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Kinetics of solid phase regrowth of self-ion-implanted amorphous SiC during low temperature furnace annealing
Nuclear Instruments and Methods in Physics Research B 206 (2003) 969–973 www.elsevier.com/locate/nimb
Kinetics of solid phase regrowth of self-ion-implanted amorphous SiC during low temperature furnace annealing O. Eryu a
a,*
, K. Abe a, K. Nakashima a, H. Harima
b
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-Cho, Showa-Ku, Nagoya 466-8555, Japan b Department of Electronics and Information Science, Kyoto Institute of Technology, Kyoto 606-8585, Japan
Abstract The solid phase regrowth (SPR) model for the amorphous SiC phase induced by self-ion-implantation is shown. This was derived from in situ time-resolved optical reflectivity (TROR) measurements, and the crystallinity evaluation method based on Raman spectrometry and Rutherford backscattering spectrometry (RBS). The roughness of an amorphous–recrystalline layer interface was monitored during a heat treatment. By the TROR measurements, it was proven that the roughness of moving interface increases during the SPR. Raman spectrometry measurements have confirmed that the polytype of the SPR layer is the same as that of the substrate. RBS channeling measurements have made it clear that the fraction of c-axis orientation in the SPR layer was about 13%. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 61.72.Dd; 78.20.Bh; 78.30.Ly Keywords: SiC; Ion-implantation; Solid phase regrowth; Raman; RBS
1. Introduction As a selective doping technique in SiC, ion implantation is a very promising technique. Especially, it is considered that high dose ion implantation is suitable for the formation of high density carrier layers for the low contact resistance electrode formation. However, high-temperature processing over 1000 °C is required in order to
*
Corresponding author. Tel.: +81-52-735-5447; fax: +81-52735-5442. E-mail address: [email protected] (O. Eryu).
activate the dopant and to eliminate the ion-implantation defects [1]. The high-temperature process leads to problems such as the formation of a C-rich phase and morphology roughening at the surface [2,3]. It is reported that high-dose implantation into (0 0 0 1)-oriented SiC leads to a regrowth of various polytype crystals during hightemperature annealing [4]. It is difficult to eliminate ion-implantation defects in (0 0 0 1)-oriented SiC, though the most usual SiC substrate is the (0 0 0 1)-oriented hexagonal substrate. It is necessary to understand the formation mechanism of defects such as dislocation loops in the implanted layer during post-ion-implantation annealing in
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00904-2
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O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
order to realize defect-free layers of recrystallized SiC. It was shown in a previous paper that the activation energy of solid phase regrowth (SPR) depends on the substrate orientation for self-implanted amorphous layers on SiC substrates [5,6]. In this paper we derive the SPR kinetics model from in situ time-resolved optical reflectivity (TROR) measurements [5]. Characteristics of the SPR layers are investigated with respect to polytype and orientation by means of Raman spectrometry and Rutherford backscattering spectrometry (RBS).
2. Experimental Samples of n-type (ND NA ’ 1:4 1018 /cm3 )(0 0 0 1) oriented 4H-SiC were implanted with Si ions (40 keV 1.5 1015 /cm2 þ 80 keV 2 1015 / cm2 þ 110 keV 5 1015 /cm2 ) and C ions (20 keV 1.5 1015 /cm2 þ 40 keV 2 1015 /cm2 þ 60 keV 4 1015 /cm2 ) to create a self-implanted amorphous layer (150 nm) at the wafer surface. Self-implantation into SiC crystals allows to minimize impurity effects on the SPR. Furnace annealing induced the SPR was accomplished on the resistively heated stage over a temperature range of 950–1000 °C in air environment. The recrystallization kinetics was observed in situ by the TROR technique. The TROR signal provides a continuous measurement of the position and the roughness of the amorphous–recrystalline layer interface (ac interface) during the SPR. A He–Ne laser (k ¼ 633 nm) was used as probe light. The spot size of the probe beam was about 100 lm in diameter. Raman spectra provide information on polytypes of SiC. The Raman spectra were observed at room temperature using a microscope with an objective lens of 50 magnification. The 488 nm line of an Ar ion laser was used for excitation. A spot size of the excitation beam was about 1 lm in diameter. Back scattering geometry from the surface of the specimen was employed. The scattered light was collected to a double monochromator and detected by a liquid-nitrogen cooled CCD camera. The crystallinity of the SPR layer was evaluated using RBS combined with channeling using 2 MeV Heþ ions.
3. Results and discussion Fig. 1 shows that the TROR signal during the SPR at 970 °C and a calculated reflectivity as a function of the position of the ac interface with a roughening width of 60 nm. A roughening width is decided from the calculation as shown in Fig. 2. The calculation of the reflectivity is carried out under the condition of the fixed the SPR rate. In the TROR technique, the time-dependent changes in the reflectivity which occur during the SPR can be used to deduce the ac interface position. In Fig. 1, it is an important point that the TROR signal shows an attenuation of the last reflectivity oscillation with annealing time. If the ac interface and the surface are flat, the reflectivity increases with annealing time as the calculated curve in Fig. 2 with a D ¼ 0 nm. This is because as the amorphous layer thickness decreases during the SPR, an attenuation of the probe beam reflected from the ac interface by absorption in the amorphous layer also decreases and thus the interference amplitude increases. We have confirmed that the surface after the SPR is optically flat by SEM. By comparing the TROR signal with calculation, the time dependence of the thickness of the SPR layer is
Fig. 1. The temperature dependence of the time-resolved optical reflectivity (TROR) signals during the solid phase regrowth (SPR).
O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
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but an optical roughness has remained actually by the roughness of ac interface increasing. Fig. 3 shows typical Raman spectra observed for an unimplanted substrate, an as-implanted substrate, and after annealing at 1000 °C for 10 min. Sharp peaks in Fig. 3 which appear at 197, 205, 777 and 798 cm1 are phonon signals of the 4H-SiC wafer [8]. These Raman spectra prove that other polytypes than 4H are not present in the SPR layer within the Raman detection limits. In addition, it is proven that defects such as dislocation
Fig. 2. Calculated reflectivity of amorphous SiC on crystalline SiC as a function of amorphous layer thickness. D is an interface roughness assumed in the calculation as shown in Fig. 5. The curves are shifted by 10% for clarity.
plotted in Fig. 1. The SPR rate in the region closer to the surface is slower in the deeper region of the amorphous layer. Fig. 2 shows the calculated reflectivity of amorphous SiC as a function of the amorphous layer thickness on crystalline SiC. The reflectivity was calculated using the assumptions of the SPR model as shown in Fig. 5. In Fig. 2, amorphous and crystalline SiC refractive indices used in the calculation are 4 0:612i and 2:63 1:2 105 i, respectively [7]. Fig. 2 gives an Rtotal (defined in Fig. 5) for different values of D (defined in Fig. 5) ranging from 0 to 100 nm. In addition, a reflectivity Rtotal was calculated by assuming that a roughening density of the ac interface linearly increases for the amorphous thickness (d as shown in Fig. 5). By comparison of TROR signals and calculated curve, it can be estimated that the roughness of the ac interface is about 60 nm. Calculation and TROR signal do not coincide where the ac interface is close to the surface. The above result may be explained by the assumption that D is assumed to be 0 nm when the SPR end in the calculation,
Fig. 3. Raman spectra observed from un-implanted, as-implanted, and after the solid phase regrowth (SPR) at 1000 °C.
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loops are included in the SPR layer, because a peak broadening and a broad Raman band (from 140 to 210 and from 750 to 820 cm1 ) are observed in the Raman spectra [9,10]. It is assumed that the SPR layer is a 4H-SiC polycrystal. On the basis of the TROR measurement and Raman spectrometry, the roughness of the ac interface increases with progressing the SPR. Fig. 4 shows the aligned [0 0 0 1] RBS spectrum after the SPR at 1000 °C for 10 min. The random spectrum is also shown. The height of the aligned spectrum indicates that the amount of dechanneling in the SPR region with a thickness of about 120 nm is anomalously high. This value agrees with the thickness of the SPR layer estimated from the TROR signal in Fig. 1. The normalized minimum yield vmin is estimated to be 87% in the SPR region in the aligned spectrum. The RBS channeling measurement has made clear that the crystallinity components exist in the SPR layer. The SPR model deduced from these experiments is shown in Fig. 5. The ac interface is flat before annealing, so the total reflectivity of an amorphous thin layer on SiC crystal substrate (Rtotal as shown in Fig. 5) can be calculated using a classical planewave analysis (Fig. 5(1)) [11]. During heat treatment at 1000 °C or less, 4H-SiC nuclei are randomly formed at the ac interface. Thus, the ac interface roughness increases by the random
Fig. 5. The solid phase regrowth model of h0 0 0 1i oriented 4HSiC.
nucleation. The interface roughening density increases with progressing the SPR, so that Rtotal changes from the case with the flat the ac interface as shown in Fig. 1. That is, the roughening density of the ac interface increases while the ac interface with roughness D moves to the surface, as shown in Fig. 5(2) and Fig. 5(3). Finally, the ac interface reaches a surface, and the SPR ends. This experiment proves that the amorphized region by high-dose ion implantation does not grow epitaxially during annealing, but the multinucleation has occurred at the ac interface.
4. Conclusions
Fig. 4. RBS spectra for the sample after annealing at 1000 °C for 10 min.
In this work we showed a kinetic model of the SPR of amorphous SiC produced by ion implantation. Using TROR measurements, the roughness of the ac interface was monitored during the heat treatment. Raman spectroscopy measurements
O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
have confirmed that the polytype of the SPR layer is the same that of the 4H-SiC substrate. These experiments clarified that ion implantation induced amorphous layers become polycrystalline with the same polytype as that of the substrate during the SPR by heat treatment up to 1000 °C. Acknowledgements The authors would like to thank the technology division I staff at Ion Engineering Center Corporation for ion implantation and RBS, and Mr. D. Matsuo for his experimental assistance. References [1] T. Kimoto, N. Inoue, H. Matsunami, Phys. Stat. Sol. (a) 162 (1997) 263.
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[2] Y. Negoro, N. Miyamoto, T. Kimoto, H. Matsunami, Mater. Sci. Forum 389–393 (2002) 783. [3] H. Nakamura, H. Watanabe, J. Yamazaki, N. Tanaka, R. Malhan, Mater. Sci. Forum 389–393 (2002) 807. [4] M. Kawamura, K. Higashi, H. Shirakura, M. Kitahara, T. Inada, Proc. 14th Symp. on Mater. Science and Engineering, Hosei Univ. Japan, 1995, p. 151. [5] O. Eryu, D. Matsuo, K. Abe, K. Nakashima, Mater. Sci. Forum 389–393 (2002) 843. [6] G.L. Olson, S.A. Kokorowski, R.A. McFarlane, L.D. Hess, Appl. Phys. Lett. 37 (1980) 1019. [7] W.J. Choyke, E.D. Palik, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids, Academic Press, Orlando, 1985, p. 587. [8] D.W. Feldman, J.H. Parker Jr., W.J. Choyke, L. Patrick, Phys. Rev. 173 (1968) 787. [9] S. Nakasima, Y. Nakatake, H. Harima, M. Katsuno, N. Ohtani, Appl. Phys. Lett. 77 (2000) 3612. [10] S. Nakashima, Y. Nakatake, Y. Ishida, T. Takahashi, H. Okumura, Mater. Sci. Forum 389–393 (2002) 629. [11] R.F. Potter, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids, Academic Press, Orlando, 1985, p. 11 (Chapter 2).
Kinetics of solid phase regrowth of self-ion-implanted amorphous SiC during low temperature furnace annealing O. Eryu a
a,*
, K. Abe a, K. Nakashima a, H. Harima
b
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-Cho, Showa-Ku, Nagoya 466-8555, Japan b Department of Electronics and Information Science, Kyoto Institute of Technology, Kyoto 606-8585, Japan
Abstract The solid phase regrowth (SPR) model for the amorphous SiC phase induced by self-ion-implantation is shown. This was derived from in situ time-resolved optical reflectivity (TROR) measurements, and the crystallinity evaluation method based on Raman spectrometry and Rutherford backscattering spectrometry (RBS). The roughness of an amorphous–recrystalline layer interface was monitored during a heat treatment. By the TROR measurements, it was proven that the roughness of moving interface increases during the SPR. Raman spectrometry measurements have confirmed that the polytype of the SPR layer is the same as that of the substrate. RBS channeling measurements have made it clear that the fraction of c-axis orientation in the SPR layer was about 13%. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 61.72.Dd; 78.20.Bh; 78.30.Ly Keywords: SiC; Ion-implantation; Solid phase regrowth; Raman; RBS
1. Introduction As a selective doping technique in SiC, ion implantation is a very promising technique. Especially, it is considered that high dose ion implantation is suitable for the formation of high density carrier layers for the low contact resistance electrode formation. However, high-temperature processing over 1000 °C is required in order to
*
Corresponding author. Tel.: +81-52-735-5447; fax: +81-52735-5442. E-mail address: [email protected] (O. Eryu).
activate the dopant and to eliminate the ion-implantation defects [1]. The high-temperature process leads to problems such as the formation of a C-rich phase and morphology roughening at the surface [2,3]. It is reported that high-dose implantation into (0 0 0 1)-oriented SiC leads to a regrowth of various polytype crystals during hightemperature annealing [4]. It is difficult to eliminate ion-implantation defects in (0 0 0 1)-oriented SiC, though the most usual SiC substrate is the (0 0 0 1)-oriented hexagonal substrate. It is necessary to understand the formation mechanism of defects such as dislocation loops in the implanted layer during post-ion-implantation annealing in
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00904-2
970
O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
order to realize defect-free layers of recrystallized SiC. It was shown in a previous paper that the activation energy of solid phase regrowth (SPR) depends on the substrate orientation for self-implanted amorphous layers on SiC substrates [5,6]. In this paper we derive the SPR kinetics model from in situ time-resolved optical reflectivity (TROR) measurements [5]. Characteristics of the SPR layers are investigated with respect to polytype and orientation by means of Raman spectrometry and Rutherford backscattering spectrometry (RBS).
2. Experimental Samples of n-type (ND NA ’ 1:4 1018 /cm3 )(0 0 0 1) oriented 4H-SiC were implanted with Si ions (40 keV 1.5 1015 /cm2 þ 80 keV 2 1015 / cm2 þ 110 keV 5 1015 /cm2 ) and C ions (20 keV 1.5 1015 /cm2 þ 40 keV 2 1015 /cm2 þ 60 keV 4 1015 /cm2 ) to create a self-implanted amorphous layer (150 nm) at the wafer surface. Self-implantation into SiC crystals allows to minimize impurity effects on the SPR. Furnace annealing induced the SPR was accomplished on the resistively heated stage over a temperature range of 950–1000 °C in air environment. The recrystallization kinetics was observed in situ by the TROR technique. The TROR signal provides a continuous measurement of the position and the roughness of the amorphous–recrystalline layer interface (ac interface) during the SPR. A He–Ne laser (k ¼ 633 nm) was used as probe light. The spot size of the probe beam was about 100 lm in diameter. Raman spectra provide information on polytypes of SiC. The Raman spectra were observed at room temperature using a microscope with an objective lens of 50 magnification. The 488 nm line of an Ar ion laser was used for excitation. A spot size of the excitation beam was about 1 lm in diameter. Back scattering geometry from the surface of the specimen was employed. The scattered light was collected to a double monochromator and detected by a liquid-nitrogen cooled CCD camera. The crystallinity of the SPR layer was evaluated using RBS combined with channeling using 2 MeV Heþ ions.
3. Results and discussion Fig. 1 shows that the TROR signal during the SPR at 970 °C and a calculated reflectivity as a function of the position of the ac interface with a roughening width of 60 nm. A roughening width is decided from the calculation as shown in Fig. 2. The calculation of the reflectivity is carried out under the condition of the fixed the SPR rate. In the TROR technique, the time-dependent changes in the reflectivity which occur during the SPR can be used to deduce the ac interface position. In Fig. 1, it is an important point that the TROR signal shows an attenuation of the last reflectivity oscillation with annealing time. If the ac interface and the surface are flat, the reflectivity increases with annealing time as the calculated curve in Fig. 2 with a D ¼ 0 nm. This is because as the amorphous layer thickness decreases during the SPR, an attenuation of the probe beam reflected from the ac interface by absorption in the amorphous layer also decreases and thus the interference amplitude increases. We have confirmed that the surface after the SPR is optically flat by SEM. By comparing the TROR signal with calculation, the time dependence of the thickness of the SPR layer is
Fig. 1. The temperature dependence of the time-resolved optical reflectivity (TROR) signals during the solid phase regrowth (SPR).
O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
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but an optical roughness has remained actually by the roughness of ac interface increasing. Fig. 3 shows typical Raman spectra observed for an unimplanted substrate, an as-implanted substrate, and after annealing at 1000 °C for 10 min. Sharp peaks in Fig. 3 which appear at 197, 205, 777 and 798 cm1 are phonon signals of the 4H-SiC wafer [8]. These Raman spectra prove that other polytypes than 4H are not present in the SPR layer within the Raman detection limits. In addition, it is proven that defects such as dislocation
Fig. 2. Calculated reflectivity of amorphous SiC on crystalline SiC as a function of amorphous layer thickness. D is an interface roughness assumed in the calculation as shown in Fig. 5. The curves are shifted by 10% for clarity.
plotted in Fig. 1. The SPR rate in the region closer to the surface is slower in the deeper region of the amorphous layer. Fig. 2 shows the calculated reflectivity of amorphous SiC as a function of the amorphous layer thickness on crystalline SiC. The reflectivity was calculated using the assumptions of the SPR model as shown in Fig. 5. In Fig. 2, amorphous and crystalline SiC refractive indices used in the calculation are 4 0:612i and 2:63 1:2 105 i, respectively [7]. Fig. 2 gives an Rtotal (defined in Fig. 5) for different values of D (defined in Fig. 5) ranging from 0 to 100 nm. In addition, a reflectivity Rtotal was calculated by assuming that a roughening density of the ac interface linearly increases for the amorphous thickness (d as shown in Fig. 5). By comparison of TROR signals and calculated curve, it can be estimated that the roughness of the ac interface is about 60 nm. Calculation and TROR signal do not coincide where the ac interface is close to the surface. The above result may be explained by the assumption that D is assumed to be 0 nm when the SPR end in the calculation,
Fig. 3. Raman spectra observed from un-implanted, as-implanted, and after the solid phase regrowth (SPR) at 1000 °C.
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O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
loops are included in the SPR layer, because a peak broadening and a broad Raman band (from 140 to 210 and from 750 to 820 cm1 ) are observed in the Raman spectra [9,10]. It is assumed that the SPR layer is a 4H-SiC polycrystal. On the basis of the TROR measurement and Raman spectrometry, the roughness of the ac interface increases with progressing the SPR. Fig. 4 shows the aligned [0 0 0 1] RBS spectrum after the SPR at 1000 °C for 10 min. The random spectrum is also shown. The height of the aligned spectrum indicates that the amount of dechanneling in the SPR region with a thickness of about 120 nm is anomalously high. This value agrees with the thickness of the SPR layer estimated from the TROR signal in Fig. 1. The normalized minimum yield vmin is estimated to be 87% in the SPR region in the aligned spectrum. The RBS channeling measurement has made clear that the crystallinity components exist in the SPR layer. The SPR model deduced from these experiments is shown in Fig. 5. The ac interface is flat before annealing, so the total reflectivity of an amorphous thin layer on SiC crystal substrate (Rtotal as shown in Fig. 5) can be calculated using a classical planewave analysis (Fig. 5(1)) [11]. During heat treatment at 1000 °C or less, 4H-SiC nuclei are randomly formed at the ac interface. Thus, the ac interface roughness increases by the random
Fig. 5. The solid phase regrowth model of h0 0 0 1i oriented 4HSiC.
nucleation. The interface roughening density increases with progressing the SPR, so that Rtotal changes from the case with the flat the ac interface as shown in Fig. 1. That is, the roughening density of the ac interface increases while the ac interface with roughness D moves to the surface, as shown in Fig. 5(2) and Fig. 5(3). Finally, the ac interface reaches a surface, and the SPR ends. This experiment proves that the amorphized region by high-dose ion implantation does not grow epitaxially during annealing, but the multinucleation has occurred at the ac interface.
4. Conclusions
Fig. 4. RBS spectra for the sample after annealing at 1000 °C for 10 min.
In this work we showed a kinetic model of the SPR of amorphous SiC produced by ion implantation. Using TROR measurements, the roughness of the ac interface was monitored during the heat treatment. Raman spectroscopy measurements
O. Eryu et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 969–973
have confirmed that the polytype of the SPR layer is the same that of the 4H-SiC substrate. These experiments clarified that ion implantation induced amorphous layers become polycrystalline with the same polytype as that of the substrate during the SPR by heat treatment up to 1000 °C. Acknowledgements The authors would like to thank the technology division I staff at Ion Engineering Center Corporation for ion implantation and RBS, and Mr. D. Matsuo for his experimental assistance. References [1] T. Kimoto, N. Inoue, H. Matsunami, Phys. Stat. Sol. (a) 162 (1997) 263.
973
[2] Y. Negoro, N. Miyamoto, T. Kimoto, H. Matsunami, Mater. Sci. Forum 389–393 (2002) 783. [3] H. Nakamura, H. Watanabe, J. Yamazaki, N. Tanaka, R. Malhan, Mater. Sci. Forum 389–393 (2002) 807. [4] M. Kawamura, K. Higashi, H. Shirakura, M. Kitahara, T. Inada, Proc. 14th Symp. on Mater. Science and Engineering, Hosei Univ. Japan, 1995, p. 151. [5] O. Eryu, D. Matsuo, K. Abe, K. Nakashima, Mater. Sci. Forum 389–393 (2002) 843. [6] G.L. Olson, S.A. Kokorowski, R.A. McFarlane, L.D. Hess, Appl. Phys. Lett. 37 (1980) 1019. [7] W.J. Choyke, E.D. Palik, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids, Academic Press, Orlando, 1985, p. 587. [8] D.W. Feldman, J.H. Parker Jr., W.J. Choyke, L. Patrick, Phys. Rev. 173 (1968) 787. [9] S. Nakasima, Y. Nakatake, H. Harima, M. Katsuno, N. Ohtani, Appl. Phys. Lett. 77 (2000) 3612. [10] S. Nakashima, Y. Nakatake, Y. Ishida, T. Takahashi, H. Okumura, Mater. Sci. Forum 389–393 (2002) 629. [11] R.F. Potter, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids, Academic Press, Orlando, 1985, p. 11 (Chapter 2).