Thin Solid Films 366 (2000) 169±174
www.elsevier.com/locate/tsf
Ultraviolet and visible Raman studies of nitrogenated tetrahedral
amorphous carbon ®lms
J.R. Shi*, X. Shi, Z. Sun, E. Liu, B.K. Tay, S.P. Lau
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
Received 3 July 1999; received in revised form 13 January 2000; accepted 13 January 2000
Abstract
Nitrogenated, tetrahedral amorphous carbon (ta-C) ®lms prepared by the ®ltered cathodic vacuum arc (FCVA) technique have been
studied using ultraviolet (UV, 244 nm) and visible (514 nm) micro-Raman scattering. The nitrogen ions were produced by a RF ion-beam
source with a nitrogen ¯ow-rate varying from 0 to 10.0 sccm, which results in a nitrogen content from 0 to 10.8 at.% in the deposited ®lms. In
the visible Raman spectra, only vibrational modes of sp 2-bonded carbon (G and D peaks) are observed, while a new wide peak, called the T
peak, located at 1090±1320 cm 21, associated with the vibrational mode of sp 3-bonded carbon, appears in the UV-Raman spectra. In the
visible Raman spectra, the G-peak width (100±113 cm 21) and the intensity ratio ID/IG (0.34±0.94) are both sensitive to the structural changes
induced by N incorporation. In the UV-Raman spectra, the G-peak position almost linearly decreases from 1665 to 1610 cm 21, and the Tpeak position increases tremendously from 1095 to 1314 cm 21 with increasing N content. The G-peak position and width, and the T-peak
position, are all sensitive to the bonding structure of the ®lms. q 2000 Elsevier Science S.A. All rights reserved.
Keywords: Raman scattering; Tetrahedral amorphous carbon
1. Introduction
Tetrahedral amorphous carbon (ta-C) ®lms, prepared by
the ®ltered cathodic vacuum arc (FCVA) method [1], have
stimulated great interest from both scienti®c and industrial
perspectives in the last decade. ta-C ®lms have interesting
and useful properties [2±6], such as extreme hardness (,70
GPa), chemical inertness, a wide Tauc optical band-gap
(,2.5 eV), smooth surface and low friction, thermal stability,
transparency in wide spectral range and ultra-thin achievable
thickness (,3 nm). Therefore, this material is important for
coating technology and electronic-device applications. It has
been shown by electron energy-loss spectroscopy (EELS)
that a signi®cant fraction (up to 87%) of the carbon atoms
in the ta-C ®lms form an amorphous, tetrahedral (sp 3) structure [5±7]. The electronic and optical properties of ta-C ®lms
were reported to be continuously adjustable by the incorporation of nitrogen during deposition [8±10]. Nitrogen ions were
ef®ciently combined into the amorphous carbon network by
nitrogen-ion bombardment (ion-assisted FCVA) [10,11].
Nitrogenated ta-C ®lms show large optical absorption coef®-
cients, high photo-response, and are a potential solar cell
material [10,12].
Raman scattering is a non-destructive technique for
measuring the bonding properties of diamond-like carbon
(DLC) and diamond. The conventional visible Raman spectrum of DLC, excited by 488- or 514-nm photons, is dominated by the G band at about 1560 cm 21 and a D feature
around 1360 cm 21, both of which are attributed to sp 2bonded carbon. Recently, direct ultraviolet (UV)-Raman
observations of the vibrational mode of the sp 3-bonded
carbon in the ta-C ®lm have been achieved using 244-nm
excitation [13±15]. In the visible Raman measurement of
the ta-C ®lm, the excitation energy (2.4 eV for 514 nm)
corresponds to the p ±p * transition at sp 2 sites, and this
leads to a resonant enhancement of the Raman cross-section
[16,17]. For UV excitation at 244 nm (5.1 eV), the energy is
suf®cient to excite the s states of both sp 2 and sp 3 sites. This
allows the Raman spectrum to show a more equally-weighted
view of the vibrational density of states for sp 2 and sp 3 sites.
In this paper, UV and visible Raman scattering studies of
nitrogenated ta-C ®lms prepared by the FCVA method are
presented. The correlation between the spectral parameters
and the nitrogen content is discussed.
* Corresponding author. Tel.: 165-790-5454; fax: 165-793-3318.
E-mail address: [email protected] (J.R. Shi)
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.
PII: S 0040-609 0(00)00732-X
170
J.R. Shi et al. / Thin Solid Films 366 (2000) 169±174
2. Experimental details
The nitrogenated ta-C ®lms were deposited by a FCVA
system which is described in detail elsewhere [6]. The carbon
plasma is produced from the arc spot on a cathode of 60-mm
diameter, 99.999% pure graphite in high vacuum. In our
system, a radial electric ®eld is introduced via the torus
duct-wall bias, and this, coupled with the curvilinear axialmagnetic ®eld on a curved toroidal duct, forms the crossed
electric-magnetic-®eld ®ltering assembly. The plasma,
steered by the ®eld through the duct to the deposition chamber, is deposited on the substrate without the unwanted
macroparticles and neutral atoms. The substrate in the
deposition chamber was negatively biased at 80 V, which
corresponds to 100 eV of impinging carbon-ion energy in
our experiment. The arc current was set to 60 A, and the
toroidal magnetic ®eld was ®xed at 40 mT. The substrates
used were cleaned k100l p-type silicon wafers, with an average thickness of 0.5 mm. The oxide layer on the silicon
surface was removed by argon ions from a RF ion-beam
source before deposition. Nitrogen gas was introduced into
the deposition chamber through the RF ion-beam source,
with an ion energy of 100 eV and ion-current density of
2.83 mA/cm 2 during deposition. The nitrogen partial pressure was varied between 2.6 £ 10 25 and 1.3 £ 10 24 Torr,
depending on the nitrogen ¯ow-rate monitored using a mass¯ow controller; the nitrogen ¯ow-rate is directly related to
the nitrogen partial pressure. A set of nitrogenated ta-C ®lms
with thicknesses of about 80 nm were prepared by the FCVA
technique at room temperature. One pure ta-C ®lm was
prepared for comparison. The nitrogen contents in the deposited ®lms were measured by Auger electron spectroscopy
(AES) and Rutherford backscattering (RBS).
The UV and visible Raman spectra were excited using the
244-nm line of a frequency-doubled Ar 1 laser (Coherent
90C FreD series) and the 514.5-nm line of an Ar 1 laser,
respectively. The UV and visible scattered light was
collected in back scattering with UV-enhanced and normal
CCD cameras, using Renishaw micro-Raman System 2000
and 1000 spectrometers. A laser output of 20 mW was used,
which resulted in an incident power at the sample of
approximately 1.5 mW. Multi-layer dielectric ®lters working in the range of UV and visible light were used for the
rejection of Rayleigh scattering light. The sample was
rotated during the UV-Raman measurement to prevent the
®lm from structural damage caused by the high photon
energy of the UV-laser. The spectral resolutions of the
UV and visible spectrometers (half-width, half-maximum)
were 4.0 and 2.0 cm 21, respectively.
Fig. 1 shows the relationship between the nitrogen atomic
fractions in the ®lm and nitrogen ¯ow-rate during the
deposition. The nitrogen content increases monotonically
as the nitrogen ¯ow-rate increases. It increases faster at a
low ¯ow-rate and becomes saturated at a high ¯ow-rate. The
®lms with nitrogen contents of 4.2, 7.9, 9.1, 9.6 and 10.8
at.% were used for Raman measurements.
The visible Raman spectra of pure and nitrogenated ta-C
®lms are shown in Fig. 2 in the range of 800±2000 cm 21. A
wide peak around 950 cm 21 is the second-order Raman peak
of the silicon substrate. The appearance of this peak is a
measure of the transparency of the ®lms near the wavelength of 514.5 nm. As the nitrogen ¯ow-rate increases,
the intensity of this peak decreases gradually. This result
agrees with the previous result that the absorption coef®cient of the ®lm increases with nitrogen content [10]. The
main asymmetric peak between 1100 and 1800 cm 21 is
attributed to the vibrational mode of sp 2-bonded carbon
clusters. The broad peak becomes more asymmetric with
increasing nitrogen ¯ow-rate. The asymmetric broad peak
could be ®tted either with two Gaussian peaks [17,18], or
with a Breit±Wigner±Fano (BWF)-shaped peak [6,19]. In
this paper, it was ®tted with two Gaussian±Lorentzian
mixed peaks, graphite (`G') and disorder (`D') peaks. The
Gaussian ratio was automatically adjusted by Grams/32
software. The ®tted G and D peaks for the ®lm containing
10.8 at.% nitrogen are shown in Fig. 2 with dashed lines. No
peak associated with nitrogen was observed in the visible
Raman spectra for all ®lms.
The ®tting results of peak positions and line-widths are
shown as a function of the nitrogen content in Fig. 3. As
the nitrogen content increases from 0 to 10.8 at.%, the Gpeak position varies in a small range from 1568 to 1572 cm 21,
while the D-peak position varies in a range from 1377 to 1412
cm 21. The changes of these peak positions are small, which
are almost within the measuring errors. With increasing
nitrogen content, the line-width (half-width at half-maxi-
3. Results and discussion
The nitrogen content in the nitrogenated ta-C ®lms, determined by AES and RBS methods, ranges from 1.7 to 11.2
at.% as the nitrogen ¯ow-rate increases from 0.5 to 12 sccm.
Fig. 1. The relationship between the nitrogen content in the ®lms and the
nitrogen ¯ow-rate during ®lm deposition.
J.R. Shi et al. / Thin Solid Films 366 (2000) 169±174
171
This situation corresponds to a big G peak and a very small
D peak, or a small value of ID/IG ratio. So the increase in the
ID/IG ratio indicates that the fraction of sp 2-bonded carbon
atoms increases with increasing nitrogen content. Based on
Robertson's `cluster model' for an amorphous carbon
network [20], the C amorphous network can be treated as a
few sp 2 clusters embedded in a sp 3-bonded matrix. The sp 3
matrix controls the mechanical properties, and p states of the
sp 2 clusters control the ®lm's electronic structure and optical
band-gap. The decreases in the hardness, the stress and the
optical band-gap [10] for the nitrogenated ®lms with a high
nitrogen ¯ow-rate are all correlated with the increase in the
sp 2-bonded carbon fraction. Since the G-line-width is partly
determined by the domain or size of the amorphous carbon
clusters, the ID/IG ratio is plotted versus G-line-width in Fig.
4b. The ID/IG ratio monotonically decreases from the largest
value of 0.94 to the smallest value of 0.34 as the G-peak width
increases from 100.5 to 113 cm 21. The low ID/IG ratio and
broad G-line-width for the ta-C ®lm indicates small sp 2 clusters in the ®lm [18]. Recent transmission electron microscope
results by Davis et al. [21] on a-C:H superlattice structures
Ê in
shows there to be no evidence of clusters greater than 5 A
r.f. plasma-deposited a-C:H ®lms. As the ID/IG ratio increases
Fig. 2. Visible (514.5 nm) Raman spectra of pure and nitrogenated ta-C
®lms. The dashed lines represent the ®tting of D and G peaks for the ®lm
with 10.8 at.% nitrogen.
mum) of the D peak varies from 149 to 173 cm 21. No obvious
trend can be obtained for the D-peak width. Meanwhile, the
line-width of the G peak monotonically decreases from 113,
for the pure ta-C ®lm, to 106 cm 21 for the ®lm with 10.8 at.%
nitrogen. The ®lm containing 9.1 at.% nitrogen has a minimum width of 100.5 cm 21. It was found that there was a
correlation between the G-peak width and the ®lm stress.
For amorphous carbon ®lms, the G-peak width almost linearly increases with the stress [18]. For nitrogenated ta-C
®lms, the stress has a small maximum for the ®lm deposited
at the 0.5 sccm nitrogen ¯ow-rate, and then decreases with
nitrogen ¯ow-rate [10]. Therefore, the G-line-width
decreases as the nitrogen content in the ®lms increases.
The variation of the G-line-width indicates that the G-linewidth is a good parameter for nitrogenated ta-C ®lms.
The intensity (®tting-area of peaks) ratio of the D peak to
the G peak is generally a measure for the zone edges or border
phonons of the carbon clusters (which depend on cluster sizes
and distributions). Fig. 4a shows the dependence of the intensity ratio ID/IG on the nitrogen content. The ID/IG ratio almost
linearly increases as the nitrogen content increases. The ®lm
containing 9.1 at.% nitrogen has the maximum ID/IG ratio of
0.94. For ta-C ®lm, it was found that the main asymmetric
peak between 1100 and 1800 cm 21 becomes quite symmetric
and has a large negative value of skewness (Q-factor) as the
fraction of sp 3-bonded carbon becomes over 80% [6,19].
Fig. 3. (a) Peak position; and (b), width of the G and D peaks of the sp 2bonded clusters at 514.5 nm as a function of nitrogen content in the ®lm.
Solid and open circles represent G-peak position and width, and solid and
open triangles represent D-peak position and width, respectively.
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J.R. Shi et al. / Thin Solid Films 366 (2000) 169±174
Fig. 4. The dependence of the ID/IG ratio on: (a), the nitrogen content in the
®lm; and (b), the G-peak width.
and the G-line-width decreases, the size of the sp 2 clusters in
the nitrogenated ta-C ®lms becomes larger.
The UV-Raman spectra for the pure and nitrogenated taC ®lms are shown in Fig. 5 in the range of 750±2000 cm 21.
The UV-Raman spectrum consists of two wide peaks,
referred as T and G bands [13,14], and a small, sharp atmospheric O2 peak at 1553 cm 21. The ®ttings with three mixed
Gaussian±Lorentzian lines are plotted for each corresponding spectrum. The decomposed T, G and O2 peaks for the
®lm with 10.8 at.% nitrogen are also shown in Fig. 5. The T
and G peaks are related with the vibrational modes of the
sp 3- and sp 2-bonded carbon clusters, respectively [13±
15,22,23]. It can be seen clearly from Fig. 5 that the height
of the T peak decreases as the nitrogen content in the ®lm
increases, and the position of the T peak moves to a higher
wave number. No peak associated with nitrogen atoms was
observed in the UV-Raman spectra. The reason for the fact
that both the visible and UV-Raman spectra are insensitive
to the vibrational mode of the nitrogen atoms may be due to
the resonance effect of the carbon-vibrational modes and
that the Raman cross-section of the vibrational mode associated with nitrogen atoms is comparatively small.
Fig. 6 shows the dependence of the peak positions and the
line-widths of the G and T bands on the nitrogen content. As
the nitrogen content increases, the G-peak position almost
Fig. 5. UV (244 nm)-Raman spectra of pure and nitrogenated ta-C ®lms.
The solid lines represent the ®ttings for each corresponding spectrum. The
dashed lines represent the ®tting of T and G peaks, and the dotted line
represents the ®tting of the peak of oxygen in the air for the ®lm with
10.8 at.% nitrogen.
linearly decreases from 1665, for the pure ta-C ®lm, to 1610
cm 21 for the ®lm containing 10.8 at.% nitrogen. The T-peak
position exhibits the opposite behaviour, increasing from
1095, for the pure ta-C ®lm, to 1314 cm 21 for the ®lm
with 10.8 at.% nitrogen. The overall position changes of
the G and T peaks are 55 and 219 cm 21, respectively. The
results for ta-C ®lm are in good agreement with the data
reported by Gilkes et al. [13,14] and Merkulov et al. [15].
With increasing nitrogen content in the ®lm, the G-linewidth decreases from 118, for the pure ta-C ®lm, to 105
cm 21 for the ®lm with 10.8 at.% nitrogen. This behaviour
is similar to that of the G-line-width determined by the
visible Raman spectra. The T-peak width (275±350 cm 21)
is much larger than the G-peak width, but no distinct trend
can be observed. The large widths of the G and T peaks
result from the small carbon-cluster size in the ®lms.
The G-peak position determined by the UV-Raman spectra is much higher than that determined by the visible
Raman spectra, and decreases obviously with increasing
nitrogen content in the ®lm. It is worth discussing the
reasons in more detail. Because of the resonance feature
of the G peak, or p ±p * transition, at sp 2 sites, small changes
in the strength of the p bonds can considerably affect the Gpeak position. This can occur either as a result of stress or
J.R. Shi et al. / Thin Solid Films 366 (2000) 169±174
Fig. 6. (a) Peak position; and (b), widths of the G and T peaks at 244 nm as
a function of nitrogen content in the ®lm. Solid and open circles represent
G-peak position and width, and solid and open triangles represent T-peak
position and width, respectively.
through changes in the clustering of sp 2 sites. Ager et al.
[24] found a strong dependence of the G position on stress in
ta-C ®lms deposited by FCVA method. They showed that by
delaminating the highly-stressed ta-C ®lms (allowing the
®lms to relax), the G position drops by about 20 cm 21. An
average stress shift of 1.9 cm 21/GPa was obtained for biaxial-plane stress in hard ta-C ®lms. For the nitrogenated ta-C
®lms, the difference in the compressive stress between the
pure ta-C ®lm and the ®lm containing 10.8 at.% nitrogen is
about 5.6 GPa. The stress-induced shift (,10.6 cm 21) could
represent one part of the total shift of the G position. As the
G-peak position, determined by the visible Raman spectra,
stays almost unchanged as the nitrogen content in the ®lm
increases (the internal stress decreases), the stress may not
be a reasonable mechanism for the large change in the Gpeak position measured by UV-Raman. Therefore, other
mechanisms are expected. One possible mechanism is that
there is a sp 2-cluster size distribution in the ®lms, and the
average sp 2-cluster size increases with increasing nitrogen
content in the ®lm. Raman scattering from sp 2-carbon clusters with various sizes could be selectively resonanceenhanced, and the varying average cluster size of sp 2 carbon
is responsible for the shift of the G-peak position. Resonant
Raman spectra of hydrogenated amorphous carbon (a-C:H)
®lms have been reported by Wagner et al. [16] and Yoshikawa et al. [25,26]. The G-peak position of sp 2-carbon clusters decreases with increasing wavelength of the excitation
173
laser. This behaviour was well interpreted in terms of p ±p *
resonant Raman scattering from sp 2-carbon clusters with
different sizes [25]. Size-dependent resonance Raman scattering was observed recently in single-wall carbon nanotubes [27,28]. Another possible mechanism is that the sp 2
bonds may not be linear, but are bent because of the high
internal compressive stress. The morphologies of nongraphitizing carbon materials reported by Harris showed
sp 2-carbon clusters made of bent sp 2 bonds [29]. The
stretching mode of the sp 2 bonds becomes higher because
of the bending of the bonds.
The T-peak position decreases tremendously as the nitrogen content in the ®lm increases. The variation of the T-peak
position is strongly correlated with the nitrogen content,
hence the content of sp 3 bonding in the ®lm. Merkulov et
al. [15] reported that the Raman peak associated with the
sp 3 bonding varies from 1150, for the ®lm with 75 at.%
sp 3-C atom, to 1400 cm 21 for the sputtered a-C with 6 at.%
sp 3-C atom. The data are in good agreement with ours. The
nearest neighbors of a given sp 3-carbon atom may be either
sp 3-, or sp 2-bonded carbon. For the ta-C ®lms with an sp 3
fraction of more than 80%, the nearest neighbors of the sp 3
site contain either zero or one sp 2-bonded carbon atom, therefore the bonds between sp 3-carbon atoms (sp 3Zsp 3 bonds)
are predominant. As the sp 3 fraction decreases, the content of
the sp 2-bonded carbon increases. The bonds between sp 3- and
sp 2-carbon atoms (sp 3Zsp 2 bonds) gradually become the
predominant bonds. This results in the upward movement
of the T peak because of the high vibrational frequency of
the sp 2-bonded atoms. When the sp 2-bonded carbon atoms
are dominant, the sp 3Zsp 2 bonds are embedded in the
sp 2Zsp 2 bonds, and the latter are the predominant bonds.
The T-peak position is coming close to the D-peak position
measured by visible Raman. As the clusters in amorphous
Ê ) [21], the vibrational
carbon ®lms are quite small (around 5 A
modes of the zone edges have a signi®cant contribution in the
Raman spectra. It is reasonable to assume that the zone edge
contribution of sp 2-carbon clusters is mixed in the T peak.
Based on this assumption, the very large T-peak width and
the shift of the T-peak position can be interpreted. The big
Fig. 7. Height ratio of IT/I1390 as a function of the nitrogen content in the
®lm.
174
J.R. Shi et al. / Thin Solid Films 366 (2000) 169±174
change in the T-peak position demonstrates that the T peak is
sensitive to the bonding structure of the amorphous carbon
®lm.
The intensity ratio of T peak to G peak varies with the
nitrogen content in the ®lm. No obvious dependence of the
IT/IG ratio on the nitrogen content was observed. This fact
further enhances the discussion above that the T peak is not
only the measure of the vibrational mode of the sp 3±sp 3
bonds, but also a measure of the vibrational modes of the
sp 3±sp 2 and sp 2±sp 2 bonds, as the sp 2 fraction of the carbon
atom increases. The count at 1390 cm 21 was used as a
reference to compare the T-peak height. Fig. 7 shows the
relationship between the height ratio, IT/I1390, and the nitrogen content in the ®lm. The IT/I1390 ratio decreases monotonically with increasing nitrogen content.
4. Conclusion
The visible Raman spectra are only sensitive to vibrational modes of sp 2-bonded carbon atoms, while a new
wide band, called the T-band, in the range of 1090±1320
cm 21, associated with sp 3-bonded carbon atoms, appears in
the UV-Raman spectra. For the visible Raman, the G- and
D-peak positions of sp 2-bonded carbon vary in a small
range, while the intensity ratio ID/IG increases from 0.34 to
0.81 as the nitrogen content in the ®lms increases. The Gline-width and the intensity ratio, ID/IG, are sensitive to the
structural changes induced by nitrogen incorporation. For
the UV-Raman, the G-peak position monotonically
decreases from 1665 to 1610 cm 21, and the T-peak position
increases tremendously from 1095 to 1314 cm 21. The Gpeak position and width, and the T-peak position, are all
sensitive to the bonding structure of the nitrogenated ta-C
®lms. The T peak is a measure of the vibrational modes of
the sp 3Zsp 3 and sp 3Zsp 2 bonds, and then the sp 2Zsp 2 bond,
as the sp 3-fraction of carbon atom decreases.
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