- Email: [email protected]
Influences of ion species (Ge+, Sb+, Au2+) on crystallization of amorphous Ge films induced by high energy ion irradiation
& *H
Nuclear Instruments and Methodsin Physics Research B 127/ 128 ( 1997) 82-85 kYlOMl
B
Beam InteractIons with Yaterials & Atoms
ELSEVIER
Influences of ion species (Ge+, Sbf, Au2+) on crystallization of amorphous Ge films induced by high energy ion irradiation Setsuo Nakao
*, Kazuo Saitoh, Masami Ikeyama,
Hiroalci Niwa, Seita Tanemura, Yoshiko Miyagawa, Soji Miyagawa
National Industrial Research Institute of Nagoya, 1-I Hirate-rho, Kita-ku, Nagoya 462, Japan
Abstract
Amorphous Ge films deposited on CaFz substrates were irradiated with Ge, Sb and Au ions and their structural changes were examined. Samples during irradiations were kept at a constant temperature about 380°C since a formation of randomly oriented crystals in amorphous matrix was closely related to temperature. It was found that the crystal quality of the epitaxial Ge films became good and the formation of randomly oriented crystals was retarded with increasing ion mass. These results presumably suggested that the crystal quality was correlated with the amount of randomly oriented crystals. Furthermore, it was assumed that the formation of randomly oriented crystals were affected by the single collision cascade whose size was increased with ion mass. From these results, it was concluded that heavy ion irradiation was effective for the enhancement of the epitaxial crystallization and the suppression of the formation of randomly oriented crystals.
1. Introduction Epitaxial crystallization of amorphous (a-) layer induced by ion irradiation has attracted much attention. Especially, there are many studies [l-6] on ion-beam-induced-epitaxial-crystallization (IBIEC) concerning Si material because it is essentially low-temperature process. In the IBIEC, it was pointed out that nuclear energy deposition was a dominant parameter [l]. However, it was recently reported that electronic energy deposition also played a significant role on the crystallization of a-Si layer [6]. In the case of other semiconductor material such as Ge, a similar tendency should be expected. We have studied on the crystallization of a-Ge films deposited on CaF, substrates, which is an example of semiconductor-on-insulator, and found that the epitaxial crystallization could be induced by high energy ion irradiation [7-91. However, the crystallization of a-Ge films were more complicated than IBIEC of Si because the crystallized Ge films included some randomly oriented crystals [8,9]. In order to control the microstructure, it is of importance to understand the effects of irradiation parameters related to energy deposition on the crystallization of the Ge films. In particular, it is of interest to know what parameter influences on the formation of randomly oriented crystals.
* Corresponding author. Fax: +81 52 916 2802; e-mail: [email protected] 0168-583X/97/$17.00
In this study, a-Ge films deposited on CaF, substrates were irradiated with various ion species and their structural changes were examined.
2. Experimental a-Ge films were deposited on air-cleaved single crystal evaporation at room temperature. The pressure was about 10m4 Pa during evaporation. Thickness of the films was about 120 nm. Ion beam irradiation and Rutherford backscattering spectrometry (RBS) combined with channeling technique were performed by tandem-type ion accelerator (NEC SSDH-II pelletron accelerator) at NIRIN. Details of experimental setup was described elsewhere [ 101. Typical irradiation conditions are summarized in Table 1. Ge+, Sb+ and Au2+ were used for irradiation. Ion irradiation was performed in 5” off ( 111) to prevent the channeling effect. Ion energy of each ion was chosen so that the projected range was about 400 nm. The projected range was much deeper than the film thickness. Sample temperature during irradiation was measured by pyrometer and was about 380°C. Time of irradiation was about 53 min. The Ge films remained entirely amorphous phase by thermal annealing at 400°C for I h [8]. Nuclear and electronic energy depositions in the Ge film calculated by TRIM code [ll] were also listed in Table 1. The ratio of the nuclear energy deposition (N) per ion among irradiations with the 3 kinds of ions, CaF,(l 11) substrates by electron-heating
0 1997 Elsevier Science B.V. All rights reserved.
HI SO1 68-583X(96)00856-7
S. Nakao et al./Nucl.
Instr. and Meth. in Phys. Res. B 127/
83
128 (1997) 82-M
N(Ge):N(Sb):N(Au), is approximately 1: 1.53. Ion Dose is chosen so that the total nuclear energy deposition (N X ion dose) is approximately the same (distribution and intensity) among the irradiations. Gn the other hand, the ratio of the electronic energy deposition (E) per ion among the irradiations, E(Ge):E(Sb):E(Au), is approximately 1:2:3. Then, the total electronic energy deposition (E X ion dose) for Ge irradiation approximately coincides with for Au irradiation. The total electronic energy deposition for Sb irradiation is larger than for others. The degree of the crystallization of a-Ge films was examined by in-situ RBS-channeling measurement. The RBS-channeling measurement was performed with 1.0 MeV He+ along with CaF, (111) axis. The surfaces of the Ge films were observed by field emission scanning microscopy (FE-SEM) at an acceleration voltage of 3 kV. Thin film x-ray diffraction (XRD) measurement was performed by 20-scan operation system with 3” incident x-rays.
3. Results and discussion Fig. 1 shows the RBS aligned spectra of the samples before and after irradiation under the different conditions. Solid line shows the RBS random spectrum for comparison. Before irradiation, an aligned yield from Ge is in agreement with the random yield due to the amorphous structure. After irradiation, the aligned yield decreases apparently. In addition, there is a clear difference between the irradiations. The aligned yield for Sb-irradiation sample is smaller than that for Ge-irradiation sample. Moreover, the further decrease in the aligned yield is observed for Au-irradiation sample. These results clearly show that the crystallization effect depends on ion species and it is enhanced with increasing ion mass. Fig. 2 shows the FE-SEM micrographs of the surfaces Fig. 2. FE-SEM micrographsof the surfaces of the Ge films after irradiations with various ion species. 9000
II 50
100
150
200
CHANNEL
Fig. 1. RBS aligned spectra of the samples before and after irradiations with various ion species under different conditions of ion energy and ion dose. Solid line shows RBS random spectrum for comparison.
of the Ge films after the irradiations. For Ge irradiation, the mottled pattern is observed on the surface of the film. For Sb irradiation, similar surface having like speckles also appears. These results indicate that both films have a mosaic structure. However, the density of the speckles for Sb-irradiation film seems to be lower than that for Geirradiation film. In contrast to these films, the uniform surface is observed for the Au-irradiation film. These results show that the crystal quality of the epitaxial films becomes good with the increase of ion mass. Fig. 3 shows the thin film (20 scan) XRD patterns of the samples. When using Ge and Sb ions, 3 peaks arising from Ge(1 11), (220) and (311) planes are clearly observed. This means that the Ge films crystallized by each irradiation include the crystals with random orientation. However, the peak intensity of the Sb-irradiation sample is
1. FUNDAMENTALS/BASICS
S. Nakao et al./Nucl. /
I
I
30
20
I
Ge(ll1)
I
I
I Ge(31 I)
Ge(220)
40 2 e (degree) /CuK a
Insrr. and Meth. in Phys. Res. B 127/128
50
60
Fig. 3. Thin film XRD patterns of the samples after irradiations
with various ion species.
slightly lower than that of the Ge-irradiation sample. In addition, full widths at half maximum of the Ge( 111) peak are about 0.7” for both samples, suggesting that the crystals have similar size in average. So, the number of randomly oriented crystals in the Sb-irradiation sample is thought to be smaller than that in the Ge-irradiation sample. While, there is no peak for Au irradiation. Therefore, it can be said that the Ge film crystallized by Au irradiation rarely includes the randomly oriented crystals. The presence of randomly oriented Ge crystals in the films irradiated with Ge and Sb ions suggests that the formation of the nuclei and the growth in amorphous matrix are conducted by the irradiations. While, the fact that Au-irradiation film rarely includes the randomly oriented Ge crystals shows the suppression of the nucleation and/or the growth of the randomly oriented Ge crystals. The difference in the amount of the randomly oriented Ge crystals can not be explained by the total nucleation energy deposition since it is approximately ,,the same among the irradiations. A possible explanation is as follows. Nucleation and growth of crystals in amorphous matrix induced by ion irradiation strongly depend on temperature during irradiation, as reported by several authors [ 12- 141. At high temperatures (> 5OO”C), ion irradiation greatly enhances the nucleation rate as well as the growth rate. At relatively low temperatures (200-5OO”C), however, the nucleation and the growth of small crystals compete against amorphization. If the temperature is constant, the survival of the
(1997) 82-85
crystals is considered to depend on the size of region related to amorphization (the size of collision cascade). C. Spinella et al. [ 121demonstrated that the pre-existing small Si crystals in a-%, whose size was comparable with the radius of collision cascade, were decreased in density by Kr ion irradiation in the range of 360-450°C. In our experiment, it is noted that the nuclear energy deposition per ion for Au irradiation is about 3 times larger than that for Ge irradiation and about 2 times larger than that by Sb irradiation, suggesting that the size of single collision cascade is increased with increasing ion mass. Therefore, it is considered that the nucleation and the growth of the randomly oriented Ge crystals are retarded at 380°C by the amorphization as the cascade volume is increased in size with increasing ion mass. H.A. Atwater et al. [15] reported that Ge grain growth in amorphous and polycrystal Ge films was enhanced by ion irradiation, and demonstrated that the grain size was increased with increasing ion mass. Their results seem to be inconsistent with our results. However, their experiments was carried out at high temperatures (500-600°C). Therefore, the effects of amorphization by ion irradiation could be negligible in their experiments. It is also observed that the crystal quality of the epitaxial Ge films becomes good with increasing ion mass, as seen in Figs. 1 and 2. From these results, the epitaxial crystallization seems to be correlated with nuclear energy deposition. However, the total nuclear energy deposition is approximately the same among the irradiations. Considering with the fact that the amount of randomly oriented crystals are decreased with ion mass, these results presumably suggest that the decrease of randomly oriented crystals contributes to the enhancement of the epitaxial crystallization. Thus, it is thought that the crystal quality is correlated with the amount of randomly oriented crystals. The results of RBS-channeling measurement (Fig. 1) show that the crystallization is more enhanced by Sb irradiation than by Ge irradiation. Moreover, the crystallization (the decrease in RBS aligned yields) for Sb irradiation are similar to that for Au irradiation in spite of the presence of randomly oriented crystals. The obtained results may support the consideration that electronic energy deposition also enhances the epitaxial crystallization of Ge films. However, the distinction of effects of electronic energy deposition is not sufficient. Further study is under way.
Table 1 Typical values for experimental condition and the results of calculation by using TRIM code Ion
Energy Sample temperature Dose rate
Ion dose
Projected range Energy deposition in Ge film hm)
Nuclear (eV cm- ’ per ion)
Electronic (eV cm- ’ per ion)
IO“
412
104
384
1.53 x IO’O 2.32 x 10”
397
4.53 x 10’0
0.848 X 10” 1.77 x 10’2 2.68 X lOI
(meVI
PC)
(ions cm-* s- ‘1 (ions cm-*)
Ge+ Sb+
0.9 1.3
380 380
18.7 x 10” 12.5 X 10”
6x 4x
Au2+
2.2
380
6.24 X 10”
2x
104
S. Nakao et al./Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 82-85
4. Conclusion
It was stated that the formation of randomly oriented crystals in amorphous matrix induced by ion irradiation would strongly proceed at high temperatures (> 500°C) but compete against amorphization in a relatively low temperature region (200-500°C). We examined the influences of the irradiations with the 3 kinds of ions (Ge+, Sb+, Au’+) on the crystallization of the Ge films at about 380°C. It was found that the crystal quality of the epitaxial Ge films became good and the amount of randomly oriented Ge crystals included in the films was decreased with increasing ion mass. These results suggest that the crystal quality is correlated with the amount of randomly oriented Ge crystals. The formation of randomly oriented Ge crystals seems to be affected by the collision cascade whose size is increased with ion mass. From these results, it is concluded that heavy ion irradiation is effective for the enhancement of the epitaxial crystallization of the Ge films and the suppression of the formation of randomly oriented Ge crystals.
Acknowledgements
The authors thank Dr. K. Baba of Technology Center of Nagasaki for the use of the field emission scanning electron microscope.
85
References
[ll J. Linnros, G. Holmen and B. Svensson, Phys. Rev. B 32 (1985) 2770.
121J. Limuos, B. Svensson and G. Holmen, Phys. Rev. B 30 (1984) 3629. 131J. Limuos and G. Holmen, J. Appl. Phys. 59 (1986) 1513. [41 A. Kinomura, A. Chayahara, Y. Horino, Y. Mokuno and K. Fujii, Nucl. Instr. and Meth. B 106 (1995) 277. [51 J. Nakata, Phys. Rev. B 43 (1991) 14643. [d J. Nakata, J. Appl. Phys. 79 (1996) 682. [71 S. Nakao, K. Saitoh, M. Ikeyama, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa, Proc. 9th Int. Conf. Ion Beam Modification of Materials (1995) p. 845. Dl S. Nakao, K. Saitoh, M. Ikeyama, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa, Thin Solid Films (in press). 191 S. Nakao, K. Saitoh, M. Ikeyama, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa, Mater. Res. Sot. Symp. Proc. 396 (19%) 191. [IO1 K. Saitoh, S. Nakao, H. Niwa, M. Ikeyama, Y. Miyagawa and S. Miyagawa, Jpn. J. Appl. Phys. 31 (1992) 4020. 1111J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. iI21 C. Spinella, F. Priolo, S. Lombardo, S.U. Campisano and E. Rimini, Nucl. Instr. and Meth. B 59/60 (1991) 363. [I31 J.S. Im and H.A. Atwater, Appl. Phys. Lett. 57 (1990) 1766. 1141J.H. Shin and H.A. Atwater, Nucl. Instr. and Meth. B 80/81 (1993) 973. 1151 H.A. Atwater, C.V. Thompson and H.I. Smith, J. Appl. Phys. 64 (1988) 2337.
Nuclear Instruments and Methodsin Physics Research B 127/ 128 ( 1997) 82-85 kYlOMl
B
Beam InteractIons with Yaterials & Atoms
ELSEVIER
Influences of ion species (Ge+, Sbf, Au2+) on crystallization of amorphous Ge films induced by high energy ion irradiation Setsuo Nakao
*, Kazuo Saitoh, Masami Ikeyama,
Hiroalci Niwa, Seita Tanemura, Yoshiko Miyagawa, Soji Miyagawa
National Industrial Research Institute of Nagoya, 1-I Hirate-rho, Kita-ku, Nagoya 462, Japan
Abstract
Amorphous Ge films deposited on CaFz substrates were irradiated with Ge, Sb and Au ions and their structural changes were examined. Samples during irradiations were kept at a constant temperature about 380°C since a formation of randomly oriented crystals in amorphous matrix was closely related to temperature. It was found that the crystal quality of the epitaxial Ge films became good and the formation of randomly oriented crystals was retarded with increasing ion mass. These results presumably suggested that the crystal quality was correlated with the amount of randomly oriented crystals. Furthermore, it was assumed that the formation of randomly oriented crystals were affected by the single collision cascade whose size was increased with ion mass. From these results, it was concluded that heavy ion irradiation was effective for the enhancement of the epitaxial crystallization and the suppression of the formation of randomly oriented crystals.
1. Introduction Epitaxial crystallization of amorphous (a-) layer induced by ion irradiation has attracted much attention. Especially, there are many studies [l-6] on ion-beam-induced-epitaxial-crystallization (IBIEC) concerning Si material because it is essentially low-temperature process. In the IBIEC, it was pointed out that nuclear energy deposition was a dominant parameter [l]. However, it was recently reported that electronic energy deposition also played a significant role on the crystallization of a-Si layer [6]. In the case of other semiconductor material such as Ge, a similar tendency should be expected. We have studied on the crystallization of a-Ge films deposited on CaF, substrates, which is an example of semiconductor-on-insulator, and found that the epitaxial crystallization could be induced by high energy ion irradiation [7-91. However, the crystallization of a-Ge films were more complicated than IBIEC of Si because the crystallized Ge films included some randomly oriented crystals [8,9]. In order to control the microstructure, it is of importance to understand the effects of irradiation parameters related to energy deposition on the crystallization of the Ge films. In particular, it is of interest to know what parameter influences on the formation of randomly oriented crystals.
* Corresponding author. Fax: +81 52 916 2802; e-mail: [email protected] 0168-583X/97/$17.00
In this study, a-Ge films deposited on CaF, substrates were irradiated with various ion species and their structural changes were examined.
2. Experimental a-Ge films were deposited on air-cleaved single crystal evaporation at room temperature. The pressure was about 10m4 Pa during evaporation. Thickness of the films was about 120 nm. Ion beam irradiation and Rutherford backscattering spectrometry (RBS) combined with channeling technique were performed by tandem-type ion accelerator (NEC SSDH-II pelletron accelerator) at NIRIN. Details of experimental setup was described elsewhere [ 101. Typical irradiation conditions are summarized in Table 1. Ge+, Sb+ and Au2+ were used for irradiation. Ion irradiation was performed in 5” off ( 111) to prevent the channeling effect. Ion energy of each ion was chosen so that the projected range was about 400 nm. The projected range was much deeper than the film thickness. Sample temperature during irradiation was measured by pyrometer and was about 380°C. Time of irradiation was about 53 min. The Ge films remained entirely amorphous phase by thermal annealing at 400°C for I h [8]. Nuclear and electronic energy depositions in the Ge film calculated by TRIM code [ll] were also listed in Table 1. The ratio of the nuclear energy deposition (N) per ion among irradiations with the 3 kinds of ions, CaF,(l 11) substrates by electron-heating
0 1997 Elsevier Science B.V. All rights reserved.
HI SO1 68-583X(96)00856-7
S. Nakao et al./Nucl.
Instr. and Meth. in Phys. Res. B 127/
83
128 (1997) 82-M
N(Ge):N(Sb):N(Au), is approximately 1: 1.53. Ion Dose is chosen so that the total nuclear energy deposition (N X ion dose) is approximately the same (distribution and intensity) among the irradiations. Gn the other hand, the ratio of the electronic energy deposition (E) per ion among the irradiations, E(Ge):E(Sb):E(Au), is approximately 1:2:3. Then, the total electronic energy deposition (E X ion dose) for Ge irradiation approximately coincides with for Au irradiation. The total electronic energy deposition for Sb irradiation is larger than for others. The degree of the crystallization of a-Ge films was examined by in-situ RBS-channeling measurement. The RBS-channeling measurement was performed with 1.0 MeV He+ along with CaF, (111) axis. The surfaces of the Ge films were observed by field emission scanning microscopy (FE-SEM) at an acceleration voltage of 3 kV. Thin film x-ray diffraction (XRD) measurement was performed by 20-scan operation system with 3” incident x-rays.
3. Results and discussion Fig. 1 shows the RBS aligned spectra of the samples before and after irradiation under the different conditions. Solid line shows the RBS random spectrum for comparison. Before irradiation, an aligned yield from Ge is in agreement with the random yield due to the amorphous structure. After irradiation, the aligned yield decreases apparently. In addition, there is a clear difference between the irradiations. The aligned yield for Sb-irradiation sample is smaller than that for Ge-irradiation sample. Moreover, the further decrease in the aligned yield is observed for Au-irradiation sample. These results clearly show that the crystallization effect depends on ion species and it is enhanced with increasing ion mass. Fig. 2 shows the FE-SEM micrographs of the surfaces Fig. 2. FE-SEM micrographsof the surfaces of the Ge films after irradiations with various ion species. 9000
II 50
100
150
200
CHANNEL
Fig. 1. RBS aligned spectra of the samples before and after irradiations with various ion species under different conditions of ion energy and ion dose. Solid line shows RBS random spectrum for comparison.
of the Ge films after the irradiations. For Ge irradiation, the mottled pattern is observed on the surface of the film. For Sb irradiation, similar surface having like speckles also appears. These results indicate that both films have a mosaic structure. However, the density of the speckles for Sb-irradiation film seems to be lower than that for Geirradiation film. In contrast to these films, the uniform surface is observed for the Au-irradiation film. These results show that the crystal quality of the epitaxial films becomes good with the increase of ion mass. Fig. 3 shows the thin film (20 scan) XRD patterns of the samples. When using Ge and Sb ions, 3 peaks arising from Ge(1 11), (220) and (311) planes are clearly observed. This means that the Ge films crystallized by each irradiation include the crystals with random orientation. However, the peak intensity of the Sb-irradiation sample is
1. FUNDAMENTALS/BASICS
S. Nakao et al./Nucl. /
I
I
30
20
I
Ge(ll1)
I
I
I Ge(31 I)
Ge(220)
40 2 e (degree) /CuK a
Insrr. and Meth. in Phys. Res. B 127/128
50
60
Fig. 3. Thin film XRD patterns of the samples after irradiations
with various ion species.
slightly lower than that of the Ge-irradiation sample. In addition, full widths at half maximum of the Ge( 111) peak are about 0.7” for both samples, suggesting that the crystals have similar size in average. So, the number of randomly oriented crystals in the Sb-irradiation sample is thought to be smaller than that in the Ge-irradiation sample. While, there is no peak for Au irradiation. Therefore, it can be said that the Ge film crystallized by Au irradiation rarely includes the randomly oriented crystals. The presence of randomly oriented Ge crystals in the films irradiated with Ge and Sb ions suggests that the formation of the nuclei and the growth in amorphous matrix are conducted by the irradiations. While, the fact that Au-irradiation film rarely includes the randomly oriented Ge crystals shows the suppression of the nucleation and/or the growth of the randomly oriented Ge crystals. The difference in the amount of the randomly oriented Ge crystals can not be explained by the total nucleation energy deposition since it is approximately ,,the same among the irradiations. A possible explanation is as follows. Nucleation and growth of crystals in amorphous matrix induced by ion irradiation strongly depend on temperature during irradiation, as reported by several authors [ 12- 141. At high temperatures (> 5OO”C), ion irradiation greatly enhances the nucleation rate as well as the growth rate. At relatively low temperatures (200-5OO”C), however, the nucleation and the growth of small crystals compete against amorphization. If the temperature is constant, the survival of the
(1997) 82-85
crystals is considered to depend on the size of region related to amorphization (the size of collision cascade). C. Spinella et al. [ 121demonstrated that the pre-existing small Si crystals in a-%, whose size was comparable with the radius of collision cascade, were decreased in density by Kr ion irradiation in the range of 360-450°C. In our experiment, it is noted that the nuclear energy deposition per ion for Au irradiation is about 3 times larger than that for Ge irradiation and about 2 times larger than that by Sb irradiation, suggesting that the size of single collision cascade is increased with increasing ion mass. Therefore, it is considered that the nucleation and the growth of the randomly oriented Ge crystals are retarded at 380°C by the amorphization as the cascade volume is increased in size with increasing ion mass. H.A. Atwater et al. [15] reported that Ge grain growth in amorphous and polycrystal Ge films was enhanced by ion irradiation, and demonstrated that the grain size was increased with increasing ion mass. Their results seem to be inconsistent with our results. However, their experiments was carried out at high temperatures (500-600°C). Therefore, the effects of amorphization by ion irradiation could be negligible in their experiments. It is also observed that the crystal quality of the epitaxial Ge films becomes good with increasing ion mass, as seen in Figs. 1 and 2. From these results, the epitaxial crystallization seems to be correlated with nuclear energy deposition. However, the total nuclear energy deposition is approximately the same among the irradiations. Considering with the fact that the amount of randomly oriented crystals are decreased with ion mass, these results presumably suggest that the decrease of randomly oriented crystals contributes to the enhancement of the epitaxial crystallization. Thus, it is thought that the crystal quality is correlated with the amount of randomly oriented crystals. The results of RBS-channeling measurement (Fig. 1) show that the crystallization is more enhanced by Sb irradiation than by Ge irradiation. Moreover, the crystallization (the decrease in RBS aligned yields) for Sb irradiation are similar to that for Au irradiation in spite of the presence of randomly oriented crystals. The obtained results may support the consideration that electronic energy deposition also enhances the epitaxial crystallization of Ge films. However, the distinction of effects of electronic energy deposition is not sufficient. Further study is under way.
Table 1 Typical values for experimental condition and the results of calculation by using TRIM code Ion
Energy Sample temperature Dose rate
Ion dose
Projected range Energy deposition in Ge film hm)
Nuclear (eV cm- ’ per ion)
Electronic (eV cm- ’ per ion)
IO“
412
104
384
1.53 x IO’O 2.32 x 10”
397
4.53 x 10’0
0.848 X 10” 1.77 x 10’2 2.68 X lOI
(meVI
PC)
(ions cm-* s- ‘1 (ions cm-*)
Ge+ Sb+
0.9 1.3
380 380
18.7 x 10” 12.5 X 10”
6x 4x
Au2+
2.2
380
6.24 X 10”
2x
104
S. Nakao et al./Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 82-85
4. Conclusion
It was stated that the formation of randomly oriented crystals in amorphous matrix induced by ion irradiation would strongly proceed at high temperatures (> 500°C) but compete against amorphization in a relatively low temperature region (200-500°C). We examined the influences of the irradiations with the 3 kinds of ions (Ge+, Sb+, Au’+) on the crystallization of the Ge films at about 380°C. It was found that the crystal quality of the epitaxial Ge films became good and the amount of randomly oriented Ge crystals included in the films was decreased with increasing ion mass. These results suggest that the crystal quality is correlated with the amount of randomly oriented Ge crystals. The formation of randomly oriented Ge crystals seems to be affected by the collision cascade whose size is increased with ion mass. From these results, it is concluded that heavy ion irradiation is effective for the enhancement of the epitaxial crystallization of the Ge films and the suppression of the formation of randomly oriented Ge crystals.
Acknowledgements
The authors thank Dr. K. Baba of Technology Center of Nagasaki for the use of the field emission scanning electron microscope.
85
References
[ll J. Linnros, G. Holmen and B. Svensson, Phys. Rev. B 32 (1985) 2770.
121J. Limuos, B. Svensson and G. Holmen, Phys. Rev. B 30 (1984) 3629. 131J. Limuos and G. Holmen, J. Appl. Phys. 59 (1986) 1513. [41 A. Kinomura, A. Chayahara, Y. Horino, Y. Mokuno and K. Fujii, Nucl. Instr. and Meth. B 106 (1995) 277. [51 J. Nakata, Phys. Rev. B 43 (1991) 14643. [d J. Nakata, J. Appl. Phys. 79 (1996) 682. [71 S. Nakao, K. Saitoh, M. Ikeyama, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa, Proc. 9th Int. Conf. Ion Beam Modification of Materials (1995) p. 845. Dl S. Nakao, K. Saitoh, M. Ikeyama, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa, Thin Solid Films (in press). 191 S. Nakao, K. Saitoh, M. Ikeyama, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa, Mater. Res. Sot. Symp. Proc. 396 (19%) 191. [IO1 K. Saitoh, S. Nakao, H. Niwa, M. Ikeyama, Y. Miyagawa and S. Miyagawa, Jpn. J. Appl. Phys. 31 (1992) 4020. 1111J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. iI21 C. Spinella, F. Priolo, S. Lombardo, S.U. Campisano and E. Rimini, Nucl. Instr. and Meth. B 59/60 (1991) 363. [I31 J.S. Im and H.A. Atwater, Appl. Phys. Lett. 57 (1990) 1766. 1141J.H. Shin and H.A. Atwater, Nucl. Instr. and Meth. B 80/81 (1993) 973. 1151 H.A. Atwater, C.V. Thompson and H.I. Smith, J. Appl. Phys. 64 (1988) 2337.