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2008, 112, 13797–13800 Published on Web 08/15/2008
Oriented Growth of Ge Nanowires with Diameters below the Bohr Radius Xihong Chen,† Myung Hwa Kim,† Xinzheng Zhang,‡ Christopher Larson,† Dapeng Yu,‡ Alec M. Wodtke,*,† and Martin Moskovits*,† Department of Chemistry & Biochemistry, UniVersity of California, Santa Barbara, California 93106, and Electron Microscopy Laboratory and State Key Laboratory for Mesoscopic Physics, School of Physics, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: June 22, 2008; ReVised Manuscript ReceiVed: July 27, 2008
Highly symmetric self-organized arrays of germanium nanowires with average diameters of ∼12 ( 3 nm were produced by chemical vapor deposition. The nanowires grew epitaxially on the faces of single-crystal Ge microcrystals produced in the same synthesis. The epitaxial growth occurred on several crystal faces with the resultant nanowire structure varying accordingly. The (111) growth direction was found to dominate, however. High-resolution TEM images of a system consisting of the NW and the substrate on which it grew epitaxially are also reported, specifically showing the interface between the two regions, thereby elucidating the growth mechanism. Introduction Semiconductor nanowires (NWs) are currently the subject of intense research both because of the fundamental new science that they help to uncover as well as for their potential applications in electronic and optoelectronic devices.1-3 Two of the key characteristics semiconductor NWs exhibit that differentiate them from bulk materials are the following: (1) Symmetry breaking: That, for example, results in direct band gap Si at the nanoscale,4 in contrast to the indirect band gap of bulk Si. This results fundamentally from the influence of spatially anisotropic surface forces that are important for nanoscale objects that are normally negligible in the bulk. (2) Quantum confinement: That, for example, may shift the energies of excited electronic states and thereby the object’s ability to interact with specific wavelengths of light when the dimensions of a nanoscale object are reduced to near or below the excitonic Bohr radius. Thus, a NW’s crystalline structure as controlled by its growth direction may be equally as important as its nanodimensions in determining its electronic and photonic properties. In this communication, we report the synthesis and characterization of Ge NWs produced by chemical vapor deposition (CVD) with diameters ∼12 ( 3 nm, well below the Bohr radius for Ge.5 Furthermore, we find that Ge NWs grow with a variety of crystalline orientations and growth directions,6 emerging epitaxially from the exposed faces of Ge microcrystals, which are spontaneously formed in the synthesis. The variety exhibited by Ge toward epitaxial NW growth from several Ge crystalline faces provides interesting possibilities for control of growth direction in NWs below the Bohr radius. The paper is organized as follows. First, we describe the CVD growth method used here and present examples of the observed * To whom all correspondence should be addressed. E-mail: mmoskovits@ ltsc.ucsb.edu (M.M.);
[email protected] (A.M.W.). † University of California, Santa Barbara. ‡ Peking University.
10.1021/jp805498q CCC: $40.75
growth motifs. We provide clear evidence of NW sizes substantially below the Bohr radius. We also present highresolution TEM images, which provide direct information on the growth direction and orientation of the NWs as well as providing valuable insight into the growth mechanism.7 Experimental Section The synthesis was carried out using an alumina boat charged with Ge powder (99.999%, Acros Organics) that was placed at the center of a 60 cm long quartz tube within a horizontal furnace. A 1 cm × 1 cm p-type Si (110) wafer covered by a 2-3 nm vapor-deposited Au film was placed approximately 16-17 cm downstream from the source. The furnace was evacuated with a rotary pump, and Ar and H2 gas were introduced separately at flow rates of 50 and 5 sccm, respectively. The furnace surface temperature was increased to 925 °C at a rate of 50 °C min-1 maintaining the total pressure at 160 Torr during the 60 min reaction time. This heats the sample to a temperature between 700 and 750 °C, which is higher than the eutectic temperature of Au-Ge (361 °C) so as to encourage the formation of liquid-phase Au-Ge alloy particles. The identity and morphology of the ensuing one-dimensional (1-D) nanostructures were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). Results Figure 1a shows a low-magnification SEM image (scale bar ) 5 µm) of a typical CVD-grown sample. The product consists of many faceted Ge microcrystals exhibiting a variety of shapes, ∼2-5 µm in size. Many of the microcrystals are covered with Ge NWs growing in well-defined directions with respect to the microcrystals’ surfaces. The NW lengths range from several hundred nanometers to several micrometers. Results of X-ray powder diffraction (XRD) (Figure 1b) show 2008 American Chemical Society
13798 J. Phys. Chem. C, Vol. 112, No. 36, 2008
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Figure 3. HRTEM of a single Ge NW. The lattice fringe and the corresponding FFT (inset) show that the Ge NWs grow along the (111) direction. The scale bar is 5 nm.
Figure 1. (a) Low-magnification SEM image of Ge microcrystals with a variety of shapes and oriented Ge NWs growing on their faces. The scale bar is 5 µm. (b) X-ray diffraction showing the (111), (220), and (311) diffraction peaks of diamond-structured Ge.
Figure 2. SEM image of aligned Ge NWs growing on the faces of (a) a square pyramidal microcrystal, (b) an apparently cubic microcrystal in which the perpendicular growth direction of the Ge NWs is clearly evident, and (c) a microplate. The scale bar for parts a, b, and c is 1 µm. (d) A higher-magnification SEM image of part c illustrates unambiguously the orientation of the Ge NWs normal to the crystal faces on which they grow. The scale bar is 200 nm.
(111), (220), and (311) features characteristic of diamondstructured Ge (PDF Card No. 04-006-2620). No other phase was detected. Higher-magnification SEM images of the sample are shown in Figure 2, displaying various observed growth motifs. Three microcrystal habits dominate: cubic, flat plate, and square pyramidal, in decreasing order of occurrence. The first and most infrequently observed is illustrated in Figure 2a. Here, one sees structures with a 4-fold axis of symmetry passing directly through the apex of what appears to be an octahedral micro-
crystal. The impossibility to image the back side of this microcrystal prevents us from claiming an octahedral structure, but the 4-fold symmetry axis is a distinct characteristic of this growth motif (Figure 2a). In Figure 2b, we show the second crystal habit, which exhibits a 3-fold symmetry axis passing through the apex and can be described as cubic. This is the most common growth motif seen in our samples. The three faces of a cube sharing a corner may either be (111), (112), and (110) or (100), (010), and (001). With either assignment, these structures demonstrate growth of Ge NWs from two or more different crystalline surfaces. The third form is shown in Figure 2c and appears as a “microplate”, that is, 2-fold rotational symmetry about the apex, where the apex of a 2-fold symmetric object is linear in form. Figure 2d shows the growth from a microplate in the highest magnification SEM of our study, more clearly showing the growth normal to the surface. In order to probe the crystalline ordering of the NWs and microcrystals, we also performed high-resolution transmission electron microscopy (HRTEM). Figure 3 shows an example of one HRTEM image of a single 14 nm diameter Ge NW and its corresponding FFT (fast Fourier transform) as an inset. The NW is a high-quality single crystal. Only a very thin amorphous layer is observed at its surface, and only trace amounts of oxygen are detected by EDX in this surface region. The FFT clearly shows (111), (311), and (220) fringes, characteristic of the diamond structure of Ge. Furthermore, the long axis of the NW, which we define as the growth direction, coincides with the (111) direction. The HRTEM data was also used to derive the precise spacing of the lattice along the (111) direction (3.26 Å), close to the bulk value (3.27 Å). Figure 4 shows HRTEM images of a nanowire still carrying the Au particle (the presence of Au was determined by EDX) that catalyzed its growth, and with the nanowire rooted to the microcrystalline growth substrate. Figure 4b shows at higher magnification the region (circumscribed by an ellipse) where the NW joins to the microcrystal. The diameter of the Aucontaining particle is ∼27 nm, which is larger than the NW’s diameter (17 nm). This size ratio was found to be typical for NWs grown in this study. We also note that the smooth surface of the catalyst particle is a characteristic feature of vapor-liquidsolid (VLS) growth.8 We conclude that this ball-like structure
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Figure 4. (a) TEM image of a single Ge NW on a Ge microcrystal and showing the Au-containing particle at the top of the NW out of which it grew, as well as the interface region between the Ge NW and the surface of the microcrystal which served as its growth template. The scale bar is 20 nm. (b) HRTEM of the interface region between the Ge NW and the Ge microcrystal shows that lattice fringes of the NW and the microcrystal are parallel and continuous as if they were sculpted from a single continuous crystal. The corresponding FFT obtained from the interface region is shown as an inset. The scale bar is 5 nm.
is a Au/Ge alloy catalyst particle from which the Ge NW grew during the CVD process likely by a VLS mechanism. The TEM image in the vicinity of the NW-microcrystal junction clearly shows the crystalline continuity between the microcrystal and the NW. We infer from this that the microcrystal serves as an epitaxial template promoting the VLS growth along a certain direction and that several growth directions are possible due to templating on various crystalline surfaces. Discussion In the previous section, we presented evidence that Ge NWs can be grown with diameters well below the excitonic Bohr radius by a VLS mechanism from Au/Ge alloy particles and that the growth direction can be varied by epitaxial templating on faceted Ge microcrystals. In this section, we discuss these key points in more detail and within the context of prior work. One of the key observations is homoepitaxial growth of Ge NWs with several growth directions in contrast to previous studies on homoepitaxial growth of Si NWs9 and heteroepitaxial growth of Ge NWs on Si,10 where preferential growth exclusively along the (111) direction is seen. A second way in which the Ge nanowires produced in this study contrast with those reported previously is their diameter range. A statistical analysis on the diameters of 32 NWs observed by HRTEM indicated that the majority clustered around an average diameter of 12 nm with a standard deviation of 3 nm. (Three outliers were found with diameters of 21, 17, and 7 nm.) Heretofore, Ge NWs with a diameter larger than ∼20 nm were reported. Largerdiameter NWs are expected to grow along directions that result in the structures with the lowest interfacial energy between the liquid and the solid;9 whereas for small-diameter NWs, the surface energy of the NW is the primary determinant of the NW’s growth direction. Consistent with these ideas, our results suggest that it may be the NWs’ small diameters that facilitate epitaxial growth on a variety of crystalline faces. We also note that the lengths of the NWs do not correlate in any simple way with their diameters in contrast to reports of Samuelson where the lengthwise growth rate increased as the
J. Phys. Chem. C, Vol. 112, No. 36, 2008 13799 NW diameter decreased.11 This suggests that the kinetics of nanowire crystallization out of the Ge/Au alloy particle, not the kinetics of Ge uptake, are rate limiting. If this is the case, it would not be surprising that several growth pathways along different growth directions could kinetically compete with one another. Control experiments were carried out in the absence of gold. Bare Si wafers or Si wafers overdeposited with 3 nm of Ni, but otherwise processed as those described in the Experimental Section, produced no NWs. However Ge microcrystals with shapes resembling the previously reported “superdome structure”12 did form on these substrates and, as shown by XRD, are crystalline diamond-structured Ge. Ge NWs similar to those described above were also obtained with a Si wafer on which a 30 nm Ge film and a 3 nm Au film were sequentially vapor deposited. Thus, the presence of Au is important not only to NW growth via a VLS mechanism, but also to the formation of facted Ge microcrystals. In the presence of gold nanoparticles, it appears that at least two basic growth processes occur, ultimately leading to oriented Ge NW growth. One is the heteroepitaxial growth of Ge microcrystals with faceted surfaces; the other is the homoepitaxial growth of Ge NWs on the surfaces of the microcrystals. The first likely occurs by the Stranski-Krastanov mechanism13 in the vicinity of the Au nanoparticles, which is likely initiated via layer-by-layer growth, followed by three-dimensional island formation. The Au nanoparticles form at high temperature through the breakup of the deposited Au film and its patchwise coalescence into gold nanoparticles.14 A published report14 shows that small amounts of deposited Au can modify surface energies sufficiently to drive the observed faceting. The fact that Au was not detectable in the faceted Ge microcrystals by EDX is not surprising in view of the small quantities of Au that can bring about such changes. In addition to the role of Au in producing the faceted structures, the structure of the Si substrate likely also plays an important role. For example, Au-patterned Si(001), Si(111), and Si(110) surfaces are reported to produce truncated pyramids, tetrahedra, and 2-fold symmetric platelet-shaped islands, respectively.14 However, the reasons why the faceted structures persist instead of becoming domes and why in our case Aucovered Si(110) produced the observed range of Ge microcrystal shapes require further study. The surfaces of the Ge microcrystallites are observed to be decorated with Au nanoparticles,15 presumably forming an Au/ Ge eutectic that in its liquid state acts as the locus of the NW synthesis as additional Ge vapor dissolves in them. In principle, the Ge atoms that contribute to NW growth can originate both from the vapor phase and from Ge ad-atoms on the microcrystal surfaces, presumably from incomplete or defective surface structures that diffuse and subsequently dissolve in the Au/Ge liquid eutectic. This “mopping-up” process is likely the reason that the Ge crystallites observed have such well-formed surfaces. Perhaps the fact that the Ge atoms that ultimately end up in the Ge NWs can originate from two sources contributes to the observed broad distribution in NW lengths. All of the Ge NWs observed in this work grow on the surfaces of Ge microcrystals, and no heteroepitaxial growth of Ge NW is seen on the Si substrate. Initially, layer-by-layer Ge growth on Si is energetically favorable due to the 4% lattice mismatch between Ge and Si.14 This is followed by three-dimensional microcrystal formation. It appears that epitaxial growth of Ge NWs only begins to dominate when the substrate is so densely
13800 J. Phys. Chem. C, Vol. 112, No. 36, 2008 covered with Ge microcrystals that the majority of additional Ge ad-atoms can only find Ge microcrystal surfaces to adsorb on. Finally, we point out that all of the structures shown in Figure 2 form self-organized structures with nanosized components appended to a microsized region. Attaching leads to the three elements of these structures could, in principle, form a device with transistor-like behavior. The band structures (and the band gaps) of Ge NWs, which depend on growth direction and diameter, are predicted to be substantially larger than the bulk value of 0.67 eV, expected for the microcrystal.4 Using the asprepared structures or by employing chemical etching techniques to reduce the average NW diameter further,16 one would obtain materials with large differences in band gaps between the NWs and the microcrystal. Such a “large/small/large” band gap structure should behave like a junction transistor. Conclusions This study produced clear evidence of Ge NW growth along at least three crystalline growth directions, producing nanowires which potentially possess unusual electronic and photonic properties arising both from their spatial anisotropy as well as their small diameters. NWs with diameters substantially less than the Bohr excitonic radius were also successfully synthesized. This is accomplished using a combination of a VLS growth mechanism catalyzed by Au/Ge alloy particles and templated growth on a homoepitaxial substrate. The templates were Ge microcrystals which grew in the same system through a single CVD process. Acknowledgment. We gratefully acknowledge the financial support from the Partnership for International Research and Education - for Electronic Chemistry and Catalysis at Interfaces - NSF grants number OISE-0530268. Support from the Institute for Collaborative Biotechnologies through grant DAAD19-03-
Letters D-0004 from the U.S. Army Research Office is gratefully noted. Extensive use was also made of the MRL Central Facilities at UCSB supported by the National Science Foundation under award nos. DMR-0080034 and DMR-0216466. We are also grateful to Dr. Seungjoon Lee for his invaluable assistance in preparing this communication. References and Notes (1) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313–1317. (2) Huang, X. M. H.; Zorman, C. A.; Mehregany, M.; Roukes, M. L. Nature 2003, 421, 496–496. (3) Huang, H. M.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897–1899. (4) Jing, M. W.; Ni, M.; Song, W.; Lu, J.; Gao, Z. X.; Lai, L.; Mei, W. N.; Yu, D. P.; Ye, H. Q.; Wang, L. J. Phys. Chem. B 2006, 110, 18332– 18337. (5) Gu, G.; Burghard, M.; Kim, G. T.; Dusberg, G. S.; Chiu, P. W.; Krstic, V.; Roth, S.; Han, W. Q. J. Appl. Phys. 2001, 90, 5747–5751. (6) Tuan, H. Y.; Lee, D. C.; Korgel, B. A. Angew. Chem., Int. Ed. 2006, 45, 5184–5187. (7) Adhikari, H.; Marshall, A. F.; Chidsey, C. E. D.; McIntyre, P. C. Nano Lett. 2006, 6, 318–323. (8) Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M. Science 2007, 316, 729–732. (9) Schmidt, V.; Senz, S.; Go¨sele, U. Nano Lett. 2005, 5, 931–935. (10) Jagannathan, H.; Deal, M.; Nishi, Y.; Woodruff, J.; Chidsey, C.; McIntyre, P. C. J. Appl. Phys. 2006, 100, 024318-10. (11) Jensen, L. E.; Bjo¨rk, M. T.; Jeppesen, S.; Persson, A. I.; Ohlsson, B. J.; Samuelson, L. Nano Lett. 2004, 4, 1961–1964. (12) Ross, F. M.; Tromp, R. M.; Reuter, M. C. Science 1999, 286, 1931– 1934. (13) Ross, F. M.; Tersoff, J.; Tromp, R. M. Phys. ReV. Lett. 1998, 80, 984–987. (14) Robinson, J. T.; Liddle, J. A.; Minor, A.; Radmilovic, V.; Yi, D. O.; Greaney, P. A.; Long, K. N.; Chrzan, D. C.; Dubon, O. D. Nano Lett. 2005, 5, 2070–2073. (15) Robinson, J. T.; Ratto, F.; Moutanabbir, O.; Heun, s.; Locatelli, A.; Mentes, T. O.; Aballe, L.; Dubon, O. D. Nano Lett. 2007, 7, 2655– 2659. (16) Ma, D. D. D.; Lee, C. S. F.; Au, C. K.; Tong, S. Y.; Lee, S. T. Science 2003, 299, 1874–1877.
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