Structure, Growth Kinetics

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Structure, Growth Kinetics, and Ledge Flowduring Vapor-Solid-Solid Growth ofCopper-Catalyzed Silicon Nanowires

C.-Y. Wen, M. C. Reuter, J. Tersoff, E. A. Stach, and F. M. Ross*,

School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, IBM T.J. Watson Research Center, Yorktown Heights, New York 10598

ABSTRACT We use real-time observations of the growth of copper-catalyzed silicon nanowires to determine the nanowire growth

mechanism directly and to quantify the growth kinetics of individual wires. Nanowires were grown in a transmission electron

microscope using chemical vapor deposition on a copper-coated Si substrate. We show that the initial reaction is the formation of a

silicide, -Cu3Si, and that this solid silicide remains on the wire tips during growth so that growth is by the vapor-solid-solid

mechanism. Individual wire directions and growth rates are related to the details of orientation relation and catalyst shape, leading

to a rich morphology compared to vapor-liquid-solid grown nanowires. Furthermore, growth occurs by ledge propagation at the

silicide/silicon interface, and the ledge propagation kinetics suggest that the solubility of precursor atoms in the catalyst is small,which is relevant to the fabrication of abrupt heterojunctions in nanowires.

KEYWORDS Si nanowires, Cu3Si catalyst, vapor-solid-solid growth mechanism, in situ transmission electron microscopy

The vapor-liquid-solid (VLS) process using Au-Si

liquid catalysts1,2 has become a routine method for

fabricating one-dimensional self-assembled Si nanow-

ires. Although Au is an excellent material for forming

nanowires with controlled morphology, attention has re-

cently focused on the use of alternative, solid catalyst

materials3-8 for two different reasons. First, the catalyst

material can be incorporated into the wire.

9

Since Au isknown to be a deep-level impurity in semiconductors, the

electrical and optical properties of nanowires could in

principle be improved using catalysts other than Au.10,11

Second, many nanowire applications could make use of

heterostructure Si/Ge or Si/SiGe nanowires in which the

composition changes along the length of the nanowires,12-14

preferably with compositionally abrupt and structurally

perfect heterointerfaces.15,16 Formation of abrupt interfaces

using the conventional Au-Si liquid catalyst, however, is

thought to be fundamentally limited because of the high

solubility of Si and Ge in the liquid, resulting in a reservoir

effect that creates a composition gradient at the heterointer-

face.17,18 It has been suggested that solid catalysts may be

advantageous in fabricating heterostructure nanowires due

to the lower solubility of the growth species in the solid.3,4,19,20

It is therefore important to understand issues that arise

during growth with solid catalysts in general.

Copper has been investigated in some detail as a potential

vapor-solid-solid (VSS) alternative to Au.5,21,22 Because Cu

is not as electronically detrimental as Au, it has potential,

for example, for forming nanowires for photovoltaic appli-

cations.11,23 Furthermore, its eutectic temperature (TE) of

802 C24 is relatively high. In principle, VSS growth should

occur, rather than VLS, if the growth temperature is below

the eutectic temperature of the semiconductor and the

metallic starting material.5,21 This implies VSS growth for the

Cu-Si system at around 500-600 C. However, experi-

mental and theoretical studies in several other materials25-28

suggest that liquid catalysts may exist far below TE, stabilized

by size effects or by the supersaturation due to the growth

process itself. In evaluation of a potential VSS material like

Cu, direct evidence of the catalyst state during growth,

determined through in situ observations, is therefore required.

In this paper, we examine in situ the use of copper as a

catalyzing material by carrying out wire growth in an ultra-

high vacuum transmission electron microscope (UHV-TEM).

From the real-time observations we determine the catalyst

structure during growth, confirming the VSS mechanism and

the presence of Cu3Si catalysts. We find a variety of mor-

phologies for copper-catalyzed Si wires and growth direc-

tions related to the catalyst orientation relation. We also

measure the growth kinetics of individual wires. Two other

VSS systems have been examined in situ, Au-Ge,25 where

growth kinetics were determined, and Pd2Si-Si,29 where

ledge flow was observed. Here, for Cu3Si-Si, we provide

quantitative measurements of an unexpectedly complex

ledge flow mechanism during growth. This process involves

rigid rotation of the solid catalyst, as well as step nucleation,

flow, and periodic pinning at the catalyst/wire interface. The

kinetics of ledge flow imply a low Si solubility in the catalyst

during VSS growth, supporting prior speculations3,4,19,20 that

* Corresponding author, [email protected].

Received for review: 10/8/2009

Published on Web: 12/30/2009

pubs.acs.org/NanoLett

2010 American Chemical Society 514 DOI: 10.1021/nl903362y | Nano Lett. 2010, 10, 514-519


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solid catalysts could be useful for the formation of abrupt

heterointerfaces.

The growth experiments were carried out in a Hitachi

H-9000 UHV-TEM with a base pressure of 2 10-10 Torr

and a maximum gas pressure during observations of 4

10-5 Torr.30 Observations were made in both plan-view

geometry, which allowed studies of the catalyst formation

and initial wire growth, and cross-sectional geometry, where

the wire morphology, catalyst and interface structure, and

wire growth rates could be determined. For plan-view

observations, chemically etched Si(111) thin foils were used.

For cross-sectional imaging, slices of n-type Si(111) wafers

were used, mounted so that the electron beam was parallel

to the substrate surface. Both types of substrate were chemi-

cally cleaned, loaded into a UHV side chamber, and flashed

at 1250 C to remove the surface oxide.31 A 0.1 nm coating

of Cu was then thermally evaporated onto the growth surface

at a pressure below 4 10-9 Torr. The substrate was

transferred under UHV to the microscope polepiece andresistively heated to the growth temperature. Si nanowires

started to grow after exposure to the chemical vapor deposi-

tion gas precursor disilane (20% diluted in helium), which

flowed into the TEM polepiece gap through a capillary.

Simultaneously, nanowire growth was recorded in bright or

dark field imaging conditions onto videotape at a rate of 30

images per second. The growth temperature ranged be-

tween 470 and 550 C and the disilane partial pressure

between 1 10-7 and 8 10-6 Torr. The sample temper-

ature was calibrated post-growth using an infrared pyrometer.

Plan-view TEM observations provide information on the

initial state and morphology evolution of the catalysts. Initialheating causes the Cu film to agglomerate (Figure 1a), and

on reaching the growth temperature, e.g., 530 C, larger

islands form within a few minutes (Figure 1b-d). These

islands have different orientations, as revealed in the variety

of moire fringes formed due to overlap of the catalyst and

Si lattices. Most islands are single-crystalline, but occasion-

ally we observe defects such as grain boundaries (Figure 1c).

After flowing disilane, nanowires start to grow from all

islands, accompanied by a gradual catalyst shape change

and appearance of sidewall facets (Figure 1e-h).

We identify the structure of the islands as -Cu3Si by

comparing electron diffraction patterns of the catalysts withthose of the Cu3Si polymorphs.32 The polymorphs in the

Cu3Si compounds differ in the stacking orders of the high-

temperature hexagonal -Cu3Si unit cell; for simplicity, we

use the unit vectors of-Cu3Si and we use Cu3Si to refer

to the catalyst phase. Furthermore, the spacing of the moire

fringes, for example in Figure 1d, is consistent with that

resulting from an overlap of Cu3Si and Si(111) lattices. Figure

1 therefore shows that the evaporated Cu layer transforms

to Cu3Si islands, which then catalyze the growth of Si

nanowires.

Figure 2a,b displays the morphology of the Si nanowires,

recorded post-growth using scanning electron microscopy

(SEM). Under these growth conditions, about 50% of the

wires are kinked (and not analyzed further), 45% are straight

and grow along the surface (wires labeled S in Figure 2b),

and 5% are straight and grow up from the surface, mostly

in 110 directions (wires labeled T). It is the T wires which

show most clearly in TEM images such as Figure 2c andwhich are analyzed in detail here. The S wires also grow

predominantly along 110, although their growth directions

appear to depend on pressure to some extent (see Support-

ing Information).

From cross-sectional TEM images recorded during growth

(Figure 2c-e), we extract information on catalyst state, wire

structure and growth kinetics for the wires that grow away

from the substrate. As is clear from their faceted shape, the

catalysts are in the solid state during wire growth, in agree-

ment with the bulk eutectic temperature given above. We

do not see measurable changes in catalyst sizes during

growth, so in principle wire diameters should remain con-

FIGURE 1. Images captured from a video recorded in plan view. (a)The Si(111) substrate at 350 C during initial heating of the sample.The dark contrast shows some of the Cu islands. (b-d) Morphologyof Cu3Si on the substrate at 530 C. The moire fringes in (d) arecaused by an overlap of the Si and Cu3Si lattices. (e-h) Images of atypical catalyst before and after flowing disilane (1 10-6 Torr) at530 C for 1, 5, and 10 min, respectively. The radiating features in(h) are due to the formation of faceted nanowire sidewalls.



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stant. The tapering observed is due to direct deposition of

Si on the sidewalls.The growth direction of the wires in Figure 2c is consistent

with 110 (shown as a dashed arrow), or with mirrored 110

after formation of a twin (dotted arrow). Detailed diffraction

pattern analysis (see the inset in Figure 2d) shows that the

epitaxial relation between the Cu3Si catalyst and silicon nano-

wire is Si(111)//Cu3Si(1101); Si[110]//Cu3Si[1120]. Twin rela-

tions are frequently seen between wires; in such cases, the Si

nanowires are mirror-symmetric in the [112] direction and the

growth directions, catalystorientations, and catalyst-nanowire

contact angles are mirror-symmetric to each other. Occasion-

ally we see wires with a different orientation relation, Si(111)//

Cu3Si(1103) (Figure 2e), and in this case the growth directionis along Si[111]. These observations suggest that the wire

growth direction is associated with the relative orientation of

the catalyst and Si substrate. The primary growth direction,

110, is different from the most common growth direction,

[111], for VLS Au-Si-catalyzed Si nanowires (e.g., ref 30) but

is the same as has been observed in some other epitaxial non-

Au-catalyzed nanowire growth on Si(111) substrates.6,7 We

speculate that VSS growth has stricter requirements for repro-

ducibility than VLS, since it requires control of wire andcatalyst

orientations, while reproducible VLS requires control only of

wire orientation. In straight wires, we occasionally see ex-

tended defects such as stacking faults, but only in planes thatare notparallel to the (111) growthplane. Theexistence of such

defectsdoes not appear to change the morphology of the wire.

We did not observe multiple twinning or any hexagonal silicon

phase as hasbeen reported elsewhere for Cu-catalyzed Si nano-

wires.5,33

In Figure 3, wire axial growth rates measured in situ show

an approximately linear relation with disilane partial pres-

sure. This suggests that growth is limited by the arrival rate

of Si from the vapor phase. Supply-limited growth is also

consistent with the observation in Figure 3 that a wire with

a higher ratio of catalyst surface area to wire cross section

grows more quickly. In contrast, VLS Au-Si-catalyzed Si

FIGURE 2. (a, b) Scanning electron microscopy plan-view and grazingangle (10) images of Cu3Si-catalyzed Si nanowires after 3 h of growthon Si(111) at 510 C and 1 10-6 Torr Si2H6. S indicates wires that

grow along the surface in 110 directions. T indicates wires growingoff the surface, like those in (c), with growth directions also 110. (c)Bright-field transmission electron microscopy (TEM) image of nanow-

ires (in projection over a

300

m strip of substrate) recorded duringgrowth at 530 C and 1 10-6 Torr Si2H6. The dashed arrow indicatesthe projection of the vector in the [011] or [101] direction, and thedotted arrow indicates the mirror symmetric directions. (d) TEM imageand electron diffraction pattern of a [101] wire. The growth plane ofthe wire is Si(111), which is labeled as A in the diffraction pattern,and the Cu3Si(0001) plane is labeled as B. The schematic illustrationshows the electron diffraction patternsof Si[110]and Cu3Si[1120] zoneaxes, open circles and solid circles, respectively, and the superpositionof Si(111) and Cu3Si(1101) reflections,labeled O. Averaged over severalwires,theCu3Si[0001] directionis 8.3(0.7to Si[111] in thediffractionpatterns, while the wire growthdirection is 18.4( 0.7 off the Si[111]axis,as expected for a wire growing out of plane in the Si[011] or Si[101]direction. (e) TEM image and electron diffraction pattern showing ananowire and catalyst with another orientation relation, Si(111)//Cu3Si(1103),labeled A and C, respectively. The growth direction is closeto Si[111].

FIGURE 3. Growth rates measured for two wires at 530 C as afunction of disilane partial pressure. The insets show the catalystmorphology. The scale bar is 50 nm. The ratio of the catalyst surfacearea to wire cross-section of wire 1 is higher than that of wire 2.Error in the growth rate measurement is within (1.8 nm/min.



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Real-time measurements of the ledge flow kinetics, such

as those shown in Figure 5, indicate that the incubation

time defined above is typically less than 20% of the time

between consecutive ledge nucleation events, and the aver-

age ledge propagation speed is relatively slow (10 nm/s). In

contrast, we have previously shown20 that VLS Au-Si-

catalyzed Si nanowires grown at the same overall rateexhibit a long incubation time followed by ledge propagation

at a rate too fast to measure (>1000 nm/s). During the

incubation time, the concentration of Si atoms in the catalyst

presumably increases, due to the incident flux, until it

becomes favorable for a ledge to nucleate. The shorter

incubation time for Cu3Si-catalyzed VSS wires compared to

Au-Si-catalyzed VLS wires suggests that a smaller amount

of excess Si in the catalyst can raise the chemical potential

enough to nucleate a ledge. This implies a lower solubility

of the precursor atom in the catalyst, as required to reduce

the reservoir effect. Of course, this particular catalyst, being

a silicide, is itself a Si reservoir and of less interest forheterostructure growth. Nonetheless, the demonstration of

low solubility in this solid catalyst is encouraging for the use

of other solid catalysts in heterostructure formation.

Finally, we also tested Cu-Si-catalyzed Si nanowire

growth in the VLS mode above the eutectic temperature. As

expected, droplets of a eutectic liquid formed and Si pre-

cipitated at the liquid/substrate interface, growing short wire

stubs. However, the droplets disappeared within a few

minutes to leave truncated Si cones (Figure 6). We assume

this is due to fast diffusion of Cu atoms in or on Si.In conclusion, we have shown directly that the growth of

silicon nanowires around 500-600 C using Cu occurs via

the VSS mechanism. We show that the catalyst is the -Cu3Si

phase and the epitaxial growth direction is predominantly

110 but depends on the relative orientation of the catalyst

and substrate. Real-time observations show that wire growth

involves rigid rotations of the catalyst particles and is by

repeated ledge nucleation and flow at the Cu3Si/Si interface,

with the ledges propagating in a jumpy manner due to

pinning by interfacial dislocations at the growth interface.

In comparison with VLS growth, wires grown by VSS may

require more stringent process control, because of thedependence of wire direction and growth rate on the details

of the catalyst shape and orientation relation. On the other

hand, this suggests the intriguing possibility that this VSS

mode could provide a wider range of morphologies if the

catalyst shape and orientation relationship could be con-

trolled. The understanding of VSS growth in this and other

systems is therefore an important goal in extending the

possibilities of materials design using catalytic growth of

nanowires.

Acknowledgment. We acknowledge financial assistance

from the NSF under Grants DMR-0606395 and DMR-0907483.

Supporting Information Available. Results of the effectsof high disilane pressure on the growth of nanowires, TEMvideos recorded during nanowire growth, and Movies 1 and2. This material is available free of charge via the Internetat http://pubs.acs.org.

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Structure, Growth Kinetics