IR characterization of a-C:H:N films sputtered in Ar/CH4/N2 plasma

Journal of Non-Crystalline Solids 331 (2003) 70–78

www.elsevier.com/locate/jnoncrysol

IR characterization of a-C:H:N films sputteredin Ar/CH4/N2 plasma

Gabriel Lazar *, Iuliana Lazar

Bacau University, Calea Marasesti 157, Bacau, Romania

Received 27 September 2002; received in revised form 6 June 2003

Abstract

Amorphous nitrogenated carbon films (a-C:H:N) were deposited by rf magnetron sputtering of a graphite target in

an Ar/CH4/N2 plasma. The films were characterized by infrared spectroscopy. An increase in the intensity of absorption

maximum at 3300–3400 and 1620 cm�1 of the IR spectra was observed. For high nitrogen concentrations, a new

maximum appears at 2150 cm�1 indicating that the C and N atoms are chemically bonded in the film. The analysis of

the broad absorption band observed between 1700 and 1000 cm�1 was made. Deconvolution of this band showed a six

band system, indicating a lack of aromatic structures. The content of nitrogen and sp2 bonded carbon in the films was

seen to increase with a growth of the N2 partial pressure in the deposition gas.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Much effort has been devoted to the study of

amorphous carbon nitride (a-C:N:H) films because

of their remarkable mechanical and tribological

properties and more recently, for their potentialapplications as electronic materials in cold cathode

displays and in electrochemical studies of water

treatments [1]. The crystalline phase of carbon

nitride, especially b-C3N4, is expected to be a very

hard material, as shown theoretically by Liu and

Cohen [2].

One of the key factors in determining the

structure and properties of hard amorphous car-bon films is the hybridization state of the atoms

* Corresponding author.

E-mail address: [email protected] (G. Lazar).

0022-3093/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.jnoncrysol.2003.09.004

that form the amorphous network [3]. Various

kinds of preparation methods have been applied to

fabricate carbon nitride films, but samples with

sufficient amount of crystallized C3N4 phase or

with mechanical properties comparable to the

predicted values have not been reported. The in-ternal stress considerably decreases with the addi-

tion of nitrogen, without sacrificing hardness, up

to an N incorporation of 2%, by the decrease of

the amount of unbound hydrogen in the film [4,5].

At higher concentrations, hardness is reduced due

to the development of graphitic domain. The in-

corporation of nitrogen could also affect the

amount of bonded hydrogen in the films [6].Franceschini et al. [3] reported that nitrogen in-

corporation strongly affects the hybridization state

of carbon atoms by increasing the sp2 fraction for

both nitrogen precursor gas N2 and NH3. At the

same time the nitrogen can be used to dope the

ed.

mail to: [email protected]

G. Lazar, I. Lazar / Journal of Non-Crystalline Solids 331 (2003) 70–78 71

material; however, the doping process remainedinefficient because the diversity of bonding con-

figuration allowed nitrogen to be incorporated in

different environments [7].

Infrared spectroscopy (IR) is a non-destructive

and widely available technique to probe the

bonding in carbon based films. IR absorption is

widely used in a-C:H films to determine the CH

bonding configuration [8]. The interpretation ofthe absorption IR spectra for nitrogen containing

carbon films is not very clear yet. The problem is

the broad band between 1700 and 1000 cm�1 that

represents a superposition of several bond contri-

butions.

In literature, three major assignments can be

found for this band: (i) the Raman �G� and �D�bands rendered infrared active by the incorpora-tion of nitrogen into the graphitic rings [9,10], (ii)

various bending modes of CC, NC, CH and NH

bonds [11,12] and (iii) a combination of the first

two [13–15].

This paper is an attempt to make this problem

clearer, starting from interpreting IR absorption

spectra for hydrogen and sp3 carbon rich, poly-

mer-like nitrogenated amorphous carbon films.

5000 4000 3000 2000 10000

20

40

60

80

100

120

140

160

180

200

1375

14501610218029103400

12% N2 8% N2 5% N2 2.5% N2 1% N2 0% N2

IR T

rans

mita

nce

(a.u

.)

wavelength (cm -1)

Fig. 1. The transmission spectra in the range 4600–650 cm�1,

for six samples deposited in argon/methane/nitrogen atmo-

sphere, with different nitrogen proportions and for a sample

deposited without nitrogen.

2. Experiment

Thin films were produced by magnetron sput-

tering in a home-built system from pure graphite

target. In brief, the system consists of a cylindrical

stainless-steel chamber with one magnetron(cathode) and two anodes [16]. In this work, a

13.56 MHz rf generator supplies power to the

system of the cathode and circular anode, and the

planar anode is biased by a dc voltage. Films, 0.2–

2.2 lm thick, were deposited on boro-silicate

(microscope slides) glass and silicon wiskers at a

target-to-substrate distance of 3 cm. The propor-

tion between Ar and methane was maintainedconstant at 1/1 and the concentration of the ni-

trogen in the gas mixture was varied from 0% to

12%. The unusual use of methane in the gas mix-

ture in a sputtering process implies a combination

of sputtering and CVD deposition. The discharge

chamber was first pumped down to 10�5 Torr and

then filled with gas. During sputtering, the gas

pressure was kept at 10�2 Torr. The substratetemperature remains below 100 �C. The appliedsubstrate bias voltage was )200 V for all samples.

IR transmission of films deposited on Si substrates

was measured with a IR double beam spectro-

photometer. Spectra were recorded in the wave-

number range from 650 to 4600 cm�1. Parts of the

spectra were fitted using Gaussian curves, centered

in the positions determined from the literature.Optical absorption of the films was measured in

the wavelength range 700–400 nm using an UV–

VIS spectrophotometer. The absorption coefficient

a was calculated using Lambert�s Law and was

explored as a function of photon energy hm. Theoptical band gap is estimated from a ðahmÞ1=2 ¼f ðhmÞ Tauc plot.

3. Experimental results

The results for nitrogen-free a-C:H films are

presented in an earlier paper [16]. Basically, the

films deposited by magnetron sputtering from

graphite target in Ar/CH4 atmosphere presents a

sp3 carbon proportion of 85–90% and an atomicconcentration of carbon-bonded hydrogen of 45%.

The great proportion of the sp3 fraction in the

films and the high amount of the hydrogen are

features of the soft polymeric films. As shown in

Fig. 1, the transmission spectrum of the nitrogen-

3100 3000 2900 2800 27000

500

1000

1500

5

4

321

α (c

m-1

)

wavenumber (cm-1)

Fig. 2. The infrared absorption spectrum for the C–H

stretching mode region for a sample obtained in following

conditions: working gas Ar/CH4/N2¼ 46%/46%/8%, gas pres-sure 10�2 Torr and substrate negative bias voltage 200 V.

Table 1

C–H stretch absorption bands deconvoluted from the Fig. 2

absorption peak

Curve

number

Observed vibration

frequency (cm�1)

Configuration

1 3027 sp2 CH2 (olefinic)

2 2998 sp2 CH (olefinic)

3 2956 sp3 CH3 (asymmetrical)

4 2921 sp3 CH2 (asymmetrical)

sp3 CH

5 2858 sp3 CH2 (symmetrical)

72 G. Lazar, I. Lazar / Journal of Non-Crystalline Solids 331 (2003) 70–78

free film presents an absorption peak at 2900 cm�1,common for all hydrogenated carbon films and

other tree peaks: at 1370, 1450 cm�1 (both as-

signed to CH bending modes [17]) and 1600 cm�1

(related to olefinic C@C bonds [18]). These three

peaks are very important for nitrogenated films,

because of their position, inside of the main ab-

sorption band (1700–1000 cm�1).

Fig. 1 shows the transmission spectra in therange 4600–650 cm�1, for six samples deposited in

argon/methane/nitrogen atmosphere, with differ-

ent nitrogen proportions and for a sample depos-

ited without nitrogen. All the films were deposited

in the following conditions: gas pressure 10�2 Torr

and substrate negative bias voltage 200 V.

The spectra show major modifications when

nitrogen concentration increases. All the nitro-genated films present a broad feature between 3700

and 3000 cm�1, associated with N–H bonds [11].

The amplitude of this absorption band increases

with the nitrogen proportion in the deposition gas

mixture indicating an increasing of the nitrogen

proportion in films.

The 2900 cm�1 absorption peak, which shows

the C–H bonds type and concentration, initiallyincreases with the growth of the nitrogen propor-

tion, then it decreases, but the position of the peak

remains the same. This behavior shows that, first,

the concentration of the carbon bonded hydrogen

is increasing and then it is decreasing, because of

the growth of the nitrogen proportion in films.

These would mean that carbon atoms were re-

placed by nitrogen atoms with an increase of theN2 partial pressure. The shape and position of the

peak are similar for all the samples, indicating a

low variation of the proportion of sp3 bonds. The

role of hydrogen in passivating the dangling bonds

or defect states is very important since it controls

the properties of film [5]. Addition of nitrogen in

the precursor may affect these defect states and

reduce the unbound hydrogen, which will effec-tively improve the adhesion of the film on the

substrate. The evidence of the very high value of

the sp3 fraction is shown in Fig. 2 where it is

presented the infrared absorption spectrum for the

C–H stretching mode region for a sample obtained

in the following conditions: working gas Ar/CH4/

N2¼ 46%/46%/8%, gas pressure 10�2 Torr and

substrate negative bias voltage 200 V. The spec-

trum can clearly be decomposed to five Gaussian

components, as shown in Fig. 2. The positions ofthese components are in very good agreement with

other results [1,16,18] and are summarized in

Table 1. From the great difference between sp3

peaks (3–5 in Fig. 2) and sp2 peaks (1 and 2 in

Fig. 2) area we may conclude that the type bond-

ing of C adjacent to H is predominantly sp3 [16].

When the proportion of the nitrogen increases,

a new absorption maximum appears at 2150 cm�1,related to CBN (nitrile) group [12,14,15] and as-

sociated to a high ion energy during the film de-

position process [6]. This peak is often observed in

the IR spectra of the samples with high nitrogen

content, suggesting that the C and N atoms are

chemically bonded in the film.

The broad band observed between 1700 and

1000 cm�1 for all nitrogenated samples evidently

Table 2

Major peaks positions in the 1700–1000 cm�1 absorption band

of hydrogenated carbon nitride films

Wavenumber

(cm�1)

Assignment Reference

1620–1650 C@C, C@N, NHx [15,17,19–22]

�1600 C@N, C@C [17,23,24]

1550–1570 Raman G, C@N [9,10,13–15,25]

1500–1510 C–N, C@N, C@C [12,14,19,26]

1450 sp3 CHx [17]

1360–1380 Raman D, CH3, CC,

C@N[9,10,13–15,17,19]

1300–1350 sp2 carbon, C–N,

C@N[12,14,23]

1220–1265 C–N (in C3N4), CC,

C@N[19,20,25,27,28]

1020–1150 C–N (aliphatic), N–H [19,29]


represents a superposition of several contributionsand the paper is focused on this feature.

At a first glance, this broad band is composed

of at least five components. For a possible as-

signment of the individual absorption bands to the

corresponding bond configuration an analysis of

the published data is necessary. Despite a large

variation in the peaks positions and assignments,

nine main bands can be identified (Table 2).Not all these maximums can be found in the

absorption spectrum for a certain film. Each peak

can be correlated with other peaks in the absorp-

tion spectrum or with the results of other experi-

ments. An attempt to deconvolute the absorption

band of the films using nine gaussians failed.

4. Discussion

The analysis of the whole absorption spectrum

and the known IR absorption properties of the

nitrogen-free films impose some conditions.

The peaks at 1600, 1450 and 1370 cm�1 must

appear in the deconvolution. The 2900 cm�1 peak

position and amplitude suggest large quantities ofsp3 bonded CH2 and CH3 groups, that implies the

presence of the 1450 and 1370 cm�1 peaks. The

presence of the C@C peak at 1600 cm�1 in nitro-

gen-free films suggests a similar position of the

peak in the nitrogenated samples spectra.

The carbon based polymer-like materials are

mainly composed of a complicated and entangled

network of cross-linked alkane-like chains withsome double or triple bonds [17]. In the absorption

spectra, the 1600 cm�1 C@C band is not accom-

panied by sharp bands in the 950–650 cm�1 region

or at 3050 cm�1, and this fact offers evidence that

sixhold aromatic structures are not major com-

ponents in these polymeric films [30]. In this

conditions, the influence of the Raman D (at 1360–

1380 cm�1) and Raman G (at 1550–1570 cm�1)peaks can be neglected but the 1370 cm�1 feature

still remains in the deconvolution. The 1500–1510

cm�1 peak is associated mainly with nitrogen

bonded in pyridine-like structures [31] and can be

neglected as well. The major contribution of the

1220–1265 cm�1 is from C–N bonds in C3N4

structures [20,31]. The great disorder of the poly-

meric films implies a small contribution of thisbond to the absorption band and we can search

the C–N bonds at 1300–1350 and 1020–1150 cm�1.

The contribution of NH bonds must also appear at

1620–1650 and 1020–1100 cm�1 [21,29]. In con-

clusion, for the investigated films, a six peaks de-

convolution must be tried.

Fig. 3 shows the 1000–1800 cm�1 absorption

band of the films presented in Fig. 1. The ab-sorption intensity clearly increases in this domain

with the nitrogen proportion in the deposition gas.

The results of the peaks deconvolution for nitro-

gen containing films are presented in Table 3.

The verification of the proposed band system

can be made by analysing the peaks position for all

samples. If the system is good, the peaks centre

must be in the proposed region and small differ-ences can appear between the peaks position for

different films. As shown in Fig. 4, the position of

the all six deconvoluted peaks has a very low

variation for all the nitrogenated films. The posi-

tion of the three peaks common for all the films

(nitrogenated or not) has a remarkably low vari-

ation around proposed values: 1375, 1450 and

1600 cm�1. This fact confirms the absence of theother absorption bands in 1450–1600 cm�1 region.

The other peaks present law amplitude variations

around a value from the proposed domain.

For an assignation of the absorption bands, the

area and proportion in the total band as a function

of the nitrogen proportion in the deposition gas

are presented for all deconvoluted peaks in Fig. 5.

0

500

1000

1500

2000

2500

3000

6

5

4

3

2

1

12 % N2

abso

rban

ce (c

m-1)

0

500

1000

1500

2000

2500

3000

8 % N2

0

500

1000

1500

2000

2500

3000

5 % N2

abso

rban

ce (c

m-1)

0

500

1000

1500

2000

2500

3000

2.5 % N2

800 1000 1200 1400 1600 18000

500

1000

1500

2000

2500

3000

1 % N2

wavenumber (cm-1)

abso

rban

ce (c

m-1)

800 1000 1200 1400 1600 18000

500

1000

1500

2000

2500

3000

0 % N2

wavenumber (cm-1)

Fig. 3. The deconvolution of the 1000–1800 cm�1 absorption band of the six films presented in Fig. 1.


The peak proportion is calculated making the ratio

between the peak area and the total band area.

The first peak, centred around 1100 cm�1, in-

creases rapidly for small proportion of the nitro-

gen in working gas, and decreases for great

nitrogen proportions. This absorption cannot be

considered to be related to the N–H bond, because

of the constant increases of the absorption in the

3300 cm�1 region with an increase of the nitrogen

partial pressure. The band would mostly be due to

the C–N bond. The decrease of the band intensity

must be correlated with an increase of the sp2

carbon proportion, as a result of the increases of

the nitrogen content in the films [29].

The intensity of the second peak increases al-

most linearly with an increase of the nitrogen

Table 3

The results of the peaks deconvolution in the 1000–1800 cm�1 domain for different proportion of nitrogen in the deposition gas

Nitrogen proportion Peak position (cm�1) Peak area (cm�2) Peak proportion in the band (%) Total band area (cm�2)

1% 1077.1 36 188 14.4 251 932

1341.3 93 357 37.1

1374.1 4731 1.9

1450.3 26 121 10.4

1599.1 36 540 14.5

1668.4 54 995 21.8

2.5% 1134.9 79 511 13.7 578 919

1332.5 159 772 27.6

1374.6 12 355 2.1

1449.5 68 001 11.7

1599.7 178 086 30.7

1635.2 81 194 14.0

5% 1120.3 61 300 10.8 565 276

1329.7 181 940 32.2

1374.4 11 941 2.1

1450.7 68 268 12.1

1608.1 17 7588 31.4

1637.7 64 239 11.4

8% 1112.5 61 491 8.1 758 607

1337.7 244 814 32.3

1375.9 11 335 1.5

1448.8 69 592 9.2

1594.6 301 118 39.7

1642.1 70 256 9.3

12% 1105.9 36 572 4.7 782 492

1357.8 322 295 41.2

1377.4 7725 1.0

1448.4 47 993 6.1

1598.6 273 677 35.0

1639.4 94 229 12.0

0 2 4 6 8 10 121000

1100

1200

1300

1400

1500

1600

1700

1800

Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6

Wav

elen

gth

(cm

-1)

Nitrogen proportion (%)

Fig. 4. The peak position for the investigated bands as a

function of nitrogen proportion in the working gas.


proportion and must be related to the C@N andsp2 carbon bonds [12,23].

There is a great similarity between the data

variation for the next two peaks. The ratio be-

tween the peaks area, shown in Fig. 6, presents a

small variation. The initial increase of the ab-

sorption, followed by a decrease can be correlated

to the same variation of the 2900 cm�1 peak. These

facts offer evidence that the two peaks are relatedto the sp3 CHx bonds.

The last two peaks must be related to the C@Cand C@N bonds. The increase of the absorption in

this region indicates that the sp2 carbon propor-

tion in the film increases with the nitrogen content.

The evidence of this fact can be the decrease of the

optical band gap. Fig. 7 shows the variation of the

0 2 4 6 8 10 121x10 4

2x10 4

3x10 4

4x10 4

5x10 4

6x10 4

7x10 4

8x10 4

Peak 1 area

Peak

are

a (c

m-2)

Nitrogen proportion (%)0 2 4 6 8 10 12

0.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105

3.5x105


Peak

are

a (c

m-2)

Peak 2 area

0 2 4 6 8 10 124.0x103

5.0x103

6.0x103

7.0x103

8.0x103

9.0x103

1.0x104

1.1x104

1.2x104

1.3x104


Peak

are

a (c

m-2)

Peak 3 area

0 2 4 6 8 10 122x104

3x104

4x104

5x104

6x104

7x104


Peak

are

a (c

m-2)

Peak 4 area

0 2 4 6 8 10 120.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105

3.5x105

4.0x105


Peak

are

a (c

m-2)

Peak 5 area

0 2 4 6 8 10 120.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105


Peak

are

a (c

m-1)

Peak 6 area

4

8

12

16

Peak proportion (%)

Peak 1 proportion

20

25

30

35

40

45

50

55

60

Peak proportion (%)

Peak 2 proportion

0

1

2

3

4

5 Peak proportion (%

)

Peak 3 proportion

4

8

12

16

20

24

Peak proportion (%)

Peak 4 proportion

0

10

20

30

40

50

Peak proportion (%)

Peak 5 proportion

10

15

20

25

30

Peak proportion (%)

Peak 6 proportion

Fig. 5. The peak area and proportion for the investigated bands as a function of nitrogen proportion in the working gas. The peak

proportion is calculated making the ratio between the peak area and the total band area.


0 2 4 6 8 10 120.10

0.15

0.20

0.25

0.30

Peak

are

a 3/

Peak

are

a 4


Fig. 6. The ratio between the area of the peaks related to the

sp3 CHx bonds as a function of nitrogen proportion in the

working gas.

0 2 4 6 8 10 12

1.9

2.0

2.1

2.2

2.3

2.4

2.5

E g (eV

)

nitrogen (%)

Fig. 7. The variation of the Tauc gap as a function of the ni-

trogen proportion in the feed gas.


Tauc gap as a function of the nitrogen proportionin the feed gas. The optical band gap decreases

from 2.5 to 1.9 eV with the growth of nitrogen flow

rate within the investigated range. Other groups

[10,11] have also reported a similar decrease in the

optical band gap of nitrogenated carbon.

5. Conclusions

Amorphous nitrogen-containing hydrogenated

carbon films are deposited from rf magnetron

sputtering of graphite target in Ar/CH4/N2 atmo-

sphere. In the IR absorption spectra, with the in-creasing of N2 proportion, there is an enlarging in

the intensity of absorption maxima at 3300–3400

and 1620 cm�1. For high nitrogen concentrations, a

newmaximum appears at 2150 cm�1 indicating that

theCandNatoms are chemically bonded in the film.

The IR spectroscopy is an easy and widely

available technique, but, because of the lack of

standards and reliable assignments, the interpre-tation of the absorption IR spectra for nitrogen

containing carbon films is not very clear yet. As one

small step in the attempt to render IR spectroscopy

appropriate for routine characterization of carbon

nitride films, the analysis of the broad absorption

band observed between 1700 and 1000 cm�1 has

been made. For polymer-like films investigated in

this paper, the deconvolution shows a six bandssystem, indicating the lack of the aromatic struc-

tures and an increase of the nitrogen and sp2 car-

bon proportion in the films with an increase of the

N2 partial pressure in the deposition gas.

Acknowledgements

The authors are grateful to Dr I. Vascan and to

Dr M. Caraman, Bacau University, for expert

technical assistance and for stimulating discussions

and comments.

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IR characterization of a-C:H:N films sputtered in Ar/CH4/N2 plasma