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Structural study of plasma enhanced chemical vapour deposited silicon carbide films
Materials Science and Engineering B75 (2000) 174 – 176 www.elsevier.com/locate/mseb
Structural study of plasma enhanced chemical vapour deposited silicon carbide films W.K. Choi * , S. Gangadharan Microelectronics Laboratory, Department of Electrical Engineering, National Uni6ersity of Singapore, 4 Engineering Dri6e 3, Singapore 117576, Singapore
Abstract The effect of the molar gas ratio (X=C2H2/(C2H2 + SiH4)) and rf power on the structural properties of plasma enhanced chemical vapour deposited hydrogenated amorphous silicon carbide films has been investigated. The deposition rate was found to increase as either X or rf power increases. The value of n reduces as either X or the rf power increases. It was concluded that high carbon concentration increases the disorder in the film and increases the gap value. The IR results show SiC bond increases with an increase in rf power or a decrease in X. The value of SiH bond decreases and CH bond increases with increases in X. Annealing increases the SiC bond but reduces the SiH and CH bonds. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Chemical vapour; Annealing; Vapour deposited
1. Introduction Hydrogenated amorphous silicon carbide (a-Si1 − x Cx :H) films have been under intense research recently. This is due to the fact that the electrical and optical properties of this material can be controlled by varying the carbon, silicon and hydrogen compositions in the film. The a-Si1 − x Cx :H films can be used as window material for amorphous silicon solar cell [1], image pickup tubes and electrophotography receptors [2]. In this paper, we present the structural properties of aSi1 − x Cx :H films prepared by the plasma enhanced chemical vapour deposition (PECVD) of silane (SiH4) and acetylene (C2H2). We will examine the deposition rate and refractive index of the films as a function of the flow rates of the reacting gases. The structural properties of the as-prepared and furnace-annealed films are investigated using the infrared (IR) technique.
2. Experimental The substrate used in this work was n-type (100) * Corresponding author. Tel.: + 65-779-6473; fax: +65-779-1103. E-mail address: [email protected] (W.K. Choi)
silicon wafer of resistivity 5–8 V cm. During PECVD deposition, the chamber pressure was pumped down to 3 × 10 − 6 Torr. C2H2 and SiH4 were then admitted into the chamber and their flow rates were set to a predetermined value. The operating pressure was 0.3 Torr. In this work, two sets of samples were made by varying the flow rate of the reacting gases or the rf power. The flow rate of the reacting gases was decided by [1,3] X=
C2H2(sccm) C2H2(sccm) + SiH4(sccm)
(1)
The samples were deposited by varying X from 0.1 to 0.7 and the rf power from 100 to 300 W. The substrate temperature was kept at 250°C and the deposition time was 10 min in all cases. The thickness of the film was measured using an Alfa-Step 500 surface profiler. The deposition rate is estimated by dividing the film thickness over the deposition time. The refractive index of the samples was measured using a Gaertner Ellipsometer at a wavelength of 633 nm. The optical gap was measured using a Shimadzu UV-3101 spectrometer with the films deposited on glass substrates. The IR study on our samples was carried out using a Fourier Transform Infrared Spectrometer (Nicolet Magna IR750).
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 5 6 - 1
W.K. Choi, S. Gangadharan / Materials Science and Engineering B75 (2000) 174–176
3. Results and discussion The deposition rate of our samples increases with either an increase in X or rf power. For a fixed rf power, n reduces as X increases in all our samples. This shows that increasing rf power does not make the film silicon rich. As X increases n reaches a saturation value of about 1.9 irrespective of the rf power used. This indicates a carbon-rich film [4,5]. The optical gap (Eo) and the edge width parameter and B 1/2 of our films were obtained from the absorption data. We found that Eo increases from 2.05 to 2.52 eV and B 1/2 decreases from 500 to 100 cm − 1/2 eV − 1/2 as X increases. As Eo and B 1/2 are parameters that indicate the amount of disorder in a film [6], it means that the film becomes more disordered as carbon incorporation increases. In our IR spectra, the main absorption peak is the Si –C stretching mode at 780 – 810 cm − 1 [5]. For films prepared with X = 0.1, the intenstiy of the peak increases with rf power indicating an increased deposition of SiC with increasing rf power. This is true for samples prepared with other X values (0.3, 0.5, 0.7). This could be due to the more effective dissociation of the reaction gases and creation of highly active radicals as rf power increases. However, for the films prepared at 100 W, the peak intensity seems to reduce with an increase in X. This could be due to the lower availability of silicon, and therefore, a decrease in the incorporation of carbon in the Si–C form [6]. A shoulder appears between 900 and 980 cm − 1 for films prepared at X =0.3, 0.5 and 0.7. This corresponds to the Si – CH3 rocking or wagging modes [6,7]. Another prominent feature in the IR spectra is the Si –H stretching peak at 2000 – 2150 cm − 1. The intensity of the peak steadily decreases as X increases. The
175
C–Hn stretching mode at 2850–2950 cm − 1 is observed only for samples with X\ 0.5. Thus, it is possible that increase in C–H bonds is reponsible for the reduction in Si–H bonding as X increases [8].The bond density of Si–C (NSi – C), Si–H (NSi – H) and C–H (NC – H) of the films were estimated from the absorption spectra as NSi − C,Si − H,C − H = A
&
a(v) d v v
(2)
where a(v) is the absorption coefficient and v is the wavelength. The values of A for Si–C, Si–H and C–H are 2.13 × 1019, 1.4 × 1020 and 1.35 × 1021 cm − 3, respectively. Figs. 1 and 2 show the variation of NSi – C and NSi – H as a function of annealing temperature for our samples. For the as-deposited samples, it is obvious that NSi – C increases as rf power increases (see Fig. 1a) and decreases as X increases (see Fig. 1b). The value of NSi – H of as-deposited samples in Fig. 2b reduces as X increases. We have deconvoluted the C= C bonds (1540 and at 1630 cm − 1), the Si–CH3 rocking and wagging mode (900–980 cm − 1), the Si–H2 stretching (2090 cm − 1) and the SiH3 stretching (2140 cm − 1) modes from the spectra. None of these modes show a consistent trend with either rf power or X. Note that in Fig. 1a, the NSi – C value for the annealed samples prepared with X=0.1 increases with annealing temperature. Choi et al.[9] have also reported an increase in NSi – C in furnace annealed rf sputtered amorphous silicon carbide films. We have observed a substantial increase in SiO bonds (1024 cm − 1) for films annealed at 800°C. This agrees with the published results [9,10]. For films deposited at 100 W but with different X values (see Fig. 1b), there seems to be very little variation in NSi – C as a function of annealing temperature.
Fig. 1. NSiC versus annealing temperature plots for as-prepared and furnace-annealed a-Si1 − x Cx :H films deposited at (a) X =0.1 and different rf power (b) 100 W and different X values.
176
W.K. Choi, S. Gangadharan / Materials Science and Engineering B75 (2000) 174–176
Fig. 2. NSiH versus annealing temperture plots for as-prepared and furnace-annealed a-Si1 − x Cx :H films deposited at (a) X =0.1 and different rf power (b) 100 W and different X values.
The value of NSi – H in Fig. 2a, b reduces with increasing annealing temperature. This is expected as hydrogen effuses out of the film with increase in annealing temperatures [11]. Note that NSi – H increases for films deposited at 100 W annealed at 400°C (see Fig. 2a). However, no increase in NSi – H was observed at 400oC for films deposited at 200 and 300 W. The SiH bonds virtually disappear at 800°C. We observed blisters and cracking of the films for films annealed at 800°C.
4. Conclusions The influence of X and rf power on the deposition rate and refractive index of PECVD a-Si1 − x Cx :H films has been investigated. The deposition rate of our films increases and the refractive index reduces as X or rf power increases. The IR results show that NSi – C increases with an increase in rf power or a decrease in X. NSi – H decreases and NC – H increases with increases in X. Annealing increases NSi – C and reduces NSi – H and NC – H.
Acknowledgements The authors wish to thank Walter Lim for his help with
.
the preparation of samples. We would also like to thank the University for a research scholarship (S.G.) and the National Science and Technology Board for a research grant (No. GR6471) for this work.
References [1] Y. Hamakawa, Proc. Mater. Res. Soc. Symp. 49 (1985) 239. [2] I. Shimizu, J. Non-Cryst. Sol. 77/78 (1985) 363. [3] S. Akita, K. Wakita, Y. Nakayama, T. Kawamura, in: G.L. Harris, C.Y.-W. Yang (Eds.), Proceedings in Physics, vol. 34, Springer, Berlin, 1987, p. 77. [4] D. Kuhman, S. Grammatica, F. Jansen, Thin Sol. Films 166 (1988) 253. [5] W.K. Choi, Y.M. Chan, C.H. Ling, Y. Lee, R. Gopalakrishnan, K.L. Tan, J. Appl. Phys. 77 (2) (1995) 827. [6] D. Della Sala, P. Fiorini, A. Frova, A. Gregori, A. Skumanich, N.M. Amer, J. Non-Cryst. Sol. 77/78 (1985) 853. [7] D.R. Mackenzie, J. Phys. D: Appl. Phys. 19 (1985) 1935. [8] J. Saraie, Y. Fujii, M. Yoshimoto, K. Yamazoe, H. Matsumani, Thin Sol. Films 117 (1984) 59. [9] W.K. Choi, T.Y. Ong, L.S. Tan, F.C. Loh, K.L. Tan, J. Appl. Phys. 83 (9) (1998) 4968. [10] S. Ray, D. Das, A.K. Barua, Sol. Energy Mater. 15 (1987) 45. [11] F. Demichelis, C.F. Pirri, E. Tresso, V. Rigato, G. Della Mea, J. Non-Cryst. Sol. 128 (1991) 133.
Structural study of plasma enhanced chemical vapour deposited silicon carbide films W.K. Choi * , S. Gangadharan Microelectronics Laboratory, Department of Electrical Engineering, National Uni6ersity of Singapore, 4 Engineering Dri6e 3, Singapore 117576, Singapore
Abstract The effect of the molar gas ratio (X=C2H2/(C2H2 + SiH4)) and rf power on the structural properties of plasma enhanced chemical vapour deposited hydrogenated amorphous silicon carbide films has been investigated. The deposition rate was found to increase as either X or rf power increases. The value of n reduces as either X or the rf power increases. It was concluded that high carbon concentration increases the disorder in the film and increases the gap value. The IR results show SiC bond increases with an increase in rf power or a decrease in X. The value of SiH bond decreases and CH bond increases with increases in X. Annealing increases the SiC bond but reduces the SiH and CH bonds. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Chemical vapour; Annealing; Vapour deposited
1. Introduction Hydrogenated amorphous silicon carbide (a-Si1 − x Cx :H) films have been under intense research recently. This is due to the fact that the electrical and optical properties of this material can be controlled by varying the carbon, silicon and hydrogen compositions in the film. The a-Si1 − x Cx :H films can be used as window material for amorphous silicon solar cell [1], image pickup tubes and electrophotography receptors [2]. In this paper, we present the structural properties of aSi1 − x Cx :H films prepared by the plasma enhanced chemical vapour deposition (PECVD) of silane (SiH4) and acetylene (C2H2). We will examine the deposition rate and refractive index of the films as a function of the flow rates of the reacting gases. The structural properties of the as-prepared and furnace-annealed films are investigated using the infrared (IR) technique.
2. Experimental The substrate used in this work was n-type (100) * Corresponding author. Tel.: + 65-779-6473; fax: +65-779-1103. E-mail address: [email protected] (W.K. Choi)
silicon wafer of resistivity 5–8 V cm. During PECVD deposition, the chamber pressure was pumped down to 3 × 10 − 6 Torr. C2H2 and SiH4 were then admitted into the chamber and their flow rates were set to a predetermined value. The operating pressure was 0.3 Torr. In this work, two sets of samples were made by varying the flow rate of the reacting gases or the rf power. The flow rate of the reacting gases was decided by [1,3] X=
C2H2(sccm) C2H2(sccm) + SiH4(sccm)
(1)
The samples were deposited by varying X from 0.1 to 0.7 and the rf power from 100 to 300 W. The substrate temperature was kept at 250°C and the deposition time was 10 min in all cases. The thickness of the film was measured using an Alfa-Step 500 surface profiler. The deposition rate is estimated by dividing the film thickness over the deposition time. The refractive index of the samples was measured using a Gaertner Ellipsometer at a wavelength of 633 nm. The optical gap was measured using a Shimadzu UV-3101 spectrometer with the films deposited on glass substrates. The IR study on our samples was carried out using a Fourier Transform Infrared Spectrometer (Nicolet Magna IR750).
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 5 6 - 1
W.K. Choi, S. Gangadharan / Materials Science and Engineering B75 (2000) 174–176
3. Results and discussion The deposition rate of our samples increases with either an increase in X or rf power. For a fixed rf power, n reduces as X increases in all our samples. This shows that increasing rf power does not make the film silicon rich. As X increases n reaches a saturation value of about 1.9 irrespective of the rf power used. This indicates a carbon-rich film [4,5]. The optical gap (Eo) and the edge width parameter and B 1/2 of our films were obtained from the absorption data. We found that Eo increases from 2.05 to 2.52 eV and B 1/2 decreases from 500 to 100 cm − 1/2 eV − 1/2 as X increases. As Eo and B 1/2 are parameters that indicate the amount of disorder in a film [6], it means that the film becomes more disordered as carbon incorporation increases. In our IR spectra, the main absorption peak is the Si –C stretching mode at 780 – 810 cm − 1 [5]. For films prepared with X = 0.1, the intenstiy of the peak increases with rf power indicating an increased deposition of SiC with increasing rf power. This is true for samples prepared with other X values (0.3, 0.5, 0.7). This could be due to the more effective dissociation of the reaction gases and creation of highly active radicals as rf power increases. However, for the films prepared at 100 W, the peak intensity seems to reduce with an increase in X. This could be due to the lower availability of silicon, and therefore, a decrease in the incorporation of carbon in the Si–C form [6]. A shoulder appears between 900 and 980 cm − 1 for films prepared at X =0.3, 0.5 and 0.7. This corresponds to the Si – CH3 rocking or wagging modes [6,7]. Another prominent feature in the IR spectra is the Si –H stretching peak at 2000 – 2150 cm − 1. The intensity of the peak steadily decreases as X increases. The
175
C–Hn stretching mode at 2850–2950 cm − 1 is observed only for samples with X\ 0.5. Thus, it is possible that increase in C–H bonds is reponsible for the reduction in Si–H bonding as X increases [8].The bond density of Si–C (NSi – C), Si–H (NSi – H) and C–H (NC – H) of the films were estimated from the absorption spectra as NSi − C,Si − H,C − H = A
&
a(v) d v v
(2)
where a(v) is the absorption coefficient and v is the wavelength. The values of A for Si–C, Si–H and C–H are 2.13 × 1019, 1.4 × 1020 and 1.35 × 1021 cm − 3, respectively. Figs. 1 and 2 show the variation of NSi – C and NSi – H as a function of annealing temperature for our samples. For the as-deposited samples, it is obvious that NSi – C increases as rf power increases (see Fig. 1a) and decreases as X increases (see Fig. 1b). The value of NSi – H of as-deposited samples in Fig. 2b reduces as X increases. We have deconvoluted the C= C bonds (1540 and at 1630 cm − 1), the Si–CH3 rocking and wagging mode (900–980 cm − 1), the Si–H2 stretching (2090 cm − 1) and the SiH3 stretching (2140 cm − 1) modes from the spectra. None of these modes show a consistent trend with either rf power or X. Note that in Fig. 1a, the NSi – C value for the annealed samples prepared with X=0.1 increases with annealing temperature. Choi et al.[9] have also reported an increase in NSi – C in furnace annealed rf sputtered amorphous silicon carbide films. We have observed a substantial increase in SiO bonds (1024 cm − 1) for films annealed at 800°C. This agrees with the published results [9,10]. For films deposited at 100 W but with different X values (see Fig. 1b), there seems to be very little variation in NSi – C as a function of annealing temperature.
Fig. 1. NSiC versus annealing temperature plots for as-prepared and furnace-annealed a-Si1 − x Cx :H films deposited at (a) X =0.1 and different rf power (b) 100 W and different X values.
176
W.K. Choi, S. Gangadharan / Materials Science and Engineering B75 (2000) 174–176
Fig. 2. NSiH versus annealing temperture plots for as-prepared and furnace-annealed a-Si1 − x Cx :H films deposited at (a) X =0.1 and different rf power (b) 100 W and different X values.
The value of NSi – H in Fig. 2a, b reduces with increasing annealing temperature. This is expected as hydrogen effuses out of the film with increase in annealing temperatures [11]. Note that NSi – H increases for films deposited at 100 W annealed at 400°C (see Fig. 2a). However, no increase in NSi – H was observed at 400oC for films deposited at 200 and 300 W. The SiH bonds virtually disappear at 800°C. We observed blisters and cracking of the films for films annealed at 800°C.
4. Conclusions The influence of X and rf power on the deposition rate and refractive index of PECVD a-Si1 − x Cx :H films has been investigated. The deposition rate of our films increases and the refractive index reduces as X or rf power increases. The IR results show that NSi – C increases with an increase in rf power or a decrease in X. NSi – H decreases and NC – H increases with increases in X. Annealing increases NSi – C and reduces NSi – H and NC – H.
Acknowledgements The authors wish to thank Walter Lim for his help with
.
the preparation of samples. We would also like to thank the University for a research scholarship (S.G.) and the National Science and Technology Board for a research grant (No. GR6471) for this work.
References [1] Y. Hamakawa, Proc. Mater. Res. Soc. Symp. 49 (1985) 239. [2] I. Shimizu, J. Non-Cryst. Sol. 77/78 (1985) 363. [3] S. Akita, K. Wakita, Y. Nakayama, T. Kawamura, in: G.L. Harris, C.Y.-W. Yang (Eds.), Proceedings in Physics, vol. 34, Springer, Berlin, 1987, p. 77. [4] D. Kuhman, S. Grammatica, F. Jansen, Thin Sol. Films 166 (1988) 253. [5] W.K. Choi, Y.M. Chan, C.H. Ling, Y. Lee, R. Gopalakrishnan, K.L. Tan, J. Appl. Phys. 77 (2) (1995) 827. [6] D. Della Sala, P. Fiorini, A. Frova, A. Gregori, A. Skumanich, N.M. Amer, J. Non-Cryst. Sol. 77/78 (1985) 853. [7] D.R. Mackenzie, J. Phys. D: Appl. Phys. 19 (1985) 1935. [8] J. Saraie, Y. Fujii, M. Yoshimoto, K. Yamazoe, H. Matsumani, Thin Sol. Films 117 (1984) 59. [9] W.K. Choi, T.Y. Ong, L.S. Tan, F.C. Loh, K.L. Tan, J. Appl. Phys. 83 (9) (1998) 4968. [10] S. Ray, D. Das, A.K. Barua, Sol. Energy Mater. 15 (1987) 45. [11] F. Demichelis, C.F. Pirri, E. Tresso, V. Rigato, G. Della Mea, J. Non-Cryst. Sol. 128 (1991) 133.