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Ion-beam-induced crystal grain nucleation in amorphous silicon carbide
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 917±921
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Ion-beam-induced crystal grain nucleation in amorphous silicon carbide A. H ofgen *, V. Heera, A. M ucklich, F. Eichhorn, W. Skorupa Forschungszentrum Rossendorf, PF 510119, D-01314 Dresden, Germany
Abstract Ion-beam-induced crystallization (IBIC) was used to produce nanocrystals in the preamorphized region of a 6H-SiC bulk crystal. The precipitation was stimulated by high dose Al implantation at temperatures from 300°C to 700°C. Using cross-sectional transmission electron microscopy (XTEM) and X-ray diraction (XRD) under grazing incidence, the morphology of the nanocrystalline phase and its dependence on the implantation parameters were investigated. After IBIC the morphology of the recrystallized material completely diers from that after thermal crystallization. Randomly oriented grains of 3C-SiC with almost spherical shape and mean diameters ranging from 5 to 20 nm are formed during Al implantation. A critical ion dose for the onset of the recrystallization is found at about 1.9 ´ 1016 Al /cm2 . Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 81.40.E; 61.43 Keywords: 6H-SiC; Ion implantation; Amorphization; Recrystallization
1. Introduction Nanocrystalline materials have attracted considerable interest during the last few years. The reduced size of the crystallites causes changes in their optical, mechanical and thermodynamic properties with respect to the bulk material, see for example [1]. In search of a manufacturing technology to produce semiconductor nanocrystals with reproducible properties, ion implantation was proved to be a well-suited method. It is well es-
*
Corresponding author. Tel.: +49-351-260-2068; fax: +49351-260-3411. E-mail address: [email protected] (A. HoÈfgen).
tablished that ion beam irradiation can stimulate the processes of nucleation and crystal grain growth in amorphous silicon at relatively low temperatures [2]. Furthermore, it has been shown that the irradiation conditions such as ion dose and irradiation temperature critically in¯uence the silicon grain size distribution. In the case of silicon carbide, with its low atomic mobilities and high thermal crystallization temperature, it has also been shown that ion implantation can stimulate the crystallization process [3]. In particular, the formation of a polycrystalline surface layer was found after 300 keV Si implantation at 480°C. However, there have been no systematic investigations of ion-beam-induced nucleation and grain growth in amorphous SiC up to now. In
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 3 7 - 4
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A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
particular, the critical parameter range for the onset of the crystallization and the kinetics of the processes are unknown. Therefore, in this investigation the dependence of the crystallization process on the implantation parameters is studied in detail. 2. Experimental Single-crystalline (c) 6H-SiC wafers with 3.5° o-axis (0 0 0 1) orientation were used as substrate material. To prevent the possible in¯uence of the crystalline±amorphous interface on the recrystallization a 1.8 lm thick amorphous surface layer was generated by 2 MeV Si implantation at room temperature. The crystallization was stimulated by high dose 300 keV Al implantation in a dose range from 3 1015 to 3 1017 Al/cm2 at temperatures from 300°C to 700°C. All implantations were carried out at a constant dose rate of about 3:3 1013 Al /cm2 /s. Al was used because it acts as an acceptor in SiC. The morphology of the
nanocrystalline phase was investigated by crosssectional transmission electron microscopy (XTEM) and X-ray diraction (XRD) under grazing incidence. The grain sizes and their depth distribution were measured directly from the XTEM micrographs. 3. Results and discussion 3.1. Morphology As demonstrated in Fig. 1, recrystallization is observed in the irradiated regions of the amorphous layer already at an implantation temperature of 300°C, well below the SiC thermal recrystallization temperature of about 800°C [4]. The thickness of the recrystallized surface layer corresponds well with the projected range of the 300 keV Al implantation (Rp 350 nm), which was calculated using the SRIM 96 code [5]. It is evident from the halo in the selected-area dirac-
Fig. 1. Cross-sectional TEM micrographs of as-amorphized and Al-implanted a-SiC layers. The corresponding diraction patterns of the Al-implanted and unimplanted regions are shown as insets.
A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
tion pattern in Fig. 1 that the unimplanted region remains amorphous up to implantation temperatures of 700°C. This clearly demonstrates that ion beam irradiation strongly enhances the kinetics of the amorphous to polycrystalline phase transition in SiC. After ion-beam-induced crystallization (IBIC), the morphology of the recrystallized material completely diers from that after thermal crystallization [6,7]. Thermal annealing above 800°C results in a mixture of columns of hexagonal SiC and twinned 3C-SiC regions in the recrystallized layer. After IBIC, SiC grains with almost spherical shape are formed in the Al-implanted regions. The diameter of the grains ranges from 5 to 20 nm. The rings in the selected-area diraction pattern (Fig. 1) of the Al-implanted region clearly show the polycrystalline structure of the recrystallized layer. Halo patterns corresponding to amorphous SiC are not observed in this region, indicating that the implanted layer is completely recrystallized. The measured radii of the diraction rings correspond to the interplanar spacings of the cubic 3C-SiC polytype. Because of the small selected area of diraction (£ 250 nm), the number of contributing particles is relatively small. Therefore, single spots are observed. Their homogenous distribution over the rings indicates that there is no preferential orientation of the nanocrystals. A further indication for the random orientation and the cubic polytype of the grains is provided by the XRD pattern shown in Fig. 2. The diraction pattern (dotted curve) of a sample implanted with 3 ´ 1017 Al/cm2 at 300°C was measured under a small (0.35°) angle of incidence which restricts the penetration depth of the X-rays to less than 1 lm. In this case, only re¯ections from the polycrystalline surface layer contribute to the diraction signal. The comparison with the diraction patterns calculated from powder diraction data of 3C and 6H-SiC (also plotted in Fig. 2) shows a very good correspondence of the measured diraction curve with the 3C-powder diraction data, taking into account the broadening of the re¯ections due to the small crystallites. In particular, the nearly identical intensity ratios of the diraction peaks verify the random orientation of the crystalline grains.
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Fig. 2. X-ray diraction curve (dotted) under grazing incidence (0.35°) of a sample irradiated with 3 1017 Al /cm2 at 300°C. The theoretical powder diraction patterns of 3C and 6H-SiC are plotted for comparison.
3.2. Kinetics In Fig. 3 the mean grain sizes of the recrystallized layers are plotted as a function of ion dose for several implantation temperatures. The results of an IBIC experiment with 300 keV Si at 480°C [3] are plotted for comparison. As seen in Fig. 3, similar results are observed for grain sizes formed after Al or Si implantation. Therefore, it can be assumed that dierences in the chemical nature of the implanted ion species do not essentially change
Fig. 3. Dependence of the average grain size of the polycrystalline layers on the implanted ion dose at several irradiation temperatures.
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A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
the basic process of IBIC. In particular, it can be excluded that metal induced crystallization [8] takes place during Al-implantation. After implantation of ion doses up to 1 ´ 1016 Al /cm2 , no sign of nucleation was detected in the implanted region, whereas after implantation of 3 ´ 1016 Al /cm2 the implanted layer was completely recrystallized. Clearly, there is a critical ion dose for the onset of crystallization. In general, this critical dose is a function of implantation parameters. It is known from IBIC in a-Si [2] that the energy deposited into nuclear collisions, as well as the ion dose rate, critically in¯uence the nucleation and growth. Therefore, the dependence of the 3CSiC grain size on the nuclear damage energy was investigated. In Fig. 4 the depth pro®le of the nuclear damage energy is plotted and compared with the depth dependence of the individual grain sizes determined from the XTEM micrographs. This plot also shows the shift of the visible boundaries of the recrystallized layer with increasing ion dose. In both cases the boundary corresponds to the same value of the damage energy. This result demonstrates that the ion beam enhancement of the crystal grain nucleation occurs above a threshold damage energy of about 5 ´ 1022
keV/cm3 . The threshold energy is approached only for Al-doses P 1:9 1016 Al/cm2 . No dependence of the crystal grain size on the damage energy pro®le was found within the statistical accuracy at damage energies above the threshold. Furthermore, as shown in Fig. 3, the average grain size shows a sublinear dependence on the implanted Al-dose and does not depend on the implantation temperature within the error limits. This result is in contrast to IBIC experiments in the ®eld of a-Si [9], which show a linear increase of the average grain size with increasing ion dose and a distinct dependence on the irradiation temperature. Both the weak grain size dependence on the implantation parameters and the always detected complete recrystallization of the implanted regions suggest that only the ®nal states of the recrystallization process are observed in the present investigation. Obviously, the nucleation and primary grain growth occurs in a very narrow dose range. The dierences in the observed grain sizes are mainly caused by secondary growth processes, such as ripening during continued irradiation [10]. Since the critical parameter range for the onset of the recrystallization has been determined, the investigation of the kinetics of the primary nucleation and grain growth processes will be possible in future experiments. 4. Conclusions
Fig. 4. Depth dependence of the individual grain sizes determined from the cross-sectional TEM micrographs (scatter graphs) in comparison to the damage energy pro®le calculated by SRIM (line + symbol graphs). The visible boundaries of the recrystallized layers are marked by vertical dashed lines. The horizontal dashed line indicates the damage energy threshold value for the onset of the ion-beam-induced crystallization.
The recrystallization behavior of amorphous SiC layers under high dose Al implantation was investigated by XTEM and grazing incidence XRD measurements. The results show that ion irradiation strongly enhances the recrystallization process in a-SiC already at 300°C, well below the thermal recrystallization temperature of about 800°C. The morphology of the recrystallized material completely diers from that after thermal crystallization. Randomly oriented grains of 3C-SiC, with almost spherical shape and mean diameters ranging from 5 to 20 nm are formed during Al implantation. A threshold value of the nuclear damage energy for the beginning of the nucleation exists at about 5 ´ 1022 keV/cm3 . This
A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
corresponds to a threshold dose of 1.9 ´ 1016 Al/cm2 . Above the threshold, the nucleation and grain growth proceeds very fast and only a weak dependence of the crystal grain size on the implantation parameters caused by secondary growth processes is found. In conclusion, the critical ion dose ranges for the processes of primary and secondary growth have been ascertained from the present experiment. Therefore the investigation of the kinetics of nucleation and crystal grain growth in a-SiC will be possible in future experiments.
Acknowledgements This work was supported by Deutsche Forschungsgemeinschaft under contract No. HE 2604/2.
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References [1] U. Woggon, Optical Properties of Semiconductor Quantum Dots, Springer Tracts in Modern Physics, vol. 136, Springer, Berlin, 1997. [2] C. Spinella, S. Lombardo, F. Priolo, J. Appl. Phys. 84 (1998) 5383. [3] V. Heera, J. Stoemenos, R. K ogler, W. Skorupa, J. Appl. Phys. 77 (1994) 2999. [4] A. H ofgen, V. Heera, F. Eichhorn, W. Skorupa, J. Appl. Phys. 84 (1998) 4769. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [6] S. Harada, M. Ishimaru, T. Motooka, Appl. Phys. Lett. 69 (1996) 3534. [7] Y. Pacaud, W. Skorupa, J. Stoemenos, Nucl. Instr. and Meth. B 120 (1996) 181. [8] M.S. Haque, H.A. Naseem, W.D. Brown, J. Appl. Phys. 79 (1996) 7529. [9] C. Spinella, F. Priolo, S. Lombardo, S.U. Campisano, E. Rimini, Nucl. Instr. and Meth. B 59/60 (1991) 363. [10] H.A. Atwater, C.V. Thompson, H.-J. Kim, Nucl. Instr. and Meth. B 39 (1989) 64.
www.elsevier.nl/locate/nimb
Ion-beam-induced crystal grain nucleation in amorphous silicon carbide A. H ofgen *, V. Heera, A. M ucklich, F. Eichhorn, W. Skorupa Forschungszentrum Rossendorf, PF 510119, D-01314 Dresden, Germany
Abstract Ion-beam-induced crystallization (IBIC) was used to produce nanocrystals in the preamorphized region of a 6H-SiC bulk crystal. The precipitation was stimulated by high dose Al implantation at temperatures from 300°C to 700°C. Using cross-sectional transmission electron microscopy (XTEM) and X-ray diraction (XRD) under grazing incidence, the morphology of the nanocrystalline phase and its dependence on the implantation parameters were investigated. After IBIC the morphology of the recrystallized material completely diers from that after thermal crystallization. Randomly oriented grains of 3C-SiC with almost spherical shape and mean diameters ranging from 5 to 20 nm are formed during Al implantation. A critical ion dose for the onset of the recrystallization is found at about 1.9 ´ 1016 Al /cm2 . Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 81.40.E; 61.43 Keywords: 6H-SiC; Ion implantation; Amorphization; Recrystallization
1. Introduction Nanocrystalline materials have attracted considerable interest during the last few years. The reduced size of the crystallites causes changes in their optical, mechanical and thermodynamic properties with respect to the bulk material, see for example [1]. In search of a manufacturing technology to produce semiconductor nanocrystals with reproducible properties, ion implantation was proved to be a well-suited method. It is well es-
*
Corresponding author. Tel.: +49-351-260-2068; fax: +49351-260-3411. E-mail address: [email protected] (A. HoÈfgen).
tablished that ion beam irradiation can stimulate the processes of nucleation and crystal grain growth in amorphous silicon at relatively low temperatures [2]. Furthermore, it has been shown that the irradiation conditions such as ion dose and irradiation temperature critically in¯uence the silicon grain size distribution. In the case of silicon carbide, with its low atomic mobilities and high thermal crystallization temperature, it has also been shown that ion implantation can stimulate the crystallization process [3]. In particular, the formation of a polycrystalline surface layer was found after 300 keV Si implantation at 480°C. However, there have been no systematic investigations of ion-beam-induced nucleation and grain growth in amorphous SiC up to now. In
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 3 7 - 4
918
A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
particular, the critical parameter range for the onset of the crystallization and the kinetics of the processes are unknown. Therefore, in this investigation the dependence of the crystallization process on the implantation parameters is studied in detail. 2. Experimental Single-crystalline (c) 6H-SiC wafers with 3.5° o-axis (0 0 0 1) orientation were used as substrate material. To prevent the possible in¯uence of the crystalline±amorphous interface on the recrystallization a 1.8 lm thick amorphous surface layer was generated by 2 MeV Si implantation at room temperature. The crystallization was stimulated by high dose 300 keV Al implantation in a dose range from 3 1015 to 3 1017 Al/cm2 at temperatures from 300°C to 700°C. All implantations were carried out at a constant dose rate of about 3:3 1013 Al /cm2 /s. Al was used because it acts as an acceptor in SiC. The morphology of the
nanocrystalline phase was investigated by crosssectional transmission electron microscopy (XTEM) and X-ray diraction (XRD) under grazing incidence. The grain sizes and their depth distribution were measured directly from the XTEM micrographs. 3. Results and discussion 3.1. Morphology As demonstrated in Fig. 1, recrystallization is observed in the irradiated regions of the amorphous layer already at an implantation temperature of 300°C, well below the SiC thermal recrystallization temperature of about 800°C [4]. The thickness of the recrystallized surface layer corresponds well with the projected range of the 300 keV Al implantation (Rp 350 nm), which was calculated using the SRIM 96 code [5]. It is evident from the halo in the selected-area dirac-
Fig. 1. Cross-sectional TEM micrographs of as-amorphized and Al-implanted a-SiC layers. The corresponding diraction patterns of the Al-implanted and unimplanted regions are shown as insets.
A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
tion pattern in Fig. 1 that the unimplanted region remains amorphous up to implantation temperatures of 700°C. This clearly demonstrates that ion beam irradiation strongly enhances the kinetics of the amorphous to polycrystalline phase transition in SiC. After ion-beam-induced crystallization (IBIC), the morphology of the recrystallized material completely diers from that after thermal crystallization [6,7]. Thermal annealing above 800°C results in a mixture of columns of hexagonal SiC and twinned 3C-SiC regions in the recrystallized layer. After IBIC, SiC grains with almost spherical shape are formed in the Al-implanted regions. The diameter of the grains ranges from 5 to 20 nm. The rings in the selected-area diraction pattern (Fig. 1) of the Al-implanted region clearly show the polycrystalline structure of the recrystallized layer. Halo patterns corresponding to amorphous SiC are not observed in this region, indicating that the implanted layer is completely recrystallized. The measured radii of the diraction rings correspond to the interplanar spacings of the cubic 3C-SiC polytype. Because of the small selected area of diraction (£ 250 nm), the number of contributing particles is relatively small. Therefore, single spots are observed. Their homogenous distribution over the rings indicates that there is no preferential orientation of the nanocrystals. A further indication for the random orientation and the cubic polytype of the grains is provided by the XRD pattern shown in Fig. 2. The diraction pattern (dotted curve) of a sample implanted with 3 ´ 1017 Al/cm2 at 300°C was measured under a small (0.35°) angle of incidence which restricts the penetration depth of the X-rays to less than 1 lm. In this case, only re¯ections from the polycrystalline surface layer contribute to the diraction signal. The comparison with the diraction patterns calculated from powder diraction data of 3C and 6H-SiC (also plotted in Fig. 2) shows a very good correspondence of the measured diraction curve with the 3C-powder diraction data, taking into account the broadening of the re¯ections due to the small crystallites. In particular, the nearly identical intensity ratios of the diraction peaks verify the random orientation of the crystalline grains.
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Fig. 2. X-ray diraction curve (dotted) under grazing incidence (0.35°) of a sample irradiated with 3 1017 Al /cm2 at 300°C. The theoretical powder diraction patterns of 3C and 6H-SiC are plotted for comparison.
3.2. Kinetics In Fig. 3 the mean grain sizes of the recrystallized layers are plotted as a function of ion dose for several implantation temperatures. The results of an IBIC experiment with 300 keV Si at 480°C [3] are plotted for comparison. As seen in Fig. 3, similar results are observed for grain sizes formed after Al or Si implantation. Therefore, it can be assumed that dierences in the chemical nature of the implanted ion species do not essentially change
Fig. 3. Dependence of the average grain size of the polycrystalline layers on the implanted ion dose at several irradiation temperatures.
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A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
the basic process of IBIC. In particular, it can be excluded that metal induced crystallization [8] takes place during Al-implantation. After implantation of ion doses up to 1 ´ 1016 Al /cm2 , no sign of nucleation was detected in the implanted region, whereas after implantation of 3 ´ 1016 Al /cm2 the implanted layer was completely recrystallized. Clearly, there is a critical ion dose for the onset of crystallization. In general, this critical dose is a function of implantation parameters. It is known from IBIC in a-Si [2] that the energy deposited into nuclear collisions, as well as the ion dose rate, critically in¯uence the nucleation and growth. Therefore, the dependence of the 3CSiC grain size on the nuclear damage energy was investigated. In Fig. 4 the depth pro®le of the nuclear damage energy is plotted and compared with the depth dependence of the individual grain sizes determined from the XTEM micrographs. This plot also shows the shift of the visible boundaries of the recrystallized layer with increasing ion dose. In both cases the boundary corresponds to the same value of the damage energy. This result demonstrates that the ion beam enhancement of the crystal grain nucleation occurs above a threshold damage energy of about 5 ´ 1022
keV/cm3 . The threshold energy is approached only for Al-doses P 1:9 1016 Al/cm2 . No dependence of the crystal grain size on the damage energy pro®le was found within the statistical accuracy at damage energies above the threshold. Furthermore, as shown in Fig. 3, the average grain size shows a sublinear dependence on the implanted Al-dose and does not depend on the implantation temperature within the error limits. This result is in contrast to IBIC experiments in the ®eld of a-Si [9], which show a linear increase of the average grain size with increasing ion dose and a distinct dependence on the irradiation temperature. Both the weak grain size dependence on the implantation parameters and the always detected complete recrystallization of the implanted regions suggest that only the ®nal states of the recrystallization process are observed in the present investigation. Obviously, the nucleation and primary grain growth occurs in a very narrow dose range. The dierences in the observed grain sizes are mainly caused by secondary growth processes, such as ripening during continued irradiation [10]. Since the critical parameter range for the onset of the recrystallization has been determined, the investigation of the kinetics of the primary nucleation and grain growth processes will be possible in future experiments. 4. Conclusions
Fig. 4. Depth dependence of the individual grain sizes determined from the cross-sectional TEM micrographs (scatter graphs) in comparison to the damage energy pro®le calculated by SRIM (line + symbol graphs). The visible boundaries of the recrystallized layers are marked by vertical dashed lines. The horizontal dashed line indicates the damage energy threshold value for the onset of the ion-beam-induced crystallization.
The recrystallization behavior of amorphous SiC layers under high dose Al implantation was investigated by XTEM and grazing incidence XRD measurements. The results show that ion irradiation strongly enhances the recrystallization process in a-SiC already at 300°C, well below the thermal recrystallization temperature of about 800°C. The morphology of the recrystallized material completely diers from that after thermal crystallization. Randomly oriented grains of 3C-SiC, with almost spherical shape and mean diameters ranging from 5 to 20 nm are formed during Al implantation. A threshold value of the nuclear damage energy for the beginning of the nucleation exists at about 5 ´ 1022 keV/cm3 . This
A. H ofgen et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 917±921
corresponds to a threshold dose of 1.9 ´ 1016 Al/cm2 . Above the threshold, the nucleation and grain growth proceeds very fast and only a weak dependence of the crystal grain size on the implantation parameters caused by secondary growth processes is found. In conclusion, the critical ion dose ranges for the processes of primary and secondary growth have been ascertained from the present experiment. Therefore the investigation of the kinetics of nucleation and crystal grain growth in a-SiC will be possible in future experiments.
Acknowledgements This work was supported by Deutsche Forschungsgemeinschaft under contract No. HE 2604/2.
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References [1] U. Woggon, Optical Properties of Semiconductor Quantum Dots, Springer Tracts in Modern Physics, vol. 136, Springer, Berlin, 1997. [2] C. Spinella, S. Lombardo, F. Priolo, J. Appl. Phys. 84 (1998) 5383. [3] V. Heera, J. Stoemenos, R. K ogler, W. Skorupa, J. Appl. Phys. 77 (1994) 2999. [4] A. H ofgen, V. Heera, F. Eichhorn, W. Skorupa, J. Appl. Phys. 84 (1998) 4769. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [6] S. Harada, M. Ishimaru, T. Motooka, Appl. Phys. Lett. 69 (1996) 3534. [7] Y. Pacaud, W. Skorupa, J. Stoemenos, Nucl. Instr. and Meth. B 120 (1996) 181. [8] M.S. Haque, H.A. Naseem, W.D. Brown, J. Appl. Phys. 79 (1996) 7529. [9] C. Spinella, F. Priolo, S. Lombardo, S.U. Campisano, E. Rimini, Nucl. Instr. and Meth. B 59/60 (1991) 363. [10] H.A. Atwater, C.V. Thompson, H.-J. Kim, Nucl. Instr. and Meth. B 39 (1989) 64.