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Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant
SUSC-20005; No of Pages 5
August 02, 2013;
Model: Gulliver 5
Surface Science xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Surface Science journal homepage: www.elsevier.com/locate/susc
Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant K.N. Romanyuk a,b, A.A. Shklyaev a,b, B.Z. Olshanetsky a,⁎ a b
Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia Novosibirsk State University, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 19 March 2013 Accepted 15 July 2013 Available online xxxx Keywords: Scanning tunneling microscopy Semiconducting surfaces Nanostructures Surfactants
a b s t r a c t pffiffiffi pffiffiffi Submonolayer Ge cluster grown by molecular beam epitaxy on the Si(111)- 3ñ 3-Bi surface were studied using scanning tunneling microscopy. The cluster of monolayer and bilayer height containing 3–4 and 9–10 atoms, respectively, have been grown at room temperature. We have found that the monolayer cluster are mobile and diffuse over Bi layer at room temperature, whilepbilayer ffiffiffi pffiffiffi cluster are epitaxial and can be classified by positions of the cluster relative to Bi trimers on the Si(111)- 3ñ 3-Bi surface. In the temperature range of 100 °C–400 °C, the cluster population consists of two types of bilayer cluster with Bi trimers in T4 and H3 positions on the cluster, correspondingly. At temperatures above 400 °C only the most stable atomic configuration with Bi trimer in H3 position on the bilayer cluster is remained on the surface. © 2013 Published by Elsevier B.V.
1. Introduction The cluster nucleating at submonolayer coverage on the Si(111) surface [1–5] have attracted considerable attention due to their potential application in nanotechnology. Such cluster are promising for the use in nanoelectronics and quantum computers [6] and can serve as model objects for studying surface atomic processes at early growth stages [5]. Comprising a fixed number of atoms they are also known as “magic” cluster [7–10]. Specifically, the cluster size can be close to the critical island size and depends on the size of the surface unit cell [3,11]. The relation between cluster structure and cluster stability is an important question since it is addressed to the functionality of the cluster and it influences the uniformity of cluster in the population. The investigation of the stable atomic configurations of surface cluster is of interest for the theoretical study, as well. However, experimental observation of the cluster structure is the challenge due to a particularly small scale. Using structural and symmetry analysis of the surface topography we were able to distinguish different atomic configurations of the cluster with a different stability in a cluster population. In this contribution, we present an experimental study of the formation and evolution p offfiffiffi Ge pffiffifficluster and cluster structure during annealing on the Si(111)- 3ñ 3 -Bi surface. The cluster of monolayer and bilayer height were grown by submonolayer pffiffiffi pffiffiffi deposition of Ge at room temperature (RT) on the Si(111)- 3ñ 3-Bi surface and were
⁎ Corresponding author. Tel.: +7 383 3333 286; fax: +7 383 333 3502. E-mail address: [email protected] (B.Z. Olshanetsky).
systematically studied at different annealing temperatures using the scanning tunneling microscope (STM) method. We used Bi surfactant since Bi suppresses the exchange intermixing of Ge and Si atoms and allows us to measure the composition pof Si/Ge nanostructures ffiffiffi p ffiffiffi [12–17]. Low intermixing on the Si(111)- 3ñ 3 -Bi surface may be important for practical applications of cluster, for instance, in quantum computers. pffiffiffi pffiffiffi Under a Bi layer of the Si(111)- 3ñ 3 -Bi surface Si atoms are arranged as on the unreconstructed Si(111)-1 × 1 surface. This allows us to analyze the cluster structure using cluster geometry and cluster environment in STM images. Taking into account the average cluster size, arrangement of Bi trimmers on top of a cluster and around it, cluster shape and cluster position related to Bi trimers on the substrate, we determined possible atomic configurations of bilayer cluster for different annealing temperatures. 2. Experiment The experiments were carried out in a UHV chamber equipped with Scanning Tunneling Microscopy (STM) (Omicron) and commercial Knudsen cells with a temperature controller. The samples were cut off Si(111) wafer doped by phosphor or boron at the concentration of 1015 cm− 3. A clean Si(111) surface was prepared by flashing at 1250 °C in vacuum of 10− 10 Torr. A Bi terminated Si(111) surface was prepared by adsorption of one atomic layer of Bi on a clean Si(111) surface pffiffiffi at pffiffiffi550–700 °C. This resulted in the formation of the Si(111)- 3ñ 3 -Bi surface structure [18]. After cooling of the sample down to room temperature (RT, 20 °C) Ge was deposited at the rate of 0.03–0.05 ML/min (1 MLGe = 7.8 × 1014 atoms/cm2, 1 BL = 2 ML). Since Bi desorption virtually
0039-6028/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.susc.2013.07.020
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
2
K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx
3. Results and discussion
a
b
c
pffiffiffi pffiffiffi Fig. 1. Monolayer and bilayer Ge cluster formed on the Si(111)- 3ñ 3-Bi surface at RT (a) and after annealing (b), unstable during scanning monolayer cluster in (a) are indicated with arrows. Cluster density in (b) is 5·1012 cm−2. (c) Height profile for monolayer and bilayer cluster in (a). Sample bias +3 V.
is zero at room temperature, Ge deposition was carried at a zero Bi flux. Composition measurements of the SiGe structures were carried out by the method based on the measurement of apparent height difference in STM images between SiGe and Si (111) surfaces [12].
pffiffiffi The pffiffiffi cluster of monolayer and bilayer height formed on a Si(111)3ñ 3-Bi surface after deposition of 0.032 BL of Ge for 1 min at room temperature are shown in Fig. 1а. The height of bilayer cluster measured by STM comprises 0.4 nm (Fig. 1 c). Therefore, in STM images, Ge bilayer cluster are visible 0.1 nm higher than the basic height of Si bilayer (0.3 nm) on Si(111) surface. As it will be shown later, the bilayer cluster are epitaxial and the observed height difference of 0.1 nm can be induced by the Bi presence [12] on top of the bilayer Ge cluster. A part of the monolayer cluster, not stable at RT during scanning, is indicated by arrows in Fig. 1а. Due to a high mobility these cluster change their position and this results in the streaks on the image along the X direction of Fig. 1а. The subsequent annealing at the temperatures above RT results in a decrease of the density of monolayer cluster leaving more stable ones and in an increase of the density of bilayer cluster. The cluster population shown in Fig. 1b is the result of annealing of the original cluster population (Fig. 1 a) at 320 °C for 10 min. These bilayer cluster seem to be identical. However, the detailed analysis shows that the bilayer cluster have different structures and different orientations on the surface. To perform the analysis of the cluster structure, we first determined the cluster size. The average size of bilayer cluster composed of 9.9 atoms was estimated from the volume of the coverage (0.032 BL) and the number of cluster per unit area 5 × 1012 cm−2 (Fig. 1b). Afterwards, the average size of the monolayer cluster composed of 3.6 atoms was estimated from the number of both monolayer and bilayer cluster per unit area (Fig. 1a). Apart from the bilayer cluster the monolayer cluster are mobile on the surface at room temperature as it can be seen from the sequence of STM images in Fig. 2. Highly mobile cluster are separated from the neighboring cluster at a certain distance. Activation energy for the diffusion ofthe mobile cluster can be estimated from diffusion pffiffiffi pffiffiffi 2 d relation: a2 ν exp −E ¼ rt , where α is a period of structure 3ñ 3 kT 12 −1 (0.6 nm), v is the attempt frequency (10 s ) and r is the displacement of the cluster (1 nm for 10 min, as estimated from the STM images in Fig. 2). For these data we evaluated the energy as Ed = 0.9 eV. To explain high diffusion mobility at RT, we havepassumed ffiffiffi pffiffiffi that monolayer cluster diffuse over a Bi layer on the Si(111)- 3ñ 3-Bi surface. We compared the value of 0.9 eV to literature data for the diffusion of single Ge atoms on the As-terminated Si(111) surface [16]. According to Ref. [16], Ge atoms diffuse over a surfactant layer with the low diffusion barrier of 0.25 eV. In the diffusion process Ge atoms do not form chemical bonds with substrate Si atoms. Otherwise, the diffusion of Ge atoms bonded to Si substrate will include re-exchange process between Ge and Bi atoms, which is characterized by a high energy barrier of 0.91 eV [16]. For the cluster composed of 3 atoms the overall energy barrier in the re-exchange process should be considerably higher than 0.91 eV. Therefore, we may conclude that the monolayer cluster do not form chemical bonds with substrate Si atoms and diffuse over the Bi layer.
Fig. 2. STM monolayer cluster images obtained at RT at intervals of 10 min (15 × 15 nm2). Several cluster with a varying arrangement are marked by ovals. Sample bias + 2.8 V, current 30 pA.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx
b
1
1
a
2 1 1
2
2 1 2
c
1
2
[112] 0
8
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15
0
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Fig. 3. Threefold symmetry bilayer cluster of the 1 and 2 orientations: (a) formed at RT; (b), (c) are the same STM image of Ge cluster observed after annealing at 300 °C with different contrast levels. The grid of trimer positions on the Si surface is imposed on the STM image in Fig. 3 (c). The locations of the cluster centers relative to Bi trimers in (c) are different for orientations 1 and 2. Sample bias +3 V.
The shape of bilayer cluster is close to triangle Fig. 3(a). We distinguished two possible orientations of triangles — 1 and 2 relative to the substrate (Fig. 3(a), (b), (c)). A detailed study of the STM images discloses that cluster of the 1 and 2 orientations also differ by their pffiffiffi p ffiffiffi locations relative to Bi trimers on the Si(111)- 3ñ 3 -Bi surface (Fig. 3(b), (c)). In the case of the 1-orientation the center of cluster is located between the trimers (Fig. 3(c))pffiffiffi at p the ffiffiffi point which is a C3v symmetry fixed point of the Si(111)- 3ñ 3 -Bi surface. In the case of the 2-orientation the trimers surrounding a cluster form a rhomb, and the center of the cluster is slightly displaced along a short rhomb diagonal (Fig. 3 (c)). We can find three rhomb orientations — all related to each other by the C3 symmetry operation which is rotation at 120° in the surface plane. The cluster of the 1 and 2-orientation have a different stability towards the annealing. At the temperature increase above 400 °C only cluster of the 1-orientation remain on the surface (Fig. 4). At the same time the process of 2D Ge island and nanostripe formation starts (Fig. 5). pffiffiffiThe pffiffiffiGe stripe with the ultimate width of 0.3 nm (one unit cell of 3ñ 3) formed after annealing at 400 °C for 1 min is shown in (Fig. 5(a)). The apparent height difference between the original Si step and the adjacent SiGe stripe Δ = 0.08 nm was measured (Fig. 5 (b)). This corresponds to Ge concentration in a SiGe stripe of about 80% (Ref. [12]). Considering that the SiGe stripe was formed as a result of transport of the material from cluster to the step edges, we can conclude that the probability of the intermixing between atoms of the Si(111) substrate and Ge atoms in the cluster is not higher than 20%. The possible cluster structures were considered using the cluster size data (9.9 atoms), symmetry and the structural analysis of the cluster topographic STM images and arrangement of the Bi trimers pffiffiffi pffiffiffi enclosing the cluster on the Si(111)- 3ñ 3 -Bi surface. The 2D islands and bilayer cluster observed in STM images (Fig. 4) were formed after annealing at 430 °C. The contrast of the STM image in Fig. 4(a) was adjusted in such a way that the Bi trimers on the top
b
a
c
3
d
pffiffiffi pffiffiffi Fig. 4. 2D islands and the cluster of 1-orientation on the Si(111)- 3ñ 3-Bi terrace after annealing at 430 °C during 20 min. (a) and (b) are the same STM image with different contrast levels: (a) trimers on top of the cluster and islands, (b) trimers on the substrate surface; (c), (d) enlarged cluster “d” and “c” in (b). Trimers enclosing cluster in (c) and (d) are marked with bright points. Sample bias +3 V.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
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K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx
b
a
Si
Si
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Ge B
Ge
15 nm
0
40 nm
height (nm)
0 0.6 0.4
A
0.2 0
B 0
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10
distance (nm)
pffiffiffi pffiffiffi Fig. 5. The apparent height difference between the atoms of the Ge stripe and Si atoms at the Si step edge: (a) the Ge stripe with the width of 0.3 nm (one 3ñ 3 unit cell), (b) the height profile of a Ge stripe and Si atoms at the Si step edge. Sample bias +3 V.
of islands were visible. 2D islands with 2, 3 and more trimers on top were also observed. Bilayer cluster (Fig. 4) differ from the islands in that the only one Bi trimer on top could be recognized. The observed cluster topography allows us to consider the bilayer Ge cluster as a small part of the Ge(111) bilayer. It should be mentioned here that the cluster of bilayer height composed of about 10 atoms can have only one Bi trimer on top. The structure of the cluster consisting of
10 Ge atoms is proposed in Fig. 6(a). The structure includes a Bi trimer on top of the cluster located in the T4 site; T4 site is directly above the second-ayer Ge (or Si) atom position [19]. The location pffiffiffiofpBi ffiffiffi trimers in the T4 position is characteristic for the Si(111)- 3ñ 3 -Bi structure [16,20,21]. In this case the trimers surrounding the cluster are arranged in the shape of a rhomb (Fig. 6(a)). The structure has a mirror plane symmetry (Cs) [22,23] and, according to general threefold
a
b
c
d
e
Fig. 6. The possible atomic structures of the cluster consisting of (a) 10 Ge atoms and (b), (c), (d) 9 Ge atoms. The structures in (c) and (d) correspond to the STM images of the cluster in Fig. 4 (c) and (d), respectively. (e) Side view of the cluster with a Bi trimer in H3.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx
Cluster coverage, relative units
4. Summary
1
2D Ge islands
bilayer clusters 1
monolayer clusters
0.5
bilayer clusters 2
0 RT
100
5
200
400
450
T,°C
Fig. 7. Cluster coverage versus temperature.
In conclusion,pthe Ge ffiffiffi p ffiffiffi cluster of monolayer and bilayer height formed on the Si(111)- 3ñ 3 -Bi surface were studied by STM at different annealing temperatures. Based on the symmetry and structural analysis of the surface topography, the possible atomic configurations of the bilayer cluster with a different stability were proposed. We have established that bilayer cluster are epitaxial while the monolayer cluster are formed and diffuse over a Bi layer. All of the considered cluster are characterized by a low Si fraction in a cluster (b20%) due to a low probability of exchange intermixing of Ge atoms with substrate Si atoms. Acknowledgments
symmetry (C3v) of the substrate, must have three orientations. Cluster with the 2-orientation formed at annealing temperatures below 400 °C (Fig. 3) satisfy pffiffiffithese pffiffiffi symmetry conditions. Trimers of the Si(111)- 3ñ 3 -Bi structure enclosing the bilayer cluster of the 1-orientation can be seen in the STM images with an adjusted contrast level (Fig. 4(b–d)). Two types of 1-oriented cluster were found (Fig. 4(c) and (d)). The observed differences between bilayer cluster in the STM images are determined by the difference in the surroundings by Bi trimers. The trimers around the cluster form a triangle with truncated corners (Fig. 4(c) and (d)). The arrangement of the trimers around cluster looks mirror symmetrical (Cs). However, the shape of the combined system of the cluster and the trimers around the cluster reveals only rotational symmetry C3 and has not any mirror (Cs) symmetry. Based on these data we considered the model where the trimer in the center of a cluster is located in the H3 — threefold hollow site [19]. The possible structures for this case are shown in Fig. 6(b), (c) and (d). The structures of cluster shown in Fig. 6(b), (c) and pffiffiffi (d) pffiffiffi differ in the positions of the center of cluster relative to Bi-( 3ñ 3 )domains. The comparison of the cluster structures and their STM images (Fig. 4(c) and (d)) confirm that the structures in Fig. 6(c) and (d) satisfy the symmetry conditions. The structure shown in Fig. 6 (b) was not found out in our experiments. In all described structures we did not consider the Bi passivation of the dangling bonds. The cluster evolution during annealing is schematically summarized in the phase diagram in Fig. 7. The cluster fractions are expressed in relative units of coverage. Based on the cluster evolution during annealing we can conclude that the cluster of 1-orientation (Fig. 6(c), (d)) are the most stable ones. One can see that the cluster of 1orientation in Fig. 6(c) and (d) occupy the smallest areas and have a minimal dangling bonds number. Thus, in the area occupied by cluster pffiffiffi depicted in Fig. 6(а) and (b) four trimers of the surface structure 3ñ p ffiffiffi 3 can be placed, while, in the area occupied by the cluster depicted in Fig. 6(c) and (d), only three trimers can be placed.
We acknowledge the financial support by Russian Foundation for Basic Research (Grants 13-02-00706-a, 11-07-00475-а, 13-0200201-а). References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Y.P. Zhang, L. Yan, S.S. Xie, S.J. Pang, H.-J. Gao, Surf. Sci. 497 (2002) L60. Z.A. Ansari, T. Arai, M. Tomitori, Surf. Sci. 574 (2005) L17. H. Asaoka, V. Cherepanov, B. Voigtlander, Surf. Sci. 588 (2005) 19. T. Sekiguchi, Sh. Yoshida, Y. Shiren, K.M. Itoh, J. Myslivecek, B. Voigtlander, J. Appl. Phys. 101 (2007) 081702; T. Sekiguchi, Sh. Yoshida, Y. Shiren, K.M. Itoh, J. Myslivecek, B. Voigtlander, Appl. Phys. Lett. 90 (2007) 013108. S. Filimonov, V. Cherepanov, Y. Hervieu, B. Voigtlander, Phys. Rev. B 76 (2007) 035428. T.D. Ladd, J.R. Goldman, F. Yamaguchi, Y. Yamamoto, E. Abe, K.M. Itoh, Phys. Rev. Lett. 89 (2002) 017901. H.H. Chang, M.Y. Lai, J.H. Wei, C.M. Wei, Y.L. Wang, Phys. Rev. Lett. 92 (2004) 066103. V.G. Kotlyar, A.V. Zotov, A.A. Saranin, T.V. Kasyanova, M.A. Cherevik, I.V. Pisarenko, V.G. Lifshits, Phys. Rev. B 66 (2002) 165401. M.Y. Lai, Y.L. Wang, Phys. Rev. B 64 (2001) 241404. M.Y. Lai, Y.L. Wang, Phys. Rev. Lett. 81 (1998) 164. F.-Ch. Chuang, B. Lui, C.-Zh. Wang, T.-L. Chan, K.-M. Ho, Surf. Sci. 598 (2005) L339. N. Paul, S. Filimonov, V. Cherepanov, M. Cakmak, B. Voigtlander, Phys. Rev. Lett. 98 (2007) 166104. B. Voigtlander, A. Zinner, T. Weber, H.P. Bonzel, Phys. Rev. B 51 (1995) 7583. M. Kawamura, N. Paul, V. Cherepanov, B. Voigtländer, Phys. Rev. Lett. 91 (2003) 096102. Y.-J. Ko, K.J. Chang, J.-Y. Yi, Phys. Rev. B 60 (1999) 1777. K. Schroeder, A. Antons, R. Berger, S. Blügel, Phys. Rev. Lett. 88 (2002) 046101. D. Kandel, E. Kaxiras, Phys. Rev. Lett. 75 (1995) 2742; D. Kandel, E. Kaxiras, Solid State Phys. 54 (2000) 219. K. Romanyuk, J. Mysliveček, V. Cherepanov, T. Sekiguchi, S. Yoshida, K.M. Itoh, B. Voigtländer, Phys. Rev. B 75 (2007) 241309(R). J.E. Northrup, Phys. Rev. Lett. 53 (1984) 683. K.J. Wan, T. Guo, W.K. Ford, J.C. Hermanson, Phys. Rev. B 44 (1991) 3471. R. Shioda, A. Kawazu, A.A. Baski, C.F. Quate, J. Nogami, Phys. Rev. B 48 (1993) 4895. K. Romanyuk, V. Cherepanov, B. Voigtländer, Phys. Rev. Lett. 99 (2007) 126103. K. Romanyuk, V. Cherepanov, B. Voigtlander, Phys. Rev. B 83 (20) (2011) 205413.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
August 02, 2013;
Model: Gulliver 5
Surface Science xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Surface Science journal homepage: www.elsevier.com/locate/susc
Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant K.N. Romanyuk a,b, A.A. Shklyaev a,b, B.Z. Olshanetsky a,⁎ a b
Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia Novosibirsk State University, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 19 March 2013 Accepted 15 July 2013 Available online xxxx Keywords: Scanning tunneling microscopy Semiconducting surfaces Nanostructures Surfactants
a b s t r a c t pffiffiffi pffiffiffi Submonolayer Ge cluster grown by molecular beam epitaxy on the Si(111)- 3ñ 3-Bi surface were studied using scanning tunneling microscopy. The cluster of monolayer and bilayer height containing 3–4 and 9–10 atoms, respectively, have been grown at room temperature. We have found that the monolayer cluster are mobile and diffuse over Bi layer at room temperature, whilepbilayer ffiffiffi pffiffiffi cluster are epitaxial and can be classified by positions of the cluster relative to Bi trimers on the Si(111)- 3ñ 3-Bi surface. In the temperature range of 100 °C–400 °C, the cluster population consists of two types of bilayer cluster with Bi trimers in T4 and H3 positions on the cluster, correspondingly. At temperatures above 400 °C only the most stable atomic configuration with Bi trimer in H3 position on the bilayer cluster is remained on the surface. © 2013 Published by Elsevier B.V.
1. Introduction The cluster nucleating at submonolayer coverage on the Si(111) surface [1–5] have attracted considerable attention due to their potential application in nanotechnology. Such cluster are promising for the use in nanoelectronics and quantum computers [6] and can serve as model objects for studying surface atomic processes at early growth stages [5]. Comprising a fixed number of atoms they are also known as “magic” cluster [7–10]. Specifically, the cluster size can be close to the critical island size and depends on the size of the surface unit cell [3,11]. The relation between cluster structure and cluster stability is an important question since it is addressed to the functionality of the cluster and it influences the uniformity of cluster in the population. The investigation of the stable atomic configurations of surface cluster is of interest for the theoretical study, as well. However, experimental observation of the cluster structure is the challenge due to a particularly small scale. Using structural and symmetry analysis of the surface topography we were able to distinguish different atomic configurations of the cluster with a different stability in a cluster population. In this contribution, we present an experimental study of the formation and evolution p offfiffiffi Ge pffiffifficluster and cluster structure during annealing on the Si(111)- 3ñ 3 -Bi surface. The cluster of monolayer and bilayer height were grown by submonolayer pffiffiffi pffiffiffi deposition of Ge at room temperature (RT) on the Si(111)- 3ñ 3-Bi surface and were
⁎ Corresponding author. Tel.: +7 383 3333 286; fax: +7 383 333 3502. E-mail address: [email protected] (B.Z. Olshanetsky).
systematically studied at different annealing temperatures using the scanning tunneling microscope (STM) method. We used Bi surfactant since Bi suppresses the exchange intermixing of Ge and Si atoms and allows us to measure the composition pof Si/Ge nanostructures ffiffiffi p ffiffiffi [12–17]. Low intermixing on the Si(111)- 3ñ 3 -Bi surface may be important for practical applications of cluster, for instance, in quantum computers. pffiffiffi pffiffiffi Under a Bi layer of the Si(111)- 3ñ 3 -Bi surface Si atoms are arranged as on the unreconstructed Si(111)-1 × 1 surface. This allows us to analyze the cluster structure using cluster geometry and cluster environment in STM images. Taking into account the average cluster size, arrangement of Bi trimmers on top of a cluster and around it, cluster shape and cluster position related to Bi trimers on the substrate, we determined possible atomic configurations of bilayer cluster for different annealing temperatures. 2. Experiment The experiments were carried out in a UHV chamber equipped with Scanning Tunneling Microscopy (STM) (Omicron) and commercial Knudsen cells with a temperature controller. The samples were cut off Si(111) wafer doped by phosphor or boron at the concentration of 1015 cm− 3. A clean Si(111) surface was prepared by flashing at 1250 °C in vacuum of 10− 10 Torr. A Bi terminated Si(111) surface was prepared by adsorption of one atomic layer of Bi on a clean Si(111) surface pffiffiffi at pffiffiffi550–700 °C. This resulted in the formation of the Si(111)- 3ñ 3 -Bi surface structure [18]. After cooling of the sample down to room temperature (RT, 20 °C) Ge was deposited at the rate of 0.03–0.05 ML/min (1 MLGe = 7.8 × 1014 atoms/cm2, 1 BL = 2 ML). Since Bi desorption virtually
0039-6028/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.susc.2013.07.020
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
2
K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx
3. Results and discussion
a
b
c
pffiffiffi pffiffiffi Fig. 1. Monolayer and bilayer Ge cluster formed on the Si(111)- 3ñ 3-Bi surface at RT (a) and after annealing (b), unstable during scanning monolayer cluster in (a) are indicated with arrows. Cluster density in (b) is 5·1012 cm−2. (c) Height profile for monolayer and bilayer cluster in (a). Sample bias +3 V.
is zero at room temperature, Ge deposition was carried at a zero Bi flux. Composition measurements of the SiGe structures were carried out by the method based on the measurement of apparent height difference in STM images between SiGe and Si (111) surfaces [12].
pffiffiffi The pffiffiffi cluster of monolayer and bilayer height formed on a Si(111)3ñ 3-Bi surface after deposition of 0.032 BL of Ge for 1 min at room temperature are shown in Fig. 1а. The height of bilayer cluster measured by STM comprises 0.4 nm (Fig. 1 c). Therefore, in STM images, Ge bilayer cluster are visible 0.1 nm higher than the basic height of Si bilayer (0.3 nm) on Si(111) surface. As it will be shown later, the bilayer cluster are epitaxial and the observed height difference of 0.1 nm can be induced by the Bi presence [12] on top of the bilayer Ge cluster. A part of the monolayer cluster, not stable at RT during scanning, is indicated by arrows in Fig. 1а. Due to a high mobility these cluster change their position and this results in the streaks on the image along the X direction of Fig. 1а. The subsequent annealing at the temperatures above RT results in a decrease of the density of monolayer cluster leaving more stable ones and in an increase of the density of bilayer cluster. The cluster population shown in Fig. 1b is the result of annealing of the original cluster population (Fig. 1 a) at 320 °C for 10 min. These bilayer cluster seem to be identical. However, the detailed analysis shows that the bilayer cluster have different structures and different orientations on the surface. To perform the analysis of the cluster structure, we first determined the cluster size. The average size of bilayer cluster composed of 9.9 atoms was estimated from the volume of the coverage (0.032 BL) and the number of cluster per unit area 5 × 1012 cm−2 (Fig. 1b). Afterwards, the average size of the monolayer cluster composed of 3.6 atoms was estimated from the number of both monolayer and bilayer cluster per unit area (Fig. 1a). Apart from the bilayer cluster the monolayer cluster are mobile on the surface at room temperature as it can be seen from the sequence of STM images in Fig. 2. Highly mobile cluster are separated from the neighboring cluster at a certain distance. Activation energy for the diffusion ofthe mobile cluster can be estimated from diffusion pffiffiffi pffiffiffi 2 d relation: a2 ν exp −E ¼ rt , where α is a period of structure 3ñ 3 kT 12 −1 (0.6 nm), v is the attempt frequency (10 s ) and r is the displacement of the cluster (1 nm for 10 min, as estimated from the STM images in Fig. 2). For these data we evaluated the energy as Ed = 0.9 eV. To explain high diffusion mobility at RT, we havepassumed ffiffiffi pffiffiffi that monolayer cluster diffuse over a Bi layer on the Si(111)- 3ñ 3-Bi surface. We compared the value of 0.9 eV to literature data for the diffusion of single Ge atoms on the As-terminated Si(111) surface [16]. According to Ref. [16], Ge atoms diffuse over a surfactant layer with the low diffusion barrier of 0.25 eV. In the diffusion process Ge atoms do not form chemical bonds with substrate Si atoms. Otherwise, the diffusion of Ge atoms bonded to Si substrate will include re-exchange process between Ge and Bi atoms, which is characterized by a high energy barrier of 0.91 eV [16]. For the cluster composed of 3 atoms the overall energy barrier in the re-exchange process should be considerably higher than 0.91 eV. Therefore, we may conclude that the monolayer cluster do not form chemical bonds with substrate Si atoms and diffuse over the Bi layer.
Fig. 2. STM monolayer cluster images obtained at RT at intervals of 10 min (15 × 15 nm2). Several cluster with a varying arrangement are marked by ovals. Sample bias + 2.8 V, current 30 pA.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
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Fig. 3. Threefold symmetry bilayer cluster of the 1 and 2 orientations: (a) formed at RT; (b), (c) are the same STM image of Ge cluster observed after annealing at 300 °C with different contrast levels. The grid of trimer positions on the Si surface is imposed on the STM image in Fig. 3 (c). The locations of the cluster centers relative to Bi trimers in (c) are different for orientations 1 and 2. Sample bias +3 V.
The shape of bilayer cluster is close to triangle Fig. 3(a). We distinguished two possible orientations of triangles — 1 and 2 relative to the substrate (Fig. 3(a), (b), (c)). A detailed study of the STM images discloses that cluster of the 1 and 2 orientations also differ by their pffiffiffi p ffiffiffi locations relative to Bi trimers on the Si(111)- 3ñ 3 -Bi surface (Fig. 3(b), (c)). In the case of the 1-orientation the center of cluster is located between the trimers (Fig. 3(c))pffiffiffi at p the ffiffiffi point which is a C3v symmetry fixed point of the Si(111)- 3ñ 3 -Bi surface. In the case of the 2-orientation the trimers surrounding a cluster form a rhomb, and the center of the cluster is slightly displaced along a short rhomb diagonal (Fig. 3 (c)). We can find three rhomb orientations — all related to each other by the C3 symmetry operation which is rotation at 120° in the surface plane. The cluster of the 1 and 2-orientation have a different stability towards the annealing. At the temperature increase above 400 °C only cluster of the 1-orientation remain on the surface (Fig. 4). At the same time the process of 2D Ge island and nanostripe formation starts (Fig. 5). pffiffiffiThe pffiffiffiGe stripe with the ultimate width of 0.3 nm (one unit cell of 3ñ 3) formed after annealing at 400 °C for 1 min is shown in (Fig. 5(a)). The apparent height difference between the original Si step and the adjacent SiGe stripe Δ = 0.08 nm was measured (Fig. 5 (b)). This corresponds to Ge concentration in a SiGe stripe of about 80% (Ref. [12]). Considering that the SiGe stripe was formed as a result of transport of the material from cluster to the step edges, we can conclude that the probability of the intermixing between atoms of the Si(111) substrate and Ge atoms in the cluster is not higher than 20%. The possible cluster structures were considered using the cluster size data (9.9 atoms), symmetry and the structural analysis of the cluster topographic STM images and arrangement of the Bi trimers pffiffiffi pffiffiffi enclosing the cluster on the Si(111)- 3ñ 3 -Bi surface. The 2D islands and bilayer cluster observed in STM images (Fig. 4) were formed after annealing at 430 °C. The contrast of the STM image in Fig. 4(a) was adjusted in such a way that the Bi trimers on the top
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pffiffiffi pffiffiffi Fig. 4. 2D islands and the cluster of 1-orientation on the Si(111)- 3ñ 3-Bi terrace after annealing at 430 °C during 20 min. (a) and (b) are the same STM image with different contrast levels: (a) trimers on top of the cluster and islands, (b) trimers on the substrate surface; (c), (d) enlarged cluster “d” and “c” in (b). Trimers enclosing cluster in (c) and (d) are marked with bright points. Sample bias +3 V.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
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pffiffiffi pffiffiffi Fig. 5. The apparent height difference between the atoms of the Ge stripe and Si atoms at the Si step edge: (a) the Ge stripe with the width of 0.3 nm (one 3ñ 3 unit cell), (b) the height profile of a Ge stripe and Si atoms at the Si step edge. Sample bias +3 V.
of islands were visible. 2D islands with 2, 3 and more trimers on top were also observed. Bilayer cluster (Fig. 4) differ from the islands in that the only one Bi trimer on top could be recognized. The observed cluster topography allows us to consider the bilayer Ge cluster as a small part of the Ge(111) bilayer. It should be mentioned here that the cluster of bilayer height composed of about 10 atoms can have only one Bi trimer on top. The structure of the cluster consisting of
10 Ge atoms is proposed in Fig. 6(a). The structure includes a Bi trimer on top of the cluster located in the T4 site; T4 site is directly above the second-ayer Ge (or Si) atom position [19]. The location pffiffiffiofpBi ffiffiffi trimers in the T4 position is characteristic for the Si(111)- 3ñ 3 -Bi structure [16,20,21]. In this case the trimers surrounding the cluster are arranged in the shape of a rhomb (Fig. 6(a)). The structure has a mirror plane symmetry (Cs) [22,23] and, according to general threefold
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Fig. 6. The possible atomic structures of the cluster consisting of (a) 10 Ge atoms and (b), (c), (d) 9 Ge atoms. The structures in (c) and (d) correspond to the STM images of the cluster in Fig. 4 (c) and (d), respectively. (e) Side view of the cluster with a Bi trimer in H3.
Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020
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In conclusion,pthe Ge ffiffiffi p ffiffiffi cluster of monolayer and bilayer height formed on the Si(111)- 3ñ 3 -Bi surface were studied by STM at different annealing temperatures. Based on the symmetry and structural analysis of the surface topography, the possible atomic configurations of the bilayer cluster with a different stability were proposed. We have established that bilayer cluster are epitaxial while the monolayer cluster are formed and diffuse over a Bi layer. All of the considered cluster are characterized by a low Si fraction in a cluster (b20%) due to a low probability of exchange intermixing of Ge atoms with substrate Si atoms. Acknowledgments
symmetry (C3v) of the substrate, must have three orientations. Cluster with the 2-orientation formed at annealing temperatures below 400 °C (Fig. 3) satisfy pffiffiffithese pffiffiffi symmetry conditions. Trimers of the Si(111)- 3ñ 3 -Bi structure enclosing the bilayer cluster of the 1-orientation can be seen in the STM images with an adjusted contrast level (Fig. 4(b–d)). Two types of 1-oriented cluster were found (Fig. 4(c) and (d)). The observed differences between bilayer cluster in the STM images are determined by the difference in the surroundings by Bi trimers. The trimers around the cluster form a triangle with truncated corners (Fig. 4(c) and (d)). The arrangement of the trimers around cluster looks mirror symmetrical (Cs). However, the shape of the combined system of the cluster and the trimers around the cluster reveals only rotational symmetry C3 and has not any mirror (Cs) symmetry. Based on these data we considered the model where the trimer in the center of a cluster is located in the H3 — threefold hollow site [19]. The possible structures for this case are shown in Fig. 6(b), (c) and (d). The structures of cluster shown in Fig. 6(b), (c) and pffiffiffi (d) pffiffiffi differ in the positions of the center of cluster relative to Bi-( 3ñ 3 )domains. The comparison of the cluster structures and their STM images (Fig. 4(c) and (d)) confirm that the structures in Fig. 6(c) and (d) satisfy the symmetry conditions. The structure shown in Fig. 6 (b) was not found out in our experiments. In all described structures we did not consider the Bi passivation of the dangling bonds. The cluster evolution during annealing is schematically summarized in the phase diagram in Fig. 7. The cluster fractions are expressed in relative units of coverage. Based on the cluster evolution during annealing we can conclude that the cluster of 1-orientation (Fig. 6(c), (d)) are the most stable ones. One can see that the cluster of 1orientation in Fig. 6(c) and (d) occupy the smallest areas and have a minimal dangling bonds number. Thus, in the area occupied by cluster pffiffiffi depicted in Fig. 6(а) and (b) four trimers of the surface structure 3ñ p ffiffiffi 3 can be placed, while, in the area occupied by the cluster depicted in Fig. 6(c) and (d), only three trimers can be placed.
We acknowledge the financial support by Russian Foundation for Basic Research (Grants 13-02-00706-a, 11-07-00475-а, 13-0200201-а). References [1] [2] [3] [4]
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Please cite this article as: K.N. Romanyuk, et al., Structure and stability of Ge cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci. (2013), http://dx.doi.org/10.1016/j.susc.2013.07.020