Published: April 18, 2011

r 2011 American Chemical Society 1934 dx.doi.org/10.1021/nl200083f |Nano Lett. 2011, 11, 1934–1940

LETTER

pubs.acs.org/NanoLett

Kinetic Effects in InP Nanowire Growth and StackingFault Formation: The Role of Interface RougheningThalita Chiaramonte,† Luiz H. G. Tizei,†,‡ Daniel Ugarte,† and Monica A. Cotta*,†

†Instituto de Física“GlebWataghin”, Universidade Estadual de Campinas, UNICAMP, 13083-859, Campinas, SP, Brazil‡Laboratório Nacional de Luz Síncrotron, C P 6192, 13083-970 Campinas, SP, Brazil

bS Supporting Information

Semiconductors nanowires (NWs) have been extensivelystudied due to their potential applications in the new gen-

eration of electronics, photonics and sensing devices.1�4 As foroptoelectronics, III�VNWs present many advantages regardingemission spectra range and carrier mobilities. These NWs havebeen successfully synthesized by the vapor�liquid�solid (VLS)5

mechanism using Au nanoparticles (NPs) as catalysts.6 Whilebulkmaterials and thin epitaxial films present zinc blende (ZB) asthe stable crystallographic phase, III�V NWs usually present anstructure where crystalline planes can be stacked as ZB orwurtzite (WZ), even if a ZB substrate is used.7�9

The formation of WZ NWs is favorable when surface energyoutweighs the volume cohesive energies; this leads to a criticaldiameter below which theWZphase is dominant.10 In general, thetransition from WZ to ZB creates stacking faults (SFs) in NWsgrown along the [111] direction. The energy barrier for the WZ/ZB transition is rather low, estimated as 3.4 meV/atom for InPNWs with diameters greater than 20 nm.10 Recently, a satisfactorycontrol of both SF occurrence and the formation of pure andmixed WZ/ZB segments has been achieved by changing growthparameters and nanoparticle diameter.11,12 By varying the crystalstructure between ZB and WZ, it is possible to develop band gapengineering in a single material along the NW axial length.12

Despite these very promising results, the physical mechanismsbehind the growth of a determined crystal structure are not yet fullyunderstood. Moreover, polytypic behavior shows striking simila-rities for several III�V materials despite the use of differentsubstrates and growth techniques.7�9,13�15 Recently, it has been

shown that the SFs density and polytypism along the NW dependon growth conditions, such as temperature, V/III ratio, and NPdiameter.12,16�18 For example, Paiman et al.19 reported that the InPNWs grown by metalorganic metal vapor deposition (MOCVD)are preferably formed in WZ phase with increasing V/III ratios(under fixed In precursor flow) or decreasing NP diameter. Also,Dick et al.,20 using metalorganic vapor phase epitaxy (MOVPE),have observed a transition fromWZ at lowTMImolar fraction to amixture ofWZ andZB at either very highmolar fraction or reducedV/III ratio. These results show that surface kinetics plays adominant role in polytypism even though detailed surface condi-tions depend on the employed growth technique.

On the basis of nucleation kinetic models, the growth ofdifferent crystal structures relies on the small energy differencebetween the two critical nuclei. At higher growth temperatures orsupersaturation conditions, larger fluctuations are expected;these factors can lead for example to the formation of bothWZ and ZB sections in NWs grown by VLS. Glas et al.21 haveproposed a nucleation-based model where the nucleus is formedat the triple phase line (TPL); under certain supersaturationconditions, the WZ phase could be favored over ZB. Morecomplete models also take into account the distortion of theNP due to the nucleus formation at the TPL and estimate thedifferent energy barriers for nucleation associated with the NPdistortion.11,17 It must be noted that most models assume a

Received: January 8, 2011Revised: April 1, 2011

ABSTRACT: InP nanowire polytypic growth was thoroughly studied usingelectron microscopy techniques as a function of the In precursor flow. Thedominant InP crystal structure is wurtzite, and growth parameters determine thedensity of stacking faults (SF) and zinc blende segments along the nanowires(NWs). Our results show that SF formation in InP NWs cannot be univocallyattributed to the droplet supersaturation, if we assume this variable to beproportional to the ex situ In atomic concentration at the catalyst particle. Animbalance between this concentration and the axial growth rate was detected forgrowth conditions associated with larger SF densities along theNWs, suggesting adifferent route of precursor incorporation at the triple phase line in that case. Theformation of SFs can be further enhanced by varying the In supply during growth and is suppressed for small diameter NWs grownunder the same conditions. We attribute the observed behaviors to kinetically driven roughening of the semiconductor/metalinterface. The consequent deformation of the triple phase line increases the probability of a phase change at the growth interface inan effort to reach local minima of system interface and surface energy.

KEYWORDS: Nanowire, InP, crystal structure, zinc blende, wurtzite, stacking fault, electron microscopy

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monolayer nucleation and subsequent layer-by-layer growthmode taking place at the NP/NW interface.

Among different possibilities to control NW VLS growth, thevariations of the vapor supply flow should provide a much bettertool since faster system responses can be achieved. This avoidsgrowth interruptions, usually associated with enhanced contam-ination and generation of wider interfaces in epitaxial thin films.In this work, we present an analysis of the morphology andcrystal structure of InP NWs grown by chemical beam epitaxy(CBE). In particular, we have investigated the effects of variationsin the vapor supply of the In precursor. Our results suggestdifferent dominant routes for precursor incorporation at the TPLas a function of flow which can lead to interface roughening anddefect formation. The model proposed from these results may beexploited to fine control NW crystal structure.

InP NWs were grown on GaAs (100) substrates in a CBEchamber, using colloidal Au NPs as the catalyst material. Thesubstrate native oxide was not desorbed prior to the InP growth.Two different particle sizes were used as catalysts (with 10 or25 nm average diameters, hereafter called small and large NPs,respectively). Trimethylindium (TMI) diluted with hydrogencarrier gas and thermally decomposed phosphine (PH3) wereused as group III and V sources, respectively. Growth tempera-ture was chosen as 420 �C in order to provide WZ NWs for theNP sizes used here.22 Two types of samples were grown. ForA-type NWs, all growth parameters were kept constant through-out the run. For B-type NWs, growth was carried out under lowTMI flow (0.15 sccm) conditions except for short segmentswhere TMI flow was stepped up or down (with a 12 s transitionperiod) to reach the intended value (1.2 sccm) without growthinterruption. Two sets of B-type samples were grown, withdifferent numbers of flow transitions. For both types of NWs,PH3 flow was kept at 15 sccm, providing the dominant back-ground pressure for CBE growth. The corresponding P2 over-pressure warrants good crystallinity in epitaxial film growth for allTMI flows used here. Samples were cooled down in a PH3

atmosphere in order to minimize any possible effects due to aresidual P concentration at the NP after growth, as suggested byprevious results.22,23 Several A-type samples were grown withTMI flow between 0.15 and 1.2 sccm; deposition time was variedinversely with TMI flow in order to keep approximately constantthe total amount of material supplied for each sample.

The samples were subsequently exposed to the environmentand prepared for morphological and structural analysis byelectron microscopy. The wires were observed with a scanningelectron microscope (SEM, model JSM 6330F) to obtaininformation on NW shape and length. The crystal structureand chemical composition of the NWs were investigated withelectron diffraction and high-resolution transmission electronmicroscopy (HRTEM) as well as Energy Dispersive Spectros-copy (EDS), using a JEM 3010 URP operated at 300 kV and aJEM 2100 ARP operated at 200 kV.

The NWs axial growth rate was estimated from SEM imagesconsidering only the longer NWs in each sample. The rate wasassumed linear and calculated by dividing the measured NWlength by the growth time. SEM and TEM images were used tocheck for the presence of constrictions and defective regions(indicative of the flow transitions, as explained further in the text)as well as the axial length between these regions in B-type NWs.The growth rates thus obtained correlate quite well with thoseobtained from SEM images for corresponding A-type samples.Axial length values are∼50 times larger than the thickness of the

InP film simultaneously deposited on the GaAs substrate and∼10 times larger than thicknesses usually found at 500 �C forplanar growth using the same TMI flow.

Figure 1 illustrates TEM images of A-type InP NWs grownunder low (0.15 sccm) and high (1.2 sccm) TMI flows for thelarge NPs. Electron diffraction indicates that these two samplesexhibited WZ crystal structure with growth axis along the [0001]direction.22 The wires grown under low flow show an almostperfect WZ structure, a slight tapering and a smooth lateralsurface. In contrast, the wires generated under high TMI flowdisplay large diameter variations along the NW length (arrows inFigure.1c); HRTEM images (Figure.1d) reveal the formation ofa high density of SFs, as well as few-monolayer-wide ZB segmentsalong the growth direction. A quantitative analysis shows that SFdensities are not statistically significant for the lower TMI flowsbut reach up to approximately 300 μm�1 for NWs grown under1.2 sccm, as shown in Figure 1e.

Figure 2 shows electron images of wires from B-type samples, forwhichTMI flow conditions vary between those used for theNWs inFigure 1. These wires also exhibit a WZ structure and a taperedshape; in particular, the wires show large diameter variations in theform of constrictions (or necks) along their length. The axial

Figure 1. (a, c) Typical TEM images of A-type InP NWs grown under0.15 and 1.2 sccm TMI flow, respectively (see text for explanations), forlarge (25 nm)NPs. Scale bars represent 1 μm. (c) Arrows indicate largervariations in diameter along the wires. (b, d) Atomic resolution images ofthe NWs. A high density of stacking faults is observed for the wire grownunder larger TMI flow (d). The plot in (e) shows the average SF densityobtained from HRTEM images for the TMI flow condition used.

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distance between these constrictions indicates that they arecorrelated with the TMI flow variations during growth.

Careful analysis of HRTEM images shows that SFs are formedmainly in the wire segments close to the constrictions (seeFigure 2a and inset). In particular, these highly defective regionsare associated with segments grown under larger TMI flow, inagreement with observations for A-type NWs (Figure 1). Theanalysis of different B-typeNWs reveals that the length and shapeof the constricted regions depend on the wire diameter (which isa function of both NP size and lateral growth). Moreover, largerSF densities can consistently be observed when the diameterincreases at these regions (see examples in Supporting Informa-tion, Figure S1). Another common feature is the correlation ofsmall variations in diameter and SF occurrence. Indeed, this iscommonly observed in NWs of many III�V materials whenZB/WZ segments are intercalated in the wires.11,12,19,20 Con-trarily, NWs grown under low TMI flow usually present diametervariations near the base (Figure 1a). These variations are mostlikely related to radial growth provided by mass transport fromthe surface which is enhanced under this particular condition(notice Supporting Information, Figure S2).

It is usually observed that defects are generated in NWs at theregions between NW and the metal NP formed during samplecool down;9,20 this is attributed to a reduction of the NPsupersaturation after the metal precursor is turned off. Moreover,for different III�V materials, the NW/NP neck region usuallyexhibits a crystal structure opposite to that of the NW; thus WZ

NWs show predominantly ZB necks and vice versa.20 Both kindsof defects, those formed close to the NW/NP interface and thoseobserved in the B-typeNWs (Figure 2) occur in a situation wherethe In supply is varying. Thus, it is interesting to note that NWsgrown under constant precursor conditions (or constant NPsupersaturation, Figure 1c,d) share similar structural character-istics (i.e., SF formation).

We must note that when the TMI flow is increased, thesupersaturation level in the whole growth system raises; however,this does not straightforwardly imply that the NP supersaturationhas been changed. In order to gather more information on themetal NP for different growth conditions, we have analyzedex situ the average In atomic concentration at the NP by EDS(Figure 3). The ex situ measurement will certainly provideinformation that differs from the actual In concentration duringgrowth (we must keep in mind that samples were cooled downunder P2 atmosphere). In our case, the set of analyzed sampleswas grown at the same temperature, with similar cool downconditions. Thus, we expect the postgrowth In concentration atthe NP to be directly related to supersaturation levels during NWgrowth. For NWs grown under constant precursor conditions(Figure 1), EDS shows statistically similar NP supersaturationlevels despite the very different TMI flows (or very distinct NWstructures) between the two samples shown in Figure.1. Thisindicates that the increase in SF density for high TMI flow cannotbe purely associated with supersaturation levels at the NP.

The relationship between NW growth and NP supersatura-tion is indicated in Figure 3, where we have also plotted theestimated NW axial growth rate. When lower TMI flows(0.1�0.5 sccm) are used for growth, both the growth rate andthe In concentration at the NP show similar trends, increasingmonotonically. Further increase of the TMI flow over 0.5 sccmgenerates a significant decrease of both parameters, what clearlydefines a maximum in both curves. For TMI flows over 0.7�0.8sccm, however, the trend of axial growth rate dissociates from theIn concentration in the NP. While growth rate shows a clear

Figure 2. TEM images of B-type InP NWs where strong TMI flowvariations were intentionally produced: (a) nanowire constrictionshowing a high density of SFs in the high flow region (inset showsHRTEM of the same region, scale bar = 2 nm); (b) typical NWmorphology displaying diameter variations. Scale bar = 2 μm.

Figure 3. Quantitative results for In atomic concentration at the NPmeasured by EDS and the NW axial growth rate obtained from SEMstatistical analysis considering only the longer NWs in each sample (andwith the same orientation). Measured lengths are projections of theactual values for the average NP diameter on our sample. The EDSmeasurements were carried out on NWs with lengths compatible withthe average value from SEM analysis. In practice it has been ratherdifficult to find long NWs conserving the NP on the TEM grid; thusthree nanowires were analyzed in each sample. The lines in the figure areguides for the eye.

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enhancement and larger dispersion with TMI flow increase, theNP In content remains constant (∼25%). The data in Figure 3clearly indicate that, for large TMI flows (>0.8 sccm), a differentroute of In precursor incorporation into the NW should becomedominant, distinct from the usually accepted Au�In alloy gener-ated at the NP, since both NW and NP diameters seem not to beaffected in this case (see Supporting Information, Figure S2).

The formation of constricted (neck) regions in B-type NWscan be partially explained on the basis of the growth rate and NPcomposition changes presented in Figure 3. When the TMI flowis raised during growth, the axial growth rate will temporarilyincrease as well; however, the lateral growth rate at the NWsidewall should not significantly change. As a result, the localtapering should be diminished since the ratio between axial andlateral growth rates will be temporarily larger. As the substratesurface acts as a large material reservoir, the tapering angle willalso vary as a function of heights along the NW. Furthermore, theIn�Au alloy NP composition changes can induce modificationsof the contact angle between semiconductor and eutectic droplet.In fact, both effects can lead to the formation of the constrictedregions shown in Figure 2; however, only a contact angle changecan account for the subsequent NW enlargement. Indeed, thisenlargement has been observed even in the absence of a longneck (see Supporting Information, Figure S1).

A qualitative evaluation of the relation between lateral andaxial growth in our samples is provided by the average base andapex diameters of NWs grown under constant precursor condi-tions, according to SEM imaging (see Supporting Information,Figure S2). As expected, the contribution of surface material forgrowth (and tapering) is reduced under larger flows.24 Incontrast, for the lower TMI flows used in our study, the basediameters are considerably larger than the apex. Moreover, in thiscase measurable incubation times are necessary to drive thesupersaturation in the dispersed NPs, similarly to the behaviorreported by Froberg et al.25 These two observations suggest thatthe TMI flow in this case is barely enough to maintain theminimum NP supersaturation for NW growth. Also, the con-tribution of material coming from the substrate should maintainNP supersaturation conditions, since the finite precursor masstransported by the vapor phase should in fact act as a limitingfactor for growth. At this point, we must consider that our resultsindicate that an intermediate TMI flow of 0.45 sccm allows amaximum In supersaturation in the NP, whose concentrationagrees with previous results for CBE growth.25 Moreover, for thisparticular TMI flow amaximum axial growth rate is observed (seeFigure 3), while SFs are still kept at low density. Further increasein TMI flow to ∼0.7 sccm induces rather unexpected results:both the In concentration at the NP and the axial growth rate falldown, but the density of SFs increases significantly (Figure 1e).

As discussed above, the correlation between ex situ NP Inconcentration and axial growth rates can thus provide a morecomplete scenario of what happens during NW growth. Wemustnote that the data included in Figure 3 also imply that super-saturation does not completely determine the structural proper-ties of InP NWs. Our TEM analysis on A-type wires indicated amonotonous increase of SF density with In precursor supply, forsamples grown with TMI flow larger than 0.6 sccm. Therefore,similar NP In concentration values (as those obtained at thelower and upper limits of the TMI flow range) can yieldcompletely different SF densities. At the same time, WZ NWswith negligible SF densities can be achieved with differentsupersaturations (in the case of TMI flows 0.15 and 0.45 sccm).

In vapor phase techniques such as CBE and MOCVD,variations of the NW axial growth rate must be related to thesurface kinetics of metalorganics catalysis.26�28 Indeed, ourresults for InP NWs show a similar trend to InAs NW grownby MOVPE.29 Attaining a rather large growth rate requires bothgroup III and V atom availability. Even for a liquid NP with anearly perfect accommodation coefficient, however, catalyticactivity could be partially suppressed at temperatures commonlyused for NW growth, as observed for TMI pyrolysis on epitaxialsubstrate surfaces.28 In that case the supersaturation which can beattained within the NP is strongly dependent on the NP surfacearea. Moreover, at In-rich growth conditions, we should considerpossible kinetic limitations imposed on the axial growth rate bygroup V atoms transport during growth. Thus the drop in axialgrowth rate at ∼0.5 sccm TMI flow (Figure.3) can be related toreduced precursor availability. In these terms, it is important toconsider that the actual source of group V atoms during growth isstill under discussion in literature.9,23,30 In fact, several phenom-ena influencing precursor availability may occur simultaneouslyduring growth, such as limited surface diffusion of species towardthe NP, issues on NP size and solubility limits, or the eventualsaturation of the metal surface catalytic activity.

We would thus expect the NW growth rate to continue to falldown with increasing TMI flow. In contrast, a further increase inTMI flow (>0.8 sccm) induces an increase of growth rate,attaining values even higher than the previous maximum valueat ∼0.5 sccm. Surprisingly, despite the significant variations ingrowth rate, our EDS analysis of In concentration at the NPremains essentially constant for TMI flows >0.8 sccm. A naturalquestion arises: how can we provide more In for NW growth ifthe NP contribution seems to be saturated? If the metal catalyticactivity is diminished or can be no longer increased, In adatomson the NW sidewalls (which represent the major adatom capturearea at shorter diffusion lengths) can contribute to solid semi-conductor growth by either directly reaching the TPL enhancingaxial growth or incorporating at the sidewalls for lateral growth.

The well accepted NW growth scenario supposes axialgrowth through a single step propagation along the NP/NWinterface.17,31 If a competition between different nucleation sitesoccurs at TPL through both NP�vapor andNP�semiconductorphases, the growth scenario can be drastically different. In fact,roughening of the interface can take place due to the simulta-neous nucleation of different monolayers around the TPL. Therelatively long sample cool down can hinder the ex situ observa-tion of such anomalous NP/semiconductor interfaces generatedduring the dynamic nuclei creation. Nevertheless, our extensiveTEM study of NW/NP interfaces has revealed that very differentNP/NW interfaces are observed when NWs show very differentSF densities. For the WZ material grown at low TMI flow (0.15sccm, Figure.4a), the HRTEM image shows sharply definedinterfaces where incomplete monolayers can be observed at theedges of the NW (inset); this nucleation causes a small distortionof the TPL with no visible phase change. In contrast, NWs grownunder large TMI flow (1.2 sccm, Figure.4b), show usually less-well-behaved interfaces (see also Supporting Information, Fig-ure.S3). In some cases, large distortions and a clear curvature ofthe interface are observed, as well as different faceting forpredominantly ZB regions.

The results discussed above suggest that even under constantgrowth conditions, the NP/NW interface can fluctuate to a muchlarger level than what is usually assumed for the propagation of asingle nuclei along the semiconductor/metal interface. This

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hypothesis is not precluded from reported in situ TEM observa-tions since they are mainly based on single element NWgrowth31,32 and under supply conditions limited by the micro-scope environment. On the other hand, Joanny and de Gennes33

have discussed the effect of roughness of a solid surface onwettability by introducing the concept of TPL elasticity andstudying its response to external forces, either arbitrary orlocalized. These authors have predicted that the irregularitiesfound in solid surfaces create random fluctuations on the work ofadhesion; these fluctuations act as a force that moves the contactline. In particular, this phenomenological model reveals anequivalence for the two most likely found experimental typesof defects: contaminants present on the surface and surfaceroughness. For the latter case, the variation of work of adhesionwith regard to the undisturbed case depends both on the actualcontact angle and on the local slope of the surface near the TPL.33

Moreover, if the droplet is at the nanoscale, a correction to thecontact angle is induced by TPL tension34 which increases withthe length/surface ratio. In these terms, the variations observedin growth rate and the NP In content as a function of TMI flow(Figure 3) result from the competition between different nucleito incorporate In and P atoms provided from different routes,(i.e., throughNP alloy and surface diffusion). As energy considera-tions do still hold, it must be physically expected that nucleationshould occur at TPL and growth proceed by step propagationalong the metal/semiconductor interface. However, if kinetic

conditions prevail such that the step propagation rate is smallerthan the nucleation rate at TPL, roughness should build up in theregion close to the TPL. A similar scenario has been recentlysuggested for the growth of Si NWs under large disilane pressuresusing Cu as catalyst.35

The kinetic origin of TPL fluctuations further suggests abehavior which is dependent on the NP size: NPs of smallerdiameters should be less susceptible to show interface rough-ening. In fact, small NPs should attain larger supersaturations forlower precursor flow conditions and the time necessary tocomplete a monolayer at the NW growth front should be muchshorter. Moreover, for the cases of both low P solubility in theNPand diffusion of P from the TPL along the NP/semiconductorinterface, we would expect a faster P incorporation at the growthfront for smaller NPs. Such effects prevent multiple nucleusformation and certainly contribute to keep a smoother NW/NPinterface. Also, more restrictive kinetic conditions should benecessary in order to change the length of the TPL when usingsmall NPs. Our experimental observations confirm these deduc-tions. Indeed, TEM images of B-type InP NWs grown with oursmaller NP diameters (10 nm), and under identical conditions asto those in Figures 2 and S1, show pure WZ structures. Nostatistically relevant SF density could be obtained for observa-tions carried out along the whole length of the NWs; diametervariations can seldom be detected, but they are not associatedwith SF occurrence (Figure 5). The measured In concentration

Figure 5. (a) TEM image of B-type InP NW grown with small (10 nm)NP with no constrictions along its length (scale bar = 1 μm). (b)HRTEM image of the marked region shows diameter variations (arrow)along the wire with no associated SFs.

Figure 4. HRTEM images (with same magnification) of A-type NP/semiconductor interfaces for (a) pure WZ and (b) WZ/ZB NW, grownunder 0.15 and 1.2 sccm TMI flow conditions, respectively. Large(25 nm) NPs were used for the growth.

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at the NP, though, is similar to those found for our large NPsdespite the larger growth rate.

It must be emphasized that our TEM results also agree verywell with previous reports on the defect density dependence onthe ratio of V/III elements. So far, the WZ�ZB transition inIII�V NWs has been mainly attributed to changes in thetemperature or V/III ratio.12,19,20 Modifications of these para-meters induce a variation of the vapor�solid nucleus surfaceenergy, which may, for example, lead to different surface recon-structions on the NW facets. However, these phenomena shouldnot solely account for the rather homogeneous response con-cerning morphologies and crystal structures of NWs generatedwith different synthesis techniques based on very dissimilargrowth environments.

When fluctuations of the TPL occur and roughness builds up,the distortions created at the NP interface should lead to theformation of fault planes in order to reach local minima of systeminterface and surface energy. In the case of WZ NWs, contactangle variation effects may be minimized and the growth frontstability may be maintained by the nucleation of {111} facetswhich are tilted toward or away from the wire axis. In theparticular case of WZ InP NWs studied here, these phenomenamust be related to the modulation of NW diameter (Figure 2b)since contact angle hysteresis is expected to occur for stronginhomogeneities causing TPL distortion.33,36 Finally, in ourinterpretation, crystal phase changes happen as a response tokinetically driven roughening, then it naturally fulfills Poissonstatistics for defect formation as observed for III�V nanowires.37

It is also interesting to notice that TPL modifications can betriggered by surface contamination.33 For example, Algra et al.11

have created InP twinning superlattices using Zn as impuritydopant. The WZ to ZB transition was attributed to the slightlydifferent solid�liquid surface free energy upon zinc addition.More recently, Wallentin et al.38 suggested increased contactangles upon addition of Zn precursor during growth; the changesin wetting were correlated to changes in surface energies whichmay affect the crystal structure of the NWs, increasing the barrierfor WZ nucleation. In these terms, we must analyze our experi-ments carefully, as a similar role could be assigned to excesscarbon atoms under the large TMI flows used here. However,electron energy loss spectroscopy experiments have not revealedthe presence of C above detection level.

In summary, on the basis of ex situ In atomic concentration atthe catalyst particle, we have shown that SF formation in InPNWs is not univocally attributed to the NP supersaturation.Moreover, we have detected that when the wires are generatedunder growth conditions associated with larger SF densities (forexample, under large TMI supply rate) there is a lack of relationbetween NP In concentration and NW axial growth rate. Ourstudy has shown that SF formation is strongly enhanced when Insupply (and NW diameter) variations occur, either from NPdepletion or from competition between different precursorsincorporation routes. In contrast, for NWs grown under thesame conditions with our small NPs, defect formation is sup-pressed. The ensemble of experimentally observed behaviors canbe explained if we take into account kinetically driven rougheningand deformation of the TPL due to a competition betweendifferent nuclei to incorporate III�V atoms from different routes,such as through NP or surface diffusion. This increases theprobability of a phase change in the system attempt to minimizeplanes at the growth interface in order to reach local minima ofsystem interface and surface energy.

’ASSOCIATED CONTENT

bS Supporting Information. TEM images of nanowires andNP/NW interfaces (S1 and S3, respectively) and measurementsof NW base and apex diameters (S2). This material is availablefree of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: monica@ifi.unicamp.br.

’ACKNOWLEDGMENT

The authors acknowledge Daniela Zanchet for discussions onNP synthesis and H. T. Obata for technical assistance. SEM andTEMmeasurements were performed at the ElectronMicroscopyFacility (LME), Brazilian Synchrotron Laboratory (LNLS). Thiswork was funded by FAPESP and CNPq.

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