Si Substrates

Article pubs.acs.org/crystal

Taper-Free and Vertically Oriented Ge Nanowires on Ge/Si Substrates Grown by a Two-Temperature Process Jung Hyuk Kim,† So Ra Moon,† Hyun Sik Yoon,† Jae Hun Jung,† Yong Kim,*,† Zhi Gang Chen,‡ Jin Zou,‡ Duk Yong Choi,§ Hannah J. Joyce,∥ Qiang Gao,⊥ H. Hoe Tan,⊥ and Chennupati Jagadish⊥ †

Department of Physics, Dong-A University, Hadan-2-dong, Sahagu, Busan 604-714, Korea Materials Engineering and Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, St. Lucia, QLD 4072, Australia § Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra ACT0200, Australia ∥ Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom ⊥ Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra ACT0200, Australia ‡

ABSTRACT: Taper-free and vertically oriented Ge nanowires were grown on Si (111) substrates by chemical vapor deposition with Au nanoparticle catalysts. To achieve vertical nanowire growth on the highly lattice mismatched Si substrate, a thin Ge buffer layer was first deposited, and to achieve taperfree nanowire growth, a two-temperature process was employed. The two-temperature process consisted of a brief initial base growth step at high temperature followed by prolonged growth at lower temperature. Taper-free and defectfree Ge nanowires grew successfully even at 270 °C, which is 90 °C lower than the bulk eutectic temperature. The yield of vertical and taper-free nanowires is over 90%, comparable to that of vertical but tapered nanowires grown by the conventional one-temperature process. This method is of practical importance and can be reliably used to develop novel nanowire-based devices on relatively cheap Si substrates. Additionally, we observed that the activation energy of Ge nanowire growth by the two-temperature process is dependent on Au nanoparticle size. The low activation energy (∼5 kcal/mol) for 30 and 50 nm diameter Au nanoparticles suggests that the decomposition of gaseous species on the catalytic Au surface is a rate-limiting step. A higher activation energy (∼14 kcal/mol) was determined for 100 nm diameter Au nanoparticles which suggests that larger Au nanoparticles are partially solidified and that growth kinetics become the rate-limiting step.

1. INTRODUCTION Nanowires have attracted considerable attention owing to their potential as nanobuilding blocks.1 A vapor−liquid−solid (VLS) growth mechanism was proposed to explain the growth of these interesting quasi-one-dimensional nanostructures.2 Among various nanowires, Ge nanowires feature high intrinsic carrier mobility and low synthesis temperature,3 which make them particularly promising for applications in microelectronics including high performance Ge nanowire field effect transistors (FETs).4−6 Vertically oriented nanowires epitaxially grown on substrates are more desirable than randomly oriented nanowires for novel device fabrication such as wrap-gate FETs7 and nanobridge devices.8−10 There has been extensive research on homoepitaxial growth of vertically oriented epitaxial Ge nanowires on Ge substrates by chemical vapor deposition (CVD).11,12 However, the growth of vertically oriented heteroepitaxial Ge nanowires on the relatively cheap Si substrates is attractive due to the © 2011 American Chemical Society

process compatibility with existing Si-based microelectronic technology. The growth of vertically oriented Ge nanowires having an epitaxial relationship with the underlying Si substrate is not straightforward due to the rapid oxidation of Si substrate. The oxide layer interferes with the growth of epitaxial [111] aligned nanowires. It is well known that the oxide-free, hydrogenterminated Si surface can only be resistant to reoxidation for a few minutes when this surface is exposed to air.13 The prevailing technique to prevent reoxidation is the deposition of a thin Au layer on Si in an ultrahigh vacuum (UHV) chamber.14−19 As an alternative to the UHV deposition/agglomeration approach for the preparation of nanosized Au catalysts, a surface treatment technique with Au colloidal solution is widely Received: July 13, 2011 Revised: October 5, 2011 Published: November 17, 2011 135

dx.doi.org/10.1021/cg2008914 | Cryst. Growth Des. 2012, 12, 135−141

Crystal Growth & Design

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Figure 1. Side view FESEM images of Ge nanowires on GeBSi substrates grown at (a) 370 °C, (b) 360 °C, (c) 350 °C, (d) 340 °C, (e) 330 °C, and (f) 320 °C. A 50 nm diameter Au colloidal solution was used. Scale bar = 500 nm. buffer layer, the Si substrates with Ge buffer layers (hereafter, this type of substrate will be denoted as the GeBSi substrate) were functionalized by dipping them in a 0.1% PLL solution for 0.5 min. After rinsing with deionized (DI) water, the substrates were blown dry with N2 gas. Three types of commercially available Au colloidal liquid solutions (Ted Pella) containing Au nanoparticles of different diameters (30 nm, 50 nm, and 100 nm) were dispersed on GeBSi substrates, and then excess nanoparticles were rinsed off with DI water. All growth and nucleation steps were carried out under GeH4 partial pressure of 0.3 Torr. For the conventional one-temperature process, Ge nanowires were grown for 20 min at various growth temperatures ranging from 320 to 370 °C. For the two-temperature process, the base growth step was carried out at 350 °C with various growth times ranging from 2 to 6 min. The base growth step was followed by a prolonged nanowire growth step for 20 min at lower growth temperatures ranging from 250 to 300 °C. Field emission scanning electron microscopy (FESEM, JEOL JSM6700F) was used to study the morphology of grown nanowires, and transmission electron microscopy (TEM, Philips F20, operated at 200 kV) was employed to determine the structural characteristics of individual nanowires. To prepare nanowire specimens for TEM observations, a Si substrate with grown nanowires was immersed into a container with ethanol and sonicated for 20 min in order to remove the nanowires from their underlying substrates. These nanowires were then dispersed onto a holey carbon film.

used. In this process, a Au colloidal solution containing Au nanoparticles of predetermined size is dispersed on the substrate functionalized by poly-L-lysine (PLL) or (aminopropyl)triethoxysilane. This technique has proven to be useful for the growth of nanowires on III−V substrates with slow oxidation rates.20 In contrast, this colloidal solution technique is more challenging for realizing vertical Ge nanowires on Si due to the rapid oxidation of Si.13 Several methods have been developed to circumvent the oxidation problem, including the introduction of a chlorine-containing precursor,21 preannealing in ambient hydrogen prior to growth,22 quick processing,23 and the use of HFcontaining Au colloidal solution.24 In our previous study, we developed a method for the growth of vertically oriented Ge nanowires on a Si substrate by employing a thin Ge buffer layer.25 We observed the most decisive factor influencing the vertical orientation was the root-mean-square (rms) roughness of the thin Ge buffer layer. A yield of vertical nanowires exceeding 96% can be routinely obtained as long as the rms roughness of the thin Ge buffer layer (thickness ∼170 nm) is as low as ∼2 nm. However, the Ge nanowires were highly tapered as a result of radial growth due to migrated species. In this study, we extend our previous work for the realization of taper-free and vertically oriented epitaxial Ge nanowires on Si substrates. In parallel with the previous strategy utilizing a thin Ge buffer layer on Si, we adopt a two-temperature process which utilizes different growth temperatures during base and subsequent nanowire growth stages. In addition, to see the effect of Au particle size on tapering behavior, we grew nanowires with different diameters utilizing 30 nm, 50 nm, and 100 nm Au nanoparticle-containing colloidal solutions.

3. RESULTS AND DISCUSSION Probably the simplest way to suppress tapering is lowering the growth temperature. Reaction species which impinge directly upon the Au nanoparticle contribute to axial growth. Additionally, Ge adatoms are adsorbed on the substrate and nanowire sidewalls and diffuse along the concentration gradient toward the growing nanoparticle−nanowire interface. These diffusing adatoms contribute to both radial and axial growth. Because radial growth is kinetically limited, diffusing adatoms are less likely to be incorporated into nanowire sidewalls at lower growth temperatures.26,27 Figure 1 shows the side view FESEM images of Ge nanowires grown on GeBSi substrates at various temperatures using 50 nm diameter Au colloidal solution. A tapering parameter, defined as Δ(diameter)/Δ(length) [nm/μm], the increase in nanometer diameter (nm) per unit nanowire length (μm) from the Au nanoparticle−nanowire interface, can be used to estimate the strength of tapering. For Ge nanowires grown at 370 °C, Δ(diameter)/Δ(length) ≈ 129 nm/μm (Figure 1a). Such a huge tapering parameter is

2. EXPERIMENTAL SECTION Si (111) substrates were hydrogen-terminated by dipping in diluted (4.8%) HF solution after the standard Radio Corporation of America (RCA) cleaning steps. The substrates were immediately loaded into a cold wall stainless-steel reaction chamber. Ge buffer layers were grown by using a lamp-heated CVD system. The processing gas was 1% H2diluted GeH4 (Voltaix). A bank of tungsten-halogen lamps allowed rapid heating of the substrates. Substrate temperatures were monitored by a thermocouple. The chamber pressure was kept at 20 Torr (GeH4 partial pressure = 0.2 Torr) during the growth of the Ge buffer layers. The Ge buffer layers were grown at 550 °C for 30 s, which is the optimized condition for the best yield of vertical nanowires as reported in our previous work.25 After the growth of an ∼170 nm-thick Ge 136



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Figure 2. Side view FESEM images of Ge nanowires on a GeBSi substrate grown by the two-temperature process. The growth temperature during the base growth stage is 350 °C, while subsequent nanowire growth is performed at 300 °C for 20 min. The growth time during the base growth stage is (a) 6 min, (b) 4 min, and (c) 2 min. A 50 nm diameter Au colloidal solution was used. The regions marked by arrows in FESEM images are the part of nanowires grown during the base growth stage. Scale bar = 500 nm.

temperature (361 °C)28 by the two-temperature process.11,29 It was hypothesized that due to the initial high temperature base growth step, the Au catalysts are still in their initial liquid state at subeutectic temperatures employed for subsequent prolonged growth, and thereby, the growth of Ge nanowire still proceeds under VLS mechanism. Kodambaka et al. have confirmed such a remarkable supercooling effect by the detailed in situ TEM observations during CVD growth using a digermane precursor gas.30 The kinetic barrier hindering Au solidification of the AuGe catalyst and/or the supersaturation of the liquid catalyst with Ge were hypothesized for explaining this remarkable phenomenon. Our primary interest is whether this idea could be applied to Ge nanowires on GeBSi substrates for achieving taper-free nanowires. Figure 2 shows the side view FESEM images of Ge nanowires grown on GeBSi substrates by the two-temperature process. In Figure 2a, we see a clear distinction between the tapered region formed during the 6 min base growth step at 350 °C and the taper-free region formed during the subsequent nanowire growth step at 300 °C for 20 min. Approximately 35% of the nanowire length is formed during base growth step, and this is close to the growth time ratio of the two steps (6 min/20 min ≈ 0.3). The fact that the growth rate at low temperature (300 °C) is comparable to that at 350 °C during the base growth step may suggest the growth is still governed by the VLS mechanism at 300 °C, considering that a slower growth rate is expected under the vaporsolid−solid mechanism.31,32 The long and tapered base region is not desirable for practical applications. As shown in Figure 2, the length of base region decreases with decreasing nucleation time. The short base region (∼200 nm) is sufficient for the formation of taper-free and vertically oriented nanowires (Figure 2c). The possibility of Ge nanowire growth at subeutectic temperatures was explored. As shown in Figure 3, the growth rate is reduced gradually with decreasing growth temperature. Remarkably, Ge nanowires are successfully grown even at 270 °C, which is 90 °C lower than the bulk eutectic

drastically reduced with decreasing growth temperature, as evidenced by the fact that Δ(diameter)/Δ(length) ≈ 22 nm/μm for Ge nanowires grown at 330 °C (Figure 1e). However, this tapering parameter is still very high for practical applications considering previously reported values from III−V nanowires.27 It is of interest to note that further decreasing the growth temperature lead to kinked nanowires (Figure 1f) accompanied with significant reduction of the growth rate. The reason for such a sudden morphological change with small change in growth temperature (only 10 °C) is probably due to the phase transition from liquid to solid state in AuGe catalysts at this temperature. This result indicates that other strategies rather than the simple control of the growth temperature are required for achieving taper-free nanowires. A two-temperature growth process which was originally proposed by Greytak et al.5 has been extensively studied by many researchers.11,12,27 During the short, high temperature growth step, a short nanowire base grew. Then the prolonged growth followed at lower temperature. The two-temperature process separates nucleation and nanowire growth stages. Thus, nanowires diameter and vertical nature can be optimized independently. Initial nucleation which has a deterministic role in the control of growth orientation occurs at temperatures near bulk eutectic. Indeed, without the prior base growth step, a low growth temperature usually produced nonvertical, kinked, and irregular nanowires as shown in Figure 1f. After forming the stable base structure/nucleation structure with ⟨111⟩ orientation, radial growth and tapering can be minimized by lowering growth temperature. Therefore, the two-temperature process increases the production yield of nanowires while preserving the taper-free nature. The nanowires grown by a two-temperature process were generally taper-free. Additionally, the formation of rotational twins is greatly suppressed as observed in III−V nanowires.27 Adhikari et al. have observed that Ge nanowires can be successfully grown at temperatures far below the bulk eutectic 137



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Figure 3. Side view FESEM images of Ge nanowires on GeBSi substrates grown by the two-temperature process. Base growth is performed at 350 °C for 2 min. Nanowires are grown at (a) 300 °C, (b) 290 °C, (c) 280 °C, (d) 270 °C, (e) 260 °C, and (f) 250 °C for 20 min. A 50 nm diameter Au colloidal solution was used. The arrow in panel e indicates kinking of the nanowire. Scale bar = 500 nm.

Figure 4. Plan view FESEM images of Ge nanowires on GeBSi substrates grown by the two-temperature growth process at (a) 260 °C, (b) 270 °C, and (c) 300 °C for 20 min. Initial base growth is carried at 350 °C for 2 min. A 50 nm diameter Au colloidal solution was used. Scale bar = 1 μm. (d) Yield of vertical nanowires vs growth temperature.

in the yield of vertical nanowires is observed for Ge nanowires grown by the two-temperature process. The bright spots in the plan view FESEM images are vertically standing nanowires. Slanted nanowires are rarely found in the FESEM images. Figure 4d shows the graph of the yield of vertical nanowires vs growth temperature. The yield of vertical nanowires is over 90% regardless of the growth temperature, confirming the decisive role of the initial nucleation step for keeping the vertical nature of nanowires. Though not shown here, the equivalent experiment of the growth of Ge nanowires by utilizing 30 nm diameter colloidal solution was conducted. The experimental results were similar: they were all taper-free and vertically standing with the yield exceeding 90% at growth

temperature (Figure 3d). All nanowires are taper-free while preserving their vertical orientation. Nanowires start to kink at 260 °C (Figure 3e), and the growth is terminated at temperatures lower than 250 °C as shown in Figure 3f. Such a rapid change in the nanowire morphology indicates that the rapid solidification of the catalyst occurs at 260−250 °C because the liquid phase cannot be maintained at this low temperature. Our primary purpose of this study is the realization of minimally tapered Ge nanowires without the loss of their vertical nature. In our previous study, for Ge nanowires grown by the conventional one-temperature process, the yield of vertical nanowires was as high as 96% under optimized growth conditions.25 As shown in Figure 4, no significant degradation 138



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temperatures above 270 °C. Like the 50 nm diameter Ge nanowires, the nanowires started to kink at 260 °C. Figure 5 shows the TEM images of unkinked and kinked nanowires grown by the two-temperature process. The taper-

no noticeable lattice defects based on the high resolution TEM (Figure 5b). This high crystalline quality is surprising because the growth temperature is 90 °C lower than the bulk eutectic temperature. The inset of Figure 5b is the corresponding fast Fourier transformation (FFT) of the high resolution TEM image, showing the nanowire having the cubic structure. Figure 5c,d shows low magnification and high resolution TEM images of a kinked Ge nanowire grown at 260 °C. The kinked part of the nanowire possesses high crystalline property. From the judgment of the FFT transform, the kinked part grows along another ⟨111⟩ direction and the interface between the catalyst and nanowire is parallel to the new {111} plane. Figure 6 shows the side view FESEM images of Ge nanowires on GeBSi substrates grown by the two-temperature process using 100 nm diameter Au colloidal solution with various growth temperatures from 300 to 260 °C. The general features are quite similar to the nanowires produced by colloidal solutions with 30 or 50 nm diameter Au nanoparticles at first glance. However, for the 100 nm case, the kinking occurs at 270 °C, rather than at 260 °C. This is expected because melting point depression is not so significant for larger diameter nanoparticles.12 A further important difference is noticed from the close inspection of the FESEM images. The growth rate decreases more rapidly with decreasing growth temperature. If the growth is dominated by surface reaction rather than the diffusion of species, the growth rate, G is conventionally described as below:

G = A exp( − Ea /RT )PGeH4

(1)

where A, Ea, R, T, and PGeH4 are the pre-exponential factor, the activation energy, the gas constant, absolute temperature, and GeH4 partial pressure, respectively. The activation energy determined from the Arrhenius plot may give a clue to identify the growth kinetics.33−35 Figure 7 shows the Arrhenius plot of the growth rate for various sizes of the Au catalysts. The activation energies were calculated from the points taken before kinking because the growth rate drops significantly after kinking. The activation energies are 4.8 and 5.6 kcal/mol for 30 and 50 nm diameter Au catalysts, respectively. These values are much smaller than 11.4 kcal/mol as reported by Lew et al.33

Figure 5. (a) Low magnification and (b) high resolution TEM images of Ge nanowires grown at 270 °C by two-temperature process. The inset in panel b is the FFT of the image. (c) Low resolution and (d) high resolution TEM images of kinked Ge nanowires grown at 260 °C by the two-temperature process. The inset in panel d is the FFT of the kinked region.

free nature of Ge nanowires grown at 270 °C is confirmed again from TEM image (Figure 5a). The nanowire grows along the ⟨111⟩ direction, which has the lowest surface energy. It has

Figure 6. Side view FESEM images of Ge nanowires on GeBSi substrates grown by the two-temperature process. Base growth is performed at 350 °C for 2 min. Nanowires are grown at (a) 300 °C, (b) 290 °C, (c) 280 °C, (d) 270 °C, and (e) 260 °C for 20 min. A 100 nm diameter Au colloidal solution is used. The arrow in panel e indicates kinking of the nanowire. Scale bar = 500 nm. 139



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can be grown even at 270 °C, which is 90 °C lower than the bulk eutectic temperature (361 °C). The yield of vertical nanowires is over 90% indicating the practical importance of this method which could be reliably used to develop novel nanowire-based devices. In addition, we observe that the activation energy of the growth rate in Ge nanowires grown utilizing 30/ 50 nm diameter Au nanoparticles is low (∼5 kcal/mol), while a higher activation energy (∼14 kcal/mol) for 100 nm diameter Au nanoparticles is obtained. The results indicate that the activation energy is dependent on Au nanoparticle size and that this in turn can determine the growth kinetics of the nanowire.

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AUTHOR INFORMATION Corresponding Author *Tel/Fax: 82-51-200-7276/7232. E-mail: [email protected].

Figure 7. Arrhenius plot of growth rate vs inverse temperature for Ge nanowires with various Au catalyst sizes.

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ACKNOWLEDGMENTS This research 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 (2009-0066463) and Australian Research Council.

According to the VLS mechanism, GeH4 decomposition occurs on the surface of Au nanoparticles. GeH4 decomposition is chemically catalyzed by the AuGe nanoparticle since the activation energy for planar Ge growth from GeH4 was reported to be 25−27 kcal/mol.34,35 Then the Au nanoparticles become supersaturated with the Ge species and form a melt. Ge species continuously supplied from gas phase species diffuse to the catalyst/nanowire interface and precipitate out to form a quasi-one-dimensional nanowire. The straight, vertical nanowires of Figures 2 and 3 suggest that the Au catalyst particle is still in a liquid phase even at subeutectic temperatures. The diffusion of Ge species through the molten catalysts would be rapid, and therefore the rate-limiting step which governs Ge nanowire growth will be the decomposition of GeH4 on the Au catalyst surface. Redwing et al. suggested the limiting step is GeH2 adsorption of the liquid catalysts because the activation energy reported for GeH2 adsorption is ∼5.5 kcal/mol.36 This value is fairly close to our activation energies for 30 and 50 nm Au catalysts. To our best knowledge, the Au catalyst-sizedependent activation energy has not been addressed before. The activation energy of 100 nm Au catalysts is 14.1 kcal/mol, almost 3 times higher than those from 30 and 50 nm diameter Au catalysts. The activation energy reported by Lew et al.33 is the value (11.4 kcal/mol) in between those of 30/50 nm diameter nanowires and 100 nm diameter nanowires and is probably an average due to size dispersion of the Au catalysts in their experiment. We hypothesize that a somewhat different growth mechanism may be considered to account for the larger activation energy for 100 nm-diameter Ge nanowires. One possibility is that because melting point depression is not so significant in the case of the 100 nm diameter Au nanoparticles, the AuGe nanoparticles are partially solidified throughout the examined temperature range.37 In this case, the relatively slow diffusion through the partially solidified AuGe catalyst compared to that of the molten AuGe catalysts may become the rate-limiting step.

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REFERENCES

(1) Lieber, C. M. MRS Bull. 2003, 28, 486. (2) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (3) Wang, D.; Dai, H. Angew. Chem., Int. Ed. 2002, 41, 4783. (4) Wang, D.; Wang, Q; Javey, A.; Tu, R.; Dai, H.; Kim, H.; McIntyre, P. C.; Krishnamohan, T.; Saraswat, K. C. Appl. Phys. Lett. 2003, 22, 2432. (5) Greytak, A. B.; Lauhon, L. J.; Gudikson, M. S.; Lieber, C. M. Appl. Phys. Lett. 2004, 84, 4176. (6) Xiang, J.; Lu, W; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489. (7) Schmidt, V.; Riel, H.; Senz, S.; Karg, S.; Riess, W.; Gosele, U. Small 2006, 2, 85. (8) Goldberger, J.; Hochbaum, A. I.; Fan, R.; Yang, P. Nano Lett. 2006, 6, 973. (9) He, R.; Gao, D.; Fan, R.; Hochbaum, A. I.; Carraro, C.; Maboudian, R.; Yang, P. Adv. Mater. 2005, 17, 2098. (10) Islam, M. S.; Sharma, S.; Kamins, T. I.; Williams, R. S. Nanotechnology 2004, 15, L5. (11) Adhikari, H.; Marshall, A. F.; Chidsey, C. E. D.; McIntyre, P. C. Nano Lett. 2006, 6, 318. (12) Adhikari, H.; McIntyre, P. C.; Marshall, A. F.; Chidsey, C. E. D. J. Appl. Phys. 2007, 102, 094311. (13) Thornton, J. M. C.; Williams, R. H. Semicond. Sci. Technol. 1989, 4, 847. (14) Jagannathan, H.; Deal, M.; Nishi, Y.; Woodruff, J.; Chidsey, C.; McIntyre, P. C. J. Appl. Phys. 2006, 100, 024318. (15) Tutuc, E.; Guha, S.; Chu, J. O. Appl. Phys. Lett. 2006, 88, 043113. (16) Dailey, J. W.; Tarachi, J.; Clement, T.; Smith, D. J.; Drucker, J.; Picraux, S. T. J. Appl. Phys. 2004, 96, 7556. (17) Jagannathan, H.; Nishi, Y.; Reuter, M.; Copel, M.; Tutuc, E.; Guha, S. Appl. Phys. Lett. 2006, 88, 103113. (18) Lugstein, A.; Hyun, Y. J.; Steinmair, M.; Dielacher, B.; Hauer, G.; Bertagnolli, E. Nanotechnology 2008, 19, 1. (19) Li, C. B.; Usami, K.; Muraki, T.; Mizuta, H.; Oda, S. Appl. Phys. Lett. 2008, 93, 041917. (20) Song, M. S.; Jung, J. H.; Kim, Y.; Wang, Y.; Zou, J.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Nanotechnology 2008, 19, 125602. (21) Hochbaum, A. I.; Fan, R.; Yang, P. Nano Lett. 2005, 5, 457. (22) Kamins, T. I.; Li, X.; Williams, R. S. Nano Lett. 2004, 4, 403. (23) Bao, X.; Soci, C.; Susac, D.; Bratvold, J.; Aplin, D. P. R; Wei, W.; Chen, C.; Deyeh, S. A.; Kavanagh, K. L.; Wang, D. Nano Lett. 2008, 8, 3755.

4. CONCLUSIONS We demonstrate a method for realizing taper-free and vertically oriented Ge nanowires on relatively cheap Si substrates. Ge nanowires are grown by chemical vapor deposition catalyzed by colloidal Au nanoparticles with different sizes (30 nm, 50 nm, and 100 nm diameters). To reduce the tapering effect, we employ a two-temperature process, consisting of a brief base growth step at high temperature followed by prolonged growth at lower temperature. By varying nanowire growth temperatures, we observe that taper-free and defect-free Ge nanowires 140



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(24) Woodruff, J. H.; Ratchford, J. B.; Goldthorpe, I. A.; McIntyre, P. C.; Chidsey, C. E. D. Nano Lett. 2007, 7, 1637. (25) Jung, J. H.; Yoon, H. S.; Kim, Y. L.; Song, M. S.; Kim, Y.; Chen, Z. G.; Zou, J.; Choi, D. Y.; Kang, J. H.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Nanotechnology 2010, 21, 295602. (26) Kim, Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Paladugu, M.; Zou, J.; Suvorova, A. Nano Lett. 2006, 6, 599. (27) Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Zhang, X.; Guo, Y.; Zou, J. Nano Lett. 2007, 7, 921. (28) Okamoto, H.; Massalski, T. B. Binary Alloy Phase Diagrams; Massaski, T. B. et al. ASM International: Materials Park, OH, 1990. (29) Adhikari, H.; Marshall, A. F.; Gordthorpe, I. A.; Chidsey, C. E. D.; McIntyre, P. C. ACS Nano 2007, 1, 415. (30) Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M. Science 2007, 316, 729. (31) McIntyre, P. C.; Adhikari, H.; Goldthorpe, I. A.; Hu, S.; Leu, P. W.; Marshall, A. F.; Chidsey, E. D. Semicond. Sci. Technol. 2010, 25, 024016. (32) Wang, Y.; Schmidt, V.; Senz, S.; Gosele, U. Nature Nanotechnol. 2006, 1, 186. (33) Lew, K. K.; Pan, L.; Dickey, E. C.; Redwing, J. M. J. Mater. Res. 2006, 21, 2876. (34) Olubuyide, O. O.; Danielson, D. T.; Kimerling, L. C.; Hoyt, J. L. Thin Solid Films 2006, 508, 14. (35) Tada, M.; Park, J. H.; Jain, J. R.; Sarawat, K. C. J. Electrochem. Soc. 2009, 156, D23. (36) Redwing, J. M.; Dilts, S. M.; Lew, K. K.; Cranmer, A.; Mohney, S. E. Proc. SPIE Int. Soc. Opt. Eng. 2005, 6003, 60030S−1. (37) Gamalski, A. D.; Tersoff, J.; Sharma, R.; Ducati, C.; Hofmann, S. Nano Lett. 2010, 10, 2972.

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