Experimental In all our experiments, Si(1 1 1) substrates were initially cleaned by an RCA standard process [7] in order to remove organic contaminations. The substrate was dipped into an HF (40%, w/v):H 2 O solution at a ratio of 1:5 to remove the native oxide from the silicon surface before inserting it into the ultra-high vacuum (UHV) chamber where the growth process took place. The nanowire growth procedure consisted of three steps: the first one was the desorption of residual oxide at a substrate temperature of 850 1C, the second one was metal evaporation from an effusion cell at a substrate temperature of 550 1C in order to form droplets on the substrate, and the last step was the evaporation of silicon at the same substrate temperature and at a rate of ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.01.011 à Corresponding author. Tel.: +49 30 6392 3050. E-mail address: kramer@ikz-berlin.de (A. Kramer). about 0.5 A ˚ /s which was accomplished by means of an electron beam evaporator. The pre-structuring of substrates was performed by focused ion beam (FIB) treatment with 30 kV Ga + ions. Distances, widths and depths of nanopores could be set up by adjusting blank time, current and dwell time of the ion beam. 3. Results and discussion The indium droplet distribution on the substrate before silicon evaporation appeared to be very inhomogeneous when a desorption step had been carried out (Fig. 1). Large droplets with diameters of about 10–20 mm and with density of about 600 mm À2 , as well as many tiny indium deposits of sizes below 100 nm, located in the free space between the larger droplets, were observed. When the desorption step was omitted, the distribution was seen to be much more homogeneous with droplet diameters of about 200 nm and density of about 7.7  10 6 mm À2 (Fig. 2). The gold distribution after a desorption step was examined by scanning electron microscopy (SEM) and transmission elect ron microscopy (TEM) measurements (Fig. 3). Again, different sizes of droplets were detected. The larger ones had diameters of about 100 nm and a density of about 1.5  10 6 mm À2 , which can be seen in the SEM image. But in contrast to the indium experiments, the distribu- tion of gold droplets did not change when skipping the desorption step. Zakharov et al. [8] also found an inhomogeneous distribution of gold droplets in the range between 10 and 300 nm under comparable experimental conditions. To explain differences in droplet formation between indium and gold, we will consider in the following the effects of different diffusion coefficients of gold and indium on silicon, the solubility of substrate atoms in the two metals, the surface tension of gold and indium and the surface energy of silicon and silicon oxide. The diffusion coefficients at temperatures around 550 1C for indium and gold on a clean Si(1 1 1) surface are 0.30 and 0.12 m 2 /s, respectively [9,10]. They are of the same order of magnitude and thus cannot account for our very different experimental results. We believe that a thin oxide layer forms during in- sertion of the sample into the UHV chamber in spite of the preceding HF-dip. There are hints in the literature [11] that deposition of gold onto a thin layer of SiO 2 on Si(1 1 1) favors the decomposition of SiO 2 , i.e. that gold contributes to cleaning the surface. This could explain why the gold distribution is the same with or without desorption step. Unfortunately, no literature data about the enhancement of decomposition of a silicon oxide layer by indium were found. From the phase diagrams In–Si and Au–Si (Fig. 4 ), it can be seen that the solubilities of silicon in gold and indium at our growth temperatures are 420 and o1 mol%, respectively. It could be argued that also the solubilities of SiO x in gold and indium are significantly different and thus that indium does not enhance the decomposition of an oxide layer. If this is actually the case, the indium distribution will then depend on whether a desorption step has been applied or not. Let us consider now the role of surface tension and surface energy of the different components of our experiment. From the phase diagram Au–Si, we expect to have a liquid Au–Si alloy at our growth temperatures with a silicon concentration of about 25 mol%. For this concentration, Naidich et al. [12] found a surface tension of about 1.0 J/m 2 at 1500 1C. No data could be found in the literature for the surface tension of indium–silicon alloys. However, as the solubility of silicon in indium at our growth temperature is less than 1 mol%, we take the surface tension of pure indium as an approximation which is 0.6 J/m 2 at its melting point (157 1C) [13]. As the surface tension of most liquids decreases in a nearly linear fashion with increasing temperature [14], there is a wide difference between the surface tension of the Au–Si alloy and the In–Si alloy at our growth temperatures. ARTICLE IN PRESS Fig. 1. SEM image of the indium droplet distribution after a desorption step had been carried out. Fig. 2. SEM image of the indium droplet distribution when the desorption step had been skipped. A. Kramer et al. / Physica E 40 (2008) 2462–2467 2463 Since the state of the substrate surface a fter desorption and the vacuum conditions are the same during evapora- tion of gold and indium, the different liquid–solid–vapor interface dynamics can be ascribed to the surface tension of the solvent. Liquids with high surface tension tend to form droplets with a small contact area with the underlying substrate whereas liquids with lower surface tension tend to wet the substrate. This cou ld explain the formation of smaller droplets in the case of gold than in the case of indium on a bare silicon surface, i.e. after desorption step for indium. Without desorption step, indium forms smaller droplets which can be explained by the different surface energies of silicon and silicon oxide. The surface energy of silicon at its melting temperature (1410 1C) is 0.9 J/m 2 and it decreases in a nearly linear way with increasing temperature [13], i.e. it is higher than 0.9 J/m 2 at our growth temperatures. As we do not know the exact composition of the surface after inserting the sample into our growth chamber, we take data of similar surfaces from the literature as an approximation. Asay and Kim [15] expect the surface energy of a not exactly specified silicon oxide surface to be higher than 0.1 J/m 2 at room temperature. Janczuk and Zdziennicka [16] determined the surface energy of quartz in the temperature range from 200 to 1000 1C and found out that it chang ed only slightly from 0.19 to 0.18 J/m 2 . This indicates that the silicon oxide surface energy is always smaller than the silicon surface energy which is not surprising if one thinks of a crystalline silicon surface and an amorphous oxide surface. This explains why the bare silicon tends to minimize the free surface by maximizing the contact area between indium and silicon. This leads to larger droplets compared to those on the silicon oxide surface. The size and distribution of gold and indium droplets on the silicon surface that we observed by SEM and TEM after cooling down (Figs. 1–3) may be therefore reasonably explained considering the influence of solubilities and surface energies on the mechanism of formation of droplets with or without desorption step. Silicon nanowires were obtained after silicon evapora- tion on substrates with indium and on those with gold. Without desorption step, however, no wire growth from indium could be realized. Furthermore, the results with indium and gold differed in direction and distribution of the wires. The sample where indium was used as solvent showed no wire growth from the large droplets, while in the space between, a not completely closed silicon layer was found. In the cavities of this layer, silicon nanowires appeared sporadically (Fig. 5). ARTICLE IN PRESS Fig. 3. TEM and SEM images of the gold distribution on a sample. The arrows indicate different sizes of droplets. 1500 1200 900 600 300 0 0 0.2 0.4 0.6 0.8 1 mole Si/(Si+In) 1500 1200 900 600 300 0 0 0.2 0.4 0.6 0.8 1 mole Si/(Si+Au) T (C) T (C) Fig. 4. Binary phase diagrams; top: In–Si and bottom: Au–Si. A. Kramer et al. / Physica E 40 (2008) 2462–24672464 Schmidt [17] studied, among other metals, indium as solvent for silicon nanowire growth. He applied an HF -dip before inserting the samples into a UHV chamber but did not apply any other cleaning steps. He got a homogeneous droplet distribution by annealing 4 nm of indium at a growth temperature of 570 1C. He did not get any nanowires after flooding the chamber with diluted silane and explained the absence of nanowires by considering the surface tension of indium and the low solubility of silicon in indium. Iacopi et al. [18] also studied indium as solvent for CVD growth of silicon nanowires. They treated the samples by H 2 plasma after electrodeposition of indium nanoparticles to reduce the surface oxidation of the metals as well as of the substrate and they obtained in this way silicon nanowires. Apart from CVD-based reports, no other works on the growth of silicon nanowires from indium by means of PVD were found. In our case, the growth of nanowires from indium seems to be rather insensitive to change of parameters like sub- strate temperature, rate of metal and silicon evaporation. On the other hand, very regular nanowires in [1 1 1] direction were obtained on the samples where gold was used as solvent (Fig. 6 ). In this case, we found unambig- uous correlations between experimental parameters and grown wires: at higher substrate temperatures, larger droplets of several 100 nm formed and led to thicke r nanowires, a higher gold evaporation rate led to smaller distances between the wires and a higher silicon evapora- tion rate led to a higher growth rate. Wires also grew when we did not perform the desorption step, which also corroborates the theory that gold is able to solve a thin surface oxide layer. There are also reports in the literature where nanowires are grown by CVD with gold as solvent on a thick oxide layer [19]. We also performed experiments with gold on a 100 nm thick thermal oxide grown on silicon to find out whether nanowires are growing or not. However, we did not get an y nanowires. Consequently, nanowire growth, at least under our experimental conditions, is only possible on a crystalline substrate, which again indicates that gold dissolves the thin native oxide on the ‘‘non-desorbed’’ substrate. To obtain a defined positioning of metal droplets, and thus a regular arrangement of nanowires, a reproducible process for the localization of single metal droplets in pre- structured nanopores was successfully developed in the course of this work. FIB treatment was applied to silicon substrates before metal evaporation. By adjusting metal evaporation rate and substrate tempe rature, ind ividual indium or gold droplets formed preferentially within the pre-structured pores (Figs. 7 and 8). For indium, this was only possibl e when we skipped the desorption step. After application of the desorption step, indium aggregates were found to be distributed randomly on the structured area, with no relation to the position of the nanopores. By contrast, when we skipped the desorption step, tiny indium droplets formed in the nanopores. This is in good agreement with the previous considerations about surface energies of silicon and silicon oxide. The conclusion of our experiments is that it is not possible to stabilize small indium droplets for silicon nanowhisker growth by pre-structuring substrates when a desorption step is carried out. ARTICLE IN PRESS Fig. 5. SEM images of a sample with indium as solvent after silicon deposition; left: overview of the sample and right: silicon nanowire grown in a cavity of the not completely closed silicon layer. Fig. 6. SEM image of a sample with gold as solvent after silicon deposition. A. Kramer et al. / Physica E 40 (2008) 2462–2467 2465 On the other hand, gold droplets could be well- distributed into nanopores after de sorption. As a next step, we tried to grow silicon nanowires from ordered gold droplets. It was actually possible to grow silicon nanowires in [1 1 1] direction from droplets which had formed in the pre-structured nanopores (Fig. 8). The successful growth of nanowires from the droplets em- bedded in the pores also means that the lattice planes which had been damaged by FIB bombardment could be healed during the growth process. A recovery of the crystalline structure around the pores is therefore possible even at the relatively low temperatures used for the wire growth. 4. Conclusions The different behavior of gold and indium on Si(1 1 1) has been described and analyzed. An explanation based on differences of solubilities of surface atoms in gold and indium and on the different surface energies of the bare and oxidized substrate as well as on the surface tension of the liquid metal alloys has been presented. Silicon nanowires have been grown via VLS mechanism with the use of gold and indium as solvent. Indium would be a favorable alternative for later electronic applications. Wires from indium could only be grown when an oxide desorption step had been applied before indium and silicon evaporation. The mechanism of wire growth from indium could not be completely understood whereas wire growth from gold was well reproducible and could be perfectly governed by the parameters of the experiment. A method to obtain a defined arrangement of the wires was successfully developed. It consisted in generating nanopores via FIB treatment on the substrate surface where metal droplets then preferentially formed. Wire growth from an ordered array of gold droplets was successfully performed. Acknowledgement The authors thank T. Remmele for the TEM measure- ments. References [1] Y. Huang, C.M. Lieber, Pure Appl. Chem. 76 (2004) 2051. [2] R. Agarwal, C.M. Lieber, Appl. Phys. A 85 (2006) 209. ARTICLE IN PRESS Fig. 7. Distribution of indium on a pre-structured substrate; top: after desorption and bottom: without desorption. Fig. 8. SEM images of structured substrates; top: nanopores on a silicon substrate, middle: gold droplets in nanopores, and bottom: nanowires grown from gold droplets in nanopores. A. Kramer et al. / Physica E 40 (2008) 2462–24672466 [3] P.D. McGary, L.W. Tan, J. Zou, B.J.H. Stadler, P.R. Downey, A.B. Flatau, J. Appl. Phys. 99 (2006) 08B310. [4] E.P.A.M. Bakkers, M.T. Borgstrom, M.A. Verheijen, MRS Bull. 32 (2007) 117. [5] P. Werner, N.D. Zakharov, G. Gerth, L. Schubert, U. Goesele, Int. J. Mater. Res. 97 (2006) 1008. [6] S.P. Anthony, J.I. Lee, J.K. Kim, Appl. Phys. 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