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Raman analysis of laser annealed nitrogen doped amorphous carbon film
Solid State Communications 123 (2002) 97–100 www.elsevier.com/locate/ssc
Raman analysis of laser annealed nitrogen doped amorphous carbon film Weili Zhang*, Yiben Xia, Jianhua Ju, Yimin Fan, Zhijun Fang, Linjun Wang, Zhiming Wang School of Materials and Engineering, Shanghai University in Jiading, Shanghai 201800, People’s Republic of China Received 31 January 2002; received in revised form 3 April 2002; accepted 24 May 2002 by A.K. Sood
Abstract A micro-Raman spectrometer has been used to study nitrogen doped amorphous hydrogenated carbon (a-C/H(N)) films, which are annealed by a focused argon ion (514.5 nm) laser operating at the power density of 300, 750 and 1500 W/mm2, respectively. The analysis shows that nitrogen atoms in the films mainly take the form of C– N bonds, which limits the forming of C –H bonds because the formation energy of C– N bond is higher than that of C– H bond. So the C – H bond in a-C/H film is easily decomposed, which increases the graphitization of film by laser annealing. On the other hand, C– N bonds are hardly decomposed in the course of laser annealing, then we consider that thermal stability of a-C/H(N) film increases with the increment of nitrogen content. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 78.30.L; 61.80.B Keywords: A. a-C/H(N) film; Laser annealing; Raman spectrum; Thermal stability; Amorphous C-film
1. Introduction There have been many reports about Raman spectra analysis of laser annealed a-C/H films. Typical research is focus on Raman analysis of D and G peaks which are obtained from a-C/H films annealed under 400 – 600 8C. The a-C/H films are prepared by radio frequency glow discharge and plasma enhanced deposition methods. The results show that the intensity ratio of D to G band (ID/IG) in Raman spectra decreases with the increment of annealing temperature [1], which also makes the positions of D and G peaks shift to 1353 and 1598 cm21, respectively. Farrow et al. obtained similar results by using Cþ irradiating a-C/H films [2]. Under these annealing conditions, H content in the films decreased, while the graphitization of films increased. Raman spectra of a-C/H films which were prepared by radio frequency coupled glow discharge method and
annealed at 600 8C for 1 h, showed that H content in the films decreased and the graphite micro-crystal phase began to appear in the films [3]. Bowden et al. [4] reported that there existed annealing effect in films irradiated by a laser beam of 1500 W/mm2. From Raman spectra analysis, laser annealing can lead to local graphitization in film (within 2 mm). Mariotto et al. [5] reported that the annealing effect on a-C/H and a-C/ H(N) films annealed under 300 and 600 8C in vacuum, in which a-C/H films injected by Nþ can change into aC/H(N) films which will accelerate graphitization of films. However, the thermal annealing behavior of a-C/ H(N) films prepared by RF-CVD is not clear. In this work, we will research the change of micro-composition and structure of the films during the relatively simple laser annealing by analyzing Raman spectra.
2. Experimental * Corresponding author. Tel.: þ86-21-6998-2305; fax: þ 86-216998-2233. E-mail address: [email protected] (W. Zhang).
Nitrogen-doped hydrogenated amorphous carbon (aC/H(N)) films were prepared by RF-CVD method in
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 2 3 7 - 5
98
W. Zhang et al. / Solid State Communications 123 (2002) 97–100
Table 1 Chemical composition and thickness of the films as function of the partial pressure of N2 in the mixture gas Partial pressure of N2 (%)
Chemical composition N (at.%)
Film thickness (mm)
Vicker’s hardness (kg N/mm2)
0 10 30 50
0 3.4 9.8 12.6
1.4 1.2 1.2 1.3
1785 1730 1552 1580
which the frequency was 13.6 MHz. The reaction gas was a mixture of CH4, Ar and N2. The silicon substrate was polished. The deposition was carried out at self-bias voltage of 850 V, negative direct current bias voltage of substrate of 150 V and a fixed pressure of 1.3 Pa. Films with different nitrogen concentration were obtained by altering the gas ratio of N2/CH4 from 0.1 to 10. Table 1 shows chemical composition and thickness of the films changing with the partial pressure of N2 in the mixture gas. Atomic percent concentration of nitrogen in the deposited films was characterized by Auger electron spectroscopy (AES). Composition and microstructure of the films were studied by FTIR. Raman scattering measurements for recording the Raman spectra were carried out at room temperature in back scattering geometry, using 514.5 nm line of argon-ion laser operating at 300 mW for 60 s. Both excitation and light collection were through microscope objective in back scattering geometry. All the power of excitation was kept on 2.4 mW at the sample surfaces after attenuation and power density was about 3000 W/mm2. In order to study annealing effects under various power densities of laser beam, several filter attenuators, such as 50, 25, 10 and 1% were used to control the power densities of the laser beam reached 1500, 750, 300 and 30 W/mm2. In the experiment, aC/H film and 12.6 at.% N content a-C/H(N) film were annealed by Arþ laser at different power densities on the same spots and analyzed by micro-Raman scattering spectrometer.
From Table 1, it is clear that the concentration of nitrogen in the deposited films increases with raising nitrogen pressure in the deposition process with RF-CVD method. However, mechanical property and micro-hardness of the films almost does not change, which is in agreement with that of films deposited by self-bias glow discharge [6]. Fig. 1 gives the FTIR absorption spectra of a-C/H(N) films as the function of N content. The spectra can be divided into three regions: C– C, C– H, C– N stretching modes from 0 to 1700 cm21, and C– N stretching mode is at 1623 cm21 which owns the highest intensity as shown in Fig. 1 This means that the nitrogen atoms introduced into the films can markedly retain C –N bond form. While intensity of the absorption of CxN stretching mode mainly within 2065–2260 cm21 is weak which means that the content of CxN is low. It is clear that, after N is doped into a-C/H films, N atoms mainly bond with C and H to form C – N and N– H bonds, the content of which is high. At the same time, there exists small amount of CxN bonds. Intensities of the three peaks increase with raising gas ratio of N2. Fig. 2 is FTIR spectra of a-C/H(N) films located within 2600–3200 cm21 and content of N is 0 and 12.6 at.%, respectively. Obviously, the absorption peak in this region is C– H stretching mode. From variations of peak intensities, it can be proved that the content of C –H bond in the film decreases obviously with raising the amount of doping N. It is sure that the ratio of sp3-CH/sp2-CH is 65% by using Gausses fit [7].
Fig. 1. FTIR spectra of a-C/H(N) films for CNH, CN and NH stretching modes as function of N content.
Fig. 2. FTIR spectra of a-C/H(N) films for C– H stretching mode as function of N content.
3. Results and discussions
W. Zhang et al. / Solid State Communications 123 (2002) 97–100
Fig. 3. Raman spectra of a-C/H films as function of power density of Arþ laser.
Fig. 3 is Raman spectra of a-C/H films under different power density of Arþ laser. It is clear from Fig. 3 that the intensity ratio of D to G band (ID/IG) in Raman spectra increases with the increment of power density of laser from 300 to 1500 W/mm2. Furthermore, with the increment of the laser power density in the annealing process, the position of G peak has been shifted from 1550 to 1598 cm21, the amount of shifting can reach 40 wavenumbers. The Raman spectrum of a-C/H film is featured by two peaks: D and G. G line situated at about 1580 cm21 for single crystal graphite results from the Raman allowed E2g mode [9], while another line appears at about 1355 cm21 for polycrystalline graphite, known as ‘D’ line which is due to the breakdown of the k vector conservation rule of the disordered lattice [10]. In the process of laser annealing C– H bonds of a-C/H films are decomposed and carbon atoms are graphitized [4], which lead to the formation of polycrystalline graphite in the a-C/H films, so the intensity ratio of ID/IG increases with the increase of laser power density. Fig. 4 gives the Raman spectra of a-C/H(N) films with 12.6 at.% N as function of annealing laser power density. The annealing laser power density in Fig. 4 from bottom to top is 300, 750 and 1500 W/mm2. With the increasing of annealing laser power density, neither the ratio of D to G
99
Fig. 5. Raman spectra of a-C/H(N) films as function of N content in the film.
band (ID/IG) nor the position of G peak in Raman spectra has obvious change. This means that nitrogen atoms form C– N bonds in DLC films after they displace hydrogen atoms. Because the bond energy of C– N is greater than that of C– H [8] so that C– N is more stable, it is difficult to decompose C– N during the laser annealing process that prevent sp2 non-crystal carbon in DLC films from graphitizing. On the other hand, the content of C –H decreases drastically in a-C/ H(N) films as shown in Fig. 2, which decreases the graphitization of films in the laser annealing process. Nitrogen introduced into the films has less effect on its mechanical property but improves the thermal stability of the films. Fig. 5 shows Raman spectra of a-C/H(N) films as function of N content (0, 3.4 and 12.6 at.%) in the films after the films were annealed by a laser beam of 1500 W/mm2 power density for 200 s. The G peak in Raman spectra from bottom to top is 1598, 1576 and 1558 cm21, respectively. This result shows further that with raising the N content, the graphitization of films is decreased and the thermal stability is improved.
4. Conclusions Comparing the results mentioned earlier, we can get the conclusions as follows: C– N bond which is more stable than C – H bond is difficult to be decomposed; nitrogen introduced into the films limits the formation of C – H which leads to decrease in graphitization of films caused by decomposition of C –H; so with the increment of N content, the graphitization of films is decreased and the thermal stability is improved.
References Fig. 4. Raman spectra of a-C/H(N) films with N content of 12.6 at.% as function of power density of Arþ laser.
[1] R.O. Dillon, J.A. Woollam, Phys. Rev. B 29 (1984) 3482.
100
W. Zhang et al. / Solid State Communications 123 (2002) 97–100
[2] L.A. Farrow, B.J. Willkens, A.S. Gozdz, D.L. Hart, Phys. Rev. B 41 (1990) 10132. [3] J. Gonzalez-Hernandez, B.S. Gozdz, D.A. Pawlik, Mater. Sci. Forum 52 and 53 (1989) 543. [4] M. Bowden, D.J. Garding, J.M. Southall, J. Appl. Phys. 71 (1992) 521. [5] G. Mariotto, F.L. Freire Jr., C.A. Achete, Thin Solid Films 241 (1994) 255. [6] D.F. Fanceschini, C.A. Achete, F.L. Freire Jr., Appl. Phys. Lett. 60 (1992) 3229.
[7] B. Racine, M. Benlahsen, K. Zellama, P. Goudeau, M. Zarrabian, G. Turban, Appl. Phys. Lett. 73 (1998) 3226. [8] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, B. Rafferty, L.M. Brown, J. Appl. Phys. 81 (1997) 2626. [9] R.J. Nemanich, G. Lucovsky, Solid State Commun. 23 (2) (1977) 117 –120. [10] V. Meenakshi, A. Sayeed, S.V. Subramanyam, Mater. Sci. Forum 223 –224 (1996) 307– 310.
Raman analysis of laser annealed nitrogen doped amorphous carbon film Weili Zhang*, Yiben Xia, Jianhua Ju, Yimin Fan, Zhijun Fang, Linjun Wang, Zhiming Wang School of Materials and Engineering, Shanghai University in Jiading, Shanghai 201800, People’s Republic of China Received 31 January 2002; received in revised form 3 April 2002; accepted 24 May 2002 by A.K. Sood
Abstract A micro-Raman spectrometer has been used to study nitrogen doped amorphous hydrogenated carbon (a-C/H(N)) films, which are annealed by a focused argon ion (514.5 nm) laser operating at the power density of 300, 750 and 1500 W/mm2, respectively. The analysis shows that nitrogen atoms in the films mainly take the form of C– N bonds, which limits the forming of C –H bonds because the formation energy of C– N bond is higher than that of C– H bond. So the C – H bond in a-C/H film is easily decomposed, which increases the graphitization of film by laser annealing. On the other hand, C– N bonds are hardly decomposed in the course of laser annealing, then we consider that thermal stability of a-C/H(N) film increases with the increment of nitrogen content. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 78.30.L; 61.80.B Keywords: A. a-C/H(N) film; Laser annealing; Raman spectrum; Thermal stability; Amorphous C-film
1. Introduction There have been many reports about Raman spectra analysis of laser annealed a-C/H films. Typical research is focus on Raman analysis of D and G peaks which are obtained from a-C/H films annealed under 400 – 600 8C. The a-C/H films are prepared by radio frequency glow discharge and plasma enhanced deposition methods. The results show that the intensity ratio of D to G band (ID/IG) in Raman spectra decreases with the increment of annealing temperature [1], which also makes the positions of D and G peaks shift to 1353 and 1598 cm21, respectively. Farrow et al. obtained similar results by using Cþ irradiating a-C/H films [2]. Under these annealing conditions, H content in the films decreased, while the graphitization of films increased. Raman spectra of a-C/H films which were prepared by radio frequency coupled glow discharge method and
annealed at 600 8C for 1 h, showed that H content in the films decreased and the graphite micro-crystal phase began to appear in the films [3]. Bowden et al. [4] reported that there existed annealing effect in films irradiated by a laser beam of 1500 W/mm2. From Raman spectra analysis, laser annealing can lead to local graphitization in film (within 2 mm). Mariotto et al. [5] reported that the annealing effect on a-C/H and a-C/ H(N) films annealed under 300 and 600 8C in vacuum, in which a-C/H films injected by Nþ can change into aC/H(N) films which will accelerate graphitization of films. However, the thermal annealing behavior of a-C/ H(N) films prepared by RF-CVD is not clear. In this work, we will research the change of micro-composition and structure of the films during the relatively simple laser annealing by analyzing Raman spectra.
2. Experimental * Corresponding author. Tel.: þ86-21-6998-2305; fax: þ 86-216998-2233. E-mail address: [email protected] (W. Zhang).
Nitrogen-doped hydrogenated amorphous carbon (aC/H(N)) films were prepared by RF-CVD method in
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 2 3 7 - 5
98
W. Zhang et al. / Solid State Communications 123 (2002) 97–100
Table 1 Chemical composition and thickness of the films as function of the partial pressure of N2 in the mixture gas Partial pressure of N2 (%)
Chemical composition N (at.%)
Film thickness (mm)
Vicker’s hardness (kg N/mm2)
0 10 30 50
0 3.4 9.8 12.6
1.4 1.2 1.2 1.3
1785 1730 1552 1580
which the frequency was 13.6 MHz. The reaction gas was a mixture of CH4, Ar and N2. The silicon substrate was polished. The deposition was carried out at self-bias voltage of 850 V, negative direct current bias voltage of substrate of 150 V and a fixed pressure of 1.3 Pa. Films with different nitrogen concentration were obtained by altering the gas ratio of N2/CH4 from 0.1 to 10. Table 1 shows chemical composition and thickness of the films changing with the partial pressure of N2 in the mixture gas. Atomic percent concentration of nitrogen in the deposited films was characterized by Auger electron spectroscopy (AES). Composition and microstructure of the films were studied by FTIR. Raman scattering measurements for recording the Raman spectra were carried out at room temperature in back scattering geometry, using 514.5 nm line of argon-ion laser operating at 300 mW for 60 s. Both excitation and light collection were through microscope objective in back scattering geometry. All the power of excitation was kept on 2.4 mW at the sample surfaces after attenuation and power density was about 3000 W/mm2. In order to study annealing effects under various power densities of laser beam, several filter attenuators, such as 50, 25, 10 and 1% were used to control the power densities of the laser beam reached 1500, 750, 300 and 30 W/mm2. In the experiment, aC/H film and 12.6 at.% N content a-C/H(N) film were annealed by Arþ laser at different power densities on the same spots and analyzed by micro-Raman scattering spectrometer.
From Table 1, it is clear that the concentration of nitrogen in the deposited films increases with raising nitrogen pressure in the deposition process with RF-CVD method. However, mechanical property and micro-hardness of the films almost does not change, which is in agreement with that of films deposited by self-bias glow discharge [6]. Fig. 1 gives the FTIR absorption spectra of a-C/H(N) films as the function of N content. The spectra can be divided into three regions: C– C, C– H, C– N stretching modes from 0 to 1700 cm21, and C– N stretching mode is at 1623 cm21 which owns the highest intensity as shown in Fig. 1 This means that the nitrogen atoms introduced into the films can markedly retain C –N bond form. While intensity of the absorption of CxN stretching mode mainly within 2065–2260 cm21 is weak which means that the content of CxN is low. It is clear that, after N is doped into a-C/H films, N atoms mainly bond with C and H to form C – N and N– H bonds, the content of which is high. At the same time, there exists small amount of CxN bonds. Intensities of the three peaks increase with raising gas ratio of N2. Fig. 2 is FTIR spectra of a-C/H(N) films located within 2600–3200 cm21 and content of N is 0 and 12.6 at.%, respectively. Obviously, the absorption peak in this region is C– H stretching mode. From variations of peak intensities, it can be proved that the content of C –H bond in the film decreases obviously with raising the amount of doping N. It is sure that the ratio of sp3-CH/sp2-CH is 65% by using Gausses fit [7].
Fig. 1. FTIR spectra of a-C/H(N) films for CNH, CN and NH stretching modes as function of N content.
Fig. 2. FTIR spectra of a-C/H(N) films for C– H stretching mode as function of N content.
3. Results and discussions
W. Zhang et al. / Solid State Communications 123 (2002) 97–100
Fig. 3. Raman spectra of a-C/H films as function of power density of Arþ laser.
Fig. 3 is Raman spectra of a-C/H films under different power density of Arþ laser. It is clear from Fig. 3 that the intensity ratio of D to G band (ID/IG) in Raman spectra increases with the increment of power density of laser from 300 to 1500 W/mm2. Furthermore, with the increment of the laser power density in the annealing process, the position of G peak has been shifted from 1550 to 1598 cm21, the amount of shifting can reach 40 wavenumbers. The Raman spectrum of a-C/H film is featured by two peaks: D and G. G line situated at about 1580 cm21 for single crystal graphite results from the Raman allowed E2g mode [9], while another line appears at about 1355 cm21 for polycrystalline graphite, known as ‘D’ line which is due to the breakdown of the k vector conservation rule of the disordered lattice [10]. In the process of laser annealing C– H bonds of a-C/H films are decomposed and carbon atoms are graphitized [4], which lead to the formation of polycrystalline graphite in the a-C/H films, so the intensity ratio of ID/IG increases with the increase of laser power density. Fig. 4 gives the Raman spectra of a-C/H(N) films with 12.6 at.% N as function of annealing laser power density. The annealing laser power density in Fig. 4 from bottom to top is 300, 750 and 1500 W/mm2. With the increasing of annealing laser power density, neither the ratio of D to G
99
Fig. 5. Raman spectra of a-C/H(N) films as function of N content in the film.
band (ID/IG) nor the position of G peak in Raman spectra has obvious change. This means that nitrogen atoms form C– N bonds in DLC films after they displace hydrogen atoms. Because the bond energy of C– N is greater than that of C– H [8] so that C– N is more stable, it is difficult to decompose C– N during the laser annealing process that prevent sp2 non-crystal carbon in DLC films from graphitizing. On the other hand, the content of C –H decreases drastically in a-C/ H(N) films as shown in Fig. 2, which decreases the graphitization of films in the laser annealing process. Nitrogen introduced into the films has less effect on its mechanical property but improves the thermal stability of the films. Fig. 5 shows Raman spectra of a-C/H(N) films as function of N content (0, 3.4 and 12.6 at.%) in the films after the films were annealed by a laser beam of 1500 W/mm2 power density for 200 s. The G peak in Raman spectra from bottom to top is 1598, 1576 and 1558 cm21, respectively. This result shows further that with raising the N content, the graphitization of films is decreased and the thermal stability is improved.
4. Conclusions Comparing the results mentioned earlier, we can get the conclusions as follows: C– N bond which is more stable than C – H bond is difficult to be decomposed; nitrogen introduced into the films limits the formation of C – H which leads to decrease in graphitization of films caused by decomposition of C –H; so with the increment of N content, the graphitization of films is decreased and the thermal stability is improved.
References Fig. 4. Raman spectra of a-C/H(N) films with N content of 12.6 at.% as function of power density of Arþ laser.
[1] R.O. Dillon, J.A. Woollam, Phys. Rev. B 29 (1984) 3482.
100
W. Zhang et al. / Solid State Communications 123 (2002) 97–100
[2] L.A. Farrow, B.J. Willkens, A.S. Gozdz, D.L. Hart, Phys. Rev. B 41 (1990) 10132. [3] J. Gonzalez-Hernandez, B.S. Gozdz, D.A. Pawlik, Mater. Sci. Forum 52 and 53 (1989) 543. [4] M. Bowden, D.J. Garding, J.M. Southall, J. Appl. Phys. 71 (1992) 521. [5] G. Mariotto, F.L. Freire Jr., C.A. Achete, Thin Solid Films 241 (1994) 255. [6] D.F. Fanceschini, C.A. Achete, F.L. Freire Jr., Appl. Phys. Lett. 60 (1992) 3229.
[7] B. Racine, M. Benlahsen, K. Zellama, P. Goudeau, M. Zarrabian, G. Turban, Appl. Phys. Lett. 73 (1998) 3226. [8] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, B. Rafferty, L.M. Brown, J. Appl. Phys. 81 (1997) 2626. [9] R.J. Nemanich, G. Lucovsky, Solid State Commun. 23 (2) (1977) 117 –120. [10] V. Meenakshi, A. Sayeed, S.V. Subramanyam, Mater. Sci. Forum 223 –224 (1996) 307– 310.