World Journal of Nano Science and Engineering, 2012, 2, 92-102 Published Online June 2012 (
X-Ray Photoelectron Spectroscopy and Raman
Spectroscopy Studies on Thin Carbon Nitride Films
Deposited by Reactive RF Magnetron Sputtering
Masao Matsuoka1*, Sadao Isotani1, Ronaldo D. Mansano2, Wilmer Sucasaire1,
Ricardo A. C. Pinto1, Juan C. R. Mittani1, Kiyoshi Ogata3, Naoto Kuratani4
1Institute of Physics, University of São Paulo, São Paulo, Brazil
2Polytechnical School, University of São Paulo, São Paulo, Brazil
3Nissin Electric Company, Ltd., Kyoto, Japan
4MEMS Development Department, Semiconductor Division, OMRON Corporation, Shiga, Japan
Email: *
Received March 23, 2012; revised April 22, 2012; accepted May 12, 2012
Thin carbon nitride (CNx) films were synthesized on silicon substrates by reactive RF magnetron sputtering of a graph-
ite target in mixed N2/Ar discharges and the N2 gas fraction in the discharge gas, FN, varied from 0.5 to 1.0. The atomic
bonding configuration and chemical composition in the CNx films were examined using X-ray photoelectron spectros-
copy (XPS) and the degree of structural disorder was studied using Raman spectroscopy. An increase in the nitrogen
content in the film from 19 to 26 at% was observed at FN = 0.8 and found to influence the film properties; normality
tests suggested that the data obtained at FN = 0.8 are not experimental errors. The interpretation of XPS spectra might
not be always straightforward and hence the detailed and quantitative comparison of the XPS data with the information
acquired by Raman spectroscopy enabled us to interpret the decomposed peaks in the N 1s and C 1s XPS spectra. Two
N 1s XPS peaks at 398.3 and 399.8 eV (peaks N1 and N2, respectively) were assigned to a sum of pyridine-like nitrogen
and CN bond, and to a sum of pyrrole-like nitrogen and threefold nitrogen, respectively. Further, the peaks N1 and N2
were found to correlate with C 1s XPS peaks at 288.2 and 286.3 eV, respectively; the peak at 288.2 eV might include a
contribution of sp 3 carbon.
Keywords: Carbon Nitride; Magnetron Sputtering; X-Ray Photoelectron; Raman Scattering
1. Introduction
The prediction of hypothetical material
-C3N4, whose
hardness might be equal or superior to that of diamond
[1], has motivated much research to synthesize and char-
acterize carbon-nitrogen materials, because this super-
hard material has not only scientific interest, but also
promising technological potential for thin-film applica-
tion. Extensive experimental effort has been made on
growing thin carbon nitride (CNx, 0 x 1.33) films
with various deposition methods, such as reactive sput-
tering [2-4], dual ion beam sputtering [5], ion beam depo-
sition [6-8], ion beam nitridation [9], laser ablation [10],
chemical vapor deposition [11].
Despite much effort to achieve the stoichiometric com-
pound, the large majority of the experiments done to date
indicate that the maximal achievable nitrogen content
structurally incorporated into amorphous CNx formed is
limited to about 30 to 40 at% less than the stoichiometric
value of the
-C3N4 phase (57 at%) [12]. Some authors
reported the presence of nanometer-sized
-C3N4 crystal-
lites buried in an amorphous CNx matrix on the basis of
electron diffraction data [13], but others claimed that
there is no definite evidence of the existence of crystal-
-C3N4 [14]. Despite the discrepancy in stoichiome-
try, the obtained CNx films prove to be interesting and
useful in material science and coating technology, because
of their high hardness, low friction, wear resistance, and
ease of fabrication.
Numerous studies have been carried out on the differ-
ent kinds of amorphous CNx; however, many questions,
fundamentally regarding the identification of carbon-ni-
trogen bonding configurations, still remain. Carbon as
well as nitrogen has some atomic bonding configurations
and the most commonly encountered configurations of
carbon and nitrogen are linear, trigonal, and tetrahedral,
which correspond to the sp, sp2, and sp3 hybridizations,
respectively. Indeed, the carbon coordination in nitrogen-
free carbon networks defines the local structural properties
*Corresponding author.
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in which hard solid materials are usually closely related
to three-dimensional four sp3 orbitals which make strong
bonds with adjacent atoms, while two-dimensional sp2
carbon forming three
bonds and one bond will lead
to much softer and somewhat brittle materials, due to its
two-dimensional structure of hexagonal carbon layers.
The addition of nitrogen atoms to the carbon network is
capable to modify the carbon coordination in different
manners and the CNx films can hence present a wide variety
of bonding configurations [15,16]. Consequently, the
optimization of the amorphous CNx properties requires
better assignment of the different types of chemical
bonding in the films.
X-ray photoelectron spectroscopy (XPS) technique
provides the information on chemical bonds and atomic
bonding, based on the chemical shifts in their core levels
in a certain chemical environment, and on the surface
chemical composition of the material [17-19]. This tech-
nique has been used extensively to study CNx films and
there are a great number of publications on XPS spectra
of CNx films. In the case of CNx films, however, XPS
studies of carbon and nitrogen suffer from the lack of
spectral resolution; the spectra have broad overlapping
peaks, due to the amorphous nature and resulting small
chemical shifts, leading sometimes to difficulties in iden-
tification and controversial assignments of different
chemical environments of carbon and nitrogen [20]. The
most difficult task is the bonding identification. On the
other hand, Raman spectroscopy is a standard and non-
destructive technique to detect the difference in energy
between incident and inelastically scattered photons
which are associated with different vibration modes and
is widely used for the characterization of all kinds of
carbon-based materials. Different from XPS, Raman
spectroscopy can show both local and collective bond
vibration modes and therefore can often supply confused
results in case of amorphous carbon. Considerable con-
fusion exists in the interpretation of the spectra obtained
by XPS and Raman for CNx materials. Therefore, it is
obvious that complementary additional techniques are
important in obtaining a more comprehensive under-
standing of CNx films. To our knowledge, such an XPS
study of CNx films in combination with Raman spec-
troscopy has not been accomplished.
In this study we have prepared thin CNx films on
Si(100) substrates, using reactive RF magnetron sputter-
ing in mixed N2/Ar discharges from a graphite target.
The N2 gas fraction in the discharge gas, as a deposition
parameter, varied from 0.5 to 1.0. We have analyzed
these deposited films in terms of the atomic bonding
structure and chemical composition using XPS. The
purpose of this paper is to combine the analysis results of
XPS with those of Raman spectroscopy in order to obtain
the consistent conclusions on the bonding structure of
carbon and nitrogen in the CNx films.
2. Experimental
The CNx films were deposited on (100)-oriented Si sub-
strates of 75 mm in diameter by reactive RF magnetron
sputtering at 13.56 MHz. The target, a high-purity
99.9999% graphite disc of 15 cm in diameter and 6 mm
in thickness, was mounted on a planar magnetron elec-
trode at a distance of 10 cm from a substrate holder. Each
substrate was cleaned with Piranha etching (4H2SO4 +
1H2O2) and dipped in dilute HF solution (1:20H2O) to
remove silicon oxide, followed by a rinse with deionized
water, and immediately loaded to the substrate holder.
The deposition chamber was evacuated with a turbo-
molecular pump to the base pressure of 3 10–4 Pa. N2
gas mixed with Ar gas (both 99.9999% purity) was admitted
into the deposition chamber through the respective mass
flow controllers, and the N2 gas fraction, FN, defined the
N2 gas flow relative to the total gas flow, varied from 0.5
to 1.0 at intervals of 0.1. Each run of deposition was
done with the discharge power of 350 W at the graphite
target, keeping the working pressure at 0.4 Pa for any
chosen FN. The substrate was maintained during the
deposition at 90˚C monitored by a thermocouple which
was attached to the substrate holder and the deposition
time was 5 min for all the films.
Conventional X-ray photoemission data of the films
were collected ex situ from the surface freshly cleaned
after a sputter using 2 keV Ar+ ion beam for 3 min using
a Shimadzu ESCA 750 spectrometer with incident Mg
radiation (1253.6 eV) and the binding energies were
calibrated with respect to the Au 4f7/2 peak at 83.8 eV,
originated from gold coverage, as external standard, de-
posited on the surface of each sample. Unpolarized Raman
spectra were measured ex situ at room temperature using
a Renishaw Raman 2000 spectrometer operating with an
Ar+ laser excitation line of 514.5 nm. The deposited film
thickness, which was measured with a Dektak 3030 sur-
face profilometer, ranged from 97 to 157 nm.
Our study includes fitting of C 1s and N 1s XPS spectra
with the Doniach-Šunjić function which has been widely
used for peak fitting [21], using the fitting procedure
described elsewhere [22]. The fit parameters were opti-
mized with a grid method [23] and the deviations due to
the fitting procedure were obtained as follows. Varying
each parameter to higher and lower values than its best-
fit parameter value, the corresponding cost functions were
calculated. The largest deviation of such a parameter
value, which gave an increase of about 20% in the cost
function, from the best-fit parameter value, was used as
the fitting deviation of the parameter [24].
3. Results and Discussion
3.1. Deposition Rate
The deposition rate of the films, evaluated by dividing
Copyright © 2012 SciRes. WJNSE
the deposited film thickness by the deposition time, is
shown in Figure 1 as a function of N2 gas fraction in the
discharge gas, FN. As FN is changed from 0.5 to 1.0, the
deposition rate increases from 0.32 to 0.52 nm/s. This FN
has a significant influence on the deposition rate of the films
and such an increase is observed already in the formation
of CNx films prepared by magnetron sputtering [25].
When N2 gas is mixed into the sputtering atmosphere,
it causes a variety of effects. Incident ions generally ac-
cumulate on the target and growing film surfaces and can
form volatile compounds with surface layer atoms, such
as CN+, HCN+, 22
, during the deposition process
[26]. The formation of these compounds lowers the sur-
face binding energy and they can be desorbed from the
sur- face (commonly referred to as “chemical sputter-
ing”). At conditions in which the ion energy (<100 eV) is
below the physical sputtering threshold, the higher the N2
gas fraction in the discharge gas, the faster the formation
rate of volatile compounds, and thereby the film forma-
tion ceases due to chemical sputtering. For our experi-
mental result the contrary is the case.
Furthermore, during the deposition of amorphous car-
bon, it is known that, when the energy of an incident ion
exceeds certain threshold energy, the ion can penetrate
the surface of the growing film and enter an interstitial
site, increasing the local density. This process is termed
“subplantation” (low energy surface implantation) and
the local bonding around this site will reform according
to the densification and thermal spike [27].
In any event, we expect that the material removal from
the surface of growing film due to chemical sputtering
may be offset by a higher deposition flux of film-forming
species at higher N2 gas fractions [26]; however, detailed
interpretation of the deposition rate is difficult due to the
complex subplantation and chemical sputtering processes
in the mixed gas system and more experimental research
is required as a result.
3.2. XPS Spectra
Three elemental species, carbon, nitrogen, and oxygen,
Figure 1. Deposition rate of CNx films as a function of N2
gas fraction.
were identified in the films by assignment of the corre-
sponding signals observed in the XPS spectra. Figures
2(a) and (b) depict the respective N 1 s and C 1 s spectra
(solid lines) observed for the film prepared at FN = 0.7
along with the individual decomposed peaks which will
be explained later. Both the N 1 s and C 1 s spectra, nor-
malized to the same maximum intensity, present asym-
metric broadening which implies the presence of some
individual peaks related to different atomic bonding con-
figurations. Diverse and, in certain case, contradictory
interpretations have been published on the XPS analysis
results of CNx films, due to the presence of complex local
environment and the lack of appropriate standard sam-
ples for reference.
The nitrogen content in each film can be estimated
from A(N)/Asum and Asum = [A(N) + A(C) + A(O)], where
A(N), A(C), and A(O) are the areas under the whole N 1s,
C 1s, and O 1s spectra, respectively, after correcting with
relative sensitive factors due to the analyzer transmission
and the photoionization cross section.
Table 1 indicates the experimental data of A(N), A(C),
A(O), Asum, A(N)/Asum, A(C)/Asum, and A(O)/Asum. Figure
3 shows the nitrogen content in the film as a function of
FN, indicating that: all the films are deficient in nitrogen,
deviating considerably from stoichiometric C3N4 (57 at%
of nitrogen); the nitrogen content is fairly constant at
around 19 at% independent of FN, except for a rise in the
nitrogen content to about 26 at% at FN = 0.8.
The nitrogen content may be determined by the com-
petition between the deposition process of nitrogen-con-
taining species by subplantation and the desorption proc-
ess of volatile species from the surface of growing film
by chemical sputtering [28]. This chemical sputtering is se-
lective, depending on bonding and stability of sites where
volatile species are incorporated, and the interpretation
Figure 2. (a) N 1s XPS spectrum and (b) C 1s XPS spectrum
of the film deposited at FN = 0.7.
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Copyright © 2012 SciRes. WJNSE
Table 1. Areas of N 1 s, C 1 s, and O 1 s XPS spectra. The units of the areas are in countseV.
FN A(N) A(C) A(O) Asum A(N)/Asum A(C)/Asum A(O)/Asum
0.5 17,175 66,981 1782 85,938 0.200 0.779 0.0207
0.6 15,636 68,362 1321 85,319 0.183 0.801 0.0155
0.7 15,223 61,902 2182 79,307 0.192 0.781 0.0275
0.8 20,545 55,708 4170 80,423 0.225 0.693 0.0519
0.9 17,757 67,005 1733 86,495 0.205 0.775 0.0200
1.0 14,072 62,229 2351 78,652 0.179 0.791 0.0299
Mean 16,000 65,300 1900 82,700
σ 3100 6300 1100 5500
p value 0.67 0.25 0.09 0.12 0.088 0.012 0.13
Figure 3. Nitrogen content in the film as a function of N2 gas
of this result needs further research. Further, from the
table, the oxygen content given by A(O)/Asum is found to
be relatively low (2 - 5 at%).
The point corresponding to FN = 0.8 in Figure 3 is
found to be removed in value from the others. Fearing
that this outlying point might be the result of an error in
measurement, the data set of Asum is examined carefully
before any further analysis. It is observed at first that: the
standard deviation of Asum (3600) is small compared with
the mean (4%); the difference between the mean and me-
dian (82871) is also small (0.2%); the means calculated
from all the subsets, which are formed by subtracting one
data from the data set, are very close to the mean of Asum.
The normality test of Anderson-Darling [29] and the fol-
lowing evaluation by the software ACTION [30] for the
normality test are used to determine whether the data set
is well-modeled by a normal distribution or not and p
value in the test is found to be 0.12.
From the considerations above mentioned and the cri-
terion of p value 0.05 for a normal distribution, the data
set of Asum is distributed normally in spite of a small
number of the data set member. Consequently, we can
conclude that the XPS measurement was run routinely
and correctly. Further, all the data of Asum fall within a
99% confidence interval and then this interval is consid-
ered as the deviation from the mean of each parameter.
Next the data sets of A(N), A(C), and A(O), also given
in Table 1, are examined and p values obtained are 0.67,
0.25, and 0.09, respectively, suggesting normal distribu-
tions for the three data sets; however, the data for FN =
0.8 are outside 99% confidence intervals for the three
data sets. To reduce the effect of random fluctuations in
A(N) and A(C), their fractional contents, i.e., A(N)/Asum
and A(C)/Asum, which are also given in the table, are ex-
amined; A(O) /Asum is excluded from the examination due
to its relatively small contribution. The p value obtained
for the data set of A(N)/Asum indicates a normal distribu-
tion (p = 0.088) and that for the data set of A(C)/Asum, a
non-normal distribution (p = 0.012); the data for FN = 0.8
are outside 99% confidence intervals for the two data sets.
Note that the subset of A(C)/Asum without the data for
FN = 0 shows a normal distribution (p = 0.45). Figures
4(a) and (b) depict the relations between the experi-
mental data and the expected data from the normal dis-
tribution for the data sets of A(N)/Asum and A(C)/Asum,
respectively, as a function of parameter Z, which is
known as a standard normal random variable and defined
by Z = (X
, where X is experimental data,
is the
mean, and
is the standard deviation. It is known that, if
the distribution of data is normal, the plotted points
should approximately lie on a straight line. All the points
except the point for FN = 0.8 in each figure arise from a
normal distribution; however, the point for FN = 0.8 in
each figure does not lie on the line determined by the
other points and hence the data for FN = 0.8 are outliers.
As shown in detail in the following Sub-Heading 3.3,
three distinct Raman parameters are evaluated from the
spectra analysis and the normality test results for each
Raman parameter indicate that: the parameter value re-
lated to the film prepared at FN = 0.8 is an outlier and the
other parameter values originate from a normal distribu-
tion in each data set. Taking into consideration that the
two measurements with XPS and Raman are independent,
we conclude that the data for FN = 0.8 are genuine results
and may indicate an extreme behavior of the corre-
sponding film.
The Knoop hardness of the films was tentatively
measured on different sites of each film using a micro-
hardness tester with a load of 10 gf. The film thickness is
insufficient for this measurement, because the hardness
Figure 4. (a) A(N)/Asum and (b) A(C)/Asum, as a function of Z.
reading obtained should be influenced by the silicon sub-
strate with hardness of 10.8 GPa. However, it is worth
mentioning here that: the apparent hardness reading indi-
cates an abrupt increase from (5.4 ± 1.4) GPa to (13.7 ±
0.8) GPa at FN = 0.8; the hardness reading for the film
prepared at FN = 0.8 is found, through the normality test,
to be an outlier. A similar abrupt increase from 18 to 25
GPa in hardness of CNx films formed by RF plasma
beam deposition using N2/Ar gas mixtures (0.1 FN 1)
has been found at FN = 0.5; however, there is not any
explanation for it [31].
The N 1s spectra have been commonly simulated with
three or four peaks based on organic molecules contain-
ing nitrogen with the well-known atomic bonding con-
figurations. Most authors have used three peaks in the
ranges of around 398 (N1), 400 (N2), and 402 eV (N3)
[6,7,9,10]. The consensus of assignment is that the peak
N3 is generally attributed to N-O bonds and/or chemi-
sorbed N2 [2,5,6,10]. Some authors have taken into ac-
count an additional peak situated at around 399 eV which
is due to sp nitrogen bonded to carbon (CN) forming a
nitrile terminating configuration [5,9].
Concerning the peaks N1 and N2, the majority of au-
thors ascribe the peaks situated below 400 eV to nitrogen
bonded to sp3 carbon (N–sp3 C), and the peaks above 400
eV, to nitrogen linked to sp2 carbon (N–sp2 C), on the
basis of the values of binding energy for urotropine
(N–sp3 C) at 399.4 eV and pyridine (N–sp 2 C) at 399.8
eV, respectively [2,5,6,10].
However, this identification scheme is not completely
accepted and alternative interpretations are published by
some authors. Ronning and co-workers proposed that the
peaks N1 and N2 are due to nitrogen bonded to two and
three neighbors, respectively [7]. Muhl and Méndez
claimed that the binding energy of nitrogen is governed
by the charge transfer of the electron lone pair from the
nitrogen according to the surrounding chemical envi-
ronment; partial or total localization of the lone pair in
the CN bond leads to a decrease in the binding energy
of the N 1s level and to an increase in that of the C 1s
level, and less or no localization, to an increase in the
binding energy of the N 1s level and to a decrease in that
of the C 1s level [12]. This charge transfer depends, of
course, on the binding nature between carbon and nitro-
gen. Accordingly, they attributed the peak N1 to two-
fold-coordinated sp 2 nitrogen in a hexagonal ring as in
pyridine (pyridine-like nitrogen) or to nitrogen linked
trigonally to three sp3 carbon atoms, and the peak N2 to
threefold-coordinated sp2 nitrogen in a pentagonal ring as
in pyrrole (pyrrole-like nitrogen). Later Sánchez-Lópes et
al. attributed the peak N1 to a sum of nitrogen bonded to
three sp2 carbon atoms (N–sp2 C) and terminating bond-
ing configurations (NH2, CN), and the peak N2, to
pyridine-like nitrogen [3]. On the other hand, Ripalda et
al. correlated the peak N1 to pyridine-like nitrogen at
edges of graphite layers or the overlap of sp and sp2 nitrogen,
and the peak N2 to substitutional nitrogen in graphite,
respectively [9]. This assignment is supported by theo-
retical calculations by Titantah and Lamoen [32]. Further,
Normand et al. proposed that the peaks N1 and N2 may be
assigned to CN bond or pyridine-like nitrogen, and to
sp3 nitrogen or pyrrole-like nitrogen, respectively, what-
ever configuration of carbon to which it is bonded [15].
Neidhardt et al. attributed the peak N1 to pyridine-like
nitrogen and the peak N2 to threefold-coordinated nitro-
gen in graphite layers [4].
The decomposition of the N 1s spectra in our work
was done accordingly in terms of the three peaks N1 - N3.
An example of the results is shown in Figure 2(a); the
peaks N1 and N2, represented by closed circles, are prin-
cipal. The contribution of the peak N3 is not included in
the figure, because of the low contamination by oxygen
as mentioned above, and hence this peak will be ignored
in the following discussion.
The best fitting parameters obtained are given in Ta-
ble 2. In this table the asymmetric parameter,
, related
to the density of states at the Fermi level, and the line
, related to the lifetime of destruction of the hole
states by electron capture are shown; both parameters are
used in the Doniach-Šunjić function. No substantial dif-
ference is observed in the respective spectra, except for
the variation in the integrated intensity of each peak.
On the other hand, the interpretation of the C 1s spectra
is even more controversial and not definitive to date. Up
to the present, the C 1s spectra have been generally ana-
lyzed by many authors in terms of four peaks around
284.5 eV (C1) and in the ranges of 285 - 286 (C2), 287 -
288 (C3), and 288 - 290 eV (C4) [5-7]. The consensus is
that the peak C1 is exclusively attributed to graphite-like
or amorphous carbon in the film and adventitious carbon
on the surface [5-7,10], and that the peak C4 is associated
with CO bonds [5,6].
Copyright © 2012 SciRes. WJNSE
Table 2. Best-fit parameters for N 1s and C 1s XPS spectra.
XPS parameters
Peak Binding
energy (eV)
(eV) Area
N1 398.31 ± 0.15 0.05 0.7 59 ± 9
N2 399.84 ± 0.14 0.05 1.4 83 ± 10
C1 284.70 ± 0.26 0.26 0.8 519 ± 44
C2 286.29 ± 0.30 0.05 1.0 81 ± 16
C3 288.19 ± 0.41 0.05 1.7 52 ± 8
Many researchers suggest that the peaks C2 and C3 are
attributed to sp2 carbon bounded to nitrogen (sp2 CN)
and to sp3 carbon linked to nitrogen (sp3 CN), respect-
tively, referring to the values of binding energy for pyri-
dine (sp2 CN; 285.5 eV) and urotropine (sp3 CN;
286.9 eV) [5]. However, this scheme is not supported by
some authors. Ronning et al. postulated that the peak C2
and C3 are ascribed to sp3 carbon bonded to one and two
nitrogen atoms, respectively, and expected for the
-C3N4 structure a single peak at 288 eV which was not
observed in their spectra [7]. Sánchez-López et al. re-
quired that the peaks C2 and C3 result from sp2 CN in-
side the aromatic structures and from sp2 CN in the
aromatic ring attached to an electronegative group (–NR2,
–NHR, –CN, where R is any other group), respectively
[3]. Normand et al. attribute the peak C2 to diamond,
diamond-like carbon, sp3 or sp2 carbon singly bonded to
nitrogen, and the peak C3 to sp3 carbon multiply bonded
to nitrogen [15].
In our study, the C 1s spectra observed were fitted ac-
cordingly with a combination of the peaks C1 - C4. Fig-
ure 2(b) indicates an example of the decomposition re-
sults; the contribution of the peak C4 is not included in
the figure, due to the low contamination by oxygen. The
best fitting parameters obtained are given in Table 2. It is
interesting to note that the asymmetric parameter, ,
leads to conducting properties for the peak C1 (graphite
or amorphous carbon) and to insulating ones for the other
peaks. A change in the N2 gas fraction does not affect the
position and width of each decomposed peak, whereas
the peak intensity is scaled.
Figure 5(a) shows the relative areas of the decom-
posed peaks in the N 1s spectra, i.e., A(N1)/A(N) and
A(N2)/A(N), where A(N1) and A(N2) are the corrected
areas of the respective peaks N1 and N2, as a function of
FN. Figure 5(b) depicts those in the C 1s spectra,
A(C1)/A(C), A(C2)/A(C), and A(C3)/A(C), where A(C1) -
A(C3) are the corrected areas of the respective peaks C1 -
C3, as a function of FN.
It should be noted from Figure 5 that: 1) there are
Figure 5. Relative areas of the decomposed peaks in: (a)
The N 1 s spectra and (b) The C 1 s spectra, as a function of
N2 gas fraction.
distinct changes in all the relative areas at FN = 0.8, indi-
cating that a certain change in bonding configurations oc-
curs at this N2 gas fraction; 2) the relative area of the
peak N2 enhances at the expense of that of the peak N1 at
FN = 0.8; 3) the relative area of the peak C1 is the major
compo- nent in all the C 1 s spectra and the relative areas
of the peaks C2 and C3 grow at the expense of the relative
area of the peak C1 at FN = 0.8. To obtain a certain cor-
relation between one of the N 1 s XPS peaks and one of
the C 1 s XPS peaks, all the combinations are checked
and two correlations between A(N2) and A(C2) and be-
tween A(N1) and A(C3) are found, as shown in Figures
6(a) and (b), respectively.
A good correlation between the peaks N2 and C2 is
observed for all the films in Figure 6(a); however, a
good correlation between the peaks N1 and C3 is found
for the films except the film formed at FN = 0.8 and his
exception will be mentioned later. These results mean
that: both of the peaks N2 and C2 are due to the same type
of a local chemical environment; both of the peaks N1
and C3, to the same type of another local chemical envi-
ronment, except for the film prepared at FN = 0.8. It is
interesting to notice from Table 2 that the binding energy
of the peak N2 is higher than that of the peak N1, while
the binding energy of the peak C2 correlating with the
peak N2 is lower than that of the peak C3 correlating with
the peak N1. These relations may be associated with the
charge transfer of the lone pair from the nitrogen in the
C-N bond, as mentioned above, that is, the peaks N1 and
C3 can originate from the C-N configuration with the
lone pair localized on the nitrogen and the peaks N2 and
C2 can arise from the C-N configuration with the lone
pair delocalized from the nitrogen.
3.3. Raman Spectra
Figure 7(a) shows the originally raw Raman spectrum
measured in the wave number region of 1000 - 1800 cm–1
Copyright © 2012 SciRes. WJNSE
Figure 6. Plots of (a) A(N2) versus A(C2) and (b) A(N1) ver-
sus A(C3).
Figure 7. (a) Raw Raman spectrum of the film deposited at
FN = 0.7; (b) The decomposition results of the spectrum
after removal of the background signal.
for the film prepared at FN = 0.7; two peaks are observed
at around 1350 and 1580 cm–1 in the spectrum. All the
Raman spectra obtained in this study indicate these two
peaks. Little difference between the Raman spectra of
nitrogen-free carbon and those of CNx is expected in this
wave number region, because of the similarity of vibra-
tional frequencies of C-C modes to those of C-N modes
[16]. There is further observed a strong photolumines-
cence background signal due to recombination of elec-
tron hole pairs within sp 2-bonded clusters [33]. To ana-
lyze the two peaks in detail, the parabolic background
signal was subtracted from each row spectrum. The sub-
traction result for the spectrum given in Figure 7(a) is
shown in Figure 7(b).
A peak located at ~1350 cm–1 is denominated the D
peak and the other at 1550 - 1590 cm–1, the G peak. The
D peak arises from breathing modes (A1g symmetry) of
sp2 clusters of hexagonal aromatic rings and can corre-
spond to a superposition of ordered and disordered car-
bon sp3 sites, which correspond to crystalline diamond
and diamond-like carbon, respectively. On the other hand,
the G peak originates from stretching vibrations (E2g
symmetry) of any pair of sp2 sites in both aromatic rings
and olefinic chains and is never observed in the absence
of sp2 graphite; this peak can provide information on the
content and configuration of sp2 network [16,27,34].
Consequently, most of the carbon-based materials show
the Raman spectra dominated by these two peaks.
Figure 7(b) depicts the fitting results, using two Gaus-
sian shapes to the two peaks for the film prepared at FN =
0.7. According to common practice, three peak parame-
ters: the G peak position, the full width at half maximum
(FWHM) of the G peak, and the area ratio of the D to G
peak, i.e., ID/IG, are considered here and these parameters
thus determined are shown in Figures 8(a)-(c), respect-
tively, as a function of FN. Note that all the peak para-
meters determined exhibit distinct changes at FN = 0.8,
namely, downward shifts in the G peak position and ID/IG,
and an upward shift in the FWHM. The data sets of G
peak position, FWHM, and ID/IG are examined using the
normality test and p values obtained are 0.15, 0.10, and
0.12, respectively, suggesting normal distributions for the
three data sets; the data for FN = 0.8 are found to be al-
ways outliers.
In order to understand the behaviors of the peak para-
meters, we use the three-stage model, which was devel-
oped by Ferrari [16] and Robertson [27] to interpret the
evolution of the Raman spectra of nitrogen-free amorphous
carbon and carbon nitride. This model is applicable to
room-temperature deposition and ion implantation of
glassy carbon [34]. Starting from perfect crystalline gra-
phite in this model, we consider the dependence of the
Raman spectra on clustering of the sp2 phase, bond-angle
and bond-length disorder, and hybridization, along the
Figure 8. (a) G peak position; (b) FWHM of the G peak;
and (c) ID/IG ratio, as a function of N2 gas fraction.
Copyright © 2012 SciRes. WJNSE
disordering trajectory ranging from ordered graphite to
tetrahedral sp3 amorphous carbon (ta-C), with three
stages. The three stages are as follows: 1) ordered sp2
graphite to nanocrystalline (nc) graphite; 2) nc graphite
to sp2 amorphous carbon (a-C); (3) sp2 a-C to ta-C or
defected diamond.
The evolution in stage 1 accompanies the progressive
clustering of the ordered graphite layers, keeping aro-
matic rings. This causes the G peak position to shift upward
from 1580 cm–1 of ordered graphite to 1600 cm–1 due to
phonon confinement and the D mode, which is forbidden
in ordered graphite, to appear and enhance. Consequently,
ID/IG starts increasing from zero according to the Tuin-
stra-Koenig relation [35]. This relation indicates that:
ID/IG is inversely proportional to the cluster size of gra-
phite and can reach ID/IG 2 with the cluster diameter
below 2 nm [36,37]. In stage 2, structural disordering of
the graphite layers due to the formation of nonsixfold
rings and distortion of sixfold rings, and loss of aromatic
bonding proceed, but keeping the pure sp2 network. This
weakens the bonds, lowers the G peak position from
1600 to 1510 cm–1 and reduces the number of the ordered
aromatic rings due to the reduction of ordered rings,
causing ID to start decreasing. The G peak retains its in-
tensity and ID/IG falls continuously from 2 to 0.1 - 0.2 as a
result. On passing from stage 2 to stage 3, sp2 configura-
tion changes gradually from mainly aromatic rings to
short olefinic sp2 chains, resulting in the absence of the D
peak, while the sp3 content rises from ~10% - 20% to
about 85%. Olefinic bonds are shorter than aromatic
bonds and have higher vibration frequencies. The G peak
position increases from 1510 to 1570 cm–1 and ID/IG is
very low or zero.
On the basis of this phenomenological three-stage
model, all the films deposited in this study can be con-
sidered to be in stage 2, taking into account the G peak
position less than that of ordered graphite (1580 cm1)
and ID/IG ranging between 1.4 and 1.8. Thus, the films
can be considered to consist of mainly sp2 sites and this
is consistent with a general remark by Ferrari et al. that
DC or RF magnetron sputtering produces sp2 a-C: N
films [16]. Accordingly, the downward shifts of the G
peak position and ID/IG at FN = 0.8, shown in Figures 8(a)
and 8(c), respectively, should be attributed to structural
disorder in the sp2 sites due to further formation of non-
sixfold rings, distortion of sixfold rings, and loss of aro-
matic bonding; the resulting wide variety of sp2-bond
length and angle distortion leads to the broadening of the
G peak, shown in Figure 8( b) , as a result [27].
3.4. Interpretation of the Decomposed Peaks in
the N 1s and C 1s XPS Spectra
As mentioned in Sub-Heading 3.2, the currently prevail-
ing interpretation of N 1s and C 1s XPS spectra, in the
base of the values of binding energy for the organic
molecules, suggests that the peaks N1 and C3 are due to
N–sp3 C and sp 3 C-N, respectively. Our finding that the
structure of all the films produced is amorphous sp2 a-C:
N (Sub-Heading 3.3), and the fact that no evidence of
large quantities of sp3 carbon is reported in the CNx films
formed by sputtering [2,4,9,16], puts this interpretation in
doubt seriously.
It is worth noting that the changes in the relative inten-
sities of the XPS peaks and those in the Raman peak pa-
rameters observed at FN = 0.8 (Figures 5 and 8, respec-
tively) take place at the maximum nitrogen content in the
film (26 at% in Figure 3). Substitutional incorporation of
nitrogen into hexagonal graphite layers forms various
defects: threefold nitrogen in hexagonal rings, pyri-
dine-like nitrogen, pyrrole-like nitrogen, CN bond, etc.
Pyridine-like twofold nitrogen can be located at an edge
of a graphite layer and at a carbon vacancy next to the
nitrogen inside a graphite layer [28]. Consequently, pyri-
dine-like nitrogen and CN bond are terminating de-
fects and their formation contributes to the clustering in
the graphite layers [12]. On the other hand, threefold
nitrogen and pyrrole-like nitrogen which forms pen-
tagonnal rings in a hexagonal mesh, promote curvature
and corrugation in the graphite layers.
The main structural change in stage 1 of the three-
stage model, as mentioned in Sub-Heading 3.3, is clus-
tering of sp2 graphite layers, and that in stage 2 is further
structural disordering due to the formation of nonsixfold
rings and distortion of sixfold rings. Accordingly, it is
expected that the increase in nitrogen content in the film
in stage 2 from 19 to 26 at% at FN = 0.8 keeps the clus-
tering of graphite layers intact, while this increase pro-
motes further formation of nonsixfold rings and distor-
tion of sixfold rings. Figure 9 exhibits the fraction of
nitrogen content due to the peak N1, given by A(N1)/Asum,
and that due to the peak N2, given by A(N2)/Asum, as a
function of FN. From the figure the former is almost in-
dependent of FN, while the latter shows a prominent rise
at FN = 0.8. From the above discussion, the experimental
finding shown in Figure 9 can be reasonably explained
by assuming that: the peak N1 is due to terminating de-
fects, such as pyridine-like nitrogen, CN bond, and the
peak N2, to pyrrole-like nitrogen and substitutional
threefold nitrogen. This identification is in accord with
insight obtained from the charge transfer of the lone pair
of nitrogen, as mentioned before, and agrees with ex-
perimental data obtained for pyridine and pyrrole by XPS
(398.6 0.3 and 400.5 0.3 eV, respectively) [38] and
by near-edge X-ray absorption fine structure (NEXAFS)
(399.9 and 403.8 eV, respectively) [39]. Further, this is
consistent with the interpretation published for the indi-
vidual peaks in the N 1s XPS spectra [4,7,9,12,15,32].
Copyright © 2012 SciRes. WJNSE
Figure 9. A(N1)/Asum and A(N2)/Asum, as a function of N2 gas
The correlation results, shown in Figure 6, suggest
that the peaks C2 and C3 can be fundamentally assigned
to a sum of pyrrole-like nitrogen and substitution three-
fold nitrogen, and to the terminating defects, respectively;
however, as shown in Figure 6(b), the point correspond-
ing to the film formed at FN = 0.8 is found to deviate in
value from the correlation, indicated by the other films,
between the peaks N1 and C3. This may be explained in
the following manner. A theoretical work shows that: a
transition from planar to corrugated graphite layers occurs at
nitrogen contents above 20 at%; with further increase in
nitrogen content to 50 at%, the corrugated clusters are
not chemically stable and the three-dimensional CNx
materials emerge [40]. Experimentally, it is reported that
CNx films formed by reactive magnetron sputtering pre-
sent fullerene-like microstructures, when the nitrogen
content reaches ~20 at% [2,41,42]. This can reasonably
explain the increase in hardness reading at FN = 0.8,
mentioned in Sub-Heading 3.2. Substituting a threefold
nitrogen atom for an sp2 carbon atom in a graphite layer
breaks a bond and leaves an unpaired electron of a re-
maining carbon atom available to form a
bond to a
carbon atom in a similar situation on an adjacent cluster.
Thus, the deviation of the film prepared at FN = 0.8 from
the correlation shown in Figure 6(b) may be attributed to
the emergence of sp3 carbon, which contributes to an
increase in intensity of the peak C3. This attribution is
consistent with the prevailing identification of the peak
C3 (sp3 C-N).
We admit that the above discussion is rather specula-
tive and that more detailed studies including hardness
and transmission electron microscopy measurements would
be necessary to get more information. However, we con-
sider that it is worthwhile to remark the detailed and
quantitative analysis of the XPS spectra and the consis-
tency in the simultaneous interpretations of Raman and
XPS spectra.
4. Conclusion
We have studied the atomic bonding configuration and
degree of structural disorder in the CNx films prepared
with reactive magnetron RF sputtering, with XPS and
Raman spectroscopy, respectively, verifying the obtained
data with the normality tests. The Raman results suggest
that all the films prepared are amorphous sp2 carbon
doped with nitrogen in stage 2 of the three-stage model,
and also indicate that the increase in the nitrogen content
in the film from 19 to 26 at%, keeping the clustering in-
tact, promotes further disorder in the graphite layer. The
peak N1 (398.3 eV) is little affected in the fraction of
nitrogen content by the increase in the nitrogen content in
the film, whereas the peak N2 (399.8 eV) is enhanced in
the fraction of nitrogen content. From the XPS results in
combination with the Raman results, it is possible to as-
sign the peak N1 to a sum of pyridine-like nitrogen and
CN bond, and the peak N2 to a sum of pyrrole-like
nitrogen and threefold nitrogen. Further, it has been
found from the XPS analysis that: the peak N1 correlates
with the peak C3 (288.2 eV) and the peak N2, with the
peak C2 (286.3 eV); the peak C3 may include a contribu-
tion of sp3 carbon. We could obtain these consistent as-
signments, when combining the results of XPS with
those of Raman spectroscopy; the assignments are in line
with the theoretical prediction for the structural evolution
induced by nitrogen incorporated into graphite.
5. Acknowledgements
This work was supported partially by Japan International
Cooperation Agency (JICA), Conselho Nacional de
Desenvolviment Científico e Tecnológico (CNPq), and
Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES).
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