The Mechanism of Ni-Assisted GaN Nanowire ... - ACS Publications

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The Mechanism of Ni-Assisted GaN Nanowire Growth Carina B. Maliakkal,* Nirupam Hatui, Rudheer D. Bapat, Bhagyashree A. Chalke, A. Azizur Rahman, and Arnab Bhattacharya Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India

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S Supporting Information *

ABSTRACT: Despite the numerous reports on the metalcatalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs−Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni−Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be Ni2Ga3. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We hence conclude that Ni-assisted GaN nanowire growth proceeds via a vapor−solid−solid mechanism instead of the conventional vapor−liquid−solid mechanism. KEYWORDS: Diffusion-induced growth, vapor−solid−solid mechanism, GaN, nanowire, MOCVD

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chemical potential or (ii) a chemical catalyst or both. Irrespective of its role, we refer to this alloy as “catalyst”, as typically followed in literature. The traditional VLS mechanism assumes that the material is collected directly from the vapor by the liquid alloy.1 This need not always be the case; it is possible that the reactants are also adsorbed on the substrate surface or the surface of the NWs and they diffuse toward the catalyst where they are incorporated into the catalyst, called diffusion-induced growth (Figure 1). The correlation between diameter and length of the NWs can give useful information on the mode in which the reactants reach the catalyst. In Si NWs grown by molecular beam epitaxy (MBE) using gold catalyst, the length was found to be inversely proportional to the diameter and was explained by adatom diffusion on the surface of the whiskers.9 Later, a similar length-diameter inverse relation was also reported in the gold-catalyzed growth of NWs of GaAs10,11 and InP.12 In GaN NW systems a similar inverse relationship was reported in growths without any external catalyst.13,14 However, an increasing growth rate as a function of diameter was observed in InP,15 Si,16,17 and Ge18 NWs and was explained on the basis of the Gibbs−Thomson effect (discussed later). Because of the opposing effects of Gibbs−Thomson effect and diffusion limited growth, it is possible that the length-diameter dependence varies nonmonotonically. This has been reported

sing semiconductor nanowires (NWs) for electronics, optics, and sensor applications requires precise control on the physical properties of the NWs, for which an understanding of the underlying growth mechanism is essential. Nanowires of many III−V semiconductors are grown using a metal-catalystmediated route. In most cases, a vapor−liquid−solid (VLS) process1,2 has been invoked to explain the growth of such NWs. While there are several reports on metal-assisted growth of gallium nitride (GaN) nanowires,2−8 there is still no clear consensus and understanding of the growth mechanisms involved. In this work, we examine two key aspects that provide insight into the mechanism of nickel catalyst mediated GaN NW growth. The first, studied by looking at the length thickness dependence of the NWs, explains how precursors reach the catalyst particle: directly from the vapor phase or also via adatom diffusion along the NW surface. The second aspect, studied via detailed postgrowth compositional analysis, attempts to verify if the growth is mediated by a solid or liquid catalyst. The catalytic NW growth process has been typically described in the following manner.1 A metal particle helps in the formation of an alloy of relatively low melting point forming liquid droplets at typical growth temperatures. This liquid droplet acts as a preferred site for deposition of precursor atoms from the vapor and eventually becomes supersaturated. This leads to the semiconductor material being precipitated out as a solid, limited in size by the droplet. This process continues as long as the supersaturation is maintained by the supply of precursors, which results in the growth of the nanowire. The liquid alloy may act as either (i) material sink due to lower © 2016 American Chemical Society

Received: August 26, 2016 Revised: November 8, 2016 Published: November 16, 2016 7632

DOI: 10.1021/acs.nanolett.6b03604 Nano Lett. 2016, 16, 7632−7638

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account the size dependence of melting point for the catalyst particle, which has not been considered previously. Experimental Details. GaN NWs were grown on various planes of sapphire substrates in a showerhead MOCVD system. Details of the growth parameters used and a comparison of NW growth on the different sapphire orientations have been reported elsewhere.21 In brief, Ni(NO3)2·6H2O solution was coated on the substrate and annealed in an H2 ambient to form metallic Ni particles in situ, which subsequently act as the catalyst. Trimethylgallium and ammonia were used as precursors. Thin NWs of triangular cross-section were grown in a nitrogen ambient. The thermocouple set point temperature during growth was ∼840 °C. The susceptor temperature was calibrated against a blackbody standard using an in situ emission-corrected pyrometer head. The Ni-coated sapphire pieces were kept on top of a dummy wafer (sapphire) in the 2 inch diameter wafer pocket. Using in situ pyrometry, we measured a temperature difference of ∼30 °C across the thickness (330 μm) of a sapphire substrate at the growth conditions. Therefore, the temperature at the growth surface is likely to be lower by ∼50−60 °C (details given in Supporting Information section A), giving a growth temperature of ∼780− 790 °C. The morphology of individual NWs was studied using a Zeiss ULTRA plus scanning electron microscopy (SEM) system with a field emission gun. The composition and phase of the catalyst particle was analyzed postgrowth using EDX. The EDX results were calibrated using a Ni2Ga3 bulk crystal grown by flux method. EELS measurements for catalyst composition were done in an FEI Titan aberration-corrected scanning transmission electron microscope at 300 kV. Results and Discussion. Length−Thickness Dependence. A side-view SEM image of a sample of GaN NWs grown on rplane sapphire is shown in Figure 2, panel a. We can clearly see that the very thick wires are relatively short, while the long NWs are very thin. To analyze the length−thickness correlation in more detail, we have measured the thickness and length of more than 130 GaN NWs; the individual data points are plotted in Figure 2, panel b. The thickness was measured close to the catalyst. (Our NWs show minimal tapering of the order of ∼1 nm/μm as discussed in Supporting Information, section B.) We have used only those nanowires at the end of which a catalyst particle is clearly visible so as to avoid any broken ones. Moreover, we have taken into account an important geometric factor to accurately estimate the wire length by looking only at the NWs that are at an angle of 30° from the substrate normal. Since the plane containing the NW growth axis and the surface normal is not in the plane obtained on cleaving, but is at about 48° rotated with the cleave plane, the projection will appear at about 22° in the SEM image, as is seen in Figure 2, panel a. The length reported here has been corrected for this geometric factor. (For further details, please refer to Supporting Information, section C.) Qualitatively, a similar length− thickness dependence was seen for NWs grown on c-plane sapphire substrates and is discussed in Supporting Information, section D. The envelope of the data points in the length-thickness plot (gray line in Figure 2b) is a function where the length increases with increasing thickness until a thickness of ∼23 nm. Beyond ∼23 nm, the length decreases as the thickness increases. This type of line shape for the length−thickness dependence has been reported and modeled for MOCVD grown Si NWs20 and chemical beam epitaxy grown InAs NWs19 using an interplay of

Figure 1. Schematic showing the three different modes by which reactants reach the catalyst particle. The supplied precursor molecules can be collected (i) from the vapor phase directly into the catalyst, (ii) from adatoms on the NW side facets diffusing into the catalyst, or (iii) from adatoms on the substrate surface that diffuse along the substrate and NW side surfaces into the catalyst.

in the Au assisted growth of InAs,19 Si, GaAs, and GaP.20 However, to the best of our knowledge, there are no reports on modes of reactant collection during growth of GaN NWs using any external catalyst. Since it is important to understand what is the rate limiting factor in catalyst assisted GaN NW growth and how the reactants are reaching the catalyst, we have studied the length versus thickness distribution of GaN NWs grown by metal-organic chemical vapor deposition (MOCVD) using a Ni-based catalyst. As these nanowires have a triangular crosssection (roughly equilateral),21 we describe the lateral dimensions of the NW by the side of triangle and refer to this as the NW “thickness”. In most reports of semiconductor nanowires, the cross-section is hexagonal, which is approximated to a cylinder and all theoretical models generally assume a cylindrical NW geometry and hence use “diameter” as the relevant lateral dimension. In most reports on metal assisted nanowire growth, the mechanism has been attributed to VLS without detailed studies on the particle phase. However, instead of a liquid alloy as in the case of VLS growth, a solid alloy can also catalyze NW growth; such a mechanism is called vapor−solid−solid (VSS) mechanism and was proposed in 2001 by Kamins et al.22 It was later verified in GaAs23 and InAs24 NWs. The growth of GaN nanowires has been reported to proceed via the VLS process using different metal-based catalysts like gold,3 nickel,2−4 indium,2 cobalt,2 iron,3 and platinum.5 However, it has also been suggested that the Ni-assisted growth of GaN by MBE very likely proceeds by a VSS mechanism based on postgrowth compositional analysis of the catalyst tip6 and by in situ reflection high-energy electron diffraction (RHEED) and mass spectroscopy.7 However, as the RHEED technique requires ultra high vacuum, it cannot be used for in situ monitoring during MOCVD growth of NWs. There is another report that proposes that the catalyst has a solid part as well as a liquid part.8 The lack of consensus on the nature of the Ni-based catalyst particle has motivated us to perform a detailed study of the composition of the catalyst particle using energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS), and transmission electron microscopy (TEM) and verify if the growth mechanism is indeed VLS or VSS. We have analyzed the composition of more than 60 catalyst particles to obtain statistically significant data, unlike previous reports, for example, Weng et al., where EDX spectra from just a couple of wires were reported.6 Further, our analysis also takes into 7633

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reach the catalyst are not very significant. However, for comparatively thicker NWs, the limiting factor is how precursor materials are being brought to the catalyst surface. According to the diffusion-induced growth model,19,20 in addition to direct absorption of material into the catalyst, the adatoms diffuse to the catalyst both from the surface of substrate and the exposed nanowire facets due to lower chemical potential. In this diffusion-dominated regime, the growth rate decreases with increasing thickness. Simplified growth models can crudely predict the length− diameter correlation of NWs in the diffusion-limited growth regime.25 Consider a cylindrical NW of radius R and let λf and λs be the adatom diffusion lengths on the side facets of the NW and on the substrate, respectively. On assuming that the adatoms are being collected from a cylindrical area of 2πRλf on the NW sidewall or a disc of radius λs on the substrate surface of area πλs2 and are being deposited on area of πR2 on top of the NW, it can be shown that the growth rate will vary inversely as a function of radius. Combined Model. We observe an initial increase in length with increasing thickness due to Gibbs−Thomson effect and then a decrease as a consequence of the diffusion-induced growth mechanism. Such a length−diameter dependence has been modeled by a couple of groups imposing approximate boundary conditions on the adatom diffusion equation, assuming that the wires are cylindrical with a hemispherical catalyst.19,20 By fitting our data with these models, we can estimate the Ga adatom diffusion length and the characteristic Gibbs−Thomson radius. A plot of the maximum lengths obtained for different thicknesses (circles) and fits using two theoretical models (lines) are shown in Figure 3, panels a and b. The blue line in Figure 3, panel a is the fitted curve using eq 4 of Fröberg et al.,19 and the line in Figure 3, panel b is obtained using eq 9 of Dubrovskii et al.,20 both assuming a growth rate independent of time. Using ref 19, we obtain a Ga adatom diffusion length of 510 nm and a characteristic Gibbs−Thomson radius of 6 nm (Figure 3a), while using ref 20, we obtain Ga adatom diffusion length on the NW as 527 nm and a characteristic Gibbs− Thomson radius of 7 nm (Figure 3b). Thus, both models seem to be in reasonable agreement with each other. Why a Distribution? For the models of diffusion-induced NW growth discussed in literature, the length−diameter plot is a smooth curve.19,20 However, they have not specified if it is the average length or the maximum length. In Figure 2, panel b, each data point is from a different NW from the ensemble growth, and we have not done any averaging. We observe that for roughly the same thickness, different NWs have different lengths. The maximum length obtained for small intervals of thickness forming the envelope of this distribution has a line shape similar to the curves reported in the literature.19,20 This difference can be attributed to several factors. First, the nonuniformity of the areal distribution of nanowires, which is inherent to the Ni compound deposition scheme (drop casting and drying), affects the growth rate. Especially in the diffusion limited regime, NWs compete with their neighboring NWs for material,26,27 and hence, a lower density of catalyst particles would result in a faster growth rate locally, whereas in regions with a dense distribution of catalyst particles, the growth rate would be reduced. The nearest neighbor distance in these NWs vary in the range of 50−300 nm. Another important factor is the existence of an initial delay time before the wire growth actually starts as discussed below.

Figure 2. (a) Side-view SEM image of the GaN NWs grown on rplane sapphire. It is evident that the thick wires are short, and the thin wires are long. Enlarged versions of a couple of representative NWs are shown at the bottom. The scale bars correspond to 1 μm. (b) Plot showing the distribution of length and thickness for more than 130 NWs. Gray curve shown is just a guide to the eye showing the envelope of the obtained distribution.

the Gibbs−Thomson effect and a simplified diffusion-based mechanism.25 Gibbs−Thomson Effect. A liquid or solid particle has an additional pressure as compared to the surrounding vapor owing to the surface energy, called the Laplacian pressure.25 It depends on system geometry and scales inversely with the size of particle. As a consequence, the equilibrium chemical potential in both these phases is also geometry dependent. This leads to an exponential increase in vapor pressure with decreasing particle size, called the Gibbs−Thomson (GT) effect.25 For nanowire growth, this would imply that, for a fixed vapor pressure, smaller catalyst particles will desorb more atoms than the larger particles and hence suppress the nanowire growth. As a consequence, growth from catalysts smaller than a certain critical radius will be blocked. Above this critical radius, as the diameter of the catalyst increases, the growth rate also will increase. (The thinnest GaN NW we have observed has a thickness of ∼14 nm, measured toward the catalyst end of the wire, corresponding to a catalyst radius of ∼7 nm.) If the reactants are directly absorbed onto the catalyst from the vapor phase only, the Gibbs−Thomson effect will dominate for all diameters and the growth rate will keep increasing with diameter. However, in Figure 2, panel b, we see that beyond ∼23 nm, the length decreases with increasing thickness. This can be explained as a consequence of a diffusion-based growth mechanism discussed below. Diffusion-Induced Growth. For very thin nanowires, as discussed above, the curvature of the catalyst particle restricts the growth rate by limiting material incorporation into the catalyst and eventually into the NW. In this regime, the growth rate increases with thickness, and the details of how precursors 7634

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EDX Analysis. EDX data were acquired in a scanning electron microscope using spot mode. The excitation was with 20 keV electrons. The system was calibrated with a cobalt standard sample. The accuracy of the Ni and Ga quantification was verified using a Ni2Ga3 single crystal. Some NWs were mechanically transferred to a clean silicon piece. The sample was then coated with a thin layer of carbon to minimize drifting induced by electrostatic charging. The EDX signal will be from a volume larger than the catalyst particle itself. Therefore, even if we position the electron beam spot well within the catalyst, it is possible that the signal obtained is partly from the NW itself and hence will show a larger Ga content. To remove this inaccuracy, we have looked at the N signal in the EDX spectrum (which would have come only from the NW), estimated the Ga contribution from the NW part, and subtracted it from the total Ga atomic percentage. Plot of atomic ratio of Ga to Ni of about 60 catalyst particles is shown in Figure 4. The ratio of atomic concentrations of Ga to Ni present in the catalyst was found to be 1.46 ± 0.26. This suggests that the catalyst is Ni2Ga3. Figure 3. Fitting of the envelope of the length−thickness distribution shown in Figure 2 using models described in (a) Fröberg et al.19 and (b) Dubrovskii et al.20 The ordinate in panel b is L/H where L is the length of the wire and H is the height of the film-like layer of GaN below the NW. Data are shown by violet dots, and the blue line is the obtained fit. The parameters obtained by the fitting shown in panel a yield a Ga adatom diffusion length of 510 nm and a characteristic Gibbs−Thomson radius of 6 nm. The blue line in panel b is with Ga adatom diffusion length of 121 nm on the substrate surface and 527 nm on the NW side facets and a characteristic Gibbs−Thomson radius of 7 nm.

If the different NWs of the same thickness start growing at random times then for the same thickness, the length of the wires which started growing early will be obviously longer than the ones that started late. This can also give rise to a distribution as seen in Figure 2. The initial delay in starting the nanowire growth can be due to either (a) the delay in collecting enough Ga to form the required Ni−Ga catalyst or (b) time lag before supersaturation in the catalyst, (c) delay between nucleation of the wires after attaining supersaturation (corresponding to the incubation time as seen in classical nucleation theory),28 or a combination of the above effects. Composition and Phase of Catalyst. So far, we have considered how reactants reach the catalyst tip and the resultant consequences on the length-thickness dependence of the NWs. We will next discuss the second aspect of understanding the growth mechanism of Ni-catalyzed GaN NWs, that is, was the growth VLS or VSS? While the composition and phase of the catalyst during the nanowire growth can, in general, be different from what it becomes after the growth, we do not have the provision to study this in situ during the growth process. However, postgrowth analysis of the catalyst particle can, in itself, provide interesting insights as discussed below. On knowing the composition of the catalyst particle and comparing it with the Ni−Ga phase diagrams (see Supporting Information, section E) in literature, one may be able to find out if the catalyst particle was a solid or a liquid at the growth temperature. The different Ga−Ni alloy phases shown in phase diagram are Ni5Ga3 rt, Ni2Ga3, Ni0.97Ga3.62, NiGa, Ni1.8Ga ht1, Ni3Ga, Ni13Ga9 rt, Ni3Ga4 rt, Ni, and Ga.29−33

Figure 4. Ga/Ni atomic ratio measured by EDX from the catalyst tip from different NWs is shown. We see that the Ga/Ni ratio is close to 1.5, which indicates that the catalyst is Ni2Ga3. As a guide to the eye, the composition Ni2Ga3 is depicted by the blue line. The Ni−Ga alloy, with the minimum Ga content, that will be liquid at the growth temperature (∼780 °C) is indicated by the green line.

EELS. An independent estimate of the catalyst composition was also obtained from the EELS spectrum that was recorded across the catalyst particle along a line (Figure 5 inset). The normalized atomic % of Ga and Ni was calculated34,35 and is shown in Figure 5. We see that in the catalyst the atomic ratio Ni/Ga is ∼1.5, which corresponds to the Ni2Ga3 phase. This composition matches well with that obtained from the EDX data. TEM Analysis. The bright field TEM image of a catalyst particle at the end of a NW (Figure 6a) shows the single crystalline nature of the catalyst. From the FFT of catalyst part of the image, we obtain a regular hexagonal pattern (Figure 6 b). The spots closest to the center arise from crystal planes with an interplanar spacing of about 2.04 Å. This FFT pattern can be fitted to only two of the Ni−Ga intermetallic phases reported in the literature: (i) Ni2Ga3 with hexagonal crystal structure having unit cell lattice parameters a = 4.054 Å and c = 4.387 Å (the closest spots in the FFT pattern arise from the {101̅0} (d = 2.03 Å)); or (ii) NiGa with Pm3̅m cubic structure where a = 2.87 Å (nearest spots from {112̅0} planes). Thus, the TEM 7635

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Figure 5. Normalized EELS quantification across the catalyst. EELS shows that the composition of the catalyst is 42 ± 3% Ni and 58 ± 3% Ga, corresponding to Ni2Ga3 phase. HAADF STEM image of the nanowire and catalyst is shown in the inset. The EELS data were collected along the dashed line indicated. Scale bar is 10 nm.

data also support the catalyst particle being in the Ni2Ga3 phase. Catalyst Composition during Growth. Thus, from the EDX, EELS, and TEM analysis, we conclude that the catalyst particle after cooling down from the growth temperature to room temperature in an N2 atmosphere is in the Ni2Ga3 phase. Purushothaman et al. had reported that the composition of the catalyst after growth becomes Ni3Ga from EDX data.36 However, in those experiments, the samples were cooled in ammonia, and hence it is possible that some Ga in the catalyst particle could have further reacted with the NH3 while cooling down. Hence, the authors speculated that the catalyst was the “catalytically active metastable Ga3Ni2 phase”. A couple of other groups also reported that the catalyst was Ni3Ga based on EDX and TEM for wires grown by MOCVD6 and based on EELS for PAMBE grown wires.37 However, they do not discuss the cooling conditions. In our case, cooling was done in N2 and not NH3, which minimized the chance of a chemical reaction in the catalyst. Hence, we believe that the catalyst composition was Ni2Ga3 during NW growth as well. Growth Mechanism: VSS or VLS? From the Ga−Ni phase diagrams, we see that Ni2Ga3 decomposes at ∼985 °C or above and will liquify completely at ∼1120 °C.31,33 (Note that the phase diagrams given in these references do not exactly match each other.29−33 The transition temperatures reported in these references also vary by about ∼100 °C. The temperature values quoted here are the lowest among the Ni−Ga phase diagrams reported in literature.) The phase diagram also shows that for a Ga−Ni alloy to be liquid at the growth temperature of ∼780 °C, the Ga content should be more than 85%.29−32 Hence, if the catalyst composition was the same during growth (which was at ∼780 °C), then the catalyst would have been solid during the NW growth also, at least for thicker NWs where size-dependent reduction of melting point can be neglected. The reported phase diagrams are based on experiments with bulk quantities and need not be strictly valid for nanoparticles. The reduction of melting point can be estimated by using another model based on the effective reduction of cohesive energy and experimentally verified for different systems.38 Accordingly, the melting point of a spherical nanoparticle, Tmnano, differs from the bulk melting point, Tmbulk by the equation

Figure 6. (a) TEM image (processed) of the NW along with catalyst particle. The crystallinity of the catalyst and the NW is evident. The Fourier transform of different parts namely (b) from the catalyst part, (c) from the NW part, and (d) from the entire area shown in panel a are also shown. (e) Higher magnification STEM image taken at the interface.

⎛ 2d ⎞ ⎟ Tm nano = Tmbulk ⎜1 − ⎝ D⎠

(1)

where d and D are the diameters of the atom and nanoparticle, respectively. So for the smallest catalyst particle we observed in TEM, with D = 14 nm, Ni2Ga3 will decompose at ∼845 °C and it will liquify completely at ∼1060 °C. The model used is for a nanosphere with a completely exposed surface, whereas in our nanowires a part of the catalyst is in contact with the nanowire. Hence, the calculations overestimate the melting point depression, that is, the phase transition temperatures will be at a higher temperature than what we just calculated. Thus, even for the thinnest NWs, the melting point of the catalyst particle is well above the growth temperatures. For thicker wires, the melting point depression is in any way not very significant. This suggests that our GaN NW growth would have most likely progressed via the VSS mechanism for all diameters. 7636

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Nano Letters “Catalytic” Role of Ni−Ga Alloy? We would like to discuss the role played by the Ga−Ni alloy droplet in growth of GaN NWs via MOCVD. For samples that are grown without any Ni coating at the optimized conditions, we do not observe any NWs, implying that the presence of Ni is a necessary condition for NW growth. If the Ni-based alloy is acting only as a seed for the nucleation, similar to nucleation at say a step on the substrate, then this alloy would be present at bottom of the NW rather that at the growing end. Since we observe the catalyst tip at the growing end of the NW, the Ni-based alloy can be either acting either as catalyst or a material collector or both. Since the Ga adatoms diffuse toward Ni droplet and form a Ni−Ga alloy, it is definitely acting as a material collector or sink with a lower chemical potential compared to the surrounding. However, we also believe that unlike the VLS growth of GaAs or InAs NWs using a Au catalyst, where the role of the Au−Ga alloy is primarily that of a material collector, in the case of GaN NW growth, the Ni-based alloy acts as a chemical catalyst as well. Conventionally, MOCVD growth of GaN films is carried out under NH3 rich conditions (i.e., very high V/III ratios in the order of 1000 and higher temperature ∼1040 °C), mainly due to the poor cracking of NH3 at lower temperatures. Here we are growing the NWs with a V/III ratio as low as ∼5 (i.e., much lesser NH3 than for planar growth). Additionally, since the growth temperature is lower by about 300 °C, the thermal decomposition of NH3 will also be significantly lower. Under such circumstances, we would expect that there will be an excess of TMGa resulting in Ga droplets on the substrate surface, but we observe GaN NWs and not Ga droplets, which suggest that NH3 decomposition or N incorporation into the NW has been catalyzed by the presence of the Ni based alloy particle. The fact that Ni based structures are being extensively used as catalyst for NH3 decomposition39,40 also supports our claim that the Ni-based alloy acts as a chemical catalyst in addition to being a material sink. Growth Model. Trimethylgallium decomposes by a complicated multistep process to leave Ga adatoms on the substrate surface. These Ga adatoms form an alloy with Ni nanoparticles left behind by the decomposition of Ni(NO3)2· 6H2O on annealing.21 Since N has very low miscibility in Ga− Ni system, hardly any N enters the catalyst particle. NH3 decomposes catalytically at the Ni−Ga alloy, gives rise to active nitrogen species that react with Ga at the triple phase line (i.e., the circumference of the NW-catalyst intersection where the NW and catalyst meet), and precipitates as GaN leading to nanowire growth. Conclusions. We have performed comprehensive structural characterization of GaN nanowires grown using a Ni-based nanoparticle seed. An analysis of the length-thickness dependence for ∼130 wires shows that as a function of thickness, the NW length initially increases (until ∼23 nm thickness) and then decreases. This shows that the Gibbs−Thomson effect is dominant at very low thickness, but the diffusion-induced growth dominates at larger thickness. Fitting this behavior to the models of Fröberg et al.19 and Dubrovskii et al.20 yields similar values of the characteristic Gibbs−Thomson radius and Ga adatom diffusion length. A postgrowth composition analysis of ∼60 catalyst particles using EDX shows it to be in the Ni2Ga3 phase, which is also confirmed by EELS and TEM measurements. Since the melting point of Ni2Ga3, even taking size dependent melting point depression into account, is at least ∼50 °C above growth temperature, we conclude that the

catalyst is a solid at the growth temperature. Hence, the Niassisted growth of GaN proceeds via a VSS mechanism.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03604. Estimation of actual growth temperature; tapering of GaN NWs; calculation of projected angle for GaN NWs grown on r-plane sapphire; length-thickness dependence on c-plane sapphire; Ni−Ga phase diagram (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: +91 22 22782517. Fax: +91 22 22804610. ORCID

Carina B. Maliakkal: 0000-0003-3169-2831 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank A. Thamizhavel for guidance in growing Ni2Ga3 crystal, Srinivasan Raghavan for discussions, and S. C. Purandare for supervision during TEM imaging. This work at TIFR was supported through internal Grant Nos. 12P0168 and 12P0169.

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REFERENCES

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DOI: 10.1021/acs.nanolett.6b03604 Nano Lett. 2016, 16, 7632−7638

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DOI: 10.1021/acs.nanolett.6b03604 Nano Lett. 2016, 16, 7632−7638