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Temperature dependence of the phase manipulation feasibility between c(4 × 2) and p(2 × 2) on the Si(1 0 0) surface
Surface Science 566–568 (2004) 767–771 www.elsevier.com/locate/susc
Temperature dependence of the phase manipulation feasibility between c(4 · 2) and p(2 · 2) on the Si(1 0 0) surface Keisuke Sagisaka *, Daisuke Fujita, Giyuu Kido, Nobuyuki Koguchi Nanomaterials Laboratory, National Institute for Materials Science, 1-2-1 Sengen Tsukuba, Ibaraki 305-0047, Japan Available online 17 June 2004
Abstract Previously, we have reported that it is possible to manipulate the Si(1 0 0) surface phases between c(4 · 2) and p(2 · 2) by precise sample bias control in scanning tunneling microscopy (STM). Electron injection into the surface from the STM tip is responsible for the phase manipulation. In the present study, the phase manipulation by STM is thoroughly studied in a wide range of temperature from 4.2 to 100 K. We have found that the manipulation is feasible below 40 K but not above 40 K. This temperature dependence is quite similar to the recent result observed by low-energy electron diffraction (LEED), which was viewed as an order–disorder phase transition of the Si(1 0 0) surface below 40 K. From the similarity of both phenomenon occurring at 40 K, we conclude that the order–disorder phase transition is induced by energetic electron irradiation onto the Si(1 0 0) surface during the LEED analysis. 2004 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Surface relaxation and reconstruction; Surface structure, morphology, roughness, and topography; Silicon
1. Introduction The topmost atoms on the Si(1 0 0) surface form dimers to reduce their dangling bonds. Evidence of a buckled dimer (asymmetric dimer) has been experimentally observed with low-energy electron diffraction (LEED) [1], angle-resolved photoelectron spectroscopy (ARPES) [2], X-ray photoemission spectroscopy (XPS) [3,4], scanning tunneling microscopy (STM) [5–7], and non-contact atomic force microscopy (NC-AFM) [8].
* Corresponding author. Tel.: +81-29-859-2741; fax: +81-29859-2701. E-mail addresses: [email protected] (K. Sagisaka), [email protected] (D. Fujita).
Nowadays, asymmetric dimer has been established as the principal feature of the reconstructed Si(1 0 0) surface. Thermal excitation of asymmetric dimers are well known as the flip–flop motion of dimers, which gives a 2 · 1 periodicity that has been directly imaged with STM [9,10] and NCAFM [11] at room temperature. On the other hand, the flip–flop motion is frozen below 200 K [1,5] and asymmetric dimers can constitute c(4 · 2) and p(2 · 2) periodicities (Fig. 1). Theoretical studies [12,13] have shown that c(4 · 2) is the minimum energy atomic structure. However, recent STM observations at very low temperature have shown different surface structures such as a single domain of p(2 · 2) [14], a mixture of c(4 · 2) and p(2 · 2) [7,15], flip–flop dimers [16], and symmetric dimers [17]. To solve this
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.062
768
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
p(2 · 2), and flip–flop dimer regions, in the image. In fact, this image was acquired after phase manipulation by utilizing the characteristic that the dimer geometry on this surface is sensitive to the sample bias voltage. This unique feature made it possible to intentionally switch c(4 · 2) to p(2 · 2) or vice versa at very low temperature using bias control. We also have suggested that the mechanism of the phase manipulation by STM is attributed to energetic electron injection from the STM tip to the surface. Accordingly, we can interpret that the order–disorder phase transition observed by LEED should be related to the phase manipulation by STM. In the present study, we examined the feasibility of the phase manipulation as a function of temperature. We repeated phase manipulations on the Si(1 0 0) surface at various temperatures between 4.2 and 100 K. The manipulation from c(4 · 2) to p(2 · 2) is not possible and c(4 · 2) remains above 40 K, whereas the c(4 · 2) dominant surface can be switched to the p(2 · 2) surface below 40 K. This result allows us to conclude that the phase transition observed by the LEED analysis was caused by irradiation of electron beam onto the surface.
Fig. 1. A typical STM image obtained at 4.2 K with a sample bias of +1.58 V (tunneling current: 30 pA) and schematics of the c(4 · 2) and p(2 · 2) asymmetric dimer geometries.
controversy, Matsumoto et al. [18] recently have performed a low-temperature LEED analysis down to 25 K. They have observed that an order– disorder phase transition occurs below 40 K. The intensities of the quarter order spots, corresponding to c(4 · 2), decrease below 40 K, but no particular stable structure is detected. They have claimed that different low temperature phases observed by STM are attributed to the disorder phase. It seems that their result is able to explain the surface at low temperature below 40 K. On the other hands, we have recently discovered that the structure observed with STM at 4.2 K strongly depends on the sample bias voltage [19]. A typical example is shown in Fig. 1. There are seen three different dimer structures; c(4 · 2),
2. Experiment The experiments were performed in an ultrahigh vacuum (UHV) system with a base pressure of 3 · 10 9 Pa. The Si(1 0 0) sample was cut from a phosphorus-doped wafer with resistivity of 0.008–0.015 X cm. A clean surface was obtained by flash to 1300 K for 10 s after degas at 900 K for 12 h. The sample surface used in this study contained 1–5% defects. Electrochemically etched tungsten wire was used as the STM tip.
3. Results and discussion Fig. 2 demonstrates the phase manipulation from c(4 · 2) to p(2 · 2) on the Si(1 0 0) surface at 4.2 K by sample bias control. Fig. 2(a) is a typical STM image of the Si(1 0 0) surface at 4.2 K before the manipulation; that is, a surface obtained immediately after cool down without perturbations. The
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
Fig. 2. Phase manipulation from c(4 · 2) to p(2 · 2) performed at 4.2 K: (a) the c(4 · 2) dominant surface before the manipulation (bias: +1.25 V, tunneling current: 30 pA), and (b) the p(2 · 2) emerged surface after the manipulation. A scan was started at +2.0 V and the sample bias was decreased slowly to +1.3 V (tunneling current: 30 pA).
surface mostly consists of the c(4 · 2) phase and some p(2 · 2) dimers are aligned vertically in the image. The p(2 · 2) dimers usually appear in the surface with some defects such as atomic vacancies and the type C defect [5], whereas a defect free surface is comprised of only the c(4 · 2) dimers [20]. The surface after the manipulation is shown in Fig.
769
2(b). The p(2 · 2) domains dramatically increased. The previous study [19] has clarified the followings: (1) the c(4 · 2) surface has lower energy than the p(2 · 2) surface; (2) the p(2 · 2) dimers appear when the surface is scanned with a positive bias that is high enough to generate flip–flop dimers. Accordingly, energetic electron injection into the surface causes the phase switch from c(4 · 2) to p(2 · 2) through the process of electron inelastic scattering in the surface; (3) the p(2 · 2) surface resumes c(4 · 2) when the surface is scanned with a negative bias voltage. More details on the phase manipulation are described elsewhere [19]. Matsumoto et al. [18] have reported using LEED that an order–disorder phase transition occurs below 40 K. According to our finding of the phase manipulation, we suspect that the phase transition was caused by electron irradiation onto the surface during a LEED measurement. Since electron injection of about 1.5 eV by STM can excite the flip–flop motion of the dimers, a LEED measurement with high incident electron energies (52 and 110 eV) should immediately perturb the surface. In order to confirm our hypothesis, we performed the phase manipulation in a wide range of temperature between 4.2 and 100 K. Fig. 3(a) and (b) show sequences of the phase manipulation conducted at 30 and 40 K, respectively. The top images confirmed that the c(4 · 2) dimers dominated the initial surface. In the middle images, a bias voltage control (+1.8 V fi +0.65 V) was carried out and the bottom images checked feasibility of the phase manipulation. The manipulations at the different temperatures resulted in distinct difference. The phase manipulation at 30 K [Fig. 3(a)] confirmed the phase change from c(4 · 2) to p(2 · 2) in a large area of the surface, the same as the result at 4.2 K in Fig. 2. On the contrary, the phase manipulation failed at 40 K [Fig. 3(b)]. Although p(2 · 2) dimers which appeared in the center of the initial image disappeared after the bias control, the c(4 · 2) dominant surface was preserved. The same procedure was repeated at various temperatures to measure the areas of the c(4 · 2) and p(2 · 2) domains before and after the manipulation. The manipulations were performed among the areas of 26 · 26–38 · 38 nm2 . Fig. 4(a) and (b) plot the ratio of the areas of the c(4 · 2) and p(2 · 2)
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K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
Fig. 4. Percentages of the c(4 · 2) and p(2 · 2) domains as a function of temperature (a) before and (b) after the phase manipulation. Filled boxes and open circles represent c(4 · 2) and p(2 · 2), respectively.
Fig. 3. A comparison of feasibility of the phase manipulation from c(4 · 2) to p(2 · 2) between (a) 30 and (b) 40 K. Phase manipulations were conducted by bias decrease from +1.8 to +0.65 V (tunneling current: 30 pA).
domains before and after the phase manipulation, respectively. Before the manipulation, c(4 · 2) was always dominant on the surface in all evaluated temperatures and small area of p(2 · 2) domains appeared around defects. However, the percentage of c(4 · 2) and p(2 · 2) after the manipulation drastically changed around 40 K. Fig. 4(b) clearly shows that the large area of p(2 · 2) emerges by the manipulation below 40 K, but the phase manipulation is not feasible above 40 K. The behavior of the c(4 · 2) percentage decrease below 40 K in Fig. 4(b) is very similar to the decrease in the quarter spots intensities of LEED [18]. From a comparison of the STM and LEED results, it is evident that the
phase transition observed by LEED was caused by the LEED analysis itself. The sample bias voltage, which decides the energy of tunneling electrons, is critical to the precise observation of the Si(1 0 0) surface with STM [15]. An inappropriate bias yields the perturbed surface bearing c(4 · 2), p(2 · 2), and flip–flop dimers together as shown in Fig. 1. The LEED analysis has perceived the absence of the p(2 · 2) spots as a disordered surface. Under the condition of a LEED measurement, irradiation of electron beam must generate the flip–flop motion and domain fluctuation between c(4 · 2) and p(2 · 2) on the Si(1 0 0) surface at very low temperature.
4. Summary Feasibility of the phase manipulation by STM was investigated in a wide rage of temperatures.
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
The phase manipulation is feasible below 40 K but not above 40 K. It is evident that electron tunneling caused phase changes. The behavior of the percentage of the c(4 · 2) domain as a function of temperature after the phase manipulation is very similar to the temperature dependence of the quarter spots observed by LEED. Therefore, we can conclude that the phase transition observed by LEED was caused by the LEED measurement itself; irradiation of energetic electrons onto the Si surface excited the flip–flop dimers and consequently resulted in the decrease in the quarter spots below 40 K.
Acknowledgements This work was performed as part of the Active Nano-Characterization and Technology Project, Special Coordination Funds of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.
References [1] T. Tabata, T. Aruga, Y. Murata, Surf. Sci. 179 (1987) L63. [2] L.S.O. Johansson, R.I.G. Uhrberg, P. M artensson, G.V. Hansson, Phys. Rev. B 42 (1990) 1305. [3] E. Landemark, C.J. Karlsson, Y.-C. Chao, R.I.G. Uhrberg, Phys. Rev. Lett. 69 (1992) 1588.
771
[4] Y. Yamashita, S. Machida, M. Nagao, S. Yamamoto, Y. Kakefuda, K. Mukai, J. Yoshinobu, Jpn. J. Appl. Phys. 41 (2002) L272. [5] R.A. Wolkow, Phys. Rev. Lett. 68 (1992) 2636. [6] H. Tochihara, T. Amakusa, M. Iwatsuki, Phys. Rev. B 50 (1994) 12262. [7] H. Shigekawa, K. Miyake, M. Ishida, K. Hata, H. Oigawa, Y. Nannichi, R. Yoshizaki, A. Kawazu, T. Abe, T. Ozawa, T. Nagamura, Jpn. J. Appl. Phys. 35 (1996) L1081. [8] T. Uozumi, Y. Tomiyoshi, N. Suehiro, Y. Sugawara, S. Morita, Appl. Surf. Sci. 188 (2002) 279. [9] R.M. Tromp, R.J. Hamers, J.E. Demuth, Phys. Rev. Lett. 55 (1985) 1303. [10] X.R. Qin, M. Lagally, Phys. Rev. B 59 (1999) 7293. [11] K. Yokoyama, T. Ochi, A. Yoshimoto, S. Morita, Jpn. J. Appl. Phys. 39 (2000) L113. [12] Z. Zhu, N. Shima, M. Tsukada, Phys. Rev. B 40 (1989) 11868. [13] A. Ramstad, G. Brocks, P.J. Kelly, Phys. Rev. B 51 (1995) 14504. [14] K. Hata, S. Yoshida, H. Shigekawa, Phys. Rev. Lett. 89 (2002) 286104. [15] K. Sagisaka, M. Kitahara, D. Fujita, Jpn. J. Appl. Phys. 42 (2003) L126. [16] T. Yokoyama, K. Takayanagi, Phys. Rev. B 61 (2000) 5078. [17] Y. Kondo, T. Amakusa, M. Iwatsuki, H. Tokumoto, Surf. Sci. 453 (2000) L318. [18] M. Matsumoto, K. Fukutani, T. Okano, Phys. Rev. Lett. 90 (2003) 106103. [19] K. Sagisaka, D. Fujita, G. Kido, Phys. Rev. Lett. 91 (2003) 146103. [20] K. Hata, T. Kimura, O. Takeuchi, H. Shigekawa, Jpn. J. Appl. Phys. 39 (2000) 3811.
Temperature dependence of the phase manipulation feasibility between c(4 · 2) and p(2 · 2) on the Si(1 0 0) surface Keisuke Sagisaka *, Daisuke Fujita, Giyuu Kido, Nobuyuki Koguchi Nanomaterials Laboratory, National Institute for Materials Science, 1-2-1 Sengen Tsukuba, Ibaraki 305-0047, Japan Available online 17 June 2004
Abstract Previously, we have reported that it is possible to manipulate the Si(1 0 0) surface phases between c(4 · 2) and p(2 · 2) by precise sample bias control in scanning tunneling microscopy (STM). Electron injection into the surface from the STM tip is responsible for the phase manipulation. In the present study, the phase manipulation by STM is thoroughly studied in a wide range of temperature from 4.2 to 100 K. We have found that the manipulation is feasible below 40 K but not above 40 K. This temperature dependence is quite similar to the recent result observed by low-energy electron diffraction (LEED), which was viewed as an order–disorder phase transition of the Si(1 0 0) surface below 40 K. From the similarity of both phenomenon occurring at 40 K, we conclude that the order–disorder phase transition is induced by energetic electron irradiation onto the Si(1 0 0) surface during the LEED analysis. 2004 Elsevier B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Surface relaxation and reconstruction; Surface structure, morphology, roughness, and topography; Silicon
1. Introduction The topmost atoms on the Si(1 0 0) surface form dimers to reduce their dangling bonds. Evidence of a buckled dimer (asymmetric dimer) has been experimentally observed with low-energy electron diffraction (LEED) [1], angle-resolved photoelectron spectroscopy (ARPES) [2], X-ray photoemission spectroscopy (XPS) [3,4], scanning tunneling microscopy (STM) [5–7], and non-contact atomic force microscopy (NC-AFM) [8].
* Corresponding author. Tel.: +81-29-859-2741; fax: +81-29859-2701. E-mail addresses: [email protected] (K. Sagisaka), [email protected] (D. Fujita).
Nowadays, asymmetric dimer has been established as the principal feature of the reconstructed Si(1 0 0) surface. Thermal excitation of asymmetric dimers are well known as the flip–flop motion of dimers, which gives a 2 · 1 periodicity that has been directly imaged with STM [9,10] and NCAFM [11] at room temperature. On the other hand, the flip–flop motion is frozen below 200 K [1,5] and asymmetric dimers can constitute c(4 · 2) and p(2 · 2) periodicities (Fig. 1). Theoretical studies [12,13] have shown that c(4 · 2) is the minimum energy atomic structure. However, recent STM observations at very low temperature have shown different surface structures such as a single domain of p(2 · 2) [14], a mixture of c(4 · 2) and p(2 · 2) [7,15], flip–flop dimers [16], and symmetric dimers [17]. To solve this
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.062
768
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
p(2 · 2), and flip–flop dimer regions, in the image. In fact, this image was acquired after phase manipulation by utilizing the characteristic that the dimer geometry on this surface is sensitive to the sample bias voltage. This unique feature made it possible to intentionally switch c(4 · 2) to p(2 · 2) or vice versa at very low temperature using bias control. We also have suggested that the mechanism of the phase manipulation by STM is attributed to energetic electron injection from the STM tip to the surface. Accordingly, we can interpret that the order–disorder phase transition observed by LEED should be related to the phase manipulation by STM. In the present study, we examined the feasibility of the phase manipulation as a function of temperature. We repeated phase manipulations on the Si(1 0 0) surface at various temperatures between 4.2 and 100 K. The manipulation from c(4 · 2) to p(2 · 2) is not possible and c(4 · 2) remains above 40 K, whereas the c(4 · 2) dominant surface can be switched to the p(2 · 2) surface below 40 K. This result allows us to conclude that the phase transition observed by the LEED analysis was caused by irradiation of electron beam onto the surface.
Fig. 1. A typical STM image obtained at 4.2 K with a sample bias of +1.58 V (tunneling current: 30 pA) and schematics of the c(4 · 2) and p(2 · 2) asymmetric dimer geometries.
controversy, Matsumoto et al. [18] recently have performed a low-temperature LEED analysis down to 25 K. They have observed that an order– disorder phase transition occurs below 40 K. The intensities of the quarter order spots, corresponding to c(4 · 2), decrease below 40 K, but no particular stable structure is detected. They have claimed that different low temperature phases observed by STM are attributed to the disorder phase. It seems that their result is able to explain the surface at low temperature below 40 K. On the other hands, we have recently discovered that the structure observed with STM at 4.2 K strongly depends on the sample bias voltage [19]. A typical example is shown in Fig. 1. There are seen three different dimer structures; c(4 · 2),
2. Experiment The experiments were performed in an ultrahigh vacuum (UHV) system with a base pressure of 3 · 10 9 Pa. The Si(1 0 0) sample was cut from a phosphorus-doped wafer with resistivity of 0.008–0.015 X cm. A clean surface was obtained by flash to 1300 K for 10 s after degas at 900 K for 12 h. The sample surface used in this study contained 1–5% defects. Electrochemically etched tungsten wire was used as the STM tip.
3. Results and discussion Fig. 2 demonstrates the phase manipulation from c(4 · 2) to p(2 · 2) on the Si(1 0 0) surface at 4.2 K by sample bias control. Fig. 2(a) is a typical STM image of the Si(1 0 0) surface at 4.2 K before the manipulation; that is, a surface obtained immediately after cool down without perturbations. The
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
Fig. 2. Phase manipulation from c(4 · 2) to p(2 · 2) performed at 4.2 K: (a) the c(4 · 2) dominant surface before the manipulation (bias: +1.25 V, tunneling current: 30 pA), and (b) the p(2 · 2) emerged surface after the manipulation. A scan was started at +2.0 V and the sample bias was decreased slowly to +1.3 V (tunneling current: 30 pA).
surface mostly consists of the c(4 · 2) phase and some p(2 · 2) dimers are aligned vertically in the image. The p(2 · 2) dimers usually appear in the surface with some defects such as atomic vacancies and the type C defect [5], whereas a defect free surface is comprised of only the c(4 · 2) dimers [20]. The surface after the manipulation is shown in Fig.
769
2(b). The p(2 · 2) domains dramatically increased. The previous study [19] has clarified the followings: (1) the c(4 · 2) surface has lower energy than the p(2 · 2) surface; (2) the p(2 · 2) dimers appear when the surface is scanned with a positive bias that is high enough to generate flip–flop dimers. Accordingly, energetic electron injection into the surface causes the phase switch from c(4 · 2) to p(2 · 2) through the process of electron inelastic scattering in the surface; (3) the p(2 · 2) surface resumes c(4 · 2) when the surface is scanned with a negative bias voltage. More details on the phase manipulation are described elsewhere [19]. Matsumoto et al. [18] have reported using LEED that an order–disorder phase transition occurs below 40 K. According to our finding of the phase manipulation, we suspect that the phase transition was caused by electron irradiation onto the surface during a LEED measurement. Since electron injection of about 1.5 eV by STM can excite the flip–flop motion of the dimers, a LEED measurement with high incident electron energies (52 and 110 eV) should immediately perturb the surface. In order to confirm our hypothesis, we performed the phase manipulation in a wide range of temperature between 4.2 and 100 K. Fig. 3(a) and (b) show sequences of the phase manipulation conducted at 30 and 40 K, respectively. The top images confirmed that the c(4 · 2) dimers dominated the initial surface. In the middle images, a bias voltage control (+1.8 V fi +0.65 V) was carried out and the bottom images checked feasibility of the phase manipulation. The manipulations at the different temperatures resulted in distinct difference. The phase manipulation at 30 K [Fig. 3(a)] confirmed the phase change from c(4 · 2) to p(2 · 2) in a large area of the surface, the same as the result at 4.2 K in Fig. 2. On the contrary, the phase manipulation failed at 40 K [Fig. 3(b)]. Although p(2 · 2) dimers which appeared in the center of the initial image disappeared after the bias control, the c(4 · 2) dominant surface was preserved. The same procedure was repeated at various temperatures to measure the areas of the c(4 · 2) and p(2 · 2) domains before and after the manipulation. The manipulations were performed among the areas of 26 · 26–38 · 38 nm2 . Fig. 4(a) and (b) plot the ratio of the areas of the c(4 · 2) and p(2 · 2)
770
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
Fig. 4. Percentages of the c(4 · 2) and p(2 · 2) domains as a function of temperature (a) before and (b) after the phase manipulation. Filled boxes and open circles represent c(4 · 2) and p(2 · 2), respectively.
Fig. 3. A comparison of feasibility of the phase manipulation from c(4 · 2) to p(2 · 2) between (a) 30 and (b) 40 K. Phase manipulations were conducted by bias decrease from +1.8 to +0.65 V (tunneling current: 30 pA).
domains before and after the phase manipulation, respectively. Before the manipulation, c(4 · 2) was always dominant on the surface in all evaluated temperatures and small area of p(2 · 2) domains appeared around defects. However, the percentage of c(4 · 2) and p(2 · 2) after the manipulation drastically changed around 40 K. Fig. 4(b) clearly shows that the large area of p(2 · 2) emerges by the manipulation below 40 K, but the phase manipulation is not feasible above 40 K. The behavior of the c(4 · 2) percentage decrease below 40 K in Fig. 4(b) is very similar to the decrease in the quarter spots intensities of LEED [18]. From a comparison of the STM and LEED results, it is evident that the
phase transition observed by LEED was caused by the LEED analysis itself. The sample bias voltage, which decides the energy of tunneling electrons, is critical to the precise observation of the Si(1 0 0) surface with STM [15]. An inappropriate bias yields the perturbed surface bearing c(4 · 2), p(2 · 2), and flip–flop dimers together as shown in Fig. 1. The LEED analysis has perceived the absence of the p(2 · 2) spots as a disordered surface. Under the condition of a LEED measurement, irradiation of electron beam must generate the flip–flop motion and domain fluctuation between c(4 · 2) and p(2 · 2) on the Si(1 0 0) surface at very low temperature.
4. Summary Feasibility of the phase manipulation by STM was investigated in a wide rage of temperatures.
K. Sagisaka et al. / Surface Science 566–568 (2004) 767–771
The phase manipulation is feasible below 40 K but not above 40 K. It is evident that electron tunneling caused phase changes. The behavior of the percentage of the c(4 · 2) domain as a function of temperature after the phase manipulation is very similar to the temperature dependence of the quarter spots observed by LEED. Therefore, we can conclude that the phase transition observed by LEED was caused by the LEED measurement itself; irradiation of energetic electrons onto the Si surface excited the flip–flop dimers and consequently resulted in the decrease in the quarter spots below 40 K.
Acknowledgements This work was performed as part of the Active Nano-Characterization and Technology Project, Special Coordination Funds of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.
References [1] T. Tabata, T. Aruga, Y. Murata, Surf. Sci. 179 (1987) L63. [2] L.S.O. Johansson, R.I.G. Uhrberg, P. M artensson, G.V. Hansson, Phys. Rev. B 42 (1990) 1305. [3] E. Landemark, C.J. Karlsson, Y.-C. Chao, R.I.G. Uhrberg, Phys. Rev. Lett. 69 (1992) 1588.
771
[4] Y. Yamashita, S. Machida, M. Nagao, S. Yamamoto, Y. Kakefuda, K. Mukai, J. Yoshinobu, Jpn. J. Appl. Phys. 41 (2002) L272. [5] R.A. Wolkow, Phys. Rev. Lett. 68 (1992) 2636. [6] H. Tochihara, T. Amakusa, M. Iwatsuki, Phys. Rev. B 50 (1994) 12262. [7] H. Shigekawa, K. Miyake, M. Ishida, K. Hata, H. Oigawa, Y. Nannichi, R. Yoshizaki, A. Kawazu, T. Abe, T. Ozawa, T. Nagamura, Jpn. J. Appl. Phys. 35 (1996) L1081. [8] T. Uozumi, Y. Tomiyoshi, N. Suehiro, Y. Sugawara, S. Morita, Appl. Surf. Sci. 188 (2002) 279. [9] R.M. Tromp, R.J. Hamers, J.E. Demuth, Phys. Rev. Lett. 55 (1985) 1303. [10] X.R. Qin, M. Lagally, Phys. Rev. B 59 (1999) 7293. [11] K. Yokoyama, T. Ochi, A. Yoshimoto, S. Morita, Jpn. J. Appl. Phys. 39 (2000) L113. [12] Z. Zhu, N. Shima, M. Tsukada, Phys. Rev. B 40 (1989) 11868. [13] A. Ramstad, G. Brocks, P.J. Kelly, Phys. Rev. B 51 (1995) 14504. [14] K. Hata, S. Yoshida, H. Shigekawa, Phys. Rev. Lett. 89 (2002) 286104. [15] K. Sagisaka, M. Kitahara, D. Fujita, Jpn. J. Appl. Phys. 42 (2003) L126. [16] T. Yokoyama, K. Takayanagi, Phys. Rev. B 61 (2000) 5078. [17] Y. Kondo, T. Amakusa, M. Iwatsuki, H. Tokumoto, Surf. Sci. 453 (2000) L318. [18] M. Matsumoto, K. Fukutani, T. Okano, Phys. Rev. Lett. 90 (2003) 106103. [19] K. Sagisaka, D. Fujita, G. Kido, Phys. Rev. Lett. 91 (2003) 146103. [20] K. Hata, T. Kimura, O. Takeuchi, H. Shigekawa, Jpn. J. Appl. Phys. 39 (2000) 3811.