Investigation of the properties of hydrogenated carbon films (a-C:H) deposited on germanium using a linear anode layer ion source

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ScienceDirect Materials Today: Proceedings 4 (2017) 11500–11504

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Investigation of the properties of hydrogenated carbon films (a-C:H) deposited on germanium using a linear anode layer ion source Alexander Zolkina,b,*, Anna Semerikovaa, Sergey Chepkasova, Maxim Khomyakova,b a

b

Novosibirsk State University, Pirogova str. 2,630090, Russian Federation Institute of Laser Physics, Siberian Branch of Russuan Academy of Science, Lavrentyev Ave. 13/3, 630090, Russian Federation

Abstract Hydrogenated carbon films (a-C:H) were deposited on single-crystal Ge with ion beams by means of a linear anode layer ion source at a flow rate of C3H8 of 1.7–4.4 sccm. An intermediate layer formed by the ion beam with an energy of 3 keV increases the adhesion of the coatings to the substrate and increases the hardness of the coatings. The highest hardness obtained with the use of the intermediate layer was 19±1.9 GPa. The transmission of the germanium substrate coated with a single layer of the aC:H coating reaches 65–67% at wavelengths of 4.5–6 µm. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2016 Russia-Japan conference Advanced Materials: Synthesis, Processing and Properties of Nanostructures. Keywords: a-С:H carbon films; linear anode layer ion source; hardness; adhesion; optical properties of a-С:H films; antireflective; Raman spectroscopy; diamond-like carbon

1. Introduction Hydrogenated carbon films with controllable properties attract a lot of attention due to their high potential as antireflective and protective coatings in IR optics [1], solar cells [2, 3], and laser systems [3, 4]. It is of great importance to increase the adhesion of the coatings while keeping the other necessary characteristics, such as optical properties and hardness, at the desired levels. Carbon films have been synthesized using ion beams with energies varying from 0.04 to 5 keV [5 – 8]. In some cases, high-quality coatings were obtained. Usually, the bonding layer between the substrate and the coating forms as the upper substrate layer * Corresponding author. Tel.: +7-913-942-33-03; fax: +7-383-363-42-80. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2016 Russia-Japan conference Advanced Materials: Synthesis, Processing and Properties of Nanostructures.

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interacts with the atoms or molecules deposited on the substrate. A thin intermediate layer is formed as the substrate atoms form bonds with the coating atoms. Stresses forming in the transition layer because of the difference in the coefficients of thermal expansion often predominate, and the coating separates from the substrate. This problem can be solved by increasing bonding between the substrate and the coating materials through implanting carbon ions into the substrate [8]. In this case, the thickness of the intermediate layer increases significantly, and the bonding between the substrate and the coating atoms becomes stronger. However, in this method, the substrate can suffer from defects, which can be detrimental for elements of micro- and nanoelectronics. These defects are not critical in the case of protective coatings on optical systems. A gridless ion source was used as it does not introduce impurities into the coatings. Sputtering of the grid material causes impurities in the film. The anode layer ion source provides an opportunity to obtain ion beams with an average energy of 0.5–5 keV. An ion source of this type allows varying the beam energy in a wide range. Highenergy beams allow achieving good adhesion, while low-energy beams ensure high quality of the coatings. The aim of this research is to investigate the effect of an intermediate adhesion layer, which is deposited by highenergy ions, on the mechanical and optical properties of the a-C:H films on Ge. 2. Experimental 2.1. Deposition of coatings The films were deposited using a linear anode layer ion source. The chamber was evacuated to the residual pressure of 10-3 Pa. After the gas was supplied, the pressure during the process was about 5×10-3 Pa. A flow of propane (C3H8) was fed into the ionization area at a rate of 1.4–4.2 sccm. Hydrogenated carbon films (a-C:H) were deposited on single-crystal Ge. Diameter of the substrate was 13 mm and the thickness was 1 mm. The substrates were processed with argon ions with an energy of 0.5–0.6 keV for 5 min in order to clean the substrates from adsorbed atoms and molecules. This treatment causes excitation of the surface states of the substrate. It allows improving chemical bonding between the coating and the substrate. In mode A, the films were deposited using ion beams with energies of 0.5–0.6 keV at an anode voltage of 1 kV and an anode current of 20 mА. The duration of the deposition was 2–3 h. In mode B, the adhesion coating was formed by gas ions having an energy of 3 keV during 0.5–1 h at an anode voltage of 5 kV and an anode current of 50–60 mА. Then, the beam energy was reduced to 0.6 keV and at this energy the deposition lasted for additional 3.5–4 h at an anode voltage of 1 kV and an anode current of 20 mА. In order to compensate the ion beam space charge, a tungsten electron emitter was used. In mode C, the films were deposited using ion beams with an energy of 3 keV at an anode voltage of 5 kV and an anode current of 50–60 mА. The duration of the deposition was 2 h. Similar to mode B, a tungsten electron emitter was used to compensate the ion beam space charge. 2.2. Coating thicknesses, hardness and adhesion The thickness of the films was measured by means of a laser ellipsometer (LEF-752) at a wavelength of 632 nm in the multi-angle mode [9]. A uniform film and substrate model was chosen to evaluate the thickness as the optical constants of the substrate are given [9], [10]. The measurement error allowed was 2–4 nm. The hardness and thickness of the films were measured by means of the nanoindentation technique and scratching using a NanoScan-3D. In order to eliminate the influence of the substrate on the resulting thickness, a model suggested in ref. [11] was used. The adhesion strength was evaluated from the critical load, at which the coating detached from the substrate during the scratching test with changing loads. The coating was scratched with a diamond pyramid indenter while increasing the load and the surface relief was scanned. The relief before and after scratching and the influence of the load and indentation were analyzed, which allowed us to obtain the load threshold, at which the plastic deformation changed to the crack formation, and measure the coating thickness at the point of detachment.

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2.3. Optical properties The Raman spectra of the films were obtained using a Jobin Yvon T64000 spectrometer at an excitation wavelength of 514.5 nm in order to study the structural features of the films. The FTIR spectra were obtained using a Fourier spectrometer FT-801- SIMECS (RU) in the 2.5–16.7 µm range in order to study the antireflective properties of the hydrogenated carbon coatings. The measurements were conducted at a resolution of 4 cm-1. 3. Results and Discussion 3.1. Film adhesion and hardness The protective coatings should be harder than the substrate – underlying base (a lens or an optical element). When Ge is used as an optical material, the hardness of the film should be greater than 10 GPa. Good adhesion is necessary to make the coating more durable. In order to improve adhesion and keep the quality of the film, we used synthesis mode B. Firstly, we implanted the ions of the working gas C3H8 with a beam energy of up to 3 keV. At this energy, the ions penetrate the surface up to a depth of several nanometers [8, 12]. The implanted ions can induce the formation of germanium carbide. Secondly, in mode B, we reduced the ion energy from 3 to 0.5–0.6 keV in order to synthesize a diamond-like carbon (DLC) coating effectively. This energy was sufficient to obtain a C–C bond between the deposited material and the intermediate (adhesion) layer. The adhesion of the a-C:H films on Ge was evaluated as acceptable. Films with an intermediate adhesion layer did not detach upon scratching with a diamond indenter at the maximum load of 50 mN. We observed no detachment in the sample that was kept indoors for as long as a year. The described deposition method allowed obtaining a hardness of 19±1.9 GPa when the deposition of the intermediate (adhesion) layer was 35 min (specimen B2) and a hardness of 14±1.4 GPa when the adhesion layer was formed for 1 h (specimen B1). We found that deposition using the ion beam with an energy of 3 keV (mode C) provided good adhesion (a detachment load of 50 mN) and fast synthesis (with a rate as high as 1.1–1.3 Å/sec). The film hardness did not exceed the hardness of germanium (10 GPa). This study showed that if the coating is deposited on Ge at 0.5–0.6 keV without an adhesion layer (mode A), the hardness ranges from 13.5±1.4 to 16±1.6 GPa. These films have a lower adhesion, as they are detached when scratched at a load of 10 mN. The coating growth rate is 0.3 Å/sec. Thus, we conclude that the adhesion layer improves the quality of the coatings. 3.2. Raman spectroscopy Raman spectra of the a-C:H films deposited by modes A, B and C at 800–2000 cm-1 are shown in Fig.1. A wide asymmetric peak of Raman scattering in the 1000–1800 cm-1 region is due to the presence of D and G bands. A broad D peak in the region of low wavenumbers (about 1340–1360 cm-1) is related to the breathing vibration modes of sp2 bonded C-rings [13 – 15]. G peak is related to scattering on stretching vibration of all pairs of sp2 carbon atoms [13 – 15]. Comparing the Raman spectra of the specimens obtained in modes A, B and C, we can see that D and G peaks shift to higher wavenumbers, while the integral intensity of D peak increases. It may be related to an increase in the concentration of the sp2-hybridized carbon and, hence, graphitization of the films [7, 14, 15]. An increase in the half width at half maximum (HWHM) of D peak indicates an increase in the concentration of disordered sp2-bonded carbon in the films [13, 14, 16].

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Fig.1. Raman spectra of a-C:H films on Ge. Spectra A (blue curve), B (red curve) and C (green curve) correspond to the specimens obtained by the use of A, B, and C modes of synthesis.

We analyzed the correlations between hardness of the coatings and such parameters of the Raman spectra as the ratio of the integral intensities ID/IG, the location and HWHM of G peak. Fig. 2 shows the correlation between the hardness of the coatings and the spectrum parameters for coatings obtained in modes A, B and C.

Fig.2. Correlation between the hardness of the a-C:H films deposited on Ge and the characteristics of the Raman spectra.

Fig. 2 (a, b, c) show that the hardness maximum corresponds to the coating with an adhesion layer (B). We observe an increase in hardness from 16±1.6 GPa to 19±1.9 GPa when the ratio of integral intensities ID/IG increases from 0.9 to 1.3. However, when the ID/IG ratio increases up to 1.9, the hardness decreases down to 10 GPa. This fact is of particular interest, as according to refs. [15, 17], an increase in the fraction of the sp3-hybridized carbon bonds may be accompanied by a decrease in ID/IG. The G peak position shifts from 1541 to 1558 cm-1, when the synthesis mode changes from A to C. The maximum of hardness corresponds to the G peak position at 1551 cm-1 and to the HWHM of 84.1 cm-1, which does not agree with the data presented in refs. [14, 17]. The shift of G peak to higher wavenumbers and a decrease in its HWHM indicate that graphitization of the film occurs. It can be explained by the presence of an adhesion sublayer, which can improve the heat transfer between the growing film and the substrate. This effect requires further investigation. 3.3. Antireflective effect Fig. 3 shows the FTIR spectra of the a-C:H films on Ge with an antireflective effect. The maximum transmittance of 65–67% is reached at the wavelength of 4.5–6 µm. These values decrease as the wavelength changes, which can be related to the change in the coating optical thickness (breaching antireflection). The spectra lack the characteristic absorption bands of C=C, C–Hn, C–H, which can be explained by the antireflective effect.

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Fig.3. FTIR spectra of the a-C:H films on Ge and the bare substrate at 2.5 – 16.7 µm. Spectrum B1 (red curve) corresponds to the specimen with a hardness of 14±1.4 GPa and a thickness of 450 nm. Spectrum B2 (blue curve) corresponds to the specimen with a hardness of 19±1.9 GPa and a thickness of 600 nm. Both B1 and B2 films were obtained by the use of B mode of synthesis and, therefore, had good adhesion.

4. Conclusions We have found out that an intermediate layer formed by the ion beam with a relatively high energy of 3 keV increases the adhesion and hardness of the coatings. The highest hardness of the coatings (19±1.9 GPa) is reached after a 2-stage synthesis process, in which the adhesion layer is first synthesized by the ion beam with an energy of 3 keV and further deposition is conducted at 0.5–0.6 keV. The transmittance of the germanium substrate having a single-side a-C:H coating reaches 65–67% at wavelengths of 4.5–6 µm. Acknowledgements We thank Dr. Vladimir. A. Volodin for his help in operating the Raman spectrometer and Dr. Galina. T. Yakushko for her help with the manuscript preparation. The support of the Novosibirsk State University Project Strategic Academic Units Nonlinear Photonics and Quantum Technologies 2016 – 2017 is highly acknowledged. References [1] S. Peng, H. Ming, Z. Feng, J. Yi-Qin, L. Hua-Song, L. Dan-Dan, L. Jian, Chin. Phys. B 24 (2015) 067803-1 – 067803-5 [2] R. A. Ismail, A. M. Mousa, M. A. Hassan, W. K. Hamoudi, Opt. Quant. Electron. 48 (2016) 16-1 – 16-11 [3] A. Grill, Thin Solid Films 355 – 356 (1999) 189 – 193 [4] J. Xu, J. Su, L. Hang, Y. Cheng, App. Surf. Sci. 265 (2013) 234 – 238 [5] M. Hakovirta, J. Sale, R. Lappalainen, A. Anttil, Phys. Let. A 205 (1995) 287 – 289 [6] W. Ryeol Kim, M. S. Park, U. C. Jung, A. R. Kwon, Y. W. Kim, W. S. Chung, Surf. Coat. Technol. 243 (2014) 15 – 19 [7] E. F. Shevchenko, V. A. Tarala, M. Yu. Shevchenko, A. A. Titarenko, Adv. in Mater. Sci. and Eng. (2014) 979450-1 – 979450-6 [8] K. Suschke, R. Hübner, P. P. Murmu, P. Gupta, J. Futter, A. Markwitz, Coat. 5 (2015), 326 – 337 [9] H. G. Tompkins and E. A. Irene (eds.), Handbook of ellipsometry, William Andrew Publishing, New York, 2005 [10] V. A. Tolmachev, J. Opt. Technol. 66 (1999) 596 – 607 [11] A. M. Korsunsky, M. R. McGurk, S. J. Bull, T. F. Page, Surf. Coat.Technol. 99 (1998) 171 – 183 [12] J.F. Gibbons, W.S. Johnson, S.W. Hylroic, Projected Range Statistics: Semiconductors and Related Materials, 2nd Edition, Halsted Press Stroudsbury, PA, USA, 1975, p. 93 [13] R. J. Nemanich, S. A. Solin, Phys. Rew. B 20 (1979), 392 – 401 [14] J. Robertson, Mater. Sci. and Eng. R 37 (2002) 129 – 281 [15] C. Casiraghi, A. C. Ferrari, J. Robertson, Phys. Rew. B 72 (2005) 085401-1 – 085401-14 [16] S. Nakao, J. Choi, J. Kim, S. Miyagawa, Y. Miyagawa, M. Ikeyama, Diamond Relat. Mater. 15 (2006) 884 – 887 [17] A.C. Ferrari, J. Robertson, Phys. Rev. B. 61 (2000) 14095 – 14107