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Influence of substrate bias on the composition of SiC thin films fabricated by PECVD and underlying mechanism
Surface & Coatings Technology 201 (2007) 6777 – 6780 www.elsevier.com/locate/surfcoat
Influence of substrate bias on the composition of SiC thin films fabricated by PECVD and underlying mechanism M. Wang a,⁎, X.G. Diao a , A.P. Huang b , Paul K. Chu b , Z. Wu c b
a Department of Physics, Beihang University, Beijing 100083, China Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China c School of Aeronautic Science and Technology, BeiHang University, Beijing 100083, China
Available online 9 November 2006
Abstract The influence of the substrate bias on the composition of SiC thin films synthesized by plasma-enhanced chemical vapor deposition was studied. Our results indicate that the ratio of Si to C in the thin films is almost stoichiometric at a bias of −300 V, whereas excessive carbon is observed in the films if the bias is lower or higher. Very little oxygen can be detected in the film produced without biasing. The effects of the bias on the composition of the thin films can be attributed to the interaction between the positive ions in the plasma and the surface atoms. The underlying mechanism is also discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Hd; 68.55.-a; 81.15.Gh Keywords: SiC; PECVD; Substrate bias; Film
1. Introduction Silicon carbide (SiC) as a wide bandgap semiconductor is recognized to be a suitable material in elevated-temperature, high-power electronic applications due to its excellent properties such as high breakdown voltage, electron mobility, thermal conductivity, and hardness [1,2]. Several methods have been explored to obtain high-quality SiC thin films on silicon substrates [3]. Plasma-enhanced chemical deposition (PECVD) is a versatile and well-established technology. The technique offers the possibility to design new structures and to change the properties associated with the microstructure of the thin films by changing the synthesis parameters [4]. SiC thin films are typically synthesized by PECVD at high temperature, and it may lead to high tensile stress and lattice defects in the films. Recently, a new method, substrate bias-assisted deposition, can effectively decrease the deposition temperature and improve the quality of the crystalline SiC thin films [5,6]. In this work, the influence of the substrate bias on the composition of the thin films is studied in detail. Our results ⁎ Corresponding author. Fax: +86 10 62080871. E-mail address: [email protected] (M. Wang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.065
indicate that the Si to C ratio in the thin film fabricated at a bias of − 300 V is nearly stoichiometric, whereas excessive carbon is observed in the thin films prepared with a lower or higher bias. In addition, very little oxygen is detected in the thin film synthesized without a bias. Our study suggests that substrate bias-assisted PECVD is an effective method to improve the composition and microstructure of the SiC thin films. 2. Experimental details SiC thin films were fabricated on n-type, 100 mm Si (100) wafers with resistivity of 4–7 Ω cm using a custom plasmaenhanced chemical vapor deposition (PECVD) instrument with a radio frequency (RF) source operated at a frequency of 13.56 MHz and power of 200 W. Prior to deposition, the substrate was etched in situ using hydrogen plasma for 10 min at 900 °C to eliminate surface oxygen. CH4 (99.999%) and SiH4 (99.999%) were used as the reactive gases, and H2 (99.999%) was introduced as the dilution gas. During deposition, the flow rates of CH4, SiH4, and H2 were fixed at 3, 3 and 50 sccm, respectively. The base pressure is 3 × 10− 3 Pa, the operating pressure was 30 Pa, and the period of 2 h was applied in the deposition process. A negative direct current (DC) bias ranging
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from 0 to − 450 V was applied to the substrate. The substrate temperature was 500 °C, which is rather low compared to the temperature used to deposit polycrystalline SiC thin films by conventional PECVD [7]. Rutherford backscattering spectrometry (RBS) was carried out using a 2-MeV 4He++ beam and a backscattering angle of 170° to determine the composition as well as the thickness of the thin films. Microstructural analyses were carried out using a Xian Fourier transform infrared (FTIR) spectrometer and Philips X-ray diffractometer in a θ–2θ configuration and Cu Kα radiation. The thickness of the as-deposited SiC thin films was measured by a Seimitzu Surfcom 480A profiler and the surface morphology was evaluated by contact-mode atomic force microscopy (AFM). Si, C and O bonding information was acquired using X-ray photoelectron spectroscopy (XPS) employing monochromatic Al Kα radiation. Prior to the XPS analyses, the sample surface was cleaned by 4 keV Ar ion bombardment for 1 min to remove atmospheric contaminants.
Fig. 2. Ratios of Si to C and thicknesses of the SiC thin films prepared at different substrate biases.
The elemental composition of the thin film influences the structure and properties of the thin films and RBS was used to characterize the materials, which can precisely detect the elemental composition in the sample [8]. Fig. 1 displays the RBS spectra that contain both the experimental and fitted results acquired from the sample produced by different substrate biases. The results indicate that only Si and C exist in the thin films and that the composition of the thin films produced under different substrate biases is basically uniform throughout the thickness. Furthermore, the Si to C ratio is smaller than 1 which means excessive carbon in all the samples as shown in Fig. 2 by deconvoluting the RBS data. It is believed to be due to the different diffusion speeds of Si and C. That is, Si diffuses faster than C making Si easier to bind with C atoms to form SiC. Meanwhile, a small amount of C–C bonds (amorphous carbon) can be detected in the thin films. It can be clearly observed in the RBS spectra that the thicknesses of the thin films under different biasing conditions are different, and the growth rate increases as
a substrate bias is applied. It is because under a negative substrate bias, SiC nucleation is increased and the diffusion and the adhesion force of the particles on the surface are enhanced, both of which are beneficial to the SiC growth [9]. Therefore, the growth rate increases with the biasing and the results are corroborated by the Seimitzu Surfcom 480A profilometry results as shown in Fig. 2. Fig. 3 shows the Fourier transform infrared (FTIR) spectra of the thin films deposited at biases from 0 to − 450 V. It can be clearly seen that all the peaks of the absorption bands are at about 800 cm− 1. It is well known that the characteristic vibration frequency of the Si–C bonds for the TO mode is around 796 cm− 1 in cubic silicon carbide (β-SiC) [10]. Our results thus indicate that the formation of Si–C bonds in the thin films, although there is a small shift to a higher wavenumber. This can be attributed to a decrease in the length of the Si–C bonds as there is compressive strain in the thin films possibly associated with the formation of nano-structures in the thin films. To further fathom the effects of the substrate bias on the composition of SiC thin films, the elemental contents and the Si
Fig. 1. RBS spectra acquired from the SiC thin films deposited at 500 °C under different substrate biases.
Fig. 3. FTIR spectra of SiC thin films prepared at different substrate biases.
3. Results and discussion
M. Wang et al. / Surface & Coatings Technology 201 (2007) 6777–6780
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and C bonding information were determined by XPS. Prior to the analysis, the sample surface was cleaned by 4 keV Ar ion bombardment for 1 min to remove atmospheric contaminants. The results show that the Si to C ratio in the thin film produced under a bias of − 300 V is nearly stoichiometric. It is in agreement with the RBS results. However, the thin films produced using lower or higher biases have excessive carbon and a small amount of oxygen can be detected from the sample prepared without a bias. Fig. 4 depicts the Si2p and C1s core level XPS spectra which exhibit apparent asymmetry. The best Gaussian fit of the Si2p spectrum acquired from the sample without biasing shows two peaks located around 100.8 eV and 101.9 eV. The former corresponds to the Si2p binding energy of SiC and the latter is believed to be that of SiOx [11]. The formation of SiOx arises from residual oxygen in the vacuum chamber. As a substrate bias is introduced, the Si2p peak becomes more symmetrical and the SiO x concentration diminishes. It means that substrate bias assistance can mitigate the formation of SiOx during deposition. The best Gaussian fitted curves of the C1s spectra under different biasing conditions are obtained with the two peaks centered at 283.4 eV and 284.9 eV. The peak at approximately 283.4 eV is due to the C–Si in SiC and that at 284.9 eV is assigned to C– C in amorphous carbon [12]. It can be clearly seen that the
Fig. 5. AFM images of SiC thin films prepared at different biases: (a) 0 V, (b) −300 V, and (c) − 450 V.
Fig. 4. XPS Si2p and C1s core level acquired from the SiC samples synthesized at different biases.
percentage of CSi–C increases with biasing, and especially at a bias of − 300 V, CC–C almost diminishes, in agreement with the RBS results. The surface morphology and roughness of the thin films determined by atomic force microscopy (AFM) in the contact mode from representative samples are displayed in Fig. 5. Uneven particles are observed on the thin films prepared without biasing as shown in Fig. 5(a). Uniform surface features are observed on the samples fabricated using substrate biasing as shown in Fig. 5(b) and (c). In particular, at a bias of − 450 V, a
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regular crystalline surface appears. It means that substrate biasing is beneficial to the formation of crystalline SiC thin films, and this behavior is similar to that with increased substrate temperature [13]. Furthermore, the surface roughness of the thin film decreases substantially with substrate biasing. It further corroborates that substrate bias assisting PECVD is an effective method to improve the composition and microstructure of SiC thin films. The above result is likely due to the negative bias attributed to the interaction of positive ions in plasma and deposited clusters on the growing surface. It leads to the enhancement of the diffusion energy of the deposited particles and relaxation of the growing surface by the collision of the accelerated positive ions in plasma. Meanwhile, it is considered that some parts of the positive ions directly become deposited particles, which are further excited by the bias. The negative substrate bias can increase the bombardment energy thereby enhancing the formation of β-SiC. The sticking probability of the particles on the surface also increases due to the formation of the active sites, and so is the mobility of the atoms, which is similar to the case with raising the substrate temperature [14]. Furthermore, although Si–C and Si–O are both covalent bonds, there is also a partial content of ionized band in the bonds of Si–C and Si–O, because of the different negative electronic affinity of Si, C and O which are 1.92, 2.54 and 3.60 eV, respectively [15]. As a result, Si presents positive, C and O negative in Si–C and Si–O bonds. There may be a local positive field around Si atoms, and a negative local field around C, and O atoms. The active ions H+ are positive, and it can induce the etching effects on the O atoms and amorphous carbon and so it is beneficial to obtain SiC films. Therefore, the bias in conjunction with the plasma hydrogenation plays the following roles: one is to induce the formation of β-SiC, and the other is to withhold the growth of the other phases like amorphous carbon and SiOx. However, excessive ion bombardment causes too much structural disorder and there exists an optimal substrate bias that provides stoichiometric SiC.
4. Conclusion Stoichiometric SiC thin films are prepared by bias-assisted PECVD on Si (100) substrates at the temperature of 500 °C. Our study suggests that substrate bias assistance in conjunction with the plasma hydrogenation is effective in promoting the formation of stoichiometric SiC and retarding the growth of amorphous carbon and oxides. Acknowledgements This work is partially supported by the project of the National Natural Science Foundation: [No. 90305026], and the Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grant (CERG) No. City U 1120/04E. References [1] H. Matsunami, S. Nishino, H. Ono, IEEE Trans. Electron Devices ED-28 (1981) 1235. [2] S. Nishino, J.A. POwll, H.A. Will, Appl. Phys. Lett. 42 (1983) 460. [3] Z. He, S. Inone, G. Garter, H. Kheyrandish, J.S. Colligon, Thin Solid Films 260 (1995) 37. [4] H. Yan, B. Wang, X.M. Song, G.H. Chen, Thin Solid Films 368 (2000) 241. [5] S.D. Wolter, B.R. Stoney, J.T. Glass, Appl. Phys. Lett. 62 (1993) 15. [6] B. Wang, W. Liu, G.J. Wang, B. Liao, J.J. Wang, M.K. Zhu, H. Wang, H. Yan, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 98 (2003) 190. [7] H. Yan, R.W.M. Kwok, S.P. Wong, Appl. Surf. Sci. 92 (1996) 76. [8] C. Jeynes, Z.H. Jafri, R.P. Webb, A.C. Kimber, M.J. Ashwin, Surf. Interface Anal. 25 (1997) 254. [9] B.B. Wang, W.L. Wang, K.J. Liao, J.L. Xiao, Phys. Rev., B 63 (2001) 085412. [10] W.G. Spitzer, D.A. Kleinman, C.J. Frosch, Phys. Rev. 113 (1959) 135. [11] A. Desalvo, F. Giorgis, C.F. Pirri, E. Tresso, P. Rava, R. Galloni, R. Rizzoli, C. Summontes, J. Appl. Phys. 81 (1997) 7973. [12] D.R. Mckenzie, J. Phys. D: Appl. Phys. 18 (1985) 1935. [13] Y.H. Lee, P.D. Richard, K.J. Bachmann, Appl. Phys. Lett. 56 (1990) 620. [14] B.R. Stoner, G.H. Ma, Phys. Rev., B 45 (1992) 11067. [15] J. Anglada, P.J. Bruna, S.D. Peyerimhoff, R.J. Buenker, J. Phys. B: Mol. Phys. 16 (1983) 2469.
Influence of substrate bias on the composition of SiC thin films fabricated by PECVD and underlying mechanism M. Wang a,⁎, X.G. Diao a , A.P. Huang b , Paul K. Chu b , Z. Wu c b
a Department of Physics, Beihang University, Beijing 100083, China Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China c School of Aeronautic Science and Technology, BeiHang University, Beijing 100083, China
Available online 9 November 2006
Abstract The influence of the substrate bias on the composition of SiC thin films synthesized by plasma-enhanced chemical vapor deposition was studied. Our results indicate that the ratio of Si to C in the thin films is almost stoichiometric at a bias of −300 V, whereas excessive carbon is observed in the films if the bias is lower or higher. Very little oxygen can be detected in the film produced without biasing. The effects of the bias on the composition of the thin films can be attributed to the interaction between the positive ions in the plasma and the surface atoms. The underlying mechanism is also discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Hd; 68.55.-a; 81.15.Gh Keywords: SiC; PECVD; Substrate bias; Film
1. Introduction Silicon carbide (SiC) as a wide bandgap semiconductor is recognized to be a suitable material in elevated-temperature, high-power electronic applications due to its excellent properties such as high breakdown voltage, electron mobility, thermal conductivity, and hardness [1,2]. Several methods have been explored to obtain high-quality SiC thin films on silicon substrates [3]. Plasma-enhanced chemical deposition (PECVD) is a versatile and well-established technology. The technique offers the possibility to design new structures and to change the properties associated with the microstructure of the thin films by changing the synthesis parameters [4]. SiC thin films are typically synthesized by PECVD at high temperature, and it may lead to high tensile stress and lattice defects in the films. Recently, a new method, substrate bias-assisted deposition, can effectively decrease the deposition temperature and improve the quality of the crystalline SiC thin films [5,6]. In this work, the influence of the substrate bias on the composition of the thin films is studied in detail. Our results ⁎ Corresponding author. Fax: +86 10 62080871. E-mail address: [email protected] (M. Wang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.065
indicate that the Si to C ratio in the thin film fabricated at a bias of − 300 V is nearly stoichiometric, whereas excessive carbon is observed in the thin films prepared with a lower or higher bias. In addition, very little oxygen is detected in the thin film synthesized without a bias. Our study suggests that substrate bias-assisted PECVD is an effective method to improve the composition and microstructure of the SiC thin films. 2. Experimental details SiC thin films were fabricated on n-type, 100 mm Si (100) wafers with resistivity of 4–7 Ω cm using a custom plasmaenhanced chemical vapor deposition (PECVD) instrument with a radio frequency (RF) source operated at a frequency of 13.56 MHz and power of 200 W. Prior to deposition, the substrate was etched in situ using hydrogen plasma for 10 min at 900 °C to eliminate surface oxygen. CH4 (99.999%) and SiH4 (99.999%) were used as the reactive gases, and H2 (99.999%) was introduced as the dilution gas. During deposition, the flow rates of CH4, SiH4, and H2 were fixed at 3, 3 and 50 sccm, respectively. The base pressure is 3 × 10− 3 Pa, the operating pressure was 30 Pa, and the period of 2 h was applied in the deposition process. A negative direct current (DC) bias ranging
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from 0 to − 450 V was applied to the substrate. The substrate temperature was 500 °C, which is rather low compared to the temperature used to deposit polycrystalline SiC thin films by conventional PECVD [7]. Rutherford backscattering spectrometry (RBS) was carried out using a 2-MeV 4He++ beam and a backscattering angle of 170° to determine the composition as well as the thickness of the thin films. Microstructural analyses were carried out using a Xian Fourier transform infrared (FTIR) spectrometer and Philips X-ray diffractometer in a θ–2θ configuration and Cu Kα radiation. The thickness of the as-deposited SiC thin films was measured by a Seimitzu Surfcom 480A profiler and the surface morphology was evaluated by contact-mode atomic force microscopy (AFM). Si, C and O bonding information was acquired using X-ray photoelectron spectroscopy (XPS) employing monochromatic Al Kα radiation. Prior to the XPS analyses, the sample surface was cleaned by 4 keV Ar ion bombardment for 1 min to remove atmospheric contaminants.
Fig. 2. Ratios of Si to C and thicknesses of the SiC thin films prepared at different substrate biases.
The elemental composition of the thin film influences the structure and properties of the thin films and RBS was used to characterize the materials, which can precisely detect the elemental composition in the sample [8]. Fig. 1 displays the RBS spectra that contain both the experimental and fitted results acquired from the sample produced by different substrate biases. The results indicate that only Si and C exist in the thin films and that the composition of the thin films produced under different substrate biases is basically uniform throughout the thickness. Furthermore, the Si to C ratio is smaller than 1 which means excessive carbon in all the samples as shown in Fig. 2 by deconvoluting the RBS data. It is believed to be due to the different diffusion speeds of Si and C. That is, Si diffuses faster than C making Si easier to bind with C atoms to form SiC. Meanwhile, a small amount of C–C bonds (amorphous carbon) can be detected in the thin films. It can be clearly observed in the RBS spectra that the thicknesses of the thin films under different biasing conditions are different, and the growth rate increases as
a substrate bias is applied. It is because under a negative substrate bias, SiC nucleation is increased and the diffusion and the adhesion force of the particles on the surface are enhanced, both of which are beneficial to the SiC growth [9]. Therefore, the growth rate increases with the biasing and the results are corroborated by the Seimitzu Surfcom 480A profilometry results as shown in Fig. 2. Fig. 3 shows the Fourier transform infrared (FTIR) spectra of the thin films deposited at biases from 0 to − 450 V. It can be clearly seen that all the peaks of the absorption bands are at about 800 cm− 1. It is well known that the characteristic vibration frequency of the Si–C bonds for the TO mode is around 796 cm− 1 in cubic silicon carbide (β-SiC) [10]. Our results thus indicate that the formation of Si–C bonds in the thin films, although there is a small shift to a higher wavenumber. This can be attributed to a decrease in the length of the Si–C bonds as there is compressive strain in the thin films possibly associated with the formation of nano-structures in the thin films. To further fathom the effects of the substrate bias on the composition of SiC thin films, the elemental contents and the Si
Fig. 1. RBS spectra acquired from the SiC thin films deposited at 500 °C under different substrate biases.
Fig. 3. FTIR spectra of SiC thin films prepared at different substrate biases.
3. Results and discussion
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and C bonding information were determined by XPS. Prior to the analysis, the sample surface was cleaned by 4 keV Ar ion bombardment for 1 min to remove atmospheric contaminants. The results show that the Si to C ratio in the thin film produced under a bias of − 300 V is nearly stoichiometric. It is in agreement with the RBS results. However, the thin films produced using lower or higher biases have excessive carbon and a small amount of oxygen can be detected from the sample prepared without a bias. Fig. 4 depicts the Si2p and C1s core level XPS spectra which exhibit apparent asymmetry. The best Gaussian fit of the Si2p spectrum acquired from the sample without biasing shows two peaks located around 100.8 eV and 101.9 eV. The former corresponds to the Si2p binding energy of SiC and the latter is believed to be that of SiOx [11]. The formation of SiOx arises from residual oxygen in the vacuum chamber. As a substrate bias is introduced, the Si2p peak becomes more symmetrical and the SiO x concentration diminishes. It means that substrate bias assistance can mitigate the formation of SiOx during deposition. The best Gaussian fitted curves of the C1s spectra under different biasing conditions are obtained with the two peaks centered at 283.4 eV and 284.9 eV. The peak at approximately 283.4 eV is due to the C–Si in SiC and that at 284.9 eV is assigned to C– C in amorphous carbon [12]. It can be clearly seen that the
Fig. 5. AFM images of SiC thin films prepared at different biases: (a) 0 V, (b) −300 V, and (c) − 450 V.
Fig. 4. XPS Si2p and C1s core level acquired from the SiC samples synthesized at different biases.
percentage of CSi–C increases with biasing, and especially at a bias of − 300 V, CC–C almost diminishes, in agreement with the RBS results. The surface morphology and roughness of the thin films determined by atomic force microscopy (AFM) in the contact mode from representative samples are displayed in Fig. 5. Uneven particles are observed on the thin films prepared without biasing as shown in Fig. 5(a). Uniform surface features are observed on the samples fabricated using substrate biasing as shown in Fig. 5(b) and (c). In particular, at a bias of − 450 V, a
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regular crystalline surface appears. It means that substrate biasing is beneficial to the formation of crystalline SiC thin films, and this behavior is similar to that with increased substrate temperature [13]. Furthermore, the surface roughness of the thin film decreases substantially with substrate biasing. It further corroborates that substrate bias assisting PECVD is an effective method to improve the composition and microstructure of SiC thin films. The above result is likely due to the negative bias attributed to the interaction of positive ions in plasma and deposited clusters on the growing surface. It leads to the enhancement of the diffusion energy of the deposited particles and relaxation of the growing surface by the collision of the accelerated positive ions in plasma. Meanwhile, it is considered that some parts of the positive ions directly become deposited particles, which are further excited by the bias. The negative substrate bias can increase the bombardment energy thereby enhancing the formation of β-SiC. The sticking probability of the particles on the surface also increases due to the formation of the active sites, and so is the mobility of the atoms, which is similar to the case with raising the substrate temperature [14]. Furthermore, although Si–C and Si–O are both covalent bonds, there is also a partial content of ionized band in the bonds of Si–C and Si–O, because of the different negative electronic affinity of Si, C and O which are 1.92, 2.54 and 3.60 eV, respectively [15]. As a result, Si presents positive, C and O negative in Si–C and Si–O bonds. There may be a local positive field around Si atoms, and a negative local field around C, and O atoms. The active ions H+ are positive, and it can induce the etching effects on the O atoms and amorphous carbon and so it is beneficial to obtain SiC films. Therefore, the bias in conjunction with the plasma hydrogenation plays the following roles: one is to induce the formation of β-SiC, and the other is to withhold the growth of the other phases like amorphous carbon and SiOx. However, excessive ion bombardment causes too much structural disorder and there exists an optimal substrate bias that provides stoichiometric SiC.
4. Conclusion Stoichiometric SiC thin films are prepared by bias-assisted PECVD on Si (100) substrates at the temperature of 500 °C. Our study suggests that substrate bias assistance in conjunction with the plasma hydrogenation is effective in promoting the formation of stoichiometric SiC and retarding the growth of amorphous carbon and oxides. Acknowledgements This work is partially supported by the project of the National Natural Science Foundation: [No. 90305026], and the Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grant (CERG) No. City U 1120/04E. References [1] H. Matsunami, S. Nishino, H. Ono, IEEE Trans. Electron Devices ED-28 (1981) 1235. [2] S. Nishino, J.A. POwll, H.A. Will, Appl. Phys. Lett. 42 (1983) 460. [3] Z. He, S. Inone, G. Garter, H. Kheyrandish, J.S. Colligon, Thin Solid Films 260 (1995) 37. [4] H. Yan, B. Wang, X.M. Song, G.H. Chen, Thin Solid Films 368 (2000) 241. [5] S.D. Wolter, B.R. Stoney, J.T. Glass, Appl. Phys. Lett. 62 (1993) 15. [6] B. Wang, W. Liu, G.J. Wang, B. Liao, J.J. Wang, M.K. Zhu, H. Wang, H. Yan, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 98 (2003) 190. [7] H. Yan, R.W.M. Kwok, S.P. Wong, Appl. Surf. Sci. 92 (1996) 76. [8] C. Jeynes, Z.H. Jafri, R.P. Webb, A.C. Kimber, M.J. Ashwin, Surf. Interface Anal. 25 (1997) 254. [9] B.B. Wang, W.L. Wang, K.J. Liao, J.L. Xiao, Phys. Rev., B 63 (2001) 085412. [10] W.G. Spitzer, D.A. Kleinman, C.J. Frosch, Phys. Rev. 113 (1959) 135. [11] A. Desalvo, F. Giorgis, C.F. Pirri, E. Tresso, P. Rava, R. Galloni, R. Rizzoli, C. Summontes, J. Appl. Phys. 81 (1997) 7973. [12] D.R. Mckenzie, J. Phys. D: Appl. Phys. 18 (1985) 1935. [13] Y.H. Lee, P.D. Richard, K.J. Bachmann, Appl. Phys. Lett. 56 (1990) 620. [14] B.R. Stoner, G.H. Ma, Phys. Rev., B 45 (1992) 11067. [15] J. Anglada, P.J. Bruna, S.D. Peyerimhoff, R.J. Buenker, J. Phys. B: Mol. Phys. 16 (1983) 2469.