Si islands with 1 ×  1 termination formed by desorption of Tl from Si(1 1 1) surface

Applied Surface Science 256 (2009) 1168–1170

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Si islands with 1  1 termination formed by desorption of Tl from Si(1 1 1) surface P. Koca´n a,b,*, H. Tochihara a a b

Department of Molecular and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holesˇovicˇka´ch 2, 180 00 Praha 8, Czech Republic

A R T I C L E I N F O

A B S T R A C T

Article history: Available online 21 June 2009

We report on the scanning tunneling microscopy observation of small Si islands with a 1  1 termination on the Si(1 1 1) surface. The islands were prepared by thermal desorption of Tl from the Tl-terminated silicon sample by means of annealing to 400–600 8C. Structure of the islands is interpreted as the dimerstacking-fault (DS) model. We propose that the otherwise unfavorable 1  1 termination is stabilized by subsurface dimers of DS. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Scanning tunneling microscopy Silicon Thallium Si(1 1 1)

1. Introduction Clean Si(1 1 1) surfaces host various metastable structures and thus represent an interesting playground for study of covalent bonding and related electronic phenomena. Detailed understanding of surface structures and their properties is crucial for fabrication of applicable nanostructured surfaces. Depending on a procedure of preparation, several reconstructions are formed on the Si(1 1 1) surface. A cleavage in vacuum at room temperature (RT) results in a 2  1 reconstruction, whose structure is known as the modified Pandey chain (PC) model [1,2]. After annealing of the 2  1 surface, dimer-adatom-stacking fault (DAS) structures are observed. Annealing at 250–400 8C gives arise to a 5  5 reconstruction [3–7] with surface density of Si atoms same as of the 2  1. Annealing at 600 8C results in formation of the famous 7  7 structure [8], which is the most stable structure on the Si(1 1 1) surface. Another metastable reconstructions can be observed on samples by a fast radiationpquenching, such as c(2  8) pffiffiffi prepared pffiffiffi ffiffiffi pffiffiffi and ( 3  3)R308(hereafter called 3  3 for simpicity) [9]. We use a reaction of the Si(1 1 1) surface with adsorbed Tl to break the 7  7 reconstruction. It is known that Tl/Si(1 1 1)-(1  1) reconstruction is formed after deposition of  1 ML of Tl and annealing to  300 8C [10–12]. In the 1  1 structure, Tl atoms occupy T4 sites of otherwise as-bulk terminated surface [11,12]. It was reported that Tl atoms desorb at  350 8C [13]. Because of relatively low desorption temperature, metastable reconstructions of the Si(1 1 1) surface are observed after desorption of Tl, as will be shown here.

Besides domains of reconstructions pffiffiffi pffiffiffi observed on cleaved or quenched surfaces (2  1 and 3  3) we observed islands with a local 1  1 termination at temperatures 450  600 8C. A 1  1 termination of Si surface is rare as it is supposed to be highly unstable because of high density of unsaturated bonds. Previously, domains of the 1  1 Si were observed after electron-stimulated desorption (ESD) from Br terminated Si(1 1 1) surface at room temperature [14]. The authors of Ref. [14] proposed that the 1  1 structure is stabilized by means of subsurface dimers originating from the 7  7 DAS structure. Here we demonstrate that the local 1  1 Si islands with height of one bilayer can be formed and preserved also during formation of another more stable structures. 2. Experimental Samples were prepared in a chamber linked directly to a UNISOKU scanning tunneling microscopy (STM) system. Base pressure was 3  1010 Torr and 2  1011 Torr in the preparation and STM chamber, respectively. Samples (Sb doped substrates with resistivity r = 0.005–0.01 V cm) were heated resistively by passing DC current, temperature was measured by infrared pyrometers. The (7  7) reconstruction was prepared in a usual way with overnight degassing at 650 8C followed by flashing to  1200 8C. Tl was evaporated from a home made Ta tube, the rate was monitored by a crystal thickness monitor. The sample was held at room temperature (RT) during deposition and STM observation. Electrochemically etched tungsten tips were used for STM experiments. 3. Results and discussion

* Corresponding author. E-mail address: [email protected] (H. Tochihara). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.078

Fig. 1 shows a constant height mode STM image of the Si(1 1 1) surface after desorption of 1 ML of thallium and annealing to

P. Koca´n, H. Tochihara / Applied Surface Science 256 (2009) 1168–1170

Fig. 1. STM image of Si(1 1 1) surface after desorption of thallium by annealing to 450 8C. A 2  1 labyrinth coexists with small domains of a local 1  1 structure. Size of image is 40  10 nm2.

450 8C. Several structures on the surface: a labyrinth of pffiffiffi pcoexist ffiffiffi 2  1 stripes, small 3  3 domains and small 2D islands with 1  1 termination. Unit cells of these reconstructions are marked pffiffiffi pffiffiffi accordingly in the figure. The 2  1 and 3  3 structures are commonly observed on cleaved [1–6,15–17] or quenched [9] surfaces. On the other hand, cleavage or quenching has not been reported to result in formation of domains with a 1  1 termination, according to our knowledge. Fist we discuss a chemical composition of the 1  1 islands. The 1  1 reconstruction is observed on the 1 ML-Tl deposited Si(1 1 1) surface. However, Tl is reported to desorb at  350 8C [13], which is much below the temperatures used in the present experiments. Another possible 1  1 termination could be the bulk terminated surface stabilized by hydrogen atoms from a residual atmosphere. It is known that H atoms saturate the dangling bond electrons of Si surfaces, so that 1  1 H-terminated surface can be observed after H dosing of the sample [18]. In Ref. [19], desorption temperature of H from the Si(1 1 1) surface is measured by thermal desorption spectroscopy and a value of 540 8C was obtained for monohydride (the highest desorption temperature) species. As we describe later, we observe 1  1 islands up to 610 8C annealing, so the Htermination can be ruled out as well. Because presence of another chemical species possibly resulting in the 1  1 termination is unreasonable, we conclude that the islands are composed purely of Si atoms. Termination by Si atoms with the 1  1 periodicity should result in a metallic state caused by a presence of a half-filled Si dangling bond. Such state should be detectable by the scanning tunneling spectroscopy (STS). Unfortunately, our experimental setup did not allow to measure STS reproducibly. On the other hand, a termination by a monovalent chemical species (e.g. H) would result in different imaging of empty and filled electron states, respectively. Such difference was not observed during our STM experiments (not shown here). For discussion of structures of the 1  1 islands we select an island in a rectangle in Fig. 1. It is clearly seen that the island is divided to four areas separated by antiphase boundaries (marked AB in the figure). Positions of protrusions with the 1  1 symmetry agree well with those of restatoms in the DAS model. Thus we propose the structure of the islands is dimer-stacking fault, i.e. DAS structure without the adatoms (denoted DS hereafter). Then the antiphase boundaries (Fig. 1) would correspond to dimer rows separating areas with or without stacking fault compared to the bulk structure. The proposed atomic model is overlaid to a magnified STM image in Fig. 2a. We note that positions of subsurface dimers cannot be strictly determined, because their imaging is difficult due to screening by the topmost layer atoms. A schematic drawing of triangular units of dimer-stacking-fault structure is shown in Fig. 2b together with their sizes and stacking orientations. We note that half unit-cells of various sizes of DAS structure were previously observed during process of the 7  7 formation [20]. Fig. 2c shows a schematic side view of a 2  1 area with p-bonded chain (left part) and with the 1  1 island (right

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pffiffiffi pffiffiffi 3  3 and with islands exhibiting

part). A situation at an antiphase boundary on the 1  1 island is demonstrated in Fig. 2d. Dimerized atoms separating areas with different stacking are marked by arrows. A (1  1) termination of Si(1 1 1) is generally supposed very unstable because of a high density of unsaturated dangling bonds (one bond per a 1  1 unit cell). As a consequence, the bulkterminated 1  1 surface reconstructs to the 2  1 PC structure even at RT. Hence it is interesting to discuss why the 1  1 is formed after Tl desorption. Our results can be compared to the structural change observed by Mochiji on surfaces prepared by electron-stimulated desorption of Br from the Si(1 1 1)(1  1)-Br surface [14]. On the samples with rest-atom layer of originally Si(1 1 1)7  7 reconstruction completely removed, the 2  1 PC structure was formed after ESD. On the other hand, if the rest-atom layer including subsurface Si dimers was not canceled, the 2  1 was not formed. Instead, a local 1  1 structure of Si rest-atoms was observed. This behavior was explained by a stabilizing effect of subsurface dimers resulting in preservation of the 1  1 local

Fig. 2. (a) Detail of the island marked in Fig. 1 by a rectangle. Proposed structural model of dimer-stacking-fault is overlaid. (b) Schematic drawing of triangular units of stacking-fault structure composing island in (a). Numbers describe sizes of the triangular units, F and U stands for faulted and unfaulted, respectively. (c) A side view of boundary between 2  1 structure and 1  1 island. (d) A side view of antiphase boundary between faulted (1  1F) and unfaulted (1  1U) domains. Arrows in (c,d) mark position of subsurface dimers.

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structure. Separation of the 1  1 island (Fig. 2) to parts with different stacking suggests a presence of stabilizing subsurface dimers in our observation as well. Here we note that origin of the dimers is different in the case of annealing Tl-desorbed surface and in the case of ESD. In the first case the dimers are formed during growth of a 1  1 island, while in the latter case the dimers are preserved from 7  7 structure. Formation of the DS islands composed of triangular units with or without stacking fault is in agreement with a simple model of energetics on the Si(1 1 1) surface proposed by Vanderbilt [21]. According to the model, formation of dimer walls separating parts with different stacking is favored even if adatoms do not occur, mainly because of elimination of one dangling bond per creation of 1ð1=2Þdimer. Next, a triangular shape of cells is simply explained by the fact that parallel dimer walls of one cell are topologically impossible [21]. Another important question is why the 1  1 islands are not further stabilized by adatoms. Presence of an adatom would considerably decrease energy by creating only one instead of originally three dangling bonds. We believe that reason for this is a kinetic one — rate of atoms hopping on top of the DS islands is low at studied temperatures, e.g. due to presence of Ehrlich-Schwoebel barrier [22]. The 1  1 islands coexisting with the 2  1 stripes are observed up to 610 8C. Observation of the 2  1 stripes up to such a high temperature deserves further attention. On cleaved surfaces, the 2  1 structure transforms to 5  5 or apparent (disordered) ‘‘1  1’’ at 250–400 8C [3–7]. What is the reason of high stability of the 2  1 stripes after Tl desorption? First we note that on the cleaved surfaces the 2  1 domains are large and with a small amount of defects, while Tl-desorbed samples show small domains with many domain boundaries. Feenstra et al. observed transition of 2  1 to the other structures on cleaved samples directly by STM [5,4]. According to their observation, the 2  1 was canceled from positions of domain boundaries. Thus we propose that boundaries between domains of 2  1 on the cleaved surfaces differ to the boundaries on Tl-desorbed samples. A possible explanation is that on the cleaved surfaces the domain boundaries are formed simply by meeting two independent domains of 2  1 during their formation, while on the Tl-desorbed surfaces the domains originate in defects of the Si(1 1 1)(1  1)-Tl reconstruction. Most likely, some of subsurface dimers from DAS structure—before (1  1)-Tl formation—are preserved up to higher temperatures, resulting in stable domain boundaries. Annealing to higher temperature is necessary to cancel the subsurface dimers and to create new dimers of DAS structure. In our experiments (not

shown here), the 2  1 labyrinth transformed to 5  5 DAS structure after annealing to 650 8C. 4. Conclusion Desorption of Tl from the Si(1 1 1)-Tl surface was used to prepare metastable terminations of the Si(1 1 1) surface. After annealing to 400–600 8C we observed islands with a local 1  1 termination. We proposed a structure of the islands is consistent with dimer-stacking model. Coexisting with the 1  1 islands, 2  1 domains were observed. The 2  1 was surprisingly stable up to 650 8C. Subsurface dimers are proposed to be stabilizing factors of both 1  1 and 2  1 terminations. In the case of 2  1, the dimers originate from imperfectly canceled DAS structures, while in the case of 1  1 the dimers are formed during formation of the islands. Acknowledgment This work was supported by the following KAKENHIs: Grant-inAid for Scientific Research (B) (19340083); Grant-in-Aid for JSPS fellows (18.06750). References [1] K.C. Pandey, Phys. Rev. Lett. 47 (1981) 1913. [2] F.J. Himpsel, P.M. Marcus, R. Tromp, I.P. Batra, M.R. Cook, F. Jona, H. Liu, Phys. Rev. B 30 (1984) 2257. [3] J.J. Lander, G.W. Gobeli, J. Morrison, J. Appl. Phys. 34 (1963) 2298. [4] R.M. Feenstra, M.A. Lutz, Surf. Sci. 243 (1991) 151. [5] R.M. Feenstra, M.A. Lutz, Phys. Rev. B 42 (1990) 5391. [6] B. Garni, D.E. Savage, M.G. Lagally, Surf. Sci. 235 (1990) 324. [7] R.I.G. Uhrberg, E. Landemark, L.S.O. Johansson, Phys. Rev. B 39 (1989) 13525. [8] K. Takayanagi, Y. Tanishiro, M. Takahashi, S. Takahashi, J. Vac. Sci. Technol. A 3 (1985) 1502. [9] F. Rose, S. Kawai, T. Ishii, H. Kawakatsu, Phys. Rev. B 73 (2006) 045309. [10] L. Vitali, F.P. Leisenberger, M.G. Ramsey, F.P. Netzer, J. Vac. Sci. Technol. A 17 (1999) 1676. [11] S.S. Lee, H.J. Song, N.D. Kim, J.W. Chung, K. Kong, D. Ahn, H. Yi, B.D. Yu, H. Tochihara, Phys. Rev. B 66 (2002) 233312. [12] T. Noda, S. Mizuno, J.W. Chung, H. Tochihara, Jpn. J. Appl. Phys. 42 (2003) 319. [13] V. Kotlyar, A. Saranin, A. Zotov, T. Kasyanova, Surf. Sci. 543 (2003) 663. [14] K. Mochiji, Phys. Rev. B 67 (2003) 113314. [15] P.P. Auer, W. Mo¨nch, Surf. Sci. 80 (1979) 45. [16] D. Zhao, D. Haneman, Surf. Sci. 418 (1998) 132. [17] J.K. Garleff, M. Wenderoth, K. Sauthoff, R.G. Ulbrich, M. Rohlfing, Phys. Rev. B 70 (24) (2004) 245424. [18] F. Owman, P. Ma¨rtensson, Surf. Sci. 303 (1994) 367. [19] G. Schulze, M. Henzler, Surface Science 124 (1983) 336. [20] W. Shimada, H. Tochihara, Surf. Sci. 526 (2003) 2199. [21] D. Vanderbilt, Phys. Rev. B 36 (1987) 6209. [22] J. Schwoebel, J. Appl. Phys. 37 (1966) 3682.