Arsenic Pathways in Self-Catalyzed Growth of GaAs Nanowires

Article pubs.acs.org/crystal

Arsenic Pathways in Self-Catalyzed Growth of GaAs Nanowires Mohammed Reda Ramdani,* Jean Christophe Harmand, Frank Glas, Gilles Patriarche, and Laurent Travers CNRS-Laboratoire de Photonique et de Nanostructures, Route de Nozay, 91640 Marcoussis, France S Supporting Information *

ABSTRACT: Self-catalyzed growth of GaAs nanowires by molecular beam epitaxy on (111)Si substrates is investigated by introducing AlxGa1−xAs time markers. The nanowire elongation rate is found to be radius-independent, constant at substrate temperatures below 650 °C and linearly increasing with the incoming arsenic flux. The basic question of which pathways are followed by the arsenic species contributing to nanowire growth is clarified. The flow rate of As atoms directly impinging on the Ga catalyst drop is significantly smaller than the As consumption by nanowire growth. Thus, supplementary As atoms are necessary to explain the actual elongation rate. We show that surface diffusion of adsorbed Asx species toward the catalyst cannot account for the missing atoms. On the other hand, the reevaporation of Asx species from the substrate and from nanowire sidewall surfaces can act as an efficient secondary arsenic source. We argue that a sufficient amount of these species can be intercepted by the Ga drop and add up with the direct As impingement to explain the actual elongation rate.

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INTRODUCTION Semiconductor nanowires (NWs) are attractive building blocks for a number of devices, such as transistors,1,2 sensors,3,4 nanolasers,5,6 or photovoltaic cells.7 They present an elegant solution to address the longstanding challenge of integrating III−V compounds on Si substrates. Their synthesis is carried out by various techniques, including epitaxial growth by vapor phase epitaxy or molecular beam epitaxy (MBE). Au particles are commonly used to catalyze their formation. However, methods replacing Au-assisted growth are being developed. One motivation is to avoid the risk that Au contaminates the NWs and creates nonradiative or scattering centers,8 which could degrade the optical and transport properties of the NWs. The use of Au is also highly undesirable in Si-CMOS technology and presents a serious obstacle to combine NWs on a Si platform. Self-catalyzed growth of III−V NWs, where the group III component serves as catalyst, is an excellent alternative regarding these problems. In recent years, several groups observed and developed the growth of Ga-catalyzed GaAs NWs on Si substrates.9−16 In this method, small liquid Ga droplets accumulate on the substrate (often covered partly by a silicon oxide) and assist the formation of NWs by vapor− liquid−solid (VLS) growth.10−12 The oxide layer can be intentionally patterned with holes to form controlled and regular arrays of NWs,13,14 or else pitlike imperfections in the Si native oxide play the role of preferential nucleation sites.10 In these studies, several features contrasting with Au-catalyzed growth were noticed. In particular, the crystal structure of the NW is predominantly zinc blende15−18 (as opposed to the wurtzite structure that dominates in Au-catalyzed growth) and the NW growth rate is independent of NW length.11 These attractive characteristics motivate further investigations of this alternative method. In the present work, we reexamine the self© 2012 American Chemical Society

catalyzed VLS growth of GaAs NWs by solid-source MBE on (111)-oriented Si substrates. We periodically insert thin (Al,Ga)As markers to measure the precise chronology of growth by observing single NWs in transmission electron microscopy. The results are analyzed, and the self-catalyzed growth mechanisms are discussed in detail. In particular, we investigate the role of the arsenic flux and discuss the longstanding question of the arsenic pathways.

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SAMPLE PREPARATION AND GROWTH CONDITIONS

The GaAs NWs were grown by solid-source MBE on 2 in. (111)oriented Si wafers that were not chemically treated. The substrates were degassed in a buffer chamber before introduction in the growth chamber. The growth temperature was measured by a pyrometer on the substrate surface before forming the NWs. The surface emissivity changes significantly when NWs develop, and consequently, the pyrometer reading evolves. For this reason, we relied essentially on the initial temperature measurements. Growth was initiated by the deposition of a small amount of Ga on the substrate surface at 400 °C. We observed that this predeposition, although not absolutely necessary to activate NW growth, leads generally to an increase of NW density. The substrate temperature was then increased to 610 °C, and Ga and As4 were supplied simultaneously to form NWs. The synthesis was carried out at a Ga flux corresponding to a rate of 0.2 nm·s−1 for standard two-dimensional layer growth. Figure 1 shows side-view scanning electron microscopy (SEM) images of a typical sample grown for 34 min. Most of the NWs are vertically aligned. A population of long NWs (about 5 μm high) with a Ga droplet at their top is observed. This population shows a low height dispersion even though Received: August 13, 2012 Revised: October 19, 2012 Published: December 4, 2012 91

dx.doi.org/10.1021/cg301167g | Cryst. Growth Des. 2013, 13, 91−96

Crystal Growth & Design

Article

Figure 1. Scanning electron micrographs of GaAs NWs grown on a (111)Si substrate by self-catalyzed VLS-MBE at 610 °C. The growth duration was 34 min. the NW diameters vary by a factor of 2. Our study will focus mainly on this type of NWs. One can also see much shorter NWs, with no Ga droplets at the top and presenting various lengths. We will see that these NWs stopped growing before the end of the MBE experiment.

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Figure 2. HAADF STEM images of a GaAs NW with periodic insertions of (Al,Ga)As markers. The growth sequence was 9 s GaAs/1 s (Al,Ga)As. (a) Observation along the ⟨211⟩ zone axis showing that the markers (dark layers) are flat over the whole NW diameter. (b) One (Al,Ga)As marker observed with atomic resolution. (c) The HAADF signal is integrated over the whole diameter, and the intensity profile is plotted along the growth axis.

MARKER TECHNIQUE In most studies, the influence of the growth parameters on NW formation is investigated by performing a series of growth experiments. A new sample is fabricated to test each value of a given parameter. This method becomes rapidly tedious and does not give access to the detailed chronology of NW growth. In particular, transient growth regimes and nonlinear NW elongation phenomena cannot be quantified. To remedy these shortcomings, we have shown that compositional modulations can be used to record an absolute time scale in the NWs. This method gives access to valuable information on the chronology of the growth.19,20 In the present work, we investigate the kinetics of self-catalyzed GaAs NW formation by inserting thin AlxGa1−xAs markers periodically during growth. The elementary sequence consists of growing GaAs for 9 s (or 14 s in some experiments) and AlxGa1−xAs for 1 s, this period being repeated as many times as necessary. We used an Al/Ga flux ratio corresponding to the formation of Al0.27Ga0.73As in standard two-dimensional layer growth. When comparing the final morphology of GaAs NWs grown with or without markers, we observed that inserting the markers from the beginning of growth can alter NW formation. For that reason, we started the growth with a stem of pure GaAs and we introduced the markers several minutes later. Using this procedure, we did not observe any strong modification of the NW morphology related to the markers (see the Supporting Information). The NWs were then removed from their substrate, transferred to a thin membrane, and observed by high-angle annular dark-field (HAADF) imaging using a Cs probe aberration-corrected JEOL 2200 FS scanning transmission electron microscope (STEM). This analysis is very sensitive to composition variations21 provided that the atomic numbers of the elemental constituents to be distinguished are sufficiently different. Al and Ga satisfy this condition, and the AlxGa1−xAs markers appear as dark layers. These AlxGa1−xAs layers are atomically flat and extend over the whole diameter of the NWs (Figure 2a). This indicates that the interface between the NW and the catalyst drop must be perfectly flat when new atomic layers condensate. In particular, there is no sign that the top facet of the NW is truncated at its periphery, beneath the liquid catalyst. This contrasts with recent reports that such truncation facets are present during the VLS growth of various materials.22−24 If this phenomenon existed in our self-catalyzed growth conditions, one would expect the marker morphology to be affected; this

does not seem to be the case. The marker extension, right to the NW sidewall, also reveals that subsequent radial growth was negligibly small in our conditions. The thickness of the markers is 3 nm (Figure 2b), and their Al concentration, determined by energy-dispersive X-ray spectroscopy, is 8%. From these values, we can estimate the total number of Al atoms incorporated in one marker and compare it to the quantity provided to the catalyst drop by direct impingement of the Al vapor flux. The direct flux represents 1/3 of the Al atoms contained in the marker. Thus, additional Al must reach the droplet via other pathways, presumably by surface diffusion on the NW sidewalls. This is qualitatively consistent with the fact that we do not observe significant radial growth of the NWs. The precise position of the marker layers along the NW axis is obtained by plotting the profile of HAADF intensity integrated over the whole NW diameter (Figure 2c). Because the distance between two successive markers is the increase of NW length during one time period, it is straightforward to deduce the elongation rate for each period. The quantity of AlAs incorporated in the NW represents about 0.24 nm per period. This length can be subtracted from the distance between two successive markers, a slight correction that gives the NW elongation rate due to GaAs growth only. This can be performed over the whole length of an NW to determine the chronology of its growth.

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TIME DEPENDENCE We first use our marker method to investigate GaAs NWs elaborated under constant growth conditions (except for the periodic supply of short Al flux pulses for the markers). In the sample of Figure 3, a pure GaAs stem was formed for 30 min. AlxGa1−xAs markers were then periodically introduced for the 20 min corresponding to the rest of the growth. The length of the GaAs stem was 2 μm, as deduced from the position of the first marker and from the thickness of the GaAs deposit on the substrate at the bottom of the NWs. Beyond this NW height, the elongation rate is nearly independent of time, as seen from 92



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Figure 4. (a) Fast Fourier transform filtered HAADF image of a single GaAs NW with thin (Al,Ga)As markers (dark contrast) introduced at constant intervals of time. During growth, the incident As4 flux is changed as indicated. This change produces an immediate increase in the NW elongation rate. (b) Correlation between As4 flux and NW elongation rate. The As4 beam equivalent pressure (red line, left-hand scale) is measured during a test experiment reproducing the As4 flux variations used for the real NW growth. The NW elongation rate variations (blue triangles, right-hand scale) are deduced from the positions of (Al,Ga)As markers introduced periodically during growth.

Figure 3. NW elongation rate versus growth time, measured with periodic markers in two different NWs of the same sample growth under constant conditions.

Figure 3 (a slight drift is observed, however) with a mean value of 2.06 nm/s. This behavior was observed consistently on the four NWs belonging to the category of “long” NWs (with a catalyst drop at their top end) that we analyzed. Previous reports based on a sample series grown for various times, have also concluded that the self-catalyzed MBE-grown GaAs NWs elongate at a constant rate.11 Nevertheless, the length of the GaAs stem indicates that this rate cannot be constant from the early stage of growth. Indeed, 30 min of growth at 2.06 nm·s−1 would result in a 3.7 μm length. This is nearly double the actual length of the GaAs stem. Hence, the stationary growth rate observed in Figure 3 must be preceded by a transient regime at a lower elongation rate. Such a behavior has previously been observed when using foreign catalysts.25−27 The transient regime was related to that part of the group III flux intercepted by the NW sidewalls that contributes to NW growth, after diffusion from the sidewalls to the catalyst drop. Indeed, this part increases with the NW length, until the latter becomes comparable to the adatom diffusion length.19 Whether a similar effect also affects the As species is an open question that is addressed in the following.

Figure 5. NW elongation rate as a function of the arsenic beam equivalent pressure. The data are obtained with periodic markers in two distinct NWs (blue up and red down triangles) from the same sample grown under various As4 fluxes. The solid line is a linear interpolation of the experimental results.

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ARSENIC-LIMITED REGIME We used the same marker method to investigate the effect of As4 pressure. This parameter was systematically varied during one growth experiment. To produce rapid flux changes, arsenic was evaporated in an effusion cell equipped with a valve. The exact sequence of As4 flux changes was measured during a test experiment by an ionization gauge replacing the sample. The record of the As4 beam equivalent pressure (BEP) is presented in Figure 4b. Flux transients of about 20 s appear after each actuation of the valve. In the actual NW growth experiment, a long GaAs stem was first grown to reach the stationary regime of elongation. Five AlxGa1−xAs markers were then introduced at each As4 flux condition. The NW elongation rate, extracted from the positions of these markers, follows the As4 flux variation remarkably well. Quasi instantaneous changes of NW elongation rate respond to the sudden changes of As4 flux (Figure 4a), and the inevitable short flux transients have even produced similar transitory variations of the growth rate (Figure 4b). Figure 5 shows the mean growth rate at each stable As4 flux condition as a function of the As4 BEP, PAs4, measured during the test experiment. The data are given for two NWs. In agreement with the result of Colombo et al.,11 the dependence is linear. The linear fit intercepts the x axis at PAs4 = 3 × 10−7 Torr. These data indicate that the self-catalyzed NW growth is

limited by the amount of arsenic reaching the droplet. The BEP extrapolated at zero growth rate is likely to correspond to the compensation of arsenic desorption from the catalyst droplet. In VLS growth, it is well-established that the group III atoms reach the droplet by surface diffusion essentially. Because the GaAs NW is stoichiometric, its growth consumes equal quantities of Ga and As atoms. We do not observe any change of diameter along the NW axis, which suggests that the Ga droplet size did not vary significantly when the As flux was changed. To maintain a constant droplet volume independent of the As4 flux, the Ga diffusion flow might be adjusted by local chemical potential variations in order to balance the quantity of As incorporated in the solid NW.

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DIRECT IMPINGEMENT OF AS IS NOT SUFFICIENT To analyze these data further, PAs4 must be converted into an absolute As atomic flux, JAs (the beam flux of As atoms impinging on the sample). To this end, we implemented several techniques based on in situ reflection high-energy electron diffraction (RHEED) observations on planar GaAs (001) surfaces (see the Supporting Information). Taking into account the angle α = 35° between the axis of the As4 source and the substrate normal, all of these techniques led to the same conversion coefficient η = 2.3 × 106 atoms·nm−2·s−1 Torr−1, where η = JAs/PAs4. 93



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Knowing the absolute As incoming flux and considering the geometry of a given NW with its apical droplet, we can evaluate the number of As atoms that directly impinge on this catalyst droplet during growth.28 The data of Figure 6 were obtained

desorption of As from the drop. In fact, this desorption must increase with PAs4. Hence, the actual slope dIAsin/dPAs4 must be even larger than dVNW/dPAs4 and the factor 2.1 is a lower limit. Let us first assume that the missing quantity of As is collected by the NW sidewalls and then diffuses to the catalyst. If the NW length is longer than the diffusion length of Asx adspecies, λfAs, the diffusion flow per unit time can be approximated by 2RNWλfAsJAs sin α. To account for the missing amount of As, this expression leads to λf ≥ 240 nm using the data of Figure 6. In the framework of our simple approximation for the As diffusion flow, this contribution to NW growth should vary as 1/RNW. Consequently, the final NW length, LNW, should vary accordingly. The experimental LNW (RNW) dependence was obtained by measuring the geometrical characteristics of a large population of NWs from the sample of Figure 1. The data from the “short” NWs, which do not present a catalyst droplet at their top, were not included. Indeed, this type of NW is not representative because they stopped their elongation before the end of the growth process (see the Supporting Information). The experimental LNW (RNW) dependence for the other NWs is shown in Figure 7. The maximum length is reached by a

Figure 6. Direct comparison between the experimental NW elongation rate and the arsenic arrival rate by direct impingement on the catalyst droplet. The open blue triangles represent the experimental growth rates measured on two different NWs (up and down symbols), and the blue line is a least-squares fit. The full red triangles and the red line reproduce the data of Figure 5 after geometrical corrections, so that they represent the contribution of the direct As impingement on the catalyst droplet. The dashed red line represents the minimum arsenic arrival rate necessary to balance the NW elongation.

from two NWs of radius RNW = 32 nm with a catalyst droplet contact angle β = 135°. Because β > 90° + α, the number of As atoms intercepted directly by the drop per time unit can be written as JAsπRNW2/sin2 β.28 After dividing this quantity by the NW top facet area, πRNW2, and by the density of As atoms in GaAs, NAs = 22.1 at·nm−3, a direct comparison with the experimental NW growth rate is possible. This comparison is shown in Figure 6: the full straight line passing through the origin represents the contribution of direct impingement of As on the catalyst droplet, that is, ηPAs4/(NAs sin2 β). This quantity is obviously not sufficient to explain the NW elongation rate, VNW, which shows a steeper dependence upon PAs4. To account for the total As consumption, an additional amount of As must reach the catalyst by at least one other pathway.

Figure 7. Dependence of NW length on NW radius. The triangles are the experimental data measured from SEM pictures. The solid line represents the dependence expected under the hypothesis of surface diffusion.

majority of NWs, independent of their radius. Several other NWs have a slightly shorter length, which may indicate a growth instability for these particular NWs (such as the early stage of a decrease of the elongation rate, for instance). The striking feature is the absence of any systematic dependence of length on radius. In particular, we do not observe a decrease of LNW with increasing RNW, whereas this behavior is clearly observed when surface diffusion plays an important role.29 To be more quantitative, we plotted the dependence of length on radius, which would be expected if the direct impinging Asx flux was complemented by the diffusion of Asx adsorbed species. The second contribution, varying as 1/RNW, is scaled with the value of λf estimated above. This model obviously fails to describe the experimental results (see Figure 7). Next, we used the marker method to study the effect of growth temperature. An NW stem was first formed at 640 °C to reach steady-state growth before changing the substrate temperature. The latter was varied between 610 and 670 °C. For each growth temperature, a series of five AlxGa1−xAs markers were introduced to measure the NW elongation rate at the stabilized temperature. After each series, the temperature was changed without growth interruption to prevent any evolution of the Ga droplet. During the temperature stabilization stages, GaAs was grown without markers for 2 min. The elongation rate decreases with temperature above a

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SURFACE DIFFUSION OF AS IS INCONSISTENT WITH THE EXPERIMENT Two recent investigations of self-catalyzed GaAs NWs have also concluded that the direct impingement of As on the catalyst drop cannot balance the As consumption by NW growth.14,18 It these works, it was proposed that the required supplementary flow reaches the catalyst drop by surface diffusion of adsorbed Asx species. Bauer et al.14 used a substrate covered with a SiO2 mask patterned with small openings, and they invoked arsenic diffusion on the mask surface. An implicit assumption is that the adsorbed Asx species must also diffuse along the NW sidewalls to reach the liquid−solid growth front. Similarly, Rudolph et al.18 considered that arsenic diffusion along the NW sidewalls contributes to the NW elongation in the VLS regime. In the following, we reexamine this hypothesis and show that it cannot account for our results. In our experiments, we estimate that the total arrival rate of As on the catalyst drop, IAsin, must be at least 2.1 times the direct impingement, IAsdir. With this factor, the slope dIAsin/ dPAs4 is equal to dVNW/dPAs4 and the offset between the two corresponding straight lines (Figure 6) represents a constant 94



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this gives a first idea of how much of the reemission from the substrate can contribute to growth. Namely, the part of the reemitted flux intercepted by the drop could be of the same order of magnitude, but probably less than the direct impingement: the geometry of the specular reflection with the two experimental angles α = 35° and β = 135° leads to IAsin/IAsdir = 1.64. In the more realistic situation where the NW has many neighbors close by (Figure 9b), most of the direct flux is intercepted by the NW sidewalls that can reemit 100% of it, since lateral growth is not observed in our conditions. In a dense NW ensemble, multiple reemission events are likely to occur and the probability that the drop intercepts additional As vapor species increases. Accordingly, the experimental ratio IAsin/IAsdir ≥ 2.1 can be reasonably explained by As reemission from the close environment of a given NW, although this is obviously very difficult to quantify precisely.

threshold of about 650 °C, which probably corresponds to the onset of significant desorption from the sample (Figure 8).

Figure 8. Dependence of NW elongation rate on substrate temperature.

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CONCLUSION In summary, the introduction of AlGaAs markers during the self-catalyzed growth of GaAs NWs was proved to be a very efficient method to investigate the growth kinetics in depth. The stationary As-limited regime that follows the initial transitory regime is characterized by a linear dependence of the NW elongation rate upon the As4 flux. However, our precise measurements of the flux of As atoms directly impinging on the Ga catalyst drop reveal that it is not sufficient to account for the NW elongation rate. We exclude that sidewall diffusion of adsorbed Asx species over hundreds of nanometers could provide the supplementary quantity of arsenic. A more realistic source is the reemission of Asx vapor species from the substrate and sidewall surfaces. This secondary source of arsenic vapor can be intercepted by the Ga drop and adds up with direct As impingement. A precise estimation of this second contribution is complex, but the result would certainly depend on the NW density.

Below this threshold, the elongation rate is temperatureindependent. If Asx species diffusing at the sidewall surfaces contributed to growth, we would expect an increase of the growth rate with increasing substrate temperature before the desorption threshold. The data of Figure 8 do not show any evidence of such a thermally activated mechanism below 650 °C. This is another argument against the possible contribution of surface diffusion of Asx species.

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ANOTHER SOURCE OF AS: THE REEMISSION FROM THE SAMPLE SURFACE The conclusion that As species do not reach the catalyst by surface diffusion is not very surprising. In the temperature range that was investigated, As2 and As4 are very volatile and the Asx species adsorbed on the sample surface may desorb very easily, so that they cannot diffuse over hundreds of nanometers. On the other hand, the sample surface reemits a significant amount of arsenic that can be captured by the catalyst droplets. This mechanism of reemission, followed by secondary adsorption, was considered for Ga atoms in NW growth on masked substrates.17,30 Building on a cursory remark by Krogstrup et al. that As species might also be reemitted by the substrate,17 we propose that As reemission from substrate and sidewalls might constitute the main source (and probably the only significant one) of the missing As atoms. The reemission is likely to be proportional to the direct vapor flux. A schematic situation where an isolated nanowire on a horizontal substrate intercepts both the direct flux and its specular reflection at the substrate surface is illustrated in Figure 9a. Even if this is probably an oversimplification of the actual geometry of As reevaporation,

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ASSOCIATED CONTENT

S Supporting Information *

Description of three growth methods and figures showing the two-dimensional (Al,Ga)As growth rate measured from RHEED oscillations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Ng, H. T.; Han, J.; Yamada, T.; Nguyen, P.; Chen, Y. P.; Meyyappan, M. Nano Lett. 2004, 4, 1247. (2) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149. (3) Hahm, J. I.; Lieber, C. M. Nano Lett. 2004, 4, 51. (4) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (5) Park, H. G.; Barrelet, C. J.; Wu, Y.; Tian, B.; Qian, F.; Lieber, C. M. Nat. Photonics 2008, 2, 622. (6) Qian, F.; Li, Y.; Gradečak, S.; Barrelet, C. J.; Wang, D.; Lieber, C. M. Nano Lett. 2004, 4, 1975. (7) Maiolo, J. R.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, D. M.; Atwater, H. A.; Lewis, S. N. J. Am. Chem. Soc. 2007, 129, 12346.

Figure 9. Schematic illustration of the possible reemission of Asx vapor species from the different sample surfaces and their subsequent capture by the catalyst drops. (a) The case of a single isolated NW, assuming a specular reflection of the Asx beam at the substrate surface. (b) The case of a dense ensemble of NWs with multiple reemissions. 95



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(8) Bar-Sadan, M.; Barthel, J.; Shtrikman, H.; Houben, L. Nano Lett. 2012, 12, 2352. (9) Jabeen, F.; Grillo, V.; Rubini, S.; Martelli, F. Nanotechnology 2008, 19, 275711. (10) Fontcuberta i Morral, A.; Colombo, C.; Abstreiter, G.; Arbiol, J.; Morante, J. R. Appl. Phys. Lett. 2008, 92, 063112. (11) Colombo, C.; Spirkoska, D.; Frimmer, M.; Abstreiter, G.; Fontcuberta i Morral, A. Phys. Rev. B 2008, 77, 155326. (12) Paek, J. H.; Nishiwaki, T.; Yamaguchi, M.; Sawaki, N. Phys. Status Solidi C 2009, 6, 1436. (13) Plissard, S.; Dick, K. A.; Larrieu, G.; Godey, S.; Addad, A.; Wallart, X.; Caroff, P. Nanotechnology 2010, 21, 385602. (14) Bauer, B.; Rudolph, A.; Soda, M.; Fontcuberta i Morral, A.; Zweck, J.; Schuh, D.; Reiger, E. Nanotechnology 2010, 21, 435601. (15) Spirkoska, D.; Arbiol, J.; Gustafsson, A.; Conesa-Boj, S.; Glas, F.; Zardo, I.; Heigoldt, M.; Gass, M. H.; Bleloch, A. L.; Estrade, S.; Kaniber, M.; Rossler, J.; Peiro, F.; Morante, J. R.; Abstreiter, G.; Samuelson, L.; Fontcuberta i Morral, A. Phys. Rev. B 2009, 80, 245325. (16) Cirlin, G. E.; Dubrovskii, V. G.; Samsonenko, Yu. B.; Bouravleuv, A. D.; Durose, K.; Proskuryakov, Y. Y.; Mendes, M.; Bowen, L.; Kaliteevski, M. A.; Abram, R. A.; Zeze, D. Phys. Rev. B 2010, 82, 035302. (17) Krogstrup, P.; Popovitz-Biro, R.; Johnson, E.; Madsen, M. H.; Nygård, J.; Shtrikman, H. Nano Lett. 2010, 10, 4475. (18) Rudolph, D.; Hertenberger, S.; Bolte, S.; Paosangthong, W.; Spirkoska, D.; Döblinger, M.; Bichler, M.; Finley, J. J.; Abstreiter, G.; Koblmüller, G. Nano Lett. 2011, 11, 3848. (19) Harmand, J. C.; Glas, F.; Patriarche, G. Phys. Rev. B 2010, 81, 235436. (20) Galopin, E.; Largeau, L.; Patriarche, G.; Travers, L.; Glas, F.; Harmand, J. C. Nanotechnology 2011, 22, 245606. (21) Carlino., E.; Grillo, V. Phys. Rev. B 2005, 71, 235303. (22) Oh, S. H.; Chisholm, M. F.; Kauffmann, Y.; Kaplan, W. D.; Luo, W.; Rühle, M.; Scheu, C. Science 2010, 330, 489. (23) Gamalski, A. D.; Ducati, C.; Hofmann, S. J. Phys. Chem. C 2011, 115, 4413. (24) Wen, C.-Y.; Tersoff, J.; Hillerich, K.; Reuter, M. C.; Park, J. H.; Kodambaka, S.; Stach, E. A.; Ross, F. M. Phys. Rev. Lett. 2011, 107, 025503. (25) Dayeh, S. A.; Yu, E. T.; Wang, J. Nano Lett. 2009, 9, 1967. (26) Tchernycheva, M.; Travers, L.; Patriarche, G.; Glas, F.; Harmand, J. C.; Cirlin, G. E.; Dubrovskii, V. G. J. Appl. Phys. 2007, 127, 094313. (27) Plante, M. C.; LaPierre, R. R. J. Appl. Phys. 2009, 105, 114304. (28) Glas, F. Phys. Status Solidi B 2010, 247, 254. (29) Dubrovskii, V. G.; Sibirev, N. V.; Suris, R. A.; Cirlin, G. E.; Ustinov, V. M.; Tchernycheva, M.; Harmand, J. C. Semiconductors 2006, 40, 1075. (30) Rieger, T.; Heiderich, S.; Lenk, S.; Lepsa, M. I.; Grützmacher, D. J. Cryst. Growth 2012, 353, 39.

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Arsenic Pathways in Self-Catalyzed Growth of GaAs Nanowires

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