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Endotaxial silicide nanowires: A review
Thin Solid Films 519 (2011) 8434–8440
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Endotaxial silicide nanowires: A review P.A. Bennett a, b,⁎, Zhian He b, David J. Smith a, b, F.M. Ross c a b c
Physics Department, Arizona State University, Tempe, AZ 85287 USA School of Materials, Arizona State University, Tempe, AZ 85287 USA IBM T. J. Watson Research Center, Yorktown Heights NY 10598 USA
a r t i c l e
i n f o
a b s t r a c t We review the topic of self-assembled endotaxial silicide nanowires on silicon. Crystallographic orientation, lattice mismatch and average dimensions are discussed for a variety of systems including Ti, Mn, Fe, Co, Ni, Pt and several rare earths on Si(100), Si(111) and Si(110) surfaces. In situ observations of growth dynamics support a constant-shape growth model, in which length, width and thickness all change in proportion as the nanowire grows, with thermally activated, facet-dependent rates. © 2011 Elsevier B.V. All rights reserved.
Available online 18 May 2011 Keywords: Silicide Nanowire Epitaxy Endotaxy
Contents 1. Introduction . . . . . . 2. Epitaxial NWs . . . . . 3. Endotaxial NWs . . . . 4. Growth mechanism . . . 5. Summary and conclusion Acknowledgments . . . . . . References . . . . . . . . .
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1. Introduction
2. Epitaxial NWs
Silicides are widely used in the microelectronics industry as contacts to silicon. They are useful because the silicide/silicon interface is chemically and structurally robust, and the Schottky barrier height can be tuned by choice of the silicide metal [1,2]. These materials will remain vital in the development of new nanoelectronic components for the same reasons. Thus, it is important to understand the behavior of silicide/silicon systems on the nanoscale. In this paper we discuss self-assembled silicide nanowires (NWs) that are attached to the silicon surface, either lying on top of the substrate, in a conventional epitaxial configuration, or partially embedded in the surface, in an “endotaxial” configuration. We review the structure and growth kinetics of both types of NWs and give a summary of the endotaxial silicide NWs known to date, along with a detailed discussion for a few illustrative systems.
Epitaxial silicide NWs were discovered by Preinesberger et al. in the Dy/Si(100) system [3]. They reported that deposition of 1ML Dy onto Si (100)-2x1 at room temperature, followed by annealing at 500 °C, created an array of long, thin islands (not yet called nanowires). Further annealing to 600 °C led to coarsening of the islands and transformation into more compact structures. Chen et al. [4], looking at the Er/Si(100) system, explained the long, thin nanowire shape as resulting from anisotropic strain, assuming a structure of hexagonal ErSi2 with orientation ErSi2 (0001)//Si(100) and ErSi2 [11–20] // Si[1–10]. This corresponds to a lattice mismatch of −1.3% and +6.5% in the long and short directions of the NW, respectively, assuming a fully coherent interface. (The hexagonal rare-earth disilicides are type AlB2 vacancy structures, with approximate stoichiometry MSi1.7 [5]. Here, we follow convention and refer to this simply as MSi2). The Rare Earth Silicide NW (RESNW) structures then are coherently-strained islands, but with uniaxial strain. It is useful to compare this NW system to a 3D strained island system, the best-studied example being Ge/Si(100). In the Ge/Si(100) system, growth has been modeled by using a chemical potential that goes to zero
⁎ Corresponding author at: Physics Department, Arizona State University, Tempe, AZ 85287 USA. Tel.: + 1 4809659623; fax: + 1 4809657954. E-mail address: [email protected] (P.A. Bennett). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.05.034
P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 1. (a) AFM image of DySi2 islands after deposition of 10ML Dy onto Si(111) at 600 °C. The inset shows a TEM diffraction pattern, with a circle marking a silicide reflection at the Si 1/3 (4–2–2) position. (b) AFM image after deposition of 1ML Dy at 700 °C onto Si(111) miscut 15° towards Si[11–2]. The inset shows a high resolution cross-section TEM image of a NW. (After Ref. [21]) (c) AFM image of DySi2 islands after deposition of 1 ml Dy onto flat Si(111) at 700 °C. The inset shows a plan-view TEM image of a similar NW, showing more fringes with 8 nm spacing (after ref [22]).
when the strain energy balances the adatom attachment energy [6]. This analysis leads to a preferred island size (that depends on lattice mismatch) and a narrow size-distribution histogram [7]. In the RE/ Si(100) NW system, it is found that the NW width across the lanthanide series scales roughly inversely with the bulk lattice mismatch, in agreement with the anisotropic strain model [8]. The width distribution histogram, however, is very broad, suggesting that the system is far from equilibrium [8]. This and other discrepancies with the coherent island model are partly explained by TEM observations that reveal a mixedphase structure for these NWs [9,10]. Several groups have studied the trends in NW structures for a variety of RESNWs on Si(100), providing information on overall morphology, electronic properties, “wetting layer” reconstructions and structural phase transitions during annealing. [8,10–13]. Tunneling spectroscopy shows that the NWs are metallic [11]. Structural details of the RESNW/Si(100) nanowire systems vary with the metal and the growth conditions. In this regard, the “best NW system” seems to be Y/Si(100), which can form very narrow (1.3 nm) isolated NWs. This is attributed to an excellent lattice match (0.1%) in the long direction of the NW, assuming the hexagonal AlB2 lattice oriented as above
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[14,15]. We note, however, that lattice match of bulk structures is not an infallible predictor for NW dimensions, since nanostructures have low-coordinated atoms at their boundaries, and can support large strains. In order to make use of epitaxial NWs, it is necessary to have some control over their location and growth direction. Most RESNW/ Si (100) systems tend to form as “bundles” of closely spaced NWs separated by narrow trenches [13]. They can be oriented into single domain structures by steps on a miscut substrate [16,17]. It is not clear, however, whether the single domain results from direct interaction of the NWs with the steps, or whether it simply reflects the single orientation of terraces induced by double-height steps. [18,19]. Here, some insight can be obtained from the RE/Si(111) system, [20] where there is no difference between terraces. Examination of RE/Si(111) systems that are identical apart from step configuration shows that steps play a direct role in determining the NW shape, and even the NW crystal structure in some cases. This is illustrated in Fig. 1 for Dy/Si(111) [21]. Deposition of Dy on flat Si(111) at 600 °C forms large, flat islands with compact, roughly hexagonal shapes (Fig. 1a). From plan-view TEM, the structure is identified as hex-DySi2 with DySi2(0001)//Si(111). The diffraction pattern indicates that this structure is fully coherent, corresponding to a (isotropic) strain of −0.2%. On stepped Si(111), however, growth at 700 °C produces NWs (Fig. 1b). These also have the hex-DySi2 structure. Cross-section TEM images of the NW/Si interface show enlarged Si(111) terraces and step coalescence onto inclined Si{111} facets. The NWs are aligned to Si [1–10], roughly parallel to the steps. Deposition at 700 °C, on flat Si(111), however, produces a mixture of hexagonal and NW shapes (Fig. 1c). [22]. Moire fringes in plan-view TEM images identify the NW as fully relaxed ortho-DySi2, with lattice mismatch +2.5% along Si [1–10], the long direction of the NW. We infer that NW growth for DySi2 on flat Si(111) does not directly result from good lattice match in the long direction of the NW. The balance of kinetic and energetic factors that influence the growth of RESNW structures remains an open topic [23–26]. The discussion above indicates that epitaxial RE/Si NWs can be grown with small dimensions but it is difficult to control their dimensions and location on the Si surface, and even the phases that form. In addition, one can expect the rare-earth containing nanostructures to be strongly reactive with oxygen, hence difficult to integrate into practical devices. This motivates the search for more robust and versatile NW systems. In this spirit, our group has explored NW formation in transition metal (TM) silicide/Si systems. In these systems NWs are formed, not by epitaxy, but rather by endotaxy, as described below. 3. Endotaxial NWs The term “endotaxy” refers to the growth of precipitate phases in a bulk matrix, with coherent interfaces surrounding the precipitate [27]. Interesting and useful structures can be formed by endotaxy, as in thermoelectric or magnetic systems [28–30]. Endotaxy is similar to “allotaxy”, a process in which crystals grow by annealing of implanted species [31]. Endotaxial silicide NWs were discovered in the Co/
Table 1 Crystal structures and interface alignment for known endotaxial silicide NW systems. NR = no report. It might be useful to add a thin or dotted horizontal line in the table below the 4th row, to guide the eye. Si(100)
Si(111)
Si(110)
Ti Mn Fe Co
NR NR α-FeSi2[38] A-type CoSi2, B-type CoSi2[22]
C49–TiSi2, B27–TiSi[33–36] MnSi1.7[37] Unknown[39] A-type CoSi2, B-type CoSi2[22]
TiSi2[36] NR s-FeSi2, β-FeSi2[39–41] [42] B-type CoSi2[22,43]
Ni Dy Pt CoPt
A-type NiSi2, B-type NiSi2 [44] [45] NR PtSi [48] Pt2Si[49] Unknown [52]
B-type NiSi2 [22] DySi2 (steps) [22] NR NR
A-type NiSi2 [22,46] DySi2 [47] PtSi[50] [51] NR
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P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 2. (a) AFM image of CoSi2 islands after deposition of 1ML Co onto a Si(111) substrate heated to 800 °C. (b and c) High resolution cross sectional TEM images taken through a triangular island and a nanowire, respectively. The white lines indicate lattice planes, and show A-type alignment under the triangle, and B-type under the NW. (After Ref. [32]).
Si(111) system, [22,32] but also occur in many other TM/Si systems, as shown in Table 1. We will discuss a few of these systems in detail, to explain the structural variants that occur. Deposition of Co onto Si(111) at 800 °C forms two types of islands, as shown in Fig. 2. One type has the shape of equilateral triangles, as expected from the 3-fold rotational symmetry of the surface. The second type has a long and thin NW shape, which is surprising since the lattice mismatch is isotropic and small (−0.5% at the growth temperature). The explanation for the NW shape is found in the crosssection TEM image (Fig. 2b) which shows that the island has grown into the substrate along inclined Si{111} planes. Importantly, one side
of the NW has a B-type {111} epitaxy, which forces the opposite side into a high-index orientation, CoSi2 {511}//Si{111}. Note that the Btype orientation would not be possible on all sides of an embedded triangular island. Thus, the NW shape involves lattice constraints on multiple, non-parallel interfaces. In this sense, endotaxial nanostructures may be considered as 3D inclusions or precipitates in the Si matrix. The lattice strain plays no role in the NW shape. This is quite different from epitaxial growth, which is governed by lattice constraints on a single interface parallel to the surface. It is useful to compare Ni/Si(111) with Co/Si(111), since they behave very similarly as thin films, [53] but show differences in NW formation.
Fig. 3. (a) AFM image of NiSi2 islands after deposition of 1 ML of Ni onto Si(111) at 750 °C. (b) Cross-section TEM image of triangular island, showing B-type {111} interface and {100} side facet. (c) AFM image of NiSi2 NWs after deposition of 2 ML of Ni at 750 °C onto Si(111) miscut 8° towards Si[− 1–12]. (d) Cross-section TEM image of a NW, with white lines indicating B-type (111) interface along the bottom, an internal twin boundary near the middle, (111) top facet and A-type {111} interface along the side (after ref[22]).
P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 3a, b shows NiSi2 islands formed on flat Si(111) at 750 °C. The islands are large and flat-topped with compact, approximately triangular shapes, and occasional second and third layers. Cross-sectional TEM shows that these triangles have a B-type (111) interface, a (111) NiSi2 facet on top, and a (100) NiSi2 facet on the side. NiSi2 NWs do not form on flat Si(111), unlike the case for Co/Si(111) (see Fig. 2). NiSi2 NWs do form, however, on stepped Si(111), as shown in Fig. 3c. AFM shows these NWs to be 20 nm wide and 2 nm high (above the surface). TEM shows a B-type (111) interface on the bottom and an A-type {111} interface along the side of the NW. The particular NW illustrated also shows an internal twin. Evidently the balance of factors affecting A-type vs B-type {111} interface formation in the (Ni, Co)/Si system is subtle. We note also that, unlike the DySi2/Si(111) NW system (see Fig. 1), the NiSi2/Si(111) NWs show no steps at the interface. This is consistent with observations of thin films of NiSi2 on Si(111) [54]. As with Co on Si(111), deposition of Co on Si(100) produces coexisting island types. These are shown in Fig. 4a–c. One island type has a compact, rectangular shape, with A-type {111} epitaxy on both sides of the island. The other island type has a narrow nanowire shape, with B-type {111} epitaxy on one side and {111}//{511} epitaxy on the opposite side. The compact islands are thicker than the NWs, and show higher contrast in plan-view TEM, but they have less height above the plane of the substrate and larger width than the NWs. Chen et al. reported similar structures for NiSi2 islands on Si(100), following nitride-mediated endotaxy [44]. Deposition of Ni or Co onto Si(110) produces a single type of island, in the form of parallel NWs aligned with Si[1–10], as shown in Fig. 4d and e. CoSi2 NWs on Si(110) show a B-type {111} facet along one bottom edge and top facets of {100} and {111}. NiSi2 NWs on Si(110), on the other hand, show A-type {111} epitaxy on both bottom edges, short {111} facets on top edges and a wide {110} facet on the top. Curiously, these NWs have a tapered shape that is observed in
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Fig. 5. AFM image of NWs on Si(100) formed by deposition of 2 ml Pt at 800 °C. Inset is an STM image (− 1 V, 1 nA) showing the self-assembled gap between orthogonal NWs.
regions of the sample that are “tilted” due to uneven sublimation during the annealing/cleaning process. The NWs point down the steps. The endotaxy structure is the same for tapered and untapered NiSi2 NWs on Si(110). The basic mechanism responsible for the tapered shape is unknown, at present. NWs formed by depositing Pt onto Si(100) are shown in Fig. 5. Early work on this system, using TEM, identified the NWs grown at 750 °C as incommensurate PtSi, with a lattice mismatch of ~ 6% [48]. More recent work, using AFM, suggests that NWs grown at 650 °C are Pt2Si, with lattice mismatch of (+ 2.6%, + 3.5%) in the long and short directions, respectively [49]. Cross-section HRTEM will be required to determine whether these NWs are epitaxial or endotaxial. This system
Fig. 4. (a) Plan-view TEM image of CoSi2 islands after deposition of 1 ml Co onto Si(100) at 750 °C, showing compact (type “A”) and nanowire (type “B”) islands. (b) Cross-section TEM image of a type “A” island, showing A-type {111} epitaxy on both sides. (c) Cross-section TEM image of a type “B” island, showing B-type {111} epitaxy on one side and {111}// {511} epitaxy on the opposite side. White lines indicate {111} planes, and also distinguish A-type vs. B-type interfaces (after Ref [32]) (d) Plan-view TEM of CoSi2 islands after deposition of 10 ml Co at 750 °C onto flat Si(110). Cross-section TEM image (inset) shows B-type (111) epitaxy on one side. (e) Plan-view TEM of NiSi2 islands after deposition of 10 ML Ni at 800 °C onto “miscut” Si(110). Cross-section TEM image (inset) shows A-type {111} epitaxy on both sides (after ref [22]).
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Fig. 6. (a, b, c) AFM images of TiSi2 NWs on Si(111) formed by deposition of 2 ml Ti at 850 °C. The structures are identified as C49 TiSi2 in (a) and B27 TiSi in (b) while (c) was not identified. (d) Bright field image of a NW similar to (a), showing moire fringes indicating an incommensurate structure. (e) Diffraction pattern from (d) showing separate silicide reflection (arrow) indicating a relaxed structure (after ref. [34]).
may be of practical importance since the NWs are structurally and chemically robust, and might be utilized in a cross-bar array for memristor or molecular switch devices [49,55]. In this regard, it is
interesting to note that self-assembled gaps with nanometer dimensions form at the intersection of orthogonal NWs, as shown in the inset of Fig. 5.
Fig. 7. (a–c) Three frames recorded during deposition of Co onto Si(110) at 750 °C, in situ in an ultra high vacuum TEM. The elapsed time since nucleation is indicated in seconds. (d and e) the position vs. time for each end of two of the NWs indicated in (a–c) (after ref. [43]).
P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 8. Scatter plot showing length and width of a collection of CoSi2 NWs grown on Si(110) at 750 °C. The lines mark the evolution of two NWs that were imaged continuously during growth, with crosses and triangles showing values every 20 s for the first 3 min of growth. The dots indicate final dimensions of NWs in the 10 μm2 field of view after 10 min growth, apart from one NW outside the plot axes with L = 150 and W = 45 nm. (after ref. [43]).
The Fe/Si system is particularly interesting because it can form semiconducting or magnetic structures [56,57]. The Fe/Si(110) system forms NWs with different crystal structures depending on temperature: deposition at 700 °C forms cubic FeSi2, while annealing this structure at 800 °C causes a phase transformation into hexagonal β-FeSi2 [40,42]. Mn/Si is another system that can be semiconducting or magnetic in thin films [58,59]. Mn deposited onto Si(111) forms two island types in the shape of triangles or NWs, analogous to Co/Si(111) [37]. The NW shape is favored by slow deposition, which in this report was 0.3ML/hr at 530 °C. No structural information was given, but it is reasonable to expect an endotaxial structure with habit planes on Si{111}, since the lattice mismatch in this configuration would be small (+1.8%). The Ti/Si(111) system forms multiple island types, as shown in Fig. 6 [34]. One island type (Fig. 6a, d) has the shape of long, thin NWs oriented along Si b220N, and is identified as C49-TiSi2; a second type (Fig. 6b) has the shape of compact rod structures oriented along Si b224N, and is identified as B27-TiSi [35]. Plan-view images and diffraction patterns show these to be fully relaxed, incommensurate structures (e.g. Fig. 6d and e). There are also occasional segmented structures, as shown in Fig. 6c. Diffraction patterns were not obtained for these segmented structures, but we guess that they grew as contiguous C49 NWs, then broke into segments, possibly upon cooling. The mechanism for the NW shapes for Ti/Si(111) is unknown since the lattice mismatch is poor (~8%) on all interfaces. 4. Growth mechanism In order to control the size and shape of endotaxial NWs, it is necessary to understand the growth mechanism in detail. This is best
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done by in situ observation of the growth dynamics. UHV-TEM is ideally suited for this purpose, since it provides dimensional information with nm-scale resolution with simultaneous strain and defect information. Here we summarize results from a detailed study of the Co/Si(110) system [43]. This system is a good choice for growth studies because, as we have seen above, the Si(110) substrate leads to a single NW orientation, which allows growth without collision between NWs. Fig. 7 shows three frames extracted from a movie recorded during the growth of CoSi2 NWs on Si(110) at 750 °C. Also shown is a plot of the position vs. time for each end of two of the NWs. For NW1, end “1a” moves quickly at first, but steadily slows. End “1b” remains fixed at first (at a small defect on the NW), but later moves at the same rate as end “1a”. The solid line shows L ~ t1/3 scaling, which we discuss later. For NW2, the “a” and “b” ends both move quickly at first, but stop after some time. The time evolution of the widths of the NWs follows the same trend as their lengths. This is shown in Fig. 8, where the growth trajectories for NW1 and NW2 during the three-minute duration of the movie follow a constant aspect ratio L/W ~ 34. The dimensions of a larger collection of NWs imaged at the end of 10 min growth are also indicated as dots on the chart. NW1 appears to be typical of the group, while NW2 is smaller than most. Similar behavior is seen for growth at 800 °C, but with a smaller aspect ratio of L/W ~ 15. We have introduced a constant-shape growth model to explain the observed L(t) and W(t)~t1/3 scaling. [43]. This is justified as follows: the length and width are seen to change in proportion, while the height (which is not measured directly) is assumed to change in proportion too, since it is constrained by the endotaxial interfaces between NW and substrate. Thus the volume increases linearly, as it must, since it is constrained by the incoming flux. At the atomic level, the anisotropic NW shape results from an unequal sticking rate for Co atoms at the ends versus the sides of the nanowire. These rates appear to be thermally activated, since the L/W aspect ratio varies strongly with temperature. This process apparently is sensitive to defects, which can cause pauses in the growth of each end independently, as seen in Fig. 7. High sensitivity to atomic details at the growing end-facet is also consistent with the relatively large spread in final dimensions for the collection of NWs grown under identical conditions, as shown in the L-W scatter plot in Fig. 8. The systematic temperature dependence is best seen from a set of AFM scans recorded after growth, as shown in Fig. 9 and Table 2 [22]. In the example shown, when the growth temperature changes from 850 °C to 700 °C, the NW width decreases by a factor 90 nm/14 nm= 6.5, the L/ W ratio increases by a factor 80/5 = 16 and the nucleation density increases by a factor 40. This temperature dependence allows us to consider how best to grow a low density of long but thin NWs, as desired for electronic applications. A low temperature is required for a high aspect ratio, but also leads to a high nucleation density, which causes close spacing and smaller size, since the total flux is then shared by many NWs. This might be countered by a slow deposition rate, but it would be difficult to overcome the exponential dependence of nucleation rate on temperature [60]. A multi-
Fig. 9. (a–c) AFM images recorded after 3ML Co deposition on Si(110) at T = 750, 800 and 850 °C respectively. The scale bar is the same for all images (after ref [22]).
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Table 2 Average dimensions for CoSi2 NWs on Si(110), formed by deposition of 3ML Co at various growth temperatures and measured by AFM. [22]. T (°C)
W (nm)
L/W
#/μm2
700 750 800 850
14 22 40 90
80 60 15 5
40 10 3 1
step approach, changing temperature and flux during growth, may be required. A further complication is that it is difficult to control surface coverage, since an ill-defined fraction of the flux “disappears” into the bulk. This occurs especially for the late TMs (Fe, Co, Ni, etc.), which have appreciable solubility and interdiffusion rates in Si for temperatures above 600 °C [61]. Chen et al. [38] have recently shown that a thin barrier layer of Si3N4 may be used to effectively slow the interdiffusion of metal and silicon during annealing of silicide nanostructures. They reported that this process forms near-equilibrium structures whose shape is determined by strain rather than growth kinetics [38]. In the case of FeSi2 NWs on Si(100), NWs with enhanced aspect ratio were obtained. A combination of the above approaches might be utilized to optimize endotaxial NW structures for electronic applications. 5. Summary and conclusion In this paper, we have reviewed the structure and growth kinetics of endotaxial silicide nanowires. Since the endotaxial structures are partially embedded in the surface, they may be considered as 3D inclusions in a silicon matrix. Endotaxial NWs form on a variety of Si surfaces, including Si (100), Si(111) and Si(110) and for a wide variety of metals including Ti, Mn, Fe, Co, Ni and Pt. In most cases, there is a good lattice match along one or more inclined {111} planes of Si or silicide, which leads to the NW shape. For Pt and Ti, however, the lattice match is not so close, yet NWs still form. Hence, it is not always possible to predict endotaxial nanowire growth based on latticematching conditions. The long, thin nanowire shape is primarily a kinetic effect due to facet-dependent reactions with unequal, thermally activated rates on the ends versus the sides of the NW. This allows tuning of the NW shape and size, with high aspect structures generally favored by lower growth temperature. Facet attachment rates might be usefully manipulated using surfactants or other surface-modifier layers. We expect that endotaxial nanowires (or compact nano-islands) can be formed by reactive deposition of metals or metal alloys [62] on other semiconductors, such as Ge [63] or GaAs. This would provide an opportunity to explore self-assembled metal/semiconductor nanostructures with interesting functionality, such as spin-polarized contacts or opto-electronic components. Acknowledgments We gratefully acknowledge the use of facilities in the J. M. Cowley Center for High Resolution Electron Microscopy and support from NSF grants ECS0304682 and DMR0503705 for much of the work reported above. References [1] K.N. Tu, J.W. Mayer, in: J.M. Poate, K.N. Tu, J.W. Mayer (Eds.), Thin Films Interdiffus. React. John Wiley, 1978, p. 359. [2] A.H. Reader, A.H. van Ommen, P.J.W. Weijs, R.A.M. Wolters, D.J. Oostra, Rep. Prog. Phys. 56 (11) (1993) 1397. [3] C. Preinesberger, S. Vandre, R. Kalka, M. Dahne-Prietsch, J. Phys. D: Appl. Phys. 31 (1998) L43.
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Zeng, P.R.C. Kent, T.H. Kim, A.P. Li, H.H. Weitering, Nat. Mater. 7 (7) (2008) 539. [16] R. Ragan, Y. Chen, D.A.A. Ohlberg, R. Medeiros-Ribeiro, R.S. Williams, J. Cryst. Growth 251 (2003) 657. [17] C. Preinesberger, et al., Appl. Phys. Lett. 87 (2005) 83107. [18] O.L. Alerhand, A.N. Berker, J.D. Joannopoulos, D. Vanderbilt, R.J. Hamers, J.E. Demuth, Phys. Rev. Lett. 64 (20) (1990) 2406. [19] X. Tong, P.A. Bennett, Phys. Rev. Lett. 67 (1991) 101. [20] J.L. McChesney, A. Kirakosian, R. Bennewitz, J.N. Crain, J.-L. Lin, F.J. Himpsel, Nanotechnol. 13 (2002) 545. [21] Z. He, D.J. Smith, P.A. Bennett, Appl. Phys. Lett. 86 (2005) 143110. [22] Z. He, PhD, Physics, Arizona State University, 2004. [23] C. Eames, M.I.J. Probert, S.P. Tear, Appl. Phys. Lett. 96/24 (2010). [24] C. Nisoli, D. Abraham, T. Lookman, A. Saxena, Phys. Rev. Lett. 102/24 (2009). [25] D. Qiu, M.X. Zhang, P.M. Kelly, Appl. Phys. Lett. 94/8 (2009). [26] A. Shinde, R.Q. Wu, R. Ragan, Surf. Sci. 604 (17–18) (2010) 1481. [27] I. Bonev, Acta Cryst. A 28/NOV1 (1972) 508. [28] D. Sakellari, N. Frangis, E.K. Polychroniadis, Phys. E-Low-Dimensional Syst. & Nanostructures 42 (5) (2010) 1777. [29] T. George, R.W. Fathauer, Appl. Phys. Lett. 59 (25) (1991) 3249. [30] T.P. Hogan, A. Downey, J. Short, J. D'Angelo, C.I. Wu, E. Quarez, J. Androulakis, P.F.P. Poudeu, J.R. Sootsman, D.Y. Chung, M.G. Kanatzidis, S.D. Mahanti, E.J. Timm, H. Schock, F. Ren, J. Johnson, E.D. Case, J. Electron. Mater. 36 (7) (2007) 704. [31] S. Mantl, J. Phys. D 31 (1) (1998) 1. [32] Z. He, D.J. Smith, P.A. Bennett, Phys. Rev. Lett. 93 (2004) 256102. [33] P.A. Bennett, B. Ashcroft, Z. He, R.M. Tromp, J. Vac. Sci. Technol. B 20 (6) (2002) 2500. [34] Z. He, M. Stevens, D.J. Smith, P.A. Bennett, Surf. Sci. 524 (1–3) (2003) 148. [35] M. Stevens, Z. He, D.J. Smith, P.A. Bennett, J. Appl. Phys. 93 (9) (2003) 5670. [36] H.C. Hsu, W.W. Wu, H.F. Hsu, L.J. Chen, Nano Lett. 7 (4) (2007) 885. [37] D. Wang, Z.Q. Zou, Nanotechnol. 20 (27) (2009) 275607. [38] S.Y. Chen, H.C. Chen, L.J. Chen, Appl. Phys. Lett. 88 (19) (2006) 193114. [39] S. Liang, PhD, Physics, Arizona State University, 2006. [40] S. Liang, R. Islam, D.J. Smith, P.A. Bennett, J. Cryst. Growth 295 (2006) 166. [41] Y. Ohira, T. Tanji, M. Yoshimura, K. Ueda, Jap. J. Appl. Phys. 47 (7) (2008) 6138. [42] S. Liang, R. Islam, D.J. Smith, P.A. Bennett, J.R. O'Brien, B. Taylor, Appl. Phys. Lett. 88 (2006) 113111. [43] P.A. Bennett, D.J. Smith, Z. He, M.C. Reuter, A.W. Ellis, F.M. Ross, Nanotechnol. (2011) submitted. [44] S.Y. Chen, L.J. Chen, Appl. Phys. Lett. 87 (25) (2005) 253111. [45] L.H. Wu, C.J. Tsai, Electrochem. Solid State Lett. 12 (3) (2009) H73. [46] J.F. Lin, J.P. Bird, Z. He, D.J. Smith, P.A. Bennett, App. Phys. Lett. 85 (2) (2004) 281. [47] Z. He, M. Stevens, D.J. Smith, P.A. Bennett, Appl. Phys. Lett. 83 (25) (2003) 5292. [48] K.L. Kavanagh, M.C. Reuter, R.M. Tromp, J. Cryst. Growth 173 (1997) 393. [49] D.K. Lim, D. 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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Endotaxial silicide nanowires: A review P.A. Bennett a, b,⁎, Zhian He b, David J. Smith a, b, F.M. Ross c a b c
Physics Department, Arizona State University, Tempe, AZ 85287 USA School of Materials, Arizona State University, Tempe, AZ 85287 USA IBM T. J. Watson Research Center, Yorktown Heights NY 10598 USA
a r t i c l e
i n f o
a b s t r a c t We review the topic of self-assembled endotaxial silicide nanowires on silicon. Crystallographic orientation, lattice mismatch and average dimensions are discussed for a variety of systems including Ti, Mn, Fe, Co, Ni, Pt and several rare earths on Si(100), Si(111) and Si(110) surfaces. In situ observations of growth dynamics support a constant-shape growth model, in which length, width and thickness all change in proportion as the nanowire grows, with thermally activated, facet-dependent rates. © 2011 Elsevier B.V. All rights reserved.
Available online 18 May 2011 Keywords: Silicide Nanowire Epitaxy Endotaxy
Contents 1. Introduction . . . . . . 2. Epitaxial NWs . . . . . 3. Endotaxial NWs . . . . 4. Growth mechanism . . . 5. Summary and conclusion Acknowledgments . . . . . . References . . . . . . . . .
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1. Introduction
2. Epitaxial NWs
Silicides are widely used in the microelectronics industry as contacts to silicon. They are useful because the silicide/silicon interface is chemically and structurally robust, and the Schottky barrier height can be tuned by choice of the silicide metal [1,2]. These materials will remain vital in the development of new nanoelectronic components for the same reasons. Thus, it is important to understand the behavior of silicide/silicon systems on the nanoscale. In this paper we discuss self-assembled silicide nanowires (NWs) that are attached to the silicon surface, either lying on top of the substrate, in a conventional epitaxial configuration, or partially embedded in the surface, in an “endotaxial” configuration. We review the structure and growth kinetics of both types of NWs and give a summary of the endotaxial silicide NWs known to date, along with a detailed discussion for a few illustrative systems.
Epitaxial silicide NWs were discovered by Preinesberger et al. in the Dy/Si(100) system [3]. They reported that deposition of 1ML Dy onto Si (100)-2x1 at room temperature, followed by annealing at 500 °C, created an array of long, thin islands (not yet called nanowires). Further annealing to 600 °C led to coarsening of the islands and transformation into more compact structures. Chen et al. [4], looking at the Er/Si(100) system, explained the long, thin nanowire shape as resulting from anisotropic strain, assuming a structure of hexagonal ErSi2 with orientation ErSi2 (0001)//Si(100) and ErSi2 [11–20] // Si[1–10]. This corresponds to a lattice mismatch of −1.3% and +6.5% in the long and short directions of the NW, respectively, assuming a fully coherent interface. (The hexagonal rare-earth disilicides are type AlB2 vacancy structures, with approximate stoichiometry MSi1.7 [5]. Here, we follow convention and refer to this simply as MSi2). The Rare Earth Silicide NW (RESNW) structures then are coherently-strained islands, but with uniaxial strain. It is useful to compare this NW system to a 3D strained island system, the best-studied example being Ge/Si(100). In the Ge/Si(100) system, growth has been modeled by using a chemical potential that goes to zero
⁎ Corresponding author at: Physics Department, Arizona State University, Tempe, AZ 85287 USA. Tel.: + 1 4809659623; fax: + 1 4809657954. E-mail address: [email protected] (P.A. Bennett). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.05.034
P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 1. (a) AFM image of DySi2 islands after deposition of 10ML Dy onto Si(111) at 600 °C. The inset shows a TEM diffraction pattern, with a circle marking a silicide reflection at the Si 1/3 (4–2–2) position. (b) AFM image after deposition of 1ML Dy at 700 °C onto Si(111) miscut 15° towards Si[11–2]. The inset shows a high resolution cross-section TEM image of a NW. (After Ref. [21]) (c) AFM image of DySi2 islands after deposition of 1 ml Dy onto flat Si(111) at 700 °C. The inset shows a plan-view TEM image of a similar NW, showing more fringes with 8 nm spacing (after ref [22]).
when the strain energy balances the adatom attachment energy [6]. This analysis leads to a preferred island size (that depends on lattice mismatch) and a narrow size-distribution histogram [7]. In the RE/ Si(100) NW system, it is found that the NW width across the lanthanide series scales roughly inversely with the bulk lattice mismatch, in agreement with the anisotropic strain model [8]. The width distribution histogram, however, is very broad, suggesting that the system is far from equilibrium [8]. This and other discrepancies with the coherent island model are partly explained by TEM observations that reveal a mixedphase structure for these NWs [9,10]. Several groups have studied the trends in NW structures for a variety of RESNWs on Si(100), providing information on overall morphology, electronic properties, “wetting layer” reconstructions and structural phase transitions during annealing. [8,10–13]. Tunneling spectroscopy shows that the NWs are metallic [11]. Structural details of the RESNW/Si(100) nanowire systems vary with the metal and the growth conditions. In this regard, the “best NW system” seems to be Y/Si(100), which can form very narrow (1.3 nm) isolated NWs. This is attributed to an excellent lattice match (0.1%) in the long direction of the NW, assuming the hexagonal AlB2 lattice oriented as above
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[14,15]. We note, however, that lattice match of bulk structures is not an infallible predictor for NW dimensions, since nanostructures have low-coordinated atoms at their boundaries, and can support large strains. In order to make use of epitaxial NWs, it is necessary to have some control over their location and growth direction. Most RESNW/ Si (100) systems tend to form as “bundles” of closely spaced NWs separated by narrow trenches [13]. They can be oriented into single domain structures by steps on a miscut substrate [16,17]. It is not clear, however, whether the single domain results from direct interaction of the NWs with the steps, or whether it simply reflects the single orientation of terraces induced by double-height steps. [18,19]. Here, some insight can be obtained from the RE/Si(111) system, [20] where there is no difference between terraces. Examination of RE/Si(111) systems that are identical apart from step configuration shows that steps play a direct role in determining the NW shape, and even the NW crystal structure in some cases. This is illustrated in Fig. 1 for Dy/Si(111) [21]. Deposition of Dy on flat Si(111) at 600 °C forms large, flat islands with compact, roughly hexagonal shapes (Fig. 1a). From plan-view TEM, the structure is identified as hex-DySi2 with DySi2(0001)//Si(111). The diffraction pattern indicates that this structure is fully coherent, corresponding to a (isotropic) strain of −0.2%. On stepped Si(111), however, growth at 700 °C produces NWs (Fig. 1b). These also have the hex-DySi2 structure. Cross-section TEM images of the NW/Si interface show enlarged Si(111) terraces and step coalescence onto inclined Si{111} facets. The NWs are aligned to Si [1–10], roughly parallel to the steps. Deposition at 700 °C, on flat Si(111), however, produces a mixture of hexagonal and NW shapes (Fig. 1c). [22]. Moire fringes in plan-view TEM images identify the NW as fully relaxed ortho-DySi2, with lattice mismatch +2.5% along Si [1–10], the long direction of the NW. We infer that NW growth for DySi2 on flat Si(111) does not directly result from good lattice match in the long direction of the NW. The balance of kinetic and energetic factors that influence the growth of RESNW structures remains an open topic [23–26]. The discussion above indicates that epitaxial RE/Si NWs can be grown with small dimensions but it is difficult to control their dimensions and location on the Si surface, and even the phases that form. In addition, one can expect the rare-earth containing nanostructures to be strongly reactive with oxygen, hence difficult to integrate into practical devices. This motivates the search for more robust and versatile NW systems. In this spirit, our group has explored NW formation in transition metal (TM) silicide/Si systems. In these systems NWs are formed, not by epitaxy, but rather by endotaxy, as described below. 3. Endotaxial NWs The term “endotaxy” refers to the growth of precipitate phases in a bulk matrix, with coherent interfaces surrounding the precipitate [27]. Interesting and useful structures can be formed by endotaxy, as in thermoelectric or magnetic systems [28–30]. Endotaxy is similar to “allotaxy”, a process in which crystals grow by annealing of implanted species [31]. Endotaxial silicide NWs were discovered in the Co/
Table 1 Crystal structures and interface alignment for known endotaxial silicide NW systems. NR = no report. It might be useful to add a thin or dotted horizontal line in the table below the 4th row, to guide the eye. Si(100)
Si(111)
Si(110)
Ti Mn Fe Co
NR NR α-FeSi2[38] A-type CoSi2, B-type CoSi2[22]
C49–TiSi2, B27–TiSi[33–36] MnSi1.7[37] Unknown[39] A-type CoSi2, B-type CoSi2[22]
TiSi2[36] NR s-FeSi2, β-FeSi2[39–41] [42] B-type CoSi2[22,43]
Ni Dy Pt CoPt
A-type NiSi2, B-type NiSi2 [44] [45] NR PtSi [48] Pt2Si[49] Unknown [52]
B-type NiSi2 [22] DySi2 (steps) [22] NR NR
A-type NiSi2 [22,46] DySi2 [47] PtSi[50] [51] NR
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Fig. 2. (a) AFM image of CoSi2 islands after deposition of 1ML Co onto a Si(111) substrate heated to 800 °C. (b and c) High resolution cross sectional TEM images taken through a triangular island and a nanowire, respectively. The white lines indicate lattice planes, and show A-type alignment under the triangle, and B-type under the NW. (After Ref. [32]).
Si(111) system, [22,32] but also occur in many other TM/Si systems, as shown in Table 1. We will discuss a few of these systems in detail, to explain the structural variants that occur. Deposition of Co onto Si(111) at 800 °C forms two types of islands, as shown in Fig. 2. One type has the shape of equilateral triangles, as expected from the 3-fold rotational symmetry of the surface. The second type has a long and thin NW shape, which is surprising since the lattice mismatch is isotropic and small (−0.5% at the growth temperature). The explanation for the NW shape is found in the crosssection TEM image (Fig. 2b) which shows that the island has grown into the substrate along inclined Si{111} planes. Importantly, one side
of the NW has a B-type {111} epitaxy, which forces the opposite side into a high-index orientation, CoSi2 {511}//Si{111}. Note that the Btype orientation would not be possible on all sides of an embedded triangular island. Thus, the NW shape involves lattice constraints on multiple, non-parallel interfaces. In this sense, endotaxial nanostructures may be considered as 3D inclusions or precipitates in the Si matrix. The lattice strain plays no role in the NW shape. This is quite different from epitaxial growth, which is governed by lattice constraints on a single interface parallel to the surface. It is useful to compare Ni/Si(111) with Co/Si(111), since they behave very similarly as thin films, [53] but show differences in NW formation.
Fig. 3. (a) AFM image of NiSi2 islands after deposition of 1 ML of Ni onto Si(111) at 750 °C. (b) Cross-section TEM image of triangular island, showing B-type {111} interface and {100} side facet. (c) AFM image of NiSi2 NWs after deposition of 2 ML of Ni at 750 °C onto Si(111) miscut 8° towards Si[− 1–12]. (d) Cross-section TEM image of a NW, with white lines indicating B-type (111) interface along the bottom, an internal twin boundary near the middle, (111) top facet and A-type {111} interface along the side (after ref[22]).
P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 3a, b shows NiSi2 islands formed on flat Si(111) at 750 °C. The islands are large and flat-topped with compact, approximately triangular shapes, and occasional second and third layers. Cross-sectional TEM shows that these triangles have a B-type (111) interface, a (111) NiSi2 facet on top, and a (100) NiSi2 facet on the side. NiSi2 NWs do not form on flat Si(111), unlike the case for Co/Si(111) (see Fig. 2). NiSi2 NWs do form, however, on stepped Si(111), as shown in Fig. 3c. AFM shows these NWs to be 20 nm wide and 2 nm high (above the surface). TEM shows a B-type (111) interface on the bottom and an A-type {111} interface along the side of the NW. The particular NW illustrated also shows an internal twin. Evidently the balance of factors affecting A-type vs B-type {111} interface formation in the (Ni, Co)/Si system is subtle. We note also that, unlike the DySi2/Si(111) NW system (see Fig. 1), the NiSi2/Si(111) NWs show no steps at the interface. This is consistent with observations of thin films of NiSi2 on Si(111) [54]. As with Co on Si(111), deposition of Co on Si(100) produces coexisting island types. These are shown in Fig. 4a–c. One island type has a compact, rectangular shape, with A-type {111} epitaxy on both sides of the island. The other island type has a narrow nanowire shape, with B-type {111} epitaxy on one side and {111}//{511} epitaxy on the opposite side. The compact islands are thicker than the NWs, and show higher contrast in plan-view TEM, but they have less height above the plane of the substrate and larger width than the NWs. Chen et al. reported similar structures for NiSi2 islands on Si(100), following nitride-mediated endotaxy [44]. Deposition of Ni or Co onto Si(110) produces a single type of island, in the form of parallel NWs aligned with Si[1–10], as shown in Fig. 4d and e. CoSi2 NWs on Si(110) show a B-type {111} facet along one bottom edge and top facets of {100} and {111}. NiSi2 NWs on Si(110), on the other hand, show A-type {111} epitaxy on both bottom edges, short {111} facets on top edges and a wide {110} facet on the top. Curiously, these NWs have a tapered shape that is observed in
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Fig. 5. AFM image of NWs on Si(100) formed by deposition of 2 ml Pt at 800 °C. Inset is an STM image (− 1 V, 1 nA) showing the self-assembled gap between orthogonal NWs.
regions of the sample that are “tilted” due to uneven sublimation during the annealing/cleaning process. The NWs point down the steps. The endotaxy structure is the same for tapered and untapered NiSi2 NWs on Si(110). The basic mechanism responsible for the tapered shape is unknown, at present. NWs formed by depositing Pt onto Si(100) are shown in Fig. 5. Early work on this system, using TEM, identified the NWs grown at 750 °C as incommensurate PtSi, with a lattice mismatch of ~ 6% [48]. More recent work, using AFM, suggests that NWs grown at 650 °C are Pt2Si, with lattice mismatch of (+ 2.6%, + 3.5%) in the long and short directions, respectively [49]. Cross-section HRTEM will be required to determine whether these NWs are epitaxial or endotaxial. This system
Fig. 4. (a) Plan-view TEM image of CoSi2 islands after deposition of 1 ml Co onto Si(100) at 750 °C, showing compact (type “A”) and nanowire (type “B”) islands. (b) Cross-section TEM image of a type “A” island, showing A-type {111} epitaxy on both sides. (c) Cross-section TEM image of a type “B” island, showing B-type {111} epitaxy on one side and {111}// {511} epitaxy on the opposite side. White lines indicate {111} planes, and also distinguish A-type vs. B-type interfaces (after Ref [32]) (d) Plan-view TEM of CoSi2 islands after deposition of 10 ml Co at 750 °C onto flat Si(110). Cross-section TEM image (inset) shows B-type (111) epitaxy on one side. (e) Plan-view TEM of NiSi2 islands after deposition of 10 ML Ni at 800 °C onto “miscut” Si(110). Cross-section TEM image (inset) shows A-type {111} epitaxy on both sides (after ref [22]).
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Fig. 6. (a, b, c) AFM images of TiSi2 NWs on Si(111) formed by deposition of 2 ml Ti at 850 °C. The structures are identified as C49 TiSi2 in (a) and B27 TiSi in (b) while (c) was not identified. (d) Bright field image of a NW similar to (a), showing moire fringes indicating an incommensurate structure. (e) Diffraction pattern from (d) showing separate silicide reflection (arrow) indicating a relaxed structure (after ref. [34]).
may be of practical importance since the NWs are structurally and chemically robust, and might be utilized in a cross-bar array for memristor or molecular switch devices [49,55]. In this regard, it is
interesting to note that self-assembled gaps with nanometer dimensions form at the intersection of orthogonal NWs, as shown in the inset of Fig. 5.
Fig. 7. (a–c) Three frames recorded during deposition of Co onto Si(110) at 750 °C, in situ in an ultra high vacuum TEM. The elapsed time since nucleation is indicated in seconds. (d and e) the position vs. time for each end of two of the NWs indicated in (a–c) (after ref. [43]).
P.A. Bennett et al. / Thin Solid Films 519 (2011) 8434–8440
Fig. 8. Scatter plot showing length and width of a collection of CoSi2 NWs grown on Si(110) at 750 °C. The lines mark the evolution of two NWs that were imaged continuously during growth, with crosses and triangles showing values every 20 s for the first 3 min of growth. The dots indicate final dimensions of NWs in the 10 μm2 field of view after 10 min growth, apart from one NW outside the plot axes with L = 150 and W = 45 nm. (after ref. [43]).
The Fe/Si system is particularly interesting because it can form semiconducting or magnetic structures [56,57]. The Fe/Si(110) system forms NWs with different crystal structures depending on temperature: deposition at 700 °C forms cubic FeSi2, while annealing this structure at 800 °C causes a phase transformation into hexagonal β-FeSi2 [40,42]. Mn/Si is another system that can be semiconducting or magnetic in thin films [58,59]. Mn deposited onto Si(111) forms two island types in the shape of triangles or NWs, analogous to Co/Si(111) [37]. The NW shape is favored by slow deposition, which in this report was 0.3ML/hr at 530 °C. No structural information was given, but it is reasonable to expect an endotaxial structure with habit planes on Si{111}, since the lattice mismatch in this configuration would be small (+1.8%). The Ti/Si(111) system forms multiple island types, as shown in Fig. 6 [34]. One island type (Fig. 6a, d) has the shape of long, thin NWs oriented along Si b220N, and is identified as C49-TiSi2; a second type (Fig. 6b) has the shape of compact rod structures oriented along Si b224N, and is identified as B27-TiSi [35]. Plan-view images and diffraction patterns show these to be fully relaxed, incommensurate structures (e.g. Fig. 6d and e). There are also occasional segmented structures, as shown in Fig. 6c. Diffraction patterns were not obtained for these segmented structures, but we guess that they grew as contiguous C49 NWs, then broke into segments, possibly upon cooling. The mechanism for the NW shapes for Ti/Si(111) is unknown since the lattice mismatch is poor (~8%) on all interfaces. 4. Growth mechanism In order to control the size and shape of endotaxial NWs, it is necessary to understand the growth mechanism in detail. This is best
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done by in situ observation of the growth dynamics. UHV-TEM is ideally suited for this purpose, since it provides dimensional information with nm-scale resolution with simultaneous strain and defect information. Here we summarize results from a detailed study of the Co/Si(110) system [43]. This system is a good choice for growth studies because, as we have seen above, the Si(110) substrate leads to a single NW orientation, which allows growth without collision between NWs. Fig. 7 shows three frames extracted from a movie recorded during the growth of CoSi2 NWs on Si(110) at 750 °C. Also shown is a plot of the position vs. time for each end of two of the NWs. For NW1, end “1a” moves quickly at first, but steadily slows. End “1b” remains fixed at first (at a small defect on the NW), but later moves at the same rate as end “1a”. The solid line shows L ~ t1/3 scaling, which we discuss later. For NW2, the “a” and “b” ends both move quickly at first, but stop after some time. The time evolution of the widths of the NWs follows the same trend as their lengths. This is shown in Fig. 8, where the growth trajectories for NW1 and NW2 during the three-minute duration of the movie follow a constant aspect ratio L/W ~ 34. The dimensions of a larger collection of NWs imaged at the end of 10 min growth are also indicated as dots on the chart. NW1 appears to be typical of the group, while NW2 is smaller than most. Similar behavior is seen for growth at 800 °C, but with a smaller aspect ratio of L/W ~ 15. We have introduced a constant-shape growth model to explain the observed L(t) and W(t)~t1/3 scaling. [43]. This is justified as follows: the length and width are seen to change in proportion, while the height (which is not measured directly) is assumed to change in proportion too, since it is constrained by the endotaxial interfaces between NW and substrate. Thus the volume increases linearly, as it must, since it is constrained by the incoming flux. At the atomic level, the anisotropic NW shape results from an unequal sticking rate for Co atoms at the ends versus the sides of the nanowire. These rates appear to be thermally activated, since the L/W aspect ratio varies strongly with temperature. This process apparently is sensitive to defects, which can cause pauses in the growth of each end independently, as seen in Fig. 7. High sensitivity to atomic details at the growing end-facet is also consistent with the relatively large spread in final dimensions for the collection of NWs grown under identical conditions, as shown in the L-W scatter plot in Fig. 8. The systematic temperature dependence is best seen from a set of AFM scans recorded after growth, as shown in Fig. 9 and Table 2 [22]. In the example shown, when the growth temperature changes from 850 °C to 700 °C, the NW width decreases by a factor 90 nm/14 nm= 6.5, the L/ W ratio increases by a factor 80/5 = 16 and the nucleation density increases by a factor 40. This temperature dependence allows us to consider how best to grow a low density of long but thin NWs, as desired for electronic applications. A low temperature is required for a high aspect ratio, but also leads to a high nucleation density, which causes close spacing and smaller size, since the total flux is then shared by many NWs. This might be countered by a slow deposition rate, but it would be difficult to overcome the exponential dependence of nucleation rate on temperature [60]. A multi-
Fig. 9. (a–c) AFM images recorded after 3ML Co deposition on Si(110) at T = 750, 800 and 850 °C respectively. The scale bar is the same for all images (after ref [22]).
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Table 2 Average dimensions for CoSi2 NWs on Si(110), formed by deposition of 3ML Co at various growth temperatures and measured by AFM. [22]. T (°C)
W (nm)
L/W
#/μm2
700 750 800 850
14 22 40 90
80 60 15 5
40 10 3 1
step approach, changing temperature and flux during growth, may be required. A further complication is that it is difficult to control surface coverage, since an ill-defined fraction of the flux “disappears” into the bulk. This occurs especially for the late TMs (Fe, Co, Ni, etc.), which have appreciable solubility and interdiffusion rates in Si for temperatures above 600 °C [61]. Chen et al. [38] have recently shown that a thin barrier layer of Si3N4 may be used to effectively slow the interdiffusion of metal and silicon during annealing of silicide nanostructures. They reported that this process forms near-equilibrium structures whose shape is determined by strain rather than growth kinetics [38]. In the case of FeSi2 NWs on Si(100), NWs with enhanced aspect ratio were obtained. A combination of the above approaches might be utilized to optimize endotaxial NW structures for electronic applications. 5. Summary and conclusion In this paper, we have reviewed the structure and growth kinetics of endotaxial silicide nanowires. Since the endotaxial structures are partially embedded in the surface, they may be considered as 3D inclusions in a silicon matrix. Endotaxial NWs form on a variety of Si surfaces, including Si (100), Si(111) and Si(110) and for a wide variety of metals including Ti, Mn, Fe, Co, Ni and Pt. In most cases, there is a good lattice match along one or more inclined {111} planes of Si or silicide, which leads to the NW shape. For Pt and Ti, however, the lattice match is not so close, yet NWs still form. Hence, it is not always possible to predict endotaxial nanowire growth based on latticematching conditions. The long, thin nanowire shape is primarily a kinetic effect due to facet-dependent reactions with unequal, thermally activated rates on the ends versus the sides of the NW. This allows tuning of the NW shape and size, with high aspect structures generally favored by lower growth temperature. Facet attachment rates might be usefully manipulated using surfactants or other surface-modifier layers. We expect that endotaxial nanowires (or compact nano-islands) can be formed by reactive deposition of metals or metal alloys [62] on other semiconductors, such as Ge [63] or GaAs. This would provide an opportunity to explore self-assembled metal/semiconductor nanostructures with interesting functionality, such as spin-polarized contacts or opto-electronic components. Acknowledgments We gratefully acknowledge the use of facilities in the J. M. Cowley Center for High Resolution Electron Microscopy and support from NSF grants ECS0304682 and DMR0503705 for much of the work reported above. References [1] K.N. Tu, J.W. Mayer, in: J.M. Poate, K.N. Tu, J.W. Mayer (Eds.), Thin Films Interdiffus. React. John Wiley, 1978, p. 359. [2] A.H. Reader, A.H. van Ommen, P.J.W. Weijs, R.A.M. Wolters, D.J. Oostra, Rep. Prog. Phys. 56 (11) (1993) 1397. [3] C. Preinesberger, S. Vandre, R. Kalka, M. Dahne-Prietsch, J. Phys. D: Appl. Phys. 31 (1998) L43.
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