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Brillouin light scattering from the Rayleigh surface phonon of thermally annealed a-C:H films
Surface and Coatings Technology, 47 (1991) 687—690
687
Brillouin light scattering from the Rayleigh surface phonon of thermally annealed a-C:H films X. Jiang, P. Grunberg, K. Reichelt and B. Stritzker Institut für Schicht- und lonentechnik and Institut fur Festkorperforschung, Forschungszentrum Jülich, D-51 70 Jülich (Germany)
Abstract Brillouin light scattering experiments are performed on amorphous diamond-like carbon films (a-C:H). Inelastic scattering from the Rayleigh surface phonon is found. Young’s modulus, the shear modulus, bulk modulus, and microhardness of a-C:H films annealed at various temperatures are evaluated combining light scattering and indentation techniques.
1. Introduction Brillouin light scattering allows the measurement of the frequency v~of the Rayleigh surface phonon [1, 2]. (For a review see ref. 3.) For backscattering experiments the Rayleigh sound velocity VR can be related to the frequency shift by VR=
(1)
21~’a0
where ).~and O~are respectively the wavelength and angle of incidence for the exciting laser light. From this the shear modulus G can then be calculated [4, 5]: G=1.2pV~
(2)
where p is the density. The load.displacement curve of a diamond nanoindenter has been proved useful for measuring the stiffness S of thin films. Young’s modulus is then derived from stiffness data using the formula [5, 6] 2 = ~1/2
(1 E
+1
(3)
A~’
where v and E are respectively Poisson’s ratio and Young’s modulus of the sample; v 0 and E0 are the corresponding parameters for the indenter (for a diamond indenter E0 = 1010 GPa and v0 = 0.213); Af is the projected area between the sample and the indenter and can be determined simultaneously by the unloading curve of the indenter. For the sample a value of v = 0.3 is assumed (S is only weakly dependent on v). 0257-8972/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
688
Based on the E and G values the bulk modulus B of the film can be calculated from the following relation: EG B=3(3GE)
(4)
2. Experimental details The a-C:H films were deposited on water.cooled silicon substrates by plasma decomposition of CH4 at a frequency of 13.56 MHz [7]. The bias voltage and CH4 gas pressure for film deposition were —400 V and 2.7 x 102 mbar, respectively. The thickness of the films was 1 pm. After deposition the films were mounted in a quartz tube furnace. Annealing was carried out under high vacuum conditions at a heating rate of 20 K mm 1 to a predetermined temperature. Cooling in vacuum was achieved by switching off the heating stage.
3. Results Figure 1 shows a sequence of Brillouin spectra of annealed a-C:H films. At temperatures lower than 400 °Conly minor shifts of the Rayleigh phonon frequency occur while the frequency decreases drastically from 15.5 to
~LiL~ ~UL~ ULr 1~c ~
~2OO~’O
Frequency shift V, IGHzl
Fig. 1. Brillouin scattering spectra of annealed a-C:H films, showing the intensity of the scattered light vs. the frequency shift v. The Rayleigh mode frequency VR is identical with the absolute value of the frequency shift.
689 200
50
I~-I—-i~4--—
150
0
I
6 I
0
200
I
400
I f/—f———
I
600
1000
Ta (
Fig. 2. Elastic moduli and microhaz~dnessas a function of annealing temperature: (a) Young’s modulus; (b) shear modulus; (c) bulk~tno~dulus; (d) hardness.
10.6 GHz for temperatures above 500 °C.Saturation is found at temperatures above 600 °C. The elastic moduli E, G and B were obtained according to eqns. (2)—(4). These data are shown in Fig. 2 together with the microhardness values obtained by hardness measurements with a nanoindenter. From Fig. 2 it is evident that, similar to the phonon phase velocity, the elastic modulus and hardness show a minor increase up to 300 °C. At higher temperatures (greater than 500 °C)a drastic decrease occurs.
4. Discussion In previous work [8] gas effusion experiments were performed, which give information about the network structure and the thermal decomposition behaviour of a-C:H. It was shown that the threshold temperature for gas effusion, and thus for the thermal decomposition process, is about 400 °Cfor films deposited under the same conditions as for the samples used in this study. This means that the a-C:H phase is thermally stable up to 400 °C. Above this temperature chemically bonded hydrogen and CH3 are desorbed and effused.
690
Nuclear magnetic resonance revealed [9] that in these materials a tetrahedral sp3 carbon atom is bonded to at least one hydrogen atom, i.e. hydrogen stabilizes sp3 hybridization. Therefore the desorption of a hydrogen atom can transform sp3.bonded carbon atoms into sp2.bonded carbon. This effect has been observed by Dischler et al. [10]. They studied annealed a-C:H films with infrared absorption and showed that heat treatment transforms sp3 sites and olefinic sp2 sites into aromatic sp2 sites [11]. The a.C:H films are thermally stable up to 400 °C. Above this temperature hydrogen will be effused and a drastic change in the bonding occurs, namely a phase transformation from diamond-like to graphite-like a-C:H.
5. Conclusion Brillouin light scattering experiments and indentation tests were performed on annealed a.C:H films. The elastic properties and hardness of these films were evaluated. These results show a connection between the variations in observed phonon frequency, elastic modulus, microhardness, and the structural changes. The a-C:H phase is thermally stable up to 400 °Cand only minor changes of the mechanical properties can be found. In the range between 400 and 600 °C the a.C:H phase transforms from diamond like to graphite like and a drastic decrease in the properties occurs.
Acknowledgment The authors would like to thank Dr. S. Manti for correction of the manuscript.
References 1 G. B. Benedek and K. Fritsch, Phys. Rev., 149 (1966) 647. 2 B. Hillebrands, P. Baumgart, R. Mock, G. Güntherodt and P. S. Bechthold, J. Appl. Phys., 58 (1985) 3166. 3 J. R. Sandercock, in M. Cardona and G. Güntherodt, (eds.), Topics in Applied Physics, Vol. 51, Springer, Berlin, 1982 p. 179. 4 X. Jiang, J. W. Zou, K. Reichelt and P. Grunberg, J. Appi. Phys., 66 (1989) 4729. 5 X. Jiang, B. Goranchev, K. Schmidt, P. Grunberg and K. Reichelt, J. Appi. Phys., 67 (1990) 6772. 6 X. Jiang, K. Reichelt and B. Stritzker, J. Appl. Phys., 68 (1989) 1018. 7 J. W. Zou, K. Reichelt, K. Schmidt and B. Dishler, J. Appl. Phys., 65 (1989) 3914. 8 X. Jiang, W. Beyer and K. Reichelt, J. Appi. Phys., 68 (1990) 1378. 9 A. Grill, B. S. Meyerson, V. V. Patel, J. A. Reimer and M. A. Petrich, J. Appl. Phys., 61 (1987) 2874. 10 B. Dischler, A. Bubenzer and P. Koidl, Solid State Commun., 48 (1983) 105. 11 B. Dischler, Editions de Physique, Paris, 1987, p. 189.
687
Brillouin light scattering from the Rayleigh surface phonon of thermally annealed a-C:H films X. Jiang, P. Grunberg, K. Reichelt and B. Stritzker Institut für Schicht- und lonentechnik and Institut fur Festkorperforschung, Forschungszentrum Jülich, D-51 70 Jülich (Germany)
Abstract Brillouin light scattering experiments are performed on amorphous diamond-like carbon films (a-C:H). Inelastic scattering from the Rayleigh surface phonon is found. Young’s modulus, the shear modulus, bulk modulus, and microhardness of a-C:H films annealed at various temperatures are evaluated combining light scattering and indentation techniques.
1. Introduction Brillouin light scattering allows the measurement of the frequency v~of the Rayleigh surface phonon [1, 2]. (For a review see ref. 3.) For backscattering experiments the Rayleigh sound velocity VR can be related to the frequency shift by VR=
(1)
21~’a0
where ).~and O~are respectively the wavelength and angle of incidence for the exciting laser light. From this the shear modulus G can then be calculated [4, 5]: G=1.2pV~
(2)
where p is the density. The load.displacement curve of a diamond nanoindenter has been proved useful for measuring the stiffness S of thin films. Young’s modulus is then derived from stiffness data using the formula [5, 6] 2 = ~1/2
(1 E
+1
(3)
A~’
where v and E are respectively Poisson’s ratio and Young’s modulus of the sample; v 0 and E0 are the corresponding parameters for the indenter (for a diamond indenter E0 = 1010 GPa and v0 = 0.213); Af is the projected area between the sample and the indenter and can be determined simultaneously by the unloading curve of the indenter. For the sample a value of v = 0.3 is assumed (S is only weakly dependent on v). 0257-8972/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
688
Based on the E and G values the bulk modulus B of the film can be calculated from the following relation: EG B=3(3GE)
(4)
2. Experimental details The a-C:H films were deposited on water.cooled silicon substrates by plasma decomposition of CH4 at a frequency of 13.56 MHz [7]. The bias voltage and CH4 gas pressure for film deposition were —400 V and 2.7 x 102 mbar, respectively. The thickness of the films was 1 pm. After deposition the films were mounted in a quartz tube furnace. Annealing was carried out under high vacuum conditions at a heating rate of 20 K mm 1 to a predetermined temperature. Cooling in vacuum was achieved by switching off the heating stage.
3. Results Figure 1 shows a sequence of Brillouin spectra of annealed a-C:H films. At temperatures lower than 400 °Conly minor shifts of the Rayleigh phonon frequency occur while the frequency decreases drastically from 15.5 to
~LiL~ ~UL~ ULr 1~c ~
~2OO~’O
Frequency shift V, IGHzl
Fig. 1. Brillouin scattering spectra of annealed a-C:H films, showing the intensity of the scattered light vs. the frequency shift v. The Rayleigh mode frequency VR is identical with the absolute value of the frequency shift.
689 200
50
I~-I—-i~4--—
150
0
I
6 I
0
200
I
400
I f/—f———
I
600
1000
Ta (
Fig. 2. Elastic moduli and microhaz~dnessas a function of annealing temperature: (a) Young’s modulus; (b) shear modulus; (c) bulk~tno~dulus; (d) hardness.
10.6 GHz for temperatures above 500 °C.Saturation is found at temperatures above 600 °C. The elastic moduli E, G and B were obtained according to eqns. (2)—(4). These data are shown in Fig. 2 together with the microhardness values obtained by hardness measurements with a nanoindenter. From Fig. 2 it is evident that, similar to the phonon phase velocity, the elastic modulus and hardness show a minor increase up to 300 °C. At higher temperatures (greater than 500 °C)a drastic decrease occurs.
4. Discussion In previous work [8] gas effusion experiments were performed, which give information about the network structure and the thermal decomposition behaviour of a-C:H. It was shown that the threshold temperature for gas effusion, and thus for the thermal decomposition process, is about 400 °Cfor films deposited under the same conditions as for the samples used in this study. This means that the a-C:H phase is thermally stable up to 400 °C. Above this temperature chemically bonded hydrogen and CH3 are desorbed and effused.
690
Nuclear magnetic resonance revealed [9] that in these materials a tetrahedral sp3 carbon atom is bonded to at least one hydrogen atom, i.e. hydrogen stabilizes sp3 hybridization. Therefore the desorption of a hydrogen atom can transform sp3.bonded carbon atoms into sp2.bonded carbon. This effect has been observed by Dischler et al. [10]. They studied annealed a-C:H films with infrared absorption and showed that heat treatment transforms sp3 sites and olefinic sp2 sites into aromatic sp2 sites [11]. The a.C:H films are thermally stable up to 400 °C. Above this temperature hydrogen will be effused and a drastic change in the bonding occurs, namely a phase transformation from diamond-like to graphite-like a-C:H.
5. Conclusion Brillouin light scattering experiments and indentation tests were performed on annealed a.C:H films. The elastic properties and hardness of these films were evaluated. These results show a connection between the variations in observed phonon frequency, elastic modulus, microhardness, and the structural changes. The a-C:H phase is thermally stable up to 400 °Cand only minor changes of the mechanical properties can be found. In the range between 400 and 600 °C the a.C:H phase transforms from diamond like to graphite like and a drastic decrease in the properties occurs.
Acknowledgment The authors would like to thank Dr. S. Manti for correction of the manuscript.
References 1 G. B. Benedek and K. Fritsch, Phys. Rev., 149 (1966) 647. 2 B. Hillebrands, P. Baumgart, R. Mock, G. Güntherodt and P. S. Bechthold, J. Appl. Phys., 58 (1985) 3166. 3 J. R. Sandercock, in M. Cardona and G. Güntherodt, (eds.), Topics in Applied Physics, Vol. 51, Springer, Berlin, 1982 p. 179. 4 X. Jiang, J. W. Zou, K. Reichelt and P. Grunberg, J. Appi. Phys., 66 (1989) 4729. 5 X. Jiang, B. Goranchev, K. Schmidt, P. Grunberg and K. Reichelt, J. Appi. Phys., 67 (1990) 6772. 6 X. Jiang, K. Reichelt and B. Stritzker, J. Appl. Phys., 68 (1989) 1018. 7 J. W. Zou, K. Reichelt, K. Schmidt and B. Dishler, J. Appl. Phys., 65 (1989) 3914. 8 X. Jiang, W. Beyer and K. Reichelt, J. Appi. Phys., 68 (1990) 1378. 9 A. Grill, B. S. Meyerson, V. V. Patel, J. A. Reimer and M. A. Petrich, J. Appl. Phys., 61 (1987) 2874. 10 B. Dischler, A. Bubenzer and P. Koidl, Solid State Commun., 48 (1983) 105. 11 B. Dischler, Editions de Physique, Paris, 1987, p. 189.