Wurtzite GaAs/AlGaAs core–shell nanowires grown by molecular beam epitaxy

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Wurtzite GaAs/AlGaAs core–shell nanowires grown by molecular beam epitaxy

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2009 Nanotechnology 20 415701 (http://iopscience.iop.org/0957-4484/20/41/415701) View the table of contents for this issue, or go to the journal homepage for more

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NANOTECHNOLOGY

Nanotechnology 20 (2009) 415701 (7pp)

doi:10.1088/0957-4484/20/41/415701

Wurtzite GaAs/AlGaAs core–shell nanowires grown by molecular beam epitaxy H L Zhou1,4 , T B Hoang1 , D L Dheeraj1, A T J van Helvoort2 , L Liu3 , J C Harmand3 , B O Fimland1 and H Weman1,4 1

Department of Electronics and Telecommunications, Norwegian University of Science and Technology, NO-7491, Trondheim, Norway 2 Department of Physics, Norwegian University of Science and Technology, NO-7491, Trondheim, Norway 3 CNRS, Laboratoire de Photonique et Nanostructures, Route de Nozay, F-91460, Marcoussis, France E-mail: [email protected] and [email protected]

Received 20 May 2009, in final form 5 June 2009 Published 16 September 2009 Online at stacks.iop.org/Nano/20/415701 Abstract We report the growth of GaAs/AlGaAs core–shell nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. Electron microscopy shows the formation of a wurtzite AlGaAs shell structure both in the radial and the axial directions outside a wurtzite GaAs core. With higher Al content, a lower axial and a higher radial growth rate of the AlGaAs shell were observed. Room temperature and low temperature (4.4 K) micro-photoluminescence measurements show a much higher radiative efficiency from the GaAs core after the NW is overgrown with a radial AlGaAs shell. (Some figures in this article are in colour only in the electronic version)

the III–V NWs, the GaAs/AlGaAs system with its small lattice mismatch (∼0.12% between GaAs and AlAs) is a very attractive candidate to form almost strain-free axial and radial heterostructured NWs. In particular, an AlGaAs shell can significantly improve the efficiency of radiative recombination in a GaAs core since it prevents carriers from reaching nonradiative states on the NW surface. The successful growth of GaAs/AlGaAs core–shell NWs has been demonstrated [8, 9], but no systematic study on the GaAs/AlGaAs core–shell NW growth has been reported so far. In this work, a detailed structural characterization of GaAs/AlGaAs core–shell NWs, grown by molecular beam epitaxy (MBE), is made by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A different growth mode was observed for the AlGaAs radial shell than for the GaAs core, induced by the shorter diffusion length of Al adatoms compared to Ga adatoms. Based on the observation of a lower density of NWs after the radial shell was grown, a possible mechanism related to the burying of NWs will be discussed. Micro-photoluminescence (µ-PL)

1. Introduction Recently there has been significant progress in developing semiconductor nanowire (NW) heterostructures for future nano-photonic and nano-electronic devices [1–3]. Such development requires a detailed understanding of the structural and opto-electronic properties of these one-dimensional (1D) heterostructures. Among these NW heterostructures, the radial core–shell and core–multishell structures offer the opportunities of controlling both the composition and the size of the core and the radial shell(s) separately. The band offset between the core and the shell(s) provides an effective way of radially confining carriers along the NW axis. Due to the specific structural design of the core–shell NWs, various device applications like light emitting diodes, laser diodes, field-effect transistors, solar cells and sensors can be realized [2, 4–7]. Most reported III–V semiconductor NWs are grown by the catalyst-assisted vapor–liquid–solid (VLS) mechanism. Within 4 Authors to whom any correspondence should be addressed.

0957-4484/09/415701+07$30.00

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H L Zhou et al

Table 1. Sample descriptions and size measurements for NW samples a –g . The GaAs core growth time for all samples was 20 min. Sample No.

Nominal Al content in shell (%)

Shell growth time (min)

Mean NW diameter (nm)

Mean NW length (nm)

NW tapering ratio

2D layer growth rate (nm s−1 )

Total 2D layer thickness (nm)

a b c d e f g

0 30 30 30 60 60 60

— 1 5 15 0.5 2 7

13 18 33 60 20 40 75

480 600 810 1900 460 500 920

0 0.012 0.025 0.025 0.015 0.054 0.067

0.14 0.18 0.17 0.10 — 0.36 0.33

170 181 220 262 — 213 307

measurements at room temperature (RT) and low temperature (4.4 K) have been performed on the same NW and show a much higher radiative efficiency from the GaAs core in the GaAs/AlGaAs core–shell NWs than from the bare GaAs core NWs.

temperature. We did not perform any growth interruption at the core–shell heterointerfaces. Morphological characterization of the NWs was performed in a Zeiss Supra field emission SEM (FE-SEM) operating at 5 kV. For TEM characterization, the NWs were scraped off from the substrate and transferred to a Cu grid with a lacey carbon film. The NWs were analyzed in a JEOL 2010F TEM (200 kV) with a scanning TEM (STEM) unit (1 nm probe size, 1 mm spherical aberration coefficient) and a high angle annular dark field (HAADF) detector (52 mrad inner detector angle). µ-PL measurements were carried out using a confocal optical microscope (Attocube CFMI), with a 633 nm laser line as the excitation source. The laser was focused onto single NWs with a spot size of ∼2 µm using a 0.65 numerical aperture objective lens. The µ-PL was dispersed by a 0.55 m focal length Jobin– Yvon spectrograph and detected by a thermal electric cooled Andor Newton Si charge-coupled device camera. The spectral resolution of the system was ∼200 µeV.

2. Experimental details The GaAs/AlGaAs core–shell NWs were grown on GaAs (111)B substrates by a Riber 32 MBE system, equipped with solid sources supplying Ga, Al atoms and an As cracker source, allowing us to fix the proportion of tetramers (As4 ). The substrate surface was first deoxidized at 620 ◦ C, and then a GaAs/Al0.3 Ga0.7 As/GaAs (60 nm/72 nm/36 nm) buffer layer was grown under growth conditions producing an atomically flat surface. The AlGaAs layer embedded in the GaAs buffer was used as a marker layer to distinguish between the GaAs buffer and the two-dimensional (2D) GaAs layer grown on the planar surface between the NWs, as shown in figure 1(h). A total amount of Au equivalent to ∼1 nm was deposited under As4 flux at 550 ◦ C using a Au effusion cell installed in the MBE growth chamber. The substrate temperature was then set to the desired value for the NW growth. This procedure results in the formation of nanoparticles containing Au alloyed with the substrate constituents. The GaAs NW growth was initiated by opening the shutter of the Ga effusion cell. The temperature of the Ga effusion cell was preset to yield a nominal planar growth rate of 0.2 nm s−1 (i.e., the growth rate on a clean and Au-free GaAs(100) substrate). For the AlGaAs shell growth, the Al shutter was also opened to supply an additional Al flux. The growth started with the GaAs core (sample a , growth time 20 min) for all core–shell NWs, after which the AlGaAs shell was grown. The temperature of the Al effusion cell was preset to what would yield an Alx Ga1−x As layer with x equal to 0.3 or 0.6 on a GaAs(100) substrate. The V/III flux ratio was kept constant for both Al fluxes used by adjusting the As4 flux. All grown samples (sample a –g ) are summarized in table 1. For the (nominal) Al0.3 Ga0.7 As shell samples, the shell growth times were 1 min, 5 min and 15 min (samples b , c, and d , respectively), whereas for the (nominal) Al0.6 Ga0.4 As shell samples, the shell growth times were 0.5 min, 2 min and 7 min (for samples e, f , and g , respectively). The Al, Ga and As4 fluxes were shut down simultaneously at the end of the AlGaAs shell growth and the substrate temperature immediately ramped down to room

3. Results The as-grown NWs were characterized by SEM to systematically investigate the morphology of the GaAs core NWs and GaAs/AlGaAs core–shell NWs with different growth times and Al contents. 45◦ -tilted view SEM images of the GaAs core NWs and the GaAs/AlGaAs core–shell NWs are shown in figures 1(a)–(g). For determination of the geometric sizes of the as-grown NWs, 40 NWs for each growth condition were randomly chosen from the SEM images. The diameter and length of the NWs were measured and the mean values are presented in table 1. The GaAs core NWs grew predominantly perpendicular to the substrate surface ([111]B direction) with a constant diameter over its length. The length of the NW is defined from the GaAs surface of the buffer to the tip of the NW, as shown in figure 1(h). The diameters of the GaAs core NWs are varying from 10 to 25 nm and the lengths of the NWs are ranging from 240 nm to about 1.2 µm (figures 2(a) and (b)). The lengths of the GaAs core NWs were observed to decrease with increasing diameter. No tapering was observed in any of the investigated GaAs NWs. However, tapering was observed in the GaAs/AlGaAs core–shell NWs. We define the nanowire tapering ratio as (DNW − Dtip )/L NW , where DNW is the mean diameter of the NWs before it tapers, Dtip the mean diameter of NWs at the tip, and L NW the mean length of the NWs. The tapering ratios of the NWs are listed in table 1; the tapering 2

Nanotechnology 20 (2009) 415701

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(a)

(b)

200 nm

200 nm (c)

(d)

200 nm (e)

200 nm (f)

200 nm

200 nm (g)

(h)

NW length

200 nm

Buffer layer

200 nm

2D layer thickness AIGaAs marker

Figure 1. 45◦ -tilted SEM images of (a) the GaAs core NWs grown in 20 min, and of the Al0.3 Ga0.7 As shells grown in (b) 1 min, (c) 5 min and (d) 15 min; as well as of the Al0.6 Ga0.4 As shells grown in (e) 0.5 min, (f) 2 min and (g) 7 min. (h) Schematic diagram of GaAs/AlGaAs core–shell NW defining ‘NW length’ and ‘total 2D layer thickness’.

and Al0.6 Ga0.4 As shell growth, we find an increased radial and a decreased axial growth rate of the AlGaAs shell with an increased Al content. At the same time, the mean volume of the GaAs/AlGaAs core–shell NWs quickly increases when the AlGaAs shell is formed, e.g. about 90 and 60 times for samples d and g , respectively, compared to the mean volume of NWs from sample a (GaAs core), as shown in figure 4(a). Using the different growth times involved, the mean growth rate for each 2D GaAs or AlGaAs layer formed during NW growth was

of the core–shell NWs becomes much more pronounced with increasing shell growth time and higher Al content. As shown in table 1 and figure 3, for Al0.3 Ga0.7 As shell growth, both the mean diameter and the mean length of the NWs are increasing with increasing shell growth time, which show that the AlGaAs shell grows in both axial and radial directions. Similarly for the Al0.6 Ga0.4 As shell growth, the increasing mean diameter and mean length of the NWs depict the radial and axial shell growth, as shown in figure 3. Comparing the Al0.3 Ga0.7 As 3

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Figure 3. Measured (a) NW diameters and (b) NW lengths versus the AlGaAs shell growth time. The mean diameter/length is indicated by an unfilled square.

Figure 2. Statistical results of 40 randomly chosen as-grown NW samples. (a) The length versus diameter for the NWs in samples a –d . The inserted black line shows a fit by the diffusion-induced growth model of Dubrovskii and Sibirev [16]. (b) The NW length versus diameter for samples a and e–g .

GaAs/AlGaAs core–shell NW (sample d ) is shown in figure 5(b). Selected area electron diffraction (SAED) patterns in figures 5(a) and (b) reveal a wurtzite (WZ) crystal structure both in the GaAs core and in the AlGaAs shell. Comparison of the HRTEM and HAADF STEM images of the same NW shown in figures 5(c) and (d), respectively, indicates that apart from radial AlGaAs growth, an axial AlGaAs is also grown on top of the GaAs core. It can also be seen that stacking faults are observed in the region where the axial AlGaAs starts to grow (by VLS) on top of the GaAs core. At the same time, it also shows that the radial AlGaAs shell exactly follows the crystal structure (including stacking faults) of the GaAs core. The interface between the GaAs core and the radial AlGaAs shell is relatively sharp and the thickness of the radial shell is highly uniform along the NW axis until the tapered AlGaAs NW region at the top. We have no indication that Al is diffusing into the GaAs NW core for either of the two studied shell compositions. For the core–shell NW shown in figure 5(b) (sample d ), the diameter of the GaAs NW core is ∼15 nm and for the whole core–shell NW ∼60 nm; which is consistent with the SEM observations. The NWs were studied by HAADF STEM to further analyze the core–shell structures. The HAADF contrast between the GaAs core and the radial AlGaAs shell is very

obtained by measuring the distance between the Al0.3 Ga0.7 As marker layer in the buffer and the top surface of the sample in the cross-sectional SEM image. A mean value for the distance was estimated from 20 different locations separated by 100 nm. The mean (and time-averaged) 2D layer growth rate versus the growth time of the AlGaAs shell is shown in figure 4(b). For samples b and f , the growth rate of the 2D layer was observed to be about 1.3 times and 2.5 times faster than the growth rate of the 2D layer for the GaAs core NW sample, respectively. The 2D layer growth rate of sample e (0.5 min shell growth time) is missing because the change in epilayer thickness is smaller than the resolution of the SEM images. The growth rate of the 2D layer is found to decrease with increasing growth time for the AlGaAs shell, which is consistent with the rapid increase of the mean NW volume with the AlGaAs growth time. A lower density of NWs was also found with increasing AlGaAs shell growth time, e.g. it is about 90 NWs µm−2 for the GaAs core NW sample and roughly 50 NWs µm−2 for sample d , as shown in figure 4(c). Figure 5(a) shows the [110] zone axis high resolution TEM (HRTEM) image of a typical GaAs core NW (14 nm diameter) from sample a . A HRTEM image of a typical 4

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Figure 4. (a) The total volume of the NWs versus AlGaAs shell growth time. The mean volume is indicated by an unfilled square. (b) Average growth rate of the 2D layer for different AlGaAs shell growth times. (c) NW density versus the AlGaAs shell growth time.

sensitive to thickness and atomic number as compared to diffraction or phase contrast which is dominating in TEM. In addition, the HAADF contrast between the core and the radial shell can be observed in random orientations of the NWs, in contrast to TEM. Since the diameter of the NW is uniform along the NWs until the tapering starts (as shown in figure 5(d)), the abrupt modulation in the HAADF image is interpreted as a compositional change between the GaAs core and the radial AlGaAs shell. Figure 5(d) reveals that there is a long (∼1 µm) AlGaAs grown above the GaAs core.

Figure 5. HRTEM images of (a) a typical NW from sample a (GaAs core) and (b) a typical NW from sample d (GaAs/AlGaAs core–shell), together with the selected area electron diffraction patterns (inset of image a and b). (c) HRTEM and (d) HAADF STEM image of a NW from sample d (same NW). A higher resolution HAADF image of a selected area indicated by a dashed rectangle in (d) is also shown. The diagonal band with lighter contrast on the NW in the middle of the image is due to the carbon support on the TEM grid. All of the NWs are oriented along the [111]B direction as indicated by the black arrows in (a)–(c).

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H L Zhou et al

substrate and/or from the sidewalls to the Au droplet at the top of the NWs, as described by Dubrovskii and Sibirev [16]. Our data are well described by this model as shown by the solid line in figure 2(a) for the growth of the GaAs core NWs. However, this relation between the length and the diameter of the NWs is changed after the AlGaAs shell starts to grow by direct nucleation on the NW sidewalls by a vapor–solid (VS) growth mechanism. The Ga and Al adatoms will be epitaxially deposited on the sidewalls of the NWs if the length of the NWs is longer than the diffusion length of Ga or Al, respectively. It is well known that Al is very reactive. Consequently, the effective surface diffusion barrier is higher for Al adatoms (∼1.74 eV for AlAs grown on AlAs(001)) than for Ga adatoms (∼1.58 eV on GaAs(001)) [17, 18]. This results in a shorter adatom diffusion length for Al compared to Ga, whereby radial sidewall deposition becomes more pronounced, in general leading to a tapered morphology for GaAs/AlGaAs core–shell NWs. The observation of the rapid increase of the NW volume with the AlGaAs growth time could be due to the shorter diffusion length of the Al adatoms as well as the increasing collection area of the sidewalls of NWs with the increasing shell growth time. NWs collect Ga and Al adatoms in three different ways; (1) diffusion from the substrate surface, (2) direct impingement on the NW sidewalls, and (3) direct impingement on the gold droplet. A larger area of the NW sidewalls (effective collection area = length × diameter) is obtained when NWs get longer, whereby the NWs can collect more Al and Ga adatoms. At the same time, the shorter diffusion length of the Al also contributes to trap more and more Al and Ga adatoms migrating on the sidewalls of the NWs, increasing the diameter. Both of these factors enhance each other, which results in a very fast increase of the NW volume. After the AlGaAs shells start to grow, the total flux impinging on the sample surface should remain constant. The AlGaAs either deposits on the NWs or on the substrate surface. Once the NW can capture more AlGaAs with a larger area of NW sidewalls, the slower growth rate of 2D layers will be expected. This can also be confirmed by the observed 2D layer growth rate, which is decreasing with the growth time of the AlGaAs shell. Next, we discuss the intriguing result of the decreasing density of NWs. When the AlGaAs growth takes place, some of the shorter NWs (e.g. the GaAs core NWs ∼250 nm) might become totally buried by the 2D layer at the initial AlGaAs shell growth stage. From figures 3 and 4(b), we see that both the radial growth rate of the AlGaAs NW shell and the growth rate of 2D AlGaAs increase with increased Al flux, whereas the NW axial AlGaAs growth rate is not significantly changed for average size NWs. The ratio between the NW axial AlGaAs growth rate and the 2D AlGaAs growth rate, of importance for NW burial, thus reduces with increase in Al flux. The ratio will be even further reduced for shorter NWs. The fact that shorter GaAs/AlGaAs core–shell NWs will get less Ga and/or Al adatoms compared to longer NWs due to both the smaller sidewall collection area and the shadowing effect. The shadowing effect is induced by the inclined molecular beam

Figure 6. Room temperature and low temperature (4.4 K) µ-PL emission spectra from a single GaAs/AlGaAs core–shell NW (sample d ).

Figure 6 shows RT and low temperature (4.4 K) µ-PL emission spectra from the same single GaAs/Al0.3 Ga0.7 As core–shell NWs (from sample d ). For the investigated GaAs core NWs from sample a , there is no detectable PL signal at RT. The RT spectrum of the NW from sample d shows only GaAs related emission at 1.431 eV (∼49 meV, full width at half maximum (FWHM)). In contrast, at 4.4 K, the spectrum of the NW from sample d shows both GaAs emission at 1.489 eV (∼12 meV, FWHM) and several PL peaks above 1.6 eV.

4. Discussion The dependence of the length versus the diameter of Si and III– V NWs grown by MBE has already been studied by several groups [10–12]. Considering the Gibbs–Thompson effect, Givargizov reported that the Si NW growth rate increases with its diameter [13] (VLS growth by chemical vapor deposition), which is not consistent with the observation of NWs VLS grown by MBE. Later, Dubrovskii et al introduced a diffusioninduced growth model for III–V NWs [14], in which the diffusion of group III adatoms from the substrate surface to the NW base and hence the diffusion of group III adatoms along the NW sidewalls to the metallic droplet was considered. This model shows that the length of the NW is a quadratic function of 1/d (where d is the NW diameter). However, this model does not consider the NW density and consequences of the inclined incident molecular beam with respect to the substrate, such as shadowing effects. Such shadowing effects will strongly influence the growth of NWs at high density. We have recently reported that due to the inclined molecular beam, the growth rate of NWs increases up to a certain length after which shadowing effects become predominant [15]. When shadowing effects take place, the growth rate can either increase, decrease or remain constant depending on the local neighboring environment [15]. In our case, the GaAs NW length is observed to decrease with an increase in diameter. It confirms that growth of the NWs is fed by diffusion of adatoms from the surface of the 6

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with respect to the substrate normal (here ∼37◦ ). Due to this effect, shorter NWs will be shadowed by longer neighboring NWs which will also lead to a reduced growth rate. A decrease in the axial growth rate of shorter NWs in combination with the increase in growth rate of the 2D layer caused by the increase in group III flux may therefore result in burying of NWs by the 2D layer, leading to a decreased NW density. Figure 6 depicts the PL spectra of the same single GaAs/AlGaAs core–shell NW at 4.4 K and at RT. We note here that the emission energy (1.489 eV) from the core GaAs is lower than the free exciton energy (1.515 eV) of ZB GaAs. This is probably due to the recombination from the type II band alignment that can occur due to stacking fault created GaAs ZB segments sandwiched in between a dominating GaAs WZ structure [19, 20]. It is important to note that no PL emission was observed from GaAs core NWs at RT. As has already been reported by several authors [9, 21], the shell in the GaAs/AlGaAs core–shell NW helps to increase the radiative recombination efficiency by approximately two orders of magnitude through the suppression of non-radiative surface recombination at the GaAs surface. As can be seen in figure 6, PL emissions from both GaAs and AlGaAs are observed at low temperature. The PL peaks above 1.6 eV are most likely related to recombination from carriers localized in the AlGaAs region (∼1 µm) at the top of the NW. The occurrence of several PL peaks is believed to be due to some local composition variations (10–15%) in this AlGaAs region. It has already been reported that for the axial VLS growth of AlGaAs on top of GaAs, a self-formed AlGaAs core–shell structure is formed with a radial variation in the Al composition [9]. The existence of AlGaAs related PL peaks from the GaAs/AlGaAs core– shell NWs suggests that there is room for improvement of the growth, especially by reducing the axial growth of AlGaAs.

that the NWs capture an increasing share of group III flux during the growth of the shell. Furthermore, a lower density of NWs is observed for long shell growth times, which we believe is due to the burying of short NWs. Micro-photoluminescence measurements at room and low temperature show a strongly enhanced radiative efficiency from the GaAs/AlGaAs core– shell NWs compared to bare GaAs core NWs.

Acknowledgments Part of this work was supported by the ‘NANOMAT’ program (Grant No. 182091) and the Norwegian-French ‘AURORA’ program (Grant No. 187692) of the Research Council of Norway.

References [1] Xiang J, Lu W, Hu Y, Wu Y, Yan H and Lieber C M 2006 Nature 441 489 [2] Duan X, Huang Y, Agarwal R and Lieber C M 2003 Nature 421 241 [3] Tchernycheva M, Cirlin G E, Patriarche G, Travers L, Zwiller V, Perinetti U and Harmand J C 2007 Nano Lett. 7 1500 [4] Lauhon L J, Gudiksen M S, Wang D and Lieber C M 2002 Nature 420 57 [5] Zhang L, Tu R and Dai H 2006 Nano Lett. 6 2785 [6] Hayden O, Greytak A B and Bell D C 2005 Adv. Mater. 17 701 [7] Tian B, Zheng X, Kempa T J, Fang Y, Yu N, Yu G, Huang J and Lieber C M 2007 Nature 449 885 [8] Noborisaka J, Motohisa J and Fukui T 2005 Appl. Phys. Lett. 86 213102 [9] Chen C, Shehata S, Fradin C, LaPierre R R, Couteau C and Weihs G 2007 Nano Lett. 7 2584 [10] Schmidt V, Senz S and G¨osele U 2007 Phys. Rev. B 75 045335 [11] Fr¨oberg L E, Seifert W and Johansson J 2007 Phys. Rev. B 76 153401 [12] Plante M C and LaPierre R R 2006 J. Cryst. Growth 286 394 [13] Givargizov E I 1975 J. Cryst. Growth 31 20 [14] Dubrovskii V G, Cirlin G E, Soshnikov I P, Tonkikh A A, Sibirev N V, Samsonenko Y B and Ustinov V M 2005 Phys. Rev. B 71 205325 [15] Dheeraj D L, Patriarche G, Zhou H, Harmand J C, Weman H and Fimland B O 2009 J. Cryst. Growth 311 1847 [16] Dubrovskii V G and Sibirev N V 2007 J. Cryst. Growth 304 504 [17] Shitara T, Neave J H and Joyce B A 1993 Appl. Phys. Lett. 62 1658 [18] Shitara T, Kondo E and Nishinaga T 1990 J. Cryst. Growth 99 530 [19] Pemasiri K et al 2009 Nano Lett. 9 648 [20] Hoang T B, Moses A F, Zhou H L, Dheeraj D L, Fimland B O and Weman H 2009 Appl. Phys. Lett. 94 133105 [21] Titova L V, Hoang T B, Jackson H E, Smith L M, Yarrison-Rice J M, Kim Y, Joyce H J, Tan H H and Jagadish C 2006 Appl. Phys. Lett. 89 173126

5. Conclusions GaAs/AlGaAs core–shell NWs with two different Al shell compositions were grown by Au-assisted molecular beam epitaxy. The core–shell NWs were found to be tapered near the end of the NW with a stronger tapering for longer AlGaAs shell growth times and higher Al content. An epitaxial radial AlGaAs shell is formed on the GaAs core by the vapor–solid growth mechanism where the shell growth rate increases with higher Al content, which is attributed to the shorter diffusion length of Al adatoms compared to Ga adatoms. In parallel with the radial AlGaAs shell growth, there is also some axial AlGaAs growth taking place on top of the GaAs NW core. The growth rate of a two-dimensional AlGaAs layer was found to decrease with the AlGaAs shell growth time, which indicates

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