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Scanning tunneling microscopy observation of Bi-induced surface structures on the Si(1 0 0) surface
Surface Science 482-485 (2001) 1440±1444
www.elsevier.nl/locate/susc
Scanning tunneling microscopy observation of Bi-induced surface structures on the Si(1 0 0) surface M. Naitoh a,*, M. Takei a, S. Nishigaki a, N. Oishi b, F. Shoji c a
Department of Electrical Engineering, Kyushu Institute of Technology, Sensui, Tobata, Kitakyushu 804-8550, Japan b Kumamoto National College of Technology, Nishigoshi, Kikuchi, Kumamoto 861-1102, Japan c Faculty of Engineering, Kyushu Kyoritsu University, Jiyugaoka, Yahatanishi, Kitakyushu 807-8585, Japan
Abstract Formation of new surface structures induced by bismuth adsorbates at the Si(1 0 0) surface has been investigated by scanning tunneling microscopy. Bismuth atoms that adsorbed on the surface at 500°C, form long linear belts in the Si(1 0 0) topmost layer. Each belt consists of two chains of bismuth dimers and substitutes for four Si-dimers per Sidimer row in the terrace. At higher coverages of bismuth (3 ML) at 450°C, long linear structures with an arrangement of bismuth chains similar to the belt on the Si terraces are formed in the bismuth terraces. When bismuth was deposited on the clean Si(1 0 0) surface at room temperature and the surface was sequentially annealed, deep pits were constructed in bismuth-covered terraces. This result is attributed to the strain in the bismuth ®lms. Ó 2001 Elsevier Science B.V. All rights reserved Keywords: Bismuth; Low index single crystal surfaces; Scanning tunneling microscopy; Silicon; Surface relaxation and reconstruction; Surface stress; Surface structure, morphology, roughness, and topography
1. Introduction The formation of metal ®lms on semiconductor surfaces has attracted much attention from both fundamental and practical viewpoints and has been investigated in many works. Among them, elucidating the growth mechanism of the group V metals such as Bi, Sb, and As on Si(1 0 0) surfaces is important for the development of heteroepitaxy technology because those can be used as surfac-
* Corresponding author. Tel.: +81-93-884-3266; fax: +81-93884-3203. E-mail address: [email protected] (M. Naitoh).
tants as well as dopants. Hanada and Kawai showed by re¯ection high-energy electron diraction that Bi atoms adsorbed on the Si(1 0 0) surface form 2 n structures, where n decreases from n 13 to n 5 with increasing the substrate temperature [1]. By scanning tunneling microscopy (STM) Park et al. demonstrated that the 2 n structures are constructed with ordered missingrow defects in the 2 1Bi overlayer [2]. In our previous paper [3], we investigated the atomic structure of Bi ®lms on the Si(1 0 0) surface by STM and low-energy electron diraction (LEED) and found that at least two layers of Bi atoms have grown at 400°C in diamond-like structure under an in¯uence of substrate structure. Moreover, we found the formation of long linear chains
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 8 6 0 - 3
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
consisting of Bi dimers. Although similar one-dimensional structures, produced arti®cially by STM tip, have been reported [4], the structure with Bidimer linear chains has been self-assembled and is not relevant to those. Techniques for forming onedimensional structures such as these Bi-dimer linear chains may be applicable to the fabrication of atomic-scale devices on a semiconductor surface. In the present paper, we report experimental results by STM regarding the reaction of Bi atoms at the Si(1 0 0) surface. Long linear structures are formed on the Bi-covered Si(1 0 0) surface. Bi atoms tend to induce selectively a certain reaction for one-dimensional restructuring on the Si(1 0 0) surface. Moreover, we found for the ®rst time the formation of pits in the Bi-layers on the Si(1 0 0) surface after Bi deposition at room temperature followed by brief annealing at 500°C, which would be attributed to strains in the Bi-®lms. We discuss a unique role of Bi atoms in the process of selfassembled surface re-structuring.
1441
3. Results and discussion In Fig. 1 we show an STM image taken after Bi deposition of 3 ML at 500°C. There are distortions in the image due to the thermal drift of the STM apparatus. In the Bi deposition at such high temperatures, adsorption and desorption of Bi atoms compete with each other at the surface [5,6].
2. Experimental The experiment was carried out in an ultrahighvacuum chamber under a base pressure of 1 10 8 Pa. The chamber was equipped with a commercial STM (JEOL JSTM-4500XT), a rear-view LEED and sample-preparation facilities including those for direct-current heating and Bi deposition. STM observations were performed at room temperature. Tips for STM were made from tungsten wires (0.3 mm in diameter). The tips were baked out before being used for scanning. Si(1 0 0) samples were cut from a p-type wafer (B-doped, 7 X cm) and cleaned in situ by heating at 900°C followed by brief ¯ashing at 1200°C. After the cleaning procedure, well-developed STM images and (2 1) LEED patterns were observed. The sample temperature was measured in the range of 250±1000°C with an optical pyrometer. Bi (99.999% purity) was deposited from a crucible made of thin Ta foil. The rate of Bi deposition was roughly 1 ML/min, where 1 ML was de®ned to contain the same amount of atoms as an ideal Si(1 0 0) layer.
Fig. 1. (a) STM image taken after Bi deposition of 3 ML at 500°C. There is a distortion in the ®gure. The image was taken at sample bias Vs 1:2 V, at tunneling current I 0:3 nA and from the area S 45 45 nm2 . (b) Local area STM image taken from (a), S 7:0 5:4 nm2 .
1442
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
Therefore, we cannot know Bi coverages quantitatively in the present stage. There appears a terrace with many straight belts and vacancy defects. The belt is imaged as consisting of adjoining two bright lines. According to our previous studies [3,7] and to the observation by Miki et al. [8], each bright line probably is a chain of Bi-dimers, the dimerization direction of which is parallel to the chain. The Bi-dimer chains are involved in the Si(1 0 0) topmost layer. The direction along the Bi chains is perpendicular to the direction of the Sidimer rows. In order to analyze precisely the structure of the Bi chains, we depicted parallel white lines on the position of the Si atoms in the ®rst layer shown in Fig. 1b. The distance between two adjacent lines is 0.38 nm, which agrees with the lattice constant of the Si(1 0 0) surface. We found that the two Bidimers substitute for four Si-dimers per Si-dimer row in the terrace. Miki et al. proposed models for Bi-dimer chains [8,9], where two Bi-dimers substitute for three Si-dimers. Those contradict the present STM results. We measured the separation between the two Bi-dimer chains in the same belt, although STM is not generally suitable for determining lateral distances. We found from Fig. 1b the separation being about 0.6 nm. By X-ray photoelectron diffraction, the separation of two Bi-dimer chains was estimated to be 0.63 nm [10], which is consistent with our measurements. Fig. 2 shows a ®lled-state STM image taken after Bi deposition of 3 ML at 450°C. We can see terraces and rectangular-shaped islands thereon. The height dierence between the brightest and the darkest regions in Fig. 2a, corresponding to ®ve atomic layers, is measured to be about 0.8 nm. At least the topmost layers of these terraces and islands should consist of Bi-dimers, by judging from the fact that we observed unresolved Bi-dimer rows in the ®lled-state STM image in contrast to the resolved Si-dimer rows. We ®nd in Fig. 2 that long linear structures with an arrangement of Bi chains, similar to the belt in the Si terraces, form in the Bi terraces. Bi atoms tend to induce selectively a certain reaction for one-dimensional restructuring on the Si(1 0 0) surface.
Fig. 2. (a) STM image taken after Bi deposition of 3 ML at 450°C. Vs 2:2 V, I 0:3 nA, S 45 45 nm2 . (b) Local area STM image around the area indicated by the white arrow in (a).
Fig. 2b shows a close-up image around the area indicated by the white arrow in Fig. 2a. We can see a group of four-bright protrusions on the Bi-dimer linear chains in the white circle in Fig. 2b. This has been observed rather frequently on the Bi chains. In the previous paper [3], we observed the formation of inlet and peninsula when the Bi chains grew across the step edge. The length of inlet or penin-
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
sula was 100 nm. From those results, we estimated that the surface migration of both Bi and Si atoms is enhanced on the Bi chains. We suggest that a group of protrusions is a key for understanding enhanced migration of the atoms along the Bi chains. Fig. 3a shows a ®lled-state STM image taken after Bi deposition at room temperature followed by annealing at 500°C for 5 s. We can see terraces with missing-row defects and many dark regions, indicated by the small white arrows, which corre-
Fig. 3. (a) STM image taken after Bi deposition of 2 ML at room temperature followed by annealing at 500°C for 5 s. There appear some pits on the surface. Vs 2:5 V, I 0:3 nA, S 45 45 nm2 . (b) Cross-sectional view at the white line in (a).
1443
spond to the pits. However, we cannot ®nd the Bi-dimer chains. The terraces should consist of Bidimers, compared with the results in the previous papers [3,7]. There are some islands in the regions around the pits. We show in Fig. 3b the depth pro®le at the white line in Fig. 3a. The depth of the pit is more than 0.25 nm. The exact value of the depth at the pit was not obtained because the top of the tip did not reach to the bottom of the pit owing to the observation condition of our STM system. The height of islands around the pits is about 0.15 nm, which corresponds to one atomic layer height. In Fig. 3a, there are few islands on the terraces apart from regions around the pits and ¯at Bilayers are formed on the surface. Though the Bi coverage of the surface is unknown, more than one layers of Bi atoms are likely grown on the Si(1 0 0) surface from the results in Fig. 2. Further experiments by, e.g., ion scattering spectroscopy are necessary to con®rm the Bi coverage. We can explain the formation process of the pits as follows: With the deposition of Bi on the Si(1 0 0) surface, the ®rst Bi layer with Bi dimers and periodically missing Bi-dimer rows is formed with (2 n) periodicity. Next, Bi atoms deposited on the surface migrate without crossing over the missing Bi-dimer rows, so that small islands with (n 2) periodicity are formed on the terraces. Finally, coalescence of small islands occurs over missing Bi-dimer rows and then the missing Bi-dimer rows disappear. Thus, the ®rst Bi layer becomes a complete (1 1) structure, and the second (top) Bi layer with Bi dimers and missing Bi-dimer rows is formed. After the completion of the second layer, the absences of dimerization and missing row in the ®rst layer should give rise to a compressive strain. To release the strain in the Bi layers, pits are to be formed on the surface at brief annealings. Louwsma et al. found square-shaped pits on the Bi-deposited Ge(1 0 0) surfaces [11]. In their observation, the depth of the square-shaped pits was typically about 5±10 atomic layer height. Ge atoms in the substrate have been removed and redeposited near the pits. However, in the present stage, we cannot infer whether the islands near the pits consist of Bi atoms and/or Si atoms in the Bi/Si system.
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M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
4. Conclusions
References
We have shown the unique character that Bi atoms possess at the Si(1 0 0) surface. Long linear chains of Bi-dimers are formed on the Si(1 0 0) surfaces and even on the Bi-covered surface. Moreover, there appear pits with depth more than 0.25 nm in the Bi terraces after brief annealings. These phenomena are caused by self-assembling of adsorbates. It would be essential to understand such unique characters of individual atoms in order to apply those to creating nano-scale electronic devices.
[1] T. Hanada, M. Kawai, Surf. Sci. 242 (1991) 137. [2] Ch. Park, R.Z. Bakhtizin, T. Hashizume, T. Sakurai, J. Vac. Sci. Technol. B 12 (1994) 2049. [3] M. Naitoh, H. Shimaya, S. Nishigaki, N. Oishi, F. Shoji, Surf. Sci. 377±379 (1997) 899. [4] T. Hashizume, S. Heike, M.I. Lutwyche, S. Watanabe, K. Nakajima, T. Nishi, Y. Wada, Jpn. J. Appl. Phys. 35 (1996) L1085. [5] A. Kawazu, T. Otsuki, G. Tominaga, Jpn. J. Appl. Phys. 20 (1981) 553. [6] K.-S. Kim, Y. Takakuwa, Y. Mori, S. Kono, Jpn. J. Appl. Phys. 35 (1996) L1695. [7] M. Naitoh, H. Shimaya, S. Nishigaki, N. Oishi, F. Shoji, Appl. Surf. Sci. 142 (1999) 38. [8] K. Miki, J.H.G. Owen, D.R. Bowler, G.A.D. Briggs, K. Sakamoto, Surf. Sci. 421 (1999) 397. [9] K. Miki, D.R. Bowler, J.H.G. Owen, G.A.D. Briggs, K. Sakamoto, Phys. Rev. B 59 (1999) 14868. [10] M. Shimomura, K. Miki, T. Abukawa, S. Kono, Surf. Sci. 447 (2000) L169. [11] H.K. Louwsma, H.J.W. Zandvliet, B.A.G. Kersten, J. Chesneau, A. van Silfhout, B. Poelsema, Surf. Sci. 381 (1997) L594.
Acknowledgements This work was partially supported by the Yoshida Foundation for the Promotion of Reading and Education.
www.elsevier.nl/locate/susc
Scanning tunneling microscopy observation of Bi-induced surface structures on the Si(1 0 0) surface M. Naitoh a,*, M. Takei a, S. Nishigaki a, N. Oishi b, F. Shoji c a
Department of Electrical Engineering, Kyushu Institute of Technology, Sensui, Tobata, Kitakyushu 804-8550, Japan b Kumamoto National College of Technology, Nishigoshi, Kikuchi, Kumamoto 861-1102, Japan c Faculty of Engineering, Kyushu Kyoritsu University, Jiyugaoka, Yahatanishi, Kitakyushu 807-8585, Japan
Abstract Formation of new surface structures induced by bismuth adsorbates at the Si(1 0 0) surface has been investigated by scanning tunneling microscopy. Bismuth atoms that adsorbed on the surface at 500°C, form long linear belts in the Si(1 0 0) topmost layer. Each belt consists of two chains of bismuth dimers and substitutes for four Si-dimers per Sidimer row in the terrace. At higher coverages of bismuth (3 ML) at 450°C, long linear structures with an arrangement of bismuth chains similar to the belt on the Si terraces are formed in the bismuth terraces. When bismuth was deposited on the clean Si(1 0 0) surface at room temperature and the surface was sequentially annealed, deep pits were constructed in bismuth-covered terraces. This result is attributed to the strain in the bismuth ®lms. Ó 2001 Elsevier Science B.V. All rights reserved Keywords: Bismuth; Low index single crystal surfaces; Scanning tunneling microscopy; Silicon; Surface relaxation and reconstruction; Surface stress; Surface structure, morphology, roughness, and topography
1. Introduction The formation of metal ®lms on semiconductor surfaces has attracted much attention from both fundamental and practical viewpoints and has been investigated in many works. Among them, elucidating the growth mechanism of the group V metals such as Bi, Sb, and As on Si(1 0 0) surfaces is important for the development of heteroepitaxy technology because those can be used as surfac-
* Corresponding author. Tel.: +81-93-884-3266; fax: +81-93884-3203. E-mail address: [email protected] (M. Naitoh).
tants as well as dopants. Hanada and Kawai showed by re¯ection high-energy electron diraction that Bi atoms adsorbed on the Si(1 0 0) surface form 2 n structures, where n decreases from n 13 to n 5 with increasing the substrate temperature [1]. By scanning tunneling microscopy (STM) Park et al. demonstrated that the 2 n structures are constructed with ordered missingrow defects in the 2 1Bi overlayer [2]. In our previous paper [3], we investigated the atomic structure of Bi ®lms on the Si(1 0 0) surface by STM and low-energy electron diraction (LEED) and found that at least two layers of Bi atoms have grown at 400°C in diamond-like structure under an in¯uence of substrate structure. Moreover, we found the formation of long linear chains
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 8 6 0 - 3
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
consisting of Bi dimers. Although similar one-dimensional structures, produced arti®cially by STM tip, have been reported [4], the structure with Bidimer linear chains has been self-assembled and is not relevant to those. Techniques for forming onedimensional structures such as these Bi-dimer linear chains may be applicable to the fabrication of atomic-scale devices on a semiconductor surface. In the present paper, we report experimental results by STM regarding the reaction of Bi atoms at the Si(1 0 0) surface. Long linear structures are formed on the Bi-covered Si(1 0 0) surface. Bi atoms tend to induce selectively a certain reaction for one-dimensional restructuring on the Si(1 0 0) surface. Moreover, we found for the ®rst time the formation of pits in the Bi-layers on the Si(1 0 0) surface after Bi deposition at room temperature followed by brief annealing at 500°C, which would be attributed to strains in the Bi-®lms. We discuss a unique role of Bi atoms in the process of selfassembled surface re-structuring.
1441
3. Results and discussion In Fig. 1 we show an STM image taken after Bi deposition of 3 ML at 500°C. There are distortions in the image due to the thermal drift of the STM apparatus. In the Bi deposition at such high temperatures, adsorption and desorption of Bi atoms compete with each other at the surface [5,6].
2. Experimental The experiment was carried out in an ultrahighvacuum chamber under a base pressure of 1 10 8 Pa. The chamber was equipped with a commercial STM (JEOL JSTM-4500XT), a rear-view LEED and sample-preparation facilities including those for direct-current heating and Bi deposition. STM observations were performed at room temperature. Tips for STM were made from tungsten wires (0.3 mm in diameter). The tips were baked out before being used for scanning. Si(1 0 0) samples were cut from a p-type wafer (B-doped, 7 X cm) and cleaned in situ by heating at 900°C followed by brief ¯ashing at 1200°C. After the cleaning procedure, well-developed STM images and (2 1) LEED patterns were observed. The sample temperature was measured in the range of 250±1000°C with an optical pyrometer. Bi (99.999% purity) was deposited from a crucible made of thin Ta foil. The rate of Bi deposition was roughly 1 ML/min, where 1 ML was de®ned to contain the same amount of atoms as an ideal Si(1 0 0) layer.
Fig. 1. (a) STM image taken after Bi deposition of 3 ML at 500°C. There is a distortion in the ®gure. The image was taken at sample bias Vs 1:2 V, at tunneling current I 0:3 nA and from the area S 45 45 nm2 . (b) Local area STM image taken from (a), S 7:0 5:4 nm2 .
1442
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
Therefore, we cannot know Bi coverages quantitatively in the present stage. There appears a terrace with many straight belts and vacancy defects. The belt is imaged as consisting of adjoining two bright lines. According to our previous studies [3,7] and to the observation by Miki et al. [8], each bright line probably is a chain of Bi-dimers, the dimerization direction of which is parallel to the chain. The Bi-dimer chains are involved in the Si(1 0 0) topmost layer. The direction along the Bi chains is perpendicular to the direction of the Sidimer rows. In order to analyze precisely the structure of the Bi chains, we depicted parallel white lines on the position of the Si atoms in the ®rst layer shown in Fig. 1b. The distance between two adjacent lines is 0.38 nm, which agrees with the lattice constant of the Si(1 0 0) surface. We found that the two Bidimers substitute for four Si-dimers per Si-dimer row in the terrace. Miki et al. proposed models for Bi-dimer chains [8,9], where two Bi-dimers substitute for three Si-dimers. Those contradict the present STM results. We measured the separation between the two Bi-dimer chains in the same belt, although STM is not generally suitable for determining lateral distances. We found from Fig. 1b the separation being about 0.6 nm. By X-ray photoelectron diffraction, the separation of two Bi-dimer chains was estimated to be 0.63 nm [10], which is consistent with our measurements. Fig. 2 shows a ®lled-state STM image taken after Bi deposition of 3 ML at 450°C. We can see terraces and rectangular-shaped islands thereon. The height dierence between the brightest and the darkest regions in Fig. 2a, corresponding to ®ve atomic layers, is measured to be about 0.8 nm. At least the topmost layers of these terraces and islands should consist of Bi-dimers, by judging from the fact that we observed unresolved Bi-dimer rows in the ®lled-state STM image in contrast to the resolved Si-dimer rows. We ®nd in Fig. 2 that long linear structures with an arrangement of Bi chains, similar to the belt in the Si terraces, form in the Bi terraces. Bi atoms tend to induce selectively a certain reaction for one-dimensional restructuring on the Si(1 0 0) surface.
Fig. 2. (a) STM image taken after Bi deposition of 3 ML at 450°C. Vs 2:2 V, I 0:3 nA, S 45 45 nm2 . (b) Local area STM image around the area indicated by the white arrow in (a).
Fig. 2b shows a close-up image around the area indicated by the white arrow in Fig. 2a. We can see a group of four-bright protrusions on the Bi-dimer linear chains in the white circle in Fig. 2b. This has been observed rather frequently on the Bi chains. In the previous paper [3], we observed the formation of inlet and peninsula when the Bi chains grew across the step edge. The length of inlet or penin-
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
sula was 100 nm. From those results, we estimated that the surface migration of both Bi and Si atoms is enhanced on the Bi chains. We suggest that a group of protrusions is a key for understanding enhanced migration of the atoms along the Bi chains. Fig. 3a shows a ®lled-state STM image taken after Bi deposition at room temperature followed by annealing at 500°C for 5 s. We can see terraces with missing-row defects and many dark regions, indicated by the small white arrows, which corre-
Fig. 3. (a) STM image taken after Bi deposition of 2 ML at room temperature followed by annealing at 500°C for 5 s. There appear some pits on the surface. Vs 2:5 V, I 0:3 nA, S 45 45 nm2 . (b) Cross-sectional view at the white line in (a).
1443
spond to the pits. However, we cannot ®nd the Bi-dimer chains. The terraces should consist of Bidimers, compared with the results in the previous papers [3,7]. There are some islands in the regions around the pits. We show in Fig. 3b the depth pro®le at the white line in Fig. 3a. The depth of the pit is more than 0.25 nm. The exact value of the depth at the pit was not obtained because the top of the tip did not reach to the bottom of the pit owing to the observation condition of our STM system. The height of islands around the pits is about 0.15 nm, which corresponds to one atomic layer height. In Fig. 3a, there are few islands on the terraces apart from regions around the pits and ¯at Bilayers are formed on the surface. Though the Bi coverage of the surface is unknown, more than one layers of Bi atoms are likely grown on the Si(1 0 0) surface from the results in Fig. 2. Further experiments by, e.g., ion scattering spectroscopy are necessary to con®rm the Bi coverage. We can explain the formation process of the pits as follows: With the deposition of Bi on the Si(1 0 0) surface, the ®rst Bi layer with Bi dimers and periodically missing Bi-dimer rows is formed with (2 n) periodicity. Next, Bi atoms deposited on the surface migrate without crossing over the missing Bi-dimer rows, so that small islands with (n 2) periodicity are formed on the terraces. Finally, coalescence of small islands occurs over missing Bi-dimer rows and then the missing Bi-dimer rows disappear. Thus, the ®rst Bi layer becomes a complete (1 1) structure, and the second (top) Bi layer with Bi dimers and missing Bi-dimer rows is formed. After the completion of the second layer, the absences of dimerization and missing row in the ®rst layer should give rise to a compressive strain. To release the strain in the Bi layers, pits are to be formed on the surface at brief annealings. Louwsma et al. found square-shaped pits on the Bi-deposited Ge(1 0 0) surfaces [11]. In their observation, the depth of the square-shaped pits was typically about 5±10 atomic layer height. Ge atoms in the substrate have been removed and redeposited near the pits. However, in the present stage, we cannot infer whether the islands near the pits consist of Bi atoms and/or Si atoms in the Bi/Si system.
1444
M. Naitoh et al. / Surface Science 482-485 (2001) 1440±1444
4. Conclusions
References
We have shown the unique character that Bi atoms possess at the Si(1 0 0) surface. Long linear chains of Bi-dimers are formed on the Si(1 0 0) surfaces and even on the Bi-covered surface. Moreover, there appear pits with depth more than 0.25 nm in the Bi terraces after brief annealings. These phenomena are caused by self-assembling of adsorbates. It would be essential to understand such unique characters of individual atoms in order to apply those to creating nano-scale electronic devices.
[1] T. Hanada, M. Kawai, Surf. Sci. 242 (1991) 137. [2] Ch. Park, R.Z. Bakhtizin, T. Hashizume, T. Sakurai, J. Vac. Sci. Technol. B 12 (1994) 2049. [3] M. Naitoh, H. Shimaya, S. Nishigaki, N. Oishi, F. Shoji, Surf. Sci. 377±379 (1997) 899. [4] T. Hashizume, S. Heike, M.I. Lutwyche, S. Watanabe, K. Nakajima, T. Nishi, Y. Wada, Jpn. J. Appl. Phys. 35 (1996) L1085. [5] A. Kawazu, T. Otsuki, G. Tominaga, Jpn. J. Appl. Phys. 20 (1981) 553. [6] K.-S. Kim, Y. Takakuwa, Y. Mori, S. Kono, Jpn. J. Appl. Phys. 35 (1996) L1695. [7] M. Naitoh, H. Shimaya, S. Nishigaki, N. Oishi, F. Shoji, Appl. Surf. Sci. 142 (1999) 38. [8] K. Miki, J.H.G. Owen, D.R. Bowler, G.A.D. Briggs, K. Sakamoto, Surf. Sci. 421 (1999) 397. [9] K. Miki, D.R. Bowler, J.H.G. Owen, G.A.D. Briggs, K. Sakamoto, Phys. Rev. B 59 (1999) 14868. [10] M. Shimomura, K. Miki, T. Abukawa, S. Kono, Surf. Sci. 447 (2000) L169. [11] H.K. Louwsma, H.J.W. Zandvliet, B.A.G. Kersten, J. Chesneau, A. van Silfhout, B. Poelsema, Surf. Sci. 381 (1997) L594.
Acknowledgements This work was partially supported by the Yoshida Foundation for the Promotion of Reading and Education.