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Effects of As+-implantation on the formation of iron silicides in Fe thin films on (1 1 1)Si
Applied Surface Science 212–213 (2003) 204–208
Effects of Asþ-implantation on the formation of iron silicides in Fe thin films on (1 1 1)Si H.T. Lu, Y.L. Chueh, L.J. Chou, L.J. Chen* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
Abstract The phase transformation of iron silicides from FeSi to b-FeSi2 on (1 1 1)Si and effects of Asþ-implantation on the transformation have been investigated by sheet resistance measurements, grazing incidence X-ray diffractometry (GIXRD), scanning transmission electron microscope (STEM) and energy dispersive analysis of X-ray (EDAX). Phase transformation from FeSi to b-FeSi2 begins at 600 8C and completes at 700 8C. The transformation is significantly enhanced by the Asþimplantation. The annealed Asþ-implanted samples show a very different sheet resistance versus annealing temperature behavior. Wider than 20 nm decorated grain boundaries were observed in the Asþ-implanted sample. EDAX data show that the dopant, As, is present only in the wide decorated grain boundary areas. # 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55.-a; 68.35.Fx; 61.16.Bg Keywords: FeSi; b-FeSi2; Asþ-implantation
1. Introduction Silicon has long been the material of choice for most microelectronic applications. But it is a poor emitter of light since silicon itself has an indirect band structure. This problem has led to numerous attempts to develop silicon-based structures with good light-emission characteristics, particularly at wavelength 1.5 mm relevant to optical fiber communication [1–3]. b-FeSi2 has been reported to possess a narrow and direct band gap of 0.85 eV [4–9], with a corresponding wavelength of 1.5 mm, which lies in the window of maximum transmission of the optical fibers. This opens the way to the Si optoelectronic engineering by mixing silicides with *
Corresponding author. Tel.: þ886-3-573-1166; fax: þ886-3-571-8328. E-mail address: [email protected] (L.J. Chen).
various bandgaps integrated on a single silicon chip. In addition, semiconducting b-FeSi2 has lately attracted scientific and technological interest for optoelectronic devices, infrared detector, thermoelectric and photovoltaic applications [10–14]. Leong et al. [15] have successfully fabricated a demonstrator silicon/iron disilicide light-emitting diode. The approach used is to incorporate directgap iron disilicide into a conventional silicon p–n junction diode, in the recombination region adjacent to one side of the depletion region, to provide a route for direct radiative recombination. Carrier injection is achieved conventionally under forward bias. Thus, it is of much interest to study the influence of dopants in silicon substrate on b-FeSi2 formation. The objective of the present study is to investigate the effects of dopants (Asþ) in silicon substrates on the formation of iron silicides.
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00363-5
H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
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2. Experimental Single crystal, 1–5 O cm, p-type (1 1 1)Si wafers were used in the present study. (1 1 1)Si wafers were implanted by 20 keV Asþ to a dose of 1 1015 atoms/ cm2. Samples were cleaned by a standard cleaning process. Thirty nanometers-thick Fe thin films were deposited onto the samples by ultrahigh vacuum (UHV) electron gun evaporation. As-deposited samples were in situ annealed in the UHV chamber at 500–800 8C for 30 min. A four-point probe was used to measure the sheet resistance. Grazing incidence X-ray diffractometry (GIXRD) was carried out for phase identification with the incident angle of X-ray fixed at 0.58. A JEOL-200 CX scanning transmission electron microscope (STEM) operating at 200 keV was conducted to study the morphology of thin films. Energy dispersive analysis of X-ray (EDAX) was utilized to determine the compositions of the samples.
3. Results and discussion Fig. 1 shows the sheet resistance data of Fe on undoped and Asþ-implanted (1 1 1)Si as a function of annealing temperature. Significant difference in sheet resistance versus annealing temperature relationship
Fig. 1. The sheet resistance data of Fe on undoped and Asþ-implanted (1 1 1)Si as a function of annealing temperature.
was observed. The sheet resistance of undoped samples increases with the annealing temperature and saturates at 700 8C. As for Asþ-implanted samples, the sheet resistance increases with a steeper slope that leads to a higher sheet resistance for the samples annealed at 600 and 700 8C. As the annealing temperature is raised to 800 8C, the sheet resistance drops from 6350 to 2740 O/sq. Fig. 2 shows the grazing angle XRD diffraction spectra of undoped samples annealed from 500 to 800 8C. FeSi was found to be the only silicide phase at 500 8C. b-FeSi2 starts to form at 600 8C, but FeSi is
Fig. 2. The grazing angle XRD diffraction spectra of undoped samples annealed from 500 to 800 8C.
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Table 1 Phase formed in undoped and Asþ-implanted samples 500 8C 600 8C Undoped
FeSi
Asþ-implanted
FeSi
700 8C
800 8C
FeSi (dominant) b-FeSi2 þ b-FeSi2 b-FeSi2 (dominant) b-FeSi2 þ FeSi
b-FeSi2 b-FeSi2
still the dominant phase. At 700 8C and above, b-FeSi2 becomes the only silicide phase. A previous study on the interfacial reactions of iron films on silicon found the presence of a small amount of a-FeSi2 in samples annealed at 700 8C and above [16]. The impurities introduced during non-UHV deposition of the films in the previous study may contribute to the formation of the phase other than b-FeSi2. b-FeSi2 is a p-type semiconductor with high resistivity [17]. In combination with XRD phase identification data shown in Fig. 2, the increase of sheet resistance is attributed to the more extensive b-FeSi2 formation. However, b-FeSi2 forms an ohmic contact to the p-type substrate in undoped samples. The measured sheet resistance is actually measuring a parallel connection of the silicide and the substrate. This explains why b-FeSi2 in the undoped samples shows a much lower sheet resistance than expected. Phase transformation in Asþ-implanted samples was also studied by XRD analysis. The results are summarized in Table 1. For samples annealed at 500 8C, the only phase present is FeSi. b-FeSi2 was predominant with the presence of FeSi in samples annealed at 600 8C. At 700 8C and above, b-FeSi2 was the only phase present. As a result, the formation of b-FeSi2 is enhanced by Asþ-implantation. On the other hand, sheet resistance reaches its highest value at 700 8C to 6350 O/sq and reduces to 2740 O/sq when annealed at 800 8C. XTEM micrograph revealed that the thickness of the silicide films is about 80 nm (Fig. 3). The resistivity of b-FeSi2 is calculated to be 0.022 O cm. From XRD data, b-FeSi2 is the only observed phase in samples annealed at a temperature higher than 600 8C. Further information is needed to clarify the reason for the sheet resistance change. Fig. 4a and b show planview TEM images of undoped, and Asþ-implanted samples annealed at 600 8C. For Asþ-implanted samples, wide decorated grain boundaries were observed. EDAX composition
Fig. 3. XTEM image of Asþ-implanted sample annealed at 800 8C.
analysis was employed to determine atomic composition difference between the grain and grain boundary regions. Fig. 5a and b are the EDAX spectra of spots inside the grain and grain boundary, respectively, as marked in Fig. 5c in a 600 8C annealed Asþ-implanted sample. The electron probe spot size was selected to be 7 nm, which is about one-third of the width of the grain boundary. From Fig. 5a and b, it can be inferred that the composition ratio of Fe/Si is obviously lower
Fig. 4. Planview TEM images of (a) undoped and (b) Asþimplanted samples annealed at 600 8C.
H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
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Fig. 6. Planview TEM images of Asþ-implanted samples annealed at (a) 700 8C and (b) 800 8C.
in the temperature range of 600–700 8C. On the other hand, the wide decorated grain boundaries disappear in samples annealed at 800 8C, as shown in Fig. 6b. It is thought that As-rich grain boundary precipitates dissolved into the silicide matrix, which led to the sheet resistance reduction in the 800 8C annealed samples.
4. Conclusions
Fig. 5. The EDAX spectra of spots (a) inside the grain and (b) grain boundary, as marked in (c) in a 600 8C annealed Asþ-implanted sample.
inside the grain and As is detected to be present only at the wide decorated grain boundaries. Fig. 6a and b are planview TEM images of Asþimplanted samples annealed at 700 and 800 8C. By comparing Figs. 4b and 6a, both grain and grain boundary are seen to grow with annealing temperature
The effects of Asþ-implantation on the phase transformation in iron films in (1 1 1)Si have been investigated by sheet resistance measurements, GIXRD, STEM and EDAX. The transformation from FeSi to b-FeSi2 begins at 600 8C and completes at 700 8C. The formation of b-FeSi2 is enhanced by Asþ-implantation. Sheet resistance increases with the fraction of high-resistivity b-FeSi2. The annealed Asþ-implanted samples have much higher sheet resistance than the undoped samples. The decreasing sheet resistance at 800 8C for Asþ-implanted samples is attributed to the dissolution of high-resistivity As-rich phase at the wide decorated grain boundaries.
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H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
References [1] D.A.B. Miller, Nature 378 (1995) 238. [2] M. Forster, U. Mantz, S. Ramminger, K. Thonke, R. Sauer, H. Kibbel, F. Schaffler, H.-J. Herzog, J. Appl. Phys. 80 (1996) 3017. [3] B. Zheng, J. Michel, F.Y.G. Ren, L.C. Kimerling, D.C. Jacobson, J.M. Poate, Appl. Phys. Lett. 64 (1994) 2842. [4] N.E. Christensen, Phys. Rev. B 42 (1990) 7148. [5] R. Eppenga, J. Appl. Phys. 68 (1990) 3027. [6] Z. Yang, K.P. Homewood, M.S. Finney, M.A. Harry, K.J. Resson, J. Appl. Phys. 78 (1995) 1958. [7] C.A. Dimitriadis, J.H. Werner, S. Logothtidis, M. Stutzmann, J. Weber, R. Nesper, J. Appl. Phys. 68 (1990) 1726. [8] M.C. Bost, J.E. Mahan, J. Appl. Phys. 64 (1988) 2034. [9] E. Arushanov, E. Bucher, Ch. Kloc, O. Kulikova, L. Kulyuk, A. Siminel, Phys. Rev. B 52 (1995) 20.
[10] J. Derrien, J. Chevrier, V. Le Thanh, J.E. Mahan, Appl. Surf. Sci. 56–58 (1992) 382. [11] H. Lange, Phys. Stat. Sol. (b) 201 (1997) 3. [12] E. Grob, M. Riffel, U. Stohrer, J. Mater. Res. 10 (1995) 34. [13] H. Katsumata, H.L. Shen, N. Kobayashi, Y. Makita, M. Hasegawa, H. Shibata, S. Kimura, A. Obara, S. Uekusa, in: Proceedings of the Ninth International Conference on Ion Beam Modification of Materials, Elsevier, New York, 1996, p. 943. [14] M. Libezny, J. Poortmans, T. Vermeulen, J. Nijs, P.H. Amesz, K. Herz, M. Powalla, in: Proceedings of the 13th European Photovoltaic Solar Energy Conference, 1995, p. 1326. [15] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [16] H.C. Cheng, T.R. Yew, L.J. Chen, J. Appl. Phys. 57 (1985) 5246. [17] J.H. Westbrook, R.L. Fleischer, Intermetallic Compounds: Principles and Practice, Wiley, New York, 1995.
Effects of Asþ-implantation on the formation of iron silicides in Fe thin films on (1 1 1)Si H.T. Lu, Y.L. Chueh, L.J. Chou, L.J. Chen* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
Abstract The phase transformation of iron silicides from FeSi to b-FeSi2 on (1 1 1)Si and effects of Asþ-implantation on the transformation have been investigated by sheet resistance measurements, grazing incidence X-ray diffractometry (GIXRD), scanning transmission electron microscope (STEM) and energy dispersive analysis of X-ray (EDAX). Phase transformation from FeSi to b-FeSi2 begins at 600 8C and completes at 700 8C. The transformation is significantly enhanced by the Asþimplantation. The annealed Asþ-implanted samples show a very different sheet resistance versus annealing temperature behavior. Wider than 20 nm decorated grain boundaries were observed in the Asþ-implanted sample. EDAX data show that the dopant, As, is present only in the wide decorated grain boundary areas. # 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55.-a; 68.35.Fx; 61.16.Bg Keywords: FeSi; b-FeSi2; Asþ-implantation
1. Introduction Silicon has long been the material of choice for most microelectronic applications. But it is a poor emitter of light since silicon itself has an indirect band structure. This problem has led to numerous attempts to develop silicon-based structures with good light-emission characteristics, particularly at wavelength 1.5 mm relevant to optical fiber communication [1–3]. b-FeSi2 has been reported to possess a narrow and direct band gap of 0.85 eV [4–9], with a corresponding wavelength of 1.5 mm, which lies in the window of maximum transmission of the optical fibers. This opens the way to the Si optoelectronic engineering by mixing silicides with *
Corresponding author. Tel.: þ886-3-573-1166; fax: þ886-3-571-8328. E-mail address: [email protected] (L.J. Chen).
various bandgaps integrated on a single silicon chip. In addition, semiconducting b-FeSi2 has lately attracted scientific and technological interest for optoelectronic devices, infrared detector, thermoelectric and photovoltaic applications [10–14]. Leong et al. [15] have successfully fabricated a demonstrator silicon/iron disilicide light-emitting diode. The approach used is to incorporate directgap iron disilicide into a conventional silicon p–n junction diode, in the recombination region adjacent to one side of the depletion region, to provide a route for direct radiative recombination. Carrier injection is achieved conventionally under forward bias. Thus, it is of much interest to study the influence of dopants in silicon substrate on b-FeSi2 formation. The objective of the present study is to investigate the effects of dopants (Asþ) in silicon substrates on the formation of iron silicides.
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00363-5
H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
205
2. Experimental Single crystal, 1–5 O cm, p-type (1 1 1)Si wafers were used in the present study. (1 1 1)Si wafers were implanted by 20 keV Asþ to a dose of 1 1015 atoms/ cm2. Samples were cleaned by a standard cleaning process. Thirty nanometers-thick Fe thin films were deposited onto the samples by ultrahigh vacuum (UHV) electron gun evaporation. As-deposited samples were in situ annealed in the UHV chamber at 500–800 8C for 30 min. A four-point probe was used to measure the sheet resistance. Grazing incidence X-ray diffractometry (GIXRD) was carried out for phase identification with the incident angle of X-ray fixed at 0.58. A JEOL-200 CX scanning transmission electron microscope (STEM) operating at 200 keV was conducted to study the morphology of thin films. Energy dispersive analysis of X-ray (EDAX) was utilized to determine the compositions of the samples.
3. Results and discussion Fig. 1 shows the sheet resistance data of Fe on undoped and Asþ-implanted (1 1 1)Si as a function of annealing temperature. Significant difference in sheet resistance versus annealing temperature relationship
Fig. 1. The sheet resistance data of Fe on undoped and Asþ-implanted (1 1 1)Si as a function of annealing temperature.
was observed. The sheet resistance of undoped samples increases with the annealing temperature and saturates at 700 8C. As for Asþ-implanted samples, the sheet resistance increases with a steeper slope that leads to a higher sheet resistance for the samples annealed at 600 and 700 8C. As the annealing temperature is raised to 800 8C, the sheet resistance drops from 6350 to 2740 O/sq. Fig. 2 shows the grazing angle XRD diffraction spectra of undoped samples annealed from 500 to 800 8C. FeSi was found to be the only silicide phase at 500 8C. b-FeSi2 starts to form at 600 8C, but FeSi is
Fig. 2. The grazing angle XRD diffraction spectra of undoped samples annealed from 500 to 800 8C.
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H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
Table 1 Phase formed in undoped and Asþ-implanted samples 500 8C 600 8C Undoped
FeSi
Asþ-implanted
FeSi
700 8C
800 8C
FeSi (dominant) b-FeSi2 þ b-FeSi2 b-FeSi2 (dominant) b-FeSi2 þ FeSi
b-FeSi2 b-FeSi2
still the dominant phase. At 700 8C and above, b-FeSi2 becomes the only silicide phase. A previous study on the interfacial reactions of iron films on silicon found the presence of a small amount of a-FeSi2 in samples annealed at 700 8C and above [16]. The impurities introduced during non-UHV deposition of the films in the previous study may contribute to the formation of the phase other than b-FeSi2. b-FeSi2 is a p-type semiconductor with high resistivity [17]. In combination with XRD phase identification data shown in Fig. 2, the increase of sheet resistance is attributed to the more extensive b-FeSi2 formation. However, b-FeSi2 forms an ohmic contact to the p-type substrate in undoped samples. The measured sheet resistance is actually measuring a parallel connection of the silicide and the substrate. This explains why b-FeSi2 in the undoped samples shows a much lower sheet resistance than expected. Phase transformation in Asþ-implanted samples was also studied by XRD analysis. The results are summarized in Table 1. For samples annealed at 500 8C, the only phase present is FeSi. b-FeSi2 was predominant with the presence of FeSi in samples annealed at 600 8C. At 700 8C and above, b-FeSi2 was the only phase present. As a result, the formation of b-FeSi2 is enhanced by Asþ-implantation. On the other hand, sheet resistance reaches its highest value at 700 8C to 6350 O/sq and reduces to 2740 O/sq when annealed at 800 8C. XTEM micrograph revealed that the thickness of the silicide films is about 80 nm (Fig. 3). The resistivity of b-FeSi2 is calculated to be 0.022 O cm. From XRD data, b-FeSi2 is the only observed phase in samples annealed at a temperature higher than 600 8C. Further information is needed to clarify the reason for the sheet resistance change. Fig. 4a and b show planview TEM images of undoped, and Asþ-implanted samples annealed at 600 8C. For Asþ-implanted samples, wide decorated grain boundaries were observed. EDAX composition
Fig. 3. XTEM image of Asþ-implanted sample annealed at 800 8C.
analysis was employed to determine atomic composition difference between the grain and grain boundary regions. Fig. 5a and b are the EDAX spectra of spots inside the grain and grain boundary, respectively, as marked in Fig. 5c in a 600 8C annealed Asþ-implanted sample. The electron probe spot size was selected to be 7 nm, which is about one-third of the width of the grain boundary. From Fig. 5a and b, it can be inferred that the composition ratio of Fe/Si is obviously lower
Fig. 4. Planview TEM images of (a) undoped and (b) Asþimplanted samples annealed at 600 8C.
H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
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Fig. 6. Planview TEM images of Asþ-implanted samples annealed at (a) 700 8C and (b) 800 8C.
in the temperature range of 600–700 8C. On the other hand, the wide decorated grain boundaries disappear in samples annealed at 800 8C, as shown in Fig. 6b. It is thought that As-rich grain boundary precipitates dissolved into the silicide matrix, which led to the sheet resistance reduction in the 800 8C annealed samples.
4. Conclusions
Fig. 5. The EDAX spectra of spots (a) inside the grain and (b) grain boundary, as marked in (c) in a 600 8C annealed Asþ-implanted sample.
inside the grain and As is detected to be present only at the wide decorated grain boundaries. Fig. 6a and b are planview TEM images of Asþimplanted samples annealed at 700 and 800 8C. By comparing Figs. 4b and 6a, both grain and grain boundary are seen to grow with annealing temperature
The effects of Asþ-implantation on the phase transformation in iron films in (1 1 1)Si have been investigated by sheet resistance measurements, GIXRD, STEM and EDAX. The transformation from FeSi to b-FeSi2 begins at 600 8C and completes at 700 8C. The formation of b-FeSi2 is enhanced by Asþ-implantation. Sheet resistance increases with the fraction of high-resistivity b-FeSi2. The annealed Asþ-implanted samples have much higher sheet resistance than the undoped samples. The decreasing sheet resistance at 800 8C for Asþ-implanted samples is attributed to the dissolution of high-resistivity As-rich phase at the wide decorated grain boundaries.
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H.T. Lu et al. / Applied Surface Science 212–213 (2003) 204–208
References [1] D.A.B. Miller, Nature 378 (1995) 238. [2] M. Forster, U. Mantz, S. Ramminger, K. Thonke, R. Sauer, H. Kibbel, F. Schaffler, H.-J. Herzog, J. Appl. Phys. 80 (1996) 3017. [3] B. Zheng, J. Michel, F.Y.G. Ren, L.C. Kimerling, D.C. Jacobson, J.M. Poate, Appl. Phys. Lett. 64 (1994) 2842. [4] N.E. Christensen, Phys. Rev. B 42 (1990) 7148. [5] R. Eppenga, J. Appl. Phys. 68 (1990) 3027. [6] Z. Yang, K.P. Homewood, M.S. Finney, M.A. Harry, K.J. Resson, J. Appl. Phys. 78 (1995) 1958. [7] C.A. Dimitriadis, J.H. Werner, S. Logothtidis, M. Stutzmann, J. Weber, R. Nesper, J. Appl. Phys. 68 (1990) 1726. [8] M.C. Bost, J.E. Mahan, J. Appl. Phys. 64 (1988) 2034. [9] E. Arushanov, E. Bucher, Ch. Kloc, O. Kulikova, L. Kulyuk, A. Siminel, Phys. Rev. B 52 (1995) 20.
[10] J. Derrien, J. Chevrier, V. Le Thanh, J.E. Mahan, Appl. Surf. Sci. 56–58 (1992) 382. [11] H. Lange, Phys. Stat. Sol. (b) 201 (1997) 3. [12] E. Grob, M. Riffel, U. Stohrer, J. Mater. Res. 10 (1995) 34. [13] H. Katsumata, H.L. Shen, N. Kobayashi, Y. Makita, M. Hasegawa, H. Shibata, S. Kimura, A. Obara, S. Uekusa, in: Proceedings of the Ninth International Conference on Ion Beam Modification of Materials, Elsevier, New York, 1996, p. 943. [14] M. Libezny, J. Poortmans, T. Vermeulen, J. Nijs, P.H. Amesz, K. Herz, M. Powalla, in: Proceedings of the 13th European Photovoltaic Solar Energy Conference, 1995, p. 1326. [15] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [16] H.C. Cheng, T.R. Yew, L.J. Chen, J. Appl. Phys. 57 (1985) 5246. [17] J.H. Westbrook, R.L. Fleischer, Intermetallic Compounds: Principles and Practice, Wiley, New York, 1995.