ARTICLE pubs.acs.org/crystal
Synthesis of AlAs and AlAsGaAs CoreShell Nanowires Ang Li,*,† Daniele Ercolani,† Lorenzo Lugani,†,# Lucia Nasi,‡ Francesca Rossi,‡ Giancarlo Salviati,‡ Fabio Beltram,† and Lucia Sorba† † ‡
NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro 12, I-56127 Pisa, Italy IMEM-CNR, Parco Area delle Scienze 37/A, I-43010 Parma, Italy ABSTRACT: The growth of free-standing AlAs nanowires on GaAs (111)B substrates by chemical beam epitaxy is presented. Nanowire growth was achieved by employing trimethylaluminium and tertiarybutylarsine as metalorganic precursors and Au as catalyst. Different temperature and group III partial pressure values were examined to investigate the growth mechanism. Furthermore, synthesis of a GaAs shell is proposed as a way to protect the AlAs nanowires from rapid oxidization. In situ reflection high energy electron diffraction analysis and ex situ scanning and transmission electron microscopy studies were performed on both AlAs and AlAsGaAs coreshell nanowires in order to assess crystal perfection and structure of the nanowires.
’ INTRODUCTION In recent years, the growth of one-dimensional semiconductor nanowires (NWs) and their heterostructures has become an active and successful research field.14 Most semiconductor NWs are grown on crystalline substrates by the metal-assisted vaporliquidsolid (VLS) mechanism. Liquid phase metal catalyst droplets are playing a role as collector of vapor phase group III materials. The liquid metal droplet is supersaturated with the growth precursors leading to solid phase semiconductor precipitation and NW growth.5,6 Notably, the VLS mechanism provides much of flexibility in terms of changeable catalyst,7 material,8,9 and substrate.911 Moreover, epitaxial techniques such as molecular or chemical beam epitaxy and chemical vapor deposition allow precise control of the growth direction,12 crystal phases,1315 and composition16 of NWs. As a consequence, NW technology is a mature platform for fundamental studies1719 as well as electronics,20,21 and photonics applications.22 AlAs is wellknown as a wide indirect-band gap material almost perfectly lattice matched to GaAs. So far the exploitation of AlAs and its alloys in NW field was mostly limited to their use as shell layer in GaAs NWs.23 This GaAsAlGaAs coreshell structure enhances the radiative recombination efficiency and carrier mobility in the GaAs.2427 AlAs/GaAs/GaP heterostructure NWs were grown on Si substrates, and compositional studies demonstrated that the AlAs segments have a wurtzite crystal structure and the optical quality of the GaAs is strongly improved by capping the NWs with AlGaAs/GaAs shell layers.28 Recent theoretical work predicts that wurzite AlAs crystals have an indirect band gap which is also larger than in zinc blende AlAs crystals.29 Furthermore, size-effects can induce an indirect direct band gap transition.30 This opens new opportunities for band gap engineering and optical applications. However, little r 2011 American Chemical Society
is known about the electronic and optical properties of pure AlAs NWs. In this paper, we present the growth of free-standing AlAs and AlAsGaAs coreshell NWs on GaAs (111)B substrates by Au-assisted chemical beam epitaxy (CBE). In situ reflection high energy electron diffraction (RHEED) analysis is reported to assess the actual NW crystal structure. Morphological scanning electron microscopy (SEM) study of pure AlAs NWs is discussed as well together with the growth mechanism and its dependence on growth parameters. We finally present optimized growth conditions of a thin GaAs shell that protects the AlAs NWs from oxidation. This allowed us to perform ex situ high resolution transmission electron microscopy (TEM) and atomic resolution scanning transmission electron microscopy (STEM) studies on AlAsGaAs coreshell NWs.
’ EXPERIMENTAL SECTION GaAs (111)B substrates were employed to grow the NWs. A 1 nm thick Au film was predeposited by thermal evaporation on the substrates which were then transferred into a Riber Compact-21 CBE chamber.31 Trimethylaluminium (TMAl) and tertiarybutylarsine (TBAs) were used as metalorganic (MO) precursors for AlAs growth. Since TBAs has a high decomposition temperature, it is cracked in the injector at 1000 °C. The substrates were heated up to the growth temperature with TBAs flowing, while nanoparticles were generated by thermal dewetting of the Au film. The substrate temperature was monitored by a pyrometer, and the growth of AlAs NWs started right after the substrate reached the growth temperature. Several growth cycles were carried out in a range from 640 to 680 °C to study the impact of the growth temperature. Received: May 17, 2011 Revised: June 29, 2011 Published: July 06, 2011 4053
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Figure 1. 45° tilted SEM image of AlAs NWs grown at 650 °C with TMAl and TBAs line pressures 0.4 and 4 Torr, respectively. (a) 20 min growth. (b) 4 min growth. Furthermore, we fixed TBAs line pressure at 4 Torr and grew AlAs NWs with TMAl line pressure varying from 0.2 to 0.6 Torr at 650 °C to study the influence of group III precursor flux. After growth the samples were cooled down to 250 °C with TBAs flowing. For AlAsGaAs coreshell NWs, triethylgallium (TEGa) was used as the gallium precursor. The core AlAs NWs were grown at 650 °C with TMAl and TBAs line pressures of 0.4 and 4 Torr, respectively. After the AlAs core growth, the samples were cooled down to the shell growth temperature under TBAs flow. GaAs shells were then grown for 2 min with TEGa and TBAs line pressures of 0.7 and 2 Torr, respectively. In order to optimize the GaAs shell growth, we varied the shell growth temperature from 460 to 370 °C at fixed TEGa and TBAs line pressures. As-grown samples were analyzed in situ by RHEED and ex situ by field emission gun SEM. Some AlAs NWs were mechanically transferred to carbon-coated copper grids and analyzed by JEOL 2200FS high resolution TEM operating at 200 keV with energy dispersive X-ray spectroscopy (EDS).
’ RESULTS AND DISCUSSION Figure 1 shows a typical 45° tilted SEM image of AlAs NWs grown at 650 °C for 20 min. The pressures of TMAl and TBAs lines employed were 0.4 and 4 Torr, respectively. For acquiring the images of Figure 1, we employed an acceleration voltage of 8 kV, a working distance of about 8 mm, and a Everhart Thornley detector. In order to eliminate the charging from the sample, we have tried to reduce the acceleration voltage, but the quality of the image was strongly reduced. Figure 1a shows AlAs NWs uniformly covering the substrate. The AlAs NWs are oriented along the [111]B direction which is perpendicular to the substrate surface. The long insulating wires are charged by electron beam during imaging and tend to bend and collapse one on top of the other as the beam is scanning. This gives the appearance of disordered growth visible in Figure 1a. Measurements show that long and straight wires in this sample have a diameter smaller than 30 nm and length exceeding 2 μm. Charging effect hindered the accurate measurement of length and diameter of long wires; therefore, we reduced the growth time from 20 to 4 min. Figure 1b shows an SEM image of AlAs NWs grown in the same condition as the sample shown in Figure 1a but for this shorter timespan. The thin wires of Figure 1a are present but also thicker; slow-growing NWs were visible in this sample. The thicker NWs were hidden by the long thin wires in the 20 min grown sample. Brighter contrast catalyst particles are visible on the tip of the NWs.
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Figure 2. Experimental growth rate (solid dots) and theoretical model (solid curve) plotted vs near-tip diameter of AlAs NWs grown at 650 °C for 4 min with TMAl and TBAs line pressures 0.4 and 4 Torr, respectively.
The study of the growth rate of the NWs as a function of their diameter can provide information on the factors limiting NW growth and on the growth mechanism itself.32 We employed this method to identify the growth mechanism of AlAs NWs. The length of AlAs NWs was measured from 45° tilted SEM images and defined as the distance from the interface of the catalyst particle to the substrate. The growth rate was calculated by dividing the NWs length by the growth time. The diameter of NWs was acquired by measuring the near-tip diameter of the NWs. The experimental growth rate versus diameter is shown in Figure 2 for a sample grown with the same growth parameters as the one shown in Figure 1b. Data show that the growth rate of AlAs NWs is widely scattered even for NWs with similar diameter. However, the data points show a non-monotonous behavior of the growth rate: for diameters less than 15 nm, the average growth rate of AlAs NWs increases from about 10 nm/ min to a maximum growth rate of 65 nm/min with increasing NW diameter. Followed a decreasing behavior of the growth rate with increasing NW diameter for NWs diameter larger than 15 nm. A similar non-monotonous growth rate behavior has been reported by different groups with different growth methods and theoretical studies.3335 In order to explain this non-monotonous behavior, they have to take into account both Gibbs Thomson (GT) effect and diffusion induced (DI) contribution. In this general model of the VLS NW growth mechanism, the NW growth rate accounts for adatom diffusion from the substrate and sidewalls into the catalyst droplet as well as the GT effect of increased chemical potential in the catalyst droplets. Considering that for most MBE/CBE growth a high surface diffusivity on the sidewalls is often observed, the NW growth rate versus NW diameter can be described as the following formula:34 2 3 d 6 K1 dL 1 2RGT 6 1 4λ 7 2λ 7 7 6 ¼ V 1 exp þ 4 5 d dt θSL θVL d d K0 2λ where d is the near-tip diameter of the NW, RGT is the GibbsThomson radius, λ is the diffusion length, Ki is a modified Bessel function of second kind of order i, V is a constant, and θSL and θVL are the supersaturation of the surface adatoms and the vapor, respectively, with respect to the liquid droplet. By limiting the diffusion length of Al atoms of the order of micrometer,36 we fitted the theoretical model (solid line in 4054
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Figure 3. The average growth rate of AlAs NWs plotted as a function of diameter and typical SEM images of NWs grown at different temperatures. The scale bar is 200 nm. (a) Plot of the mean growth rate vs diameter with different growth temperatures indicated by different colors. (bd) SEM images of AlAs NWs grown at 640, 650, and 680 °C, respectively.
Figure 2) to our experimental data (solid dots in Figure 2). From the fitting curve, we extract a reasonable value of RGT ∼ 1.7 nm and λ ∼ 1 μm in agreement with other reported IIIV NWs grown by MBE.34 Before studying the growth of the AlAs NWs, we carried out several experiments to anneal the Au film and performed ex situ SEM measurements to investigate the diameter distribution of the catalyst. However, the size distribution of catalyst observed ex situ is different from the size distribution of the AlAs NWs after growth, and it is very difficult to find a direct relation between the two distributions. This could be due to several factors. First, due to the relatively high growth temperature of AlAs NWs the catalyst is in the liquid phase, and therefore during the temperature ramp to cool down the sample prior to unloading it from the growth chamber the droplet size distribution evolves and can be modified. Second, the particle size distribution acquired ex situ also depends on the contact angle between the droplet and the substrate as well as the particle composition. Therefore, it is very difficult to relate the ex situ observed particle size distribution to the NW diameter distribution. Because of the large scattering of the growth rate of AlAs NWs in Figure 2, in what follows we calculate the average growth rate of NWs for a given diameter, in order to compare different growth conditions. Different growth runs were carried out by fixing TMAl and TBAs line flux to 0.4 and 4 Torr, respectively, and varying growth temperature (from 640 to 680 °C in 10 steps). The average growth rate plotted as a function of near-tip diameter is shown as rectangular points in Figure 3a (solid lines are guide to the eye). For temperatures in the range of 650660 °C, the average growth rate is approximately 3 times higher than at lower or higher temperatures. At temperatures of 680 °C, only very thick wires grow (see Figure 3d) and with an extremely slow growth rate, while no wires grow below 640 °C.
Figure 4. Experimental and calculated AlAs NWs RHEED patterns. Experimental (a) and calculated (b) RHEED pattern from diffraction of the electron beam perpendicular to {1010} planes. Experimental (c) and calculated (d) RHEED pattern from diffraction of the electron beam perpendicular to {1120} planes. The lattice used in the calculation is wurtzite. (e) Unit cell of hexagonal wurtzite AlAs NWs, with the [0001] direction parallel to GaAs substrate [111] direction. Red color indicates {1010} planes corresponding to the diffraction patterns (a) and (b), while gray color indicates {1120} planes corresponding to the patterns (c) and (d).
We performed SEM analysis for all NW samples grown at different temperatures. SEM images for samples grown at 640, 650, and 680 °C are shown in Figure 3bd, respectively. From the SEM images, it appears that the surface becomes rougher with decreasing temperature. This is probably due to the decreased mobility of Al atoms at low temperature so that before being incorporated into Au particles they form small islands on the substrate. Furthermore, we observed a maximum value of wire density of ∼40 μm2 at optimal growth temperature, that is, 650 °C. The density of NWs will decrease to ∼15 μm2 with either decreasing or increasing growth temperature. Normally, it is expected that at lower density, the axial growth of NWs should be higher because of material conservation in a certain area. On the contrary, we observed the opposite result. This makes us believe that with a fixed III/V ratio, the growth of AlAs NWs is strongly influenced by the growth temperature but not by the density. In order to explore the growth dependence on the MO precursor pressures, we have grown samples at different III/V flux ratios. Because of the lower flexibility in variation of the TBAs flux with the CBE technique, we have varied the III/V flux ratio by fixing the TBAs line pressure to 4 Torr and changing the TMAl flux. For a TMAl flux lower than 0.3 Torr, a low growth 4055
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Figure 5. TEM image of pure AlAs NWs. The solid red lines highlight the crystalline part inside an amorphous wire.
rate is observed with the maximum growth rate below 10 nm/ min. This is probably because at high temperature Al atoms have a high probability of re-evaporation from the surface36 leading to lower amounts of Al in the catalyst droplet. The catalyst droplets hardly reach the supersaturating condition necessary for VLS growth, and the mean growth rate is sharply reduced. With increasing TMAl flux (up to 0.5 Torr), the growth rate reaches the highest value with a maximum of 65 nm/min. Furthermore, with higher TMAl flux (0.6 Torr), the growth rate starts to decrease and the NWs start to show defects such as kinks and an appreciable lateral growth. Thanks to its rapid feedback and capability of providing crystal-structure information during growth, RHEED has already shown great value in the field of NW growth.37 We performed in situ RHEED measurements to investigate the crystal structure of AlAs NWs. The diffraction pattern of AlAs NWs was as shown in Figure 4a; following a 30 °C rotation around the (111) axis perpendicular to the sample surface, the diffraction pattern changed to the pattern shown in Figure 4c, but it reverts to the pattern in Figure 4a after another 30 °C rotation. In order to determine the crystal structure of AlAs NWs from the RHEED pattern, we used the EMS On Line software38 to perform a RHEED simulation. The simulation was based on the assumption that the wurtzite (zinc blende) AlAs NWs grow with their [0001] ([111]) direction perpendicular to the GaAs (111)B surface. We chose an acceleration voltage equal to 14 kV with the camera distance of 500 mm corresponding to our experimental setup. Depending on whether the crystal structure of NWs is wurtzite or zinc blende, the simulation gives two series of very different patterns at different rotation angles, making its identification rather straightforward.7 Figure 4 shows an excellent agreement between the experimental RHEED pattern and the wurtzite NW simulation results. In this geometrical configuration, the simulation for the wurtzite gives rise to only two possible diffraction patterns: either from a beam direction perpendicular to {1010} planes (shown as Figure 4b) or from a beam direction perpendicular to {1120}
Figure 6. STEM HAADF and EDS analysis of AlAsGaAs coreshell NW with a shell growth temperature of 460 °C. (a) HAADF STEM image of NW. (bd) Elemental mapping of the NW by EDS. (e) Integration of different element signals from the white rectangular area shown in (a).
(shown as Figure 4d). As indicated by red and gray color respectively in Figure 4e, these two families of planes have 30° in-plane angle in agreement with the 30° periodicity observed from experimental diffraction patterns. On the other hand, zinc blende NWs show patterns clearly different from the experimentally observed ones.7 Considering that most IIIV NWs grown by various techniques often adopt the hexagonal wurtzite structure39,40 in contrast to their bulk counterparts, it can be derived from the in situ RHEED analysis that also the AlAs NWs are crystalline and mainly wurtzite structure. In order to investigate in more detail also the intensity of the RHEED patterns, detailed calculations using dynamical diffraction theory would be required. However, this is beyond the scope of the present work: the wurtzite and zinc blende structures have so different symmetries that they can be clearly distinguished one from the other by just looking at the position of the diffraction spots alone, without the need of intensity considerations. After growth, some AlAs NWs were mechanically transferred to a copper grid (with carbon supporting film) to perform TEM analysis. AlAs is very reactive and indeed rapidly oxidizes hindering the study of the NW crystal structure. We tried several ways to protect the samples before transferring them into the TEM chamber, but they appeared to oxidize right after dismounting from sample holder. Figure 5 shows one of the few TEM images in which we could find a residual crystalline part inside the wire. All the oxidized wires are amorphous with a polycrystalline catalyst on top. The red solid lines in Figure 5 define the tiny region inside the wire which remained crystalline. From all samples, we always found that structure is wurtzite, in agreement with the RHEED results described earlier. 4056
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Figure 7. TEM images of AlAsGaAs coreshell NWs. (a) High resolution TEM image of a typical coreshell NW. The inset shows selected area electron diffraction pattern from the body of the NW. (b) Atomic resolution HAADF image of the wire. Because of the Z-contrast, the AlAs core appears dark and it is clearly distinguished the bright GaAs outer layer with a thickness of 1 nm.
We also performed EDS measurements to determine the NW chemical composition. As expected, we observed a high quantity of oxygen in the body of wires, especially in the amorphous segments. No Ga signal was recorded from either the NW body or the catalyst particle, even though the Au film was thermally dewetting on the GaAs substrate before the AlAs NWs growth. In order to prevent AlAs oxidation, we deposited a GaAs shell around the AlAs NWs by employing a two-temperature growth procedure. We first grew the AlAs core at 650 °C for 20 min, and then the sample was cooled to 480 °C with TBAs flowing. Then the TEGa flow was opened for 2 min for the growth of the shell, followed by the usual cool down to 250 °C under TBAs flow. Since we observed identical RHEED diffraction patterns before,
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during, and after the shell deposition, we deduce that the crystal structure of AlAs NWs was not modified by the shell. In order to determine the chemical composition of the AlAsGaAs coreshell NW sample, we performed STEM high angle annular dark field (HAADF) analysis, as shown in Figure 6a. The sample shows clear Z-contrast difference caused by the material changing. The darker part of the NW indicated lower-Z AlAs region and the brighter region near the catalyst tip is a GaAs segment axially grown over the coreshell structure. This agrees well with elemental mappings of Al, As, and Ga signals acquired by the EDS detector (Figure 6bd, respectively). From the elemental mappings, we conclude that the top part of NW is pure GaAs, and the interface of AlAs/GaAs is quite sharp shown by a cross-section composition analysis of the NW area indicated by the white rectangle in Figure 6a, from which a shell thickness of about 6 nm can be estimated. Optimization of the shell growth was performed in order to reduce the axial growth of GaAs segment on top of the NWs. We found that with fixed TEGa and TBAs flux, by decreasing the deposition temperature from 480 to 370 °C, the axial growth rate of GaAs can be reduced from around 50 nm/min to less than 10 nm/min. Correspondingly, the lateral growth rate of the shell decreases from about 4 nm/min to less than 1 nm/min. In order to check whether a thin GaAs shell is protecting the AlAs NWs without deteriorating the crystal structure of the AlAs core, we performed TEM analysis on GaAsAlAs coreshell NWs. Figure 7 shows high resolution TEM images of a sample grown with optimized GaAs shell growth temperature of 370 °C. Figure 7a is taken in the [2110] zone axis. The inset of Figure 7a shows selected area electron diffraction (SAED) pattern carried out from the body of the wire. From the high resolution TEM and SAED analysis, we can determine that the AlAs NWs have wurtzite structure with lattice constants a = 3.9 ( 0.1 Å and c = 6.5 ( 0.1 Å. It has been shown theoretically41 that surface relaxation modifies the structural parameters only in NWs with diameters smaller than some tens of Å. Our values are derived in AlAs NWs with diameters of some hundreds of Å which are expected to be quasi-bulk. Assuming that the lattice constants of the hexagonal unit cell can be derived from geometric conversion of the cubic bulk lattice constants by shifting layers from the cubic to the hexagonal close-packed arrangement, for√ the ideal wurtzite crystal the lattice constant is given by awz = azb/ 2 and the lattice constant along the c axis perpendicular to the hexagon is related to the in-plane lattice constant by c = awz(8/3)1/2. From these geometric conversions, one would expect a = 4.003 Å and c = 6.537 Å, which match the TEM measurements within the experimental accuracy. The slight discrepancy in the a value can also be explained by considering that it has been shown experimentally and by ab initio calculations that such geometrical conversion is not extremely accurate to obtain wurtzite lattice parameters due to the differences in third nearest-neighbors in wurtzite and zincblende lattices,42 which tend to stretch the c axis and compress the a constant, in agreement with our TEM observations.
’ CONCLUSION By optimizing the growth parameters of AlAs NWs, we identified the best growth temperature window in the range from 650 to 660 °C, where the average growth rate can reach about 40 nm/min. By fixing the growth temperature, we varied the TMAl flux to study growth flux dependence. The optimized 4057
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Crystal Growth & Design V/III ratio of AlAs NWs growth at 650 °C is in the 46.6 range. From in situ RHEED analysis, the AlAs NWs are confirmed to be crystalline and the crystal structure of the AlAs NWs is mainly wurtzite, with its [0001] axis perpendicular to the (111)B surface of the substrate. A two-temperature growth procedure was employed to deposit a thin GaAs shell on the AlAs NWs to protect the latter from rapid oxidation. By optimizing the GaAs shell growth temperature with fixed TEGa and TBAs flux, a 1 nm thin GaAs shell was grown epitaxially on the sidewall of AlAs NWs. We found that such a GaAs shell can prevent the AlAs NWs from oxidization without deteriorating the crystal structure of the AlAs core. HRTEM and STEM characterization show that the AlAs NWs have a high quality wurtzite structure and the measured lattice constant of the AlAs NW is a = 3.9 ( 0.1 Å and c = 6.5 ( 0.1 Å. The AlAsGaAs coreshell structure opens the way to further ex situ experiments on AlAs NWs and to fabricate AlAs-based nanomaterials and nanostructures.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses #
LASPE, ICMP, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland.
’ ACKNOWLEDGMENT We acknowledge financial support from Monte dei Paschi di Siena with the project “Implementazione del laboratorio di crescita dedicato alla sintesi di nanofili a semiconduttore”, the bilateral project of Ministero degli Affari Esteri “Nanocharacterization of nanowires, nanomagnets and laser diodes for sensors, optoelectronics and data storage (N3)”, and the FIRB project prot. RBIN067A39_002. ’ REFERENCES (1) Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. J. Phil. Trans. R. Soc. Lond. A 2004, 362, 1247. (2) Dick, K. A. Prog. Cryst. Growth Charact. 2008, 54, 138. (3) Law, M.; Goldberger, J.; Yang, P. Annu. Rev. Mater. Res. 2004, 34, 83. (4) Harmand, J. C.; Patriarche, G.; Pere-Laperne, N.; MeratCombes, M. N.; Travers, L.; Glas, F. Appl. Phys. Lett. 2005, 87, 203101. (5) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (6) Hiruma, K.; Yazawa, M.; Katsuyama, T.; Ogawa, K.; Haraguchi, K.; Koguchi, M.; Kakibayashi, H. J. Appl. Phys. 1995, 77, 447. (7) Heun, S.; Radha, B.; Ercolani, D.; Kulkarni, G. U.; Rossi, F.; Grillo, V.; Salviati, G.; Beltram, F.; Sorba, L. Cryst. Growth Des. 2010, 10, 4197. (8) Samuelson, L. Physica E 2004, 21, 560. (9) Martensson, T.; Patrik, C.; Svensson, T.; Wacaser, B. A.; Larsson, M. W.; Seifert, W.; Deppert, K.; Gustafsson, A.; Reine, L.; Samuelson, L. Nano Lett. 2004, 4, 1987. (10) Roddaro, S.; Caroff, P.; Biasiol, G.; Rossi, F.; Bocchi, C.; Nilsson, K.; Fr€oberg, L.; Wagner, J. B.; Samuelson, L.; Wernersson, L.; Sorba, L. Nanotechnology 2009, 20, 285303. (11) Tomioka, K.; Motohisa, J.; Hara, S.; Fukui, T. Nano Lett. 2008, 8, 6. (12) Fortuna, S. A.; Li, X. Semicond. Sci. Technol. 2010, 25, 024005. (13) Dick, K. A.; Deppert, K.; Samuelson, L.; Wallenberg, L. R.; Ross, F. M. Nano Lett. 2008, 8, 4087.
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(14) Patriarche, G.; Glas, F.; Tchernycheva, M.; Sartel, C.; Largeau, L.; Harmand, J.; Cirlin, G. E. Nano Lett. 2008, 8, 1638. (15) Algra, R. E.; Verheijen, M. A.; Borgstr€ om, M. T.; Feiner, L. F.; Immink, G.; van Enckevort, W. J. P.; Vlieg, E.; Bakkers, E. P. A. M. Nature 2008, 456, 369. (16) Lim, S. K.; Tambe, M. J.; Brewster, M. M.; Gradeak, S. Nano Lett. 2008, 8, 1386. (17) Roddaro, S.; Pescaglini, A.; Ercolani, D.; Sorba, L.; Beltram, F. Nano Lett. 2011, 11, 1695. (18) Roddaro, S.; Pescaglini, A.; Ercolani, D.; Sorba, L.; Giazotto, F.; Beltram, F. Nano Res. 2011, 4, 259. (19) Nilsson, H. A.; Caroff, P.; Thelander, C.; Larsson, M.; Wagner, J. B.; Wernersson, L.-E.; Samuelson, L.; Xu, H. Q. Nano Lett. 2009, 9, 3151. (20) Egard, M.; Johansson, S.; Johansson, A.-C.; Persson, K.-M.; Dey, A. W.; Borg, B. M.; Thelander, C.; Wernersson, L.-E.; Lind, E. Nano Lett. 2010, 10, 809. (21) Roddaro, S.; Nilsson, K.; Astromskas, G.; Samuelson, L.; Wernersson, L.-E.; Karlstr€om, O.; Wacker, A. Appl. Phys. Lett. 2008, 8, 92. (22) Svensson, C. P. T.; Martensson, T.; Tr€agardh, J.; Larsson, C.; Rask, M.; Hessman, D.; Samuelson, L.; Ohlsson, J. Nanotechnology 2008, 18, 305201. (23) Shtrikman, H.; Popovitz-Biro, R.; von Huth, P.; Kretinin, A.; Heiblum, M. Core-shell GaAs-AlAs Nanowires Grown by MBE. In Proceedings of Nanoelectronics Conference (INEC), 2010 3rd International, Hongkong, China, January 38, 2010; IEEE: New York, 2010; pp 103104. (24) Chen, C.; Shehata, S.; Fradin, C.; LaPierre, R.; Couteau, C.; Weihs, G. Nano Lett. 2007, 7, 6. (25) Lucot, D.; Jabeen, F.; Harmand, J. C.; Patriarche, G.; Giraud, R.; Faini, G.; Mailly, D. Appl. Phys. Lett. 2011, 98, 142114. (26) Parkinson, P.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Zhang, X.; Zou, J.; Jagadish, C.; Herz, L. M.; Johnston, M. B. Nano Lett. 2009, 9, 3349. (27) Persano, A.; Nabet, B.; Taurino, A.; Prete, P.; Lovergine, N.; Cola, A. Appl. Phys. Lett. 2011, 98, 153106. (28) Zhang, G.; Tateno, K.; Gotoh, H.; Sogawa, T.; Nakano, H. Jpn. J. Appl. Phys. 2010, 49, 015001. (29) De, A.; Pryor, C. E. Phys. Rev. B 2010, 81, 155210. (30) Shiri, D.; Kong, Y.; Buin, A.; Anantram, M. P. Appl. Phys. Lett. 2008, 93, 073114. (31) Ercolani, D.; Rossi, F.; Li, A.; Roddaro, S.; Grillo, V.; Salviati, G.; Beltram, F.; Sorba, L. Nanotechnology 2009, 20, 505605. (32) Dubrovskii, V.; Cirlin, G.; Soshnikov, I.; Tonkikh, A.; Sibirev, N.; Samsonenko, Y.; Ustinov, V. Phys. Rev. B 2005, 71, 205325. (33) Fr€oberg, L.; Seifert, W.; Johansson, J. Phys. Rev. B 2007, 76, 153401. (34) Dubrovskii, V.; Sibirev, N.; Cirlin, G.; Soshnikov, I.; Chen, W. H.; Larde, R.; Cadel, E.; Pareige, P.; Xu, T.; Grandidier, B.; Nys, J. P.; Stievenard, D.; Moewe, M.; Chuang, L.; Chang-Hasnain, C. Phys. Rev. B 2009, 79, 205316. (35) Dubrovskii, V.; Sibirev, N.; Harmand, J. C.; Glas, F. Phys. Rev. B 2008, 78, 235301. (36) Tanaka, M.; Suzuki, T.; Nishinaga, T. Jap. J. Appl. Phys. 1990, 29, L 706. (37) Tchernycheva, M.; Harmand, J. C.; Patriarche, G.; Travers, L.; Cirlin, G. E. Nanotechnology 2006, 17, 4025. (38) http://cecm.insa-lyon.fr/CIOLS/crystal1.pl (39) Persson, A. I.; Larsson, M. W.; Stenstr€om, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Nat. Mater. 2004, 3, 677. (40) Soshnikov, I. P.; Cirlin, G. E.; Tonkikh, A. A.; Samsonenko, Y. B.; Dubovskii, V. G.; Ustinov, V. M.; Gorbenko, O. M.; Litvinov, D.; Gerthsen, D. Phys. Solid State 2005, 47, 2213. (41) Cicero, G.; Ferretti, A.; Catellani, A. Phys. Rev. B 2009, 80, 201304. (42) Kriegner, D.; Panse, C.; Mandl, B.; Dick, K. A.; Keplinger, M.; Persson, J. M.; Caroff, P.; Ercolani, D.; Sorba, L.; Bechstedt, F.; Stangl, J.; Bauer, G. Nano Lett. 2011, 11, 1483.
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