Applied Surface Science 226 (2004) 191–196
Formation of nanoclusters on silicon from carbon deposition
V. Palermo*, D. Jones
ISOF, Istituto per la Sintesi Organica e Fotoreattivita’, CNR, via Gobetti 101, I-40129 Bologna, Italy
Abstract
Changes in the structure of silicon surfaces can be induced by adsorption of carbon-containing molecules followed by thermal
treatments. Clean Si(111) surfaces, prepared in vacuum and exposed to different adsorbants such as methanol or carbon
monoxide, change their structures with the formation of self-organised nanostructures (15–50 nm diameter) after suitable UHV
annealing procedures. Evolution of the size and density per unit area over different heating periods indicates that the structures
are nucleated by carbon atoms present on the surface while their growth derives from mobile surface silicon atoms during the
annealing process. Methanol adsorbs dissociatively on silicon at room temperature thus leading to a high density of nucleation
centres, but when the process is applied to partially oxide-masked silicon surfaces using CO as adsorbant the nanostructures
form preferentially at the Si/SiO2 interface around the mask border thus offering the possibility to grow more ordered selforganised nanoscale patterns. Monte Carlo simulations of this process correlate well with STM measurements.
# 2003 Elsevier B.V. All rights reserved.
PACS: 68.35.-p; 61.16.Ch; 81.07.-b; 81.16.Rf
Keywords: STM; Nanostructures; Silicon; Adsorption
1. Introduction
The continuous drive for extreme miniaturisation
and the vicinity of the physical limits of classical
lithographic techniques in the microelectronics industry has led to increased efforts to find novel fabrication
techniques. The use of self-organisation phenomena is
one of the most promising, simple and economic
methods for producing nanometric structures on surfaces. Small lines and ordered structures can be selfassembled on surfaces and, in particular, on silicon
[1,2]. It has been previously shown how contaminant
molecules can adsorb onto silicon surfaces and nucle-
*
Corresponding author. Tel.: þ39-051-639-8336;
fax: þ39-051-639-8349.
E-mail address: [email protected] (V. Palermo).
ate the growth of small, protruding nanostructures
(‘‘nanoislands’’) on the otherwise flat silicon surface
during UHV annealing [3].
When surface oxide layers are removed from silicon
by UHV thermal annealing at 700–800 8C, the process
begins through the creation of small oxide-free areas
called ‘‘voids’’ which enlarge radially until the whole
surface is oxide free [4–7]. The above-mentioned
nanoislands grow at void nucleation points with concomitant radial enlargement of the voids during
annealing. Carbon-containing compounds present in
UHV systems as well as extrinsic surface impurities
have been cited as the source of this contamination
[3,8,9]. The nanoislands form over the whole surface
and their formation mechanism is still not clear.
This paper compares the results obtained by adsorbing carbon monoxide and methanol on silicon at room
temperature to nucleate nanoisland growth. Many
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2003.11.021
192
V. Palermo, D. Jones / Applied Surface Science 226 (2004) 191–196
different molecules were tested, but only the most
significant results are reported here. The molecules are
adsorbed on clean silicon surfaces prepared in
vacuum, and the nanoisland growth process is followed using STM. Furthermore, samples partially
masked by an oxide layer are used to have a selective
growth of the nanoislands into voids of the oxide layer.
Finally, Monte Carlo simulations are used to reproduce the island growth process.
temperature but, after degassing overnight at 150 8C,
heated for 10 min at 800 8C to create the voids [3].
These samples were then directly exposed to CO in the
vacuum system. Further annealing at 800 8C for
10 min led to the radial expansion of the voids with
loss of the oxide and the creation of the first series of
nanoislands around the original void perimeter.
Repeating this process led to the formation of a
second, larger concentric circular arrangement of
nanoislands.
2. Experimental
3. Results and discussion
Samples of Si(111) (p-doped, 0.015 O cm) were
degassed in vacuum for 5 h at 650 8C and then flashed
repeatedly at 1200 8C to clean the surface. Surface
morphology was observed in situ between heating
cycles with an Omicron UHV-STM operating at a
base pressure of 1011 Torr. Typical measurement
parameters were sample bias þ1.2 V, tunnelling current 50 pA. The adsorbant gases were introduced into
the vacuum system using a leak valve and the pressure
controlled with an ion gauge. The molecules tested
were oxygen, water, methanol, carbon monoxide,
carbon dioxide and methane. During gas adsorption
the surface was observed periodically with STM.
Between the measurements, the STM tip was retracted
to avoid shadowing effects. After gas adsorption, the
silicon was resistively heated and nanoisland growth
monitored periodically with the STM.
For the experiments with oxide-free voids in the
native oxide layer the samples were not flashed at high
Methanol is known to interact strongly with the
Si(111) 7 7 reconstructed surface dissociating upon
adsorption [10,11]. The adsorption of methanol can, in
fact, be followed by the corresponding darkening of
dangling bonds in the STM image. The well-known
7 7 unit cell has many highly reactive dangling
bonds; in particular each cell has 12 symmetric dangling bonds on so-called ‘‘adatoms’’ and six on ‘‘rest’’
atoms (see Fig. 1a). The CH3OH molecule reacts with
an adatom of the 7 7 unit cell forming Si–OCH3
[10]. The remaining hydrogen atom reacts with a Si
rest atom. Since there are 12 adatoms and six rest
atoms in the unit cell, only half of the adatoms are
darkened to STM observations, and the rest remain
visible (Fig. 1b). After half of the adatoms have
formed SiOCH3 the surface saturates and no more
molecules adsorb. The surface reaches saturation after
only a few langmuirs; even after more than 30 L the
Fig. 1. (a) STM image of the clean Si(1 1 1) surface before methanol adsorption, 8 nm 8 nm. An half unit cell is highlighted, with adatoms
(circles) and rest atoms (crosses). Rest atoms are not visible. (b) The same surface after 30 l exposure to methanol, 20 nm 20 nm. (c) Zoom
out, 80 nm 80 nm.
V. Palermo, D. Jones / Applied Surface Science 226 (2004) 191–196
193
Fig. 2. (a) Nanoislands grown after methanol adsorption and heating 5 s at 800 8C, 80 nm 80 nm. The typical silicon 7 7 pattern has been
restored. (b) After 1 min at 8008, 200 nm 200 nm. (c) After 18 min at 8008, 200 nm 200 nm. All images have been gradient-filtered.
Although their positions are quite random, there is a
preponderance of nucleated structures along step
edges and domain boundaries (Fig. 2). This indicates
that most of the carbon present on the surface desorbs
during the heating, but carbon atoms adsorbed at
defect sites (i.e. steps or domain boundaries) are more
stable and nucleate nanoisland growth.
The same adsorption procedure was repeated using
carbon monoxide instead of methanol. CO is known to
adsorb onto and desorb from silicon surfaces in its
neutral molecular state without reacting chemically
with the surface. STM images of the silicon surface,
even after 180 L of CO exposure did not, in fact, even
show the presence of molecular adsorption at room
temperature. This can be due to selective preferential
adsorption of CO at surface defect sites which were
too few to be observed with certainty, or to the
elevated mobility of CO molecules on the surface,
2500
160
140
100
1500
80
1000
60
40
Volume
20
Islands /um 2
2000
120
V (nm 3)
STM images like the one in Fig. 1b remain exactly the
same. This leads to a very uniform coverage equivalent to 9:6 1013 carbon atoms/cm2. The process has
been described in detail in [10].
Observing the surface on a larger scale (Fig. 1c), it
is possible to note that the adsorption is quite uniform
on the whole surface. The only irregularities are
observed at the border between different 7 7
domains, already present on the clean surface, which
remain visible as dark lines on the contaminated one.
Following methanol adsorption the samples were
heated to 800 8C, allowed to cool and then examined
again with the STM. Although the heating cycle
restored the crystalline surface with the 7 7 reconstruction, a large number of nanoislands was observed
over the whole surface (Fig. 2a). The island density is
1500 60 mm2.
It is known that annealing alkyl monolayers on
silicon surfaces at 500 8C leads to the disappearance
of C–H and Si–H vibrational modes and the appearance of a Si–C vibrational mode [12]. Although
photoemission results [11] indicate a uniform dispersion of SiC over the surface, the real space STM
images in Fig. 2 show a quite clean 7 7 surface
with the contaminating atoms concentrated into the
nanoislands.
Heating for longer times the number of islands per
unit area remains the same but their dimensions
increase (see Fig. 2b and c and Fig. 3). During the
heating no more carbon contaminants are provided to
the surface, so island growth must be due to silicon
atoms diffusing on the surface and feeding the islands.
500
Isl. Density
0
0
90
150
330
450
1050
Heating time (s)
Fig. 3. Evolution of nanoisland volume and density with heating
time.
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V. Palermo, D. Jones / Applied Surface Science 226 (2004) 191–196
which excludes their detection with the STM
tip. However, some CO adsorption must have occurred
because nanoisland nucleation and growth was indeed
observed after the subsequent heating step (10 min at
800 8C). At 230 L CO exposure of silicon, Chamberlain et al. did, in fact, observe on silicon the presence
of small amounts of CO in their TPD spectra [13].
The island distribution density was only 360 30
mm2 (25% of the density obtained with methanol)
and approximate island dimensions, given the limitations of such measurements with STM, much larger
with respect to methanol-nucleated islands (510 60
nm3 versus 120 20 nm3).
Similar treatment of blank samples with no exposure to CO gave no significant production of surface
nanoislands. Analogous experiments using other
molecules, like oxygen, water, carbon dioxide or
methane, also did not show any nanoisland growth.
Given that CO did, in some way, react at some
particular sites on the surface, we exposed partially
oxide-masked silicon surfaces to the gas with the aim
of presenting active sites to the adsorbant. These
surfaces, prepared in UHV [3,14] contain surface
oxide voids of 50–100 nm diameter with naked silicon
delimited by an exposed Si/SiO2 interface (see
Fig. 4a). At the centre of these voids there is a
nanostructure produced during the original radial
enlargement of the void. After cooling, these samples
were exposed to 180 L of CO at room temperature and
then heated again to 800 8C for 10 min. During this
second heating cycle, with consequent further radial
enlargement of the voids with oxide decomposition,
new series of nanostructures were formed in a circular
arrangement corresponding to the original perimeter
of the oxide void with the Si/SiO2 interface during CO
exposure (see Fig. 4b). When the same samples were
heated without having been exposed to CO, there was
simply void enlargement without the formation of
secondary nanostructures around the void perimeter.
This suggests that, although the whole surface
(oxide and naked silicon), is exposed to CO, the
gas reacts chemically only at the Si/SiO2 interface
around the oxide-free void perimeter. These reaction
sites thus become nucleation sites, leading to growth
of the circular arrangements of nanoislands during
subsequent annealing. A similar phenomenon of preferential CO decomposition at the perimeter of oxygen
islands has been already observed [15].
On repeating the CO exposure/heating cycle, a
second circle of nanoislands formed around the newly
expanded surface void perimeter (Fig. 4c). This process can, in principle, be repeated to obtain a series of
concentric circular arrangements of nanostructures,
similar to a nanoscopic ‘‘dartboard’’, the only limit
being the minimum distance between neighbouring
voids. The island circles are very irregular and their
dimension quite variable.
However, the most interesting prospect which this
site-dependant chemical reaction, leading to the
nucleation and growth of nanostructures, brings to
light its possible use for producing surface nanostructures by exploiting this ‘‘edge effect’’. The use of
selective adsorption along the edge of an oxide pattern
followed by the formation of nanostructures only
Fig. 4. (a) Nanoscopic void in the surface oxide layer, 120 nm 120 nm. (b) Nanoisland circle created after CO adsorption and heating at
800 8C, 400 nm 400 nm. (c) After another cycle of CO adsorption and heating, 590 nm 590 nm.
V. Palermo, D. Jones / Applied Surface Science 226 (2004) 191–196
18
195
400
(a)
16
(b)
350
14
300
12
250
10
200
8
150
6
100
4
50
2
0
0
0
200
400
600
800
1000
0
200
400
600
800
1000
Fig. 5. (a) Real STM profile of islands grown in a void. (b) Simulated profile of island growth, arbitrary units.
along that edge through self-organisation processes
would allow a significant reduction in feature dimensions whilst still using classical processing techniques.
The exploitation of this ‘‘reactive adsorption edge
effect’’ to produce nanoscale structures is analogous
to the ‘‘light propagation edge effect’’ exploited by Li
et al. [16].
For nanoisland growth, silicon atoms have to move
uphill on the surface. Similar mound growth on various surfaces has been explained as a consequence of
the Ehrlich–Schwöbel (ES) barrier effect where a
mobile surface atom when approaching a step from
above finds a potential barrier at the step and is
reflected. This leads to a net uphill flux of atoms on
crystal steps, thus being a major cause of roughening
[17,18].
The presence of the ES barrier effect on silicon
surfaces is controversial: STM measurements during
epitaxial growth suggested that the barrier is weak or
even absent on Si(111) [19], but thermal relaxation
measurements of hillocks and craters at high temperature showed the presence of a significant ES barrier
effect [20]. Thus, it would seem that the ES barrier is a
sufficient but not a necessary condition [21] for mound
growth on silicon surfaces.
In order to assess the effect of the ES barrier on
nanoisland growth in our system, Monte Carlo simulations of surface roughening were carried out.
A simple, mono-dimensional system was used
where the probability of each atom of diffusing
depends upon the number of nearest neighbours and
upon an interaction energy Es [22]. Details of the
algorithm and the parameters used will be reported
elsewhere [23].
The starting configuration was a flat surface with a
central peak, like the one shown in Fig. 4a, and
simulation temperature was set to 800 8C. Fig. 5
compares a real STM profile taken from a void with
the simulated one. Best results were obtained using an
Es interaction energy of 0.4 eV.
We can see that, even though the complexity and the
dimensionality of the simulated system is much simpler than the real one, in both cases sharp mounds
develop on the otherwise flat surface. This indicates
that the presence of an ES barrier is sufficient to
produce structures similar to those observed experimentally, although further studies, using more complex, 2-D systems, are necessary.
4. Conclusions
Carbon-containing molecules which react with the
silicon surface nucleate the growth of nanoislands
during UHV annealing. Island dimension and number
depends strongly on molecule–surface interaction.
Methanol, which adsorbs dissociatively over the
whole silicon surface, leads to a large number of
structures whereas CO reacts preferentially at Si/
SiO2 interfaces.
The presence of carbon in the nucleating molecule
is a condition necessary but not sufficient to nucleate
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V. Palermo, D. Jones / Applied Surface Science 226 (2004) 191–196
island growth. Molecules without carbon (oxygen,
water) or interacting weakly with silicon (methane,
CO2), do not nucleate island growth. The carbon
adsorbed on the surface through the contaminating
molecules is a main component of the central island
core, but further growth is due to a flux of silicon
atoms from the surrounding surface. The site-specific
reactivity of CO offers the possibility of producing
ordered arrangements of nanostructures by exploiting
this surface adsorption edge effect.
The initial results from Monte Carlo simulations
indicate that the presence of an Ehrlich–Schwöbel
barrier can cause the growth of well-defined, protruding nanoislands as those observed experimentally.
Acknowledgements
A Ph.D. grant (V.P.) from the CNR in Rome is
gratefully acknowledged. Thanks are also due to Mr.
G. Bragaglia for technical assistance with the STM
instrumentation.
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