Carbon 43 (2005) 835–840

www.elsevier.com/locate/carbon

Prevention of Si-contaminated nanocone formation duringplasma enhanced CVD growth of carbon nanotubes

Dong-Wook Kim a,b,*, L.-H. Chen b, J.F. AuBuchon b, I.-C. Chen b, Soo-Hwan Jeong a,In K. Yoo a, S. Jin b

a Samsung Advanced Institute of Technology, P.O. Box 111, Suwon, Kyongki 440-600, South Koreab University of California at San Diego, La Jolla, CA 92093-0411, USA

Received 15 September 2004; accepted 8 November 2004

Available online 30 December 2004

Abstract

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by

direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si

substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of

tapered-diameter nanocones containing Si. Either low bias voltage (�500 V) or deposition of a protective layer (tungsten or titaniumfilm with 10–200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylin-

drical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field

emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the

nanocone formation.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon nanotubes; B. Chemical vapor deposition; C. Scanning electron microscopy, Transmission electron microscopy

1. Introduction

Carbon nanotubes (CNTs) have been shown to be

useful for a variety of applications such as field emission

displays (FEDs) [1–3], nanoscale electromechanical actu-

ators [4], field-effect transistors (FETs) [5,6], nanointer-

connects [7], and scanning probe microscope (SPM)tips [8]. It is advantageous to grow CNTs directly on sil-

icon (Si) to realize future large-scale integrated nano-

electronic devices. Among several growth techniques,

plasma enhanced chemical vapor deposition (PECVD)

has attracted great attention owing to its ability to

grow vertically aligned CNTs at relatively low tempera-

0008-6223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2004.11.018

* Corresponding author. Tel.: +82 31 280 9398; fax: +82 31 280

9349.

E-mail address: peterkim@ucsd.edu (D.-W. Kim).

tures, which makes the synthesis more compatible with

conventional semiconductor fabrication processes [9–

16].

The low temperature growth and complicated plas-

ma-related behaviors may cause a large concentration

of defects and some difficulty in the control of the pro-

cess [11,12]. Under certain growth conditions, a CNTno longer maintains the intended cylindrical shape and

changes into a defective, cone-like structure, which often

exhibits larger diameters and much Si contamination

[11,12]. The high aspect ratio and the excellent electrical

conductance properties, inherent in the cylindrical tube

geometry (as compared with the cone-geometry often

associated with a large-diameter base), are essential

characteristics for efficient operation of CNT field emit-ter devices and other electrical devices, such as FETs

and interconnects. Therefore, the ability to control the

836 D.-W. Kim et al. / Carbon 43 (2005) 835–840

morphology and reduce the defect concentration of

synthesized CNTs is important for ensuring satisfactory

device performance.

In this paper, we report how plasma etching of an Si

substrate influences the morphology and composition of

CNTs during a PECVD growth process. We reveal thatappropriate bias voltage or selective deposition of a pro-

tective layer prevents the unwanted Si etching and the

subsequent formation of Si-contaminated nanocone

structures.

Fig. 1. SEM images and schematic cross-sectional diagrams of (a)

nanocones grown on a bare Si substrate and (b) nanotubes grown on a

10 nm thick Ti-coated Si substrate.

2. Experimental

A direct current (DC) PECVD system was used to

grow vertically aligned CNTs on an Si substrate at a

temperature of about 700 �C [15,16]. A mixed gas of

acetylene (C2H2) and ammonia (NH3) at a ratio of 1:5

was used for the process. The gas pressure was held at

3 Torr during the growth. A DC bias voltage of 500–

600 V was applied between the anode and the cathode,

which maintained the plasma during the growth.For catalyst fabrication, a 10 nm thick nickel (Ni)

catalyst layer was sputter deposited onto an n-type

Si(100) substrate. Catalyst islands of �200 nm diameter

were fabricated on the Si substrates using resist (poly-

methyl methacrylate, PMMA) coating and e-beam

lithography followed by a lift-off process. We chose

the 200 nm diameter catalyst size so as to nucleate and

grow only a single CNT from each catalyst island [9–14].

3. Results and discussion

It has been realized that CNTs grown on bare Si

often result in a nanocone morphology while a buffer

layer-coated Si produces more cylindrical CNTs. Here-

after, we will call the cylindrical CNT �nanotubes� com-pared with the cone shaped �nanocones�. 1 Fig. 1(a) and(b) shows scanning electron microscope (SEM) images

and schematic cross-sectional diagrams of CNTs grown

at the bias voltage of 550 V on a bare Si substrates and a

10 nm thick titanium (Ti) coated Si substrate, respec-

tively. A Ni catalyst layer was patterned into islands

of 200 nm diameter and 2 lm spacing. Although the

deposition conditions were nearly identical for the twosamples, their geometry showed a dramatic difference.

1 In general, the CNTs synthesized by DC PECVD processing are

of �nanofiber� type multiwall nanotubes with the graphene layer wallspositioned at some angles rather than having the walls parallel to the

long axis. The nanotubes grown by microwave plasma CVD or

thermal CVD tend to have the carbon walls parallel to the long axis.

The carbon nanostructures with inclined walls are still denoted as

�CNT� in this paper in a broad sense, as many of them still have hollow

cores in the middle.

The CNTs grown on a bare Si substrate are cone-like,

but those on a Ti layer exhibit a more cylindrical mor-

phology. The nanocones were significantly contami-

nated with Si while the cylindrical CNTs were not, as

will be discussed later.

Fig. 2 shows an SEM image and schematic cross-sec-tional diagram of a CNT array grown on e-beam pat-

terned Ni-on-Ti catalyst islands. These islands were

prepared by e-beam patterning and subsequent sputter

deposition of a 10 nm thick Ti film on an Si substrate

followed by 10 nm thick Ni sputter deposition. Fig. 2

shows that the CNTs grown on patterned Ti islands

are cone-like instead of cylindrical, like the CNTs shown

in Fig. 1(b). From the results of Fig. 1, it could bethought that the Ti layer serves as a diffusion barrier

that prevents the diffusion of Si from the substrate to

the growing CNT during the high temperature growth

process at �700 �C, and as a result, the nanotubes hav-ing the desirable cylindrical morphology are formed in

the presence of the Ti buffer layer. The Ti islands in

Fig. 2. SEM image and schematic cross-sectional diagram of carbon

nanocones grown on Ni/Ti islands.

D.-W. Kim et al. / Carbon 43 (2005) 835–840 837

Fig. 2, which are originally directly underneath the Ni

catalyst islands, and separate the CNT base from the

Si substrate, should have prevented or minimized possi-

ble diffusion reactions between the CNTs and the Si sub-

strate [11]. However, the SEM images in Fig. 2 clearly

show that the nanocone formation still occurs implyingthat another factor, not the Si diffusion, should be influ-

encing the CNT morphology.

We also noted that morphology of CNTs could be

significantly altered by adjusting the applied bias voltage

during the PECVD process. Fig. 3(a) shows an example

of carbon nanocones grown on an Si substrate at an ap-

plied bias voltage of 600 V. In this case, the catalyst

layer was deposited as a uniform thin film (which breaksinto islands on heating to the CVD temperature), rather

than e-beam patterned islands. The base diameter of the

nanocones is as large as �400 nm, and the cone angle is�15–20�. In contrast, nanotubes are formed on the samebare Si substrate by PECVD at a lower bias voltage of

500 V, as shown in Fig. 3(b). The diameter of the nano-

Fig. 3. Effect of bias voltage on CNT morphology grown by DC

PECVD at 700 �C. (a) Nanocones obtained at 600 V and (b)

nanotubes at 500 V.

tubes does not significantly change from the bottom to

top.

To further investigate the growth mechanism of the

CNTs, a high-resolution transmission electron micro-

scope (TEM) was used to study the microstructure and

composition of our CNTs. Fig. 4(a) and (b) shows typi-cal TEM images of carbon nanocones and nanotubes,

respectively, with the two samples prepared under differ-

ent bias voltages. The nanocone has an inner cone,

which is covered by a thinner outer layer. The cone is

crystalline with internal inclusions. Energy dispersive

spectroscopy (EDS) analysis indicates that the cone ma-

trix is mostly silicon and carbon, with the Si content esti-

mated to be at least 40 at.%, and the tiny inclusions aremainly composed of Ni. Si is found in the cone every-

where, which indicates that the precipitation of Si into

the nanocone occurs throughout the duration of growth.

The nanotubes shown in Fig. 4(b) have an almost con-

stant diameter and Ni catalyst particles cap on their

tops. The EDS analysis on nanotube samples in Fig.

4(b) shows that no detectable amount of Si is found

for most parts of the sample. A trace of Si is only foundat the very bottom part of the nanotube implying that a

slight diffusion of Si into the base may have occurred.

Elemental composition distributions of the nano-

cones and nanotubes grown on e-beam patterned cata-

lyst islands, like the samples shown in Figs. 1 and 2,

were also acquired. Since these nanocones were grown

under a moderate bias voltage of 550 V and on pat-

terned Ti buffer islands, the Si concentration is less than20 at.% in the body. However, it is noticeable that a

Fig. 4. TEM micrographs for (a) nanocone, (b) nanotube.

Fig. 5. SEM image and schematic diagram of a CNT array grown on

an Si substrate with a patterned Ti layer. A 10 nm thick Ti film covers

most of Si surface and the only exposed Si area is a 60 · 60 lm2 square

minus a 20 · 20 lm2 Ti square at the center.

838 D.-W. Kim et al. / Carbon 43 (2005) 835–840

finite amount of Si can be found even at the very top of

the nanocone by the EDS analysis. The cylindrical

nanotubes grown on a uniform Ti layer do not have

detectable amounts of Si, except for at the bottoms

where it is presumably caused by some solid-state diffu-

sion of Si from the substrate. All these results imply thatthe CNT morphology is related to its Si defect concen-

tration. They also indicate that while some diffusion of

Si from the substrate may contribute to the CNT con-

tamination, the main cause for the contamination is

the precipitation of plasma etched Si onto the CNTs.

The CNTs are grown in both the vertical and lateral

directions: the vertical growth is related to the diffusion

of carbon (C) through the Ni catalyst particles, and thelateral growth results from the precipitation of both C

and Si. The DC discharge may induce etching of sub-

strate materials and some portion of the grown CNTs

[10,11]. A proper gas ratio of C2H2:NH3 and bias volt-

age can equilibrate growth of graphene sheets and etch-

ing of amorphous carbon, resulting in growth of

cylindrically shaped carbon nanotubes along the applied

electric field direction [11]. Also, it should be noted thatthe catalyst particles suffer from sputtering and frag-

mentation during growth [17]. Prolonged growth may

result in loss of the catalyst particle and termination of

the vertical growth; however, the lateral growth can con-

tinue if the C or Si supply is maintained. In our growth

conditions, a rather low C2H2:NH3 ratio of 1:5 was uti-

lized, which may allow a somewhat fast etching rate of

an Si substrate [12]. Moreover, either exposure of a bareSi surface or higher bias voltage is likely to enhance Si

etching and incorporation of Si atoms into the growing

CNTs in the plasma. The availability of such etched Si

in the plasma can promote the lateral growth, resulting

in nanocone formation as shown in Figs. 1–3. Optimiz-

ing the bias voltage or adding a protective metal thin

film on the Si surface could suppress the Si etching

and enable the growth of the nanotubes with minimaldefects. Regarding the protective layers, we tested sev-

eral kinds of thin films such as Ti, Ti nitride (TiN), tung-

sten (W), and silicon dioxide (SiO2). Most of these metal

thin films worked well as a protective layer that pre-

vented Si etching, except for SiO2. It seems that acti-

vated ions in the plasma are able to etch the SiO2 film

as well as the Si substrate during the DC discharge

[18]. The structural and chemical analyses of our carbonnanocones and nanotubes support the above interpre-

tations.

Fig. 5 shows an SEM image and a schematic diagram

of a CNT array grown on an Si substrate with a pat-

terned Ti layer. As illustrated, a 10 nm thick Ti film cov-

ers most of the Si surface, and the only exposed Si

surface is a picture frame shaped area with a

60 · 60 lm2 sized square minus a 20 · 20 lm2 sized Tisquare at the center. As shown in Fig. 5, cylindrical

nanotubes can be grown even on the bare Si surface with

the aid of the surrounding Ti protective layer. This SEM

image clearly confirms that etching and subsequent Siincorporation, rather than Si diffusion, dominantly

modifies the CNT morphology. This kind of surround-

ing protection layer might slightly increase the Si defect

concentration compared with the uniform protection

layer case, such as shown in Fig. 1(b). Physical proper-

ties of an underlying layer and a resulting interfacial

layer may be critical for device performance. A direct

growth of CNTs on an Si substrate without any metallicinterfacial layer is desirable in some cases [13]. For such

a case, surrounding area protection can suffice to allow

the direct growth of CNTs on Si, as the very limited Si

surface exposed to the plasma will minimize Si etching

and incorporation into the CNTs, thus allowing an opti-

mization of their electrical and structural properties

simultaneously.

Fig. 6(a) and (b) shows exemplary SEM images andcross-sectional diagrams of a gated CNT field emitter di-

rectly grown on Si substrates. The insulator and gate

materials were 1.5 lm thick SiO2 and 0.2 lm thick W,

respectively. We used a self-aligned fabrication process,

similar to that of Prior et al., except for the gate material

[9]. Thin films of W and SiO2 were etched by SF6-based

plasma and buffered oxide etch solution (6 parts 40%

NH4F and 1 part 49% HF), respectively. During theCNT PECVD, the W gate electrode covers most of the

SiO2 surface and protects it from etching. As a result,

we can grow cylindrical CNTs directly on Si substrates.

In this example, we find that a properly chosen gate

metal layer can be used simultaneously for the purpose

of Si surface protection during the PECVD process

and also for the device operation after the fabrication.

A CNT field emitter is a superior electron sourcecompared with conventional Schottky or field emission

Fig. 7. (a) SEM image of a single CNT grown at the center of SiO2hole and (b) a schematic cross-sectional diagram of an example vertical

transistor using a CNT.

Fig. 6. (a) SEM image and (b) cross-sectional diagram of a gated CNT

field emitter directly grown on Si substrates.

D.-W. Kim et al. / Carbon 43 (2005) 835–840 839

sources since it has high brightness electron beams

(109 A m�2 sr�1 V�1 s), a small energy spread (0.2–0.3 eV), and long-term stability [1]. However, poor emis-

sion uniformity due to the extreme sensitivity of the

Fowler–Nordheim tunneling process to small variations

in nanotube radii and heights, gate diameters, position

of the gate holes, and the work function, just like vari-

ous other field emission sources, limits commercial

device application of the CNT field emitter. An intro-

duction of a resistive layer between the field emitterand the emitter lines helps to mitigate the uniformity

problem [19]. A lateral resistor mesh is used to homog-

enize the emission current and prevent a short-circuit

effect by limiting the electrical current. In general,

doped-Si, e.g. having a resistivity in the level of

�105 X cm and a thickness of �0.1 lm, can be used asthe resistive layer and is compatible with the manufac-

turing process of the emitters. We expect that such astructure would show a better emission uniformity com-

pared with field emitter arrays grown on metal elec-

trodes (e.g., TiW/Mo/TiW) [9]. Moreover, the high

aspect ratio of the cylindrical CNTs is likely to show

lower turn-on voltages than carbon nanocones, which

generally exhibit lower aspect ratios [12].

Desirable single CNTs with minimal defects, not

nanocones contaminated with Si, can also be grown in-side a small aperture hole in SiO2, as shown in Fig. 7(a).

As mentioned earlier, SiO2 is also susceptible to etching

during the PECVD process, thus it can supply Si ele-

ment to the plasma, resulting in carbon nanocone for-

mation. Small holes, such as a gate cavity, might

significantly affect the nanotube growth process due to

the possible impediment of gas flow to the structure or

local variation of electric field strength due to the pres-

ence of the gate structure. In spite of such concerns, a

�single� and �cylindrical� CNT was successfully grown

at the center of 600 nm gate hole. This structure can also

be useful for vertical electronic components such as

diodes/transistors and nanointerconnects [6,12,13]. Fig.

7(b) shows a schematic cross-sectional diagram of anexemplary vertical transistor [6]. For such electrical

applications, minimal chemical and structural defects

(such as the formation of Si-contaminated nanocones)

are crucial for achieving low electrical resistance and

desirable field effect switching.

4. Conclusions

We investigated the plasma etching effects on the

morphology and chemistry of carbon nanotubes (CNTs)

grown by DC plasma CVD. The formation of Si-con-

taminated nanocone-type defective structure was pre-

vented by suppressing the etching of the Si substrate

material during the plasma processing by using either

a low bias voltage (�500 V) or deposition of a protectivemetal layer on the Si surface such as Ti and W, which

provided strong etching resistance to plasma. Proper

choice of materials also allowed us to use a device ele-

ment itself, such as a gate metal electrode in a field emit-

ter array as the protective layer during the PECVD

growth. Control of the CNT morphology demonstrated

840 D.-W. Kim et al. / Carbon 43 (2005) 835–840

by our Si-fab-compatible techniques can be very useful

for improving the device performance.

Acknowledgments

We acknowledge the support of the work by Univer-

sity of California Discovery Fund under Grant No.

ele02-10133/Jin, NSF NIRTs under Grant No. DMI-

0210559 and DMI-0303790. The authors would like to

thank Dr. Alexander Tikhonovsky for assistance in

TEM studies and helpful discussions.

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