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Applied Surface Science 253 (2006) 1480–1483

AFM investigation on surface evolution of amorphous

carbon during ion-beam-assisted deposition

X.D. Zhu a,*, F. Ding a, H. Naramoto b, K. Narumi b

a CAS Key Laboratory of Basic Plasma Physics, Department of Modern Physics,

University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of Chinab Advanced Science Research Center, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan

Received 18 May 2005; received in revised form 20 January 2006; accepted 14 February 2006

Available online 31 March 2006

Abstract

Hydrogen-free amorphous carbons (a-C) have been prepared on mirror-polished Si(1 1 1) wafers through thermally evaporated C60 with

simultaneous bombardments of Ne+ ions. The time evolution of film surfaces has been characterized by atomic force microscopy (AFM) at two

temperatures of 400 and 700 8C, respectively. Based on the topography images and the root-mean-square (rms) roughness analysis, it is found that

the a-C surfaces present roughening growth at the initial stage. With increasing growth time, the cooperative nucleation of the islands and pits

appears on the surfaces, suggesting three-dimensional growth, and then they continue to evolve to irregular mounds at 400 8C, and elongated

mounds at 700 8C. At the steady growth stage, these surfaces further develop to the structures of bamboo joints and ripples corresponding to these

two temperatures, respectively. It is believed that besides ion sputtering effect, the chemical bonding configurations in the amorphous carbon films

should be taken into considerations for elucidating the surface evolutions.

# 2006 Elsevier B.V. All rights reserved.

PACS: 68.37.Ps; 68.55.Jk; 81.05.Uw

Keywords: Amorphous carbon; Surface morphology; Ion beam

1. Introduction

A major focus of research on nonequilibrium processes at

surfaces is the evolution of surface morphology, which has been

a topic of continuing research for several decades [1]. Because

the usefulness of a growing film depends heavily on the nature

of surface morphology, including its height variations, rough-

ness, or patterns. It is therefore important to understand the

evolution of the surface morphology during processing.

Ion bombardment of solid surfaces is known to create

curious morphologies ranging from self-affine surface rough-

ness to ripples [2–4]. Ion-beam-assisted deposition (IBAD)

combines this advantage with film growth. A series of chemical

reactions in IBAD can be generated due to collisions between

incident ions and source species, which causes lots of atoms,

clusters and radicals. Further, these species react with the

substrate physically and chemically to form film growth. ‘On

* Corresponding author. Tel.: +86 5513601168; fax: +86 5513601164.

E-mail address: xdzhu@ustc.edu.cn (X.D. Zhu).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.02.034

the other hand, the growing surface is simultaneously

bombarded by incident ion beam, which provides a possibility

of patterning a surface.

In our previous work, we have used IBAD to deposit

successfully amorphous carbon films with various textured

surfaces, such as ripples, mounds [5,6]. In this article, we focus

on the surface evolutions of time-dependent deposits for

amorphous carbons obtained in C60 vapor by simultaneous

bombardments of Ne+ ions at middle and relatively high growth

temperatures.

2. Experiment

Experiments were carried out on an ion-beam-assisted

deposition device. This machine was equipped with an ion gun

and a sublimator. C60 powder with the purity of 99.99% was

placed in a pyrolytic BN crucible of the sublimator. The

background pressure in the chamber was less than 2 � 10�6 Pa.

The Si(1 1 1) wafers used as substrates were rinsed ultra-

sonically with de-ionized water, acetone, and ethanol,

respectively, before they were placed on the substrate holder.

X.D. Zhu et al. / Applied Surface Science 253 (2006) 1480–1483 1481

Fig. 2. Root-mean-square (rms) roughness as a function of growth times. The

samples are deposited at 400 8C.

C60 vapor was produced by heating electrically the sublimator

up to 400 8C, and the growing film was simultaneously

bombarded by Ne+ ions with ion incident angle of 608 from

substrate normal. The emission of the filament in the ion gun is

fixed at 20 mA. The working pressure was maintained around

6 � 10�4 Pa in the chamber. After the deposition, the carbon

films were analyzed by micro-Raman spectroscopy, and atomic

force microscopy (AFM). Micro-Raman spectra were recorded

at room temperature using the 514 nm line of Ar+ ion laser.

3. Results and discussion

Fig. 1 shows Raman spectra of the films deposited at

different growth times at 400 8C. At the initial growth stage of

20 min, the strong peak centered at 980 cm�1 can still be

observed, which is assigned to Si from the substrate. With

increasing the growth time, a broad Raman signal centered at

1547 cm�1 appears and enhances, but Raman spectra of Si

become weaker, and then vanished. From 100 min, the Raman

spectra of the deposited films present similar structures,

suggesting a steady growth. Raman spectra in Fig. 1 manifest

the characteristics of amorphous diamond-like carbon films,

i.e., a broad peaks at energies�1547 cm�1 (symmetric G peak)

with a small shoulder at �1353 cm�1 (asymmetric D peak)

[7,8].

The root-mean-square (rms) roughness as a function of

growth times is shown in Fig. 2. The rms roughness firstly

increases gradually, followed by rapid increase with increasing

growth times. Fig. 3 shows the surface evolution of the

deposited films with growth time. At the initial growth stage of

10 min, the film surface is shown to be undulate.

For the film growth, three growth modes have been, in

principle, classed: layer-by-layer growth in lattice matched

system (Frank–van-der Merwe), the island growth mode

(Volmer–Weber) and the isolated island growth (Stranski–

Kratanow) in lattice mismatched systems [9]. In the early

Fig. 1. Raman spectra of the films deposited at different growth times. The

samples are prepared at 400 8C.

deposition stage, the rms roughness increases gradually, shown

in Fig. 2, and also the films present undulate. So, it is reasonable

to conclude that these are characterized by dynamic rough-

ening. It is considered that the growing surface also faces ion

bombardment. Roughening of surfaces by sputtering has long

been observed, especially at off-normal incidence [10]. The

ability of ion bombardment to affect the dynamics of surface

processes arises by virtue of the energy deposited at or near the

surface during impact. Material removal by sputtering is the

most obvious process that can lead to development of interface

morphology. It is very likely that the gradual increase of the

surface roughness in amorphous carbon films in Fig. 2 is

induced by Ne+ ion sputtering on the growing films.

As the growth time is increased, the three-dimensional

growth takes place, the islands and pits appear on the surfaces.

With a further increase to the growth time, and the topographies

of the a-C films are shown to be a set of continuous irregular

mounds. Simultaneously, there is a rapid increase in rms

roughness as shown in Fig. 2. But this kind surface is unstable,

and with further increase of deposition time, the surface

develops to ‘‘bamboo joint’’ structures at the stable growth

stage.

Fig. 4 shows the surface evolution of the films deposited at

700 8C. Fig. 5 displays the their root-mean-square (rms)

roughness as a function of growth times. In the initial stage, the

surface presents a similar growth as like at 400 8C, and the rms

roughness increases gradually. After 30 min, rms roughness

increases rapidly. At 60 min, the three-dimensional elongated

mounds appear. Different from bamboo joint structures at

400 8C, the periodical ripples with the wave vector parallel to

the projection of incident ion beam form at steady growth stage.

The surface evolution of a-C films is a complicated

nonequilibrium processes. Concerning the formation and

development of far-from-equilibrium interfaces, there have

been a great number of theoretical and experimental

investigations aiming at understanding the mechanism

involved, but experimental verification is still underway. In

our previous work, we have investigated the surfaces of

amorphous carbon films grown by ion-beam-assisted deposi-

tion at 200 8C growth temperature and 2.0 keV ion energy. The

films present mound surfaces at the steady growth stage. In this

X.D. Zhu et al. / Applied Surface Science 253 (2006) 1480–14831482

Fig. 3. AFM images of the films deposited at 400 8C, showing the surface evolution of the deposited films with growth times. (a), (b), (c), (d), and (e) refer to 20, 45,

100, 200, and 250 min, respectively. Insets are enlarged images taken from the centers. The scanning size is 400 nm � 400 nm.

Fig. 4. AFM images of the films deposited at 700 8C, showing the surface evolution of the deposited films with growth times. The scanning size is 400 nm � 400 nm.

X.D. Zhu et al. / Applied Surface Science 253 (2006) 1480–1483 1483

Fig. 5. Root-mean-square (rms) roughness as a function of growth times. The

samples are deposited at 700 8C.

case, the stable structure evolves to bamboo joint structures at

the middle growth temperature 400 8C, and to ripple at high

growth temperatures 700 8C.

Two processes should be noted in IBAD, gas phase and

surface reaction. Ne+ ions extracted from ion gun firstly react

with C60 molecules in vapor environment. We have demon-

strated that the dissociation of C60 clusters may be induced as

ion energy is increased to above 500 eV [6]. To date, the actual

mechanism of fragmentation is not completely clear yet. But it

is known that containing-carbon species including C2 should be

produced during the collisions between C60 clusters and Ne+

ions [11,12].

Since the incident ion energy, ion to atom ratio, ion density

all are independent in IBAD, it is reasonable to suppose that the

gas phase processes should be similar at different growth

temperatures. In this case, surface reaction should be taken into

consideration for the difference of the stable structures

observed at various temperatures. It is generally acknowledged

that film properties, such as composition and morphology are

generally strong functions of temperature.

From Raman spectra (no shown), the chemical bonding

configuration varies distinctly with increasing substrate

temperatures. With elevated growth temperatures, separated

G and D lines appear and become clearer, suggesting an sp2

phase development and organization of small graphite clusters.

These c–c bonding variations strongly affect the surface

diffusion during film growths, which is contributed to various

surface characteristics with growth temperatures.

4. Conclusion

In summary, we have carried kinetic investigation of the

topography of ion-beam-assisted a-C films at two temperatures

400 and 700 8C, respectively. At the initial stage the a-C

surfaces present roughening growth, following by three-

dimensional growths. With increasing growth time, the surfaces

show the cooperative nucleation of the islands and pits, and

then they further develop to three-dimensional irregular

mounds at 400 8C, and elongated mounds at 700 8C. At the

steady growth stage, these mounds evolve to the structures of

bamboo joints and ripples with respect to these two

temperatures, respectively. It is believed that besides ion

sputtering, the chemical bonding configurations in the

amorphous carbon films should be taken into considerations

for elucidating the surface evolutions.

Acknowledgement

One of authors (Zhu) wishes to express appreciations for the

project sponsored by the National Natural Science Foundation

(No. 50472010), the Inertial Confinement Fusion Technology

Exploration Foundation.

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