Stopping and Resuming at Will the Growth of GaAs Nanowires

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STOPPING AND RESUMING AT WILL THE GROWTH OF GaAs NANOWIRES Giacomo Priante, Stefano Ambrosini, Vladimir G. Dubrovskii, Alfonso Franciosi, and Silvia Rubini Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400701w • Publication Date (Web): 31 Jul 2013 Downloaded from http://pubs.acs.org on August 4, 2013

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Crystal Growth & Design

STOPPING AND RESUMING AT WILL THE GROWTH OF GaAs NANOWIRES G. Priante,†, % S. Ambrosini,†, ø

,‡ ,# ,‰

V. G. Dubrovskii,‡,§ A. Franciosi, † ,ø, þ and S. Rubini†, *

† Istituto Officina dei Materiali CNR, Laboratorio TASC, S.S. 14 km 163.5, I-34149, Trieste, Italy ø Università di Trieste, Dipartimento di Fisica, Via Valerio 2, I-34127, Trieste, Italy ‡ St. Petersburg Academic University, Khlopina 8/3, 194021 St. Petersburg, Russia # Durham University, Stockton Rd, DH1 3L Durham, UK § Ioffe Physical Technical Institute of the Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia þ Sincrotrone Trieste S.C.p.A., Elettra Laboratory, S.S. 14 km 163.5, 34149, Trieste, Italy

ABSTRACT. We report on the possibility of interrupting and resuming at will the self-assisted growth of GaAs nanowires by molecular beam epitaxy. The Ga nanoparticles assisting nanowire growth on Sitreated GaAs(111)B wafers were consumed by exposure to an As flux. Condensation of new Ga nanoparticle on the top (111)B facets of the existing GaAs nanowires was achieved by either resuming GaAs growth in Ga-rich conditions or by exposing the nanowires to a Ga flux. The new Ga nanoparticles were found to assist the growth of new GaAs nanowires in epitaxial relation with the previous nanowires. The growth and the regrowth processes of the nanowires are jointly described by an analytical model that is able to reproduce the observed experimental time-dependence of nanowire length and diameter.

*Corresponding author: Dr. Silvia Rubini Istituto Officina dei Materiali CNR, Laboratorio TASC S.S. 14 Km. 163,5 I-34149 TRIESTE ITALY phone +39 040 3756 437 / 433 fax +39 040 226767 e-mail: [email protected]

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STOPPING AND RESUMING AT WILL THE GROWTH OF GaAs NANOWIRES G. Priante,†, % S. Ambrosini,†, ø

,‡ ,# ,‰

V. G. Dubrovskii,‡,§ A. Franciosi, † ,ø, þ and S. Rubini†, *

† Istituto Officina dei Materiali CNR, Laboratorio TASC, S.S. 14 km 163.5, I-34149, Trieste, Italy ø Università di Trieste, Dipartimento di Fisica, Via Valerio 2, I-34127, Trieste, Italy ‡ St. Petersburg Academic University, Khlopina 8/3, 194021 St. Petersburg, Russia # Durham University, Stockton Rd, DH1 3L Durham, UK § Ioffe Physical Technical Institute of the Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia þ Sincrotrone Trieste S.C.p.A., Elettra Laboratory, S.S. 14 km 163.5, 34149, Trieste, Italy

ABSTRACT. We report on the possibility of interrupting and resuming at will the self-assisted growth of GaAs nanowires by molecular beam epitaxy. The Ga nanoparticles assisting nanowire growth on Sitreated GaAs(111)B wafers were consumed by esposure to an As flux. Condensation of new Ga nanoparticle on the top (111)B facets of the existing GaAs nanowires was achieved by either resuming GaAs growth in Ga-rich conditions or by exposing the nanowires to a Ga flux. The new Ga nanoparticles were found to assist the growth of new GaAs nanowires in epitaxial relation with the


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previous nanowires. The growth and the regrowth processes of the nanowires are jointly described by an analytical model that is able to reproduce the observed experimental time-dependence of nanowire length and diameter.



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INTRODUCTION. Semiconductor nanowires (NWs) are attracting great interest for their potential use as building blocks in novel electronic and optoelectronic nanodevices. They are commonly grown in molecular beam epitaxy (MBE), chemical beam epitaxy (CBE) or metal organic vapour phase epitaxy (MOVPE) by using metal nanoparticles (NPs) preformed on a suitable substrate to act as collectors for the incoming material and promote the highly anisotropic NW growth. Gold nanoparticles have been used in the vast majority of cases, and the growth mechanism is described according to the vapor-liquid-solid (VLS) model.1 The well-known incompatibility of gold with Si technology, as well as the aim of avoiding possible incorporation of Au into the NW body during the growth,2 stimulated great effort to the development of Au-free NW growth protocols. Effective results have been obtained, by metal-free selective area (SA) methods, exploiting the selectivity of MOVPE in the growth on patterned GaAs substrates.3,4 Regular arrays of InAs5 and GaAs6 nanowires were obtained also on patterned Si(111) and eventually the possibility of integrating III-V vertical NW-based transistors on Si was demonstrated.7 Selective area growth on Si was also obtained by MBE for InAs and In-rich InGaAs NWs. 8 A different approach for the growth of GaAs NWs is given by the so-called self-assisted methods,9,10 where Ga NPs, preformed by a suitable choice of growth conditions and/or substrates, are used to induce the anisotropic growth. Au-free self-assisted GaAs NWs have thus been obtained by MBE on treated GaAs9,11, on Si10,12-17 wafers, and on patterned Si substrates.18 In all cases the substrate surface was covered initially by a thin layer of Si oxide. Superior optical properties of self-assisted, compared to Au-assisted, GaAs NWs were demonstrated by photoluminescence spectroscopy.19 Even though the Au- and Ga-assisted MBE growth of GaAs NWs was described in both cases within the VLS framework, an important difference between the two is that while the size of the metal NP 4 Environment ACS Paragon Plus

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remains largely constant during Au-assisted growth, during self-assisted NW growth the size of Ga NP varies substantially with the As/Ga flux ratio employed. When a balanced As/Ga ratio is used, the diameter of the NPs is found to increase linearly with time, together with the NW diameter, that is uniform along the NW axis,20,21 and the contact angle β between the NP and the NW top remains constant and larger than 110°.21 In Ga-rich conditions, the volume of the NP increases more quickly than the NW diameter, and NWs with a diameter larger at the tip than at the bottom are obtained.20 In As-rich growth conditions, the NP volume shrinks with time, and eventually the NPs is completely consumed.22 The consumption of the NP was found to be correlated to changes in the crystal structure of the NW. NWs obtained from MBE self-assisted growth typically display pure zincblende (ZB) structure, affected only by rotational twins.11,15,16 When the growth conditions led to a size reduction of the NP, with its diameter becoming comparable to that of the NW (contact angle β ≅ 90°), a prevalent WZ structure was observed.22 This change in crystal structure was related to the migration of the nucleation site from the center of the NW body, favoring the ZB phase, to the triple phase line, favoring WZ structure.14,22-24 A direct indication about the relation between As/Ga flux ratio and crystal structure was recently obtained by in-situ x-ray diffraction studies during the growth of self-assisted GaAs nanowires.25 During growth in Ga-rich conditions, an increase in the As/Ga ratio was found to correspond to an increase in the WZ rate of formation, although with long response time. The results were interpreted in terms of an important role of the morphology of the growth front, and specifically of the relative size of nanowire and Ga nanoparticle, in favoring one or the other crystal structure. WZ segments in self assisted ZB wires were also obtained by Ketterer et al.26 by varying the As/Ga flux ratio. Recently, a growth model was developed, relating the structure of the NW to the shape, relative volume and contact angle of the metal nanoparticle.27 This dependence of the prevalent NW crystal



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structure on the NP-NW contact angle was exploited to synthesize GaAs with engineered crystal phase. Changes in the V/III flux ratio28 and flux interruption followed by exposure to Ga or to As29 were used to obtain ZB-defected and WZ-defected superlattices28 as well as alternate ZB and WZ segments.29 Extinction of the Ga NP during self-assisted NW growth can be obtained by exposing the NW to an As flux. The final morphology of the wire tip depends on the details of the procedure,30 but a flat (111) top facet is commonly observed,30,31 analogous to that of metal-free GaAs NWs obtained by SA MOVPE.32 This process has been used to terminate the axial VLS growth and promote the formation of a shell around the nanowires.12,31,33-35 However, overgrowth after the extinction of the Ga NP was reported to lead to the transformation of the NW tip, that finally displays three {110} facets.31,35 The same morphology was reported by different groups10,22,36 and ascribed to a growth process not mediated by the presence of a NP on the NW tip. In this Article we report the successful regrowth of GaAs NWs on the tip of previous NWs obtained by self-assisted MBE growth followed by complete consumption of the Ga NP. We achieved condensation of a new Ga nanoparticle on the top (111)B facet of the existing GaAs NW either by resuming GaAs growth in Ga-rich conditions or by exposing the NWs to a Ga flux. The new Ga-NP assists the growth of a new GaAs NW in epitaxial relation with the previous NW. The signature of the combined growth process is found in the crystal structure of the resulting NW, where a WZ segment, produced during the NP consumption phase, is found to be sandwiched between two ZB sections developed during the Ga-assisted growth. The growth and the regrowth processes are both described by a unified analytical model that is able to reproduce the observed peculiar timedependence of NW length and diameter, providing insights on the growth mechanism. The demonstrated possibility to stop and resume the axial growth has great potential for the realization of tailored axial and radial NW heterostructures, where the nanowire composition and its electrical properties are varied axially and radially, and of nanowires with designed radial modulations.37 6 Environment ACS Paragon Plus

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EXPERIMENTAL SECTION. GaAs NWs were grown by solid-source MBE on Si-treated11 GaAs(111)B wafers. Self-catalyzed NW growth was carried out at 640 °C with a V/III ratio of 5. The Ga flux was set to a beam equivalent pressure (BEP) of 3.6×10-7 Torr, as measured by an ion gauge placed next to the substrate holder. The As4 flux was set to a BEP value of 1.8×10-6 Torr. The Ga flux was chosen to give a GaAs layer-bylayer growth rate of 1 µm per hour in standard conditions on a GaAs(001) surface, as determined by RHEED intensity oscillations. During the growth the substrate was rotated at 3 rpm. Growth was initiated by simultaneous exposure of the substrate to Ga and As4 fluxes. After 60 minutes, the Ga shutter was closed and the sample kept for 10 minutes at the growth temperature under As flux to obtain the complete consumption of the nanoparticle, as confirmed by post-growth morphological analysis. After this step, the Ga shutter was opened again for different times. The regrowth was in all cases stopped by shutting off both beams and the sample was then cooled down to room temperature in ultra-high-vacuum outside the growth chamber. The NW morphology was studied by scanning electron microscopy (SEM), using a Zeiss SUPRA 40, field-emission instrument operated at 10 keV, with a nominal point resolution of 1.5 nm. Nanowires were investigated both by direct imaging of the as-grown samples and after being mechanically transferred to an n-doped Si(111) wafer. To obtain structural information, NW were mechanically transferred onto lacey carbon-coated copper grids and examined by transmission electron microscopy (TEM), using a Jeol 2100F microscope (Cs=1 mm) operated at 200 keV in bright-field mode at the Durham University Science Site in Durham, UK.

RESULTS AND DISCUSSION. Figure 1 shows the evolution of the NW tip after different exposure times to Ga and As beams. 7 Environment ACS Paragon Plus


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Figure 1. Collage of SEM images picturing the topmost region of GaAs nanowires observed just after the consumption of the Ga nanoparticle (A) and following subsequent exposure to Ga and As beams for 30 s (B-D), 60 s (E), 150 s (F), 300 s (G) and 420 s (H), respectively. The three different NPs observed in NWs coming from the same sample (B-D) give an indication of the local fluctuations in the process. A marker at the left of each wire indicates the position of the diameter discontinuity as described in the text. Inset: tilted view (45°) of a sample after 150 s of re-growth. The marker in the inset corresponds to 1 µm.

Wire A represents the starting point: after the complete consumption of the Ga nanoparticle the tip appears flat and a slight reduction in the NW diameter is observed some 200 nm below the tip, as indicated by the marker. This morphology is characteristic of the NP consumption by exposure to 8 Environment ACS Paragon Plus

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As.28,30,31 The distance between the diameter reduction and the NW top was found to be quite reproducible within a given sample, with fluctuation less than 10%, and in the 150-200 nm range in all samples. We speculate that the segment above the diameter reduction roughly corresponds to the portion of the wire grown without Ga flux supply. A minimum As exposure time of 150 s was found to be necessary to obtain the consumption of the particles. We can estimate the axial growth rate, in this peculiar growth mode, to be 60-80 nm/min, substantially lower than the stationary value of 130 nm/min. This effect can be qualitatively explained by remembering that self-assisted growth is Aslimited 20 and that all the incoming As is collected by the NP surface, because the sidewall diffusion of As is negligible.38 As the NPs shrinks, the effective incoming As flux decreases, giving a reduction of the growth rate. After 30 s of exposure to Ga and As beams (wires B, C and D, from the same sample) the morphology of the tip changes. A hemispheric NP, with diameter smaller than that of the wire, becomes visible, sitting on top of a pyramidal structure, with an overall morphology resembling that of NW terminating with {110} facets.10,22,36 Different stages of this process can be appreciated in wires B-D, displaying NPs with different size. We speculate that such differences between NWs of the same sample should be due to local fluctuations22 in the effective beam fluxes (due for instance to shadowing effects), enhanced by the slow substrate rotation (only 1-2 revolutions in a 30 s growth), that cannot average effectively the angle of incidence of the incoming fluxes. These fluctuations and are not observed for longer growth times. After 60 s (wire E) a new wire portion has grown and the NP has already reached its equilibrium geometry, with a contact angle exceeding 110°, in agreement to what is expected for a stable, selfcatalyzed growth.21,30 The resumption of Ga-assisted growth after consumption of the Ga nanoparticle was accidentally observed by Dimakis and coworkers39 in a small number of wires of their samples, but the authors considered this phenomenon not reliable or sufficiently reproducible, and declared it 9 Environment ACS Paragon Plus


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“practically impossible”. The growth parameters of their experiment were not reported but we speculate that a slightly too high As/Ga ratio or a low growth temperature reduced the nucleation probability of the new Ga nanoparticles on the NW tips. For increasing deposition times (wire F, 150 s, and G, 300 s) the NW regrowth continues on top of the original wire and the diameter of the regrown NW appears to increase gradually. A small reverse tapering is typically observed, with the NW diameter close to the Ga NP being slightly larger than at the base of the regrown section. Nanowires B-F show two diameter discontinuities along their bodies: the first related to the NP consumption, as in A, and the second due to the growth resumption with smaller diameter. With increasing regrowth time, the separation between the two discontinuities tends to decrease and after 420 s, (wire H), only one discontinuity is observed, with an irregular shape. At the same time the continuous increase of the diameter of the bottom part of nanowire can be observed. As commonly observed in Ga-assisted growth,20,21 also for the regrown wires the length increase is accompanied by a diameter increase.


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Figure 2. (a) Time dependence of the regrowth length of GaAs NWs as measured from the interface with the preexisting NW (triangles) and of the length of GaAs NWs grown on GaAs(111)B surfaces as measured from the substrate surface (squares) during Ga-assisted growth in the same conditions. (b) Time dependence of the diameter of the regrown NW (triangles) and of the diameter of NWs grown on the GaAs(111)B substrate under the same conditions (squares). The solid lines show the results of theoretical fits to the data using the model described in the text.



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In Figure 2 the length and diameter of the NW portion due to regrowth on preexisting GaAs NWs are plotted together with the length and diameter of GaAs NWs fabricated on GaAs(111)B substrates by means of GaAs-assisted MBE growth (regular growth). For regrown wires, the diameter is measured immediately below the Ga nanoparticle. Each value is the result of an average across a representative sample of N ≈ 20 wires, with error bars representing the standard deviation. In Figure 2a the standard deviation is as large as, or smaller than, the symbol size. Fits to the experimental data obtained according to the model described below are also plotted. From the left panel of Figure 2 we derive a NW regrowth rate onto the preexisting NWs of 130±5 nm/min, fully consistent with the corresponding value observed during regular Ga-assisted NW growth on GaAs(111)B substrates (133±1 nm/min). The increase in NW radius during regrowth on pre-existing NWs (triangles) in Figure 2b is apparently much faster than what is observed during Ga-assisted NW growth on GaAs(111)B substrates (squares). During regular growth of self-assisted NWs the diameter growth rate is constant (1.5±0.1 nm/min) starting from a radius at nucleation of about 20 nm. In contrast to this, the radial growth rate during regrowth is approximately 9 nm/min in the early stages, and slows down with increasing regrowth time. From Figure 2b we observe that a regrowth of 10 min is sufficient to recover the initial NW diameter (115±6 nm in Figure 1), and that after a 15 minute regrowth the NW diameter close to the NW tip is indistinguishable from that one would obtain after 75 min of uninterrupted Gaassisted growth (135 nm). The phenomenon of regrowth, shown in Figure 1 and schematized in Figure 3, is addressed theoretically in the following. In particular, two main issues will be discussed: (i) the linear lengthtime L(t ) and radius-time R(t ) dependencies observed during regular GaAs NW growth on GaAs (111)B substrates shown in Figure 2; and (ii) the non-linear radius-time r(t ) dependence of the regrown NW segment shown in Figure 2b. 12 Environment ACS Paragon Plus

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Figure 3. Schematic of the nanowire growth process as described in the text. (a) Regular self-assisted growth, (b) nucleation of a Ga nanoparticle, (c) and (d) nanowire regrowth. (i) During regular self-assisted growth (Figure 3a), the nanowires are schematized as cylinders of time-dependent length L and radius R. The latter equals the base radius of a spherical cap NP with a fixed contact angle β . Let us consider the As and Ga fluxes reaching the catalyst nanoparticle. The net flux of As atoms reaching the NP equals dN As = J As ,eff πR 2 dt

(1)

where J As,eff summarizes adsorption, desorption and re-emission of arsenic and neglects its surface diffusion.38 For Ga atoms, in contrast, lateral diffusion is taken into account, while diffusion from the substrate surface is neglected, because of the much greater nanowire length as compared to the Ga diffusion length on the sidewalls λ at a given growth temperature. The λ value is of the order of several hundreds of nm at 640 oC, according to Ref. (40). The net influx of Ga atoms is therefore given by



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 θ dNGa = J Ga,eff πR2 + J GasinαGa 2Rλ1 − l  θ dt f 

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  (2)  

with J Ga,eff summarizing adsorption, desorption and possible re-emission of gallium. The diffusioninduced term contains the impingement rate of Ga onto the sidewalls, J Ga sinαGa (with J Ga the direct Ga flux and αGa the incidence angle of Ga beam), onto the equivalent surface area 2Rλ seen by the beam. The ratio of gallium activities in the liquid phase ( θl ) and on the sidewalls ( θ f ) accounts for the reverse diffusion flux from the droplet.40-43 Since in our experiments the diameter of the Ga NP increases with time, we consider the case of an excessive Ga influx,

dN Ga dN As . In this case, the nanowire elongation rate is arsenic-limited and is > dt dt

simply proportional to the effective atomic As influx, therefore πR 2 dL / dt = ΩGaAsdN As / dt , where

ΩGaAs is the elementary volume of a GaAs pair in the solid phase. Using Eq. (1) and integrating, we obtain the observed linear length-time dependence

L = ΩGaAsJ As,eff t ≡ Ct . (3) Eq. (3) is used to fit the experimental data, as shown in Figure 2a, with C ≅ 130 nm/min. The excessive gallium influx leads to the increase of the liquid volume V according to

dV / dt = ΩGa (dNGa / dt − dNAs / dt) , where ΩGais the elementary volume of Ga in the liquid phase. Using Eqs. (1), (2), and the condition β = constant, we arrive at the differential equation dR B = A + . (4) dt R

The coefficients A and B are given by

A=

Ω 2λJ Ga sinαGa  θl ΩGa 1− ( J Ga,eff − J As ,eff ) ; B = Ga  θ f (β ) π f (β ) f 

   

(5)


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with f ( β ) = [(1 − cos β )( 2 + cos β )] /[(1 + cos β ) sin β ] reflecting the relationship between the liquid volume and the droplet contact angle for a given R .16,44 The parameter A is proportional to the Ga/As effective flux imbalance, while B is proportional to the diffusion flux going into the NP, or out of it. Its sign, corresponding to the direction of the diffusion flux, is determined by the relative activities of the Ga adatoms into the liquid phase and on the sidewalls.42 If the activity of the adatoms on the facets is larger than that of the liquid, the diffusion flux is directed towards the NP, and B is positive. Let us assume that B is positive. The A value in Eq. 4 can be of either signs: A0 (excess J Ga,eff ). In the former case, large NPs with R > − B / A shrink and small NPs with

R < − B / A grow in size, until the steady-state condition is reached at Rs = −B / A. This state can only be achieved by Ga diffusion assistance, which compensates the excessive As NP flux. In the latter case, all NPs grow regardless of their initial size. The obtained radius-time relationship depends drastically on the values of A and B . Indeed, integration of Eq. (5) with the initial condition R(t = 0 ) = R0 readily yields At = R − R0 +

B  R0 + B / A  ln  A  R+B/ A 

(6)

Whenever the ratio B /(AR) is large compared to one, the radius scales with time as R 2 = R02 + 2Bt (actually regardless of the sign of A ). In the opposite case (negligible diffusion, B /(AR) J As,eff and that the contribution of sidewall diffusion of is negligible. Regarding an effective absence of Ga diffusion we should note that the value of λ can be considerably reduced by sidewall incorporation. Indeed, a cylindrical NW shape can only be preserved by the successive nucleation and growth of shells around the NW, a process occurring simultaneously with the droplet growth. The shells most probably start from the base, since the rectangular nanowire/substrate interface gives a perfect spot to create the step, and then propagate along the NW length to the top via the step flow mechanism.46,47 This process will be studied in detail elsewhere. The smallness of the B coefficient may also be due to the factor 1 − θl / θ f , which tends to zero (or even negative) when the chemical potentials of Ga atoms in the liquid phase and on the sidewalls are close to each other.41,42,43 (ii) Considering the regrowth process, after a short incubation time required to reach the stationary droplet configuration (which is actually reached after only 60 s of deposition), the conditions for the As droplet flux are equivalent to the regular growth. Therefore we can conclude that NW length depends linearly on the growth time ( l = Ct ) at any time with the same time constant, in agreement with the experimental data in Figure 2a. 16 Environment ACS Paragon Plus

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However, if we consider the NW diameter during regrowth, the experimental r (t ) correlation is drastically different from R(t ) for short growth times. The only difference between the two configurations shown in Figure 3a and 3c is the additional diffusion flux coming from Ga impinging onto the top facet, which definitely dominates at small enough l , shorter than Ga diffusion length λ. Indeed, Ga atoms arriving there can subsequently reach the droplet and contribute to the droplet inflation. We will therefore use the equation

 θ  dv ≅ ΩGaπr 2 ( J Ga , eff − J As , eff ) + ΩGaπ ( R 2 − r 2 ) J Ga cosαGa 1 − l  , r < R dt  θs 

(7)

for the increase of the droplet volume v on top of the secondary nanowire. The first term here is the same as at the regular growth stage. The second term contains the factor 1 − θ l / θ s , accounting for the supersaturation of the surface Ga adatoms with respect to the droplet.40,41,42 The term π ( R 2 − r 2 ) accounts for the free portion of the top facet and vanishes at r = R , i.e., when the radius of the top section reaches that of the stem. Diffusion of atoms impinging on the sidewall is ignored according to the results obtained for the regular growth process. Using the relationship dv / dt = f ( β )πr 2 dr / dt at

β = const , we arrive at dr R2 = ( A − b) + b 2 dt r , r < R; dr = A, r ≥ R, dt

(9)

with b=

 θ Ω Ga J Ga cos α Ga  1 − l f (β )  θs

  . 

(10)

The solution to the first Eq. (10) formula contains two distinctly different regimes, namely the linear regime r = r0 + ( A − b)t and the non-linear regime (at R ≅ const) 17 Environment ACS Paragon Plus


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r 3 = r03 + 3bR 2t .

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(11)

The resulting r(t) correlation depends on which of the two terms in the right hand side of Eq. (9) dominates. Since our experimental r (t ) curve is essentially non-linear, the b value should be large enough compared to A. Red line in Figure 2b shows the fit obtained from Eq. (11) with b = 2.34 nm/min, which is noticeably larger than A , R = 55 nm, and r0=0. Finally, when r reaches the R value, since there is no contribution from Ga atoms impinging on the remaining area of the top facet, the conditions are exactly identical to the regular growth stage, and the kinetic equations for dr/ dt is the same as given by Eqs. (4) and (5). This explains the tendency for saturation observed in the experimental r (t ) dependence of Figure 2b. We emphasize here that general NW growth theory40,41,42 reveals the independence of the substrate and sidewall diffusion fluxes where one can be negligible and another substantial. The large b value that follows from fitting the non-linear r(t) dependence at the re-growth stage indicates that the diffusion flux from the top facet to the Ga NP of the secondary NWs is much larger than that of the sidewall adatoms. Our model treats a single nanowire and does not account for cooperative growth effects such as shadowing47,48 or competition of neighboring NWs for the diffusion fluxes,49 leading to a dependence of the growth rates on the NW density. Since the sidewall diffusion has been shown to be ineffective for the regular growth stage and the diffusion flux of Ga originates from the top facet (which is fully exposed to the beam) at the re-growth stage, these effects are not expected to modify considerably our modeling scheme, but to affect only the sharpness of the size distribution of secondary wires.

The crystal structure of two representative kinds of nanowires is investigated in Figure 4. In Figure 4a, we show TEM results for the structure of the tip of a nanowire after the consumption of the Ga NP


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in As flux (such as wire A in Figure 1). We can recognize different regions: going from the bottom to top, the nanowire firstly presents a faulted section, then an 136 ± 5 nm long wurtzite portion, and finally a 31 ± 5 nm long zincblende tip, separated by a 12 ± 5 nm long faulted segment. Boxed insets show the fast Fourier transform (FFT) calculated from high resolution images in the corresponding segments. We notice a 20 nm diameter reduction at the bottom of the WZ section. Finally, we can see that the top head is flat and perpendicular to the growth direction, therefore exposing a (111)B surface. A similar sequence of structures, namely a high-quality wurtzite stretch sandwiched by two regions with a high density of defects, and followed by a high-quality zincblende tip, was already reported in the literature. Jabeen and coworkers3 observed this structure for wires having no Ga NP at the end of the growth. These NWs showed a pyramid-like tip, with {110} facets. This morphology and structure was also reported by Plissard et al.,6 who ascribed the absence of the Ga NP to its crystallization upon cooling in As flux. In our previous work22 we also found the same structural sequence and tip morphology in NWs found without Ga NP after the growth, while we found a high-quality WZ stretch at the interface with the Ga nanoparticle in NWs where the NP contact angle was close to 90°. These results were interpreted as evidences of wurtzite formation during the extinction phase of the NP. Very recently, the same crystal lattice sequence was also observed by Kim and coworkers,30 for NWs where the Ga NP was intentionally consumed by exposure to As flux. In that case, however, the ZB NW tip was found to be flat, and parallel to the (111) planes as in the present case.



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Figure 4. (a) TEM image of a NW after the consumption of the Ga NP. In the insets, fast Fourier transforms of high resolution TEM images (not shown) acquired in the same regions along the [110] zone axis. (b) and (c) Lower magnification TEM image of a NW after 15’ of regrowth. Scale bar is 0.5 µm. The image in (c) has been colored to distinguish between the different crystalline sections. For each a representative FFT pattern of a higher resolution image recorded along the [110] zone axis is reported in the insets (d), (e), (f) and (g). Indexation of the pattern in (g) is reported in the supplementary information. In Figure 4 (b-g) the crystal structure of a representative NW obtained by 15 min of regrowth is shown. Panel (b) shows the nanowire as imaged by low resolution bright field TEM. In (c) the same 20 Environment ACS Paragon Plus

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image is reproduced in false colors, according to the different crystal phases coded as the boxed FFTs in panels (d-g), obtained from high resolution images, recorded along the nanowire body (not shown). Going from bottom to top, we see the alternation of two ZB domains, whose FFTs are displayed in panels (c) and (d), separated by twin planes. There are three zincblende twinned segments and another short series of stacking faults, whose representative FFT, displayed in panel (e), is given by the superposition of the two ZB patterns. A 180 ± 5 nm long wurtzite segment with no defects (FFT in inset (f)) leads to another faulted region in the center of which a 25 ± 5 nm thick ZB region is present, marked by an arrow. Then we find a 630 ± 5 nm long defect-free zincblende segment where a diameter reduction is visible, 235 ± 5 nm above the wurtzite part. Above this, nine twinned zincblende segments lead to the Ga droplet at the tip. Along this twinned part, neither wurtzite stacking, nor stacking faults were detected. We underline that while the details of the structure sequence, such as the length of the different regions, depend on the imaged nanowire, the lattice sequence, and especially i) the presence of a long defect-free zincblende region above the high-quality wurtzite part, separated from this by a defective region, and ii) the presence in this ZB region of a diameter discontinuity, were observed in all the imaged nanowires. By comparing the observed sequence of structures with those in Figure 4a and on the basis of the observation in Refs. (22) and (28), we conclude that the WZ section corresponds to the growth during the extinction of the Ga nanoparticle and that the short ZB region just above it, marked by the arrow, corresponds to the last portion of wire grown at the end of the extinction process. The observed sequence of structures is analogous to those obtained by changing the As/Ga flux ratio during the growth25,26,28 as well as by interrupting the growth and modifying the NP/NW contact angle by supplying only As or only Ga.29 However, the length of the WZ segment is in our case dictated by the volume of the Ga NP that is consumed, since no other Ga supply is present in our process, and is 21 Environment ACS Paragon Plus


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highly reproducible in our samples. We notice that a highly defective transition region is always reported at each crystal structure switch.25,26,29 Further analysis of Figure 4 reveals details about the smoothing of the diameter discontinuities in Figure 1 and of the overall regrowth process. We notice that the diameter discontinuity observed in Figure 4a at the bottom interface between ZB and WZ has been cancelled by the lateral growth. The HRTEM images (not shown), where the FFT in panels (c-g) were recorded, reveal that lateral growth during the regrowth process took place with the same crystal structure of the NW core. A similar phenomenon was exploited by Shtrickman and coworkers to synthesize pure WZ GaAs NWs by Auassisted growth, i.e., by enhancing the lateral growth on thin WZ GaAs nanowires.50 A diameter discontinuity, reminiscent of the regrowth process, is still visible, although the NW diameter below the WZ section is the same as that we measure close to the Ga NP. This is due to the small inverse tapering of the regrown wire. However, the discontinuity is shifted more than 300 nm above the end of the WZ section, i.e., well beyond the position where the regrowth started. This implies that, during the regrowth process, lateral growth proceeds from the bottom of the wire and progressively smoothens and pushes the discontinuity toward the NW tip. The details of this process will be analysed elsewhere.


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Figure 5. SEM images of the evolution of the NW tip morphology after exposure to Ga flux. (a) after consumption of the Ga NP, and after (b) 1’, (c) 2’ 30” and (d) 5’ of exposure to a Ga flux. Images (a), (c) and (d) were obtained from NWs transferred onto a Si substrate, while image (b) is a 45° degrees view of an as-grown sample. Wires in (a) were slightly tilted to show the shape of the tip. To better display the triangular geometry of the top surface red lines were added at its boundary. Scale bars are 100 nm long in Figures 5a-d. (e) Plan view of the entire wire shown in (d). The formation process of the Ga NP on the preexisting NW tip can be studied in more detail by depositing only gallium on the flat tip of wires grown with the same procedure used before. SEM images of representative wires are reported in Figure 5. Figure 5a shows the morphology of the wire tips before exposure to Ga. We see that the flat top facet is triangularly-shaped. After 1' (Figure 5b) we observe that a nanoparticle is again present on top of the wires. Its diameter is smaller than that of the wire. It is interesting to observe that, at this early stage, the NP base is also triangularly-shaped, exactly covering the (111)B top facet. After 2'30'' of Ga deposition (Figure 5c), the NP is fully developed and its contact angle is approaching rapidly the steady-state value that is observed during balanced growth. However, it is possible to notice that the NP is not covering the entire NW tip, but is still sitting on the flat part. After 5' (Figure 5d) the NP is now covering the entire NW tip, and its shape is now indistinguishable from that observed during regular self-assisted growth of GaAs NW on GaAs(111)B substrates for similar NW length and diameter. We observe that the maximum size of the NP appears to be limited, in the sense that when the equilibrium NP geometry is reached, the NP stops growing. No NP with contact angle larger than 130° were ever observed. Along with the growth of the top NP the nucleation of additional NPs on the NW sidewalls is found to take place (Figure 5e), at least at 640 °C.



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Although the size of the Ga NPs formed after a given Ga deposition time varies across the NW population, due to local fluctuations or to the formation of sidewall NPs, when perfect matching of the top NP and the NW body is found, i.e., the NP has a contact angle larger than 110° and a diameter larger than that of the NW, the regrowth takes place leaving a minimal sign of the growth interruption. An example is shown in Figure 6, where a NW obtained after a 5’ Ga deposition followed by 5’ of GaAs regrowth is observed. In Figure 6b we can see a magnification of the portion of NW body where this small discontinuity is visible.

Figure 6. SEM image of a single NW transferred on a Si substrate. The sample was exposed to 5' Ga before resuming GaAs growth for 5’. In this case, it is evident that growth resumed with a nanoparticle covering almost all the NW tip. The indentations visible in Figure 6b are present every other NW edge, reflecting the 3-fold symmetry of the tip after the NP consumption, as shown before in Figure 5a. To explain the presence of these indentations even after 5’ of regrowth we can consider that regrowth with such a large NP does not allow for an extra Ga flux coming from the top facets, i.e., it proceeds as in the case of steady-state growths. Therefore the radial growth rate is constant along the whole NW, and equal to the steady-state rate of 1.5 nm/min, and the discontinuity grows together with the NW


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facets. Furthermore, the indentations are not able to collect more material than the facets per unit area, because they are shadowed by the wire segment above.

CONCLUSIONS. In summary, we have shown that the VLS growth of self-catalyzed GaAs nanowires can be resumed after the extinction of the Ga nanoparticle. Nucleation of a new Ga-nanoparticle on the flat nanowire tip was obtained either in standard Ga-rich growth condition or by exposure to a Ga flux only. The Ga nanoparticle assists the epitaxial regrowth of a nanowire, with a diameter that rapidly recovers the original value of the preexisting NW. Supplying only gallium allowed us us to tune the starting diameter of the nanoparticle and to induce a NW regrowth starting with the same diameter of the underlying nanowire. The structural difference of the NW grown in steady-state self-assisted mode (ZB) and during the consumption of the nanoparticle (WZ) allowed to recognize by TEM the different growth stages in the grown nanostructures. Nucleation and growth of a Ga NP on the top surface of the preexisting NWs represents a puzzling phenomenon since no formation of Ga droplets able to trigger NW growth has been reported on GaAs(111)B surfaces in the absence of an oxidize Si layer. We speculate that the small surface area on the NW tip acts like a hole in a patterned substrate, preventing the droplet from running over the surface,51 and promoting nucleation and growth. The growth and regrowth processes was described by a simple analytical model taking into account the contributions to the growth of atoms impinging on the nanowires sidewalls and on the nanowire tips underlining the importance of the latter for the rapid recovery of the NW diameter during the regrowth. The model allowed us to reproduce the experimental dependence of the nanowire length and diameter both during standard growth and the regrowth.



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The mechanisms discussed and modeled here for the first time could be exploited to grow nanowires with controlled axial and radial heterostructures by simply stopping and resuming the axial growth.

Acknowledgments The authors wish to thank G.E. Cirlin, D. Zeze and B. Mendes for providing access to and assistance in the electron microscopy characterization. VGD would like to thank Frank Glas for helpful discussions.

Supporting Information Available. Additional SEM images of samples shown in Fig. 1, along with a brief description, and a larger version of Fig. 4g. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. %This author performed the nanowire growth and the characterization by scanning electron microscopy. ‰ This author performed the transmission electron microscopy and took care of the collaboration with the theoretical group.


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For Table of Contents Use Only.

Self-catalyzed GaAs nanowire growth can be resumed after the consumption of the catalyst nanoparticle. In suitable conditions, a new nanoparticle condensates on the flat end of the NW, assisting further NW growth. The regrown part quickly recovers its original diameter, behavior explained by the geometry of the system and by diffusivity of Ga adatoms. The Ga nanoparticle nucleation occurs on an oxide-free, clean surface.


Stopping and Resuming at Will the Growth of GaAs Nanowires

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