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Characterization of amorphous and nanocrystalline carbon films

April 2006
Materials Chemistry and Physics 96(2-3):253-277

April 2006
96(2-3):253-277

DOI:10.1016/j.matchemphys.2005.07.048

Authors:

Beihang University (BUAA)

Amorphous and nanocrystalline carbon films possess special chemical and physical properties such as high chemical inertness, diamond-like properties, and favorable tribological proprieties. The materials usually consist of graphite and diamond microstructures and thus possess properties that lie between the two. Amorphous and nanocrystalline carbon films can exist in different kinds of matrices and are usually doped with a large amount of hydrogen. Thus, carbon films can be classified as polymer-like, diamond-like, or graphite-like based on the main binding framework. In order to characterize the structure, either direct bonding characterization methods or the indirect bonding characterization methods are employed. Examples of techniques utilized to identify the chemical bonds and microstructure of amorphous and nanocrystalline carbon films include optical characterization methods such as Raman spectroscopy, Ultra-violet (UV) Raman spectroscopy, and infrared spectroscopy, electron spectroscopic and microscopic methods such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, transmission electron microscopy, and electron energy loss spectroscopy, surface morphology characterization techniques such as scanning probe microscopy (SPM) as well as other characterization methods such as X-ray reflectivity and nuclear magnetic resonance. In this review, the structures of various types of amorphous carbon films and common characterization techniques are described.

Schematic diagram of the principle of Raman scattering [44]: (a) inelastic scattering of an optical quantum hitting the materials; (b) term diagram; (c) Raman spectra. Because vibration of atoms in the excited state is much less than that of the ground state atoms, the Stokes line is stronger than anti-Stokes line.

…

Typical Raman spectra acquired from different kinds of carbon films (our data plus refs. [48,53,54,67]).

Relationship between the film density and substrate bias [43]: (a) density as a function of the G line width, and (b) G peak position as a function of substrate bias.

…

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Content uploaded by Liuhe Li

Content may be subject to copyright.

Materials Chemistry and Physics 96 (2006) 253–277

Characterization of amorphous and nanocrystalline carbon ﬁlms

Paul K. Chu∗, Liuhe Li1

Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong

Received 13 May 2005; accepted 13 July 2005

Abstract

Amorphousandnanocrystallinecarbonﬁlmspossessspecial chemical and physical properties such as high chemical inertness, diamond-like

properties,and favorabletribologicalproprieties.The materials usually consist of graphite anddiamond microstructures and thus possess prop-

ertiesthat lie between the two. Amorphous and nanocrystalline carbon ﬁlmscanexistindifferentkindsofmatrices and are usually doped with a

largeamountofhydrogen.Thus,carbonﬁlmscanbeclassiﬁedaspolymer-like, diamond-like, or graphite-likebasedonthemainbindingframe-

work.Inorder to characterize the structure, eitherdirect bonding characterization methods or the indirectbonding characterization methods are

employed. Examples of techniques utilized to identify the chemical bonds and microstructure of amorphous and nanocrystalline carbon ﬁlms

include optical characterization methods such as Raman spectroscopy, Ultra-violet (UV) Raman spectroscopy, and infrared spectroscopy,

electron spectroscopic and microscopic methods such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS),

Auger electron spectroscopy, transmission electron microscopy, and electron energy loss spectroscopy, surface morphology characterization

techniquessuch as scanning probe microscopy (SPM) as well asother characterization methods such as X-ray reﬂectivity and nuclear magnetic

resonance. In this review, the structures of various types of amorphous carbon ﬁlms and common characterization techniques are described.

Keywords: Amorphous carbon; Diamond-like carbon; Characterization

1. Introduction

There has been a lot of interest on amorphous and

nanocrystalline carbon ﬁlms in the past twenty years because

they exhibit beneﬁcial chemical and physical properties such

as high chemical inertness, diamond-like properties [1–5],

andfavorabletribological proprieties [6,7] suitable for indus-

trial use. Their unique properties can be attributed to the

special and interesting properties of the microstructures. The

materials usually comprise a hybrid of graphite and diamond

microstructures and thus possess properties that lie between

the two. Amorphous and nanocrystalline carbon ﬁlms can

exist in different kinds of matrices and are usually doped

with a large amount of hydrogen thereby making the mate-

rials even more diverse. The most common chemical bonds

∗Corresponding author. Tel.: +852 27887724; fax: +852 27889549.

E-mail address: paul.chu@cityu.edu.hk (P.K. Chu).

1Present address. Department of 702, School of Mechanical Engineer-

ing and Automation, Beijing University of Aeronautics and Astronautics,

Beijing 100083, China.

in amorphous and nanocrystalline carbon are sp3and sp2

hybridizations. In the sp3conﬁguration, a carbon atom forms

four sp3orbitals making a strong ␴bond to the adjacent

atom. In some of the carbon ﬁlms with high hardness, the

sp3content is sometimes in excess of 80% [8–15]. These

ﬁlms are commonly referred to as tetrahedral amorphous car-

bon or ta-C ﬁlms. In these ﬁlms, sp3dominates to establish a

diamond-likebase framework.In the sp2conﬁguration, acar-

bon atom forms three sp2orbitals forming three ␴bonds and

the remaining p orbital forms a ␲bond. The ␲orbital geo-

metrically lies normal to the ␴bond plane and is the weaker

bond so that it is closer to the Fermi level, Ef. The three

␴bonds and ␲bond usually constitute a ring plane in sp2

clusters. A variety of larger clusters can be formed by fus-

ing double bonds and rings [16]. Under the constraint of the

rigid sp3network, the rings can coexist with each other or

may be deformed. Robertson and O’Reilly have suggested a

model[16] for amorphouscarbon that consistsof sp2-bonded

clusters interconnected by a random network of sp3-bonded

atomic sites. Taking advantage of extensive characterization

doi:10.1016/j.matchemphys.2005.07.048

254 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

and research, the structure and properties of carbon ﬁlms

have become more understood. Several other structural mod-

els have also been proposed, such as the one based on three

dimensional all-sp2bonding introduced by Cˆ

ot´

eetal.[17]

and Liu et al. [18,19]. In their model, they propose an all-sp2

structure that can yield a high bulk modulus and low bulk

modulus as determined by their bond lengths. Experimental

results have conﬁrmed the possibility of the ﬁlm structure to

be composed of a matrix of dispersed cross-linked sp2sites

that provide the network with rigidity [20–22]. In addition

to sp2and sp3, there is some evidence of the presence of sp

bonds as well [23]. In the sp conﬁguration, a carbon atom

forms two sp orbitals thus two ␴bonds and the other valence

electrons constitute two perpendicular ␲orbitals.

Amorphous and nanocrystalline carbon ﬁlms can exist in

threefold (sp2bonding) and fourfold (sp3) bonding coordina-

tion,andunlikeitscrystallinecounterpart,theymayconsistof

any mixture of the two bonding types [13]. At the beginning,

it was generally believed that the high hardness arises from

thesp3rigidframework,butrecentstudies haveindicated that

sp2bonds can also render carbon ﬁlms with relatively high

hardness.Thoughthe sp3and sp2bonds donotshowthe same

long distance order characteristics as they do in crystalline

diamond or graphite, the bonds sometimes can intermix and

exhibit extended order on a nano scale. When the sizes of

sp2and sp3clusters become large enough, nanocrystalline

graphite and diamond structures can be observed. They have

been detected not only in as-deposited carbon ﬁlms (usually

prepared under a high temperature) [24–26] but also after

annealing, mechanical rubbing in friction tests, or even irra-

diation [27–29].

In amorphous carbon and nanocrystalline carbon ﬁlms,

dopedhydrogen usually plays an important role. Recentstud-

ies have revealed experimental evidence of the existence of

transpolyacetylene (TPA) chains in a-C:H ﬁlms free of nano

crystalline diamond [30]. In a related study, it was found

that a considerable amount of hydrogen (up to ≈60 at.%)

could be incorporated in the ﬁlms leading to soft polymer-

likeamorphous carbon (PLC) [31]. The hydrogenated carbon

ﬁlms are characterized by a wide optical band gap and they

can exhibit special optical absorption, intense photolumines-

cence, and electron afﬁnity. The synthesis, properties, and

thermal stability of hydrogenated carbon ﬁlms have aroused

enormous research interest due to their potential applica-

tions [31–36]. Though the only bond for hydrogen in carbon

ﬁlms is C H, various combinations of the C H bond make

characterization quite difﬁcult and complicated. It has been

proposed that in carbon ﬁlms either in the amorphous state

or nanocrystalline state, there are variable-sized and compli-

cated polyacetylene and/or oleﬁnic chains and/or aromatic

clusters [16,30]. Above all, though amorphous and nanocrys-

talline carbon ﬁlms commonly only consist of carbon and

hydrogen, they are interconnected in many ways to yield

materials with distinctly different properties. Carbon ﬁlms

can be classiﬁed as polymer-like, diamond-like, or graphite-

like based on the main binding framework [31]. In order

to characterize the structure, either direct bonding charac-

terization methods or the indirect bonding characterization

methods are employed. Examples of techniques utilized to

identifythechemical bonds and microstructure of amorphous

and nanocrystalline carbon ﬁlms include optical character-

ization methods such as Raman spectroscopy, UV Raman

spectroscopy, and infrared spectroscopy, electron spectro-

scopic and microscopic methods such as scanning elec-

tron microscopy (SEM), X-ray photoelectron spectroscopy

(XPS), Auger electron spectroscopy, transmission electron

microscopy, and electron energy loss spectroscopy, surface

morphology characterization techniques such as scanning

probe microscopy (SPM) as well as other characterization

methods such as X-ray reﬂectivity and nuclear magnetic res-

onance.

It should be noted that though previous studies have ver-

iﬁed the presence of both sp2and sp3bonds and the lack

of long-range order, short order in such ﬁlms has not been

directly observed due to the lack of techniques with sufﬁcient

resolution and sensitivity to the atomic structure in the thin

ﬁlms [23]. In addition, the use of multiple characterization

techniques is preferred as comparison can better disclose the

detailed structures of the ﬁlms. In this paper, techniques that

are commonly employed to study carbon ﬁlms are described.

2. Optical characterization methods

2.1. Raman spectroscopy

Raman spectroscopy is a popular nondestructive, ambient

probingtool to characterizethe structure andusually imposes

very little constraint on the substrate size [37–43]. When a

light quantum hν0hits a surface, an elastic scattering pro-

cess, that is, Rayleigh scattering of quanta with energy hν0

ensues. This process has the highest probability. However,

there also exists an inelastic process in which the vibra-

tional energy is altered by hνs. The inelastic process is called

Ramanscatteringand quanta of energyhν0±hνsareemitted.

Because vibration of the atoms in the excited state is much

lessthanthatofthegroundstateatomsatambienttemperature

according to Boltzmann’s law, it is more efﬁcient to excite

ground-state atoms to a vibrationally excited state than to

receivethedecayenergyfromthevibrating atoms. Hence, the

emittedquantahavingenergyofhν0−hνsare more prevalent

than the emitted quanta with energy of hν0+hνs. The Raman

linescorrespondingtothequantawithenergyofhν0−hνsare

referredto as the Stokes lineswhereas the higher energy lines

(hν0+hνs) are called the anti-Stokes lines. As the intensities

of the anti-Stokes lines are lower, only the Stokes lines are

usually recorded in the Raman spectrum [44,45]. The light

scattering process is illustrated in Fig. 1 [44].

Raman spectroscopy is a very effective way to investigate

the detailed bonding structure of carbon ﬁlms. Though there

is still debate on the exact relationship between the atom

vibration and Raman spectra, the method is the most widely

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 255

Fig. 1. Schematic diagram of the principle of Raman scattering [44]: (a)

inelastic scattering of an optical quantum hitting the materials; (b) term

diagram; (c) Raman spectra. Because vibration of atoms in the excited state

is much less than that of the ground state atoms, the Stokes line is stronger

than anti-Stokes line.

used to distinguish the bonding type, domain size, and sen-

sitivity to internal stress in amorphous and nanocrystalline

carbon ﬁlms [46]. Raman spectra are usually discussed in

the context of diamond versus graphite as carbon ﬁlms are

composed of short distance ordered sp3and sp2bonds.

Diamond has a single Raman active mode at 1332cm−1,

which is a zone center mode of T2g symmetry [47]. The

other Raman line occurs at 1575cm−1reﬂecting the zone-

center E2g mode of perfect graphite. Different orientation

of the sample with respect to the incident laser beam does

not alter the spectrum [48] and it is usually designated as the

“G” peak for graphite. However, in multi-crystalline graphite

such as commercial grade graphite, an additional sharp peak

shows up in the Raman spectrum. It occurs at a wave num-

ber of 1355cm−1and represents a zone-edge A1g mode due

to the disorder. It is usually designated as the “D” peak that

means disorder [47,48]. For all other kinds of amorphous

and nanocrystalline carbon ﬁlms, the Raman spectrum typ-

ically shows a G peak centered around 1550cm−1andaD

peak centered at 1360 cm−1[49]. Ferrari and Robertson have

suggestedthat the G and Dpeaks are due to sp2only.Accord-

ing to them, the G peak is due to the bond stretching of all

pairs of sp2atoms in both rings and chains. This mode does

not require the presence of sixfold rings, and so it occurs

at all sp2sites, not only those in rings. It always lies in the

range 1500–1630cm−1, as it does in aromatic and oleﬁnic

molecules. The D peak is due to the breathing modes of A1g

symmetry involving phonons near the K zone boundary. This

modeisforbiddeninperfectgraphiteandonlybecomesactive

inthe presence ofdisorder [40]. To extract usefulinformation

from the Raman spectra, the following methods are usually

adopted.

2.1.1. Interpretation of Raman spectra

A novice researcher may initially be confused with the

designations used to describe various types of carbon ﬁlms

according to the structure or/and properties and that some of

them have overlapping and even conﬂicting meanings. Fol-

lowing are some of the established nomenclatures for various

types of carbon ﬁlms:

•a-C ﬁlms: softer carbon ﬁlms without hydrogen usually

formed at low energy or higher temperature.

•a-C:H ﬁlms: softer carbon ﬁlms with hydrogen.

•ta-C ﬁlms: tetrahedral amorphous carbon ﬁlms with high

content of sp3bonding and without hydrogen.

•ta-C:H ﬁlms: tetrahedral amorphous carbon hydrogen

ﬁlms with high content of sp3bonding.

•Nanocrystalline diamond ﬁlms: carbon ﬁlms with nano

diamond crystalline structure [50–56].

•Nanocrystalline graphite ﬁlms: carbon ﬁlms with nano

graphite crystal, usually prepared by annealing at higher

temperature after prolonged mechanical scrubbing, depo-

sition at a high temperature, or high-energy post-

irradiation [57–60].

•Glassy carbon ﬁlms: an interesting form of disordered car-

bon that microscopically consists of a mixture of graphite-

like ribbons or micro ﬁbrils. One may consider glassy

carbon as having a level microstructural disorder between

that of amorphous carbon and single-crystal graphite

[59,61–64].

•Polymeric a-C:H ﬁlms: softer amorphous carbon ﬁlms

with a high hydrogen content.

•High hardness graphite-like carbon ﬁlms: ﬁlms possess-

ing a graphite-like structure and relatively high hardness,

toughness and wear resistance [17,22,21,65–67].

The typical Raman shifts in diamond, single crystalline

graphite, commercial graphite, activated charcoal, and the

aforementioned amorphous and nanocrystalline carbon ﬁlms

are depicted in Fig. 2. However, it should be noted that the

appearance of the Raman spectrum depends on the wave-

length of the excitation laser. To enable easier comparison,

the abscissa ranges for all the Raman lines are made the same

from 1000cm−1to 2000 cm−1or 800cm−1to 2000cm−1.

The Raman peak composed of the D and G components

is one of the most widely investigated attributes in amor-

phous carbon ﬁlms. In graphite nanocrystalline carbon ﬁlms,

the D peak usually stands out and the width is smaller (only

256 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

the high temperature annealed nanographite ﬁlm is shown

in Fig. 2). As shown in Fig. 2, the ﬁrst peak is a sharp one

at 1332 cm−1with the typical shouldered amorphous carbon

peak as the background, and the other one has four separate

peaks located at around 1150 cm−1, 1350cm−1, 1500cm−1,

and 1580cm−1. The peak at around 1150 cm−1is assigned

to the nanocrystalline phase of diamond, 1500cm−1to dis-

ordered sp3carbon, and 1350cm−1and 1580 cm−1to the

popularly known D and G bands. The Raman spectrum of

glassy carbon ﬁlms resembles that of commercial graphite

Fig. 2. Typical Raman spectra acquired from different kinds of carbon ﬁlms (our data plus refs. [48,53,54,67]).

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 257

Fig. 2. (Continued).

but the intensity of D and G peaks is very different. Fig. 2

onlydisplaysschematicallytheRamanspectra expectedfrom

different kinds of amorphous carbon ﬁlms. In order to obtain

more details, more data analysis is necessary.

2.1.2. Peak ﬁtting

To reveal more information about the structure of the

carbon ﬁlms, the Raman spectra should be deconvoluted.

However, owing to the myriad of combinations of the car-

bon and carbon–hydrogen structures in the materials, ﬁtting

ofthe Raman spectrais not aneasy task andrequires different

processes.

The most common Raman spectra ﬁtting method is to

employ two Gaussian peaks with linear background or non-

linear background subtraction [46,68,69] with the G peak

centered at 1360 cm−1and the D peak at around 1580cm−1.

Itis based on the assumption that the amorphouscarbon ﬁlms

are composed of disordered sp2and sp3networks. The ﬁtted

peak intensity of D and G is used to evaluate Lathat reﬂects

thein-planecrystallitesize.Thesecondspectraﬁttingmethod

proposed by Prawer et al. [70] utilizes a Breit–Winger–Fano

(BWF) line shape and linear background subtraction meth-

ods. The BWF line shape is described by:

I(ω)=I0[1 +2(−0)/QΓ ]2

1+[2(−0)/Γ ]2(1)

where I() is the intensity as a function of frequency, I0the

maximumpeak intensity,0andΓthe peak positionand full

width at half-maximum (FWHM), respectively, and Qis the

258 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

BWF coupling coefﬁcient. In the limit that 1/Qapproaches

zero,the Lorentzian line shapeis acquired. Thus, atotal of six

parameters are required to ﬁt the data, namely I0,0,Q,Γ

and two parameters for the linear background. Employing

this ﬁtting method, the additional D peak is usually sub-

merged.Thisispossiblesincethereisnoindependentspecies,

for instance, well-isolated sp2and sp3domains in the ﬁlms

[70–73].Apart from theG and D peaks,some other peakscan

be observed occasionally [74], and to obtain a better ﬁt, addi-

tional peaks are usually introduced [75–78], for example, in

polymeric amorphous carbon ﬁlms or ﬁlms with adventitious

elements such as O, F or N [76–78].

It should be noted that based on the discovery that the

1100cm−1UV Raman peak may reveal information per-

taining to sp3bonding, visible Raman peaks should also be

used to probe sp3bonding. Chen et al. [79] used their results

and theoretical analysis and experimental results from others

[80–82] to conclude that two peaks at around 1168 cm−1and

1271cm−1are associated with sp3diamond bonding in ta-C

ﬁlms. Hence, the Raman spectra contain composite informa-

tion that must be deconvoluted by careful curve ﬁtting.

2.1.3. Peak position and width

Though the origin of both the D and G bands is not well

understood [20,43,70,83], the position and width of these

two peaks are usually used as a reference to determine the

deposition parameters, ﬁlm properties as well as structure.

The overall spectrum is characteristic of each type of carbon

but a single-wavelength Raman spectrum may be indistin-

guishable [37].Fig. 3(a) shows the density of a-C:H, a-C

(ﬁlm formed roughly at 10eV), and i-C (hydrogen-free ﬁlm

formed at 50–200 eV) ﬁlms as a function of the G line width

and Fig. 3(b) reveals the position of Raman G line as a func-

tion of the substrate bias for a-C:H ﬁlms [43].

Fig. 3. Relationship between the ﬁlm density and substrate bias [43]: (a)

density as a function of the G line width, and (b) G peak position as a

function of substrate bias.

It should be mentioned that even for the same sample, the

Raman peak position and width vary with the laser excitation

wavelength. Fig. 4 depicts the variation of dispersion of the

G peak using different excitation wavelengths. The G peak

disperses in more disordered carbon and the dispersion is

proportional to the degree of disorder, but it does not disperse

in graphite, glassy carbon, or nano crystalline graphite [37].

The FWHM of the peak decreases with increasing excitation

energy because of the gap variation of the carbon ﬁlms. A

Fig. 4. Effects of the excitation wavelength on the G peak position and FWHM [37]: (a) dispersion of G peak versus excitation wavelength and (b) dispersion

of FWHM.

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 259

higher excitation energy can excite a wider gap at the same

wavenumber. That is to say, to excite the same gap, a smaller

packet of wavenumber is needed for higher energy photon,

thus resulting in a smaller width [37,41]. The width of the

peak also increases due to the site-to-site variation in the

number of next nearest neighbors, that is, when sp2C atoms

have sp3C neighbors.

2.1.4. ID/IG

The ID/IGratio (intensity of the D peak to that of the G

peak) is known to vary in DLC ﬁlms synthesized by differ-

ent methods and parameters and sometimes even for ﬁlms

fabricated by the same method. It is believed that ID/IGis

related to the size of the graphite planes in the DLC ﬁlms

[69,84],andthishas been used to analyze the thermalstability

and frictional properties of the materials [77,85–88]. The D

peak intensity (ID) usually increases after annealing at above

300◦C or after a long friction test. This phenomenon is gen-

erally ascribed to the conversion of sp3bonds to sp2bonds,

desorption of hydrogen, and conversion of carbon structure

to nanocrystalline graphite [87].

The use of ID/IGto evaluate the size of the graphite cluster

size was proposed by Tuinstra and Koenig [48]. They stud-

ied the relationship between the ID/IGratio measured from

the Raman spectrum and the graphitic in-plane microcrys-

tallite size, La, obtained from X-ray diffraction. Based on a

series of measurements on microcrystalline graphite samples

with varying microcrystalline sizes, they observed a linear

correlation between ID/IGand 1/La[40,48,89,90]:

IG=Cλ

La(2)

where is C515.5nm is ∼44 ˚

The ID/IGratio has been successfully used to evaluate

Lain the study of properties such as the degree of disor-

der in amorphous carbon, nano graphite crystal size, and

the domain for glassy carbon ﬁlms. However, direct corre-

lation is not as straightforward when the ID/IGvalue is larger

than the maximum value observed in the microcrystalline

graphite studies. For instance, Cho et al. [91] have observed

thatwhenthevalueisover1.1,thelinearrelationshipbetween

ID/IGand the inverse of the microcrystallite size 1/Lamay no

longer hold. They ascribe the increase in the ID/IGratio to the

increase of the number of graphite microcrystallites with less

defects in lieu of the increase in La. This also explains the

increase in the intensity of D band for most amorphous ﬁlms

after annealing at a higher temperature [92–94]. A higher

annealing temperature increases the short-range order but

not La.

2.2. UV Raman

The success of conventional visible Raman spectroscopy

to differentiate different types of amorphous carbon ﬁlms

[95–97]hasledtothe developmentofRamanexcitationusing

ultraviolet light. This technique is a powerful method to char-

acterize the carbon ﬁlm structures and is considered to be

a future method to characterize amorphous and nanocrys-

talline carbon ﬁlms especially with respect to sp3C atoms

[98–100]. The advantage of UV Raman is its higher excita-

tion energy than visible Raman. The Raman intensity from

sp3C can be increased while previously dominant resonance

Raman scattering from sp2C atoms is suppressed. Since the

overall Raman intensity is proportional to ω4, where is

the frequency of incident photons, UV excitation is more

advantageous considering the weak Raman signal from thin

DLC ﬁlms [99]. Hence, UV Raman spectroscopy is the pre-

ferred technique to probe the surface structure because UV

only excites the topmost ∼10–15 nm of the sample surface at

224 nm [37]. For lower excitation energies, the probed depth

is larger.

Gilkes et al. [98] and Merkulov et al. [99] have proposed

that if there are sp3structures in the carbon ﬁlms, there will

be UV excitation at around 1060–1100 cm−1due to the C–C

sp3vibration. This transition only manifests under UV irra-

diation [40].Fig. 5 displays the UV Raman spectra (244 nm)

of sputtered a-C ﬁlms with different sp3contents from 6% to

75% based on the results of Merkulov et al. The intermediate

frequency peak at 1150cm−1is assigned to sp3C excitation

with some sp2contributions. It is observed that:

1. one intermediate frequency peak at ∼1180cm−1appears

in the UV Raman spectra of the diamond like amorphous

carbon ﬁlms;

2. the G peak shifts from ∼1580 cm−1to ∼1620 cm−1as

the fraction of sp3C increases from ∼6 at.% to ∼75 at.%;

3. the peak widths also increase with higher sp3fraction.

Using peak ﬁtting, it is found that sp3C contributions to

the peak at 1150cm−1have a striking resemblance to the

theoretical phonon density of state (PDOS) of the sp3net-

Fig. 5. UV Raman spectra of the carbon ﬁlms containing ∼6–75at.% sp3

carbon atoms [99].

260 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

work calculated by Wang and Ho [80]. A conclusion can

thus be drawn that the peak shift and widening are due to the

site-to-site variation in the number of the sp2C and sp3C

neighbors.

Consistent with those observed by Merkulov, Gilkes’s

results reveal an additional peak, width increase, and shift

to higher wavenumber of the G peak. It is also found that the

peakat 1100 cm−1isclose to a main peak as suggested bythe

vibrational density of states (VDOS) calculated by Beeman

etal. [81] fora fully sp3-bondedcontinuous random network.

They attribute the shift in the G peak to a larger contribution

from oleﬁnic (chain) sp2groups.

It should be noted that in UV Raman, the sensitivity for

the sp2Raman vibrations is still stronger than that of the sp3

by a factor of 5–10 [12]. Hence, in the analysis of carbon

ﬁlms by UV Raman vibration, the sp2peak excitation can

still dominate. It is thus too early to draw the conclusion that

UV Raman is the most powerful method to determine the sp3

fractionfrom any kindof carbon ﬁlms because therehas been

relatively little literature on this topic.

2.3. Infrared spectroscopy

Infrared spectroscopy is about 100 years old whereas

Raman spectroscopy is more than 70. Although the exci-

tation mechanism is somewhat similar, infrared and Raman

spectroscopies have evolved separately. While infrared spec-

troscopy has quickly become the workhorse of vibrational

spectroscopy in the industry and analytical laboratories,

Raman spectroscopy has hitherto been limited to research

activitiesprimarily[44].Infraredspectra are usually recorded

by measuring the transmittance of light quanta. The frequen-

cies of the absorption bands are proportional to the energy

difference between the vibrational ground and excited states

as illustrated in Fig. 6. The absorption bands due to the vibra-

tional transitions are found in the wavenumber region of

ν=4000–10 cm−1[44].

Fig. 6. Principle of infrared absorption: (a) quanta of the energy hν1,hνs

and hν2impact the molecule and only hνsis absorbed; (b) term diagram; (c)

infrared absorption spectrum [44].

Fig. 7. IR spectrum of an a-C:H ﬁlm [102].

Amorphous carbon ﬁlms consist of different percentages

of sp2,sp

3and even sp1carbon atoms and hydrogen atoms

are commonly incorporated into the carbon network. Owing

to the localized nature of C H vibrations and the sensitiv-

ity of their frequency to the nature of the C C bonds, the

analysis of the C H vibrational modes using infrared spec-

troscopy can be used to characterize the amorphous carbon

ﬁlms,especially for hydrogenated carbon ﬁlms [47,101]. The

ﬁrst assignments of vibrational frequencies to distinguish

CH modes were made by Dischler et al. [102,103]. The

IR absorption spectrum consists of C H stretching modes at

2800–3300cm−1and C C modes and C H bending modes

below 2000cm−1are exhibited in Fig. 7 [102]. Dischler

published a complete list of all possible C H vibrations

(10 stretching, 17 bending, and 1 torsion) for polymeric

anddiamond-like amorphous carbon [104]. Summarizing the

resultsof Dischler [104] andHeitz et al. [105],Robertson for-

mulated the assignments of the various modes in a-C:H, as

shown in Table 1.

Aside from qualitative characterization the spxC–Hy

types, efforts have been made to quantitatively determine

the sp2/sp3ratio, analyze the C H vibrations in terms of

their absorption strength, and derive the total amount of H

atoms [101,105–110]. However, accurate quantitative deter-

mination of the amorphous composition by these methods is

difﬁcult because a model of amorphous carbon ﬁlm structure

is quite complicated and has not been conclusively derived.

In addition, the IR spectra acquired from the ﬁlms tend to be

a composite of many different vibrations of C H bonds.

Another common utility of IR spectra is to examine the

optical gap of the carbon ﬁlm. The optical gap of the amor-

phous ﬁlm can be assessed using IR transmission and reﬂec-

tion measurements [102,111–113]. By extrapolation using

the following relation (Tauc relation), the abscissa can yield

the optical gap Eopt [102,113,114].

(αE)1/2=G(E−Eopt) (3)

where Eis phonon energy (hν), Gis the gradient of line; Eis

the photon energy, αis the coefﬁcient. In addition,

T=(1 −R)2exp(−αd) (4)

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 261

Table 1

Assignments of IR vibrational frequencies in a-C:H [47]

Wavenumber(cm−1) Conﬁguration Oleﬁnic or

aromatic Symmetrical or

asymmetrical

3300 sp1

3085 sp2CH2Oleﬁnic A

3035 sp2CH Aromatic

2990–3000 sp2CH Oleﬁnic S

2975 sp2CH2Oleﬁnic S

2955 sp3CH3A

2920 sp3CH2A

2920 sp3CH

2885 sp3CH3S

2855 sp3CH2S

1480 sp3CH3A

1450 sp3CH2A

1430 sp2CH Aromatic

1415 sp2CH2Oleﬁnic

1398 sp3(CH3)3S

1375 sp3CH3S

C–C

2180

1640 sp2Oleﬁnic

1580 sp2Aromatic

1515 sp2/sp3

1300–1270 sp2/sp3

1245 sp2/sp3

where Tand Rare, respectively, the transmission and

reﬂection coefﬁcients measured in the photon energy range

between 2 and 6eV, and dis the ﬁlm thickness. It should be

noted that the optical gap can also be deﬁned as the energy

at which the optical absorption coefﬁcient reaches 104cm−1

[111,115,116].

3. Electron spectroscopy and microscopy

3.1. X-ray photoelectron spectroscopy (XPS)

When the surface is irradiated with X-rays, electron emis-

sion results due to the photoelectric and Auger effects. The

photoelectron emission process is illustrated in Fig. 8. The

Fig. 8. Schematic of the emission process of photoelectrons excited by X-

rays.

kineticenergyoftheemittedphotoelectronsisgivenby[117]:

EK=hν −Eb−Ws(5)

wherehνis the energy of the photon, Ebthe bindingenergyof

the atomic orbital from which the electron originates, and Ws

is the spectrometer work function. The technique is called

X-ray photoelectron spectroscopy (XPS) and also referred

to as electron spectroscopy for chemical analysis (ESCA) in

older literature. Because each element has a unique set of

binding energies, XPS can be used to identify and determine

the concentration of the elements within the escape depth of

thephotoelectronsin the near surface region. Variationsinthe

elemental binding energies (the chemical shifts) arise from

the differences in the chemical potential and polarizability of

the compounds. These chemical shifts can be used to identify

the chemical state of the materials [117].

Compared with optical analysis methods, XPS is not as

frequently used in the analysis of carbon ﬁlms since it cannot

detect hydrogen. However, because it can reveal the binding

energy of the carbon atoms and discern the hybrid sp3and

sp2bonds, it is a very powerful method to evaluate the struc-

ture of amorphous carbon ﬁlms without causing excessive

damage to the materials [118–127]. Furthermore, because of

the lack of direct binding energy information from optical

characterization methods, XPS is complementary to optical

characterization methods [119,128]. Owing to its high sensi-

tivity to chemical shifts or the chemical environment of the

probed atom, it is a very useful tool to characterize the struc-

tures of the doped amorphous carbon ﬁlms [125,129–132].

The C1s peak position in diamond is 285.50eV that is

about 1.35 eV higher than that in graphite (284.15eV) [133].

The ionization cross-sections in the core levels are exclu-

sively dependent on the atomic factors. The intensity of the

core-level peaks is then directly proportional to the density of

atoms [118]. If the C1s core-level binding energies of the sp3

and sp2hybrids in a-C are different, as in graphite [134] and

diamond [135], the C1s peak position will vary and can be

deconvoluted into two subpeaks, and so proper peak ﬁtting

can reveal the sp3to sp2ratio. Fig. 9 shows the C1s spec-

tra acquired from amorphous carbon ﬁlms deposited using

different vacuum arc deposition parameters shown in Table 2

[119].Theshiftof the peak position is obvious.Bycomparing

with the Raman shift result, Li et al. have drawn a conclusion

that XPS is very useful in supplementing the optical charac-

terization (Raman) of amorphous carbon ﬁlms [119].

As aforementioned, XPS is a useful method to assess the

sp3and sp2components. It should be noted that it is not nec-

essary to use reference samples since the chemical shifts of

the C1s photoelectron spectra due to sp3and sp2do not bear

a direct relationship with the matrix [118]. The intensity of

the binding energy is linearly proportional to the fraction of

sp3and sp2bonds and the ionization cross sections are inde-

pendent of the chemical state of the atoms for X-ray photons

with energies well above the ionization threshold [120,136].

Diazet al. [118] proposed a sp3and sp2identiﬁcation method

262 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

Table 2

DLC ﬁlms preparation conditions [119]

Sample Focusing coils

current (A) Filter coils

current (mA) Arc current (A) Arc voltage (V) Gases (SCCM) Working vacuum (Pa) Bias current

(mA) Deposition

time (min)

1 6/2 – 78 40–52 Ar:130 3–4×10−204

2 7/3 – 100 25–30 Ar:40 6×10−2100 3

3 7/3 – 66 20–30 Ar:60 6.4–7×10−2150 1

4 7/2.5 – 82 20–30 Ar:70; C2H2:40 7×10−21000 5

5 7/2.5 – 82 28–32 Ar:70 7×10−21000 10

6 5/2 220 50 30–32 Ar:40 8×10−2150 20

7 5/2 220 50 30–32 Ar:126 5×10−2600 20

by assuming a higher binding energy due to sp3hybrids and

the higher binding energy of the C1s core level in diamond

[137] than in graphite [134,135].

In order to deconvolute the C1s spectra, the diamond sur-

faceandbulkC1sspectradecompositionmethodproposedby

Morar et al. [135] can be used. In this method, the measured

spectra are ﬁrst ﬁtted by using a Lorentzian-shaped peak to

represent the broadened C1s level. The Lorentzian peak is

convoluted with a Gaussian-shaped broadening function and

added to a modeled background. The Gaussian represents

broadening resulting from variations in the position of the

Fermi level within the band gap as well as any vibrational

broadening. The modeled background includes an intrinsic

secondary electron component with a contribution propor-

tional to the integral under the photoemission peak and an

extrinsic secondary electron component that is not directly

associated with the C1s photoemission process.

Fig.9. XPSspectraacquiredfromDLCﬁlmswiththediamondandgraphite

C1s peak position shown as dotted lines [119].

By subtracting the background, Diaz decomposed the C1s

spectra peak using ﬁve variable parameters: Gaussian width,

binding energies of sp3and sp2, and the singularity index for

the sp2component. In the calculation performed by Haerle

et al. [120], four additional parameters are added: intensities

of the two components plus two used for deﬁning a Shirley

background proportional to the integrated peak intensity. For

the sp2component, the Doniac–Sunjic function is usually

used [118,120,138]. In this process, convolution of a Gaus-

sian and a Lorentzian with an additional parameter allowing

for asymmetry in the line shape is employed. This function

is used to account for the asymmetric line shapes resulting

from the screening due to electron-hole pair excitations at the

Fermi energy.

Because some of the amorphous carbon ﬁlms have poor

conductance,specialcareshouldbeexercisedtoavoid sample

surface charging during the XPS analysis. The success can

be judged by showing that the peak position is independent

of the intensity of the incident X-rays.

3.2. Auger electron spectroscopy (AES)

Auger electron emission is initiated by the creation of an

ion with an inner shell vacancy induced by bombardment of

electrons, X-rays, or ions. Electron beam excitation is most

common due to the ability to focus an electron beam onto

a small area for microanalysis, as illustrated in Fig. 10. The

kinetic energy of an Auger electron is equal to the energy

difference of the singly ionized state and the double ionized

ﬁnal state. For an arbitrary ABC transition in an atom of

atomic number z, the Auger electron kinetic energy is given

by the difference in the binding energies of energy levels A,

BandC[139]:

EABC(z)=EA(z)−EB(z)−E∗

C(z)−Ws(6)

where Wsis the spectrometer work function and E*is the

binding energy of a level in the presence of a core hole and

is greater than the binding energy of the same level in a neu-

tral atom. Auger transitions are typically designated by the

energy of the electrons involved using X-ray spectroscopy

nomenclature. The ﬁrst label corresponds to the energy level

of the initial core hole, whereas the second and third terms

refer to the initial energy levels of the two electrons involved

in the Auger transition. AES is usually used to determine the

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 263

Fig. 10. Schematic diagram of Auger electron spectroscopy. (II-a) KLIILIII transition, or simply a KLL transition and X-ray ﬂuorescence (II-b).

elemental composition and in some cases also the chemical

state of the atoms in the surface region of a solid material

[139]. Coupled with scanning capabilities, AES can be used

to map the distribution of the elements in the near surface

region within the escape depth of the Auger electron. Com-

positional depth proﬁles can also be obtained by performing

ion sputtering in concert with AES.

AES can be used to characterize amorphous carbon ﬁlms

[121,140–148], although it is not as common as other non-

destructivetechniques.Mizokawaetal.[149] haveconﬁrmed

that high-energy electron bombardment can affect the sur-

face structure of the carbon ﬁlms. After long time electron

irradiation, the AES spectra from DLC ﬁlms resemble that of

softamorphouscarbon[149,144].WhileX-rayexcitedAuger

electron spectra (XAES) can be used to study the amorphous

carbon ﬁlms structures and be used as a ﬁngerprint of the

carbon state and the characterization of diamond-like carbon

ﬁlms [149,141,145], it does not mean that electron excited

AES cannot be used. In fact, AES has been used to character-

ize the chemical states of the carbon ﬁlms [150] and to derive

the sp2/sp3ratio [122], provided that electron beam damage

is understood.

3.3. Transmission electron microscopy (TEM)

A TEM works like a slide projector. The electron beam

is the light beam. The electrons pass through the thinned

materials (slide) and the output (image) is modulated by

the structure (opaqueness) of the materials. The intensity

of the transmitted electron beam varies with the sample

structure. The transmitted image is magniﬁed and projected

onto the viewing screen. The resolving power is deﬁned by

d=λ(nsinα), and since the wavelength of electron beam is

much less than that of the visible light (wavelength of green

lightis 500 nm versusλ= 0.004 nm for100 keV electron) and

thereisverylittleelectron deﬂection through a thinspecimen,

muchhigher resolution (atomicscale) than that achievableby

optical microscopy and even conventional scanning electron

microscopy can be obtained. With respect to the charac-

terization of amorphous carbon and nanocrystalline carbon

ﬁlms, three kinds of TEM imaging techniques are usually

used.

3.3.1. Electron diffraction (ED)

When the atoms plane space satisﬁes Bragg’s Law

d=nλ

2 sinθand some other conditions, the electron

diffraction pattern can be obtained. The simplest application

is to identify crystalline substances based on the spacing of

atomic planes within their structures. More detailed analy-

sis of the ED patterns can provide important information on

the orientation relationship between crystalline phases (such

as coatings on substrates, precipitates in materials, etc.), the

nature of defects in solids, order–disorder effects, and crys-

tallite size analysis.

3.3.2. Dark ﬁeld (DF) imaging

The technique utilizes a single diffracted beam to form

the image in a TEM. This causes all regions of the specimen

not of the same crystal structure and orientation in the region

that produces the diffracted beam to appear dark in the ﬁnal

image. The method allows visual phase differentiation in the

TEM.

3.3.3. Bright ﬁeld (BF) imaging

It is a imaging mode in a TEM that uses only unscat-

tered electrons to form the image. Contrast in such an

264 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

image is due entirely to thickness and density variations in a

sample.

3.3.4. High resolution imaging

Imaging in the HRTEM mode is accomplished by allow-

ing some of the diffraction image to overlay the bright ﬁeld

image thereby enhancing the contrast along the lattice lines.

Itallowsdirectmeasurement of lattice parameters,inspection

of individual defects and grain orientation.

Becausethecarbon atoms are randomlydisorderedor pos-

sess very short range order in the ﬁlms, a TEM image of

amorphous carbon ﬁlms usually exhibits the same homoge-

neous brightness image either in the DF or BF mode, just like

that of other amorphous materials. The DF patterns show a

darkblurred ring without abright spot. TEMimages acquired

from amorphous carbon ﬁlms do not convey much structural

information, but it is a powerful tool to judge whether the

ﬁlmisamorphous, i.e. whether the carbonatomshaveenough

short distance order or whether its ED pattern manifests as a

diffuse intensity ring [145,151].

Though TEM images do not reveal much information

about amorphous carbon ﬁlms, it is very useful to assess

nanocrystalline carbon ﬁlms. It can be used to determine

the nanocrystalline size, structure, distribution [152–154],

whether or not the nano particles are nano diamond crys-

tals or nano graphite crystals, the ﬁlm thickness by means of

cross-sectional transmission electron microscopy [151,155],

and to study the nucleation and growth mode through the

studies of nano crystal orientation and the relationship with

the substrate [156]. It can also be used to study the amor-

phous→nanocrystalline transition. After high temperature

annealing or long term chemical rubbing, it has been shown

that crystallization of the amorphous carbon has occurred by

TEM [157].

3.4. Electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy is a very powerful

method to unveil the detailed structures and have been used

largely to research amorphous and nanocrystalline carbon

ﬁlms[36,47,158–163]. When an electron beam interacts with

a ﬁlm, it can give up all or part of its energy in the process.

The energy loss corresponds to the ﬁlm electron arrangement

of the solid. By collecting the electron energy loss informa-

tion, the bonding, oscillation, or vibrational information of

the solid can be obtained [44].

Electron energy loss spectroscopy and EELS are the gen-

eral name and acronym for the techniques whereby a beam of

electronsisallowedtointeractwith the materials and thescat-

tered beam of electrons is spectroscopically analyzed to pro-

vide the electron energy spectrum after the interaction [164].

EELS can be classiﬁed as transmission EELS which is usu-

ally conducted in a transmission electron microscopy (TEM)

environmentandis usually referred toby the acronymsEELS

using serial spectrometer or parallel EELS (PEELS) using

parallel spectrometer, and surface EELS which is solely con-

cernedwith the interaction of anelectron beam with a surface

of a material and usually has the acronym CEELS (core-

electron energy loss spectroscopy), ELS [electron (energy)

loss spectroscopy], or HREELS (high resolution electron

energy loss spectroscopy)] [164,44,165]. Because the pri-

mary electron energy in HREELS is typically 2–5eV, its

energy is so low that it is usually used only for the analy-

sis of molecules attached to the surface and the very shallow

structural or vibrational message. It is therefore rarely used

to unravel the details of carbon ﬁlm structures [166–169].

A typical EELS spectrum can be divided into three sepa-

rate regions:

•The zero-loss peak formed by the electrons of unscattered

and elastically scattered electrons: in the analysis of the

structural of the carbon ﬁlms, the width of the zero-loss

signalcanprovidethe resolution of thespectrometer[170].

•The low-loss region formed by the stimulated electronic

transitions within the valence band of the solid, i.e. of only

a few eV, and by stimulated collective oscillations of the

electron sea of the solid: the energy loss usually extends to

several tens of eV and for carbon ﬁlms, it usually ranges

from 0eV to 40 eV [47,59,161,158].

•Higher energy losses resulting from electron energy losses

due to the ionization under the interaction between the

primary electrons and the inner shell electrons: the higher

energy loss distribution usually has a long tail stretching

to zero energy. For carbon ﬁlms, the K edge is usually at

285eV and above.

EELS has been shown to be a very useful tool in

the unambiguous identiﬁcation of diamond and graphite

[162,140,171]. For diamond in the higher energy loss region,

only the K edge peak at around 290eV due to the excitation

of ␴*states of sp3appears, but the peak at 285eV due to

excitation to ␲*states cannot be observed. Both the 285eV

and 290eV peaks will appear in graphite EELS. The latter

peak is due to the ␴*states of sp2. It can be postulated that

in amorphous carbon ﬁlms or in the nanocrystalline carbon

ﬁlms, the 285eV peak and 290 eV peak will appear because

the ﬁlm structures are usually composed of either sp2or sp3.

Typical carbon K edge electron energy loss spectra acquired

from different kinds of carbon ﬁlms are shown in Fig. 11

[47,172].

Aside from the qualitative characterization of the amor-

phous carbon ﬁlm structures, the high loss region in the

carbon ﬁlm EELS spectrum is usually used to yield quan-

titative information of the sp3and sp2contents. The most

widely used quantitative sp3and sp2analysis method is pro-

posed by Berger et al. [9]. In this method, graphite is used as

a 100% sp2standard reference in the form of combined test

specimen for electron microscopy. In the 100% sp2graphite,

the ratio of the integrals under the 285eV and 290eV peaks

is kept the same (1:3). For diamond (100% sp3), the 285eV

peak does not appear at all. By considering that the inten-

sity of ␲*the feature is taken to be directly proportional to

the number of ␲-bonded electrons in the material, the sp2

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 265

Fig. 11. Typical carbon K edge electron energy loss spectra measured from

different types of carbon ﬁlms [47,172].

fraction is calculated as follows:

x=[I␲/I␲+␴]ﬁlm

[I␲/I␲+␴]reference (7)

where I␲and I␲+␴are the integrated intensity of the ␲*and

␲*+␴*features, respectively, and the subscripts “ﬁlm” and

“reference” refer to the ratio determined from the ﬁlm and

the 100% sp2reference.

The graphite crystalline reference will result in an orienta-

tion dependence side effect, and to eliminate it, Alexandrou

et al. have proposed to use C60 as a reference to eliminate

the orientation problem completely [21,173]. By using the

known 1:3 ratio of ␲to ␴bonds for 100% sp2and 0:4 ratio

of ␲to ␴bonds for 100% sp3, Cuomo et al. [160] have pro-

posed another formula to calculate the atomic fraction of sp2

bonded carbon:

[I␲/I␴]ﬁlm

[I␲/I␴]reference =3x

4−x

where xis the atomic fraction of sp2bonded carbon. I␲and

I␴are the integrated intensity in the range between 284eV

and 289eV and from 290 eV to 305eV, respectively.

UsingKedge EELS to calculatethesp2fraction, the selec-

tion of the proper energy windows (the integration boundary)

needs particular attention. Berger et al. [9] have shown that

the sp2fraction reaches a stable value for an energy window

greater than ∼50 eV. In addition, Robertson has proposed the

following points [47]:

1. the ﬁlms thickness should be appropriate;

2. the ␲*peak in graphite is excitonically enhanced com-

pared to the simple ␲*peak in the conduction band DOS;

3. the reference sample should be randomly oriented micro-

crystalline graphite because the ␲*peak is highly polar-

ized [174];

4. additional peaks between 285 eV and 290eV may appear

in hydrocarbon ﬁlms and fullerenes [175–179].

By assuming that the carbon ﬁlm is composed entirely

of sp2and sp3carbon and considering N␲/N␴, the ratio of

the number of ␲and ␴orbitals for 100% sp2and 100% sp3

bondedcarbon,thenumberfractionof sp3bonded atoms (FN)

is given by Pappas et al. [34] as follows:

FN=1−3N␲/N␴

1+N␲/N␴

(8)

where

N␲

N␴=[I␲/I␴]ﬁlm

3[I␲/I␴]reference (9)

The integrated boundaries for I␲and I␴are from 284eV to

289eV and 290 eV to 305eV.

The low energy loss region is usually from 0eV to 40eV

(there is usually a peak at 63±2 eV due to double loss to

bulk plasmons), and the shifts in the position of the loss

peak associated with the collective excitation modes of the

valence electrons can be correlated to the change in the elec-

tron density [64]. Graphite displays a peak at 27.0±0.5 eV

attributable to (␴+␲) electron excitations while the peak

at about 6–8eV is due to ␲electron excitations [158,180].

The low energy loss region of diamond has been studied

in details by Armon and Sellschop [181], Lurie and Wil-

son [140], Roberts and Walker [182], Himpsel et al. [183],

and others. The typical low energy loss spectra are shown

in Fig. 12 and the possible meaning of the peaks is given in

Table 3.

The plasmon oscillations of the valence electrons reveal

a broad peak in the low energy EELS spectra, and this has

been used to evaluate the mass density of the carbon ﬁlms

and the sp3fraction in the carbon ﬁlms [173]. It should be

noted that the peak position of the bulk plasmon mode in

low-energy electron energy loss spectroscopy may vary with

the substrate type, even if ﬁlm deposition is carried out at the

same temperature [160].

4. Surface morphology

4.1. Atomic force microscopy (AFM)

Scanning probe microscopy (SPM) is a powerful tech-

nique for the accurate measurement of surface morphol-

ogy and properties. Two of the more common SPM tech-

niques are atomic force microscopy (AFM) and scanning

266 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

Table 3

Low electron energy loss peaks of diamond (type-IIa single-crystal diamond)

Elastic Peak

P1P3P4P5P6P7

Energy Loss (eV) 0 6.5 ±1.0 12.5±0.5 16.5 ±0.8 22.7 ±0.8 34.0 ±0.5 63 ±2

Relative intensitiesa75 0.71 0.93 0.43 1.36 1 0.05

Connotation Elastic peak Γ25 →Γ15 Can be assigned to several electronic

transitions between crystalline levels:

Σ2→Σ3

L

3→L3

X4→X1

L

3→L1

Γ25 →Γ2or X1→X1Γ25 →Γ1and/or loss to surface

plasmons or contaminationbLoss to Bulk

plasmons Double loss to

bulk plasmons

aAssume that the intensity of P6is 1 and the relative intensity is only given as a reference data. It will vary with different measurement situation.

bP5is still not well understood. It has been attributed to surface plasmons, interband transition, or contamination [161].

Fig. 12. Typical low energy loss spectra acquired from various carbon ﬁlms.

tunneling microscopy (STM). AFM is a three-dimensional

surface topography imaging technique whereas STM pro-

vides pictures of atoms on surfaces. STM and AFM pro-

vide sub-nanometer resolution in all three dimensions, but

because a voltage is exerted onto the sample in STM, the

technique is limited to conducting and semiconducting sam-

ples. On account of the low conductance of most amor-

phous or nanocrystalline carbon ﬁlms, AFM is more widely

used.

AFM measures the local attractive or repulsive forces

betweentheprobetip and sample surface [184,185].AnAFM

instrument uses a micro-machined cantilever with a tip at the

end to sense the sample surface. As the tip is repelled by

or attracted to the surface, the cantilever beam deﬂects. The

magnitude of the deﬂection is captured by a laser that reﬂects

at an oblique angle from the very end of the cantilever. As

the tip is rastered over the sample, the vertical deﬂection are

recorded and displayed to produce an AFM image. AFM can

achieve a resolution of 0.01nm, and unlike electron micro-

scopes, can image samples in air or liquids.

AFM is used in many applications and materials includ-

ing thin and thick coatings, ceramics, composites, glasses,

synthetic and biological membranes, metals, polymers, and

semiconductors. With regard to the morphology of amor-

phous carbon and nanocrystalline carbon ﬁlms, the appli-

cation is quite straightforward. AFM study of carbon ﬁlms

deposited from mass selected C+ions reveals a clear corre-

lation between the surface roughness and degree of diamond

like(sp3)properties[186].Byvaryingthedeposition parame-

ters,thecorrelationbetween the carbon ﬁlm surfacemorphol-

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 267

Fig. 13. AFM images of (a) VAD sample and (b) PIII-D sample.

ogy and the deposition parameters can be readily determined

[186–188].

Besidesmorphological observation and surface roughness

calculation [189], AFM is also an excellent tool to investi-

gate the nucleation and growth mechanism of amorphous

carbon ﬁlms [190,191]. By comparing the small difference

revealed by the AFM images displayed in Fig. 13 and consid-

ering the different ﬁlm growth phenomena observed for dia-

mond like carbon ﬁlms fabricated by vacuum arc deposition

(VAD) and plasma immersion ion implantation-deposition

(PIII-D), Li et al. have adopted a statistical formation theory

to eluciate the possible nucleation and growth mechanisms

[190].

4.2. Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is perhaps the most

widely used analytical technique. The basic components in a

scanning electron microscope are depicted in Fig. 14 [192].

In an SEM instrument, a voltage is applied to an electron

emitting ﬁlament typically made of W or LaB6to produce

an electron beam. The electrons are collimated and focused

268 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

Fig. 14. Schematic diagram showing the basic components of the SEM

[192].

by electron optics including condenser lenses and objective

lenses and eventually rastered by a set of scanning coils

onto the sample surface. The electrons interact with the top

few nanometers to several microns of the sample. Secondary

electrons are emitted from the sample surface carrying infor-

mation of the surface topography and they are then detected,

ampliﬁed, processed, displayed, and stored.

SEM can be used to assess the surface morphology of

amorphous carbon ﬁlms and nanocrystalline carbon ﬁlms

[157,193–195]. However, because amorphous carbon ﬁlms

usually display a smooth morphology, especially when a

semiconductor wafer is used as the substrate, SEM does

not convey much detail about the ﬁlm structure. Moreover,

because the electrical conductance of amorphous carbon

ﬁlms is often low, surface charging can occur and distort

the resulting SEM image. In general, SEM is used in the

following situations:

1. SEM can be used to determine nano-structured com-

pounds or particles formed on the amorphous carbon

ﬁlm surface or ﬁlm delamination due to mismatch of the

ﬁlm and substrate. An example is shown in Fig. 15(a)

[193,194,196–199].

2. The surface morphology of amorphous and nanocrys-

talline carbon ﬁlms can be monitored, especially for the

latter ones and an example is illustrated in Fig. 15(b)

[153,200–203].

3. It can be used to observe the cross section of a ﬁlm to

determine the ﬁlm thickness, to deduce the ﬁlm growth

rate, or to investigate the growth mechanism. It is also

proper to observe the cross section of multi-layered ﬁlms

and an example is displayed in Fig. 15(c) [157,204,205].

4. It is an excellent tool to observe the friction tracks and

analyze the debris after a friction test on the amorphous

Fig. 15. SEM images of various carbon ﬁlms to demonstrate the versatility of SEM: (a) RF plasma assisted pulsed laser deposited carbon ﬁlms [197]; (b)

nano-diamond ﬁlms deposited by direct current glow discharge assisted chemical vapor deposition [153]; (c) 1.5␮m DLC ﬁlm deposited at 60◦ion incidence

[205]; (d) corrosion spots on a disk after corrosion tests [206].

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 269

ornanocrystalline carbon ﬁlms. Itis thus suitableto assess

theresults of mechanical orchemicaltreatment. An exam-

ple is shown in Fig. 15(d) [157,206,207].

5. Nuclear magnetic resonance

Many isotopes such as 13C and 1H possess nuclear spin

or angular momentum. Since a spinning charge generates

a magnetic ﬁeld, and the angular momentum results in a

magnetic moment. When placed in a magnetic ﬁeld B0, the

spinning nuclei do not all align with their magnetic moments

in the ﬁeld direction. By applying a radio-frequency ﬁeld of

an appropriate frequency orthogonal to B0, the nuclear spins

will experience resonance and become aligned. At this res-

onance frequency, the nuclear magnetic dipoles in the lower

energy state ﬂip over and by detecting the ﬂipping, struc-

tural information can be gained by the technique of nuclear

magnetic resonance (NMR) spectroscopy [208].

It is generally accepted that NMR yields quantitative

results of the sp3/sp2ratio in amorphous and nanocrystalline

carbon ﬁlms [209,210]. In fact, it has been used as a stan-

dard method to calibrate the sp2/sp3ratio derived from other

methods [211]. In the sp2/sp3ratio measurement, the sp2and

sp3carbon NMR spectra show two separate peaks at differ-

ent chemical shift positions. The sp2and sp3peak chemical

shifts vary slightly in different types of amorphous carbon

ﬁlms. Some of the sp2and sp3NMR peak positions of the

carbon ﬁlms are summarized in Table 4.

NMR is thus a relatively simple and powerful method to

determine the sp2/sp3ratio because each nucleus gives rise

to the same integrated NMR signal intensity regardless of

its chemical environment. A typical NMR spectrum is dis-

Fig. 16. NMR spectra of carbon ﬁlms [47,218,220–222].

played in Fig. 16. The sp3and sp2atoms show two separate

peaks, especially in cross polarization magic angle spinning

(CPMAS) NMR spectra, and the ratio of sp2to sp3is equal to

the peak integrated radio. By combining the results from 13C

NMR and 1H NMR, the hydrogen content as well as detailed

CH bonding information can be obtained [216].

The limitation of NMR is that the natural abundance of

13C is relatively low (about 1.1%) [220], and a large amount

Table 4

Examples of peak positions of sp2and sp3in NMR spectra acquired from carbon ﬁlms

Film types sp2Peak position (ppm) sp3Peak position (ppm) sp2/sp3H (at.%) Deposition Ref.

Nature diamond powder 120 36 ±2∼0/100 [212]

Graphite 135 – 100/0 0 [213]

DLC 140 50 1.5–1.7 0.3–0.4 [214]

a-C:H 130 40 0.55 – [211]

0.34

0.19

a-C:H 130 40 1.44 0.35 [215]

a-C:H 130 40 2.35 0.34 [216]

1.85 0.40

1.25 0.42

a-C:H 140 50 1.63 – [217]

a-C 130±5 62 14.6 <0.15 [218]

a-C:H 130 40 0.16 0.61 [219]

0.25 0.58

1.0 0.47

1.25 0.31

1.63 0.35

a-C:H 140 39 – [220]

270 P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277

of materials (typically more than 30mg) is needed for the

analysis [211]. Although the technique is simple and infor-

mative, it is not a very common practice and the researchers

must be aware of the following ambiguities and precautions:

•In two cases, particular carbon atoms may be undetected.

Firstly, the lifetime of carbon atoms near unpaired elec-

trons may be broadened beyond detectability, and sec-

ondly, carbons present in hydrogen deﬁcient structures

may be difﬁcult to detect due to excessively long cross

polarization and spin-lattice relaxation times. However,

this does not affect the derivation of the sp2/sp3ratio

because the detected 13C atoms distribute in the whole

ﬁlm evenly [219].

•Thepresenceofhydrogeninthesampleis required to allow

efﬁcient use of cross-polarization techniques [211].

•When acquiring the NMR spectrum from carbon ﬁlms,

the time should be long enough to allow all the spin-lattice

relaxation signals to be fully detected.

•For the strained carbon bonding in tetrahedral amorphous

carbon ﬁlms, except the sp2peak at 140ppm and the sp3

peak at 39ppm, an additional peak at 66 ppm (related to

TMS) corresponds to compressed graphitic C [220].

6. X-ray reﬂectivity

X-ray reﬂectivity (XRR) is a nondestructive method to

examine the amorphous carbon ﬁlm density, thickness, and

microscopy roughness [223–227]. In this technique, the X-

ray reﬂectivity is measured while varying the X-ray incident

angle θfrom a low value (e.g. at a glancing angle of ∼0.1◦)to

a higher value (e.g. 3◦). During the incident angle increase,

the recorded reﬂectivity intensity gradually increases due

to the sin(θ) dependence of the incident intensity. After a

critical angle, θc, has been reached, the detected reﬂectiv-

ity will decrease rapidly. The reﬂection of X-rays from a

layered, dielectric medium was discussed by Parratt who

derived a recursion formula to calculate the reﬂected inten-

sity from successive interfaces [228]. The X-ray reﬂectivity

is determined by the density as well as surface and interface

roughness of the ﬁlms, and in turn, these ﬁlms properties can

bederivedfromtheX-rayreﬂectivity[223,225,226,229].The

refraction index of a material with elements jof atomic num-

ber Zj, molar masses Mj, density ρjat X-ray wavelength λis

slightly smaller than 1 and given by [173]:

n=1−δ−iβ (10)

where the dispersion δand absorption βcan be described by

the following formulae, respectively:

δ=r0λ2

2

ρj

Mj(Zj+f

j) (11)

β=NA

2πr0λ2

ρj

f =µλ

4π(12)

where NAis the a Avogadro number, µthe linear absorption

coefﬁcient, r0the classical electron radius, fand f are the

dispersion and absorptive corrections, and at a given λ,f

and f can be calculated for different atoms [226]. Applying

Snell’s law at the air/ﬁlm interface, the critical angle can be

derived:

θc=√2δ=λ





NAr0

π

ρj

Mj(Zj+f

j) (13)

Using the above formula, the ﬁlm density can be obtained.

An accurate measurement of the density of carbon ﬁlms is

important,since this relates to thepresence of microvoids, the

molecular structure of the ﬁlm, and how the carbon atoms are

bonded to each other [223]. The ﬁlm density has also been

found to vary with the deposition parameters. Typical X-ray

reﬂectivity acquired from amorphous carbon ﬁlms is shown

in Fig. 17.

The modulation in the reﬂectivity arises from the interfer-

ence between X-rays scattered from the carbon/air and the

carbon/substrate interfaces and so the period of this oscil-

lation is directly related to the thickness of the carbon ﬁlm

[230,231]. For a thin ﬁlm on a substrate, there will be two

contributions to the reﬂectivity: the air–ﬁlm interface and the

ﬁlm–substrateinterface.Considering that real interfaces have

some roughness and there is a gradual change in the electron

density, the X-ray reﬂected from the two interfaces interferes

and the reﬂectivity is approximately given by [223,232]:

R=



rs+riexp(iQt)

1+rsriexp(−iQt)



2(14)

Fig. 17. Typical X-ray reﬂectivity from amorphous carbon ﬁlms.

P.K. Chu, L. Li / Materials Chemistry and Physics 96 (2006) 253–277 271

where rsand riare the reﬂection amplitudes (including

roughness) at the surface (air-ﬁlm) and the inner interface

(ﬁlm-substrate), respectively, tis the ﬁlm thickness, and

Q=(4π/λ)sin θis the scattering vector. Using this relation-

ship, the reﬂectivity intensity decay and the fringes above

θccan be ﬁtted [223], and the surface thickness and the sur-

face roughness (two interface) can subsequently be derived

[4,223,225,226].

7. Conclusion

Carbon, together with hydrogen, perhaps is the most mag-

ical element in the world. They are the backbone of the organ

materials and many inorganic materials. Carbon atoms form

threekinds of hybrid bonds and differentC C and C Hbond

arrangement and orders in a three dimensional network make

the structure of carbon-containing ﬁlms very complex. The

structural characterization methods described in this paper

are the most frequently used ones. It should be borne in

mind that the information gained from the amorphous car-

bon ﬁlms is usually “statistical” in nature since at the present,

we still lack a direct atomic characterization method that can

be used over a large scale. In addition, because of the com-

plex structure of carbon ﬁlms, inner stress, dangling bonds,

and dislocations, characterization of the carbon ﬁlm struc-

ture may not be deﬁnite. In most cases, the use of multiple

techniques to tackle the problem from several perspectives is

recommended, especially when it is the ﬁrst time to analyze

a “new” carbon ﬁlm.

Several techniques are not discussed here due to the

length of the article. For example, mechanical characteriza-

tion methods such as nano indenter hardness can also convey

a very important message. Other notable ones include X-ray,

neutron and electrons diffraction, ellipsometry, elastic recoil

detection,andreaders are suggested to peruse othertextbooks

and papers or articles cited in this paper.

Acknowledgement

The work was sponsored by Hong Kong Research Grants

Council (RGC) Competitive Earmarked Research Grant

(CERG) No. City U1120/04E.

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Diamond-like carbon (DLC) coatings are effective in protecting the key components of marine equipment and can greatly improve their short-term performance (1.5~4.5 h). However, the lack of investigation into their long-term (more than 200 h) performance cannot meet the service life requirements of marine equipment. Here, three multilayered DLC coatings, namely Ti/DLC, TiCx/DLC, and Ti-TiCx/DLC, were prepared, and their long-term corrosion resistance was investigated. Results showed that the corrosion current density of all DLC coatings was reduced by 1–2 orders of magnitude compared with bare 316L stainless steel (316Lss). Moreover, under long-term (63 days) immersion in a 3.5 wt.% NaCl solution, all DLC coatings could provide excellent long-term corrosion protection for 316Lss, and Ti-TiCx/DLC depicted the best corrosion resistance; the polarization resistances remained at ~3.0 × 107 Ω·cm2 after immersion for 63 days, with more interfaces to hinder the penetration of the corrosive media. Meanwhile, during neutral salt spray (3000 h), the corrosion resistance of Ti/DLC and TiCx/DLC coatings showed a certain degree of improvement because the insoluble corrosion products at the defects blocked the subsequent corrosion. This study can provide a route to designing amorphous carbon protective coatings for long-term marine applications in different environments.

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The influence of surface hydrogen coverage on the electron field emission of diamond films was investigated by high-resolution electron energy loss spectroscopy. It was found that hydrogen plasma treatment increased the surface hydrogen coverage while annealing caused hydrogen desorption and induced surface reconstruction. Field electron emission measurements manifested that increase of surface hydrogen coverage could improve the field emission properties, due to the decrease of electron affinity of the diamond surface by hydrogen adsorption.

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Characterisation of Diamond and Diamond-Like Films

Chapter

Jan 1991

Paul Chalker

The development of both CVD and PVD technologies for the deposition of diamond and diamond-like carbon (DLC) films has provided a wide range of materials for potential applications in optical, electronic and tribological fields. The range of materials produced by these deposition technologies arise partly from the allotropy of carbon and also from the possibility of forming hydrogenated materials which are widely classified as α-C:H ceramics. Analytical techniques for characterising these materials are important in as much as they may define the product of a set of particular process parameters and that they relate macroscopic properties of the films such as hardness, electrical and thermal conductivity, or optical transparency to microscopic properties such as phase composition, defect structures, impurities and microstructure.By understanding the relationships between macroscopic and microscopic properties, films with optimal behaviour can be produced to meet the requirements of a specific application.

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Mar 1982

NMR AND IR STUDIES HAVE BEEN CARRIED OUT ON GLOW-DISCHARGE-DEPOSITED A-SI//1// - //XC//X: H FILMS. THE HYDROGEN CONTENTINCREASES WITH AN INCREASE IN X UP TO 0. 4 MAINLY IN GATHERED FORM CONTRIBUTING TO THE BROAD COMPONENT OF THE NMR LINE.BY COMBINING THE NMR AND IR RESULTS, THE PROPORTIONALITY BETWEEN THE NUMBER OF H BONDED TO C AND THE INTENSITY OF IR ABSORPTION DUE TO THE C-H STRETCHING MODE VIBRATION IS ASCERTAINED.

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J. Roberston

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Ray Egerton

ELECTRON ENERGY LOSS SPECTROSCOPY.

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Jan 1977

B. Jouffrey

Many calculations have been done for determining the ratio I//E/I//I which can give an idea on the loss of information due to the importance of I//I. But for understanding this ratio it is necessary to know the different types of excitation which give energy losses as a function of the energy of the incident electrons. This viewpoint is connected with the loss of information due to energy losses. These troubles can be avoided by using filtered images, but it is important to know if the same resolution as with elastic electrons can be obtained with electrons which have suffered a given loss. In STEM the chromatic aberration is nonexistent and most of the problems will have their origin not in the energy loss itself but in the scattering. A positive aspect of energy losses is the possibility of obtaining information on the sample.

Characterization of amorphous and nanocrystalline carbon films

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