- Email: [email protected]
TED observation of Ag islands grown on Si(111 )3×3 -Ag surface
Surface Science 493 (2001) 366±372
www.elsevier.com/locate/susc
UHV-TEM/TED observation p p of Ag islands grown on Si(1 1 1) 3 3-Ag surface Yoshifumi Oshima *, Hiroyuki Nakade, Sinya Shigeki, Hiroyuki Hirayama, Kunio Takayanagi Graduate school at Nagatsuta, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8502, Japan Received 18 October 2000; accepted for publication 26 January 2001
Abstract
p p Growths of Ag islands on Si(1 1 1) 3 3-Ag surface at room temperature were observed by UHV transmission electron microscopy and diraction. The Ag islands grown after six monolayer deposition had neither (1 0 0) nor (1 1 0) orientation, but had two complex epitaxial dominantly. One was striped islands which gave rise to a porientations p diraction pattern commensurate with the 3 3 lattice of the Si(1 1 1) surface. The other was the coagulated islands whose diraction pattern indicated the Ag(1 3 4) sheet grown parallel to the Si(1 1 1) surface. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Electron±solid diraction; Silicon; Silver
1. Introduction Surface and interface structures have attracted much interest in fundamental and applied sciences. Metal±silicon interfaces are particularly of importance in device technologies, and a number of researches have been done. From these works well known is the fact that metals deposited on silicon surfaces generate a speci®c surface layer for each metal of one or sub-monolayer thickness. Monolayer thick metal deposited on the Si(1 1 1) surfaces reconstructed to the 7 7 structure has often
* Corresponding author. Tel.: +81-45-924-5637; fax: +81-45924-5685. E-mail address: [email protected] (Y. Oshima).
commensurate lattices with the silicon surface, with some exceptions of incommensurate lattices for Cup[1,2]. Among those commensurate lattices, p 3 3 structures appear p forpAu [3], Ag [4], Pd [5], Sn [6], Bi [7,8]. The 3 3 structure of Ag has the honeycomb-chained structure [9,10], saturated at the coverage of 1 ML (monolayer 7:8 1014 atoms=cm2 ), changes the metallic surface state of the Si(1 1 1)7 7 into a semi-conductive state [11]. The semiconductive surface results from termination of the Si dangling bonds by 5s electrons of Ag [12]. Such termination of the dangling bonds aects electronic properties originated from interface structures between metals and silicon. Apparent change of the Schottky barrier height was found for the Pb±Si(1 1 1) interface, depending on whether Pb ®lm is deposited on pthepclean Si(1 1 1)7 7 surface or on a Si(1 1 1) 3 3-Pb
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 2 4 1 - 9
Y. Oshima et al. / Surface Science 493 (2001) 366±372
surface [13]. Similar change was also found for Ag±Si(1 1 1) interfaces [14] and others [15]. However, any relationship between dangling bond density and Schottky barrier height has not been clari®ed yet because of the lack of knowledge of their interfacial structures. Because the surface structures formed at the beginning by metal deposition may change after the subsequent ®lm deposition on it. Interface structures are often dicult to be analyzed even by modern techniques in surface science, which are sensitive only for a few atomic layers from the top surface layer. Recently, some groups investigated the interface using X-ray pdiraction p after growing thin Ag ®lm on Si(1 1 1) 3 3-Ag surface at room ptemperature. Aburano et al. re p ported that the 3 3 periodicity disappeared at the interface and Ag ®lms were oriented with Ag{1 1 1}, Ag{2 0 0} and Ag{3 1 1} parallel to the Si(1 1 1) surface p p[16], while Akimoto et al. reported that the 3 3 periodicity remained at the interface and Ag ®lms mainly had Ag{1 1 0} plane parallel to the Si(1 1 1) surface [17]. We applied UHV transmission electron microscope and diffraction (TEM/TED) technique for interface structure analyses in order to investigate structure of each Ag island and its buried interface. We reported brie¯y that two p kinds p of Ag islands were grown on the Si(1 1 1) 3 3-Ag. One is a striped Ag ®lm which gives rise to thepdiraction spots p commensurate with the Si(1 1 1) 3 3 lattice in addition to incommensurate spots. The other is Ag islands of which (1 3 4) plane is parallel to the Si(1 1 1) surface.
2. Experimental We used an ultrahigh-vacuum transmission electron microscope (JEM-2000V). A Si(1 1 1) wafer (Sb-doped 0.02 X cm) was cut to a size of 1 7 mm and thinned mechanically to a 0.2 mm. After rinsing, it was cleaned in the UHV microscope by ¯ashing at 1200°C, and subsequently thinned by heating at 1000°C to have local areas with 10±20 nm thickness. Ag was deposited in situ on the clean Si(1 1 1)7 7 surface at 300°C. By
367
the deposition at a rate of 0.1 ML/s, diraction intensities of the 7 7 surface to reach p decreased p constant values, while the 3 3 intensity increased for the saturation value. p pWe thus con®rmed the saturation of the 3 3 structure at one monolayer (ML), which covers only the top Si(1 1 1) surface. We subsequently deposited Ag of six MLs at room temperature with the deposition rate of 0.1 ML/sec. After this deposition, the 7 7 reconstruction at the bottom surface had disappeared. We suppose that a part of Ag atoms migrate to the bottom surface from the top surface to convert the 7 7 structure into 1 1 structure. During the deposition of the Ag islands, the specimen was not irradiated by the imaging electron beam in order to avoid any pelectron p beam eect. Ag islands grown on the 3 3 surface were observed by bright±dark ®eld electron microscopy images and selected area diraction patterns to determine structures of Ag islands and their interface structures. 3. Results and discussion pThe pAg islands were grown on the Si(1 1 1) 3 3-Ag surface with two structures. One is a ¯at island extended laterally, and the other is a three-dimensional and thick. We call the former island striped island, and the latter (1 3 4) island, hereafter, and describe their features observed by TEM and TED. 3.1. Striped Ag island Fig. 1a and b show typical bright-®eld TEM images of striped Ag islands. The striped Ag island in Fig. 1a is formed at the center of terrace. These islands were not formed from the step indicated by dashed lines. They are elongated largely along the terrace in the [1 1 2] direction with alternating bright and dark stripes. The stripes are parallel to the [1 1 0] direction. The average width of a couple of the bright and dark stripes was 7.7 nm, although the widths of the bright and dark stripes are not regular. Striped Ag island in Fig. 1b has a dierent morphology from Fig. 1a. It is not elongated along
368
Y. Oshima et al. / Surface Science 493 (2001) 366±372
Fig. 1. (a) A typical bright-®eld TEM image of a stripe Ag island. This island has alternating bright and dark stripes pattern parallel to the [1 1 0] direction. The average width of a pair of the bright and dark stripes was 7.7 nm. The dashed lines indicate the steps of the Si(1 1 1) surface. bright-®eld TEM image of another stripe Ag island. This island was grown on a wide terrace with p(b) A ptypical some holes of the 3 3 structure [18]. The dot-dashed lines indicate the holes, whose diameter is about 70±100 nm.
the [1 1 2] direction, but it has straight edge parallel to the [1 1 2]pdirection p and so on. This island is grown on the 3 3 surface which had some hole regions on a wide ¯at terrace. These holes were formed by Ag deposition on the wide terrace of the Si(1 p1 1)7 p 7 surface [18], which are lower than the 3 3 domains on the Si(1 1 1) surface by monoatomic layer of the Si(1 1 1) atomic sheet. The holes were found by contrast as shown in the inserted image which corresponds to the region indicated by white line in the TEM image. The edges of these holes are indicated by dotdashed lines in Fig. 1b. The stripes of the Ag island are seen to be discontinuous at the edges of the holes. Therefore, the striped Ag islands had nucleated p pin each hole and on the terrace of the 3 3 domains, and grew to the edge of the hole. The growth of this island is less in¯uenced by the edge of the holes, since the edge of the island is pstraight p regardless of the hole region of the 3 3 structure. In this island, the average width of a pair of the bright and dark stripes was
5.0 nm. From several TEM images of the striped islands, the average width was found to be 5±7 nm. Fig. 2a and b show TED patterns of the striped Ag island in Fig. 1a. The former was taken with incident electron beam normal to the Si(1 1 1) surface, the latter, with incident electron beam tilted about 4° along the [1 1 0] direction from the surface normal. Fig. 2a shows six {2 2 0} re¯ection spots of Si substrate indicated by the dashedline and the spots corresponding to the p exagon p 3 3 structure. Some of the latter spots are split along the [1 1 2] direction as shown by the spots a and b. Also, the spots c andpc0 are pintense, which cannot be explained by the 3 3 structure at the interface. Therefore, we notice two dierent reciprocal unit A and B indicated by solid and dashed lines, respectively, since the distance between the spot b02 and a02 is twice larger than between the spots b01 and a01 as shown in the inserted intensity pro®le of Fig.p2a. Some of the p fractional-order spots of the 3 3 structure are split due to these two structures. The reciprocal
Y. Oshima et al. / Surface Science 493 (2001) 366±372
369
Fig. 2. (a) A TED pattern obtained from the striped Ag island of Fig. 1a. This pattern was taken with incident beam normal to the Si(1 1 1) surface. Six Si{2 2 0} spots are indicated by hexagon with p p dashed lines. The (hk) spots are indicated by unit cell of the Si(1 1 1)(1 1). Except the spots of bulk Si crystal and the 3 3 structure, two reciprocal units, A and B are observed. The spots ai (i 1, 2) appear on the solid lines, the spots bi , on the dashed lines. The inserted intensity pro®le shows the split spacing between a02 and b02 is twice larger than between a01 and b01 . (b) This pattern was taken with incident beam tilted about 4° from the Si(1 1 1) surface.
unit A and B have the common spots along the [1 1 0] direction as indicated by the spots c and c0 . The spot c (or c0 ) corresponds to the ( 4/3, 8/3) fractional order spot. Since the ratio of the lattice constants between bulk Ag and Si crystal is 3:4, the {2 2 0} spot of Ag crystal coincides with the ( 4/3, 8/3) spot. Therefore, the spot c (or c0 ) is the {2 2 0} spot of Ag crystal. On the other hand, the reciprocal unit A and B have dierent spots along the [1 1 2] direction. In Fig. 2a, the spot a coincides with the (2 0) spot and the spot b appear shorter
than the spot a. The unit cell of structure B elongates against structure A. We do not ®nd any indices of the spots a and b which are perpendicular to the (2 2 0) re¯ection spot indicated by c (c0 ). We do not decide on the crystallographic orientation of Ag islands. The previousp results that the pshowed Ag islands grown on the 3 3 structure has the (1 1 1) or (1 1 0) plane parallel to the surface [16,17]. But, the striped islands do not agree with these results, because the {2 2 4} spot of Ag crystal has to appear further outside the (2 0) spot and the
370
Y. Oshima et al. / Surface Science 493 (2001) 366±372
{0 0 2} spot, inside. In Fig. 2b, the reciprocal unit A appears, but the reciprocal unit B disappears. But, the reciprocal unit B was sometimes observed in the diraction pattern with the same condition as this ®gure. In Fig. 2b, it is noteworthy that pthe fractional-order spots are produced by the 3 p 3 structure. Because the dynamical eect on the intensity of the re¯ection spots are almost ignored [19], when the incident beam is tilted about 4° from the Si(1 1 1) surface normal. The additional spots produced by double diraction do not appear in this incident beam condition. Fig. 3a shows a bright-®eld TEM image obtained from a striped Ag island. In addition to the stripes, fringes perpendicular to the stripes and those inclined to them appeared. Fourier transformed pattern obtained from this TEM image shows the spots corresponding to the vertical and
oblique fringes observed in Fig. 3a. It is noticed that the oblique fringes give split spots indicated by the spots A A0 and B B0 . The interference fringes between the center spot and A A0 spot correspond to that appeared in a region of Fig. 3a. The interference fringes between the center spot and B B0 spot is from the rest of the regions in Fig. 3a. The interference fringes in a and b regions are not parallel but tilted 1±2° which corresponds to the splitting of the spots A and B in Fig. 3b. Fig. 3c schematically illustrates the TED pattern from the striped islands. The Si{2 2 0} spots are shown by gray circles forming a hexagonal arrangement. Solid circles are the spots due to the Ag island. Open circles are the double diraction spots of the Ag islands and Si crystal. Thus, moire fringe in Fig. 3a corresponds to interference fringes between the center spot and the spots indicated
Fig. 3. (a) A typical bright-®eld TEM image of a striped Ag island. Since the interference fringes in regions A and B are not parallel but tilted 1±2°, two regions a and b are observed as shown. Moire fringes in the region b # is slightly tilted from the region b. (b) A Fourier transformed pattern obtained from the TEM image of (a). The oblique fringes of (a) give split spots A A0 and B B0 . (c) Illustration of TED pattern from striped Ag island and Si. The Double diraction between the Si{2 2 0} and Ag spots produce spots A, B in (c), which correspond to the spots A, B in (b).
Y. Oshima et al. / Surface Science 493 (2001) 366±372
371
by open circles. Being compared with Fig. 3c and b, we verify that the striped Ag island has two domains a and b which give two reciprocal unit A and B, respectively. The p domain p a is commensurate with the Si(1 1 1) 3 3 structure. On the other hand, domain having about 5% pb ispnot, mismatch with the 3 3 structure in the island of Fig. 2a. We do not understand why both commensurate and incommensurate lattices are formed together in one striped island. The boundary between the a and b is not clear. p domains p The 3 3 structure remain at the interface, because p p the Ag island grown on the Si(1 1 1) 3 3-Ag surface is commensurate at some regions. This result is agreement withpAkimoto p et al. But, we do not know whether the 3 3 structure is honeycomb-chained trimer structure or not. In this study, Ag island is so thin (several monolayer) that it modi®es the structure from bulk Ag crystal. In previous results, Ag ®lms is about a in thick. These Ag ®lms must few hundreds A become the same structure as the bulk crystal. 3.2. Ag(1 3 4) island Fig. 4a shows a typical bright-®eld TEM image of another type of Ag island and Fig. 4b, a TED pattern taken from the Ag island at the center. This island grew three-dimensionally. We call these islands (1 3 4) island, since they have Ag crystalline structure and their (1 3 4) lattice plane is parallel to the Si(1 1 1) surface. The large island in Fig. 4a shows complex contrast. In the diraction pattern of Fig. 4b, we found the systematic re¯ection spots from Ag(1 1 1) and ( 1 1 1). We also recognize the six Si{2 2 0} spots from the Si(1 1 1) surface. Spots such as indicated by arrows are explained by double diraction between the Ag{1 1 1} and Si spots as indicated by arrows P and Q. The spots which do not appear by double diraction are Ag(3 5 3), Ag(4 4 2) and Ag(5 3 1) spots which are seen at high diraction angles. Provided that the two reciprocal vectors of Ag( 1 1 1) and Ag(4 4 2) are in the plane parallel to the Si(1 1 1) surface, the orientation of the Ag island concerned [uvw] is determined to be [1 3 4] direction. Therefore, this Ag island is oriented with Ag(1 3 4) plane parallel to the Si(1 1 1) surface.
Fig. 4. (a) A typical bright-®eld TEM image of Ag(1 3 4) island. This island is grown three dimensionally, and its shape is not regular. (b) A TED pattern of Ag(1 3 4) island. Two reciprocal vectors of Ag(1 1 1) and Ag(4 4 2) spots appear except the spots of Si crystal and double diraction spots between Si{2 2 0} and Ag spots. Arrows indicate double diraction spots produced by the Ag{1 1 1} and Si spots, for example, P and Q.
On the other hand, the p p re¯ection spots corresponding to the 3 3 structure p are in not pseen the TED pattern. Therefore, any 3 3 layer does not remain at this interface. As shown in Fig. 5, atomic positions within the p unit pcell of Ag(1 3 4) plane resemble that of 3 3 structure. p Fig. p 5b shows the relationship between the 3 3 structure and the unit cell of the Ag island indicated by bold lines. It is noteworthy that slight rotation and compression of the Ag(1 3 4) lattice p brings pto commensurate Ag(1 3 4) lattice to the 3 3 unit cell.
372
Y. Oshima et al. / Surface Science 493 (2001) 366±372
two regions. On the other hand, the Ag(1 3 4) islands are grown three dimensionally. Except the spots of Si crystal and double diraction spots between the Si{2 2 0} and Ag spots, the systematic re¯ection spots from Ag(1 1 1) and Ag(4 4 2) appear in the TED pattern. These two reciprocal vectors indicate that the Ag island is oriented with Ag(1 3 4) plane parallel to the Si(1 1 1) surface. Acknowledgements This work was supported partially by Grant-in Aid from Ministry of Education of Japan (no. 09NP1201). References
Fig. 5. (a) Atomic structure of the (1 3 4) plane. Circles indicate the atoms. Atomic coordination is indicated. The angle between the vector [1 1 1] and [1 128°. (b) The p3 2] is pabout relative relationship between the 3 3 structure and the (1 3 4) plane is shownpschematically. The shaded lines indi p cate the unit cell of the 3 3 structure, while the bold lines indicate the (1 3 4) lattice plane.
4. Conclusion We observed the structure of Ag island and the interface between the island and the Si(1 1 1) substrate using UHV-TEM/TED. Ag islands are p p grown on the Si(1 1 1) 3 3-Ag surface at room temperature by 6 ML Ag deposition. We found two types of Ag islands, striped Ag island and Ag(1 3 4) island. The striped Ag islands have alternating bright and dark stripes parallel to the [1 1 0] direction. The average width of a pair of the bright and dark stripes is 5±7 nm, although each width of the bright and dark stripe is not regular. The striped Ag island has two structure p units, p one of which is commensurate with the 3 3 structure, while the other has a unit cell elongated along the [1 1 2] direction about ®ve percent relative to the commensurate lattice. Moire pattern in the bright-®eld TEM images demonstrated these
[1] E. Daugy, P. Mathiez, F. Salvan, J.M. Layet, Surf. Sci. 154 (1985) 267. [2] H. Kemmann, F. Muller, H. Neddermeyer, Surf. Sci. 192 (1987) 11. [3] S. Takahashi, Y. Tanishiro, K. Takayanagi, Surf. Sci. 242 (1991) 73. [4] G. LeLay, Surf. Sci. 132 (1983) 169. [5] K. Akiyama, K. Takayanagi, Y. Tanishiro, Surf. Sci. 205 (1988) 177. [6] K.M. Conway, J.E. Macdonald, Surf. Sci. 215 (1989) 555. [7] T. Takahashi, K. Izumi, T. Ishikawa, S. Kikuta, Surf. Sci. 191 (1987) L825. [8] Y. Haga, K. Takayanagi, Ultramicroscopy 45 (1992) 95. [9] T. Takahashi, S. Nakatani, N. Okamoto, T. Ishikawa, S. Kikuta, Surf. Sci. 242 (1991) 54. [10] T. Takahashi, S. Nakatani, Surf. Sci. 282 (1993) 17. [11] K.J. Wan, E.F. Lin, J. Nogami, Phys. Rev. B 45 (1992) 9509. [12] H. Aizawa, M. Tsukada, Phys. Rev. B 59 (1999) 10923. [13] D.R. Heslinga, H.H. Weitering, D.P. van der Werf, T.M. Klapwijk, T. Hibma, Phys. Rev. Lett. 64 (1990) 1589. [14] R.F. Schmitsdorf, T.U. Kampen, W. Monch, Surf. Sci. 324 (1995) 249. [15] R.T. Tung, Phys. Rev. Lett. 52 (1984) 461. [16] R.D. Aburano, H. Hong, J.M. Roesler, K. Chung, D.-S. Lin, P. Zschack, H. Chen, T.-C. Chiang, Phys. Rev. B. 52 (1995) 1839. [17] K. Akimoto, M. Lijadi, S. Ito, A. Ichimiya, Surf. Rev. Lett. 5 (1998) 719. [18] A. Shibata, Y. Kimura, K. Takayanagi, Surf. Sci. 303 (1994) 161. [19] Y. Tanishiro, K. Takayanagi, Ultramicroscopy 27 (1989) 1.
www.elsevier.com/locate/susc
UHV-TEM/TED observation p p of Ag islands grown on Si(1 1 1) 3 3-Ag surface Yoshifumi Oshima *, Hiroyuki Nakade, Sinya Shigeki, Hiroyuki Hirayama, Kunio Takayanagi Graduate school at Nagatsuta, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8502, Japan Received 18 October 2000; accepted for publication 26 January 2001
Abstract
p p Growths of Ag islands on Si(1 1 1) 3 3-Ag surface at room temperature were observed by UHV transmission electron microscopy and diraction. The Ag islands grown after six monolayer deposition had neither (1 0 0) nor (1 1 0) orientation, but had two complex epitaxial dominantly. One was striped islands which gave rise to a porientations p diraction pattern commensurate with the 3 3 lattice of the Si(1 1 1) surface. The other was the coagulated islands whose diraction pattern indicated the Ag(1 3 4) sheet grown parallel to the Si(1 1 1) surface. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Electron±solid diraction; Silicon; Silver
1. Introduction Surface and interface structures have attracted much interest in fundamental and applied sciences. Metal±silicon interfaces are particularly of importance in device technologies, and a number of researches have been done. From these works well known is the fact that metals deposited on silicon surfaces generate a speci®c surface layer for each metal of one or sub-monolayer thickness. Monolayer thick metal deposited on the Si(1 1 1) surfaces reconstructed to the 7 7 structure has often
* Corresponding author. Tel.: +81-45-924-5637; fax: +81-45924-5685. E-mail address: [email protected] (Y. Oshima).
commensurate lattices with the silicon surface, with some exceptions of incommensurate lattices for Cup[1,2]. Among those commensurate lattices, p 3 3 structures appear p forpAu [3], Ag [4], Pd [5], Sn [6], Bi [7,8]. The 3 3 structure of Ag has the honeycomb-chained structure [9,10], saturated at the coverage of 1 ML (monolayer 7:8 1014 atoms=cm2 ), changes the metallic surface state of the Si(1 1 1)7 7 into a semi-conductive state [11]. The semiconductive surface results from termination of the Si dangling bonds by 5s electrons of Ag [12]. Such termination of the dangling bonds aects electronic properties originated from interface structures between metals and silicon. Apparent change of the Schottky barrier height was found for the Pb±Si(1 1 1) interface, depending on whether Pb ®lm is deposited on pthepclean Si(1 1 1)7 7 surface or on a Si(1 1 1) 3 3-Pb
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 2 4 1 - 9
Y. Oshima et al. / Surface Science 493 (2001) 366±372
surface [13]. Similar change was also found for Ag±Si(1 1 1) interfaces [14] and others [15]. However, any relationship between dangling bond density and Schottky barrier height has not been clari®ed yet because of the lack of knowledge of their interfacial structures. Because the surface structures formed at the beginning by metal deposition may change after the subsequent ®lm deposition on it. Interface structures are often dicult to be analyzed even by modern techniques in surface science, which are sensitive only for a few atomic layers from the top surface layer. Recently, some groups investigated the interface using X-ray pdiraction p after growing thin Ag ®lm on Si(1 1 1) 3 3-Ag surface at room ptemperature. Aburano et al. re p ported that the 3 3 periodicity disappeared at the interface and Ag ®lms were oriented with Ag{1 1 1}, Ag{2 0 0} and Ag{3 1 1} parallel to the Si(1 1 1) surface p p[16], while Akimoto et al. reported that the 3 3 periodicity remained at the interface and Ag ®lms mainly had Ag{1 1 0} plane parallel to the Si(1 1 1) surface [17]. We applied UHV transmission electron microscope and diffraction (TEM/TED) technique for interface structure analyses in order to investigate structure of each Ag island and its buried interface. We reported brie¯y that two p kinds p of Ag islands were grown on the Si(1 1 1) 3 3-Ag. One is a striped Ag ®lm which gives rise to thepdiraction spots p commensurate with the Si(1 1 1) 3 3 lattice in addition to incommensurate spots. The other is Ag islands of which (1 3 4) plane is parallel to the Si(1 1 1) surface.
2. Experimental We used an ultrahigh-vacuum transmission electron microscope (JEM-2000V). A Si(1 1 1) wafer (Sb-doped 0.02 X cm) was cut to a size of 1 7 mm and thinned mechanically to a 0.2 mm. After rinsing, it was cleaned in the UHV microscope by ¯ashing at 1200°C, and subsequently thinned by heating at 1000°C to have local areas with 10±20 nm thickness. Ag was deposited in situ on the clean Si(1 1 1)7 7 surface at 300°C. By
367
the deposition at a rate of 0.1 ML/s, diraction intensities of the 7 7 surface to reach p decreased p constant values, while the 3 3 intensity increased for the saturation value. p pWe thus con®rmed the saturation of the 3 3 structure at one monolayer (ML), which covers only the top Si(1 1 1) surface. We subsequently deposited Ag of six MLs at room temperature with the deposition rate of 0.1 ML/sec. After this deposition, the 7 7 reconstruction at the bottom surface had disappeared. We suppose that a part of Ag atoms migrate to the bottom surface from the top surface to convert the 7 7 structure into 1 1 structure. During the deposition of the Ag islands, the specimen was not irradiated by the imaging electron beam in order to avoid any pelectron p beam eect. Ag islands grown on the 3 3 surface were observed by bright±dark ®eld electron microscopy images and selected area diraction patterns to determine structures of Ag islands and their interface structures. 3. Results and discussion pThe pAg islands were grown on the Si(1 1 1) 3 3-Ag surface with two structures. One is a ¯at island extended laterally, and the other is a three-dimensional and thick. We call the former island striped island, and the latter (1 3 4) island, hereafter, and describe their features observed by TEM and TED. 3.1. Striped Ag island Fig. 1a and b show typical bright-®eld TEM images of striped Ag islands. The striped Ag island in Fig. 1a is formed at the center of terrace. These islands were not formed from the step indicated by dashed lines. They are elongated largely along the terrace in the [1 1 2] direction with alternating bright and dark stripes. The stripes are parallel to the [1 1 0] direction. The average width of a couple of the bright and dark stripes was 7.7 nm, although the widths of the bright and dark stripes are not regular. Striped Ag island in Fig. 1b has a dierent morphology from Fig. 1a. It is not elongated along
368
Y. Oshima et al. / Surface Science 493 (2001) 366±372
Fig. 1. (a) A typical bright-®eld TEM image of a stripe Ag island. This island has alternating bright and dark stripes pattern parallel to the [1 1 0] direction. The average width of a pair of the bright and dark stripes was 7.7 nm. The dashed lines indicate the steps of the Si(1 1 1) surface. bright-®eld TEM image of another stripe Ag island. This island was grown on a wide terrace with p(b) A ptypical some holes of the 3 3 structure [18]. The dot-dashed lines indicate the holes, whose diameter is about 70±100 nm.
the [1 1 2] direction, but it has straight edge parallel to the [1 1 2]pdirection p and so on. This island is grown on the 3 3 surface which had some hole regions on a wide ¯at terrace. These holes were formed by Ag deposition on the wide terrace of the Si(1 p1 1)7 p 7 surface [18], which are lower than the 3 3 domains on the Si(1 1 1) surface by monoatomic layer of the Si(1 1 1) atomic sheet. The holes were found by contrast as shown in the inserted image which corresponds to the region indicated by white line in the TEM image. The edges of these holes are indicated by dotdashed lines in Fig. 1b. The stripes of the Ag island are seen to be discontinuous at the edges of the holes. Therefore, the striped Ag islands had nucleated p pin each hole and on the terrace of the 3 3 domains, and grew to the edge of the hole. The growth of this island is less in¯uenced by the edge of the holes, since the edge of the island is pstraight p regardless of the hole region of the 3 3 structure. In this island, the average width of a pair of the bright and dark stripes was
5.0 nm. From several TEM images of the striped islands, the average width was found to be 5±7 nm. Fig. 2a and b show TED patterns of the striped Ag island in Fig. 1a. The former was taken with incident electron beam normal to the Si(1 1 1) surface, the latter, with incident electron beam tilted about 4° along the [1 1 0] direction from the surface normal. Fig. 2a shows six {2 2 0} re¯ection spots of Si substrate indicated by the dashedline and the spots corresponding to the p exagon p 3 3 structure. Some of the latter spots are split along the [1 1 2] direction as shown by the spots a and b. Also, the spots c andpc0 are pintense, which cannot be explained by the 3 3 structure at the interface. Therefore, we notice two dierent reciprocal unit A and B indicated by solid and dashed lines, respectively, since the distance between the spot b02 and a02 is twice larger than between the spots b01 and a01 as shown in the inserted intensity pro®le of Fig.p2a. Some of the p fractional-order spots of the 3 3 structure are split due to these two structures. The reciprocal
Y. Oshima et al. / Surface Science 493 (2001) 366±372
369
Fig. 2. (a) A TED pattern obtained from the striped Ag island of Fig. 1a. This pattern was taken with incident beam normal to the Si(1 1 1) surface. Six Si{2 2 0} spots are indicated by hexagon with p p dashed lines. The (hk) spots are indicated by unit cell of the Si(1 1 1)(1 1). Except the spots of bulk Si crystal and the 3 3 structure, two reciprocal units, A and B are observed. The spots ai (i 1, 2) appear on the solid lines, the spots bi , on the dashed lines. The inserted intensity pro®le shows the split spacing between a02 and b02 is twice larger than between a01 and b01 . (b) This pattern was taken with incident beam tilted about 4° from the Si(1 1 1) surface.
unit A and B have the common spots along the [1 1 0] direction as indicated by the spots c and c0 . The spot c (or c0 ) corresponds to the ( 4/3, 8/3) fractional order spot. Since the ratio of the lattice constants between bulk Ag and Si crystal is 3:4, the {2 2 0} spot of Ag crystal coincides with the ( 4/3, 8/3) spot. Therefore, the spot c (or c0 ) is the {2 2 0} spot of Ag crystal. On the other hand, the reciprocal unit A and B have dierent spots along the [1 1 2] direction. In Fig. 2a, the spot a coincides with the (2 0) spot and the spot b appear shorter
than the spot a. The unit cell of structure B elongates against structure A. We do not ®nd any indices of the spots a and b which are perpendicular to the (2 2 0) re¯ection spot indicated by c (c0 ). We do not decide on the crystallographic orientation of Ag islands. The previousp results that the pshowed Ag islands grown on the 3 3 structure has the (1 1 1) or (1 1 0) plane parallel to the surface [16,17]. But, the striped islands do not agree with these results, because the {2 2 4} spot of Ag crystal has to appear further outside the (2 0) spot and the
370
Y. Oshima et al. / Surface Science 493 (2001) 366±372
{0 0 2} spot, inside. In Fig. 2b, the reciprocal unit A appears, but the reciprocal unit B disappears. But, the reciprocal unit B was sometimes observed in the diraction pattern with the same condition as this ®gure. In Fig. 2b, it is noteworthy that pthe fractional-order spots are produced by the 3 p 3 structure. Because the dynamical eect on the intensity of the re¯ection spots are almost ignored [19], when the incident beam is tilted about 4° from the Si(1 1 1) surface normal. The additional spots produced by double diraction do not appear in this incident beam condition. Fig. 3a shows a bright-®eld TEM image obtained from a striped Ag island. In addition to the stripes, fringes perpendicular to the stripes and those inclined to them appeared. Fourier transformed pattern obtained from this TEM image shows the spots corresponding to the vertical and
oblique fringes observed in Fig. 3a. It is noticed that the oblique fringes give split spots indicated by the spots A A0 and B B0 . The interference fringes between the center spot and A A0 spot correspond to that appeared in a region of Fig. 3a. The interference fringes between the center spot and B B0 spot is from the rest of the regions in Fig. 3a. The interference fringes in a and b regions are not parallel but tilted 1±2° which corresponds to the splitting of the spots A and B in Fig. 3b. Fig. 3c schematically illustrates the TED pattern from the striped islands. The Si{2 2 0} spots are shown by gray circles forming a hexagonal arrangement. Solid circles are the spots due to the Ag island. Open circles are the double diraction spots of the Ag islands and Si crystal. Thus, moire fringe in Fig. 3a corresponds to interference fringes between the center spot and the spots indicated
Fig. 3. (a) A typical bright-®eld TEM image of a striped Ag island. Since the interference fringes in regions A and B are not parallel but tilted 1±2°, two regions a and b are observed as shown. Moire fringes in the region b # is slightly tilted from the region b. (b) A Fourier transformed pattern obtained from the TEM image of (a). The oblique fringes of (a) give split spots A A0 and B B0 . (c) Illustration of TED pattern from striped Ag island and Si. The Double diraction between the Si{2 2 0} and Ag spots produce spots A, B in (c), which correspond to the spots A, B in (b).
Y. Oshima et al. / Surface Science 493 (2001) 366±372
371
by open circles. Being compared with Fig. 3c and b, we verify that the striped Ag island has two domains a and b which give two reciprocal unit A and B, respectively. The p domain p a is commensurate with the Si(1 1 1) 3 3 structure. On the other hand, domain having about 5% pb ispnot, mismatch with the 3 3 structure in the island of Fig. 2a. We do not understand why both commensurate and incommensurate lattices are formed together in one striped island. The boundary between the a and b is not clear. p domains p The 3 3 structure remain at the interface, because p p the Ag island grown on the Si(1 1 1) 3 3-Ag surface is commensurate at some regions. This result is agreement withpAkimoto p et al. But, we do not know whether the 3 3 structure is honeycomb-chained trimer structure or not. In this study, Ag island is so thin (several monolayer) that it modi®es the structure from bulk Ag crystal. In previous results, Ag ®lms is about a in thick. These Ag ®lms must few hundreds A become the same structure as the bulk crystal. 3.2. Ag(1 3 4) island Fig. 4a shows a typical bright-®eld TEM image of another type of Ag island and Fig. 4b, a TED pattern taken from the Ag island at the center. This island grew three-dimensionally. We call these islands (1 3 4) island, since they have Ag crystalline structure and their (1 3 4) lattice plane is parallel to the Si(1 1 1) surface. The large island in Fig. 4a shows complex contrast. In the diraction pattern of Fig. 4b, we found the systematic re¯ection spots from Ag(1 1 1) and ( 1 1 1). We also recognize the six Si{2 2 0} spots from the Si(1 1 1) surface. Spots such as indicated by arrows are explained by double diraction between the Ag{1 1 1} and Si spots as indicated by arrows P and Q. The spots which do not appear by double diraction are Ag(3 5 3), Ag(4 4 2) and Ag(5 3 1) spots which are seen at high diraction angles. Provided that the two reciprocal vectors of Ag( 1 1 1) and Ag(4 4 2) are in the plane parallel to the Si(1 1 1) surface, the orientation of the Ag island concerned [uvw] is determined to be [1 3 4] direction. Therefore, this Ag island is oriented with Ag(1 3 4) plane parallel to the Si(1 1 1) surface.
Fig. 4. (a) A typical bright-®eld TEM image of Ag(1 3 4) island. This island is grown three dimensionally, and its shape is not regular. (b) A TED pattern of Ag(1 3 4) island. Two reciprocal vectors of Ag(1 1 1) and Ag(4 4 2) spots appear except the spots of Si crystal and double diraction spots between Si{2 2 0} and Ag spots. Arrows indicate double diraction spots produced by the Ag{1 1 1} and Si spots, for example, P and Q.
On the other hand, the p p re¯ection spots corresponding to the 3 3 structure p are in not pseen the TED pattern. Therefore, any 3 3 layer does not remain at this interface. As shown in Fig. 5, atomic positions within the p unit pcell of Ag(1 3 4) plane resemble that of 3 3 structure. p Fig. p 5b shows the relationship between the 3 3 structure and the unit cell of the Ag island indicated by bold lines. It is noteworthy that slight rotation and compression of the Ag(1 3 4) lattice p brings pto commensurate Ag(1 3 4) lattice to the 3 3 unit cell.
372
Y. Oshima et al. / Surface Science 493 (2001) 366±372
two regions. On the other hand, the Ag(1 3 4) islands are grown three dimensionally. Except the spots of Si crystal and double diraction spots between the Si{2 2 0} and Ag spots, the systematic re¯ection spots from Ag(1 1 1) and Ag(4 4 2) appear in the TED pattern. These two reciprocal vectors indicate that the Ag island is oriented with Ag(1 3 4) plane parallel to the Si(1 1 1) surface. Acknowledgements This work was supported partially by Grant-in Aid from Ministry of Education of Japan (no. 09NP1201). References
Fig. 5. (a) Atomic structure of the (1 3 4) plane. Circles indicate the atoms. Atomic coordination is indicated. The angle between the vector [1 1 1] and [1 128°. (b) The p3 2] is pabout relative relationship between the 3 3 structure and the (1 3 4) plane is shownpschematically. The shaded lines indi p cate the unit cell of the 3 3 structure, while the bold lines indicate the (1 3 4) lattice plane.
4. Conclusion We observed the structure of Ag island and the interface between the island and the Si(1 1 1) substrate using UHV-TEM/TED. Ag islands are p p grown on the Si(1 1 1) 3 3-Ag surface at room temperature by 6 ML Ag deposition. We found two types of Ag islands, striped Ag island and Ag(1 3 4) island. The striped Ag islands have alternating bright and dark stripes parallel to the [1 1 0] direction. The average width of a pair of the bright and dark stripes is 5±7 nm, although each width of the bright and dark stripe is not regular. The striped Ag island has two structure p units, p one of which is commensurate with the 3 3 structure, while the other has a unit cell elongated along the [1 1 2] direction about ®ve percent relative to the commensurate lattice. Moire pattern in the bright-®eld TEM images demonstrated these
[1] E. Daugy, P. Mathiez, F. Salvan, J.M. Layet, Surf. Sci. 154 (1985) 267. [2] H. Kemmann, F. Muller, H. Neddermeyer, Surf. Sci. 192 (1987) 11. [3] S. Takahashi, Y. Tanishiro, K. Takayanagi, Surf. Sci. 242 (1991) 73. [4] G. LeLay, Surf. Sci. 132 (1983) 169. [5] K. Akiyama, K. Takayanagi, Y. Tanishiro, Surf. Sci. 205 (1988) 177. [6] K.M. Conway, J.E. Macdonald, Surf. Sci. 215 (1989) 555. [7] T. Takahashi, K. Izumi, T. Ishikawa, S. Kikuta, Surf. Sci. 191 (1987) L825. [8] Y. Haga, K. Takayanagi, Ultramicroscopy 45 (1992) 95. [9] T. Takahashi, S. Nakatani, N. Okamoto, T. Ishikawa, S. Kikuta, Surf. Sci. 242 (1991) 54. [10] T. Takahashi, S. Nakatani, Surf. Sci. 282 (1993) 17. [11] K.J. Wan, E.F. Lin, J. Nogami, Phys. Rev. B 45 (1992) 9509. [12] H. Aizawa, M. Tsukada, Phys. Rev. B 59 (1999) 10923. [13] D.R. Heslinga, H.H. Weitering, D.P. van der Werf, T.M. Klapwijk, T. Hibma, Phys. Rev. Lett. 64 (1990) 1589. [14] R.F. Schmitsdorf, T.U. Kampen, W. Monch, Surf. Sci. 324 (1995) 249. [15] R.T. Tung, Phys. Rev. Lett. 52 (1984) 461. [16] R.D. Aburano, H. Hong, J.M. Roesler, K. Chung, D.-S. Lin, P. Zschack, H. Chen, T.-C. Chiang, Phys. Rev. B. 52 (1995) 1839. [17] K. Akimoto, M. Lijadi, S. Ito, A. Ichimiya, Surf. Rev. Lett. 5 (1998) 719. [18] A. Shibata, Y. Kimura, K. Takayanagi, Surf. Sci. 303 (1994) 161. [19] Y. Tanishiro, K. Takayanagi, Ultramicroscopy 27 (1989) 1.