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ELSEVIER Surface Science 377-379 (1997) 1006-1009
Nucleation of homoepitaxial Si chains on Si( 001)
at room temperature
J. van Wingerden, M.J. Haye, P.M.L.O. Scholte *, F. Tuinstra
Delft University of Technology, Applied Physics Department, Lorentzweg I, 2628 CJ De& The Netherlands
Received 1 August 1996; accepted for publication 15 October 1996
Abstract
Results are presented of a microscopic study of the formation of epitaxial Si dimer chains. We lind that dimers adsorb on a
limited number of sites only. Interactions between dimers and diffusing adatoms lead to the formation of three-atom clusters which
can be extended to diluted lines of dimers. Only two types of lines are observed: lines along [ 1 lo] and lines along [310]. These lines
connect to each other and form a random network. Upon further deposition, the diluted dinner lines transform into epitaxial dimer
rows. This transition starts at the end of the lines by reorienting the dimers in the line and adding mobile adatoms.
Keywords: Adatoms; Clusters; Epitaxy; Growth; Models of surface kinetics; Molecular beam epitaxy; Nucleation; Scanning tunneling
microscopy; Self-assembly; Semiconducting tilms; Silicon; Single crystal epitaxy; Stepped single crystal surfaces; Sticking; Surface
diffusion; Surface energy; Surface structure, morphology, roughness, and topography; Vicinal single crystal surfaces
1. Introduction In an alternative approach, structures with
atomically sharp boundaries can be made by using
With the ongoing miniaturisation in electronic the self-assembly of atoms during deposition.
devices, standard lithographic techniques such as Examples are the spontaneous formation of Ge
electron-beam lithography and masking are near- hut clusters during the epitaxial growth of Ge on
ing their limits. To be able to produce conducting Si(OO1) [3], and SiGe quantum dots grown by the
structures, even with atomic dimensions, new tech- codeposition of Si and Ge [4]. The morphology
niques have to be developed which do not involve and crystal structure of the dots and clusters are
the use of statistical processes such as etching. At determined by preferred adsorption sites, diffusion
the moment, considerable effort is going on to properties of the adatoms and lattice mismatch.
investigate the potential of scanning probe micro- Also, one-dimensional structures can be formed
scopes to manipulate atoms and clusters on a by processes which involve self-assembly. In partic-
surface into ordered structures (see, for example, ular, during the first stages of the growth of group
the quantum corrals of Eigler et al. [l]), or to III, IV and V materials onto Si and Ge (001)
modify the surface locally with atomic resolution surfaces, chains of dimers are formed [5-71.
VI* In the present paper, we present a detailed study
of the formation of Si chains on a Si(OO1) surface.
* Corresponding author. Fax: +31 15 278 3251; We will discuss the formation of the chains starting
e-mail: scholte@duttncb.tn.tudelft.nl from the formation of single dimers. In Section 2
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved
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2. J. van Wingerden et al. j Surface Science 377-379 (1997) 1006-1009
the experimental details are given. Then we will
discuss the preferred adsorption sites of single
dimers and their extension to long chains, before
proceeding to a brief discussion of the electronic
structure of a single dimer chain.
2. Experimental
The experiments were performed in a UHV
system with a base pressure of 5 x lo-l1 Torr. A
commercial beetle-type scanning tunnelling micro-
scope (STM) was used for measuring constant-
current STM images. The surface dynamics at
room temperature was studied by capturing STM
images at 10 s intervals and combining them into
a movie. Clean Si(OOl)-(2 x 1) surfaces with low
defect density and monolayer steps were prepared
by flashing the surface to 1250°C. Submonolayers
Fig. 1. Filled ((a) and (c)) and empty ((b) and (d)) state images
of Si were deposited using a commercial miniature
of isolated Si dimers on a Si(OOl)-(2 x 1) substrate. The letters
electron-beam evaporator. For studies of the room- refer to the adsorption sites of Fig. 2. Tunnelling conditions:
temperature growth processes, we waited at least (a)-(d) Itunnel=0 4 nA; (a) and (c) Vbias=1.3 V, (b)
4 h before deposition after flashing. V,,ias=-1.3V, (d) Vbias=-1.15V.
3. Results
The mobility of Si adatoms is too high to be
able to observe them with STM at room temper-
ature. Only adatoms which are adsorbed at surface
steps or other surface defects can be observed in
empty-state images. Therefore, the pathway for
the formation of dimers from single adatoms is
not directly accessible by STM. Indirect evidence
for this pathway can be obtained from the relative
abundance of the preferred adsorption sites of the Fig. 2. Schematic drawings of the orientations of isolated
dimers. However, discussion of this formation adsorbed dimers and clusters of two dimers on neighbouring
process is beyond the scope of this paper, and will substrate dimer rows with their notations.
be undertaken elsewhere [8].
In Fig. 1 both empty-state and filled-state images 67% are at C-positions, while the rest are at either
of single Si dimers are shown. The notation for A or B positions. The ratio between the occupan-
indicating the different adsorption sites is indicated cies of A and B sites is approximately 1:lO.
in Fig. 2. At higher coverages, the dimers start to form
At room temperature, the abundance of D clusters. In Fig. 3 two important types of clusters
dimers is negligible. Furthermore, we have only are shown: the twin and the cross [9]. The twin
observed D dimers in the direct neighbourhood of can be considered as a rearranged BB couple,
other dimers or of surface steps, i.e. they are never where two new substrate dimer bonds are formed
isolated. Of all the isolated dimers, approximately (see inset in Fig. 3b). It is characterised by the

3. J. van Wingerden et al. / Surface Science 377-379 (1997) 1006-1009
Fig. 4. A V-shaped chain of C dimers along [310] directions.
(a) Filled-state image, (b) empty-state image. (c) Metastable
adsorption sites for adatoms near to a C dimer. Tunnelling
conditions: Itunnel= 0.4nA, (a) Vbias=1.3V, (b) Vbias=
-1.3 v.
Fig. 3. Filled- and empty-state images of a twin and a cross
structure. (a) Filled-state image of a twin, (b) empty-state image
of a twin, (c) filled-state image of a cross structure, (d) empty-
state image of a cross structure. The insets in (b) and (d) show this process will lead to a chain of C dimers in the
a schematic drawing of a twin and a cross. The horizontal grey [ 3 lo] direction [ 81. An example is shown in Fig. 4.
lines indicate substrate dimer bonds. Upon further deposition, the diluted dimer rows
are transformed into epitaxial chains of BD dimers.
fainter appearance of the B dimers which are We observed that this conversion takes place exclu-
involved in the twin, relative to non-bonded B sively at the end of the [ 1lo] dimer rows. Both B
dimers (see Fig. 3a). and D dimers were observed as the last dimers in
The cross structure is essentially a C dimer with the chain. This corroborates a model in which the
two adatoms attached to it, as shown in the inset conversion initiates by forming a B dimer from an
of Fig. 3d. The twin is a “closed” structure which atom of the outer C dimer with the adatom at the
cannot be easily extended into a chain of dimers. end of the diluted dimer row. Repeating this
In contrast, the cross structure can. By adding process once more results in the BB twin. Finally,
extra atoms at the adatoms of the cross, a so-called a collapse of the twin leads to the formation of an
diluted dimer row of C dimers along [ 1101 is epitaxial BD segment, yielding an end sequence of
formed [9]. At the ends of the diluted dimer row, C-adatom-B-D.
single adatoms can be attached. It should be noted The conductivity of the chains demands special
that this diluted dimer row is essentially non- attention, since it is not trivial that a single row
epitaxial, since the orientation of the dimers is of atoms will be conducting or semiconducting,
parallel to the substrate dimers. even if the element is. For example, ab-initio
Also, the C dimers may form ordered structures. calculations by Brocks et al. [lo] indicate that a
There are two preferred adsorption sites for single single chain of Al dimers may be semiconducting.
adatoms in the neighbourhood of a C dimer. The On the other hand, in the case of Au on Si( 11l),
first was indicated above in the cross structure. It 1D conduction in a cluster of lines has been
is in line with the C-dimer bond (see Fig. 3d). The observed [ 111.
second preferred adsorption site is indicated in We have calculated the band structure of a single
Fig. 4c. If two adatoms are subsequently adsorbed epitaxial dimer chain on a Si(OO1) surface, using
at this site, a new C dimer is formed. Repeating a Car-Parrinello ab-initio method. A detailed

4. J. van Wingerden et al. / Surface Science 377-379 (1997) 1006-1009 1009
discussion of the results will be published elsewhere along the row. These states appear to be localized
[ 121. From the electron density obtained, the corre- on the chain. However, the chains are semicon-
sponding STM image can be computed. The result ducting and not metallic. Our calculations show
is shown in the inset of Fig. 5. The surface was that it is possible to fill the empty states by doping
prepared by depositing Si at a slightly elevated the Si chains with alkali atoms. We find that the
temperature, causing the dimers to line up in dimer band structure of the chain essentially does not
rows. The calculation result, placed in line with a alter, while the Fermi level shifts upward, resulting
dimer row, shows a good agreement with the in a Si dimer chain which conducts in the direction
experiment. We find that the epitaxial dimer chains along the chain [ 131.
have empty electron states with a high dispersion
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