NANO LETTERS
Position-Controlled Interconnected InAs Nanowire Networks
2006 Vol. 6, No. 12 2842-2847
Kimberly A. Dick,*,† Knut Deppert,† Lisa S. Karlsson,‡ Werner Seifert,† L. Reine Wallenberg,‡ and Lars Samuelson† Solid State Physics, Lund UniVersity, Box 118, S-221 00 Lund, Sweden, and Polymer & Materials Chemistry/nCHREM, Lund UniVersity, Box 124, S-221 00 Lund, Sweden Received August 29, 2006; Revised Manuscript Received October 27, 2006
ABSTRACT We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or “nanotrees”. A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.
Research in semiconductor nanostructures has indicated great promise for applications of these structures in the electronics, materials science, and life science fields.1,2 The step from production of building blocks, such as nanoparticles and nanowires, to more complex nanostructured systems is key to the realization of this potential.3,4 We have previously demonstrated that controlled formation of axial heterostructures in InAs nanowires allows for fabrication of resonant tunneling diodes,5 single-electron transistors,6 few-electron quantum dots,7 field effect transistors,8 and few-electron memories.9 Also, position-controlled nanowire arrays have been demonstrated, using electron beam lithography10,11 nanoimprint lithography,12 atomic force microscopy manipulation of aerosol particles,13 and selected-area masking with SiO2.14 The next step is the production of increasingly complex architectures of one-dimensional components. A variety of novel branched structures have been presented, including sequentially seeded branched structures,15,16 tetrapods,17,18 self-assembled hierarchical structures,19-21 and interconnected networks.22 Greater control is needed, however, in order to make commercial use of these promising structures. The method discussed here presents the possibility to interconnect position-controlled epitaxial nanowires, incorporate functional elements into specified positions, and prepare the structure to be connected electrically to allow for its incorporation into more complex devices. * Corresponding author. E-mail:
[email protected]. † Solid State Physics. ‡ Polymer & Materials Chemistry/nCHREM. 10.1021/nl062035o CCC: $33.50 Published on Web 11/10/2006
© 2006 American Chemical Society
This is achieved by a unique combination of top-down lithographic processing, which defines the positions of the first-generation nanowires, and bottom-up epitaxial growth, which results in highly crystalline nanowires of selected length, diameter, and orientation. Sequential levels of semiconductor nanowires are grown to form a treelike structure (“nanotree”), with each level seeded by metal nanoparticles. These particles act as collectors for the vaporphase group III material, which combines beneath the particle with group V material from the vapor. This mechanism is commonly referred to as vapor-liquid-solid23 or vaporsolid-solid24 depending on the state of the particle. On the basis of the temperatures used and the observed composition of the particles, we will assume in this case that the seed particle is solid.25 The nanowire structures presented here are composed of InAs, which is a preferred material because of its narrow (direct) band gap, high electrical mobility, and absence of surface electron depletion.26 However, this technique is not in principle material-dependentsit can be extended to any system in which the position of the firstgeneration nanowires can be controlled and the second generation is seeded by metal nanoparticles. Positioncontrolled branched structures of InP and GaP have already been demonstrated.15 A thorough understanding of the morphology and crystallography of structures formed by the different III-V materials is essential. We have previously reported27 that InAs nanowires have a wurtzite structure when grown under the conditions used in this study; trunk nanowires grow in the [0001h] direction (normal to the (1h1h1h) zinc blende substrate), and nanowire branches grow outward from the trunk in the
Figure 1. SEM image of a large-scale position-controlled InAs nanowire network. This network is composed of two generations of nanowires, sequentially seeded by Au nanoparticles. The first generation is seeded by Au particles produced in an ordered pattern by electron beam lithography. These nanowires (“trunks”) grow normal to the substrate in the [111]B direction in the cubic cell or the [0001h] direction in the hexagonal cell. The second generation of nanowires (branches) is seeded by randomly deposited Au aerosol nanoparticles. These nanowire branches grow outward at right angles to the trunks in the 〈1h100〉 directions (in the hexagonal cell). Proper positioning of the first generation trunk nanowires ensures that branches grow toward neighboring trunks, connecting them together.
six 〈1h100〉 directions. Thus, by lithographically positioning the trunk seed particles along the 〈2h11〉 directions (with respect to the substrate), branches will grow toward neighboring trunks and connect them together. The substrate cleavage planes are known to be the (1h10) planes, as these are the lowest-order nonpolar surfaces. An example of a large-scale interconnected InAs nanowire network is shown in Figure 1. If the pattern of dots is oriented with reference to the sample edge, produced by electron beam lithography, the crystallographic directions of the dots with respect to each other can be selected (Figure 2a,c). Au is used for the trunk seed particles, but for branches, prealloyed binary Au-In particles are used.28 These particles reduce the particle-trunk interaction, which has been shown to have a detrimental effect on the growth of InAs branches.29 In order to produce interconnected nanotrees, positioncontrolled Au particles are needed.10-12 Patterns of Au particles were produced on InP (111)B substrates, with a diameter defined at 60 nm (see Figure 2a). InP substrates are preferred because InAs nanowires grown on these substrate tend to be less tapered than those grown on InAs substrates.30 InAs nanowires were grown from the positioncontrolled Au seed particles by metal-organic vapor-phase epitaxy (MOVPE), performed in a 10 kPa hydrogen atmosphere (6 L/min). Samples were heated first to 420 °C under a constant flow of phosphine (PH3), which was needed to hinder the decomposition of the InP substrates. Then a flow of trimethylindium (TMI) was turned on; this along with the PH3 served as a precursor gas for the component elements, Nano Lett., Vol. 6, No. 12, 2006
Figure 2. Schematic illustration of the procedure for producing interconnected nanowire networks. (a) Top view of InP (111)B sample with position-controlled InAs nanowires. These wires were seeded with lithographically positioned Au particles (on top of the wires after growth). (b) Tilted view of substrate with positioncontrolled InAs nanowires. Note that nanowires grow perpendicular to the (111)B substrate. Since these wires have a wurtzite structure, the growth direction is the corresponding [0001h] direction. (c) Crystallographic directions in the zinc blende (cubic) cell indicated in black, with the corresponding wurtzite (hexagonal) directions indicated in blue; both are indicated because the InAs substrates have a zinc blende structure, while the nanowire structures are wurtzite. These directions are illustrated with respect to the topview images (a) and (e). On comparison of parts a and c it can be seen that the cleavage planes forming the edges of the substrate are (1h10) planes in the cubic cell. The zinc blende 〈2h11〉 directions are also indicated, which correspond to wurtzite 〈1h100〉 directions. (d) Tilted view of sample as in (a), showing InAs nanowires after deposition of Au aerosol particles. Note that the particles are distributed randomly along wires and on the substrate. These particles will serve as seeds for second-generation nanowire (branch) growth. (e) Top view of the sample after growth of branches, which followed deposition of Au aerosol particles (part c). Note that branches grow parallel to cleavage planes (1h10) of the zinc blende substrate; that is, they grow in the 〈1h100〉 directions in the hexagonal cell (parallel to the 〈2h11〉 directions in the cubic cell). If the first generation nanowires have been positioned along the correct directions, the structures will become interconnected by their branches. (f) Tilted view of sample after growth of branches. The total number of branches shown has been reduced (compared to (e)) to make the image more clear.
and thus growth initiated. The optimum molar fractions of these precursors were found to be 3.0 × 10-6 for the TMI and 7.5 × 10-3 for the PH3. This allowed for the growth of a short InP nanowire “base” which improved the nucleation of nanowires on the substrate. The growth time for this base was 1 min, yielding an epitaxial nanowire of about 250 nm length. Following this, the flow of TMI was turned off, and the samples were heated to the desired InAs growth temperature (420, 440, 460, or 480 °C). When growth temperature was reached, the PH3 flow was turned off, and TMI and arsine (AsH3) were turned on, allowing the growth of InAs nanowires from the InP nanowire bases. We have previously reported than InP-InAs heterostructures can be grown despite the lattice mismatch;5 the wires in this study did not exhibit any kinks or nucleation problems related to the InP base. The optimum molar fraction of the AsH3 precursor was 2843
found to be 2.4 × 10-4. Conditions were chosen to yield optimum (rodlike) morphologysthat is, to reduce growth of the side facets during vertical nanowire growth.26 Under a given set of growth conditions, the wire growth rate was constant; typical InAs growth times were chosen between 4 and 8 min, yielding lengths on the order of 1.5-3 µm (see Figure 2b). The next step, after the production of ordered nanowire arrays, was to add nanowire branches connecting these “trunks” to each other. Aerosol nanoparticles31 were used as seeds for this second generation of nanowires (Figure 2d); Au-In binary particles were used as they have been shown to result in better nucleation of InAs branches.28 The sizeselected Au-In particles are deposited onto the arrays of InAs nanowires in an electrostatic precipitator.32 The trajectory of the particles toward the substrate depends on Brownian motion, the overall electric field, electrical forces, van der Waals forces, and the drag force; when particles approach nanowires on the surface, one must also consider image forces and van der Waals forces between the incoming particle and the wires; as well there may also be an increase of the electric field strength in the vicinity of the wire tip, a dipole field around the wires due to polarization.33 When such forces are taken into account, it can be determined that the distribution of particles on nanowires will depend on such factors as the diameter, spacing and length of the nanowires, the diameter of the nanoparticles, and the strength of the electric field used for deposition. With the parameters chosen in this study, particles were distributed fairly equally on the InP substrate and the trunks nanowires, resulting in secondgeneration nanowires on the substrate in addition to branches. Nanowire branches were grown by a procedure similar to that described for the trunks. The primary difference was that the samples were heated to the InAs growth temperature (460 °C) under a flow of AsH3 (rather than PH3), as the InP base was not required and prevention of InAs trunk nanowire decomposition was considered more important than prevention of InP substrate decomposition. Flows were as above, with a typical growth time of 3 min. Again, overgrowth of the structure was minimized by careful selection of parameters. If the initial pattern of Au particles is correctly oriented, branches will grow toward the neighboring trunks and connect with them (parts e and f of Figure 2). Figure 3 shows scanning electron microscopy (SEM) images of position-controlled interconnected InAs nanotrees. Figure 3a shows a magnified view of a large-scale interconnected network (similar to the one shown in Figure 1), while parts b-d of Figure 3 show smaller groups of interconnected nanotrees: two in Figure 3b (tilted view) and four in Figure 3c (top view) and Figure 3d (tilted view). It can be confirmed that all branches grow in the desired 〈1h100〉 directions, since trunks are positioned along these directions with respect to each other. It can be noted, however, that since there are six such directions, not all branches connect with an adjacent nanowire trunk when trunks are not positioned in all six directions (parts b-d of Figure 3). These extra branches do not affect the properties of the interconnected structure and are thus of no concern at this point. Further study, however, 2844
Figure 3. SEM images of InAs nanowire networks. (a) Magnified view of large-scale network, viewed from above. (b) Two interconnected InAs nanotrees, viewed at an angle of 45° to the surface normal. (c) Four interconnected InAs nanotrees, viewed from above. (d) Four interconnected InAs nanotrees as in (c), viewed at an angle of 45° to the surface normal.
may lead to more selective growth of branches. One possibility would be to change the selected trunk orientation; trunks grown on [001] substrates, for example, yield only four branch directions.15 It can be observed in Figure 3d that the growth rate (and thus final length) of individual branches depends on their position on the trunk. Specifically, branches closer to the substrate have a higher growth rate and are thus longer, while branches near the top of the trunk have a lower growth rate. This can be understood by considering that the substrate acts as a collector for vapor-phase precursor materials, which then reach the Au seed particles by surface diffusion. This means that quantitative analysis of the growth rates of branches (as a function of temperature, precursor pressures and particle size) is very difficult and beyond the scope of this paper. As branch distance from the substrate increases, the available collection area decreases. Once a branch is further from the substrate than the diffusion length of the furthest-diffusing precursor, the collection area and growth rate will be constant. This was previously reported to be the case for growth of GaP nanotrees;15 it has been noted that the diffusion length of Ga on GaP is significantly shorter than that of In on InAs.27 This length discrepancy is not a problem for this experiment and thus is not addressed. The growth rate could be more accurately controlled by limiting the number of branches and their position on the trunks. This could be achieved by burying the trunks and substrate in a polymer during Au-In particle deposition, leaving only a short length of trunk exposed to particles. The polymer is then lifted off after deposition and before branch growth, leaving only a small section of trunk with particles on it, which are thus all Nano Lett., Vol. 6, No. 12, 2006
Figure 4. TEM images of a pair of connected InAs nanotrees. (a) Low magnification overview of the two connected nanotrees. In this case the tops of the trunks have broken off during transfer to the TEM grid; one of these tops is visible between the trunks. The growth direction of the trunks is indicated by an arrow. The branch originates at the trunk on the left and grows toward the right. Stacking faults are visible as dark lines perpendicular to the trunks. A large tapered region is visible between the branch and the trunk on the left. (b) Magnification of the area indicated in red in (a). It is evident here that stacking faults from the trunk have continued out into the branch. Also, the large triangular section below the branch on the right side of the image (indicated by an arrow) appears to originate from the connection of this branch with the second trunk. This section has a zinc blende crystal structure, while the rest of the nanotree pair has a wurtzite structure. (c) Magnification of the area indicated in green in (a). This section shows the branch wrapping around the second trunk, then continuing to grow beyond it. It is clear here that the zinc blende section below the branch is epitaxially related to both the branch and the second trunk, forming an interface between then. (d) Magnification of the tip of the branch, indicated in blue in (a). It can be seen here that the zinc blende connecting section below the branch has been incorporated into the branch itself and continues to the tip.
approximately the same height from the substrate. This procedure (which will be published elsewhere)34 could also be used (with a much thinner polymer layer) to prevent AuIn particles from landing on the substrate and seeding a second generation of “trunk” nanowires. It is noted that trunks continue to grow during the branch growth phase, and so particles positioned near the top of the trunks would not result in branches very close to the final top of the trunk. The microstructure of the nanowire networks was investigated by transmission electron microscopy (TEM). Samples were prepared by gently breaking connected nanowires off of the substrate close to the base, to deposit them onto TEM grids. Since this process can damage the structures, samples were prepared with pairs of connected nanowires (as in Figure 3b), rather than large-scale network. A set of connected nanowires is shown in Figure 4a. In this case, the Nano Lett., Vol. 6, No. 12, 2006
tops of the two “trunk” nanowires have been broken off, but the branch section is intact. Here, the branch has wrapped around the second trunk perpendicular to its length and then continued in its original growth direction. The region indicated in red in Figure 4a is magnified in Figure 4b. Analysis of this high-resolution image indicates that this region has a primarily wurtzite structure with several stacking faults, visible as dark lines parallel to the branch growth direction. These stacking faults occurred during trunk growth and have been carried out into the branch. Also, there is a large triangular single crystalline section below the branch on the right-hand side of the image. This section has grown between the second trunk and the branch. Since it is epitaxially related to the branch, it evidently grew after the branch came into contact with the trunk. Unlike the rest of the structure, this segment is primarily zinc blende in structure, possibly because it has grown by a different mechanism (bulk growth) than the nanowire structure. Magnification of the area shown in green in Figure 4a is shown in Figure 4c. It is clear that branch and trunk have primarily a wurtzite structure with stacking faults but that the “connecting” section beneath the branch is primarily zinc blende. Finally, in Figure 4d, the tip of the branch is magnified (from the region in blue in Figure 4a). Stacking faults from the first trunk are clearly seen to continue to this point, demonstrating that only one branch (originating from the first trunk) is present in this image. Also, the zinc blende connecting section has been entirely incorporated into the branch. It thus appears that this section serves to secure an epitaxial relationship between the branch and the second trunk, providing a good contact between them. Investigation of a variety of pairs of interconnected structures indicated that not all exhibited such a “clean” interface between the growing branch and the second trunk. That is, in some cases the growing branch changed direction numerous times after encountering the neighboring trunk, often wrapping around it. However, in all cases we examined there was a microstructural relationship between the branch and the second trunk, indicating epitaxial growth even in cases where the morphology was not so uniform. The mixture of wurtzite and zinc blende structural phases may have important and interesting consequences for the electrical and optical properties of the structure. Calculations indicate that the band gap of wurtzite InAs is about 50 meV higher than that of zinc blende InAs;35 the Fermi level is still pinned in the conduction band. This change in band gap has also been observed experimentally.36,37 In a structure with significantly long segments of both zinc blende and wurtzite, all carriers will relax to zinc blende segments and this is what will be visible optically. This suggests that some areas of the nanowire will be optically unavailable, which could be a problem in components. However, a structure containing only very short zinc blende sections will not exhibit this behavior; the zinc blende segments will instead act as extremely shallow quantum wells. If the Fermi level is pinned in the conduction band (as noted above), there will be less obvious difference in the optical behavior of the different structures. With respect to electron transport, the alternating 2845
band gaps of the two structures may result in increased electrical scattering between segments; such effects have not been observed in electron transport experiments conducted to date. Practical applications of the interconnected nanowires presented here range from studies of, and development of better understanding of, self-assembly of matter into complex three-dimensional structures, to the use of interconnected nanowires for sensor arrangements and, in a longer perspective, to the realization of solid-state-based artificial neural networks. Such networks have been proposed to be used for efficient pattern recognition or data processing (filtering).38 So far, artificial neural networks have been created either based on software implementation or as integrated circuits mimicking the multiple interconnections in biological neural networks. In this report we demonstrate the creation of a new type of artificial network which, in many ways, resembles the biological neural networks; we first define the nodes, or the neurons, as the vertical nanowires (“trunks”) and the interconnection between the nodes are formed as branches growing out from each vertical nanowire (trunk) and joining with the neighboring trunk. The resulting interconnected network is thus a monolithic and highly crystalline structure. The ability to control the diameter and length scales of each section of the structure independently, as well as the vertical positions of branches,34 increases the applicability of these structures. We can produce either a small or very large number of neuron nodes each interconnected with its nearest neighbors (in our example, each node connects to three neighboring nodes). The choice of InAs as basic building material is quite ideal given its reproducible and very good quasi-metallic conductance properties. It may be possible to develop conditions for “firing” of the signals between the nodes, for example by positioning double-barrier tunneling structures in the branches,5,6 which only allow firing between neighboring nodes when sufficient charge-based biasing between the nodes has been built up. In comparison to other artificial neural network implementations, it should be noted that the weights of interconnects between the nodes are not, in our implementation of the network, easy to alter or program. To summarize, we have demonstrated a technique for the formation of complex position-controlled interconnected networks composed of multiple generations of InAs nanowires. By lithographically determining the position of the first generation of nanowires and controlling the growth conditions for subsequent generations, we arrange the structure in such a way that the structures interconnect by self-assembly. The growth of each generation of nanowires is controlled independently, allowing for the incorporation of functional elements into different parts of the network structure. Finally, we examine the crystal structure of these nanowire networks and show that good contact is established between the growing branch and the nanowire to which it becomes connected. Acknowledgment. The authors thank T. Mårtensson for assistance with electron beam lithography, and Z. Zanolli, J. Tra¨gårdh, and C. Thelander for valuable insights and 2846
discussions. This work was performed within the Nanometer Structure Consortium at Lund University, and supported by the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), the Knut and Alice Wallenberg Foundation, and the European Community (EU Contract No. 015783 NODE). References (1) Samuelson, L.; Thelander, C.; Bjo¨rk, M. T.; Borgstro¨m, M.; Deppert, K.; Dick, K. A.; Hansen, A. E.; Mårtensson, T.; Panev, N.; Persson, A. I.; Seifert, W.; Sko¨ld, N.; Larsson, M. W.; Wallenberg, L. R. Physica E 2004, 25, 313. (2) Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Philos. Trans. R. Soc. London 2004, 362, 1247. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) Samuelson, L. Mater. Today 2003, 6, 22. (5) Bjo¨rk, M. T.; Ohlsson, B. J.; Thelander, C.; Persson, A. I.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2002, 81, 4458. (6) Thelander, C.; Mårtensson, T.; Bjo¨rk, M. T.; Ohlsson, B. J.; Larsson, M. W.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2003, 83, 2052. (7) Bjo¨rk, M. T.; Thelander, C.; Hansen, A. E.; Jensen, L. E.; Larsson, M. W.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2004, 4, 1621. (8) Bryllert, T.; Wernersson, L. -E.; Fro¨berg, L. E.; Samuelson, L. IEEE Electron DeVice Lett. 2006, 27, 323. (9) Thelander, C.; Nilsson, H. A.; Jensen, L. E.; Samuelson, L. Nano Lett. 2005, 5, 635. (10) Mårtensson, T.; Borgstro¨m, M.; Seifert, W.; Ohlsson, B. J.; Samuelson, L. Nanotechnology 2003, 14, 1255. (11) Jensen, L. E.; Bjo¨rk, M. T.; Jeppesen, S.; Persson, A. I.; Ohlsson, B. J.; Samuelson, L. Nano Lett. 2004, 4, 1961. (12) Mårtenson, T.; Carlberg, P.; Borgstro¨m, M.; Montelius, L.; Seifert, W.; Samuelson, L. Nano Lett. 2004, 4, 699. (13) Ohlsson, B. J.; Bjo¨rk, M. T.; Magnusson, M. H.; Deppert, K.; Samuelson, L.; Wallenberg, L. R. Appl. Phys. Lett. 2001, 79, 3335. (14) Noborisaka, J.; Motohisa, J.; Fukui, T. Appl. Phys. Lett. 2005, 86, 213102. (15) Dick, K. A.; Deppert, K.; Larsson, M. W.; Mårtensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380. (16) Wang, D.; Qian, F.; Yang, C.; Zhong, Z.; Lieber, C. M. Nano Lett. 2004, 4, 871. (17) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 122, 12700. (18) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (19) Gao, P.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 12653. (20) Lao, Y. L.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (21) May, S. J.; Zheng, J. G.; Wessels, B. W.; Lauhon, L. J. AdV. Mater. 2005, 17, 298. (22) Zhou, J.; Ding, Y.; Deng, S. Z.; Gong, L.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2005, 14, 2107. (23) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (24) Persson, A. I.; Larsson, M. W.; Stenstro¨m, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Nat. Mater. 2004, 3, 677. (25) Dick, K. A.; Deppert, K.; Mårtensson, T.; Mandl, B.; Samuelson, L.; Seifert, W. Nano Lett. 2005, 5, 761. (26) Milnes, A. G.; Polyakov, A. Y. Mater. Sci. Eng. B 1993, 18, 237. (27) Dick, K. A.; Deppert, K.; Samuelson, L.; Seifert, W. J. Cryst. Growth, in press. (28) Dick, K. A.; Geretovszky, Z.; Mikkelsen, A.; Karlsson, L. S.; Lundgren, E.; Malm, J.-O.; Andersen, J. N.; Samuelson, L.; Seifert, W.; Wacaser, B. A.; Deppert, K. Nanotechnology 2006, 17, 1344. (29) Dick, K. A.; Deppert, K.; Karlsson, L. S.; Wallenberg, L. R.; Samuelson, L.; Seifert, W. AdV. Funct. Mater. 2005, 15, 1603. (30) Dick, K. A.; Deppert, K.; Samuelson, L.; Seifert, W. J. Cryst. Growth, in press. (31) Magnusson, M. H.; Deppert, K.; Malm, J.-O.; Bovin, J.-O.; Samuelson, L. J. Nanopart. Res. 1999, 1, 243. (32) Deppert, K.; Schmidt, F.; Krinke, T.; Dixkens, J.; Fissan, H. J. Aerosol Sci. 1996, 27, S151. (33) Bayer, K.; Dick, K. A.; Krinke, T. J.; Deppert, K., submitted.
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