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Si multilayers with ion beams
Nuclear Instruments and Methods in Physics Research B 178 (2001) 279±282
www.elsevier.nl/locate/nimb
Coherent amorphization of Ge/Si multilayers with ion beams E. Alves
a,*
, A.D. Sequeira a, N. Franco a, M.F. da Silva a, J.C. Soares a, N.A. Sobolev b, M.C. Carmo b a
Deptartamento de Fõsica, Instituto Tecnol ogico e Nuclear, EN10, 2686-953 Sacav em, Portugal b Departamento de Fõsica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Abstract Dierent Ge/Si superlattices were irradiated with 150 keV Ar ions at room temperature with ¯uences in the range 1012 to 5 1015 cm 2 . Defect production was studied with Rutherford backscattering/channeling spectrometry and Xray diraction (XRD). The evolution of the damage with ion ¯uence reveals the existence of three distinct regimes. During the ®rst regime the concentration of defects increases slowly until the minimum yield reaches a value of 20%. The onset of the second regime occurs at a ¯uence of 2 1013 cm 2 and is characterized by a fast buildup of the damage. This regime develops in a narrow interval of ¯uences 2±5 1013 cm 2 and leads to a saturation of the measured damage. The third regime corresponds to the formation of a continuous amorphous layer that widens with the increase of ¯uence. A large asymmetry of the diuse X-ray scattering arises near the critical dose of the crystallineto-amorphous transition and then relaxes again. The presence of three regimes was also observed in pure silicon irradiated simultaneously with the superlattices, but with a damage saturation threshold nearly one order of magnitude higher 3 1014 cm 2 . We suppose the driving force of the transition is the excess of strain in the implanted region. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Superlattice; Ion implantation; Amorphization
1. Introduction Ion±solid interactions have been studied intensively during the last decades. The technological impact of these investigations with the incorporation of ion implantation in the processing technologies of the semiconductor industry explains the large amount of research in this area [1,2]. The
*
Corresponding author. Tel.: +351-21-9946086; fax: +35121-9941525. E-mail address: [email protected] (E. Alves).
ballistic nature of ion implantation leads to the formation of dierent defect structures through the entire implanted layer. The knowledge of these structures and their stability is of crucial importance for the technological applications. Nevertheless, although all the eort and the large number of experimental techniques available to study defects and defect interactions, there are still some gaps to ®ll in order to get the detailed picture of all the aspects related with these processes. Recently the possibility to amorphize selectively multilayered semiconductor structures (SiGe/Si and AlAs/GaAs) were pointed out by some
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 8 0 - 8
280
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 279±282
authors [3,4]. The selectivity of the amorphization process allows the formation of alternating amorphous±crystalline layers in quantum-well structures. However, this selectivity disappears when the thickness of individual layers is only a few monoatomic layers [5,6]. In this case, despite all the research eorts, the role played in the amorphization process by the compositional changes due to ion mixing or thermal spikes and by the buildup of internal strain is still a matter of debate. In this work, we used Rutherford backscattering spectrometry (RBS) and X-ray diraction (XRD) to investigate the amorphization of SiGe multilayers with dierent periods under argon ion irradiation. In both cases we found a coherent amorphization of the Si and Ge layers at a threshold value similar to the one of pure Ge and one order of magnitude lower than the value found for the pure silicon.
laxed SiGe alloy ®lm were grown before the deposition of the Si and Ge layers. The Si5 Ge5 and Si6 Ge4 superlattices with 360 and 100 periods, respectively, were deposited at 520°C. Further details and a complete characterisation of the structures can be found elsewhere [7±9]. The samples were irradiated at room temperature with Ar ions with ¯uences in the range 1 1012 ±5 1014 cm 2 . The energy was 150 keV and the current was kept below 0:5 lA cm 2 to avoid beam heating eects. Defect production was followed with RBS in the channeling mode and X-ray analysis. The backscattered particles were detected at 140° with respect to the beam direction using a silicon surface barrier detector located in the standard IBM geometry and with resolution of 13 keV. The x±2h reciprocal space mappings and the integral x-scan were performed using a high-resolution high-temperature double-crystal X-ray diractometer, the Hotbird, at ITN [10].
2. Experimental
3. Results
The SiGe superlattices were grown by MBE on (1 0 0) Si substrates. A buer layer of step-graded SiGe alloy with a thickness of 650 nm or a thin re-
The RBS results shown in Fig. 1 were obtained for the Si5 Ge5 superlattice. The aligned spectrum along the (1 0 0) axis of the virgin sample indicates a
Fig. 1. Random and aligned RBS spectra for the Si5 Ge5 multilayer irradiated with the Ar ¯uences shown.
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 279±282
good single crystalline quality of the multilayer. Up to a dose of 2 1013 the damage increases linearly. Since at room temperature the point defects are mobile both in silicon and germanium it is expected to ®nd essentially clusters of interstitials and vacancies formed during their migration during this stage. After this regime the amount of defects increases rapidly and the damage peak reaches the random level for a ¯uence of 5 1013 cm 2 . The TEM studies reveal that at this ¯uence, although heavily damaged, the implanted region is not yet amorphous [5,6]. After reaching the amorphization the increase of the ¯uence only widens the amorphous layer. These results are well summarized in Fig. 2 where we plot the normalized minimum yield versus the ¯uence. Comparing the curves obtained for pure silicon and for the individual multilayer components we observe two main dierences. First, we see that during the ®rst regime the number of defects in the implanted region increases faster in the multilayer structure. Second, the amorphization threshold for silicon in the superlattice was reduced one order of magnitude compared to the bulk material and is equal to that of germanium. The accelerated growth of defects observed for the superlattice could be related with the presence of some residual strain that can be released by the stabilization of the point defects into complex defect structures. On the other hand, the layered structure of the superlattice can accommodate large strains
Fig. 2. Normalized minimum yield versus the implanted ¯uence. The solid lines are guides to highlight the trends.
281
compared to pure silicon. This could be the explanation for dierent values found for the triggering of the second damage regime. In fact we see that the second regime starts at a minimum yield of 10% for silicon while for the superlattice layer it starts at 25%. However, in both cases the transition from the lightly damaged region to the amorphous state occurs within a narrow range of ¯uences. It seems that the same physical mechanism is responsible for this transition. One of the models for the crystallineto-amorphous transition in pure silicon assumes that it happens due to the collapse of heavily damage regions [11]. This collapse is driven by the strain created in the implanted region. The same behavior was found for the Si6 Ge4 multilayers. XRD was used to measured the strain in the implanted layers and follow its evolution with the implanted ¯uence. The results shown in Fig. 3 are the integral x-scans (Fig. 3(a)) and x±2h reciprocal space mappings (Fig. 3(b)) of the (4 0 0) Bragg peak, obtained for the Si5 Ge5 multilayer as function of the ¯uence. Fig. 3(a) shows the formation of a satellite peak on the left of the (4 0 0) peak that is associated with the expansion of the lattice parameter (perpendicular to the surface) on the implanted region produced by the accumulation of defects. For a ¯uence of 1:5 1014 cm 2 we observe the presence of a broad peak around 68:2° due to the diuse scattering from the amorphous layer. To have a ®ne picture of the transition to the amorphous state, a detailed study is under way for the samples implanted with intermediate ¯uences in the range of 5 1013 ±2 1014 cm 2 . The structural information is more evident from the two-dimensional reciprocal-space maps (Fig. 3(b)). In Fig. 3(b) we show the reciprocal space maps around the (4 0 0) re¯ection corresponding to the same ¯uences of Fig. 3(a). We observe an increase in the diuse scattering on the left side of the peak with the dose. The scattering becomes highly anisotropic for a ¯uence of 7 1013 cm 2 with the appearance of a `streak' at lower values of the scattering angle. This indicates a non-uniform expansion of the lattice. The lattice parameter in this region varies from the value of the unimplanted material towards larger lattice parameters. When this region collapses the diusion scattering from the amorphous layer originates the broad peak centered at 68:2°. These
282
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 279±282
4. Conclusions Our results show the existence of three regimes during the crystalline to amorphous transition in SiGe superlattices with few monoatomic layers at room temperature. The ®rst regime corresponds to the formation of stable point defect clusters. After this regime the defect concentration rises very fast and the amorphization occurs. We propose that the excess of strain in the implanted region originate the driving force for the transition. Acknowledgements We wish to thank H. Presting for supplying the samples and numerous valuable discussions and C. Marques and J. Rocha for carrying out the Ar implantations. References
Fig. 3. (a) Sequence of integral x-scans from the (4 0 0) diffraction planes as a function of the implanted dose. (b) The XRD x±2h reciprocal space maps around the (4 0 0) re¯ection.
results clearly show that the transition from the crystalline-to-amorphous state in the multilayer is related with the strain developed in the implanted region due to the accumulation of defects. When the maximum value of strain is exceeded the implanted region collapses to become amorphous.
[1] J.F. Gibbons, Y. Sugimoto, Nucl. Instr. and Meth. B 21 (1987) 83. [2] J.W. Corbett, J. Karins, T.Y. Tan, Nucl. Instr. and Meth. 182/183 (1981) 457. [3] D.J. Eaglesham, J.M. Poate, D.C. Jacobson, M. Cerullo, L.N. Pfeier, K. West, Appl. Phys. Lett. 58 (1991) 523. [4] M. Vos, C. Wu, I.V. Mitchell, T.E. Jackman, J.M. Baribeau, J. McCarey, Appl. Phys. Lett. 58 (1991) 951. [5] N.A. Sobolev, K. Gartner, U. Kaiser, U. K onig, H. Presting, B. Weber, E. Wendler, W. Wesch, Mater. Sci. Forum 248±249 (1997) 289. [6] N.A. Sobolev, U. Kaiser, I.I. Khodos, H. Presting,, U. K onig, in: S.J. Zinkle et al., Microstructural Processes in Irradiated Materials, Mat. Res. Soc. Symp. Proc., Vol. 540, 1999. [7] U. Mencziger, G. Abstreiter, J. Olajos, H. Grimmeis, H. Kibbel, H. Presting, E. Kasper, Phys. Rev. B 44 (1991) 9118. [8] W. Jager, D. Stenkamp, P. Ehrhart, K. Leifer, W. Sybertz, H. Kibbel, H. Presting, E. Kasper, Thin Solid Films 222 (1992) 221. [9] H. Presting, H. Kibbel, M. Jaros, R.M. Turton, U. Menczigar, G. Abstreiter, H.G. Grimmeis, Semicond. Sci. Technol. 7 (1992) 1127. [10] A.D. Sequeira, N. Franco, J. Neves, Mater. Sci. Forum, in press. [11] G. Bai, M.A. Nicolet, J. Appl. Phys. 70 (1991) 649.
www.elsevier.nl/locate/nimb
Coherent amorphization of Ge/Si multilayers with ion beams E. Alves
a,*
, A.D. Sequeira a, N. Franco a, M.F. da Silva a, J.C. Soares a, N.A. Sobolev b, M.C. Carmo b a
Deptartamento de Fõsica, Instituto Tecnol ogico e Nuclear, EN10, 2686-953 Sacav em, Portugal b Departamento de Fõsica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Abstract Dierent Ge/Si superlattices were irradiated with 150 keV Ar ions at room temperature with ¯uences in the range 1012 to 5 1015 cm 2 . Defect production was studied with Rutherford backscattering/channeling spectrometry and Xray diraction (XRD). The evolution of the damage with ion ¯uence reveals the existence of three distinct regimes. During the ®rst regime the concentration of defects increases slowly until the minimum yield reaches a value of 20%. The onset of the second regime occurs at a ¯uence of 2 1013 cm 2 and is characterized by a fast buildup of the damage. This regime develops in a narrow interval of ¯uences 2±5 1013 cm 2 and leads to a saturation of the measured damage. The third regime corresponds to the formation of a continuous amorphous layer that widens with the increase of ¯uence. A large asymmetry of the diuse X-ray scattering arises near the critical dose of the crystallineto-amorphous transition and then relaxes again. The presence of three regimes was also observed in pure silicon irradiated simultaneously with the superlattices, but with a damage saturation threshold nearly one order of magnitude higher 3 1014 cm 2 . We suppose the driving force of the transition is the excess of strain in the implanted region. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Superlattice; Ion implantation; Amorphization
1. Introduction Ion±solid interactions have been studied intensively during the last decades. The technological impact of these investigations with the incorporation of ion implantation in the processing technologies of the semiconductor industry explains the large amount of research in this area [1,2]. The
*
Corresponding author. Tel.: +351-21-9946086; fax: +35121-9941525. E-mail address: [email protected] (E. Alves).
ballistic nature of ion implantation leads to the formation of dierent defect structures through the entire implanted layer. The knowledge of these structures and their stability is of crucial importance for the technological applications. Nevertheless, although all the eort and the large number of experimental techniques available to study defects and defect interactions, there are still some gaps to ®ll in order to get the detailed picture of all the aspects related with these processes. Recently the possibility to amorphize selectively multilayered semiconductor structures (SiGe/Si and AlAs/GaAs) were pointed out by some
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 8 0 - 8
280
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 279±282
authors [3,4]. The selectivity of the amorphization process allows the formation of alternating amorphous±crystalline layers in quantum-well structures. However, this selectivity disappears when the thickness of individual layers is only a few monoatomic layers [5,6]. In this case, despite all the research eorts, the role played in the amorphization process by the compositional changes due to ion mixing or thermal spikes and by the buildup of internal strain is still a matter of debate. In this work, we used Rutherford backscattering spectrometry (RBS) and X-ray diraction (XRD) to investigate the amorphization of SiGe multilayers with dierent periods under argon ion irradiation. In both cases we found a coherent amorphization of the Si and Ge layers at a threshold value similar to the one of pure Ge and one order of magnitude lower than the value found for the pure silicon.
laxed SiGe alloy ®lm were grown before the deposition of the Si and Ge layers. The Si5 Ge5 and Si6 Ge4 superlattices with 360 and 100 periods, respectively, were deposited at 520°C. Further details and a complete characterisation of the structures can be found elsewhere [7±9]. The samples were irradiated at room temperature with Ar ions with ¯uences in the range 1 1012 ±5 1014 cm 2 . The energy was 150 keV and the current was kept below 0:5 lA cm 2 to avoid beam heating eects. Defect production was followed with RBS in the channeling mode and X-ray analysis. The backscattered particles were detected at 140° with respect to the beam direction using a silicon surface barrier detector located in the standard IBM geometry and with resolution of 13 keV. The x±2h reciprocal space mappings and the integral x-scan were performed using a high-resolution high-temperature double-crystal X-ray diractometer, the Hotbird, at ITN [10].
2. Experimental
3. Results
The SiGe superlattices were grown by MBE on (1 0 0) Si substrates. A buer layer of step-graded SiGe alloy with a thickness of 650 nm or a thin re-
The RBS results shown in Fig. 1 were obtained for the Si5 Ge5 superlattice. The aligned spectrum along the (1 0 0) axis of the virgin sample indicates a
Fig. 1. Random and aligned RBS spectra for the Si5 Ge5 multilayer irradiated with the Ar ¯uences shown.
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 279±282
good single crystalline quality of the multilayer. Up to a dose of 2 1013 the damage increases linearly. Since at room temperature the point defects are mobile both in silicon and germanium it is expected to ®nd essentially clusters of interstitials and vacancies formed during their migration during this stage. After this regime the amount of defects increases rapidly and the damage peak reaches the random level for a ¯uence of 5 1013 cm 2 . The TEM studies reveal that at this ¯uence, although heavily damaged, the implanted region is not yet amorphous [5,6]. After reaching the amorphization the increase of the ¯uence only widens the amorphous layer. These results are well summarized in Fig. 2 where we plot the normalized minimum yield versus the ¯uence. Comparing the curves obtained for pure silicon and for the individual multilayer components we observe two main dierences. First, we see that during the ®rst regime the number of defects in the implanted region increases faster in the multilayer structure. Second, the amorphization threshold for silicon in the superlattice was reduced one order of magnitude compared to the bulk material and is equal to that of germanium. The accelerated growth of defects observed for the superlattice could be related with the presence of some residual strain that can be released by the stabilization of the point defects into complex defect structures. On the other hand, the layered structure of the superlattice can accommodate large strains
Fig. 2. Normalized minimum yield versus the implanted ¯uence. The solid lines are guides to highlight the trends.
281
compared to pure silicon. This could be the explanation for dierent values found for the triggering of the second damage regime. In fact we see that the second regime starts at a minimum yield of 10% for silicon while for the superlattice layer it starts at 25%. However, in both cases the transition from the lightly damaged region to the amorphous state occurs within a narrow range of ¯uences. It seems that the same physical mechanism is responsible for this transition. One of the models for the crystallineto-amorphous transition in pure silicon assumes that it happens due to the collapse of heavily damage regions [11]. This collapse is driven by the strain created in the implanted region. The same behavior was found for the Si6 Ge4 multilayers. XRD was used to measured the strain in the implanted layers and follow its evolution with the implanted ¯uence. The results shown in Fig. 3 are the integral x-scans (Fig. 3(a)) and x±2h reciprocal space mappings (Fig. 3(b)) of the (4 0 0) Bragg peak, obtained for the Si5 Ge5 multilayer as function of the ¯uence. Fig. 3(a) shows the formation of a satellite peak on the left of the (4 0 0) peak that is associated with the expansion of the lattice parameter (perpendicular to the surface) on the implanted region produced by the accumulation of defects. For a ¯uence of 1:5 1014 cm 2 we observe the presence of a broad peak around 68:2° due to the diuse scattering from the amorphous layer. To have a ®ne picture of the transition to the amorphous state, a detailed study is under way for the samples implanted with intermediate ¯uences in the range of 5 1013 ±2 1014 cm 2 . The structural information is more evident from the two-dimensional reciprocal-space maps (Fig. 3(b)). In Fig. 3(b) we show the reciprocal space maps around the (4 0 0) re¯ection corresponding to the same ¯uences of Fig. 3(a). We observe an increase in the diuse scattering on the left side of the peak with the dose. The scattering becomes highly anisotropic for a ¯uence of 7 1013 cm 2 with the appearance of a `streak' at lower values of the scattering angle. This indicates a non-uniform expansion of the lattice. The lattice parameter in this region varies from the value of the unimplanted material towards larger lattice parameters. When this region collapses the diusion scattering from the amorphous layer originates the broad peak centered at 68:2°. These
282
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 178 (2001) 279±282
4. Conclusions Our results show the existence of three regimes during the crystalline to amorphous transition in SiGe superlattices with few monoatomic layers at room temperature. The ®rst regime corresponds to the formation of stable point defect clusters. After this regime the defect concentration rises very fast and the amorphization occurs. We propose that the excess of strain in the implanted region originate the driving force for the transition. Acknowledgements We wish to thank H. Presting for supplying the samples and numerous valuable discussions and C. Marques and J. Rocha for carrying out the Ar implantations. References
Fig. 3. (a) Sequence of integral x-scans from the (4 0 0) diffraction planes as a function of the implanted dose. (b) The XRD x±2h reciprocal space maps around the (4 0 0) re¯ection.
results clearly show that the transition from the crystalline-to-amorphous state in the multilayer is related with the strain developed in the implanted region due to the accumulation of defects. When the maximum value of strain is exceeded the implanted region collapses to become amorphous.
[1] J.F. Gibbons, Y. Sugimoto, Nucl. Instr. and Meth. B 21 (1987) 83. [2] J.W. Corbett, J. Karins, T.Y. Tan, Nucl. Instr. and Meth. 182/183 (1981) 457. [3] D.J. Eaglesham, J.M. Poate, D.C. Jacobson, M. Cerullo, L.N. Pfeier, K. West, Appl. Phys. Lett. 58 (1991) 523. [4] M. Vos, C. Wu, I.V. Mitchell, T.E. Jackman, J.M. Baribeau, J. McCarey, Appl. Phys. Lett. 58 (1991) 951. [5] N.A. Sobolev, K. Gartner, U. Kaiser, U. K onig, H. Presting, B. Weber, E. Wendler, W. Wesch, Mater. Sci. Forum 248±249 (1997) 289. [6] N.A. Sobolev, U. Kaiser, I.I. Khodos, H. Presting,, U. K onig, in: S.J. Zinkle et al., Microstructural Processes in Irradiated Materials, Mat. Res. Soc. Symp. Proc., Vol. 540, 1999. [7] U. Mencziger, G. Abstreiter, J. Olajos, H. Grimmeis, H. Kibbel, H. Presting, E. Kasper, Phys. Rev. B 44 (1991) 9118. [8] W. Jager, D. Stenkamp, P. Ehrhart, K. Leifer, W. Sybertz, H. Kibbel, H. Presting, E. Kasper, Thin Solid Films 222 (1992) 221. [9] H. Presting, H. Kibbel, M. Jaros, R.M. Turton, U. Menczigar, G. Abstreiter, H.G. Grimmeis, Semicond. Sci. Technol. 7 (1992) 1127. [10] A.D. Sequeira, N. Franco, J. Neves, Mater. Sci. Forum, in press. [11] G. Bai, M.A. Nicolet, J. Appl. Phys. 70 (1991) 649.