structure and stability of ge cluster on si(111) surface in the presence of bi surfactant

Surface Science xxx (2013) xxx–xxx

SUSC-20005; No of Pages 5 August 02, 2013; Model: Gulliver 5

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Surface Science

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Structure and stability of Ge cluster on Si(111) surface in the presence ofBi surfactant

K.N. Romanyuk a,b, A.A. Shklyaev a,b, B.Z. Olshanetsky a,⁎a Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russiab Novosibirsk State University, Novosibirsk 630090, Russia

⁎ Corresponding author. Tel.: +7 383 3333 286; fax: +E-mail address: [email protected] (B.Z. Olshanetsky).

0039-6028/$ – see front matter © 2013 Published by Elsehttp://dx.doi.org/10.1016/j.susc.2013.07.020

Please cite this article as: K.N. Romanyuk, et(2013), http://dx.doi.org/10.1016/j.susc.2013

a b s t r a c tffiffiffip ffiffiffip

a r t i c l e i n f o

Article history:Received 19 March 2013Accepted 15 July 2013Available online xxxx

Keywords:Scanning tunneling microscopySemiconducting surfacesNanostructuresSurfactants

SubmonolayerGe cluster grown bymolecular beam epitaxy on the Si(111)- 3ñ 3-Bi surfacewere studied usingscanning tunneling microscopy. The cluster of monolayer and bilayer height containing 3–4 and 9–10 atoms, re-spectively, have been grown at room temperature.We have found that themonolayer cluster aremobile and dif-fuse over Bi layer at room temperature, while bilayer cluster are epitaxial and can be classified by positions of thecluster relative to Bi trimers on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface. In the temperature range of 100 °C–400 °C, the

cluster population consists of two types of bilayer clusterwith Bi trimers in T4 andH3 positions on the cluster, cor-respondingly. At temperatures above 400 °C only the most stable atomic configuration with Bi trimer in H3 po-sition 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) sur-face [1–5] have attracted considerable attention due to theirpotential application in nanotechnology. Such cluster are promisingfor the use in nanoelectronics and quantum computers [6] and canserve as model objects for studying surface atomic processes at earlygrowth stages [5]. Comprising a fixed number of atoms they are alsoknown as “magic” cluster [7–10]. Specifically, the cluster size can beclose to the critical island size and depends on the size of the surfaceunit cell [3,11].

The relation between cluster structure and cluster stability is animportant question since it is addressed to the functionality of thecluster and it influences the uniformity of cluster in the population.The investigation of the stable atomic configurations of surfacecluster is of interest for the theoretical study, as well. However,experimental observation of the cluster structure is the challengedue to a particularly small scale. Using structural and symmetryanalysis of the surface topography we were able to distinguishdifferent atomic configurations of the cluster with a differentstability in a cluster population.

In this contribution, we present an experimental study of theformation and evolution of Ge cluster and cluster structure duringannealing on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface. The cluster of monolayer

and bilayer height were grown by submonolayer deposition of Ge atroom temperature (RT) on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface and were

7 383 333 3502.

vier B.V.

al., Structure and stability of G.07.020

systematically studied at different annealing temperatures usingthe scanning tunnelingmicroscope (STM)method.Weused Bi surfactantsince Bi suppresses the exchange intermixing of Ge and Si atoms andallows us to measure the composition of Si/Ge nanostructures[12–17]. Low intermixing on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface may be

important for practical applications of cluster, for instance, in quantumcomputers.

Under a Bi layer of the Si(111)-ffiffiffi3

pñ

ffiffiffi3

p-Bi surface Si atoms are

arranged as on the unreconstructed Si(111)-1 × 1 surface. This allowsus to analyze the cluster structure using cluster geometry and clusterenvironment in STM images. Taking into account the average clustersize, 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 fordifferent annealing temperatures.

2. Experiment

The experiments were carried out in a UHV chamber equippedwith Scanning Tunneling Microscopy (STM) (Omicron) and com-mercial Knudsen cells with a temperature controller. The sampleswere cut off Si(111) wafer doped by phosphor or boron at theconcentration of 1015 cm−3. A clean Si(111) surface was preparedby flashing at 1250 °C in vacuum of 10−10 Torr. A Bi terminatedSi(111) surface was prepared by adsorption of one atomic layer ofBi on a clean Si(111) surface at 550–700 °C. This resulted in theformation of the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface structure [18]. After

cooling of the sample down to room temperature (RT, 20 °C) Gewas 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

e cluster on Si(111) surface in the presence of Bi surfactant, Surf. Sci.

http://dx.doi.org/10.1016/j.susc.2013.07.020

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a

b

c

Fig. 1.Monolayer and bilayer Ge cluster formed on the Si(111)-ffiffiffi3

pñ

ffiffiffi3

p-Bi surface at RT (a)

and after annealing (b), unstable during scanning monolayer cluster in (a) areindicated with arrows. Cluster density in (b) is 5·1012 cm−2. (c) Height profile formonolayer and bilayer cluster in (a). Sample bias +3 V.

2 K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx

is zero at room temperature, Ge deposition was carried at a zero Biflux. Composition measurements of the SiGe structures were carriedout by the method based on the measurement of apparent heightdifference in STM images between SiGe and Si (111) surfaces [12].

Fig. 2. STMmonolayer cluster images obtained at RT at intervals of 10 min (15 × 15 nm2).current 30 pA.

Please cite this article as: K.N. Romanyuk, et al., Structure and stability of G(2013), http://dx.doi.org/10.1016/j.susc.2013.07.020

3. Results and discussion

The cluster of monolayer and bilayer height formed on a Si(111)-ffiffiffi3

pñ

ffiffiffi3

p-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 mea-sured 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 Sibilayer (0.3 nm) on Si(111) surface. As it will be shown later, the bi-layer cluster are epitaxial and the observed height difference of0.1 nm can be induced by the Bi presence [12] on top of the bilayerGe cluster. A part of the monolayer cluster, not stable at RT duringscanning, is indicated by arrows in Fig. 1а. Due to a high mobilitythese cluster change their position and this results in the streaks onthe image along the X direction of Fig. 1а.

The subsequent annealing at the temperatures above RT results ina decrease of the density of monolayer cluster leaving more stableones and in an increase of the density of bilayer cluster. The clusterpopulation shown in Fig. 1b is the result of annealing of the originalcluster population (Fig. 1 a) at 320 °C for 10 min. These bilayer clusterseem to be identical. However, the detailed analysis shows that thebilayer cluster have different structures and different orientationson the surface. To perform the analysis of the cluster structure, wefirst determined the cluster size. The average size of bilayer clustercomposed of 9.9 atomswas 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 com-posed of 3.6 atomswas estimated from the number of both monolay-er and bilayer cluster per unit area (Fig. 1a).

Apart from the bilayer cluster the monolayer cluster are mobileon the surface at room temperature as it can be seen from thesequence of STM images in Fig. 2. Highly mobile cluster are separatedfrom the neighboring cluster at a certain distance. Activation energyfor the diffusion of the mobile cluster can be estimated from diffusionrelation: a2ν � exp −Ed

kT

� �¼ r2

t , where α is a period of structureffiffiffi3

pñ

ffiffiffi3

p

(0.6 nm), v is the attempt frequency (1012 s−1) and r is thedisplacement of the cluster (1 nm for 10 min, as estimated from theSTM images in Fig. 2). For these data we evaluated the energy asEd = 0.9 eV. To explain high diffusion mobility at RT, we have assumedthat monolayer cluster diffuse over a Bi layer on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi

surface. We compared the value of 0.9 eV to literature data for thediffusion of single Ge atoms on the As-terminated Si(111) surface[16]. According to Ref. [16], Ge atoms diffuse over a surfactant layerwith the low diffusion barrier of 0.25 eV. In the diffusion processGe atoms do not form chemical bonds with substrate Si atoms.Otherwise, the diffusion of Ge atoms bonded to Si substrate willinclude re-exchange process between Ge and Bi atoms, which ischaracterized by a high energy barrier of 0.91 eV [16]. For the clustercomposed of 3 atoms the overall energy barrier in the re-exchangeprocess should be considerably higher than 0.91 eV. Therefore, wemay conclude that the monolayer cluster do not form chemicalbonds with substrate Si atoms and diffuse over the Bi layer.

Several cluster with a varying arrangement are marked by ovals. Sample bias +2.8 V,



1

1

1

1

12

2

2

2

b

c

0 15nm

0 15nm

0 8

a

nm

1

2

[ 2]11

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 withdifferent contrast levels. The grid of trimer positions on the Si surface is imposed on theSTM 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.

a

c d

Fig. 4. 2D islands and the cluster of 1-orientation on the Si(111)-ffiffiffi3

pñ

ffiffiffi3

p-Bi terrace after annea

levels: (a) trimers on top of the cluster and islands, (b) trimers on the substrate surface; (c), (d)bright points. Sample bias +3 V.

3K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx


The shape of bilayer cluster is close to triangle Fig. 3(a). Wedistinguished two possible orientations of triangles — 1 and 2 relativeto the substrate (Fig. 3(a), (b), (c)). A detailed study of the STM imagesdiscloses that cluster of the 1 and 2 orientations also differ by theirlocations relative to Bi trimers on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface

(Fig. 3(b), (c)). In the case of the 1-orientation the center of clusteris located between the trimers (Fig. 3(c)) at the point which is aC3v symmetry fixed point of the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface. In the

case of the 2-orientation the trimers surrounding a cluster form arhomb, and the center of the cluster is slightly displaced along ashort rhombdiagonal (Fig. 3 (c)).We can find three rhomb orientations— all related to each other by the C3 symmetry operation which is rota-tion at 120° in the surface plane. The cluster of the 1 and 2-orientationhave a different stability towards the annealing. At the temperature in-crease above 400 °C only cluster of the 1-orientation remain on the sur-face (Fig. 4). At the same time the process of 2D Ge island andnanostripe formation starts (Fig. 5). The Ge stripe with the ultimatewidth of 0.3 nm (one unit cell of

ffiffiffi3

pñ

ffiffiffi3

p) formed after annealing at

400 °C for 1 min is shown in (Fig. 5(a)). The apparent height differ-ence between the original Si step and the adjacent SiGe stripe Δ =0.08 nm was measured (Fig. 5 (b)). This corresponds to Ge concen-tration in a SiGe stripe of about 80% (Ref. [12]). Considering thatthe SiGe stripe was formed as a result of transport of the materialfrom cluster to the step edges, we can conclude that the probabilityof the intermixing between atoms of the Si(111) substrate and Geatoms in the cluster is not higher than 20%.

The possible cluster structures were considered using the clustersize data (9.9 atoms), symmetry and the structural analysis of thecluster topographic STM images and arrangement of the Bi trimersenclosing the cluster on the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface. The 2D

islands and bilayer cluster observed in STM images (Fig. 4) wereformed after annealing at 430 °C. The contrast of the STM image inFig. 4(a) was adjusted in such a way that the Bi trimers on the top

b

ling at 430 °C during 20 min. (a) and (b) are the same STM image with different contrastenlarged cluster “d” and “c” in (b). Trimers enclosing cluster in (c) and (d) aremarkedwith



0 40

a

0 15

A

B

Ge

Sib

nm

nm 0 5 10

0.2

0

0.6

0.4

distance (nm)

heig

ht (

nm)

A

B

Ge

Si

Fig. 5. The apparent height difference between the atoms of the Ge stripe and Si atoms at the Si step edge: (a) the Ge stripewith thewidth of 0.3 nm (oneffiffiffi3

pñ

ffiffiffi3

punit cell), (b) the height

profile of a Ge stripe and Si atoms at the Si step edge. Sample bias +3 V.

4 K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx

of islands were visible. 2D islands with 2, 3 and more trimers on topwere also observed. Bilayer cluster (Fig. 4) differ from the islands inthat the only one Bi trimer on top could be recognized. The observedcluster topography allows us to consider the bilayer Ge cluster as asmall part of the Ge(111) bilayer. It should be mentioned here thatthe cluster of bilayer height composed of about 10 atoms can haveonly one Bi trimer on top. The structure of the cluster consisting of

a

c

e

Fig. 6. The possible atomic structures of the cluster consisting of (a) 10 Ge atoms and (b), (c), (dFig. 4 (c) and (d), respectively. (e) Side view of the cluster with a Bi trimer in H3.


10 Ge atoms is proposed in Fig. 6(a). The structure includes a Bi trimeron top of the cluster located in the T4 site; T4 site is directly above thesecond-ayer Ge (or Si) atom position [19]. The location of Bi trimers inthe T4 position is characteristic for the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi struc-

ture [16,20,21]. In this case the trimers surrounding the cluster arearranged in the shape of a rhomb (Fig. 6(a)). The structure has a mirrorplane symmetry (Cs) [22,23] and, according to general threefold

b

d

) 9 Ge atoms. The structures in (c) and (d) correspond to the STM images of the cluster in



mon

olay

ercl

uste

rs

bilayerclusters 2

bilayerclusters 1

2D G

e is

land

s

RT 100 200 400 4500

0.5

1

T,°C

Cluster coverage,relative units

Fig. 7. Cluster coverage versus temperature.

5K.N. Romanyuk et al. / Surface Science xxx (2013) xxx–xxx

symmetry (C3v) of the substrate, must have three orientations.Cluster with the 2-orientation formed at annealing temperaturesbelow 400 °C (Fig. 3) satisfy these symmetry conditions.

Trimers of the Si(111)-ffiffiffi3

pñ

ffiffiffi3

p-Bi structure enclosing the bilayer

cluster of the 1-orientation can be seen in the STM images with anadjusted contrast level (Fig. 4(b–d)). Two types of 1-oriented clusterwere found (Fig. 4(c) and (d)). The observed differences betweenbilayer cluster in the STM images are determined by the differencein the surroundings by Bi trimers. The trimers around the clusterform a triangle with truncated corners (Fig. 4(c) and (d)). The arrange-ment of the trimers around cluster looksmirror symmetrical (Cs). How-ever, the shape of the combined system of the cluster and the trimersaround the cluster reveals only rotational symmetry C3 and has notany mirror (Cs) symmetry. Based on these data we considered themodel where the trimer in the center of a cluster is located in the H3

— threefold hollow site [19]. The possible structures for this case areshown in Fig. 6(b), (c) and (d). The structures of cluster shown inFig. 6(b), (c) and (d) differ in the positions of the center of clusterrelative to Bi-(

ffiffiffi3

pñ

ffiffiffi3

p)domains. The comparison of the cluster struc-

tures and their STM images (Fig. 4(c) and (d)) confirm that the struc-tures in Fig. 6(c) and (d) satisfy the symmetry conditions. Thestructure shown in Fig. 6 (b) was not found out in our experiments. Inall described structures we did not consider the Bi passivation of thedangling bonds.

The cluster evolution during annealing is schematically summarizedin the phase diagram in Fig. 7. The cluster fractions are expressed inrelative units of coverage. Based on the cluster evolution duringannealing 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 1-orientation in Fig. 6(c) and (d) occupy the smallest areas and have aminimal dangling bonds number. Thus, in the area occupied by clusterdepicted in Fig. 6(а) and (b) four trimers of the surface structure

ffiffiffi3

pñffiffiffi

3p

can be placed, while, in the area occupied by the cluster depictedin Fig. 6(c) and (d), only three trimers can be placed.


4. Summary

In conclusion, theGe cluster ofmonolayer and bilayer height formedon the Si(111)-

ffiffiffi3

pñ

ffiffiffi3

p-Bi surface were studied by STM at different

annealing temperatures. Based on the symmetry and structural analysisof the surface topography, the possible atomic configurations of the bi-layer cluster with a different stability were proposed. We haveestablished that bilayer cluster are epitaxial while the monolayercluster are formed and diffuse over a Bi layer. All of the consideredcluster are characterized by a low Si fraction in a cluster (b20%) due toa low probability of exchange intermixing of Ge atoms with substrateSi atoms.

Acknowledgments

We acknowledge the financial support by Russian Foundationfor Basic Research (Grants 13-02-00706-a, 11-07-00475-а, 13-02-00201-а).

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structure and stability of ge cluster on si(111) surface in the presence of bi surfactant

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