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Large area dimer vacancy array on the Si(100) surface studied by scanning tunneling microscope
Surface Science 412/413 (1998) 236–241
Large area dimer vacancy array on the Si(100) surface studied by scanning tunneling microscope H.Q. Yang a,b,*, C.X. Zhu a, J.N. Gao a, Z.Q. Xue b, S.J. Panga a Beijing Laboratory of Vacuum Physics, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People’s Republic of China b Department of Radio-Electronics, Peking University, Beijing 100871, People’s Republic of China Received 31 March 1998; accepted for publication 21 May 1998
Abstract Large area dimer vacancy arrays can be formed on the Si(100) surface when Si atoms are deposited on the Si(100)-2×1 surface followed by quenching from 1200°C. The dimer vacancy lines (DVLs) of the dimer vacancy array run perpendicular to the dimer rows and the basic building cells of the DVLs are: (i) a cluster of two missing dimers; (ii) a complex of one missing dimer and a cluster of two missing dimers. Small isolated islands with DVLs like that of the Si(100) surface are observed on the surface. The precursors of the islands are investigated. The formation mechanism of the island is that the interaction between the stress field of the dimers on the Si(100) surface and that of the island causes the dimer vacancies in the dimer rows of the island and the attraction between the dimer vacancies in adjacent dimer rows aligns the dimer vacancies. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscope; Silicon; Surface defects
1. Introduction The Si(100) surface has been the subject of many experimental and theoretical studies [1–4] for the last three decades due not only to its technological importance in semiconductor industry but also to its complex reconstructions under various conditions of sample preparation. It is well known that dimer vacancies are always observed on the Si(100)-2×1 surface. Theoretical calculation by Pandey [5] indicated that dimer vacancies could exit on equilibrium surfaces due to their low energy of formation. Dimer vacancies can be introduced on the Si(100)-2×1 surface by many methods [6–22]. Zandvliet et al. [7,8] observed that * Corresponding author. Fax: +86 10 62556598.
random defects created by energetic ion sputtering ordered into line defects running perpendicular to the dimer rows upon annealing at elevated temperatures. Chander et al. [10,11] observed missing dimer rows along the dimer row direction by etching the Si(100)-2×1 surface with chlorine, bromine, etc. Niehus et al. [16 ] found on the Si(100)-2×1 surface that several types of dimer vacancy defects which were induced by Ni contamination aligned perpendicular to the dimer rows. When the Si(100)-2×1 surface was covered by germanium or phosphorus, dimer vacancy lines (DVLs) running perpendicular to the dimer rows could be introduced [19–21] and their formation was attributed to the surface stress caused by lattice-mismatch. Recently, DVLs running perpendicular to the dimer rows were found on the
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Si(100) surface after high temperature quenching and the formation of the DVLs was due to the interaction between dimer vacancies [22,23]. In this paper, we report a new method to form DVLs running perpendicular to the dimer rows on the Si(100) surface. The morphology of the Si(100) surface and the details of the DVLs are investigated by scanning tunneling microscope (STM ). Small isolated islands having the same feature as the Si(100) surface are observed. The precursor and the formation mechanism of the islands are studied.
2. Experiment The experiment was performed in an ultra-high vacuum chamber equipped with an Omicron STM1 system. The base pressure of the system was ~5×10−11 Torr. STM tips were chemically etched aluminum wires and were carefully cleaned in the ultra-high vacuum chamber. The samples used in our experiment were cut from a Sb-doped Si(100) wafer with a resistivity of 0.05 V cm and size of 12 mm×2 mm×0.5 mm. They were outgassed overnight at 600°C and quickly heated to 1200°C for several seconds. The temperatures were measured with an infrared pyrometer. The pressure in the chamber was below 3×10−10 Torr during heating. This procedure resulted in a surface with terraces of alternating 1×2 and 2×1 dimer reconstructions and the defect density of the surface was less than 1%. Another Si(100) sample prepared by the same procedure was used as an evaporant. During the deposition, the Si(100)-2×1 sample was positioned close to the evaporant which was resistively heated and the pressure in the chamber was raised to 6×10−10 Torr. The temperature of the Si(100)-2×1 sample was kept at room temperature during the deposition. After the deposition, the Si(100)-2×1 surface was immediately observed in situ with STM and we found that the surface was covered by small Si clusters. Then, the sample was quickly heated to 1200°C and kept for 10 s, then the heating current of the sample was turned down quickly and the quenching rate was ~150°C/s. After several cycles of temperature, the heating
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current was turned off. After cooling down to room temperature, the sample was transferred in situ to the Omicron STM for imaging. All images presented here were acquired with constant tip– sample current mode.
3. Results and discussion The STM images of the sample at the three preparation stages are shown in Fig. 1. Fig. 1a is a typical STM image of the Si(100)-2×1 surface with single-layer step A (dimer rows on the upper terrace run parallel to the step edge) and step B (dimer rows on the upper terrace run perpendicular to the step edge) [24]. The density of dimer vacancies is less than 1%. The scan area is 40 nm×40 nm. After the deposition, the surface is covered with small Si clusters, as shown in Fig. 1b. The size of the clusters is ~6 nm. After quenching from 1200°C, the feature of the Si(100) surface is shown in Fig. 1c. The scan area is 200 nm×200 nm. Fig. 1d is an enlarged image of Fig. 1c. The scan area is 20 nm×20 nm. From Fig. 1d, we can see that the white lines in Fig. 1c are dimer bands (DBs) running perpendicular to dimer rows. They are composed of short dimer rows of three to five dimers. The dark lines in Fig. 1c are dimer vacancy lines (DVLs). The DBs and DVLs run from one step edge to the next step edge without breaking. This kind of structure is formed on the whole Si(100) surface and the result is repeatable. The result is different from previous results [25–28]. This is mainly because of the difference in the amount of Si deposition and the quenching temperature. When the deposition amount is higher, a Si(100)-2×1 surface with high density of defects will form. When the deposition amount is lower, the results are similar to those reported by Mo et al. [25]. To identify the basic building cells of the (DVLs), the types of dimer vacancies of the DVLs are classified. We find that two types of dimer vacancies mainly contribute to the building cells of the DVLs. They are: (i) a cluster of two neighbor dimer vacancies (2-DV ), as marked by C in Fig. 1a; (ii) a complex of a single dimer vacancy (1-DV ) and a 2-DV separated by a dimer
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Fig. 1. STM images of the three stages of sample preparation. (a) Si(100)-2×1 surface before deposition. The scan area is 40 nm×40 nm. A and B mark the single-layer step A and B, respectively. C and D mark a 2-DV and a (1+2)-DV complex, respectively. (b) Si(100) surface after deposition. The scan area is 125 nm×125 nm. (c) Si(100) surface after quenching from 1200°C. The scan area is 200 nm×200 nm. (d ) An enlarged image of the structure of the quenched surface. The scan area is 20 nm×20 nm. All the images are acquired at −1.89 V sample bias and 0.65 nA.
[(1+2)-DV ], as marked by D in Fig. 1a. This is because these two kinds of defects have the lowest formation energy relative to the other defects [2]. Furthermore, we observed that small isolated islands with DVLs formed on the Si(100) surface. Fig. 2 shows a typical STM image of the islands. The preferable shape of the islands is a rectangle with the long edges running parallel to the DVLs of the Si(100) surface. The islands are composed
of DVLs and DBs alternatively. The DVLs run from one long edge to the other long edge of the islands without breaking. Fig. 3a is a close look at an island. The island contains five mini-islands and crosses one DVL of the Si(100) surface. A DVL is inserted between two neighbor miniislands. Fig. 3b is a profile of the line LL∞ in Fig. 3a. The AB, BC and CD parts of the line profile are measured over a 2-DV, a mini-island
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Fig. 2. An STM image of the quenched Si(100) surface covered by several isolated islands with the same feature as the Si(100) surface. The white lines are DBs and the dark lines are DVLs. The scan area is 145 nm×145 nm. The image is acquired at −1.89 V sample bias and 0.78 nA.
and a (1+2)-DV, respectively. The asymmetry of the AB part is caused by the rebonding of the exposed second layer atoms [22,23]. The peak of the CD part stands for the dimer separating the 1-DV and the 2-DV of the (1+2)-DV complex. It is relatively lower due to the relaxation of the rebonded second-layer atoms of the (1+2)-DV complex [2,5]. From the line profile, we can see that the height of the island and the depth of the DVLs on the Si(100) surface are single atom layer. To understand the formation mechanism of the islands, the precursors of the islands are observed. Fig. 4a shows the precursors of two islands. Fig. 4b shows a line profile across the up-left island. From Fig. 4, we can clearly see that the up-left island is a dimer row with five dimers. The two dimers at the ends of the dimer row are separated from the three center dimers by one dimer vacancy. The down-right island has the same structure as the up-left island but with seven dimers. Fig. 5 is a schematic diagram of the precursors of the two islands. Fig. 5a and b associate with the up-left and the down-right island, respectively. The surface atoms of the Si(100) surface between the separated ad-dimers and the center ad-dimers rebond [2,5], as shown by the dashed lines in Fig. 5. A possible formation mechanism of the
Fig. 3. (a) An STM image of an island. The scan area is 17 nm×17 nm. The image is acquired at −1.89 V sample bias and 0.65 nA. (b) The profile of line LL∞ in (a). The AB, BC and CD parts of the line profile are measured over a 2-DV, a mini-island and a (1+2)-DV, respectively.
island is proposed. The separation of the two ad-dimers from the center ad-dimers is caused by the stress field of the dimers of the Si(100) surface and that of the ad-dimers because the Si(100) surface is under tensile stress along the x direction and under compressive stress along the y direction, while the ad-dimer rows are under tensile stress along the y direction and under compressive stress along the x direction [29]. This condition is similar to the Si(100) surface covered by germanium [19,20]. To release the elastic energy caused by the two stress fields, a dimer vacancy is introduced. The rebonded configuration has the same number of dangling bonds as that of the dimer rows without dimer vacancies and leads to increased p-bonding between the dangling bonds
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Fig. 5. Schematic diagrams of the two initial islands: (a) the up-left island; (b) the down-right island. The black circles are adatoms. The shaded circles are atoms of the Si(100) surface. The sizes of the circles decrease with atoms away from the surface. The dashed lines show the rebonding of the surface atoms of the Si(100) surface. Fig. 4. An STM image of the precursors of two islands. The scan area is 15 nm×15 nm. The image is acquired at −1.45 V sample bias and 0.65 nA. (b) The profile of line AB over the up-left island.
of the adjacent dimers to the vacancy [2,30] and lowers the formation energy of the dimer rows. The energetically favorable configuration is one dimer vacancy at every fourth site [5]. This coincides with our observation. When the island grows, arriving atoms prefer attaching to the two ends of the island because of an anisotropic accommodation coefficient for arriving atoms [25]. The interaction between the stress field of the dimers of the Si(100) surface and that of the ad-dimer will modulate the ad-dimers and repeatedly create dimer vacancies several dimers away and form a dimer row with several 1+DVs. With the growth of the dimer row, the number of 1+DVs will increase and the formation energy of the dimer row will increase. To reduce the formation energy of the dimer row, the 1+DVs will collect by
diffusion along the dimer row to form a 2-DV or (1+2)-DV complex because of their lower formation energy [2]. When the island widens, a similar structure will form in adjacent dimer rows. But the DVs in the latter dimer rows will be pinned by the DVs in an adjacent former dimer row because of the attraction between missing dimer vacancies in adjacent rows [3,19,20]. So the DVLs grows perpendicular to the dimer rows and the islands are formed, as shown in Figs. 2 and 3a. To understand the details of the island growth and control the array, more experiments need to be done.
4. Conclusion We find that large area dimer vacancy arrays can be formed on the Si(100) surface when the Si(100)-2×1 surface is covered by Si clusters followed by quenching from 1200°C. The DBs and
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DVLs run perpendicular to the dimer rows. The DBs are composed of short dimer rows of three to five dimers and the basic building cells of the DVLs are 2-DV and (1+2)-DV complexes. Small isolated islands with DVLs are observed on the quenched Si(100) surface and the precursors of the islands are studied. The DVs of the island are caused by the interaction between the stress field of the dimers of the Si(100) surface and that of the island. The attraction between the DVs in adjacent dimer rows makes the DVs align perpendicular to the dimer rows.
References [1] R.J. Hamers, R.M. Tromp, J.E. Demuth, Phys. Rev. B 34 (1986) 5343. [2] J. Wang, T.A. Arias, J.D. Joannopoulos, Phys. Rev. B 47 (1993) 10497. [3] P.C. Weakliem, Z. Zhang, H. Metiu, Surf. Sci. 336 (1995) 303. [4] H.J.W. Zandvliet, H.K. Louwsma, P.E. Hegeman, B. Poelsema, Phys. Rev. Lett. 75 (1995) 3890. [5] K.C. Pandey, in: D.J. Chadi, W.A. Harrison (Eds.), Proc. 17th Int. Conf. on the Physics of Semiconductors, Springer, New York, 1985, p. 55. [6 ] P. Bedrossian, Surf. Sci. 301 (1994) 223. [7] H.J.W. Zandvliet, H.B. Elswijk, E.J. van Loenen, I.S.T. Tsong, Phys. Rev. B 46 (1992) 7581. [8] H. Fei, H.J.W. Zandvliet, M.H. Tsai, J.D. Dow, I.S.T. Tsong, Phys. Rev. Lett. 69 (1992) 3076. [9] P. Bedrossian, T. Klitsner, Phys. Rev. Lett. 68 (1992) 646.
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[10] M. Chander, D.A. Goetsch, C.M. Aldao, J.H. Weaver, Phys. Rev. B 52 (1995) 8288. [11] M. Chander, Y.Z. Li, D. Rioux, J.H. Weaver, Phys. Rev. Lett. 71 (1993) 4154. [12] D. Rioux, R.J. Pechman, M. Chander, J.H. Weaver, Phys. Rev. B 50 (1994) 4430. [13] K. Wurm, R. Kliese, Y. Hong, B. Rottger, H. Neddermeyer, I.S.T. Tsong, Phys. Rev. B 50 (1994) 1567. [14] Y. Wei, L. Li, I.S.T. Tsong, Appl. Phys. Lett. 66 (1995) 1818. [15] J.A. Martin, D.E. Savage, W. Moritz, M.G. Lagally, Phys. Rev. Lett. 56 (1986) 1936. [16 ] H. Niehus, U.K. Kohler, M. Copel, J.E. Demuth, J. Microsc. 152 (1988) 735. [17] T. Aruga, Y. Murata, Phys. Rev. B 34 (1986) 5654. [18] J.Y. Koo, J.Y. Yi, C. Hwang, D.H. Kim, S. Lee, D.H. Shin, Phys. Rev. B 52 (1995) 17269. [19] J. Tersoff, Phys. Rev. B 45 (1992) 8833. [20] X. Chen, F. Wu, Z. Zhang, M.G. Lagally, Phys. Rev. Lett. 73 (1994) 850. [21] L. Kipp, R.D. Bringans, D.K. Biegelsen, J.E. Northrup, A. Garcia, L.E. Swartz, Phys. Rev. B 52 (1995) 5843. [22] F.K. Men, A.R. Smith, K.J. Chao, Z. Zhang, C.K. Shih, Phys. Rev. B 52 (1995) R8650. [23] A.R. Smith, F.K. Men, K.J. Chao, Z. Zhang, C.K. Shih, J. Vac. Sci. Technol. B14 (1996) 909. [24] D.J. Chadi, Phys. Rev. Lett. 59 (1987) 1691. [25] Y.W. Mo, B.S. Swartzentruber, R. Kariotis, M.B. Webb, M.G. Lagally, Phys. Rev. Lett. 63 (1989) 2393. [26 ] R.J. Hamers, U.K. Kohler, J.E. Demuth, J. Vac. Sci. Technol. A8 (1990) 195. [27] A.J. Hoeven, J.M. Lenssinck, D. Dijkkamp, E.J. van Loenen, J. Dieleman, Phys. Rev. Lett. 63 (1989) 1830. [28] Y.W. Mo, R. Kariotis, D.E. Savage, M.G. Lagally, Surf. Sci. 219 (1989) L551. [29] O.L. Alerhand, D. Vanderbilt, R.D. Meade, J.D. Joannopoulos, Phys. Rev. Lett. 61 (1988) 1973. [30] N. Roberts, R.J. Needs, Surf. Sci. 236 (1990) 112.
Large area dimer vacancy array on the Si(100) surface studied by scanning tunneling microscope H.Q. Yang a,b,*, C.X. Zhu a, J.N. Gao a, Z.Q. Xue b, S.J. Panga a Beijing Laboratory of Vacuum Physics, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People’s Republic of China b Department of Radio-Electronics, Peking University, Beijing 100871, People’s Republic of China Received 31 March 1998; accepted for publication 21 May 1998
Abstract Large area dimer vacancy arrays can be formed on the Si(100) surface when Si atoms are deposited on the Si(100)-2×1 surface followed by quenching from 1200°C. The dimer vacancy lines (DVLs) of the dimer vacancy array run perpendicular to the dimer rows and the basic building cells of the DVLs are: (i) a cluster of two missing dimers; (ii) a complex of one missing dimer and a cluster of two missing dimers. Small isolated islands with DVLs like that of the Si(100) surface are observed on the surface. The precursors of the islands are investigated. The formation mechanism of the island is that the interaction between the stress field of the dimers on the Si(100) surface and that of the island causes the dimer vacancies in the dimer rows of the island and the attraction between the dimer vacancies in adjacent dimer rows aligns the dimer vacancies. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscope; Silicon; Surface defects
1. Introduction The Si(100) surface has been the subject of many experimental and theoretical studies [1–4] for the last three decades due not only to its technological importance in semiconductor industry but also to its complex reconstructions under various conditions of sample preparation. It is well known that dimer vacancies are always observed on the Si(100)-2×1 surface. Theoretical calculation by Pandey [5] indicated that dimer vacancies could exit on equilibrium surfaces due to their low energy of formation. Dimer vacancies can be introduced on the Si(100)-2×1 surface by many methods [6–22]. Zandvliet et al. [7,8] observed that * Corresponding author. Fax: +86 10 62556598.
random defects created by energetic ion sputtering ordered into line defects running perpendicular to the dimer rows upon annealing at elevated temperatures. Chander et al. [10,11] observed missing dimer rows along the dimer row direction by etching the Si(100)-2×1 surface with chlorine, bromine, etc. Niehus et al. [16 ] found on the Si(100)-2×1 surface that several types of dimer vacancy defects which were induced by Ni contamination aligned perpendicular to the dimer rows. When the Si(100)-2×1 surface was covered by germanium or phosphorus, dimer vacancy lines (DVLs) running perpendicular to the dimer rows could be introduced [19–21] and their formation was attributed to the surface stress caused by lattice-mismatch. Recently, DVLs running perpendicular to the dimer rows were found on the
0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 04 3 1 -2
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Si(100) surface after high temperature quenching and the formation of the DVLs was due to the interaction between dimer vacancies [22,23]. In this paper, we report a new method to form DVLs running perpendicular to the dimer rows on the Si(100) surface. The morphology of the Si(100) surface and the details of the DVLs are investigated by scanning tunneling microscope (STM ). Small isolated islands having the same feature as the Si(100) surface are observed. The precursor and the formation mechanism of the islands are studied.
2. Experiment The experiment was performed in an ultra-high vacuum chamber equipped with an Omicron STM1 system. The base pressure of the system was ~5×10−11 Torr. STM tips were chemically etched aluminum wires and were carefully cleaned in the ultra-high vacuum chamber. The samples used in our experiment were cut from a Sb-doped Si(100) wafer with a resistivity of 0.05 V cm and size of 12 mm×2 mm×0.5 mm. They were outgassed overnight at 600°C and quickly heated to 1200°C for several seconds. The temperatures were measured with an infrared pyrometer. The pressure in the chamber was below 3×10−10 Torr during heating. This procedure resulted in a surface with terraces of alternating 1×2 and 2×1 dimer reconstructions and the defect density of the surface was less than 1%. Another Si(100) sample prepared by the same procedure was used as an evaporant. During the deposition, the Si(100)-2×1 sample was positioned close to the evaporant which was resistively heated and the pressure in the chamber was raised to 6×10−10 Torr. The temperature of the Si(100)-2×1 sample was kept at room temperature during the deposition. After the deposition, the Si(100)-2×1 surface was immediately observed in situ with STM and we found that the surface was covered by small Si clusters. Then, the sample was quickly heated to 1200°C and kept for 10 s, then the heating current of the sample was turned down quickly and the quenching rate was ~150°C/s. After several cycles of temperature, the heating
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current was turned off. After cooling down to room temperature, the sample was transferred in situ to the Omicron STM for imaging. All images presented here were acquired with constant tip– sample current mode.
3. Results and discussion The STM images of the sample at the three preparation stages are shown in Fig. 1. Fig. 1a is a typical STM image of the Si(100)-2×1 surface with single-layer step A (dimer rows on the upper terrace run parallel to the step edge) and step B (dimer rows on the upper terrace run perpendicular to the step edge) [24]. The density of dimer vacancies is less than 1%. The scan area is 40 nm×40 nm. After the deposition, the surface is covered with small Si clusters, as shown in Fig. 1b. The size of the clusters is ~6 nm. After quenching from 1200°C, the feature of the Si(100) surface is shown in Fig. 1c. The scan area is 200 nm×200 nm. Fig. 1d is an enlarged image of Fig. 1c. The scan area is 20 nm×20 nm. From Fig. 1d, we can see that the white lines in Fig. 1c are dimer bands (DBs) running perpendicular to dimer rows. They are composed of short dimer rows of three to five dimers. The dark lines in Fig. 1c are dimer vacancy lines (DVLs). The DBs and DVLs run from one step edge to the next step edge without breaking. This kind of structure is formed on the whole Si(100) surface and the result is repeatable. The result is different from previous results [25–28]. This is mainly because of the difference in the amount of Si deposition and the quenching temperature. When the deposition amount is higher, a Si(100)-2×1 surface with high density of defects will form. When the deposition amount is lower, the results are similar to those reported by Mo et al. [25]. To identify the basic building cells of the (DVLs), the types of dimer vacancies of the DVLs are classified. We find that two types of dimer vacancies mainly contribute to the building cells of the DVLs. They are: (i) a cluster of two neighbor dimer vacancies (2-DV ), as marked by C in Fig. 1a; (ii) a complex of a single dimer vacancy (1-DV ) and a 2-DV separated by a dimer
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Fig. 1. STM images of the three stages of sample preparation. (a) Si(100)-2×1 surface before deposition. The scan area is 40 nm×40 nm. A and B mark the single-layer step A and B, respectively. C and D mark a 2-DV and a (1+2)-DV complex, respectively. (b) Si(100) surface after deposition. The scan area is 125 nm×125 nm. (c) Si(100) surface after quenching from 1200°C. The scan area is 200 nm×200 nm. (d ) An enlarged image of the structure of the quenched surface. The scan area is 20 nm×20 nm. All the images are acquired at −1.89 V sample bias and 0.65 nA.
[(1+2)-DV ], as marked by D in Fig. 1a. This is because these two kinds of defects have the lowest formation energy relative to the other defects [2]. Furthermore, we observed that small isolated islands with DVLs formed on the Si(100) surface. Fig. 2 shows a typical STM image of the islands. The preferable shape of the islands is a rectangle with the long edges running parallel to the DVLs of the Si(100) surface. The islands are composed
of DVLs and DBs alternatively. The DVLs run from one long edge to the other long edge of the islands without breaking. Fig. 3a is a close look at an island. The island contains five mini-islands and crosses one DVL of the Si(100) surface. A DVL is inserted between two neighbor miniislands. Fig. 3b is a profile of the line LL∞ in Fig. 3a. The AB, BC and CD parts of the line profile are measured over a 2-DV, a mini-island
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Fig. 2. An STM image of the quenched Si(100) surface covered by several isolated islands with the same feature as the Si(100) surface. The white lines are DBs and the dark lines are DVLs. The scan area is 145 nm×145 nm. The image is acquired at −1.89 V sample bias and 0.78 nA.
and a (1+2)-DV, respectively. The asymmetry of the AB part is caused by the rebonding of the exposed second layer atoms [22,23]. The peak of the CD part stands for the dimer separating the 1-DV and the 2-DV of the (1+2)-DV complex. It is relatively lower due to the relaxation of the rebonded second-layer atoms of the (1+2)-DV complex [2,5]. From the line profile, we can see that the height of the island and the depth of the DVLs on the Si(100) surface are single atom layer. To understand the formation mechanism of the islands, the precursors of the islands are observed. Fig. 4a shows the precursors of two islands. Fig. 4b shows a line profile across the up-left island. From Fig. 4, we can clearly see that the up-left island is a dimer row with five dimers. The two dimers at the ends of the dimer row are separated from the three center dimers by one dimer vacancy. The down-right island has the same structure as the up-left island but with seven dimers. Fig. 5 is a schematic diagram of the precursors of the two islands. Fig. 5a and b associate with the up-left and the down-right island, respectively. The surface atoms of the Si(100) surface between the separated ad-dimers and the center ad-dimers rebond [2,5], as shown by the dashed lines in Fig. 5. A possible formation mechanism of the
Fig. 3. (a) An STM image of an island. The scan area is 17 nm×17 nm. The image is acquired at −1.89 V sample bias and 0.65 nA. (b) The profile of line LL∞ in (a). The AB, BC and CD parts of the line profile are measured over a 2-DV, a mini-island and a (1+2)-DV, respectively.
island is proposed. The separation of the two ad-dimers from the center ad-dimers is caused by the stress field of the dimers of the Si(100) surface and that of the ad-dimers because the Si(100) surface is under tensile stress along the x direction and under compressive stress along the y direction, while the ad-dimer rows are under tensile stress along the y direction and under compressive stress along the x direction [29]. This condition is similar to the Si(100) surface covered by germanium [19,20]. To release the elastic energy caused by the two stress fields, a dimer vacancy is introduced. The rebonded configuration has the same number of dangling bonds as that of the dimer rows without dimer vacancies and leads to increased p-bonding between the dangling bonds
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Fig. 5. Schematic diagrams of the two initial islands: (a) the up-left island; (b) the down-right island. The black circles are adatoms. The shaded circles are atoms of the Si(100) surface. The sizes of the circles decrease with atoms away from the surface. The dashed lines show the rebonding of the surface atoms of the Si(100) surface. Fig. 4. An STM image of the precursors of two islands. The scan area is 15 nm×15 nm. The image is acquired at −1.45 V sample bias and 0.65 nA. (b) The profile of line AB over the up-left island.
of the adjacent dimers to the vacancy [2,30] and lowers the formation energy of the dimer rows. The energetically favorable configuration is one dimer vacancy at every fourth site [5]. This coincides with our observation. When the island grows, arriving atoms prefer attaching to the two ends of the island because of an anisotropic accommodation coefficient for arriving atoms [25]. The interaction between the stress field of the dimers of the Si(100) surface and that of the ad-dimer will modulate the ad-dimers and repeatedly create dimer vacancies several dimers away and form a dimer row with several 1+DVs. With the growth of the dimer row, the number of 1+DVs will increase and the formation energy of the dimer row will increase. To reduce the formation energy of the dimer row, the 1+DVs will collect by
diffusion along the dimer row to form a 2-DV or (1+2)-DV complex because of their lower formation energy [2]. When the island widens, a similar structure will form in adjacent dimer rows. But the DVs in the latter dimer rows will be pinned by the DVs in an adjacent former dimer row because of the attraction between missing dimer vacancies in adjacent rows [3,19,20]. So the DVLs grows perpendicular to the dimer rows and the islands are formed, as shown in Figs. 2 and 3a. To understand the details of the island growth and control the array, more experiments need to be done.
4. Conclusion We find that large area dimer vacancy arrays can be formed on the Si(100) surface when the Si(100)-2×1 surface is covered by Si clusters followed by quenching from 1200°C. The DBs and
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DVLs run perpendicular to the dimer rows. The DBs are composed of short dimer rows of three to five dimers and the basic building cells of the DVLs are 2-DV and (1+2)-DV complexes. Small isolated islands with DVLs are observed on the quenched Si(100) surface and the precursors of the islands are studied. The DVs of the island are caused by the interaction between the stress field of the dimers of the Si(100) surface and that of the island. The attraction between the DVs in adjacent dimer rows makes the DVs align perpendicular to the dimer rows.
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