OPTICAL AND STRUCTURAL PROPERTIES OF

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Nitrogenated diamond-like carbon films have been deposited on glass and p-type Si (100) sub- strates by ... to remove the resistive native oxide formed over the.

June 27, 2006 19:42 00775

Surface Review and Letters, Vol. 13, No. 1 (2006) 1–6 c World Scientific Publishing Company 

OPTICAL AND STRUCTURAL PROPERTIES OF NITROGENATED DIAMOND-LIKE CARBON FILMS PREPARED BY r.f. PECVD M. RUSOP∗,¶ , S. ABDULLAH∗ , J. PODDER† , T. SOGA‡ and T. JIMBO§ Institute of Science, Universiti Teknologi Mara, 40450 Shah Alam, Selangor, Malaysia † Department of Physics, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh ‡ Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan



§

Research Center for Nano-Device and System, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan ¶ [email protected] Received 17 July 2005

Nitrogenated diamond-like carbon films have been deposited on glass and p-type Si (100) substrates by radio frequency (r.f.) plasma-enhanced chemical vapor deposition (PECVD) with a frequency of 13.56 MHz at room temperature using CH4 as precursor of carbon source and H2 as a carrier gas. The deposition was performed at a different flow rate of nitrogen from 0 to 12 sccm under a constant r.f. power. The effect of nitrogen incorporation on the bonding states and growth kinetics of the deposited films have been investigated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy and optical properties by UV spectroscopy measurement. Our experimental results show that the incorporation of nitrogen has a considerable effect on the properties of the deposited films. FTIR spectra show that the nitrogen is bonded to carbon and hydrogen as C=N, C≡N, N–H and C–H bonding configurations in the as-deposited film. The incorporation of nitrogen is found to shift the Raman G peak toward the higher wave number and to increase the Raman ID /IG ratio demonstrating the graphitic character of the hydrogenated amorphous carbon–nitrogen films. Band gap is found to reduce with the increase in nitrogen concentration. Keywords: Optical properties; bonding properties; nitrogenated; nitrogen incorporation; amorphous carbon; PECVD.

1. Introduction

and biocompatibility. The a-C:H:N films, either in polymer-like phase or in diamond-like phase, have been deposited by several techniques1−3 to find a wide range of technological applications. The structural modification of a-C:H films can be done by the addition of nitrogen or silicon to hydrocarbon

The study of hydrogenated amorphous carbon– nitrogen (a-C:H:N) films has generated much interest in recent years for unique tribological characteristics such as high hardness, infrared transparency, chemical inertness, low friction coefficients, ¶

Corresponding author. 1

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M. Rusop et al.

2. Experimental

precursors to reduce the internal stress and the mechanical properties overall. Nitrogen incorporation induces a-C:H films to be more graphitic and strongly decrease the defect density, enabling its use as a semiconductor material.4 Nitrogen incorporation was also found to decrease the threshold electric field in electron field emission process,5 enabling the use of a-C:H:N films as a overcoat on emission tips in flat panel display devices.6 Nitrogen incorporation results in a strong decrease in the sp3 carbon atom fraction,7 which is mainly responsible for a-C:H film rigidity. And finally, nitrogen incorporation results in the preferential bonding of hydrogen atoms to the nitrogen atoms, which makes strong carbon–nitrogen extended network formation, and adds terminating groups to the amorphous network. The interest in amorphous carbon (a-C) and amorphous hydrogenated carbon (a-C:H) films has been extended to their nitrogen-containing modifications. Nitrogen atoms are commonly incorporated into the film during the growth process using a gas mixture of N2 and a hydrocarbon precursor such as CH4 .8,9 The influence of nitrogen incorporation on the structural modification of amorphous carbon films has also been subject for both theoretical and experimental investigations. In recent years, much attention has been paid to the doping of these aC:H films with nitrogen.10 The aim of the present work is to report the concerned effect of nitrogen incorporation on the film growth kinetics and modification of a-C:H:N film structure and optical properties.

Boron–nitrogen co-doped incorporation on a-C:H films (denoted as a-C:H:N) were grown on glass and Si (100) substrates from a mixture of N2 and CH4 binary gases at fixed flow rates and inner partial pressure of CH4 mixed with H2 carrier gas using radio frequency (r.f.) plasma-enhanced chemical vapor deposition (PECVD) reactor with a capacitively coupled parallel plate operating at 13.56 MHz. The glass and n-Si substrates were placed onto the electrode, which is connected to the r.f. oscillator. Before deposition, the glass and n-Si substrates were cleaned ultrasonically with acetone and methanol each for 10 min, respectively. After cleaning, they were etched with HF : H2 O (1 : 10) for 1 min to remove the resistive native oxide formed over the surface and then quickly transferred into the r.f. CVD chamber. The deposition is performed over different flow rates of nitrogen (viz., 0, 6, 8, 10, and 12 sccm) under a constant r.f. power of 300 W and a partial pressure of mixed gases (CH4 + H2 + N2 ) of 10 Pa. Effects of the nitrogen incorporation on the bonding states, optical properties, and thickness of deposited films were examined by Raman scattering analysis (Raman), Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy and Alpha-step 500 surface profiler. Chemical bonding and elemental composition of the deposited films have been determined by X-ray photoelectron spectroscopy (XPS) measurements (SSX-100 XPS system of Surface Science Instrument) using Al Kα (1486.6 eV) radiation as an X-ray source under high

Table 1. Growth conditions and physical properties of as-deposited a-C:H:N thin films grown by r.f. PECVD at 300 W r.f. power and 10 Pa partial pressure. Sample CH4 (sccm) H2 (sccm) N2 (sccm) D position (cm−1 ) D FWHM (cm−1 ) G position (cm−1 ) G FWHM (cm−1 ) ID /IG Thickness (nm) Optical gap (eV)

A

B

C

D

E

10 50 0 1337 311 1555 149 2.94 130 1.62

10 50 6 1347 301 1556 156 1.85 90 1.46

10 50 8 1356 283 1562 146 1.91 105 1.44

10 50 10 1360 320 1567 155 2.69 115 1.38

10 50 12 1375 318 1574 147 3.15 124 1.35

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Optical and Structural Properties of Nitrogenated Diamond-Like Carbon Films by r.f. PECVD

vacuum conditions of about 10−10 Torr. The chemical bonding state in the films was analyzed after the 0.5-keV Ar+ ion etching of the film surface for 3 min.

3

D-Peak G-Peak (E)

Raman was used to provide complementary information about the bonding states of various phases of carbon and structural quality of the amorphous carbon films. The Raman spectra of the most-disordered carbons tend to be dominated by the G and D modes of graphite, even when the carbons do not have particular graphitic ordering. Figure 1 shows the Raman spectra of nitrogen-doped film (except sample A). In the analysis of the Raman spectrum, the Gaussian and Lorentzian distribution functions are applied to identify D- and G-peak positions from the deconvolution of the spectra, and obtain the intensity ratio (ID /IG ) as well. A slight frequency shift toward higher-G-peak position was found in our specimens when the nitrogen gas flow rate was increased. The decrease in the sp3 content due to increase in the nitrogen content of the coating film would cause the rise of the ID /IG ratio and the up-shift of the G peak. This behavior was ascribed to an increase in the size or number of the graphite clusters present in the films. This tendency agrees with that exhibited in the study of Tamor et al.11 Mariotto et al.12 also studied the Raman spectra of a-C:H:N films deposited by r.f. PECVD in methane and nitrogen atmospheres. They found a continuous increase in the ID /IG band ratio upon nitrogen incorporation, and a shift of the G-band peak position toward that of crystalline graphite. In Fig. 1, undoped nitrogen sample (A) shows both D and G peaks relatively at lower wave number and higher ID /IG ratio compared with the nitrogen-doped samples. But when nitrogen flow rate is increased, a shift of the G peak toward the higher wave number a relatively narrower band width of the G peak, and an increase of the ID /IG ratio are observed. Therefore, it is expected that nitrogen incorporation resulted in an increase of graphitization. Only the nitrogen-doped sample shows clearly a typical variation in the Raman spectra in Fig. 2. Although the Raman spectra of nitrogen-doped samples show similar trend of shifting and narrowing of the G peak, there is still a small change and

Raman intensity (a.u.)

3. Results and Discussion (D)

(C) (A)

(B)

1000

1200

1400

1600

1800

Raman shift (cm−1)

Fig. 1. Raman spectra of boron–nitrogen co-doped aC:H:N thin films deposited from different flow rates of nitrogen at 300 W r.f. power and 10 Pa partial pressure.

Fig. 2. Raman spectra of nitrogen-doped a-C:H:N thin films deposited from different flow rates of nitrogen at 300 W r.f. power and 10 Pa partial pressure.

difference that are observed if we compare the Raman spectra with nitrogen-doped, without nitrogen-doped, and boron–nitrogen co-doped samples. However, the change is small, but there is a clear indication of the effect of the boron that is co-doped in the films. The processes involved in plasma deposition of a-C:H films are quite complex. Also, each species

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from the plasma interacts with the growing layer in a different way.13 At first, we have positively charged ions, mainly the carbon-carrying ones and nitrogen ions, which are extracted from the hydrogen– nitrogen plasmas and accelerated toward the substrate. They are responsible, by energy deposition, for the activation of C–C bond formation, among other consequences. Carbon-carrying slow neutral radicals contribute mainly to the film growth itself, by sticking to dangling bonds at the film-growing surface. Hydrogen fast ions and slow neutral ions can be involved in dangling-bond creation or saturation, among other processes. Hence, nitrogen and boron addition to the deposition atmosphere may, besides altering the plasma chemistry combined, also alter the surface process acting in the growing layer. FTIR has been used to provide information about the bonding configuration in a-C:H:N films. Figure 3 illustrates the following chemical bonding: the band appearing at the wave number in the range of 1050–1300 cm−1 corresponds to C–O bond; the band appearing in the range of 1500–1600 cm−1 for C=C bond (sp2 bonding). The C–H bond generally appears at two wave number ranges, one at 1340– 1470 cm−1 and the other at 2850–3100 cm−1 (not shown in Fig. 2). The band appearing at the wave number around 2100–2260 cm−1 represents here the C≡C bond (sp3 bonding). In Fig. 3, all the three types of chemical bonds appear. However, the frequency range (2210– 2280 cm−1 ) for the C≡N bond (sp3 bonding) almost overlaps the frequency range (2100–2260 cm−1 ) for the C≡C bonding in the FTIR spectrum.14 Therefore, it is actually difficult to identify clearly the bonding difference between them only by the spectra. Since C–H bond is found more prominent at around 1340–1470 cm−1 , more attachment of H bond in the film is expected. The C≡N absorption is an obvious indicator of the incorporated nitrogen atoms presented as terminal bonds in the amorphous network. However, a singly bonded C–H stretching mode and peak N–H centered at about 2920 and 3400 cm−1 , respectively, were not detected in our samples. The broad absorption of the C–H stretching mode decreased, whereas the intensities of the N–H and C≡N stretching modes increased with increasing N concentration. A sharp peak located in the range 1590–1600 cm−1 might be attributed to C=N or

C-H

(E)

C=N, C=O C=C, C≡C

C-O

N-H

CO2

(D)

C≡C

Absorbance (a.u.)

4

(C)

(B )

(A)

600

1000

1400

1800

2200

2600

Wave number

3000

3400

3800

(cm−1)

Fig. 3. FTIR spectra of boron–nitrogen co-doped aC:H:N thin films deposited from different flow rates of nitrogen at 300 W r.f. power and 10 Pa partial pressure.

C=O. Since the oxygen atomic fraction, obtained from XPS analysis, is in the range of 3.0–9.0% for all a-C:H:N films, it is expected that the peak around 1600 cm−1 could be due to incorporated nitrogen, resulting in the C=N absorption or C=O overlaps. Moreover, IR results provide an evidence of incorporated atomic nitrogen, which substitutes atomic carbon in the carbon network of the aC:H:N films. These absorption peaks are found in good agreement with those reported in previous papers.15,16 The atomic percentage of C, N and O in the film (at.%) was determined by XPS and displayed in Figs. 4–6. The broadening peaks of C 1s and N 1s were found more symmetric with increasing nitrogen concentration in the film (Fig. 5). There is a clear indication of the nitrogen atom being involved in the chemical bonds with carbon in three possible distinct chemical states: C–N, C=N, and C≡N.17 The nitrogen incorporation is found to increase with the higher flow rate of nitrogen. The atomic percentage of nitrogen is found to be in the range of 2.9–6.6 and that of oxygen from 3% to 9%, respectively. The binding energies of about 285.1, 286.2, and 287.3 eV

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Optical and Structural Properties of Nitrogenated Diamond-Like Carbon Films by r.f. PECVD

5

C 1s

8 (E)

(B) (C) (D)

Intensity (a.u.)

6

(α hν )1/2

O 1s

(C)

B 1s

N 1s

(A)

(D)

2

(E)

0

200

400

600

800

1000

Binding energy (eV)

Fig. 4. XPS spectra of boron–nitrogen co-doped aC:H:N thin films deposited for different flow rates of nitrogen at 300 W r.f. power and 10 Pa partial pressure.

Fig. 5. XPS spectra of boron–nitrogen co-doped aC:H:N thin films deposited for 12 sccm flow rate of nitrogen at 300 W r.f. power and 10 Pa partial pressure.

(a)

(A1)

4

(b)

Fig. 6. Deconvoluted C 1s and N 1s spectra of a-C:H films deposited for 12 sccm flow rate of nitrogen at 300 W r.f. power and 10 Pa partial pressure.

0

1

1.5

2

2.5

3

3.5

4

4.5

Photon energy (eV)

Fig. 7. Optical band gap of boron–nitrogen co-doped a-C:H:N thin films deposited from different flow rates of nitrogen at 300 W r.f. power and 10 Pa partial pressure (sample A1 for nitrogen-doped only).

correspond to pure carbon network denoted as C–C, C=N, C–N, or C≡N, and C–O bonded network, respectively (Fig. 5). Similarly, N 1s peak is also deconvoluted into three assignments corresponding to N–C or N≡C, N=C, and N–O bond for 399.4, 398.4, and 400.3 eV, respectively (Fig. 6). These deconvoluted assignments are found to be in good agreement with the previous reports.18 The C–O and N–O assignments are probably related to the incorporation of oxygen into the films due to their exposure to the atmosphere prior to the XPS analyses. The optical band gap of the as-deposited a-C:H:N films are estimated from the optical transmittance and reflectance spectra in the range of 300–2000 nm using UV–visible spectroscopy and shown in Fig. 7. Tauc relationship was used to evaluate the energy gap (Eg ). The optical band gap is obtained from the extrapolation of the linear part of the curve at the absorption coefficient α = 0 using the Tauc relation.19 In our experimental results, we have found that the energy band gap is reduced with the incorporation of nitrogen. For nitrogen undoped boron-doped samples (A), the band gap is found to be 1.62 eV and then it gradually decreases to 1.35 eV with the increase of nitrogen flow rate. From the energy gap values, it is clear that there is an influence of nitrogen in the as-deposited films. The optical band gap is expected to decrease due to the increase of the sp2 fraction in these as-deposited a-C:H:N films. This is related to

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the fact that with the increase of nitrogen concentration, the film starts to promote the formation of additional clustering of aromatic rings, thus affecting mainly the band gap.

4. Conclusions The as-deposited a-C:H thin films grown by r.f. PECVD using different flow rates of nitrogen under 300 W r.f. power and 10 Pa partial pressure have been evaluated by the Raman, FTIR, and XPS analyses. The nitrogen incorporation has affected the chemical compositions, bonding configuration, and band gap of the deposited films. The increase in the flow rate of nitrogen concentration in the gas mixture of CH4 and H2 reduces the sp3 content, thus resulting in the upshift of the Raman G peak and the rise of the intensity ratio ID /IG . Energy band gap is found to decrease with the increase of nitrogen concentration, which might be due to the formation of graphitic domains. The atomic percentage of nitrogen and oxygen is calculated by XPS measurement in the range of 2.9–6.6 and 3–9%, respectively.

Acknowledgment This work is supported by the carbon solar cell project under the Japanese Governmental Organization of the New Energy and Industrial Technology Development Organization (NEDO).

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