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Characterization of air-exposed surface of β-FeSi2 fabricated by ion beam sputter deposition method
Nuclear Instruments and Methods in Physics Research B 206 (2003) 321–325 www.elsevier.com/locate/nimb
Characterization of air-exposed surface of b-FeSi2 fabricated by ion beam sputter deposition method T. Saito a, H. Yamamoto a, K. Yamaguchi a, T. Nakanoya a, K. Hojou a,*, M. Haraguchi b, M. Imamura c, N. Matsubayashi c, T. Tanaka c, H. Shimada
c
a
Japan Atomic Energy Research Institute, 2-4 Shirakatashirane, Tokai, Naka, 319-1195 Ibaraki, Japan Graduate School of Science and Engineering, Ibaraki University, 2-1-1, Bunkyo, Mito, 310-8512 Ibaraki, Japan National Institute of Advanced Industrial Science and Technology, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b c
Abstract The surface composition and chemical states of b-FeSi2 on a Si(1 0 0) substrate after exposure to the ambient atmosphere was investigated by using X-ray photoelectron spectroscopy with synchrotron radiation (SR-XPS). The analysis revealed that about 70% of the Si substrate was covered with b-FeSi2 . In addition, it was found that a SiO2 overlayer with a thickness of about 1 nm covered the whole surface of the sample. These results suggest that Si was segregated during the IBSD process and that the readily formed SiO2 layer behaved as a protective layer against further oxidation of b-FeSi2 surface. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 73.20.)j; 73.61.Le Keywords: Iron silicide; Ion beam sputter deposition; Synchrotron radiation; XPS; Non-destructive; Depth profiling analysis
1. Introduction b-FeSi2 has many unique electronic properties such as a narrow direct band gap of 0.85 eV [1]. In addition, this silicide does not contain harmful elements. From these points of view, b-FeSi2 is one of the suitable candidates for optical and electric devices in the near future. In our previous study [2], we have obtained a highly oriented b-FeSi2 to h1 0 0i direction on a Si(1 0 0) surface by ion beam sputter deposition
*
Corresponding author. Tel.: +81-29282-5479; fax: +81-29282-6716. E-mail address: [email protected] (K. Hojou).
(IBSD) method. We also reported that the surface of the b-FeSi2 was stable in the ambient atmosphere [3]. From this result we have speculated that the surface of b-FeSi2 was covered with a thin oxide layer that behaves as a protective layer against further oxidation. However, detailed discussion could not be conducted because the oxide layer was supposed to be very thin (1 nm). X-ray photoelectron spectroscopy (XPS) has often been used for the determination of surface composition and chemical states of many materials. However, information depth of conventional XPS analyses with an MgKa or AlKa anode is relatively large (2 nm). Because of recent developments of the synchrotron radiation (SR) instrumentation, X-rays with the energy range of
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00754-7
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T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
200–1000 eV have become available. The tunability of the excitation energy enables us to perform surface sensitive XPS analysis and non-destructive depth profiling analysis [4]. In the present study we have analyzed the surface compositions and chemical states of an airexposed surface of a b-FeSi2 formed by IBSD by using of SR-XPS. The in-depth profiles of Fe/Si and SiO2 /Si ratios were obtained and the thickness and structure of the surface SiO2 layer were discussed.
2. Experimental The XPS analysis was performed at the beamline 13C [5] of the Photon Factory (PF). The XPS spectra were recorded with a hemispherical electron analyzer (PHI1600C). The photoelectron take-off angle was fixed at 54.7° relative to the surface normal. The substrate used was a n-type CZ-Si(1 0 0) wafer. Before Fe deposition, the native oxide layer of the substrate was removed by the annealing at 773 and 1073 K for 1 h followed flashing at 1273 K for 3 min. In the IBSD method Arþ ions with an acceleration voltage of 35 kV were bombarded into an Fe target (99.998 at.%). The sputtered Fe atoms with a deposition rate of 0.09 nm/s were deposited on the Si(1 0 0) substrate at a substrate temperature of 973 K for 60 min. The pressure in the IBSD chamber during the deposition was kept below 3 105 Pa. The details of the IBSD apparatus were described elsewhere [2]. After the fabrication of b-FeSi2 , the sample was taken out of the vacuum chamber and kept in the ambient atmosphere for two weeks before the XPS analysis.
3. Results and discussion Fig. 1 shows the Fe 2p(A) and Si 2p(B) XPS spectra of b-FeSi2 on Si(1 0 0) after exposure to the ambient atmosphere. To record spectra with similar information depth, the excitation energies were set at 890 eV (a), 1050 eV (b) and 1250 eV (c) for Fe 2p spectra and 270 eV (a), 450 eV (b) and 690 eV (c) for Si 2p spectra, respectively. All the Fe 2p
Fig. 1. Fe 2p (A) and Si 2p (B) XPS spectra of air-exposed bFeSi2 on Si(1 0 0) surface. Excitation energies used were 890 eV (a), 1050 eV (b) and 1250 eV (c) for Fe2p spectra and 270 eV (a), 450 eV (b) and 690 eV (c) for Si 2p spectra.
spectra exhibited two sharp features, which were assigned to Fe 2p3=2 and 2p1=2 features of b-FeSi2 [6]. A small feature at about 730 eV was assigned to a characteristic plasmon feature of b-FeSi2 [6]. With increasing the excitation energy (a ! c), the intensity of a small feature at about 710 eV is decreased. This feature was assigned to Fe 2p3=2 of Fe oxide. This result suggests that surface Fe was partly oxidized but the change was relatively small. In each Si 2p spectrum (B), two features at about 103 and 99 eV were observed. These were assigned to Si 2p of SiO2 and b-FeSi2 . With increasing the excitation energy (a ! c) the intensity of the feature assigned to SiO2 is decreased. This result suggests that Si was oxidized to a higher degree than Fe during the exposure to air. In the previous studies [2,3], we investigated the surface structure of b-FeSi2 on a Si substrate by using scanning electron microscope (SEM) and transmission electron microscope (TEM). The results showed that about 75% of the substrate surface was covered with b-FeSi2 islands. Therefore, two oxidation processes can be deduced from the present results; one is that a thin uniform SiO2 layer covered the almost entire surface of the sample (model 1 in Fig. 2) and the other is that
T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
only the exposed Si substrate surface was oxidized (model 2 in Fig. 2). To clarify the oxidation phenomena, we have performed non-destructive depth profiling analysis
Fig. 2. Schematic structural models of the sample surface. (A) Model 1, (B) model 2.
Fig. 3. Comparison of experimentally obtained SiSiO2 =ðSiSi þ SiFeSi2 Þ, Fe/SiTotal and Fe/SiSiO2 ratios with simulated profiles as a function of excitation energy.
323
by using SR-XPS. Fig. 3 shows excitation energy dependence of the SiSiO2 =ðSiSi þ SiFeSi2 Þ, Fe/SiTotal and Fe/SiSiO2 ratios of the oxidized sample. The Fe/ SiTotal atomic ratios were calculated using the peak areas and photo-ionization cross-sections of Fe 3p and Si 2p spectra at each excitation energy [7]. The peak areas of SiSiO2 and SiSi were calculated by deconvoluting each Si 2p spectrum by a leastsquare curve-fitting method using mixed Gaussian–Lorentzian envelopes after subtracting a Shirley-type background [8]. The feature assigned to the b-FeSi2 could not be separated from that assigned to the Si substrate because the chemical shift of b-FeSi2 from Si is relatively small (0.3 eV) [6]. In Fig. 3, the Fe/SiTotal ratio decreased with decreasing the excitation energy. This indicates that the atomic concentration of Si near the uppermost surface was larger than that in the bulk. The SiSiO2 =ðSiSi þ SiFeSi2 Þ ratio steeply increased with decreasing the excitation energy suggesting the existence of a thin surface SiO2 layer on the sample. The analysis with the lowest excitation energy gave almost the same Fe/SiTotal value as Fe/ SiSiO2 . This indicates that almost all of the Si at the uppermost surface of the sample is present as SiO2 . In order to discuss in detail, the experimental results in Fig. 3 were compared with simulated profiles using model 1 and model 2 shown in Fig. 2. In the simulations, following two assumptions were made [4]; (1) attenuation length of the photoelectrons is identified by inelastic mean free path (IMFP), (2) attenuation of photoelectrons is exponential in the escape direction. The IMFP value of the photoelectrons at each excitation energy was calculated using the theoretical equation in the literature [9]. The each ratio at each excitation energy (hm) in each model can be given by the equations shown in Appendix A. The simulated profiles using model 1 (solid lines) with a parameter set of a ¼ 0:7 and t ¼ 0:85 nm (solid lines) are consistent with the experimental data, whereas simulation using model 2 with a parameter set of a ¼ 0:3 and t ¼ 1:2 nm (dashed lines) does not give consistent results with the experimental data. Note that the coverage of b-FeSi2 in model 1 (a ¼ 0:7) agreed with that
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obtained in a previous study (0.75) [3]. From these results, we conclude that a thin SiO2 over-layer covers not only the exposed region of the Si substrate surface but also the b-FeSi2 surface. The results in Fig. 1 indicated that only Si is oxidized after exposure to the atmosphere. This suggests that a thin surface Si layer is formed during IBSD and protects the surface of b-FeSi2 from being oxidized. Alvarez et al. [10] investigated the fabrication process of b-FeSi2 on a Si(1 1 1) surface using ultra-violet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). They reported that the b-FeSi2 surface fabricated on a Si substrate was covered with a Si layer by annealing at a high temperature such as 973 K. This is consistent with our discussion; the segregation of Si from b-FeSi2 took place during the IBSD process by surface diffusion and the segregated Si at the uppermost surface was subsequently oxidized. The SiO2 layer at the uppermost surface behaved as a protective layer for further oxidation due to the dense structure that prevents inward diffusion of oxygen.
Appendix A A.1. Model 1
SiSiO2 SiSi þ SiFeSi2 hm Z t ¼ expð z=kSiO2 sin hÞdz 0 Z 1 ð1 aÞexpð t=kSiO2 sin hÞexpð ðz tÞ t Z 1 2a=3expð t=kSiO2 sin hÞ =kSi sin hÞdz þ t expð ðz tÞ=kFeSi2 sin hÞdz
¼ kSiO2 ½1 expð t=kSiO2 sin hÞ ðkSi ½ð1 aÞexpð t=kSiO2 sin hÞ þ kFeSi2 ½2a=3 expð t=kSiO2 sin hÞ Þ;
Fe SiTotal Z ¼
hm 1
a=3 expð z=kSiO2 sin hÞ expð ðz tÞ Z 1 =kFeSi2 sin hÞ dz ð1 aÞ expð t t
t
=kSiO2 sin hÞ expð ðz tÞ=kSi sin hÞ dz Z t expð z=kSiO2 sin hÞ dz þ Z0 1 2a=3 expð t=kSiO2 sin hÞ þ t expð ðz tÞ=kFeSi2 sin hÞ dz
4. Conclusion The surface composition and chemical states of a b-FeSi2 fabricated on a Si(1 0 0) substrate by IBSD were investigated. The SR-XPS analysis revealed that b-FeSi2 covered about 70% of the Si(1 0 0) substrate. This was quite consistent with our previous result (75%) obtained by using SEM and TEM [3]. It was also found that a SiO2 overlayer with a thickness of about 1 nm covered the entire surface of the sample including the b-FeSi2 surface. Si was presumably segregated during the IBSD process and the readily formed SiO2 layer behaved as a protective layer against oxidation of b-FeSi2 surface.
¼ kFeSi2 ½a=3 expð t=kSiO2 sin hÞ ðkSi ½ð1 aÞ expð t=kSiO2 sin hÞ þ kSiO2 ½1 expð t=kSiO2 sin hÞ þ kFeSi2 ½2a=3 expð t=kSiO2 sin hÞ Þ;
Fe SiSiO2 Z ¼
hm 1
a=3 expð z=kSiO2 sin hÞ expð ðz tÞ Z t =kFeSi2 sin hÞ dz expð z=kSiO2 sin hÞ dz t
Acknowledgements
0
This work was done under the approval of the PF Advisory Committee (no. 2002G280).
kFeSi2 ½a=3 expðt=kSiO2 sin hÞ ¼ : kSiO2 ½1 expðt=kSiO2 sin hÞ
T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
A.2. Model 2
SiSiO2 SiSi þ SiFeSi2 hm Z t ¼ ð1 aÞ expð z=kSiO2 sin hÞdz 0 Z 1 ð1 aÞ expð t=kSiO2 sin hÞ expð ðz tÞ t Z 1 =kSi sin hÞdz þ 2a=3 expð z=kFeSi2 sin hÞdz
Fe SiSiO2 Z ¼
325
hm 1
a=3 expð z=kSiO2 sin hÞ
t
expð ðz tÞ=kFeSi2 sin hÞ dz Z t expð z=kSiO2 sin hÞ dz 0
kFeSi2 ½a=3 expðt=kSiO2 sin hÞ ¼ : kSiO2 ½1 expðt=kSiO2 sin hÞ
0
¼ kSiO2 ð1 aÞ½1 expðt=kSiO2 sin hÞ =ðkSi ½ð1 aÞ expð t=kSiO2 sin hÞ þ 2=3akFeSi2 Þ;
Fe SiTotal hm Z 1 ¼ a=3 expð z=kFeSi2 sin hÞ dz 0 Z 1 ð1 aÞ expð t=kSiO2 sin hÞ t
expð ðz tÞ=kSi sin hÞ dz Z t ð1 aÞ expð z=kSiO2 sin hÞ dz þ Z0 1 2a=3 expð z=kFeSi2 sin hÞ dz þ 0
¼ kFeSi2 ½a=3 expðt=kSiO2 sin hÞ =ðkSi ½ð1 aÞ expðt=kSiO2 sin hÞ þ kSiO2 ½1 expðt=kSiO2 sin hÞ þ kFeSi2 ½2a=3 expðt=kSiO2 sin hÞ Þ;
References [1] N.E. Christensen, Phys. Rev. B 42 (1990) 7148. [2] M. Sasase, T. Nakanoya, H. Yamamoto, K. Hojou, Thin Solid Films 401 (2001) 73. [3] T. Saito, H. Yamamoto, M. Sasase, T. Nakanoya, K. Yamaguchi, M. Haraguchi, K. Hojou, Thin Solid Films 415 (2002) 138. [4] H. Shimada, N. Matsubayashi, M. Imamura, T. Sato, A. Nishijima, Catal. Lett. 39 (1999) 125. [5] H. Shimada, N. Matsubayashi, M. Imamura, T. Sato, Y. Yoshimura, T. Hayakawa, K. Takehira, A. Toyoshima, K. Tanaka, A. Nishijima, Rev. Sci. Instrum. 66 (1995) 1780. [6] K. R€ uhrnschopf, D. Borgmann, G. Wedler, Thin Solid Films 280 (1996) 171. [7] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 155. [8] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [9] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interf. Anal. 17 (1991) 911. [10] J. Alvarez, J.J. Hinarejos, E.G. Michel, R. Miranda, Surf. Sci. 287–288 (1993) 490.
Characterization of air-exposed surface of b-FeSi2 fabricated by ion beam sputter deposition method T. Saito a, H. Yamamoto a, K. Yamaguchi a, T. Nakanoya a, K. Hojou a,*, M. Haraguchi b, M. Imamura c, N. Matsubayashi c, T. Tanaka c, H. Shimada
c
a
Japan Atomic Energy Research Institute, 2-4 Shirakatashirane, Tokai, Naka, 319-1195 Ibaraki, Japan Graduate School of Science and Engineering, Ibaraki University, 2-1-1, Bunkyo, Mito, 310-8512 Ibaraki, Japan National Institute of Advanced Industrial Science and Technology, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b c
Abstract The surface composition and chemical states of b-FeSi2 on a Si(1 0 0) substrate after exposure to the ambient atmosphere was investigated by using X-ray photoelectron spectroscopy with synchrotron radiation (SR-XPS). The analysis revealed that about 70% of the Si substrate was covered with b-FeSi2 . In addition, it was found that a SiO2 overlayer with a thickness of about 1 nm covered the whole surface of the sample. These results suggest that Si was segregated during the IBSD process and that the readily formed SiO2 layer behaved as a protective layer against further oxidation of b-FeSi2 surface. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 73.20.)j; 73.61.Le Keywords: Iron silicide; Ion beam sputter deposition; Synchrotron radiation; XPS; Non-destructive; Depth profiling analysis
1. Introduction b-FeSi2 has many unique electronic properties such as a narrow direct band gap of 0.85 eV [1]. In addition, this silicide does not contain harmful elements. From these points of view, b-FeSi2 is one of the suitable candidates for optical and electric devices in the near future. In our previous study [2], we have obtained a highly oriented b-FeSi2 to h1 0 0i direction on a Si(1 0 0) surface by ion beam sputter deposition
*
Corresponding author. Tel.: +81-29282-5479; fax: +81-29282-6716. E-mail address: [email protected] (K. Hojou).
(IBSD) method. We also reported that the surface of the b-FeSi2 was stable in the ambient atmosphere [3]. From this result we have speculated that the surface of b-FeSi2 was covered with a thin oxide layer that behaves as a protective layer against further oxidation. However, detailed discussion could not be conducted because the oxide layer was supposed to be very thin (1 nm). X-ray photoelectron spectroscopy (XPS) has often been used for the determination of surface composition and chemical states of many materials. However, information depth of conventional XPS analyses with an MgKa or AlKa anode is relatively large (2 nm). Because of recent developments of the synchrotron radiation (SR) instrumentation, X-rays with the energy range of
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00754-7
322
T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
200–1000 eV have become available. The tunability of the excitation energy enables us to perform surface sensitive XPS analysis and non-destructive depth profiling analysis [4]. In the present study we have analyzed the surface compositions and chemical states of an airexposed surface of a b-FeSi2 formed by IBSD by using of SR-XPS. The in-depth profiles of Fe/Si and SiO2 /Si ratios were obtained and the thickness and structure of the surface SiO2 layer were discussed.
2. Experimental The XPS analysis was performed at the beamline 13C [5] of the Photon Factory (PF). The XPS spectra were recorded with a hemispherical electron analyzer (PHI1600C). The photoelectron take-off angle was fixed at 54.7° relative to the surface normal. The substrate used was a n-type CZ-Si(1 0 0) wafer. Before Fe deposition, the native oxide layer of the substrate was removed by the annealing at 773 and 1073 K for 1 h followed flashing at 1273 K for 3 min. In the IBSD method Arþ ions with an acceleration voltage of 35 kV were bombarded into an Fe target (99.998 at.%). The sputtered Fe atoms with a deposition rate of 0.09 nm/s were deposited on the Si(1 0 0) substrate at a substrate temperature of 973 K for 60 min. The pressure in the IBSD chamber during the deposition was kept below 3 105 Pa. The details of the IBSD apparatus were described elsewhere [2]. After the fabrication of b-FeSi2 , the sample was taken out of the vacuum chamber and kept in the ambient atmosphere for two weeks before the XPS analysis.
3. Results and discussion Fig. 1 shows the Fe 2p(A) and Si 2p(B) XPS spectra of b-FeSi2 on Si(1 0 0) after exposure to the ambient atmosphere. To record spectra with similar information depth, the excitation energies were set at 890 eV (a), 1050 eV (b) and 1250 eV (c) for Fe 2p spectra and 270 eV (a), 450 eV (b) and 690 eV (c) for Si 2p spectra, respectively. All the Fe 2p
Fig. 1. Fe 2p (A) and Si 2p (B) XPS spectra of air-exposed bFeSi2 on Si(1 0 0) surface. Excitation energies used were 890 eV (a), 1050 eV (b) and 1250 eV (c) for Fe2p spectra and 270 eV (a), 450 eV (b) and 690 eV (c) for Si 2p spectra.
spectra exhibited two sharp features, which were assigned to Fe 2p3=2 and 2p1=2 features of b-FeSi2 [6]. A small feature at about 730 eV was assigned to a characteristic plasmon feature of b-FeSi2 [6]. With increasing the excitation energy (a ! c), the intensity of a small feature at about 710 eV is decreased. This feature was assigned to Fe 2p3=2 of Fe oxide. This result suggests that surface Fe was partly oxidized but the change was relatively small. In each Si 2p spectrum (B), two features at about 103 and 99 eV were observed. These were assigned to Si 2p of SiO2 and b-FeSi2 . With increasing the excitation energy (a ! c) the intensity of the feature assigned to SiO2 is decreased. This result suggests that Si was oxidized to a higher degree than Fe during the exposure to air. In the previous studies [2,3], we investigated the surface structure of b-FeSi2 on a Si substrate by using scanning electron microscope (SEM) and transmission electron microscope (TEM). The results showed that about 75% of the substrate surface was covered with b-FeSi2 islands. Therefore, two oxidation processes can be deduced from the present results; one is that a thin uniform SiO2 layer covered the almost entire surface of the sample (model 1 in Fig. 2) and the other is that
T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
only the exposed Si substrate surface was oxidized (model 2 in Fig. 2). To clarify the oxidation phenomena, we have performed non-destructive depth profiling analysis
Fig. 2. Schematic structural models of the sample surface. (A) Model 1, (B) model 2.
Fig. 3. Comparison of experimentally obtained SiSiO2 =ðSiSi þ SiFeSi2 Þ, Fe/SiTotal and Fe/SiSiO2 ratios with simulated profiles as a function of excitation energy.
323
by using SR-XPS. Fig. 3 shows excitation energy dependence of the SiSiO2 =ðSiSi þ SiFeSi2 Þ, Fe/SiTotal and Fe/SiSiO2 ratios of the oxidized sample. The Fe/ SiTotal atomic ratios were calculated using the peak areas and photo-ionization cross-sections of Fe 3p and Si 2p spectra at each excitation energy [7]. The peak areas of SiSiO2 and SiSi were calculated by deconvoluting each Si 2p spectrum by a leastsquare curve-fitting method using mixed Gaussian–Lorentzian envelopes after subtracting a Shirley-type background [8]. The feature assigned to the b-FeSi2 could not be separated from that assigned to the Si substrate because the chemical shift of b-FeSi2 from Si is relatively small (0.3 eV) [6]. In Fig. 3, the Fe/SiTotal ratio decreased with decreasing the excitation energy. This indicates that the atomic concentration of Si near the uppermost surface was larger than that in the bulk. The SiSiO2 =ðSiSi þ SiFeSi2 Þ ratio steeply increased with decreasing the excitation energy suggesting the existence of a thin surface SiO2 layer on the sample. The analysis with the lowest excitation energy gave almost the same Fe/SiTotal value as Fe/ SiSiO2 . This indicates that almost all of the Si at the uppermost surface of the sample is present as SiO2 . In order to discuss in detail, the experimental results in Fig. 3 were compared with simulated profiles using model 1 and model 2 shown in Fig. 2. In the simulations, following two assumptions were made [4]; (1) attenuation length of the photoelectrons is identified by inelastic mean free path (IMFP), (2) attenuation of photoelectrons is exponential in the escape direction. The IMFP value of the photoelectrons at each excitation energy was calculated using the theoretical equation in the literature [9]. The each ratio at each excitation energy (hm) in each model can be given by the equations shown in Appendix A. The simulated profiles using model 1 (solid lines) with a parameter set of a ¼ 0:7 and t ¼ 0:85 nm (solid lines) are consistent with the experimental data, whereas simulation using model 2 with a parameter set of a ¼ 0:3 and t ¼ 1:2 nm (dashed lines) does not give consistent results with the experimental data. Note that the coverage of b-FeSi2 in model 1 (a ¼ 0:7) agreed with that
324
T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
obtained in a previous study (0.75) [3]. From these results, we conclude that a thin SiO2 over-layer covers not only the exposed region of the Si substrate surface but also the b-FeSi2 surface. The results in Fig. 1 indicated that only Si is oxidized after exposure to the atmosphere. This suggests that a thin surface Si layer is formed during IBSD and protects the surface of b-FeSi2 from being oxidized. Alvarez et al. [10] investigated the fabrication process of b-FeSi2 on a Si(1 1 1) surface using ultra-violet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). They reported that the b-FeSi2 surface fabricated on a Si substrate was covered with a Si layer by annealing at a high temperature such as 973 K. This is consistent with our discussion; the segregation of Si from b-FeSi2 took place during the IBSD process by surface diffusion and the segregated Si at the uppermost surface was subsequently oxidized. The SiO2 layer at the uppermost surface behaved as a protective layer for further oxidation due to the dense structure that prevents inward diffusion of oxygen.
Appendix A A.1. Model 1
SiSiO2 SiSi þ SiFeSi2 hm Z t ¼ expð z=kSiO2 sin hÞdz 0 Z 1 ð1 aÞexpð t=kSiO2 sin hÞexpð ðz tÞ t Z 1 2a=3expð t=kSiO2 sin hÞ =kSi sin hÞdz þ t expð ðz tÞ=kFeSi2 sin hÞdz
¼ kSiO2 ½1 expð t=kSiO2 sin hÞ ðkSi ½ð1 aÞexpð t=kSiO2 sin hÞ þ kFeSi2 ½2a=3 expð t=kSiO2 sin hÞ Þ;
Fe SiTotal Z ¼
hm 1
a=3 expð z=kSiO2 sin hÞ expð ðz tÞ Z 1 =kFeSi2 sin hÞ dz ð1 aÞ expð t t
t
=kSiO2 sin hÞ expð ðz tÞ=kSi sin hÞ dz Z t expð z=kSiO2 sin hÞ dz þ Z0 1 2a=3 expð t=kSiO2 sin hÞ þ t expð ðz tÞ=kFeSi2 sin hÞ dz
4. Conclusion The surface composition and chemical states of a b-FeSi2 fabricated on a Si(1 0 0) substrate by IBSD were investigated. The SR-XPS analysis revealed that b-FeSi2 covered about 70% of the Si(1 0 0) substrate. This was quite consistent with our previous result (75%) obtained by using SEM and TEM [3]. It was also found that a SiO2 overlayer with a thickness of about 1 nm covered the entire surface of the sample including the b-FeSi2 surface. Si was presumably segregated during the IBSD process and the readily formed SiO2 layer behaved as a protective layer against oxidation of b-FeSi2 surface.
¼ kFeSi2 ½a=3 expð t=kSiO2 sin hÞ ðkSi ½ð1 aÞ expð t=kSiO2 sin hÞ þ kSiO2 ½1 expð t=kSiO2 sin hÞ þ kFeSi2 ½2a=3 expð t=kSiO2 sin hÞ Þ;
Fe SiSiO2 Z ¼
hm 1
a=3 expð z=kSiO2 sin hÞ expð ðz tÞ Z t =kFeSi2 sin hÞ dz expð z=kSiO2 sin hÞ dz t
Acknowledgements
0
This work was done under the approval of the PF Advisory Committee (no. 2002G280).
kFeSi2 ½a=3 expðt=kSiO2 sin hÞ ¼ : kSiO2 ½1 expðt=kSiO2 sin hÞ
T. Saito et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 321–325
A.2. Model 2
SiSiO2 SiSi þ SiFeSi2 hm Z t ¼ ð1 aÞ expð z=kSiO2 sin hÞdz 0 Z 1 ð1 aÞ expð t=kSiO2 sin hÞ expð ðz tÞ t Z 1 =kSi sin hÞdz þ 2a=3 expð z=kFeSi2 sin hÞdz
Fe SiSiO2 Z ¼
325
hm 1
a=3 expð z=kSiO2 sin hÞ
t
expð ðz tÞ=kFeSi2 sin hÞ dz Z t expð z=kSiO2 sin hÞ dz 0
kFeSi2 ½a=3 expðt=kSiO2 sin hÞ ¼ : kSiO2 ½1 expðt=kSiO2 sin hÞ
0
¼ kSiO2 ð1 aÞ½1 expðt=kSiO2 sin hÞ =ðkSi ½ð1 aÞ expð t=kSiO2 sin hÞ þ 2=3akFeSi2 Þ;
Fe SiTotal hm Z 1 ¼ a=3 expð z=kFeSi2 sin hÞ dz 0 Z 1 ð1 aÞ expð t=kSiO2 sin hÞ t
expð ðz tÞ=kSi sin hÞ dz Z t ð1 aÞ expð z=kSiO2 sin hÞ dz þ Z0 1 2a=3 expð z=kFeSi2 sin hÞ dz þ 0
¼ kFeSi2 ½a=3 expðt=kSiO2 sin hÞ =ðkSi ½ð1 aÞ expðt=kSiO2 sin hÞ þ kSiO2 ½1 expðt=kSiO2 sin hÞ þ kFeSi2 ½2a=3 expðt=kSiO2 sin hÞ Þ;
References [1] N.E. Christensen, Phys. Rev. B 42 (1990) 7148. [2] M. Sasase, T. Nakanoya, H. Yamamoto, K. Hojou, Thin Solid Films 401 (2001) 73. [3] T. Saito, H. Yamamoto, M. Sasase, T. Nakanoya, K. Yamaguchi, M. Haraguchi, K. Hojou, Thin Solid Films 415 (2002) 138. [4] H. Shimada, N. Matsubayashi, M. Imamura, T. Sato, A. Nishijima, Catal. Lett. 39 (1999) 125. [5] H. Shimada, N. Matsubayashi, M. Imamura, T. Sato, Y. Yoshimura, T. Hayakawa, K. Takehira, A. Toyoshima, K. Tanaka, A. Nishijima, Rev. Sci. Instrum. 66 (1995) 1780. [6] K. R€ uhrnschopf, D. Borgmann, G. Wedler, Thin Solid Films 280 (1996) 171. [7] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 155. [8] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [9] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interf. Anal. 17 (1991) 911. [10] J. Alvarez, J.J. Hinarejos, E.G. Michel, R. Miranda, Surf. Sci. 287–288 (1993) 490.