- Email: [email protected]
Synthesis of very long ErSi2 nanowires on Si substrates
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e2263–e2267 www.elsevier.com/locate/jcrysgro
Synthesis of very long ErSi2 nanowires on Si substrates S. Harakoa, K. Kounoa, S. Komurob, A. Ohatac, X. Zhaoa, a
Department of Physics, Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan b Faculty of Engineering, Tokyo University, Kawagoe, Saitama 350-8585, Japan c Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Available online 24 December 2004
Abstract ErSi2 nanowires with a length of several micrometers have been fabricated on Si substrates. The starting samples were highly Er-doped amorphous Si thin layers formed by laser ablating an Si:Er2O3 mixture target. After post-annealing in vacuum, the nanowires were formed on the Si(0 0 1) surfaces along h1 1 0i directions. The average length and width of the wires depended on the annealing temperature and time. The height of the wires, in contrast, remained equal to be 0.6 nm. The longest wire achieved here was 20 mm. A large aspect ratio of 500 was obtained. X-ray diffraction and TEM observation indicated the hexagonal ErSi2 formation. Our results provide a way to synthesize Si-based nanowires having enough length for further device applications without an UHV system. r 2004 Elsevier B.V. All rights reserved. PACS: 68.66.La; 61.46.+w; 68.55.Ac; 68.37.Ps Keywords: A1. Nanostructures; Nanowires; A3. Self-assembly growth
1. Introduction The synthesis of rare-earth silicide nano-structures on Si substrate has been an interesting topic because of their potential in forming low-dimensional devices [1–3]. One approach for fabricating such nanostructures is a self-assembled growth in an ultra-high vacuum (UHV) system. Epitaxial growth techniques such as molecular beam epitaxy and metalorganic chemical vapor deposition have Corresponding author. Fax: +81 35261 1023.
E-mail address: [email protected] (X. Zhao).
made the formation of quantum wells (twodimension) and quantum dots (zero-dimension) possible at the scale of a few nanometers (nm). The fabrication of one-dimensional wires with a width less than 10 nm has still been a difficult task by either the above-mentioned growth methods or lithographic techniques. Self-assembled growth, therefore, have been used to form wire-like structures based on the lattice mismatch between the wires and the semiconductor substrates. This technique allows the formation of very narrow wires. But the average length of those wires was in the range of 100 nm, which making such wires in
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.362
ARTICLE IN PRESS e2264
S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
device applications difficult. Indeed, most of the reports focused on the formation mechanism and investigation in vacuum such as by using a scan tunneling microscope [1–5]. Most of the studies confined to surface physics are more correct. Fabrication of long nanowires with controlled crystal growth direction is very important for clarifying the physical and electronic properties in such one-dimensional system and for device applications. In this paper, we present a successful process that allows the formation of very long erbium disilicide (ErSi2) nanowires on Si(0 0 1) substrates. Due to its low resistivity (3.5 107 Om in bulk) and low schottky barrier height to Si [6,7], ErSi2 nanowires grown on Si substrates have been studied extensively [1–5,8,9]. The lattice mismatch of the hexagonal type ErSi2 in h0 0 0 1i direction (caxis) with Sih1 1 0i axis is 6.5%, and that between ErSi2 h1 1 2 0i and Sih1 1 0i is 1.3%, respectively. This allows the ErSi2 nanocrystallites growing on a-axis (h1 1 2 0i direction) along the Sih1 1 0i directions to form nanowires. In our experiments, we used laser ablation and post-annealing processes to form ErSi2 nanowires on Si(0 0 1) surface. The longest wire achieved in our works was 20 mm. A large aspect ratio of 500 was obtained. Our results provide a possible way to fabricate ErSi2 nanowires having enough length for further device applications without the UHV system.
1 105 Torr. The crystalline structure and orientation of the ErSi2 nanowires were investigated by X-ray diffraction (XRD) measurement. An atomic force microscope (AFM) was used to investigate the wires’ shape, density, and their average length, width and height. The microstructure of the wires was observed by a transmission electron microscope (TEM) equipped with an energy-disperse Xray spectrometer (EDS).
3. Results and discussion Fig. 1 shows a typical morphology of ErSi2 nanowires formed on the Si(0 0 1) surface measured by AFM. The sample was annealed at 1100 1C for 6 min. The area size of the AFM image was 5 mm2. Very long nanowires have been observed in the view. All of the wires were along the perpendicular Sih1 1 0i and h1 1¯ 0i crystal directions, as shown by the arrows in the figure. A detailed investigation revealed that the perpendicular wires were grown on different (1 0 0) terraces of the Si(0 0 1) surface. The lattice mismatch of the hexagonal type ErSi2 in h0 0 0 1i direction with the Sih1 1 0i axis is 6.5%, and that
2. Experimental procedure The starting material is a ceramic target, a mixture of Si and a prescribed amount of 30 wt% Er2O3. The Er density included in the target is simply calculated to be 2.2 1021 cm3. A Qswitched YAG laser (4o0 ¼ 266 nm, 5 ns, 10 Hz, 1 J/cm2) was used to ablate the target in a vacuum chamber with a background pressure of 5 107 Torr. The detailed ablation process has been reported in our previous work [10,11]. Highly Er-doped amorphous Si thin layers of approximately 2 nm thick were formed on Si(0 0 1) substrates at room temperature. After the deposition, the thin films were annealed at temperatures from 1000 to 1200 1C in vacuum with a pressure of
Fig. 1. An AFM 5 5 mm2 image of ErSi2 nanowires formed on Si(0 0 1) surface. All of the wires are along the perpendicular Sih1 1 0i directions (see arrows in the figure).
ARTICLE IN PRESS S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
104 Si:Er2O3 30 wt% 6 min 103 Size of Nanowire (nm)
Length 102
Width
10
Height
1
0.1 950
1000
(a)
1050 1100 1150 1200 Annealing Temperature (ºC)
1250
Si:Er2O3 30 wt% ErSi2 (100) XRD Intensity (arb. units)
between ErSi2 h1 1 2 0i and the Sih1 1 0i is 1.3%, respectively. The wires were running on h1 1 0i direction and were perpendicular to the dimer rows on the h1 1¯ 0i direction in one terrace. The length of the nanowires in Fig. 1 ranged from 1 to 4 mm and the width from 10 to 40 nm, respectively. The height of the wires, in contrast, was almost the same and ranged from 0.3 to 1 nm. The longest wire formed in this work was about 20 mm with an aspect ration length over a width of 500. It was also observed that some wider wires having a width near 100 nm were separated by several parallel narrow lines. These results indicated that the ErSi2 wires were running along the h1 1 2 0i direction. The average measured height of 0.6 nm reflected the diameter of the ErSi2 hexagonal unit cell. Our experiments do not need an oxygen-free environment and the UHV system, which provides a way to form nanowires at a low cost. Shown in Fig. 2(a) is the annealing temperature dependence of the sizes of the ErSi2 nanowires. The data are summed from five AFM observation views with an image region size of 5 5 mm2. At temperatures lower than 950 1C, no nanowires were observed on the Si(0 0 1) surface. At temperatures higher than 1000 1C, ErSi2 nanowires appeared. The average length reached a maximum at 1100 1C, which suggests an evaporation of the wires at high temperatures such as 1200 1C. The measured average height of the wires, in contrast, remained almost constant for all annealing temperatures. Fig. 2(b) shows the XRD diffraction patterns of the same samples used in Fig. 2(a). At temperatures lower than 950 1C, all samples showed amorphous nature. No detectable diffraction peak could be seen. At temperatures higher than 1000 1C, a sharp and intense peak named as ErSi2(1 0 0) was observed. This peak reflects the diffraction of ErSi2 h1 1 2 0i direction (ErSi2 a-axis) growth. Diffraction from other crystal planes was out of the detection limit. The most intense diffraction peak that occurred at 1100 1C suggested the longest wire grown in this temperature, as shown in Fig. 2(a). The reduction of the XRD signal from a sample annealed at 1200 1C revealed the evaporation of the wires in such high temperature.
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6 min
ErSi2 (200) 1200ºC
1100ºC
1000ºC
20 (b)
25
30
35 40 45 2Theta (degree)
50
55
60
Fig. 2. (a) Annealing temperature dependence of the average sizes of ErSi2 nanowires formed on Si(0 0 1) surface. The bar in the figure shows the size distribution of the wires measured from five AFM 5 5 mm2 images. (b) XRD diffraction patterns of the same samples used in (a) annealed at 1000, 1100 and 1200 1C.
Fig. 3(a) shows the annealing time dependence of the average sizes of the ErSi2 nanowires. The samples were annealed at 1100 1C in vacuum. For an annealing time shorter than 3 min, no ErSi2 nanowire formation was observed. The sample
ARTICLE IN PRESS S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
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104 Si:Er2O3 30 wt% 1100 ºC Size of Nanowire (nm)
103 Length 102 Width 101
Height
100
10-1
2
3
4
(a)
5
6
7
8
9
10
Annealing Time (min)
Si:Er203 30 wt% 1100ºC
ErSi2 (100) XRD Intensity (arb, units)
ErSi2 (101) 9min
ErSi2 (200)
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4min
3min 1min 20
(b)
25
30
35
40
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recognizable due to the evaporation of the wires and also due to the substrates that caused a distortion on the morphology of the sample surfaces. The XRD patterns for the above-mentioned samples are shown in Fig. 3(b). The results for annealing times of 4, 6 and 9 min are in good agreement with the AFM observation. Because of the high density of the nanowires, we could observe the X-ray diffraction peaks from the samples by long time measurements. Beside the sharp peak of ErSi2(1 0 0) plane, a weak peak named as ErSi2(1 0 1) was also observed. It is suggested that the precipitates were grown threedimensionally at a very early step of the nucleation. Afterwards, the wires were formed in selfassembly by an anisotropic deposition and evaporation process. The samples annealed for 3 min and less exhibited three-dimensional nanocrystallites formation in the films. The fast evaporation of amorphous Si than ErSi2 nanocrystallites might be the reason for the precipitates on the surface of the 4 min annealed sample. For investigating the microstructure of the ErSi2 nanowires, TEM and EDS measurements have been carried out. It is shown that hexagonal ErSi2 has been formed on the Si(0 0 1) surface. The resistivity of the ErSi2 nanowires has also been measured using an AFM system. An average resistivity value of 6.5 106 Om has been demonstrated. This is 20 times higher than that of the bulk ErSi2 as mentioned before. This large resistivity is caused by the one-dimensional conduction in the ErSi2 nanowires. Details of the TEM and electric measurements will be reported elsewhere.
2Theta (degree)
Fig. 3. (a) Annealing time dependence of the average sizes of ErSi2 nanowires formed on Si(0 0 1) surface. The bar in the figure shows the size distribution of the wires measured from five AFM 5 5 mm2 images. (b) XRD diffraction patterns of the same samples used in (a) compared with samples annealed for 1 and 3 min.
annealed for 4 min gave rise to rectangular and/or L-type ErSi2 nuclei on its surface. The nuclei grew on perpendicular h1 1 0i directions for longer annealing times. For an annealing time longer than 10 min, the ErSi2 nanowires became hardly
4. Conclusions We have fabricated very long ErSi2 nanowires on Si substrates. The starting samples were highly Er-doped amorphous Si thin layers formed by laser ablation. A post-annealing process gave rise to the ErSi2 nanowire formation on the Si(0 0 1) surface. The ErSi2 nanowires were along the perpendicular Sih1 1 0i directions. The average length and width of the wires depended on the annealing temperature and time. The height of the
ARTICLE IN PRESS S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
wires remained to be 0.6 nm reflecting one unit cell growth of the hexagonal ErSi2. The longest wire achieved here was 20 mm. A large aspect ratio of 500 was obtained. Our results provide a way to form Si-based nanowires having enough length that is important for further device applications.
Acknowledgements We would like to thank Prof. T. Kimura and Dr. H. Isshiki of University of Electro-Commun. of Tokyo for stimulating discussions. The work was partially supported by Grant-in-Aid for Research from Asahi Glass Foundation. References [1] C. Preinesberger, S. Vandre, T. Kalka, M. Dahne-Prietsch, J. Phys. D 31 (1998) L43.
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[2] C. Ohbuchi, J. Nogami, Phys. Rev. B 66 (2002) 165323. [3] M. Kuzmin, P. Laukkanen, R.E. Perala, R.-L. Vaara, I.J. Vayrynen, Appl. Surf. Sci. 222 (2004) 394. [4] C. Preinesberger, S.K. Becker, S. Vandre, T. Kalka, M. Dahne, J. Appl. Phys. 91 (2002) 1695. [5] Yong Chen, A.A.O. Douglas, G.M. Ribeiro, Y. Austin Chang, R.S. Williams, Appl. Phys. Lett. 76 (2000) 4004. [6] H. Norde, J. de Sousa Pires, F. d’Heurle, F. Pesavento, S. Petersson, P.A. Tove, Appl. Phys. Lett. 38 (1981) 865. [7] K.N. Tu, R.D. Thompson, B.Y. Tsaur, Appl. Phys. Lett. 76 (1981) 626. [8] R. Ragan, Y. Chen, D. Ohlberg, G. M-Ribero, R. Williams, J. Crystal Growth 251 (2003) 657. [9] G. Chen, J. Wan, J. Yang, X. Ding, L. He, X. Wang, Surf. Sci. 513 (2002) 203. [10] S. Komuro, S. Maruyama, T. Morikawa, X. Zhao, H. Isshiki, Y. Aoyagi, Appl. Phys. Lett. 69 (1996) 3896. [11] X. Zhao, S. Komuro, H. Isshiki, Y. Aoyagi, T. Sugano, Appl. Phys. Lett. 74 (1999) 120.
Journal of Crystal Growth 275 (2005) e2263–e2267 www.elsevier.com/locate/jcrysgro
Synthesis of very long ErSi2 nanowires on Si substrates S. Harakoa, K. Kounoa, S. Komurob, A. Ohatac, X. Zhaoa, a
Department of Physics, Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan b Faculty of Engineering, Tokyo University, Kawagoe, Saitama 350-8585, Japan c Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Available online 24 December 2004
Abstract ErSi2 nanowires with a length of several micrometers have been fabricated on Si substrates. The starting samples were highly Er-doped amorphous Si thin layers formed by laser ablating an Si:Er2O3 mixture target. After post-annealing in vacuum, the nanowires were formed on the Si(0 0 1) surfaces along h1 1 0i directions. The average length and width of the wires depended on the annealing temperature and time. The height of the wires, in contrast, remained equal to be 0.6 nm. The longest wire achieved here was 20 mm. A large aspect ratio of 500 was obtained. X-ray diffraction and TEM observation indicated the hexagonal ErSi2 formation. Our results provide a way to synthesize Si-based nanowires having enough length for further device applications without an UHV system. r 2004 Elsevier B.V. All rights reserved. PACS: 68.66.La; 61.46.+w; 68.55.Ac; 68.37.Ps Keywords: A1. Nanostructures; Nanowires; A3. Self-assembly growth
1. Introduction The synthesis of rare-earth silicide nano-structures on Si substrate has been an interesting topic because of their potential in forming low-dimensional devices [1–3]. One approach for fabricating such nanostructures is a self-assembled growth in an ultra-high vacuum (UHV) system. Epitaxial growth techniques such as molecular beam epitaxy and metalorganic chemical vapor deposition have Corresponding author. Fax: +81 35261 1023.
E-mail address: [email protected] (X. Zhao).
made the formation of quantum wells (twodimension) and quantum dots (zero-dimension) possible at the scale of a few nanometers (nm). The fabrication of one-dimensional wires with a width less than 10 nm has still been a difficult task by either the above-mentioned growth methods or lithographic techniques. Self-assembled growth, therefore, have been used to form wire-like structures based on the lattice mismatch between the wires and the semiconductor substrates. This technique allows the formation of very narrow wires. But the average length of those wires was in the range of 100 nm, which making such wires in
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.362
ARTICLE IN PRESS e2264
S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
device applications difficult. Indeed, most of the reports focused on the formation mechanism and investigation in vacuum such as by using a scan tunneling microscope [1–5]. Most of the studies confined to surface physics are more correct. Fabrication of long nanowires with controlled crystal growth direction is very important for clarifying the physical and electronic properties in such one-dimensional system and for device applications. In this paper, we present a successful process that allows the formation of very long erbium disilicide (ErSi2) nanowires on Si(0 0 1) substrates. Due to its low resistivity (3.5 107 Om in bulk) and low schottky barrier height to Si [6,7], ErSi2 nanowires grown on Si substrates have been studied extensively [1–5,8,9]. The lattice mismatch of the hexagonal type ErSi2 in h0 0 0 1i direction (caxis) with Sih1 1 0i axis is 6.5%, and that between ErSi2 h1 1 2 0i and Sih1 1 0i is 1.3%, respectively. This allows the ErSi2 nanocrystallites growing on a-axis (h1 1 2 0i direction) along the Sih1 1 0i directions to form nanowires. In our experiments, we used laser ablation and post-annealing processes to form ErSi2 nanowires on Si(0 0 1) surface. The longest wire achieved in our works was 20 mm. A large aspect ratio of 500 was obtained. Our results provide a possible way to fabricate ErSi2 nanowires having enough length for further device applications without the UHV system.
1 105 Torr. The crystalline structure and orientation of the ErSi2 nanowires were investigated by X-ray diffraction (XRD) measurement. An atomic force microscope (AFM) was used to investigate the wires’ shape, density, and their average length, width and height. The microstructure of the wires was observed by a transmission electron microscope (TEM) equipped with an energy-disperse Xray spectrometer (EDS).
3. Results and discussion Fig. 1 shows a typical morphology of ErSi2 nanowires formed on the Si(0 0 1) surface measured by AFM. The sample was annealed at 1100 1C for 6 min. The area size of the AFM image was 5 mm2. Very long nanowires have been observed in the view. All of the wires were along the perpendicular Sih1 1 0i and h1 1¯ 0i crystal directions, as shown by the arrows in the figure. A detailed investigation revealed that the perpendicular wires were grown on different (1 0 0) terraces of the Si(0 0 1) surface. The lattice mismatch of the hexagonal type ErSi2 in h0 0 0 1i direction with the Sih1 1 0i axis is 6.5%, and that
2. Experimental procedure The starting material is a ceramic target, a mixture of Si and a prescribed amount of 30 wt% Er2O3. The Er density included in the target is simply calculated to be 2.2 1021 cm3. A Qswitched YAG laser (4o0 ¼ 266 nm, 5 ns, 10 Hz, 1 J/cm2) was used to ablate the target in a vacuum chamber with a background pressure of 5 107 Torr. The detailed ablation process has been reported in our previous work [10,11]. Highly Er-doped amorphous Si thin layers of approximately 2 nm thick were formed on Si(0 0 1) substrates at room temperature. After the deposition, the thin films were annealed at temperatures from 1000 to 1200 1C in vacuum with a pressure of
Fig. 1. An AFM 5 5 mm2 image of ErSi2 nanowires formed on Si(0 0 1) surface. All of the wires are along the perpendicular Sih1 1 0i directions (see arrows in the figure).
ARTICLE IN PRESS S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
104 Si:Er2O3 30 wt% 6 min 103 Size of Nanowire (nm)
Length 102
Width
10
Height
1
0.1 950
1000
(a)
1050 1100 1150 1200 Annealing Temperature (ºC)
1250
Si:Er2O3 30 wt% ErSi2 (100) XRD Intensity (arb. units)
between ErSi2 h1 1 2 0i and the Sih1 1 0i is 1.3%, respectively. The wires were running on h1 1 0i direction and were perpendicular to the dimer rows on the h1 1¯ 0i direction in one terrace. The length of the nanowires in Fig. 1 ranged from 1 to 4 mm and the width from 10 to 40 nm, respectively. The height of the wires, in contrast, was almost the same and ranged from 0.3 to 1 nm. The longest wire formed in this work was about 20 mm with an aspect ration length over a width of 500. It was also observed that some wider wires having a width near 100 nm were separated by several parallel narrow lines. These results indicated that the ErSi2 wires were running along the h1 1 2 0i direction. The average measured height of 0.6 nm reflected the diameter of the ErSi2 hexagonal unit cell. Our experiments do not need an oxygen-free environment and the UHV system, which provides a way to form nanowires at a low cost. Shown in Fig. 2(a) is the annealing temperature dependence of the sizes of the ErSi2 nanowires. The data are summed from five AFM observation views with an image region size of 5 5 mm2. At temperatures lower than 950 1C, no nanowires were observed on the Si(0 0 1) surface. At temperatures higher than 1000 1C, ErSi2 nanowires appeared. The average length reached a maximum at 1100 1C, which suggests an evaporation of the wires at high temperatures such as 1200 1C. The measured average height of the wires, in contrast, remained almost constant for all annealing temperatures. Fig. 2(b) shows the XRD diffraction patterns of the same samples used in Fig. 2(a). At temperatures lower than 950 1C, all samples showed amorphous nature. No detectable diffraction peak could be seen. At temperatures higher than 1000 1C, a sharp and intense peak named as ErSi2(1 0 0) was observed. This peak reflects the diffraction of ErSi2 h1 1 2 0i direction (ErSi2 a-axis) growth. Diffraction from other crystal planes was out of the detection limit. The most intense diffraction peak that occurred at 1100 1C suggested the longest wire grown in this temperature, as shown in Fig. 2(a). The reduction of the XRD signal from a sample annealed at 1200 1C revealed the evaporation of the wires in such high temperature.
e2265
6 min
ErSi2 (200) 1200ºC
1100ºC
1000ºC
20 (b)
25
30
35 40 45 2Theta (degree)
50
55
60
Fig. 2. (a) Annealing temperature dependence of the average sizes of ErSi2 nanowires formed on Si(0 0 1) surface. The bar in the figure shows the size distribution of the wires measured from five AFM 5 5 mm2 images. (b) XRD diffraction patterns of the same samples used in (a) annealed at 1000, 1100 and 1200 1C.
Fig. 3(a) shows the annealing time dependence of the average sizes of the ErSi2 nanowires. The samples were annealed at 1100 1C in vacuum. For an annealing time shorter than 3 min, no ErSi2 nanowire formation was observed. The sample
ARTICLE IN PRESS S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
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104 Si:Er2O3 30 wt% 1100 ºC Size of Nanowire (nm)
103 Length 102 Width 101
Height
100
10-1
2
3
4
(a)
5
6
7
8
9
10
Annealing Time (min)
Si:Er203 30 wt% 1100ºC
ErSi2 (100) XRD Intensity (arb, units)
ErSi2 (101) 9min
ErSi2 (200)
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4min
3min 1min 20
(b)
25
30
35
40
45
50
55
60
recognizable due to the evaporation of the wires and also due to the substrates that caused a distortion on the morphology of the sample surfaces. The XRD patterns for the above-mentioned samples are shown in Fig. 3(b). The results for annealing times of 4, 6 and 9 min are in good agreement with the AFM observation. Because of the high density of the nanowires, we could observe the X-ray diffraction peaks from the samples by long time measurements. Beside the sharp peak of ErSi2(1 0 0) plane, a weak peak named as ErSi2(1 0 1) was also observed. It is suggested that the precipitates were grown threedimensionally at a very early step of the nucleation. Afterwards, the wires were formed in selfassembly by an anisotropic deposition and evaporation process. The samples annealed for 3 min and less exhibited three-dimensional nanocrystallites formation in the films. The fast evaporation of amorphous Si than ErSi2 nanocrystallites might be the reason for the precipitates on the surface of the 4 min annealed sample. For investigating the microstructure of the ErSi2 nanowires, TEM and EDS measurements have been carried out. It is shown that hexagonal ErSi2 has been formed on the Si(0 0 1) surface. The resistivity of the ErSi2 nanowires has also been measured using an AFM system. An average resistivity value of 6.5 106 Om has been demonstrated. This is 20 times higher than that of the bulk ErSi2 as mentioned before. This large resistivity is caused by the one-dimensional conduction in the ErSi2 nanowires. Details of the TEM and electric measurements will be reported elsewhere.
2Theta (degree)
Fig. 3. (a) Annealing time dependence of the average sizes of ErSi2 nanowires formed on Si(0 0 1) surface. The bar in the figure shows the size distribution of the wires measured from five AFM 5 5 mm2 images. (b) XRD diffraction patterns of the same samples used in (a) compared with samples annealed for 1 and 3 min.
annealed for 4 min gave rise to rectangular and/or L-type ErSi2 nuclei on its surface. The nuclei grew on perpendicular h1 1 0i directions for longer annealing times. For an annealing time longer than 10 min, the ErSi2 nanowires became hardly
4. Conclusions We have fabricated very long ErSi2 nanowires on Si substrates. The starting samples were highly Er-doped amorphous Si thin layers formed by laser ablation. A post-annealing process gave rise to the ErSi2 nanowire formation on the Si(0 0 1) surface. The ErSi2 nanowires were along the perpendicular Sih1 1 0i directions. The average length and width of the wires depended on the annealing temperature and time. The height of the
ARTICLE IN PRESS S. Harako et al. / Journal of Crystal Growth 275 (2005) e2263–e2267
wires remained to be 0.6 nm reflecting one unit cell growth of the hexagonal ErSi2. The longest wire achieved here was 20 mm. A large aspect ratio of 500 was obtained. Our results provide a way to form Si-based nanowires having enough length that is important for further device applications.
Acknowledgements We would like to thank Prof. T. Kimura and Dr. H. Isshiki of University of Electro-Commun. of Tokyo for stimulating discussions. The work was partially supported by Grant-in-Aid for Research from Asahi Glass Foundation. References [1] C. Preinesberger, S. Vandre, T. Kalka, M. Dahne-Prietsch, J. Phys. D 31 (1998) L43.
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[2] C. Ohbuchi, J. Nogami, Phys. Rev. B 66 (2002) 165323. [3] M. Kuzmin, P. Laukkanen, R.E. Perala, R.-L. Vaara, I.J. Vayrynen, Appl. Surf. Sci. 222 (2004) 394. [4] C. Preinesberger, S.K. Becker, S. Vandre, T. Kalka, M. Dahne, J. Appl. Phys. 91 (2002) 1695. [5] Yong Chen, A.A.O. Douglas, G.M. Ribeiro, Y. Austin Chang, R.S. Williams, Appl. Phys. Lett. 76 (2000) 4004. [6] H. Norde, J. de Sousa Pires, F. d’Heurle, F. Pesavento, S. Petersson, P.A. Tove, Appl. Phys. Lett. 38 (1981) 865. [7] K.N. Tu, R.D. Thompson, B.Y. Tsaur, Appl. Phys. Lett. 76 (1981) 626. [8] R. Ragan, Y. Chen, D. Ohlberg, G. M-Ribero, R. Williams, J. Crystal Growth 251 (2003) 657. [9] G. Chen, J. Wan, J. Yang, X. Ding, L. He, X. Wang, Surf. Sci. 513 (2002) 203. [10] S. Komuro, S. Maruyama, T. Morikawa, X. Zhao, H. Isshiki, Y. Aoyagi, Appl. Phys. Lett. 69 (1996) 3896. [11] X. Zhao, S. Komuro, H. Isshiki, Y. Aoyagi, T. Sugano, Appl. Phys. Lett. 74 (1999) 120.