Graphitization of amorphous diamond-like carbon films by laser irradiation

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Optical Materials 30 (2008) 749–752 www.elsevier.com/locate/optmat

Graphitization of amorphous diamond-like carbon films by laser irradiation Alfonsas Grigonis a,*, Artur Medvid b, Pavels Onufrijevs b, Jurgis Babonas c, Alfonsas R_eza c a

Physics Department, Kaunas University of Technology, Studentu 50, LT-3031 Kaunas, Lithuania b Riga Technical University, 14 Azenes Street, LV-1048 Riga, Latvia c Semiconductor Physics Institute, A. Gosˇtauto 11, LT-01108 Vilnius, Lithuania Available online 28 March 2007

Abstract Amorphous diamond-like hydrogenated carbon, or a-C:H, films were deposited on Si wafers by means of direct ion beam deposition from acetylene or from a gaseous mixture of acetylene and hydrogen. The samples were irradiated by the second harmonic (k = 532 nm) of a Q-switched YAG:Nd laser. The effect of laser irradiation was studied with Raman and infrared spectroscopy and spectroscopic ellipsometry. Changes in the optical spectra of samples of various H content were investigated depending on laser intensity. Experimental data were interpreted taking into account the structural transformations of a-C:H films.  2007 Elsevier B.V. All rights reserved. Keywords: Amorphous diamond-type carbon films; Laser irradiation; Optical properties

1. Introduction Amorphous carbon films are perspective materials for many applications [1]. The physical properties of plasmadeposited amorphous hydrogenated carbon (a-C:H) films are mainly determined by the carbon sp3/sp2 bonding ratio [1] and the hydrogen content [2]. Laser irradiation leads to graphitization, spallation and evaporation of diamond-like carbon (DLC) films. Laserinduced graphitization of the surface layer has the lowest threshold [3]. Modeling has shown (cf. [4]) that nanosecond laser irradiation of DLC resulted in the development of stratified graphitization layers. For quasi-stationary annealing, a graphitization threshold of 500 C was obtained for various a-C:H films, whereas the sublimation temperature for graphite was 4000 C [5]. To the best of the authors’ knowledge, the influence of the amount of hydrogen on phase transitions and optical properties of a-C:H films has not been investigated yet. *

Corresponding author. Tel.: +370 37 300323; fax: +370 37 456472. E-mail address: [email protected] (A. Grigonis).

0925-3467/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.02.027

The aim of the present work has been to study the optical properties of laser-irradiated a-C:H films of various hydrogen contents. The critical values of irradiation have been estimated for essential structural transformations. The corresponding changes in bonding and concentration of sp3 bonds are discussed.

2. Experimental Amorphous hydrogenated carbon (a-C:H) films up to 250 nm thick were formed on Si (1 0 0) and (1 1 1) wafers by means of direct ion-beam deposition at room temperature. Prior to deposition, the substrates were cleaned with hydrogen plasma for 1 min to 10 min. The films were formed during 30 min from acetylene or an acetylenehydrogen gaseous mixture at ion energy of 1000 eV, ion current density of 0.12 mA/cm2 and pressure below 10 2 Pa. (Details of the deposition procedure are given in [6]. The properties of non-irradiated samples, a-C:H films on Si, are discussed in [7].) Prior to laser irradiation, an SiO2 layer was formed on the DLC/Si samples.

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The samples were irradiated in a scanning mode with a step of 25 lm by the second harmonic (k = 532 nm) of a Q-switched YAG:Nd laser with pulses of duration s = 10 ns and the repetition rate of 12.5 Hz. The diameter of the laser beam spot at height 1/e2 was 2.5 mm. The intensity of the laser pulse, IL, varied in the range of 1.8– 10.2 MW/cm2. The optical properties of laser-irradiated a-C:H films were studied by Raman scattering (RS), infrared (IR) spectroscopy, null- and spectroscopic ellipsometry (SE). RS was investigated using an Ar laser beam (k = 514.5 nm, 20 mW) with a 2 mm spot. Experimental RS curves were fitted by two Gaussian-shape lines in the spectral range of 1000–1900 cm 1. IR absorption and reflection spectra were measured in the 100–4000 cm 1 and 670–4000 cm 1 ranges using a Spectrum GX Perkin Elmer spectrometer. The thickness and refractive index of surface layers were determined using a Gaertner L115 automatic rotatingpolarizer null-ellipsometer operating with an He–Ne laser (632.8 nm). Spectroscopic ellipsometry studies were performed with a photometric ellipsometer with rotating analyzer in the 1–5 eV range [8].

3. Results and discussion 3.1. Structural studies As followed from structural studies, which have been carried out with the atomic force microscopy (AFM) technique, the surface of the initial DLC/Si samples was quite smooth, with roughness of a few nanometers (Fig. 1a). Irradiation with a low-intensity laser beam up to 4 MW/cm2 resulted in the appearance of grains 100– 200 nm in size and a slight increase in surface roughness (up to 10–15 nm, see Fig. 1b). The changes in morphology were due to structural transformations related to graphitization of the a-Si:H film. At medium intensity of the laser beam (IL 5–7 MW/cm2), large, sub-micrometer-size grains developed (50–70 nm in height, see Fig. 1c). The observed process was close to that expected for spallation of surface layers. At the highest laser intensity, IL  10 MW/cm2, extensive structural transformations occurred and the grain structure disappeared (see Fig. 1d). Both spallation and evaporation of material were responsible for the observed pattern.

Fig. 1. AFM micrographs of a 204 nm thick DLC film (a) before and after irradiation with a laser beam of (b) 3.5 MW/cm2, (c) 7.1 MW/cm2 and (d) 9.8 MW/cm2 intensity. The size of the area shown is 5 · 5 lm2. The full vertical scale equals (a) 3.7 nm, (b) 47 nm, (c) 74 nm and (d) 104 nm.

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3.2. Raman scattering and IR spectroscopy

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Two main Raman modes D (at 1360 cm 1) and G (at 1560 cm 1) were observed in initial DLC/Si samples (Fig. 2a). Regularities in the Raman spectra of laser-irradiated samples depend on the a-C:H films’ composition. For films with high hydrogen concentration (43%), weak Raman modes at 1260 cm 1 and 950 cm 1 assigned to diamond nanocrystals and SiC, respectively [9], were observed at IL in the range of (1.8–3.5) MW/cm2. At IL > 7 MW/cm2, Raman spectra were considerably different. The samples obtained at a relatively low hydrogen content (35%) were characterized [7] by a high percentage (P70%) of sp3 bonds in a-C:H films. The shape of Raman spectra of these samples was not altered significantly up to the moderate laser intensity. At a higher intensity, 5 MW/cm2, the D mode decreased with respect to the G mode and shifted towards the lower frequencies, whereas the position of the G mode remained almost unchanged. The red shift of the D mode may be due to the influence of the mode at 1260 cm 1 attributed to diamond nanocrystals (see [9]) and could indicate mutual sp2 M sp3 transformations between bonds of different types. At IL > 4 MW/cm2, along with graphitization process, the microchannels were formed [10] in which the temperature and pressure increased locally, creating favorable condi-

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Fig. 3. IR reflectance spectra of a sample with 70 nm thick DLC film irradiated at laser intensities (1) 3.8 MW/cm2 and (2) 5.1 MW/cm2 with respect to reflectance of non-irradiated sample.

tions for the formation of diamond nanocrystals. Additionally, the possible formation of SiC should also lead to an increase in sp3 bonds and the amount of Si in a-C:H films [11]. In a-C:H films with a higher content of the graphite phase (H 6 27%) and dominant sp2-type bonds [9], glass carbon formed at IL  4–7 MW/cm2. In Raman spectra, two modes were distinctly resolved at 1350 cm 1 and 1600 cm 1 (see Fig. 2b). The irradiation dose necessary for the formation of glassy carbon depended on the amount of sp3 bonds and hydrogen in a-C:H films. In samples of higher hydrogen contents, transformation to glassy C occurred at lower laser intensities. A further increase of IL led to intensive spallation. Irradiation with IL at 1.8–3.8 MW/cm2 resulted in a slight (2–7%) decrease in IR transmittance, though reflectance remained almost unchanged (see Fig. 3). At IL > 3.8 MW/cm2, an increase in the valence vibration modes was observed at 2850 cm 1 and 2920 cm 1, as well as deformation modes at 1400–1500 cm 1, which have been attributed to methyl. However, the band at 1500– 1700 cm 1 assigned to the vibrations of C@C bonds, increased simultaneously. Thus, as a result of irradiation, hydrogen either evaporated or migrated joining free carbon bonds and thus increasing the amount of CH2 in the sp3 hybridization. This process did not increase the amount of DLC as the transformation of carbon in sp3 bonds into sp2 bonds was more probable at increased temperatures of a-C:H films. The films with higher hydrogen contents were more sensitive to laser irradiation.

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Fig. 2. Experimental (points) and fitted Gaussian-shape (curves) lines in Raman spectra of a DLC/Si sample with a higher content of the graphite phase (a) before and (b) after irradiation at IL = 10 MW/cm2.

The spectral dependence of optical response for silicacoated irradiated DLC/Si samples was explained with the same model of the effective DLC film as for samples without the SiO2 layer [7,12]. The dielectric function of the irradiated a-C:H films was determined by solving the

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Our structural and optical investigations of laser-irradiated silica-coated a-C:H films formed on Si substrates have shown that films with higher hydrogen content are more sensitive to irradiation of nanosecond laser pulses. Laser irradiation leads to graphitization of a-C:H films and the formation of glassy carbon. Glassy carbon has not occurred in a-C:H films with higher content of sp3-type CAC bonds. As a result of laser irradiation, SiC and diamond-like nanocrystals have been formed.

This work was partially supported by the Lithuanian State Science and Studies Foundation.

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4. Conclusions

Acknowledgement

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up to 26% [15]. When laser intensity was increased to 3.6 MW/cm2, stratification of the DLC layer manifested itself and experimental data could be fitted by model calculations taking into account the inhomogeneity of a-C:H film. The e(E) spectra changed significantly at high laser intensities. A possible graphitization of the DLC film could be responsible for the increase in e(E) values at lower photon energies, while the contribution at 3.6–4.5 eV can be assigned to the formation of glassy carbon and sp2-bonded C atoms in diamond [13].

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E, eV Fig. 4. Spectra of the imaginary part of the dielectric function of (a) thin (70 nm) and (b) thick (204 nm) a-C:H films irradiated at various laser intensities. Curves 2a, 3a and 2b, 3b are the data for the upper and lower sublayers of a-C:H film, respectively.

multilayer model with the transfer matrix technique (cf. [8], see Fig. 4a). At low laser intensities (IL = 2.4 MW/cm2) the dielectric function, e(E), of a thin DLC film (d = 70 nm) was close to that for a-C (cf. [7]). At IL = 4.8 MW/cm2, the contribution of a thin DLC film was interpreted as broadened e(E) for a-C:H. At high intensities, two contributions to the optical response were clearly noticeable: one, with the peak at 3.7 eV, was close to that due to sp2-bonded C atoms in a diamond film, while the other, with the maximum at 4.4 eV, originated from glassy carbon [13]. It should be noted that glass-like materials are usually formed under laser irradiation with pulse duration of 10 7 s and intensity of 10 MW/cm2 (cf. [14]). The contribution of a thicker a-C:H film (d = 204 nm) to the optical response (see Fig. 4b) was well approximated by that of meta-stable amorphous carbon films with a high (76%) percentage of diamond-like components and porosity of

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