Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering

Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering

    Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering ¨ Ozlem Duyar ...

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    Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering ¨ Ozlem Duyar Cos¸kun, Taner Zerrin PII: DOI: Reference:

S0925-9635(15)00086-2 doi: 10.1016/j.diamond.2015.04.004 DIAMAT 6400

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

7 November 2014 16 April 2015 21 April 2015

¨ Please cite this article as: Ozlem Duyar Co¸skun, Taner Zerrin, Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering, Diamond & Related Materials (2015), doi: 10.1016/j.diamond.2015.04.004

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ACCEPTED MANUSCRIPT Optical, structural and bonding properties of diamond-like amorphous carbon films deposited by DC magnetron sputtering Özlem Duyar Coşkuna*, Taner Zerrina Hacettepe University, Department of Physics Engineering, Thin Film Preparation and Characterization Laboratory, Ankara, Turkey

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Abstract

In the present study effects of working pressure on the optical, structural and bonding properties of diamond-like amorphous carbon (DLC) thin films prepared by direct current (DC) magnetron sputtering technique were investigated. XPS analysis is used to determine the bonding properties and optical spectroscopy to determine optical

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constants. Increasing the working pressure from 2 to 50 mTorr is shown to strongly influence the optical and structural properties due to a large increase in the sp3/sp2 ratio from 0.51 to 2.81. With increasing pressure, the real part of the

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refractive index decreases from 2.4 to 1.6 at 550 nm, while the imaginary part also decreases from 0.38 to 0.02 at 550 nm, leading to a large increase in optical transmission in the visible region from 10 % to 80 % for films of approximately equal thickness, while the Tauc gap of the films increases from 0.80 eV to 2.06 eV. Rms surface

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roughnesses of the films increased from 0.95 nm to 4.69 nm with increasing working pressure. These results indicate that the sp3 sites mainly consist of C-H bonds at higher working pressures and the optical and structural properties are

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consistent with a change from graphitic to a more polymer-like DLC. The resulting strong increase of transparency in the visible range extends the optically useful range for DLC films into the visible part of the spectrum, for applications

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such as low-e coatings for windows.

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Keywords: Diamond-like amorphous carbon, magnetron sputtering, optical properties, surface topography, bonding properties, band gap Corresponding author: Özlem Duyar Coşkun Address: Hacettepe University

Physics Engineering Department 06800 Beytepe/Ankara Turkey Phone: +90 312 297 7250 Fax: +90 312 299 20 37 E-mail: [email protected]

ACCEPTED MANUSCRIPT 1. Introduction Diamond-like amorphous carbon (DLC) or hydrogenated amorphous carbon (a-C:H) films have outstanding properties such as hardness, chemical inertness, high wear resistance, high optical transmittance, smoothness of surface, etc [1,2]. All of these properties make these films as a promising alternative material for optical coatings, hard coatings, cutting tools, biomedical components, low-k dielectrics and microelectromechanical system (MEMS) devices [1-5].

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DLC films consist of a mixed phase of tetrahedrally bonded (sp3) and trigonally bonded (sp2) carbon atoms. Relative

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amounts of these two phases and also the hydrogen content in the films strongly depend on the deposition parameters.

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The optical and electrical properties of the films are determined by the fraction and orientation of sp2 states which form the conduction and valence band edges close to the Fermi level. These sp 2 sites form clusters consisting of several atoms which are embedded in the sp3 bonded matrix. The relative amounts of sp2 and sp3 bonded carbons or namely the sp3/sp2 ratio determine the film properties [6,7]. DLC is not one material, but rather a family of used

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carbon or amorphous hydrogenated carbon films ranging from mechanically soft films with a diamond-like optical gap to low optical gap films with diamond-like mechanical properties [8]. Thus the properties of the films can be tailored

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for different applications. Increase of sp2 cluster in the film reduces the diamond-like properties. It was found that small amounts of hydrogen help to stabilize the diamond-like character of the films, while large hydrogen content leads to more polymeric character [9-14]. DLC films can be prepared by physical or chemical deposition methods

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including filtered arc, laser ablation, direct ion beam, sputtering and plasma assisted chemical vapour deposition techniques [15-27]. Magnetron sputtering is an extremely versatile technique that has been widely used for the

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preparation of both hydrogenated and hydrogen-free DLC films [28-37]. The superiority of this technique lies in its easier adaptability to industrial applications, as homogenous films on large-area substrates can be grown with high

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deposition rates. In magnetron sputtering, a noble gas, usually Ar, is introduced as the sputtering gas. Ar pressure is an important experimental parameter in the technique and influences the energy of the incident particles on the substrate surface, affecting film properties [38]. In magnetron sputtering chamber pressures of a few mTorr are typical. In this

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work the influence of Ar deposition pressure on the optical, bonding and surface properties of DLC films is investigated over a wide range of pressure (2-50 mTorr). The correlation between microstructure and the physical properties provides information on the understanding of the potential of DLC films in optical coating applications. It is shown that working pressure has an important influence on the bonding properties, strongly influencing the optical properties of the deposited films. The change of the optical and structural properties are correlated with the change of the microstructure of DLC films characterized by XPS analysis. DLC films produced at low pressure are limited to optical applications in the mid IR, as they are opaque in the visible. Films produced at high pressures have high optical transparency in the visible extending the possible applications of this remarkable family. In particular, such films are remarkably promising candidates for dielectric layers in optical applications such as e-low coatings, antireflection coatings and photovoltaic-thermal panels etc. 2. Experimental details DLC thin films were deposited on 1737F glass substrates using direct current (DC) magnetron sputtering technique at room temperature. The target is a pure (99.995 %) graphite circular disk with 2 inch diameter and 1/4 inch thickness and is fitted with a moveable shutter. The base pressure of the chamber was about 2.0 × 10-6 Torr for all deposition processes. Pure argon gas is introduced to the chamber as sputtering gas via a mass flow controller. The argon flow

ACCEPTED MANUSCRIPT was changed to give chamber pressures between 2 - 50 mTorr with a fixed throttle position on the vacuum pump. A stable plasma was established with an applied DC power of 60 W to the magnetron gun. After presputtering of the graphite target for 5 minutes, the shutter was removed and the deposition started. The substrates were not heated during the deposition and there was no substrate bias. The target to substrate distance was 50 mm. The sputtering pressure (PAr), deposition time (t), film thickness (nm) and the surface resistance of as deposited films (R□) are given

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in Table 1.

PAr (mTorr)

t (min)

d (nm)

R (kΩ/□)

DLC 1

60

2

35

254.2

500

DLC 2

60

11

30

267.4

*

DLC 3

60

20

30

258.6

*

DLC 4

60

50

30

281.2

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Sample Name

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Table 1 Deposition parameters for the films prepared at different working pressures. (*Electrical resistance of the film was higher than 20 MΩ).

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Optical transmission and reflection of the films from exactly the same spot were measured simultaneously using an Aquila nkd-8000e spectrophotometer over the spectral range of 350 nm to 1100 nm at 30 °C angle of incidence. The

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wavelength dependancy of refractive index n(λ), extinction coefficient k(λ) and the thickness of the films were determined by simultaneous fitting of the Forouhi-Bloomer Model [39] to the photospectroscopic measurements. Optical absorption spectra of the films were obtained from a Hitachi U-0080D UV-VIS-NIR spectrophotometer at normal incidence. The energy dependancy of absorption coefficient α(E) and E04 gaps of the films were determined

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using the optical absorption data.

The film structure was studied by X-ray diffraction (XRD) using a Rigaku Dmax X-ray Diffractometer with a Cu Kα radiation source. The surface chemistry and bonding properties of the films were investigated by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific™ K-Alpha™ spectrometer with an Al Kα source after applying sequential etching processes. Thus, changes in bonding properties and hybridization types through the inner regions of the films were also studied. AFM measurements were performed in contact mode, using a Park System XE-100 microscope. Typical scan areas were 1 μm

1μm and scan rate was 1 Hz. RMS surface roughness values were

obtained from a complete scan, using the software supplied with the AFM. 3. Results and discussions In this study, the deposition pressure was increased up to 50 mTorr without any problem for the sample adherence to the substrate. The adherence of the films was tested by using the Scotch tape method. After rapidly stripping the tape from the film surface, the surfaces of the films were examined visually and then by optical microscope and no loss of adhesion was seen.

ACCEPTED MANUSCRIPT XRD patterns of the films deposited at different working pressures are shown in Figure 1. The patterns show that the films are amorphous in structure. All the films exhibited broad scattering peaks lying in the 10° - 30° two theta range

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originating from the glass substrate.

Figure 1. XRD patterns of DLC films prepared at different working pressures on glass substrates at room temperature.

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XPS measurements were first performed without etching the surface. Then the surface of the films were etched by argon ions for a total of 120 seconds with XPS spectra being obtained after each 60 seconds. Thus, three different measurements were made for each sample as a function of etching time at 0, 60 and 120 seconds to study the bonding

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properties through the inner regions of the films. XPS survey spectra and narrow scan C 1s and O 1s spectra of the

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samples were collected after each etching processes. As an example, survey spectra for the film prepared at 2 mTorr working pressure is given as a function of etching time in Figure 2.

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C 1s and O 1s peaks, originating from the photoelectrons ejected from the core levels of carbon and oxygen elements, significantly appeared in the survey spectrum. The O 1s peak at 532.1 eV is mainly attributed to the adsorbed oxygen on the film surface. After an etching process of 60 seconds, the O 1s peak intensity drastically decreased. After further

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etching of the sample surface for a total etch time of 120 seconds, the O 1s peak intensity remained nearly constant. This decrease of O 1s peak intensity to a nearly constant level after the first etching process indicates not only the presence of an oxide layer at the surface but also residual oxygen in the chamber originating from water vapour contamination during the deposition process. The relative height of the C1s peak is higher than the O1s peak. Due to the low content of oxygen [48], the calculated contents of sp2 and sp3 in the film are considered to be reliable. Using the narrow scan C 1s spectra, it is possible to get information about the hybridization types in the films by deconvolution of the C 1s peak to obtain sp2-C and sp3-C components that are known to lie at different binding energies [49-53]. The areas under the sp2-C and sp3-C peaks in C 1s spectra were used to determine the sp3/sp2 ratio of the films. Because sp2 hybridization is only related with graphitic structure, the sp3/sp2 ratio gives information about diamond-like properties of the films [54-58]. The C 1s narrow scan spectrum obtained after an etching process of 120 seconds for the film deposited at 2 mTorr working pressure is given in Figure 3.

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Figure 2. XPS survey spectra for the film prepared at 2 mTorr as a function of etching time.

Figure 3. C 1s narrow scan spectrum of the film prepared at 2 mTorr. Data were collected after etching the film surface by argon ions for 120 seconds. The C 1s peak is first deconvoluted into three components of 30 % Gaussian and 70 % Lorentzian nature. The binding energies, full width at half maximum (FWHM) values and the areas under the peaks are then obtained using CASA XPS software. The calculated peak positions and areas of sp2-C, sp3-C and C-O peaks, sp3/sp2 ratio and concentration of atomic oxygen (%) of the films deposited at all working pressures used are presented in Table 2 for those samples

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etched for 120 seconds. The sp3/sp2 ratio increases with increasing argon pressure as seen in Table 2. In hydrogen-free films this indicates more diamond-like properties of the films. However, it is known that hydrogen incorporation into the films also increases the number of sp3 bonds, as hydrogen connects to the many dangling bonds in an amorphous structure. This helps to stabilize the films [9-14] and with hydrogen-rich precursors as used in PECVD, produces the hydrogenated amorphous carbon form of DLC known as a-C:H. Now, DLC films sputtered from graphitic targets

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normally contain low levels of hydrogen and are opaque, with sp3/sp2 ratios that are much lower than found in this work. This suggests that the main effect of increasing chamber pressure in these experiments is to increase the

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hydrogen content of the films. The sp3 component obtained from narrow scan C 1s spectra arises from both C-C sp3

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and C-H sp3 bonds and unfortunately it is not possible from XPS data to distinguish C-C sp3 and C-H sp3 bonding types. The evidence obtained from our optical measurements is then crucial in determining the exact nature of these films.

Peak Position (eV)

Area (cps.(eV))

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PAr (mTorr)

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Table 2 Peak positions, areas under the peaks, sp3/sp2 ratios and oxygen concentration (%) obtained after deconvolution of XPS C 1s spectra for the films prepared at different working pressures.

sp3-C

C-O

sp2-C

2

284.1

285.0

286.3

25602.3

11

284.1

284.8

286.7

21588.8

20

283.9

284.6

286.8

50

283.8

284.4

285.9

C-O

12983.6

11207.0

0.51

2.45

14851.2

3107.5

0.69

3.56

18485.3

16565.4

7021.9

0.90

4.03

8286.5

23289.5

9540.6

2.81

4.14

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Oxygen Concentration(%)

sp3-C

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sp2-C

sp3/sp2

The optical transmission of the films with increasing working pressure are given in Figure 4 and the films deposited at

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higher pressures are shown to be much more transparent than the films deposited at lower pressures. The transmission of the film prepared at 2 mTorr is 9.4 % at the wavelength of 550 nm. Whereas for 50 mTorr pressure, the optical transmission increased to 82 % at 550 nm. Over the visible range, 400-700 nm, these DLC films show a transmission of between 56–89%, considerably higher than in previously reported work[40].

ACCEPTED MANUSCRIPT Figure 4. Optical transmission spectra of DLC films prepared at different working pressures on glass substrates at room temperature. The refractive indices and extinction coefficients of the films obtained from the fitting procedure are shown in figures 5a and 5b. Both show a large decrease with increasing working pressure, indicating less dense and more transparent

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films.

Figure 5. a) Refractive index and b) extinction coefficient as a function of wavelength for DLC films prepared at different working pressures on glass substrates at room temperature. The optical band gaps E04 and Tauc gaps (Eg) of the films are derived from the measured absorption spectra of the samples deposited on glass substrates. The optical gap, Eg, is deduced from the known Tauc relation [62] by extrapolating the linear behaviour of (αE)1/2 as a function of photon energy E, as shown in Figure 6. E04 gaps of the films were obtained by using α(E) curves to determine when the absorption coefficient reached a value of 104 cm-1, as shown in Figure 7. As seen in Figure 7 and Figure 8, optical gap and sp3/sp2 ratio of the films increases with increasing working pressure. Calculated Tauc gaps were 0.80 eV, 1.18 eV, 1.40 eV and 2.06 eV for the films prepared at 2 mTorr, 11 mTorr, 20 mTorr and 50 m Torr, respectively.

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Figure 6. Tauc plots of DLC films prepared at different working pressures on glass substrates at room temperature.

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Figure 7. Absorption coefficients as a function of energy for DLC films prepared at different working pressures on

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glass substrates at room temperature. E04 gaps of the films are also shown in the figure.

Figure 8. Variation of Tauc gap and sp3/sp2 ratio of the films with varying argon pressure.

The refractive index of crystalline diamond is 2.42, with near zero extinction coefficient at 550 nm. The refractive index values of dense hydrogen-free DLC films are in the range of 2.0 - 2.6 at 550 nm, but with significant extinction coefficient, making films of such material opaque in the visible region. DLC films containing hydrogen have lower refractive indices (<1.8 at 550 nm) and normally have lower extinction coefficients [44-46]. The extreme case of hydrogen-rich material would be a polymer such as polythene, with refractive index around 1.51 and essentially zero

ACCEPTED MANUSCRIPT extinction coefficient at 550 nm. Graphite has zero band gap and crystalline diamond has a band gap of 5.5 eV and polythene has a band gap of 6.0 eV. Hydrogen-free amorphous diamond like carbon films tend to have optical band gaps between 1.6-2.2 eV and hydrogenated forms have optical band gaps of 1.7 to 4.0 eV [44]. Consideration of the optical results along with the sp3/sp2 ratios from XPS strongly suggest that the main effect of increasing the working pressure in these experiments is to incorporate increasing levels of hydrogen. The high sp 3/sp2

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ratios and increased transparency could point towards the hard, diamond end of the DLC family, however the low

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values of refractive index and band gap are typical of the softer, hydrogen-rich region.

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It has been reported that DC magnetron sputtering deposited DLC films could contain hydrogen due to water vapour contamination in the chamber even if no hydrogen gas is deliberately introduced into the chamber [41-43]. This would seem to be the obvious explanation for the results shown above. Thus, we suppose that hydrogen content in the films is increased with increasing working pressure, forming stable sp 3 bonds leading to a more loosely connected network,

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resulting in decreased film density and decreased refractive index. At the highest pressures the films become more polymeric in nature with even lower refractive index and extinction coefficient.

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Surface topographies and rms roughness values of the films are presented in Figure 9. Rms surface roughnesses increased from 0.95 nm to 4.69 nm with increasing argon pressure and dispersed small domes appeared on the film

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film produced at 2 mTorr is much smoother.

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surface as shown in Figure 9. It can be seen that while the films produced at higher pressures are relatively rough, the

These surface effects are strongly dependent on the deposition conditions and can be explained in terms of a few basic processes. At higher argon pressures, collisions between the particles in the chamber increase, thus carbon atoms lose

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some kinetic energy before they arrive to the substrate. This low energy process will result in a diffusion mechanism at the surface of the substrates/films [64].

Because low energy carbon atoms can not be able to penetrate the film surface [65], diffusion in the surface layers of

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DLC tends to generate ordered clusters with high sp2 contents. This phase is also a thermodynamically stable graphite phase. Because sp3 fraction in the films prepared at high working pressures mostly consists of sp3 C-H bonds, the number of sp3 C-C bonds will be smaller than the number of sp2 C-C bonds at the surface, even though the film has a large sp3 fraction. Clusters of sp2 bonded atoms form preferentially on the surface and this leads to an increase in the surface roughness. Impinging carbon atoms and/or argon ions with lower energy than the atomic displacement threshold energy remain in surface which are responsible of surface diffusion. Thus, this mechanism leads to major clustering of ordered sp2 (graphitic) sites and promote surface roughness [66-67]. On the other hand, if the argon pressure is low during the deposition process, collisions between the sputtered species and argon atoms/ions decrease. Thus, the energy of the sputtered atoms and argon ions increase. The penetration of the carbon atoms to subsurface layers, which leads to denser films at low pressures. So the film prepared at 2 mTorr with the smallest rms roughness of 0.95 nm should contain more sp3 C-C bonds than those of prepared at higher argon pressures although XPS analysis indicates the least sp3 fraction for that film due to the lower hydrogen content in the structure originating from the lower working pressure during the deposition process.

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Figure 9. Surface topographies of DLC films prepared at a) 2 mTorr, b) 11 mTorr, c) 20 mTorr, d) 50 mTorr argon pressures.

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In addition, the sputtered carbon atoms and/or argon ions arriving at the subsrate can sputter pre-deposited atoms from the surface layers if their energy is high enough. Back-scattered argon atoms can also efficiently remove material from the surface of growing film. The back-sputtering process in the high energy/low pressure processes leads to smoother surface topographies, since outgrowth parts on the films surface are usually sputtered preferentially [65-66]. This could be another reason of low rms roughness for the film prepared at the lowest working pressure of 2 mTorr.

ACCEPTED MANUSCRIPT 4. Conclusions DLC thin films were deposited on 1737F glass substrates using DC magnetron sputtering technique with different argon pressures at room temperature. With increasing working pressure from 2 to 50 mTorr, optical transmission in the visible region increased from 10 % to 80 %, approximately. Decrease of refractive index from 2.4 to 1.6 at 550 nm

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with increasing sp3 fraction shows that sp3 sites mainly consist of C-H bonds at higher working pressures. Optical

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properties of the films changed from graphite-like to polymer-like as sp3/sp2 ratio increased from 0.51 to 2.81 and

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Tauc gaps of the films increased from 0.80 eV to 2.06 eV with increasing working pressure. Rms surface roughnesses of the films increased from 0.95 nm to 4.69 nm with increasing working pressure. These results are attributed to increased incorporation of hydrogen into the films, arising from cracking of residual water vapour in the chamber by

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the argon plasma.

These results are of great importance as the films produced at high working pressure showed very high transparency with good adhesion to the glass substrates. Such films are easily prepared by the method of DC magnetron sputtering

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and can extend the potential applications of DLC family films into the visible range of the spectrum.

Acknowledgements

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This work is supported by the research grant of the Scientific and Technological Research Council of Turkey

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(TÜBİTAK) under the project number 113F318. We would especially like to thank Professor Dr. Frank Placido, of the University of the West of Scotland, for numerous discussions and helpful comments. We also thank

measurements. References

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Superconductivity and Nanotechnology Laboratory of Hacettepe University Physics Engineering Department for XRD

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ACCEPTED MANUSCRIPT Figure Captions Figure 1. XRD patterns of DLC films prepared at different working pressures on glass substrates at room temperature. Figure 2. XPS survey spectra for the film prepared at 2 mTorr as a function of etching time.

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Figure 3. C 1s narrow scan spectrum of the film prepared at 2 mTorr. Data were collected after etching the film surface by argon ions for 120 seconds.

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Figure 4. Optical transmission spectra of DLC films prepared at different working pressures on glass substrates at room temperature. Figure 5. a) Refractive index and b) extinction coefficient as a function of wavelength for DLC films prepared at different working pressures on glass substrates at room temperature. Figure 6. Tauc plots of DLC films prepared at different working pressures on glass substrates at room temperature.

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Figure 7. Absorption coefficients as a function of energy for DLC films prepared at different working pressures on glass substrates at room temperature. E04 gaps of the films are also shown in the figure.

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Figure 8. Variation of Tauc gap and sp3/sp2 ratio of the films with varying argon pressure.

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Figure 9. Surface topographies of DLC films prepared at a) 2 mTorr, b) 11 mTorr, c) 20 mTorr, d) 50 mTorr argon pressures.

ACCEPTED MANUSCRIPT Table Captions Table 1. Deposition parameters for the films prepared at different working pressures. (*Electrical resistance of the film was higher than 20 MΩ).

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Table 2. Peak positions, areas under the peaks, sp3/sp2 ratios and oxygen concentration (%) obtained after deconvolution of XPS C 1s spectra for the films prepared at different working pressures.

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Prime Novelty Statement DLC films were deposited on glass substrates by DC magnetron sputtering technique via changing in a wide range of chamber pressures between 2 and 50 mTorr. The optical performance of amorphous diamond-like films, for instance transmission in the visible region increased from 10% to 80% with increasing Ar pressure. The results showed that bonding characteristics and optical properties of diamond-like films could be tailored by adjusting deposition pressure.

ACCEPTED MANUSCRIPT Highlights  DLC thin films were deposited on glass substrates at different working pressures.  Optical transmission increased from 10% to 80% with increasing working pressure.  The films with refractive indices varying between 1.6-2.4 at 550 nm were deposited.  Refractive index decreased as Tauc gaps increased with increasing sp3 fraction.

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 The film with diamond-like optical property has a lower film density and a higher roughness.