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MEDICAL PHYSICS IN <strong>THE</strong> BALTIC STATES 7 (2009) Proceedings of International Conference “Medical Physics 2009” 8 - 10 October 2009, Kaunas, Lithuania PHOTON INDUCED MODIFICATION <strong>OF</strong> PROTECTIVE a-C:H J. LAURIKAITIENĖ*, J. PUIŠO* , **, D. ADLIENĖ*, Š. MEŠKINIS** , *, V.ŠABLINSKAS***, *Physics Department, Kaunas University of Technology, Studentų g.50, LT-51368, Lithuania **Institute of Physical Electronics, KTU, Savanorių 271, LT-50131 Kaunas, Lithuania ***Dept. of General Physics and Spectroscopy, Faculty of Physics, Vilnius University, Vilnius, Lithuania Abstract: Diamond like carbon (DLC) films were prepared by direct ion beam method on quartz (SiO2) wafers, using different films synthesis parameters. The structural and optical properties of the amorphous hydrogenated DLC films have been studied irradiating them with high energy X-ray photons in order to study the radiation effects. Raman spectroscopy was used to evaluate the structural properties of the initial and irradiated sample series. The most significant changes in transparency of the irradiated samples were observed in less dense and highly hydrogenated (more then 35 at.%) DLC films. Changes of optical properties are analyzed in relationship with the hydrogen reduction mechanism introduced during irradiation of samples by X-ray photons and the possibility to use DLC as protective coatings in optical devices is discussed. Keywords: photons, amorphous DLC films, radiation effects, bonding structure, optical properties 1. Introduction Technological applications of diamond like carbon (DLC) films are very promising due to their suitable physical and chemical properties, such as tissue equivalence, good chemical inertness, high radiation hardness, absolute non-toxicity, high optical transparency, mechanical hardness, etc...DLCs are used in different electronic devices, as protective and antireflective coatings in solar cells, in space research or in nuclear reactors [1-3]. Properties of the hydrogenated amorphous DLC films strongly depend on the deposition conditions and parameters, and hydrogen content in the films [3, 4]. However, the properties of the DLC films also can be modified irradiating them by various types of particles (i.e. ions, neutrons, photons) [5-8]. Behavior of polycrystalline DLC structures upon their exposure by X-ray are widely discussed, since they are used as active elements in radiation detectors [1, 2]. However there is a lack of information about the interaction of X-ray photons with the amorphous DLC films. Due to this reason different types DLC films produced at room temperature using direct ion beam method were irradiated by high energy X-ray photons and, optical properties of experimental structures were investigated with the aim to assess radiation induced structural changes in the films. 2. Experiment Hydrogenated amorphous diamond like carbon films 117 were synthesized by direct ion beam method on SiO2 wafers. Films were formed in pure acetylene (C2H2) gas. Sample deposition conditions were described in [9, 10] Series of the experimental samples were divided into groups: sample series of the first group (No. 1G) were deposited at 240 eV and ion beam current density 20 µA/cm 2 , samples of the second, and the third groups (1 and 5) were deposited using the same ion beam energy 800 eV, but different ion beam current density: No. 1 – 50 µA/cm 2 , No. 5 – 125 µA/cm 2 . The main optical properties of the initial (not irradiated) sample series are presented in [11, 12]. All sample series (No.1, No.5, No.1G) were irradiated at the same time in medical linear accelerator Clinac 2100C (VARIAN) with high energy (average energy 10.8 MeV [13]) photons. All samples were irradiated up to 4 fractions (2 Gy dose per fraction). After each irradiation fraction the main optical characteristics of samples were measured. Laser ellipsometer GAERTNER 117 operating with a He-Ne laser (λ =632.8 nm) was used for the estimation of the thickness and refractive index of investigated films. Optical transmittance and absorbance spectra of samples were measured by Ultraviolet and Visible Absorption Spectrometer SPECORD UV/VIS as well. Absorption coefficient was calculated from UV–VIS transmittance spectra using Lambert– Beer law. Bonding structure of the irradiated carbon films were analyzed using GX FT-IR (PERKIN ELMER) spectrometer and Raman spectrometer with Spectra Physics Stabilite 2017 argon laser (λ = 514.5 nm). Hydrogen content was re-calculated from Raman spectra using approximation method from [5], or
J. Laurikaitienė et al. / Medical Physics in the Baltic States 7 (2009) 117 - 120 determined by elastic-recoil detection analysis (ERDA) using primary ion beam of 2.0 MeV He + ions. Surface morphology of the films was investigated by atomic force microscope (AFM) NANOTOP-206. Measurements were performed in tapping mode using V-shaped ultra sharp Si cantilever tip with the radius of curvature less than 10 nm. Applied force constant was 1.5 N/m. 3. Results and discussions Optical properties of DLC films were analyzed before and after irradiation of samples. Ellipsometric measurements have shown the decreasing tendency of the refractive index and increasing transparency (Fig.1) in all irradiated samples as compared to initial samples. It is interesting to note, that the increase of the optical transparency after the first irradiation of samples, has a tendency to decrease after further irradiation. According to the authors [21] optical properties are related to the hydrogen content and density of the DLC films. We have found that samples with a lower density, and higher H content were more transparent in wavelength range 200-900 nm after the irradiation. This fits well with the results of other authors, where the similar relationship between the transparency and hydrogen content in DLC was indicated [8]. Transmittance, % 60 50 40 30 20 10 No. 1 before and after irradiation 0 200 300 400 500 600 700 800 Wave length, nm Fig. 1. Transmittance spectra of DLC film No. 1 before (0 – 0 Gy) and after (1 – 2 Gy, 2 – 4 Gy, 3 – 6 Gy, 4 – 8 Gy) irradiation with high energy photons Close relationship between changes of the optical band gap (determined as Tauc gap) after the irradiation and the hydrogen content was found as well (Fig.2), since the hydrogen content in all as prepared samples was different. 2 6 4 8 0 118 (αE) 1/2 , (eV/cm) 1/2 8x10 5 7x10 5 6x10 5 5x10 5 4x10 5 3x10 5 2x10 5 1x10 5 0 No. 5 before irradiation after irradiation 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Energy, eV Fig. 2. DLC film No.5 before and after irradiation: (α⋅E) 1/2 dependence on photon energy E (ETauc = 1,75 eV, before irradiation and ETauc = 1,90 eV, after irradiation) Increase of the optical band gap with an increase of the irradiation dose (γ photons) could be explained by the decreasing number and size of sp 2 clusters in DLC films [22] after the irradiation. On the other hand, if the E04 and ETauc has a tendency to decrease [23, 24], it is related to the increase of the refractive index n. The most significant increase of the refractive index was observed for the irradiated sample series No.1G. Refractive index was increased from 1.9 to 2.5 as the hydrogen concentration decreased from 36 to 24 at %. Changes of the optical properties of high energy X-ray photon irradiated DLC films were more evident for less dense, and higher hydrogenated samples (sample No.1G), which were synthesized using the higher ion beam energy and ion beam current density. No dramatic changes of DLC optical properties were observed after multiple irradiation. In general, irradiation of DLC films with high energy Xray photons was responsible for the increased optical transparency in all investigated samples, indicating higher increasing tendency for the DLC samples produced at low ion beam energies and ion beam current density (No. 1G). Changes of the optical properties of the samples are related to the structural changes of DLC films. Due to this reason investigation of Raman spectra has been performed. It was found that after 8 Gy irradiation the shift of G and D peaks to the lower wave numbers in Raman spectra, were observed as compared to the Raman spectra obtained after the first 2Gy fraction of irradiation of the samples. The decrease of ID/IG ratio in heavier irradiated samples was observed as well. ID/IG decreased from 0.86 to 0.64 for a sample No. 1 (Fig.3), however no significant changes of ID/IG ratio were identified for a sample No.5. These results correspond to the fact, that the first irradiation fraction (when significant) modifies and stabilizes the irradiated structure of DLC film. Low doses cause mores significant structural changes.
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2009 PROCEEDINGS OF THE 7 th INTERN
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Executive editor: D.Adlienė CONFER
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