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STM study of structural changes on Si(100)2×1-Sb surface induced by atomic hydrogen
Applied Surface Science 169±170 (2001) 93±99
STM study of structural changes on Si(100)2 1-Sb surface induced by atomic hydrogen O. Kubo*, J.-T. Ryu, H. Tani, T. Harada, T. Kobayashi, M. Katayama, K. Oura Department of Electronic Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Received 2 August 1999; accepted 12 October 1999
Abstract Using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we have studied the structural changes of the Si(100)2 1-Sb surface caused by hydrogen adsorption at both room temperature (RT) and 3008C. We have found that the ordering of a 2 1-Sb surface is more stable against atomic hydrogen exposure at 3008C than at RT, and that some Sb atoms desorb during atomic hydrogen exposure at 3008C. However, upon hydrogen exposure at both temperatures, we have observed neither three-dimensional islands nor the hydrogen terminated Si substrate which were reported for hydrogen interaction with the other metal/Si systems. On the 2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT followed by 5508C annealing, long bright lines similar to those reported for the Bi/Si(100) system have also been found. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Antimony; Hydrogen; Scanning tunneling microscopy (STM); Incorporation; Desorption; Line structure
1. Introduction Atomic hydrogen interaction with adsorbate/Si systems has recently received considerable attention in connection with deliberate use of the various effects of hydrogen on the growth of the thin ®lms on the Si substrate [1±7]. In particular, several works [8±14] have been published concerning the adsorption of atomic hydrogen on the metal/Si systems, in which metal atoms form a two-dimensional (2D) layer. In the most cases, the interaction of those systems with the
*
Corresponding author. Tel.: 81-6-6879-7779; fax: 81-6-6879-7780. E-mail address: [email protected] (O. Kubo).
atomic hydrogen results in the transformation of the uniform metal layer into separate 3D islands. Consequently, hydrogen atoms replace metal atoms, forming a hydrogenated Si surface. Such atomic hydrogeninduced structural changes of the metal/Si systems have been reported for group-I (Ag) [8,9], -III (Al, In) [10±13], and -IV (Pb) [14] metals. In contrast, the interaction of atomic hydrogen with Si surfaces terminated by group-Velements has scarcely been examined. Among the group-V adsorbates, adsorption of Sb on to the Si surfaces have recently received especial attention in connection with three technologically important phenomena: surfactant-mediated epitaxy [15], construction of delta-doping layers [16], and improvement of the quality of III±V compound epitaxial growth on Si substrates [17]. Thus, it is
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important to study the interaction of hydrogen with Sb terminated Si surfaces from both the technological and fundamental points of view. On the other hand, when Sb atoms are adsorbed on a Si(100) surface, two kinds of reconstructed surface phases, 2 1-Sb [18±20] and c(4 4)-Sb [21,22], are formed depending on both Sb coverage and substrate temperature. The 2 1-Sb phase has been reported to be formed by deposition of 0.6±2.0 ML Sb on to the clean Si(100) surface held at 3758C followed by 5408C annealing [18], while the c(4 4)-Sb phase has been reported to be formed by Sb deposition of 1± 2 ML on to the Si(100) surface at RT followed by annealing at the temperature of 570±6208C [21,22]. The absolute Sb coverage in the 2 1-Sb and c(4 4)-Sb surface phases has been reported to be 0.7±0.9 monolayer (ML: 1 ML 6:8 1014 atoms/ cm2, the ideal Si surface atomic density in the (100) plane) [19] and 0.25 ML, respectively [21,22]. In this study, using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we have investigated the interaction of atomic hydrogen with the Si(100)2 1-Sb surface phase at both RT and 3008C. We have found that the ordered structure of the 2 1-Sb surface is more stable against atomic hydrogen exposure at 3008C than at RT. However, upon hydrogen exposure at both temperatures, we have observed neither 3D islands nor the hydrogen terminated Si substrate which were reported for the other metal/Si systems [8±14]. After 5508C annealing of the 2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT, we have found the long bright lines similar to those reported for the Bi/Si(100) system [23,24]. 2. Experimental All experiments were carried out in an ultrahighvacuum (UHV) chamber with STM and LEED facilities. The base pressure in the UHV chamber was typically 8 10ÿ11 Torr. The STM tips were fabricated by electrochemical etching from a W wire (f 0:25 mm) and cleaned by in situ annealing. The n-type Si(100) wafers with 0.05 O cm resistivity were used. An atomically ¯at Si(100)2 1 surface was obtained after outgassing at 6008C for over 12 h, by several times ¯ashing to 12008C for 10 s each (by
direct current ¯owing through the sample), followed by slow cooling. Sb was deposited from a tantalum foil tube by direct current heating. For atomic hydrogen exposure, H2 gas was admitted into the chamber and dissociated into atomic hydrogen with an 18008C tungsten ®lament, 7 cm from the Si surfaces. Since the arrival rate of atomic hydrogen is unknown, the dose of molecular hydrogen is speci®ed, and expressed in units of Langmuir (1 L 1 10ÿ6 Torr s). To form a completely hydrogen terminated Si(100)2 1-H surface, a clean Si(100)2 1 surface was exposed to 500 L hydrogen at 3008C. All STM observations were made at RT. 3. Results and discussions Fig. 1(a) shows the Si(100)2 1-Sb surface prepared by evaporating more than 1 ML of Sb at RT followed by 5508C annealing. From this surface, a 2 1 LEED pattern was observed as shown in the upper-right inset in Fig. 1(a), in which half-order spots demonstrate a streak-like character. Such a 2 1-Sb surface contains a large number of antiphase boundaries (indicated by arrows in the lower-right inset in Fig. 1(a)) [18]. The clean Si(100) surface is known to be free of such an antiphase boundaries. This result indicates that most of the surface area is covered by Sb dimers. On the other hand, when the 2 1-Sb surface was prepared by evaporating Sb atoms at 5508C followed by annealing at 6208C, a more rough surface than that of Fig. 1(a) was formed as shown in Fig. 1(b). From this surface, a conventional 2 1 LEED pattern was observed as shown in the upper-right inset in Fig. 1(b). In this image, one can see many defects and a few 2D islands. According to the report by Garni et al. [20], when submonolayer amounts of Sb are evaporated on to the Si(100) surface at the temperature of 250± 5008C, Sb atoms are incorporated into the top layer of Si atoms, forming defects and 2D islands. Moreover, we observed some rows of buckled dimer and quite small areas of the c(4 4) surface phase with 0.25 ML Sb (see the lower-right inset in Fig. 1(b)) [21,22]. We speculate that some Sb atoms are incorporated into the top layer of Si atoms and the amount of Sb is less on the surface shown in Fig. 1(b) than that shown in Fig. 1(a).
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Ê ) of Si(100)2 1 surfaces, (a) prepared by evaporating more than 1 ML Sb on to a clean Si(100) surface Fig. 1. STM images (500 A 500 A at RT followed by 5508C annealing, (b) prepared by evaporating more than 1 ML Sb at 5508C followed by 6208C annealing. Upper-right insets in (a) (taken at 44 eV) and (b) (taken at 54 eV) show the LEED patterns from the corresponding surfaces, respectively. Lower-right Ê ) and (b) (125 A Ê ) show close-up STM images of parts of (a) and (b), respectively. These images were insets in (a) (100 A 100 A 125 A taken at a sample bias of ÿ2.6 V (a), ÿ1.9 V (b).
First, we exposed the 2 1-Sb surface shown in Fig. 1(a), which has the ¯atter morphology than that shown in Fig. 1(b), to atomic hydrogen at RT. In this case, a 2 1 LEED pattern already changes to a 1 1 pattern after hydrogen exposure of 15 L as shown in the upper-right inset in Fig. 2(a). A ®lled state STM image of the 2 1-Sb surface exposed to atomic hydrogen of 50 L at RT is shown in Fig. 2(a). As one can see, the surface becomes more disordered due to atomic hydrogen adsorption, with the increased number of voids and small clusters, which have a Ê . We speculate that some Sb diameter below 10 A atoms were expelled from the original position by atomic hydrogen interaction, consequently the number of voids and small clusters of Sb which were expelled increases. However, the surface is still ¯at and the dimer structure remains in spite of the fact that the surface displays a 1 1 LEED pattern. In the detailed observation, it was revealed that the domains of ordered dimer row structure have a width of less Ê (see the lower-left inset than 5 or 6 dimer rows, 40 A in Fig. 2(a)). This is probably due to the displacement of some Sb atoms or the breaking of Sb dimer bonds.
When the 2 1-Sb surface shown in Fig. 1(a) was exposed to atomic hydrogen of 300 L at RT, the voids became larger but the number of small clusters did not grow noticeably as one can see in Fig. 2(b). The rather large unformed clusters indicated by arrows in Ê in height and 30 A Ê in diameter) Fig. 2(b) (10 A are thought to be hydrogen-induced Si clusters [25]. Upon further hydrogen exposure (1000 L), the surface becomes rough with increasing the number of the unformed clusters similar to those shown in Fig. 2(b). However, neither ¯at 2D terrace nor 3D islands were observed. When Si(100)4 3-In surface is exposed to atomic hydrogen at RT, three-dimensional In island are formed on the surface and the Si substrate surface emerged between the In clusters is terminated by hydrogen atoms, resulting in new ¯at 2D structures appearing [8±14]. On the contrary, we could observe neither 3D islands nor new ¯at 2D structures in the present case. We suppose that Si substrate was etched by hydrogen so that no 2D structures could be observed, i.e. the stability of underlying Si layer in Si(100)2 1-Sb structure during atomic hydrogen exposure is less than that in Si(100)4 3-In structure.
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Ê ) of the Si(100)2 1-Sb surface as shown in Fig. 1(a) after atomic hydrogen exposure of (a) 50 and (b) Fig. 2. STM images (500 A 500 A Ê ) followed by annealing at 5508C of Si(100)2 1-Sb surface exposed to atomic hydrogen of 300 L at RT. (c) STM image (500 A 500 A Ê ) of a Si(100)2 1-Sb surface of Fig. 1(b) after 50 L hydrogen exposure at RT. Upper-right 1000 L at RT. (d) STM image (3000 A 3000 A insets in (a) (taken at 44 eV) and (c) (taken at 45 eV) show the LEED patterns from the corresponding surfaces, respectively. Lower-left insets Ê ) of parts of (a) and (d), respectively. These images were taken at a sample bias of in (a) and (d) show close-up STM images (100 A 100 A ÿ2.3 V (a), ÿ2.9 V (b), ÿ2.1 V (c), and ÿ1.5 V (d).
Although it is considered that the short diffusion length on the etched Si surface causes no 3D islands appearing, we speculate another story which will be discussed later.
Fig. 2(c) shows a ®lled state STM image of the Si(100)2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT and annealed then at 5508C. On this surface, some c(4 4) areas are observed in spite of
O. Kubo et al. / Applied Surface Science 169±170 (2001) 93±99
the low annealing temperature of 5508C, at which the Si(100)2 1-Sb surface of Fig. 1(a) was prepared (a weak c(4 4) LEED pattern coexisted with a 2 1 LEED pattern was also observed as shown in the inset in Fig. 2(c)). Taking into account that Sb coverage of the c(4 4)-Sb is only 0.25 ML [21,22], the observation of this reconstruction suggests that some Sb atoms were removed from the surface either during hydrogen exposure or at subsequent annealing. Ryu et al. [26,27] reported that Sb atoms desorb from the surface during hydrogen exposure at RT. This is consistent with the result that no 3D islands were observed on the surface exposed to 1000 L atomic hydrogen at RT. An interesting phenomenon has been found on this surface, namely, the formation of the long bright lines. These line structures are metastable and disappear after annealing at 6008C. It is remarkable that the similar line structures were also observed on the Si(100)2 1-Sb surface shown in Fig. 1(b) after atomic hydrogen exposure of 10±100 L at RT (some Sb atoms were incorporated into the top layer of Si on the original 2 1-Sb surface). The lines are running perpendicular to the surrounding dimer rows. As a result, the length of the lines on the A-terraces (on which the dimer rows are parallel to the step edge) is
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limited by the terrace width. In contrast, the length of the lines on the B-terraces (with the dimer rows perpendicular to the step edge) seems to have no external limitations and lines might be very long. Indeed, we observed the lines with a length as large Ê on the B-terrace. The line is clearly imaged as 3000 A as two bright lines separated by about 2a (a is the period of the unreconstructed Si(100)1 1 surface, Ê ). From high resolution STM images (see the 3.84 A inset in Fig. 2(d)), we infer that each bright line consists of a dimer chain, in which the direction of the dimerisation is parallel to the direction along the line. Miki et al. [23] and Naitoh et al. [24] observed similar lines for the Bi/Si(100) system but without hydrogen exposure. However, we think that the key processes to form the line structures for the Sb/Si(100) system, are not only Sb incorporation into the Si substrate but also hydrogen adsorption. However, a more detailed discussion on the lines is dif®cult at present. When the Si(100)2 1-Sb surface shown in Fig. 1(a), which had the ¯attest morphology, was exposed to atomic hydrogen of 75 L at 3008C, the ordering of the surface remained essentially intact as shown in Fig. 3(a). This surface revealed a clear 2 1
Ê ) of the Si(100)2 1-Sb surface as shown in Fig. 1(a) after atomic hydrogen exposure (a) 75 and (b) Fig. 3. STM images (500 A 500 A 2250 L at 3008C. The inset in (b) shows the LEED pattern from the corresponding surface. These images were taken at a sample bias of ÿ1.6 V and ÿ2.6 V.
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LEED pattern was still clearly observed and the surface morphology did not change, except for the presence of the slightly larger voids than those present before hydrogen exposure. This result indicates that the Sb dimers are more stable against atomic hydrogen interaction at 3008C than against that at RT. With increasing the hydrogen exposure, the voids became larger. Fig. 3(b) shows the 2 1-Sb surface exposed to atomic hydrogen of 2250 L at 3008C. This surface is characterised by a slightly weaker 2 1 LEED pattern than that before hydrogen exposure as shown in the inset in Fig. 3(b) and about 50% of the surface was covered with voids, indicating the displacement of Sb atoms. It is well known that 3D islands composed of the metal atoms are formed on the surface when an uniform metal/Si system is exposed to atomic hydrogen at 200±3008C [8±14]. Ryu et al. [13] reported that 3D In islands which formed on the Si(100)4 3-In surface exposed to atomic hydrogen became larger with increasing the temperature during hydrogen exposure since the diffusion length of expelled In atoms became longer with increasing the temperature. On the other hand, for the Si(100)2 1-Sb, no 3D islands were observed in spite of the increase of the size of the voids. In this case, small Sb clusters and hydrogen-induced Si clusters cannot be observed while they can be observed on the 2 1-Sb surface exposed to atomic hydrogen at RT as shown in Fig. 2(a) and (b). This result suggests that hydrogen adsorption at 3008C induces complete desorption of expelled Sb atoms from the surface and the appearing Si surface is not etched by atomic hydrogen (probably terminated by hydrogen as Si(100)2 1H). Upon annealing the surface shown in Fig. 3(b) at 5508C, we did not ®nd the line structures shown in Fig. 2(c). 4. Summary Using STM and LEED, the structural changes of the Si(100)2 1-Sb surface caused by H adsorption at both RT and 3008C have been studied. It has been found that at the initial stage of atomic hydrogen exposure at RT, the interaction of atomic hydrogen with the surface takes place randomly all over the surface and a 2 1 periodicity is already destroyed. However, upon further hydrogen exposure, we have
observed neither 3D islands nor the hydrogen terminated Si substrate which were reported for the other metal/Si systems with such metals as group-I (Ag), III (Al, In), and -IV (Pb) elements. On the Si(100)2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT followed by 5508C annealing, we have observed the c(4 4) structure with 0.25 ML Sb, indicating that some Sb atoms desorb from the surface plausibly already during hydrogen exposure. On this surface, we have found the line structures similar to those reported for the Bi/Si(100) system. We suppose that the key processes to form the line structures are Sb incorporation into the Si top layer and hydrogen adsorption. Sb dimer structure on a Si(100)2 1-Sb surface has been found to be more stable against hydrogen exposure at 3008C than against that at RT. We have con®rmed that some Sb atoms desorb during atomic hydrogen exposure at 3008C. Acknowledgements The authors would like to thank Prof. A.V. Zotov for a valuable discussion. This work was supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science and Culture, Japan.
References [1] M. Copel, R.M. Tromp, Appl. Phys. Lett. 58 (1991) 2648. [2] T. Fuse, K. Kawamoto, T. Shiizaki, E. Tazou, M. Katayama, K. Oura, Jpn. J. Appl. Phys. 37 (1998) 2625. [3] K. Sumitomo, T. Kobayashi, F. Shoji, K. Oura, Phys. Rev. Lett. 66 (1991) 1193. [4] M. Naitoh, F. Shoji, K. Oura, Surf. Sci. 242 (1991) 152. [5] Y. Tanaka, T. Kinoshita, K. Sumitomo, F. Shoji, I. Katayama, K. Oura, Appl. Surf. Sci. 60/61 (1992) 195. [6] A. Hirata, K. Saiki, A. Koma, Appl. Phys. Lett. 65 (1994) 3182. [7] Y. Okada, J.S. Harris Jr., J. Vac. Sci. Technol. B 14 (1996) 1725. [8] K. Oura, H. Ohnishi, Y. Yamamoto, I. Katayama, Y. Ohba, J. Vac. Sci. Technol. B 14 (1996) 988. [9] H. Ohnishi, Y. Yamamoto, K. Oura, I. Katayama, Y. Ohba, J. Vac. Sci. Technol. A 13 (1995) 1438. [10] M. Naitoh, H. Ohnishi, Y. Ozaki, F. Shoji, K. Oura, Appl. Surf. Sci. 60/61 (1992) 190. [11] A.A. Saranin, E.A. Khramtsova, K.V. Ignatovich, V.G. Lifshits, T. Numata, O. Kubo, M. Katayama, K. Oura, Phys. Rev. B 55 (1997) 5353.
O. Kubo et al. / Applied Surface Science 169±170 (2001) 93±99 [12] M. Katayama, T. Numata, O. Kubo, H. Tani, A.A. Saranin, K. Oura, Appl. Phys. A 66 (1998) S 985. [13] J.-T. Ryu, T. Fuse, O. Kubo, H. Tani, T. Fujino, T. Harada, A.A. Saranin, A.V. Zotov, M. Katayama, I. Katayama, K. Oura, J. Vac. Sci. Technol. B 17 (1999) 983. [14] K. Oura, Y. Tanaka, H. Morishita, F. Shoji, I. Katayama, Nucl. Instrum. Methods Phys. Res. B 85 (1994) 439. [15] M. Copel, M.C. Reuter, M. Horn von Hoegen, R.M. Tromp, Phys. Rev. B 42 (1990) 11682. [16] A.V. Zotov, A.A. Saranin, V.G. Lifshits, E.A. Khramtsova, Surf. Sci. Lett. 230 (1990) L 147. [17] R. Hunger, N. Blick, N. Esser, M. Arens, W. Richter, V. Wagner, J. Geurts, Surf. Sci. 307±309 (1994) 1061. [18] J. Nogami, A.A. Baski, C.F. Quate, Appl. Phys. Lett. 58 (1991) 475.
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[19] W.F.J. Slijkerman, P.M. Zagwijn, J.F. van der Veen, D.J. Gravesteijn, G.F.A. van de Walle, Surf. Sci. 262 (1992) 25. [20] B. Garni, I.I. Kravchenko, C.T. Salling, Surf. Sci. 423 (1999) 43. [21] R.J. Dixon, C.F. McConville, S.J. Jenkins, G.P. Srivastava, Phys. Rev. B 57 (1998) R12701. [22] J. Wasserfall, W. Ranke, Surf. Sci. 331±333 (1995) 1099. [23] K. Miki, J.H.G. Owen, D.R. Bowler, G.A.D. Briggs, K. Sakamoto, Surf. Sci. 421 (1999) 397. [24] M. Naitoh, H. Shimaya, S. Nishigaki, N. Oishi, F. Shoji, Surf. Sci. 377±379 (1997) 899. [25] J.J. Boland, Phys. Rev. Lett. 65 (1990) 3325. [26] J.-T. Ryu, K. Kui, M. Katayama, K. Oura, Appl. Surf. Sci. 121/122 (1997) 223. [27] J.-T. Ryu, K. Kui, K. Noda, M. Katayama, K. Oura, Jpn. J. Appl. Phys. 36 (1997) 4435.
STM study of structural changes on Si(100)2 1-Sb surface induced by atomic hydrogen O. Kubo*, J.-T. Ryu, H. Tani, T. Harada, T. Kobayashi, M. Katayama, K. Oura Department of Electronic Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Received 2 August 1999; accepted 12 October 1999
Abstract Using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we have studied the structural changes of the Si(100)2 1-Sb surface caused by hydrogen adsorption at both room temperature (RT) and 3008C. We have found that the ordering of a 2 1-Sb surface is more stable against atomic hydrogen exposure at 3008C than at RT, and that some Sb atoms desorb during atomic hydrogen exposure at 3008C. However, upon hydrogen exposure at both temperatures, we have observed neither three-dimensional islands nor the hydrogen terminated Si substrate which were reported for hydrogen interaction with the other metal/Si systems. On the 2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT followed by 5508C annealing, long bright lines similar to those reported for the Bi/Si(100) system have also been found. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Antimony; Hydrogen; Scanning tunneling microscopy (STM); Incorporation; Desorption; Line structure
1. Introduction Atomic hydrogen interaction with adsorbate/Si systems has recently received considerable attention in connection with deliberate use of the various effects of hydrogen on the growth of the thin ®lms on the Si substrate [1±7]. In particular, several works [8±14] have been published concerning the adsorption of atomic hydrogen on the metal/Si systems, in which metal atoms form a two-dimensional (2D) layer. In the most cases, the interaction of those systems with the
*
Corresponding author. Tel.: 81-6-6879-7779; fax: 81-6-6879-7780. E-mail address: [email protected] (O. Kubo).
atomic hydrogen results in the transformation of the uniform metal layer into separate 3D islands. Consequently, hydrogen atoms replace metal atoms, forming a hydrogenated Si surface. Such atomic hydrogeninduced structural changes of the metal/Si systems have been reported for group-I (Ag) [8,9], -III (Al, In) [10±13], and -IV (Pb) [14] metals. In contrast, the interaction of atomic hydrogen with Si surfaces terminated by group-Velements has scarcely been examined. Among the group-V adsorbates, adsorption of Sb on to the Si surfaces have recently received especial attention in connection with three technologically important phenomena: surfactant-mediated epitaxy [15], construction of delta-doping layers [16], and improvement of the quality of III±V compound epitaxial growth on Si substrates [17]. Thus, it is
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important to study the interaction of hydrogen with Sb terminated Si surfaces from both the technological and fundamental points of view. On the other hand, when Sb atoms are adsorbed on a Si(100) surface, two kinds of reconstructed surface phases, 2 1-Sb [18±20] and c(4 4)-Sb [21,22], are formed depending on both Sb coverage and substrate temperature. The 2 1-Sb phase has been reported to be formed by deposition of 0.6±2.0 ML Sb on to the clean Si(100) surface held at 3758C followed by 5408C annealing [18], while the c(4 4)-Sb phase has been reported to be formed by Sb deposition of 1± 2 ML on to the Si(100) surface at RT followed by annealing at the temperature of 570±6208C [21,22]. The absolute Sb coverage in the 2 1-Sb and c(4 4)-Sb surface phases has been reported to be 0.7±0.9 monolayer (ML: 1 ML 6:8 1014 atoms/ cm2, the ideal Si surface atomic density in the (100) plane) [19] and 0.25 ML, respectively [21,22]. In this study, using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we have investigated the interaction of atomic hydrogen with the Si(100)2 1-Sb surface phase at both RT and 3008C. We have found that the ordered structure of the 2 1-Sb surface is more stable against atomic hydrogen exposure at 3008C than at RT. However, upon hydrogen exposure at both temperatures, we have observed neither 3D islands nor the hydrogen terminated Si substrate which were reported for the other metal/Si systems [8±14]. After 5508C annealing of the 2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT, we have found the long bright lines similar to those reported for the Bi/Si(100) system [23,24]. 2. Experimental All experiments were carried out in an ultrahighvacuum (UHV) chamber with STM and LEED facilities. The base pressure in the UHV chamber was typically 8 10ÿ11 Torr. The STM tips were fabricated by electrochemical etching from a W wire (f 0:25 mm) and cleaned by in situ annealing. The n-type Si(100) wafers with 0.05 O cm resistivity were used. An atomically ¯at Si(100)2 1 surface was obtained after outgassing at 6008C for over 12 h, by several times ¯ashing to 12008C for 10 s each (by
direct current ¯owing through the sample), followed by slow cooling. Sb was deposited from a tantalum foil tube by direct current heating. For atomic hydrogen exposure, H2 gas was admitted into the chamber and dissociated into atomic hydrogen with an 18008C tungsten ®lament, 7 cm from the Si surfaces. Since the arrival rate of atomic hydrogen is unknown, the dose of molecular hydrogen is speci®ed, and expressed in units of Langmuir (1 L 1 10ÿ6 Torr s). To form a completely hydrogen terminated Si(100)2 1-H surface, a clean Si(100)2 1 surface was exposed to 500 L hydrogen at 3008C. All STM observations were made at RT. 3. Results and discussions Fig. 1(a) shows the Si(100)2 1-Sb surface prepared by evaporating more than 1 ML of Sb at RT followed by 5508C annealing. From this surface, a 2 1 LEED pattern was observed as shown in the upper-right inset in Fig. 1(a), in which half-order spots demonstrate a streak-like character. Such a 2 1-Sb surface contains a large number of antiphase boundaries (indicated by arrows in the lower-right inset in Fig. 1(a)) [18]. The clean Si(100) surface is known to be free of such an antiphase boundaries. This result indicates that most of the surface area is covered by Sb dimers. On the other hand, when the 2 1-Sb surface was prepared by evaporating Sb atoms at 5508C followed by annealing at 6208C, a more rough surface than that of Fig. 1(a) was formed as shown in Fig. 1(b). From this surface, a conventional 2 1 LEED pattern was observed as shown in the upper-right inset in Fig. 1(b). In this image, one can see many defects and a few 2D islands. According to the report by Garni et al. [20], when submonolayer amounts of Sb are evaporated on to the Si(100) surface at the temperature of 250± 5008C, Sb atoms are incorporated into the top layer of Si atoms, forming defects and 2D islands. Moreover, we observed some rows of buckled dimer and quite small areas of the c(4 4) surface phase with 0.25 ML Sb (see the lower-right inset in Fig. 1(b)) [21,22]. We speculate that some Sb atoms are incorporated into the top layer of Si atoms and the amount of Sb is less on the surface shown in Fig. 1(b) than that shown in Fig. 1(a).
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Ê ) of Si(100)2 1 surfaces, (a) prepared by evaporating more than 1 ML Sb on to a clean Si(100) surface Fig. 1. STM images (500 A 500 A at RT followed by 5508C annealing, (b) prepared by evaporating more than 1 ML Sb at 5508C followed by 6208C annealing. Upper-right insets in (a) (taken at 44 eV) and (b) (taken at 54 eV) show the LEED patterns from the corresponding surfaces, respectively. Lower-right Ê ) and (b) (125 A Ê ) show close-up STM images of parts of (a) and (b), respectively. These images were insets in (a) (100 A 100 A 125 A taken at a sample bias of ÿ2.6 V (a), ÿ1.9 V (b).
First, we exposed the 2 1-Sb surface shown in Fig. 1(a), which has the ¯atter morphology than that shown in Fig. 1(b), to atomic hydrogen at RT. In this case, a 2 1 LEED pattern already changes to a 1 1 pattern after hydrogen exposure of 15 L as shown in the upper-right inset in Fig. 2(a). A ®lled state STM image of the 2 1-Sb surface exposed to atomic hydrogen of 50 L at RT is shown in Fig. 2(a). As one can see, the surface becomes more disordered due to atomic hydrogen adsorption, with the increased number of voids and small clusters, which have a Ê . We speculate that some Sb diameter below 10 A atoms were expelled from the original position by atomic hydrogen interaction, consequently the number of voids and small clusters of Sb which were expelled increases. However, the surface is still ¯at and the dimer structure remains in spite of the fact that the surface displays a 1 1 LEED pattern. In the detailed observation, it was revealed that the domains of ordered dimer row structure have a width of less Ê (see the lower-left inset than 5 or 6 dimer rows, 40 A in Fig. 2(a)). This is probably due to the displacement of some Sb atoms or the breaking of Sb dimer bonds.
When the 2 1-Sb surface shown in Fig. 1(a) was exposed to atomic hydrogen of 300 L at RT, the voids became larger but the number of small clusters did not grow noticeably as one can see in Fig. 2(b). The rather large unformed clusters indicated by arrows in Ê in height and 30 A Ê in diameter) Fig. 2(b) (10 A are thought to be hydrogen-induced Si clusters [25]. Upon further hydrogen exposure (1000 L), the surface becomes rough with increasing the number of the unformed clusters similar to those shown in Fig. 2(b). However, neither ¯at 2D terrace nor 3D islands were observed. When Si(100)4 3-In surface is exposed to atomic hydrogen at RT, three-dimensional In island are formed on the surface and the Si substrate surface emerged between the In clusters is terminated by hydrogen atoms, resulting in new ¯at 2D structures appearing [8±14]. On the contrary, we could observe neither 3D islands nor new ¯at 2D structures in the present case. We suppose that Si substrate was etched by hydrogen so that no 2D structures could be observed, i.e. the stability of underlying Si layer in Si(100)2 1-Sb structure during atomic hydrogen exposure is less than that in Si(100)4 3-In structure.
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Ê ) of the Si(100)2 1-Sb surface as shown in Fig. 1(a) after atomic hydrogen exposure of (a) 50 and (b) Fig. 2. STM images (500 A 500 A Ê ) followed by annealing at 5508C of Si(100)2 1-Sb surface exposed to atomic hydrogen of 300 L at RT. (c) STM image (500 A 500 A Ê ) of a Si(100)2 1-Sb surface of Fig. 1(b) after 50 L hydrogen exposure at RT. Upper-right 1000 L at RT. (d) STM image (3000 A 3000 A insets in (a) (taken at 44 eV) and (c) (taken at 45 eV) show the LEED patterns from the corresponding surfaces, respectively. Lower-left insets Ê ) of parts of (a) and (d), respectively. These images were taken at a sample bias of in (a) and (d) show close-up STM images (100 A 100 A ÿ2.3 V (a), ÿ2.9 V (b), ÿ2.1 V (c), and ÿ1.5 V (d).
Although it is considered that the short diffusion length on the etched Si surface causes no 3D islands appearing, we speculate another story which will be discussed later.
Fig. 2(c) shows a ®lled state STM image of the Si(100)2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT and annealed then at 5508C. On this surface, some c(4 4) areas are observed in spite of
O. Kubo et al. / Applied Surface Science 169±170 (2001) 93±99
the low annealing temperature of 5508C, at which the Si(100)2 1-Sb surface of Fig. 1(a) was prepared (a weak c(4 4) LEED pattern coexisted with a 2 1 LEED pattern was also observed as shown in the inset in Fig. 2(c)). Taking into account that Sb coverage of the c(4 4)-Sb is only 0.25 ML [21,22], the observation of this reconstruction suggests that some Sb atoms were removed from the surface either during hydrogen exposure or at subsequent annealing. Ryu et al. [26,27] reported that Sb atoms desorb from the surface during hydrogen exposure at RT. This is consistent with the result that no 3D islands were observed on the surface exposed to 1000 L atomic hydrogen at RT. An interesting phenomenon has been found on this surface, namely, the formation of the long bright lines. These line structures are metastable and disappear after annealing at 6008C. It is remarkable that the similar line structures were also observed on the Si(100)2 1-Sb surface shown in Fig. 1(b) after atomic hydrogen exposure of 10±100 L at RT (some Sb atoms were incorporated into the top layer of Si on the original 2 1-Sb surface). The lines are running perpendicular to the surrounding dimer rows. As a result, the length of the lines on the A-terraces (on which the dimer rows are parallel to the step edge) is
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limited by the terrace width. In contrast, the length of the lines on the B-terraces (with the dimer rows perpendicular to the step edge) seems to have no external limitations and lines might be very long. Indeed, we observed the lines with a length as large Ê on the B-terrace. The line is clearly imaged as 3000 A as two bright lines separated by about 2a (a is the period of the unreconstructed Si(100)1 1 surface, Ê ). From high resolution STM images (see the 3.84 A inset in Fig. 2(d)), we infer that each bright line consists of a dimer chain, in which the direction of the dimerisation is parallel to the direction along the line. Miki et al. [23] and Naitoh et al. [24] observed similar lines for the Bi/Si(100) system but without hydrogen exposure. However, we think that the key processes to form the line structures for the Sb/Si(100) system, are not only Sb incorporation into the Si substrate but also hydrogen adsorption. However, a more detailed discussion on the lines is dif®cult at present. When the Si(100)2 1-Sb surface shown in Fig. 1(a), which had the ¯attest morphology, was exposed to atomic hydrogen of 75 L at 3008C, the ordering of the surface remained essentially intact as shown in Fig. 3(a). This surface revealed a clear 2 1
Ê ) of the Si(100)2 1-Sb surface as shown in Fig. 1(a) after atomic hydrogen exposure (a) 75 and (b) Fig. 3. STM images (500 A 500 A 2250 L at 3008C. The inset in (b) shows the LEED pattern from the corresponding surface. These images were taken at a sample bias of ÿ1.6 V and ÿ2.6 V.
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LEED pattern was still clearly observed and the surface morphology did not change, except for the presence of the slightly larger voids than those present before hydrogen exposure. This result indicates that the Sb dimers are more stable against atomic hydrogen interaction at 3008C than against that at RT. With increasing the hydrogen exposure, the voids became larger. Fig. 3(b) shows the 2 1-Sb surface exposed to atomic hydrogen of 2250 L at 3008C. This surface is characterised by a slightly weaker 2 1 LEED pattern than that before hydrogen exposure as shown in the inset in Fig. 3(b) and about 50% of the surface was covered with voids, indicating the displacement of Sb atoms. It is well known that 3D islands composed of the metal atoms are formed on the surface when an uniform metal/Si system is exposed to atomic hydrogen at 200±3008C [8±14]. Ryu et al. [13] reported that 3D In islands which formed on the Si(100)4 3-In surface exposed to atomic hydrogen became larger with increasing the temperature during hydrogen exposure since the diffusion length of expelled In atoms became longer with increasing the temperature. On the other hand, for the Si(100)2 1-Sb, no 3D islands were observed in spite of the increase of the size of the voids. In this case, small Sb clusters and hydrogen-induced Si clusters cannot be observed while they can be observed on the 2 1-Sb surface exposed to atomic hydrogen at RT as shown in Fig. 2(a) and (b). This result suggests that hydrogen adsorption at 3008C induces complete desorption of expelled Sb atoms from the surface and the appearing Si surface is not etched by atomic hydrogen (probably terminated by hydrogen as Si(100)2 1H). Upon annealing the surface shown in Fig. 3(b) at 5508C, we did not ®nd the line structures shown in Fig. 2(c). 4. Summary Using STM and LEED, the structural changes of the Si(100)2 1-Sb surface caused by H adsorption at both RT and 3008C have been studied. It has been found that at the initial stage of atomic hydrogen exposure at RT, the interaction of atomic hydrogen with the surface takes place randomly all over the surface and a 2 1 periodicity is already destroyed. However, upon further hydrogen exposure, we have
observed neither 3D islands nor the hydrogen terminated Si substrate which were reported for the other metal/Si systems with such metals as group-I (Ag), III (Al, In), and -IV (Pb) elements. On the Si(100)2 1-Sb surface exposed to atomic hydrogen of 1000 L at RT followed by 5508C annealing, we have observed the c(4 4) structure with 0.25 ML Sb, indicating that some Sb atoms desorb from the surface plausibly already during hydrogen exposure. On this surface, we have found the line structures similar to those reported for the Bi/Si(100) system. We suppose that the key processes to form the line structures are Sb incorporation into the Si top layer and hydrogen adsorption. Sb dimer structure on a Si(100)2 1-Sb surface has been found to be more stable against hydrogen exposure at 3008C than against that at RT. We have con®rmed that some Sb atoms desorb during atomic hydrogen exposure at 3008C. Acknowledgements The authors would like to thank Prof. A.V. Zotov for a valuable discussion. This work was supported by a Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science and Culture, Japan.
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