The Mechanism of Ni-Assisted GaN Nanowire Growth - Nano Letters

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The mechanism of Ni-assisted GaN nanowire growth Carina B Maliakkal, Nirupam Hatui, Rudheer D. Bapat, Bhagyashree Abhay Chalke, A Azizur Rahman, and Arnab Bhattacharya Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03604 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016

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

Abstract Despite the numerous reports on the metal-catalyzed 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 eect dominant in very thin nanowires and a diusion 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 Ni2 Ga3 . 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. 1

ACS Paragon Plus Environment

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Diusion-induced growth, Vapor-Solid-Solid mechanism, GaN, nanowire, MOCVD

Introduction Using semiconductor nanowires (NWs) for electronics, optics and sensors 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-catalyst-mediated route. In most cases a "Vapor-Liquid-Solid" (VLS) process

1,2

has been invoked to explain the growth of such NWs.

While there are several

reports on metal-assisted growth of gallium nitride (GaN) nanowires,

28

clear consensus and understanding of the growth mechanisms involved.

there is still no

In this work, we

examine two key aspects that provide insight into the mechanism of nickel catalyst mediated GaN NW growth. The rst, 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 diusion along the NW surface. The second aspect, studied via detailed post-growth 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, that is liquid and forms a droplet 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, resulting in the growth of the nanowire. The liquid alloy may act as either (i) as material sink due to lower chemical potential and

/

or (ii)

a chemical catalyst. Irrespective of its role we refer to this alloy as `catalyst', as typically followed in literature.

2


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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 diuse towards the catalyst where it is incorporated into the catalyst, called diusion-induced growth (Figure 1). The correlation between diameter and length of the NWs can give useful information of 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 diusion 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 GaAs,

10,11

and InP.

12

In GaN NW systems also a similar inverse relationship was

reported in growths without any external catalyst. a function of diameter was observed in InP,

15

Si

13,14

16,17

But, an increasing growth rate as

and Ge

the basis of the Gibbs-Thomson eect (discussed later).

18

NWs and was explained on

Due to the opposing eects of

Gibbs-Thomson eect and diusion limited growth it is possible that the length-diameter dependence varies non-monotonically. This has been reported 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 MOCVD using a Ni-based catalyst. As these nanowires have a triangular cross section (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

3


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Directly from vapour TMGa

TMGa

Ga

Diffusing along NW facets

Diffusing from substrate surface Ga

Figure 1:

A schematic showing the three dierent 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 diusing into the catalyst, and/or (iii) from adatoms on the substrate surface which diuse along the substrate and NW side surfaces into the catalyst.

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 was later veried in GaAs

23

and InAs

24

et al. 22

It

NWs.

The growth of GaN nanowires has been reported to proceed via the VLS process using dierent metal-based catalysts like gold,

3

nickel,

24

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 post-growth compositional analysis of the catalyst tip

6

spectroscopy.

and by in-situ reection high-energy electron diraction (RHEED) and mass

7

But as the RHEED technique requires ultra high vacuum it cannot be used

for in-situ monitoring MOCVD growth of NWs.

There is another report which 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

4


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verify if the growth mechanism is indeed VLS or VSS. We have analyzed the composition of more than 60 catalyst particles to obtain statistically signicant data, unlike previous reports e.g.

Weng

et al.,

where EDX spectra from just a couple of wires were reported.

6

Further, our analysis also takes into 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 elsewere.

21

In brief, Ni(NO 3 )2 .6H2 O solution

was coated on the substrate and annealed in an H 2 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 setpoint temperature during growth was

∼ 840 ◦ C. The susceptor temperature

is calibrated against a black-body 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 diameter wafer pocket. Using in-situ pyrometry we measured a temperature dierence of

∼ 30 ◦ C

across the thickness (330

µm)

of a sapphire substrate at the growth conditions. So

the temperature at the growth surface is likely to be lower by

∼ 50 − 60 ◦ C

Supporting Information section A), giving a growth temperature of

(details given in

∼ 780 − 790 ◦ C.

The morphology of individual NWs was studied using a Zeiss ULTRA plus scanning electron microscopy (SEM) system with a eld emission gun. The composition and phase of the catalyst particle was analyzed post-growth, using energy dispersive x-ray spectroscopy (EDX). The EDX results were calibrated using a Ni 2 Ga3 bulk crystal grown by ux method. Electron energy loss spectroscopy (EELS) measurements for catalyst composition were done in an FEI Titan aberration-corrected scanning transmission electron microscope at 300 kV.

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Results and discussion Length - thickness dependence (a)

1 µm

(b)

6

Length (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 2 0 0

Figure 2:

20

40

60

80

100

Nanowire Thickness (nm)

120

140

(a) A side-view SEM image of the GaN NWs grown on r-plane sapphire. It is

evident that the thick wires are short and the thin wires are long. An enlarged version of a couple of representative NWs are shown at the bottom. The scale bars correspond to 1 (b) Plot showing the distribution of length and thickness for more than 130 NWs.

µm.

Grey

curve shown is just a guide to the eye showing the envelope of the obtained distribution.

A side-view SEM image of a sample of GaN NWs grown on r-plane sapphire is shown in Figure 2a. We can clearly see that the very thick wires are relatively short while the long NWs are very thin. In order 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

6


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points are plotted in Figure 2b. 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 in order to accurately estimate the wire length, 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 2a. The length reported here has been corrected for this geometric factor. (For further details please refer 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 (grey line in Figure 2b) is a function where the length increases with increasing thickness till a thickness Beyond

∼ 23 nm the length decreases with increasing thickness.

∼

23 nm.

This type of lineshape for the

length-thickness dependence has been reported and modeled for MOCVD grown Si NWs and chemical beam epitaxy grown InAs NWs

19

20

using an interplay of the Gibbs-Thomson

eect and a simplied diusion-based mechanism.

25

Gibbs-Thomson eect 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) eect.

25

For nanowire growth this would imply that, for a xed vapor pressure, smaller

catalyst particles will desorb more atoms than the larger particles, and hence suppress the

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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 towards 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 eect will dominate for all diameters and the growth rate will keep increasing with diameter.

However, in Figure 2b we see that beyond

∼

23 nm

the length decreases with increasing thickness. This can be explained as a consequence of a diusion-based growth mechanism discussed below.

Diusion-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 reach the catalyst is not very signicant.

However, for comparatively thicker

NWs, the limiting factor is how precursor materials are being brought to the catalyst surface. According to the diusion-induced growth model,

19,20

in addition to direct absorption of

material into the catalyst, the adatoms diuse to the catalyst both from the surface of substrate and the exposed nanowire facets due to lower chemical potential. In this diusion dominated regime the growth rate decreases with increasing thickness. Simplied growth models can crudely predict the length-diameter correlation of NWs in the diusion-limited growth regime. and

λs

25

Consider a cylindrical NW of radius R and let

λf

be the adatom diusion 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 2π Rλf on the NW sidewall or a disc of radius being deposited on area of

πR2

λs

on the substrate surface of area

πλ2s

are

on top of the nanowire it can be shown that the growth rate

will vary inversely as a function of radius.

8


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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 diusion-induced growth mechanism. Such a length-diameter dependence has been modeled by a couple of groups imposing approximate boundary conditions on the adatom diusion equation, assuming that the wires are cylindrical with a hemispherical catalyst.

19,20

By tting our data with these models we can

estimate the Ga adatom diusion length and the characteristic Gibbs-Thomson radius. A plot of the maximum lengths obtained for dierent thicknesses (circles) and ts using two theoretical models (lines) are shown in Figure 3 a and b. The blue line in Figure 3a is the tted curve using Equation 4 of using Equation 9 of Dubrovskii Using Ref.

19

Fröberg et al. 19

et al., 20

and the line in Figure 3b is obtained

both assuming a growth rate independent of time.

we obtain a Ga adatom diusion length of 510 nm and a characteristic Gibbs-

Thomson radius of 6 nm (Figure 3a).

While using Ref.

20

we obtain Ga adatom diusion

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 diusion-induced NW growth discussed in literature, the length-diameter plot is a smooth curve.

19,20

But they have not specied if it is the average length or the

maximum length. In Figure 2b each data point is from a dierent NW from the ensemble growth and we have not done any averaging. We observe that for roughly the same thickness dierent NWs have dierent lengths. The

maximum

length obtained for small intervals of

thickness forming the envelope of this distribution has a lineshape similar to the curves reported in literature.

19,20

This dierence can be attributed to several factors. Firstly, the

non-uniformity of the areal distribution of nanowires, which is inherent to the Ni compound deposition scheme (drop casting and drying), aects the growth rate.

Especially in the

diusion limited regime, NWs compete with their neighboring NWs for material

9


26,27

and

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Length (µm)

(a) 8 6 4 2 0

0

20

0

20

(b)

40

60

80

120 140

100


30 25 20 15

L/H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 5 0

Figure 3:

40

60

80

100

120


140

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 (b) is L/H where L is the length of the wire and H is the height of the lm-like layer of GaN below the NW. Data is shown by violet dots and the blue line is the obtained t. The parameters obtained by the tting shown in (a) yields a Ga adatom diusion length of 510 nm and a characteristic Gibbs-Thomson radius of 6 nm. The blue line in (b) is with Ga adatom diusion 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.

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 to 300 nm. Another important factor is the existence of an initial delay time before the wire growth actually starts as discussed below. If the dierent 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

10


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than the ones which 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 eects.

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 i.e.

was the growth

VLS or VSS? While the composition and phase of the catalyst during the nanowire growth can, in general, be dierent from what it becomes after the growth, we do not have the provision to study this in-situ during the growth process.

However, post-growth 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 nd out if the catalyst particle was a solid or a liquid at the growth temperature. The dierent Ga-Ni alloy phases shown in phase diagram are Ni 5 Ga3 rt, Ni2 Ga3 , Ni0.97 Ga3.62 , NiGa, Ni1.8 Ga ht1, Ni3 Ga, Ni13 Ga9 rt, Ni3 Ga4 rt, Ni and Ga.

2933

EDX Analysis EDX data was 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 quantication was veried using a Ni 2 Ga3 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

11


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6.0 5.5

Ga/Ni in atomic %

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5.0 4.5

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Figure 4:

0

10

20

30

40

NW number

50

60

Ga:Ni atomic ratio measured by EDX from the catalyst tip from dierent NWs

is shown in the gure. We see that Ga:Ni ratio is close to 1.5 indicating that the catalyst is Ni2 Ga3 . As a guide to the eye the composition Ni 2 Ga3 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.

signal will be from a volume larger than the catalyst particle itself. So 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 Ni 2 Ga3 .

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 calculated 34,35

in the catalyst the atomic ratio Ni /Ga is

and is shown in Figure 5. We see that

∼1.5 which corresponds to the Ni 2 Ga3

composition matches well with that obtained from the EDX data.

12


phase. This

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Ni% Ga%

70 60

Ga/Ni in atomic %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 40 30 20 10 0 0

Figure 5:

10

20

30

40

Distance (nm)

50

60

Normalized EELS quantication across the catalyst. EELS shows that the com-

position of the catalyst is 42

±

3

%

Ni and 58

±

3

%

Ga, corresponding to Ni 2 Ga3 phase.

HAADF STEM image of the nanowire and catalyst is shown in the inset. The EELS data was collected along the dashed line indicated. Scale bar is 10 nm.

TEM Analysis The bright eld 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 inter-planar spacing of about 2.04 Å. This FFT pattern can be tted to only two of the Ni-Ga intermetallic phases reported in the literature: (i) Ni 2 Ga3 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 arises from the

P m3m

(d=2.03Å)) or (ii) NiGa with

cubic structure where a =2.87 Å (nearest spots from [11¯ 20] planes). Thus, the TEM

data also supports the catalyst particle being in the Ni 2 Ga3 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 N 2 atmosphere is in the Ni2 Ga3 phase.

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(a)

2 nm (b)

catalyst (c)

NW

(d)

(e)

NW

catalyst Figure 6:

1 nm

TEM image (processed) of the NW along with catalyst particle is shown in (a).

The crystallinity of the catalyst and the NW is evident. The Fourier transform of dierent parts namely (b) from the catalyst part, (c) from the NW part and (d) from the entire area shown in (a) are also shown. (e) Higher magnication STEM image taken at the interface.

Purushothaman

et al.

had reported that the composition of the catalyst after growth

becomes Ni3 Ga from EDX data.

36

But 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 NH 3 while cooling down. Hence the authors speculated that the catalyst was

14


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the "catalytically active metastable Ga 3 Ni2 phase". A couple of other groups also reported that the catalyst was Ni 3 Ga based on EDX and TEM for wires grown by MOCVD on EELS for PAMBE grown wires.

37

6

and based

But they do not discuss about cooling conditions. In

our case cooling was done in N 2 and not NH3 , minimizing the chance of a chemical reaction in the catalyst.

Hence we believe that the catalyst composition was Ni 2 Ga3 during NW

growth as well.

Growth Mechanism: VSS or VLS? From the Ga-Ni phase diagrams we see that Ni 2 Ga3 decomposes at and will liquify completely at

∼ 1120 ◦ C. 31,33

references do not exactly match each other. these references also vary by about

lowest

∼ 100

◦

∼ 985

◦

C or above

(Note that the phase diagrams given in these

2933

The transition temperatures reported in

C. The temperature values quoted here are the

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 content should be more than 85 %. during growth (which was at

2932

∼ 780 ◦ C),

∼ 780

◦

C, the Ga

Hence, if the catalyst composition was the same 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. Unlike some other reports,

36

we had stopped both N and Ga precursors

ows simultaneously after wire growth. So we expect the catalyst composition to remain the same during the cool down. 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 eective reduction of cohesive energy and experimentally veried for dierent systems.

38

Accordingly, the melting point of a spherical nanoparticle,

Tmnano , diers from the bulk melting point, T mbulk by the equation

Tmnano = Tmbulk (1 −

2d ) D

15


(1)

Nano Letters

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Page 16 of 29

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, Ni2 Ga3 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 overestimates the melting point depression, i.e. the these 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 anyway not very signicant. This suggests that our GaN NW growth would have most likely progressed via the VSS mechanism for all diameters.

"Catalytic" role of the Ni-Ga alloy? We would like to discuss the role played by the Ga-Ni alloy droplet in growth of GaN NWs via MOVPE. For samples which 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 diuse towards Ni droplet and form a Ni-Ga alloy, it is denitely 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, MOVPE growth of GaN lms is carried out under NH 3 rich conditions (i.e. very high V/III ratios in the order of 1000 and higher temperature

∼ 1040

16


◦

C), mainly due to the poor

Page 17 of 29

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Nano Letters

cracking of NH 3 at lower temperatures. Here we are growing the NWs with a V/III ratio as low as

∼ 5

(i.e.

much lesser NH 3 than for planar growth).

growth temperature is lower by about

Additionally, since the

300 ◦ C the thermal decomposition of NH 3

will also be

signicantly 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 suggests that NH 3 decomposition or /and 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 NH 3 decomposition

39,40

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(NO 3 )2 .6H2 O on annealing.

21

Since N has very low miscibility in

Ga-Ni system, hardly any N enters the catalyst particle. NH 3 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 (till

∼

23 nm

thickness) and then decreases. This shows that the Gibbs-Thomson eect is dominant at very low thickness but the diusion induced growth dominates at larger thickness. Fitting

17


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

this behavior to the models of

Fröberg et al. 19

and Dubrovskii

Page 18 of 29

et al. 20

yields similar values

of the characteristic Gibbs-Thomson radius and Ga adatom diusion length. A post growth composition analysis of

∼

60 catalyst particles using EDX shows it to be in the Ni 2 Ga3

phase, which is also conrmed by EELS and TEM measurements. Since the melting point of Ni2 Ga3 , 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 Ni-assisted growth of GaN proceeds via a VSS mechanism.

Supporting Information The following topics are discussed in the Supporting Information:

•

Estimating 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

Acknowledgement The authors are thankful to A. Thamizhavel for guidance in growing Ni 2 Ga3 crystal; to Srinivasan Raghavan for discussions; and to S.C. Purandare for supervision during TEM imaging. This work at TIFR was supported through internal grants 12P0168 and 12P0169.

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Nano Letters

Table of Contents Graphic Directly from Ni2Ga3 vapour

TMGa

SOLID

6

Ga

Diffusing from substrate surface


4

2

0

Ga

GT dominated

TMGa

8

Length (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

0

Diffusion limited

40

80

120


22


1 TMGa 2 3 4 5 6 TMGa 7 8 9 10 11 12 13 14

Ni2Ga3 SOLID

6

Ga

Diffusing from substrate surface Ga


4

2

GT dominated

vapour

8 Nano Letters

Length (µm)

Directly Page 23 of 29from

Diffusion limited

ACS Paragon Plus Environment 0 0 40

80

120


Directly fromNano Letters vapour

Page 24 of 29

1 TMGa Ga 2 3 4 Diffusing along 5 6 NW facets TMGa 7 Diffusing 8 from 9 substrate 10 11 surface ACS Paragon Plus Environment 12 13 Ga 14

(a) Page 25 of 29

1 µm

Length (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 (b) 16 17 6 18 19 20 4 21 22 23 2 24 25 26 0 27 28 0 29

Nano Letters

ACS Paragon Plus Environment 20

40

60

80

100


120

140

(a) 8

Page 26 of 29

Length (µm)

6 1 2 3 4 4 5 6 2 7 8 9 0 0 10 11 (b) 1230 13 1425 15 1620 1715 18 1910 20 215 22 230 24 0 25

Nano Letters

40

60

80

100

120 140


L/H

20

ACS Paragon Plus Environment 20 40 60 80 100 120


140

6.0

Ga/Ni in atomic %

Page 5.5 27 of 29

Nano Letters

5.0 4.5

1 4.0 2 3.5 3 4 3.0 5 2.5 6 2.0 7 1.5 8 1.0 9 0.5 100.0 11 12


10

20

30

40

NW number

50

60

Ni% Nano Letters Ga%

70

Page 28 of 29

Ga/Ni in atomic %

60

1 2 50 3 4 40 5 30 6 7 20 8 9 10 10 0 11 12 13


10

20

30

40

Distance (nm)

50

60

(a) Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 216 nm 17 18 (b) 19 20 21 22 23 24 25 26 27 28 29 (d) 30 31 32 33 34 35 36 37 38

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catalyst (c)

NW

(e)

NW


catalyst

1 nm

The Mechanism of Ni-Assisted GaN Nanowire Growth - Nano Letters

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