Morphology of perylene thin films on SiOx/Si(1 0 0) and SiO2/Si(1 0 0): A spectroscopic and microscopic study of the influence of the preparation parameters

Article abstract of DOI:10.1016/j.cplett.2009.08.015

Thin films of perylene on SiO_x/Si(1 0 0) and SiO₂/Si(1 0 0) substrates have been studied by X-ray photoelectron spectroscopy and atomic force microscopy. These investigations reveal that structure, morphology, and growth modes depend

Full text of DOI:10.1016/j.cplett.2009.08.015

Chemical Physics Letters 479 (2009) 76–80
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Morphology of perylene thin films on SiO /Si(1 0 0) and SiO /Si(1 0 0): A
x
2
spectroscopic and microscopic study of the influence of the preparation parameters
a,b,*
, X. Yu , S. Schmitt , C. Heske b,c, E. Umbach b,d
b
b
M.B. Casu
a
University of Tübingen, Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 8, 72076 Tübingen, Germany
Experimentelle Physik II, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4003, USA
FZ Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Thin films of perylene on SiO /Si(1 0 0) and SiO /Si(1 0 0) substrates have been studied by X-ray photo-
electron spectroscopy and atomic force microscopy. These investigations reveal that structure, morphol-
ogy, and growth modes depend on the preparation parameters. By varying the deposition rate between
x
2
Received 6 April 2009
In final form 5 August 2009
Available online 11 August 2009
0.8 and 16 nm/min, a transition from island growth mode, with large and isolated crystallites, to a homo-
geneous film growth is observable.
We can predict the growth of perylene on technologically relevant substrates: homogeneous smooth
films at lower substrate temperature (below 200 K) and single grains, enough for single nanodevices,
when the film is grown at higher substrate temperature.
Ó 2009 Elsevier B.V. All rights reserved.
1
. Introduction
Organic electronic devices offer an interesting alternative to
metals such as zirconium or lanthanum [14,15]. In a bottom-gate
device, the organic semiconductor is deposited on the insulator;
therefore, it is very important to gain detailed knowledge of
the interface between an organic layer and its insulator substrate.
In particular, an understanding of the growth mechanisms is
necessary in order to control the preparation of optimized
devices.
inorganic semiconductor electronics due to low-cost deposition
methods, flexible substrates, and simple packaging [1–4]. Organic
field effect transistors (OFETs) are part of this rapidly developing
field, but remain a technological challenge [5–8]. The most impor-
tant part of an OFET is represented by the organic layer. Organic
single crystals of tetracene [9] and pentacene [10] have been used
in OFETs, reaching relatively high room temperature mobility val-
Perylene (C20H12) is one of the simplest aromatic hydrocarbons
and its crystal and molecular structure has been thoroughly stud-
ied already in the 1960s [16,17]. Today, its high room temperature
2
2
À1 À1
2
À1 À1
ues of 0.4/0.5 cm /V s. However, the growth of single crystals for
(RT) hole (0.4 cm V
s
) and electron mobility (5.5 cm V
s
use in OFETs is not an easy task. Organic single crystals are often
fragile and small. Their integration in a device is difficult, in partic-
ular regarding the organic/insulator interface and the organic/me-
tal contacts. A promising alternative is given by thin films of small
molecules [7]. Usually, in a thin film transistor, the molecules can
be vapor-deposited under vacuum, spin-coated, dip-coated, or
printed on the insulator material. All of these deposition tech-
niques are of significant importance for low-cost electronics.
An appropriate choice of the insulator, i.e., one that optimally
fulfils the main role of insulating the gate contact from the active
layer, is very important. The most commonly used insulators are
silicon-based [7–11], some alternatives are given by organic insu-
lators [12,13] or metal oxides mixed with transition or rare-earth
when the electric field is parallel to the crystallographic b axis)
[18] make perylene an interesting model system. In a recent paper
we have analyzed the growth mechanisms of perylene on Si(1 1 1)
and Si(1 0 0) using a multi-technique approach [19]. In the present
Letter we report an investigation of perylene thin films on SiO
x
/
Si(1 0 0) and SiO /Si(1 0 0) substrates. Particular emphasis will be
2
on the morphology of the perylene films, as investigated by atomic
force microscopy (AFM) and X-ray photoelectron spectroscopy
(XPS). Apart from the investigation in [19], only a few works have
studied the perylene thin film growth dynamics or the interaction
with different metal substrates [20–24], and very little work has
been done on oxide substrates [25–27].
2
. Experiment
*
Corresponding author. Address: University of Tübingen, Institute of Physical
and Theoretical Chemistry, Auf der Morgenstelle 8, 72076 Tübingen, Germany. Fax:
Substrate preparation and organic thin film deposition were
+49 7071 29 5490.
E-mail address: benedetta.casu@uni-tuebingen.de (M.B. Casu).
performed in an ultra-high vacuum (UHV) chamber (base pressure
0
009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.08.015

M.B. Casu et al. / Chemical Physics Letters 479 (2009) 76–80
77
À10
5
 10
mbar) equipped with an ion sputter gun, two separate
to probe a wide range of different preparations and their influence
on the film morphology.
The AFM measurements were performed under ambient condi-
tions in non-contact mode with a Topometrix scanning probe
microscope.
gas dosing systems for argon and oxygen, respectively, a low
energy electron diffraction (LEED) optics, a standard twin anode
X-ray source (the presented spectra are taken with Al K radiation),
a
and a hemispherical electron analyzer (VG CLAM II). The prepara-
tion section of this chamber contains a home-made Knudsen cell
and is separated from the measuring section by means of a shutter
to preserve the base pressure.
3
. Results
The substrates were single-side polished p-Si(1 0 0) wafers
CrysTec, Germany) with an undoped resistivity of 2 kX cm. The
3
.1. Perylene thin films on SiO /Si(1 0 0)
x
(
cleaning of the Si(1 0 0) surface was performed according to a stan-
dard recipe [28]. Without any previous ex situ treatment, it was
outgassed in UHV below the temperature at which the oxide is
removed. Then the temperature was raised rapidly to 1520 K for
Fig. 1 shows 10
thick perylene films deposited on SiO
temperatures and using different deposition rates. Fig. 1a shows a
typical AFM image of a film grown with = 1 nm/min and a sub-
strate temperature of 200 K. Perylene forms very small grains.
The maximum peak-to-valley height is 701 nm and the average
film roughness is 118 nm. An increase of the deposition rate
l
m  10
l
m AFM images of nominally 120 nm-
x
/Si(1 0 0) at various substrate
U
1
–2 min. Finally, the substrate was slowly cooled down to room
temperature. The obtained clean Si(1 0 0) surface is flat and the
disorder is minimized, as found in our AFM and LEED characteriza-
tion. We prepared two different types of oxidized substrates. One
was prepared by exposing the cleaned Si(1 0 0) surface to molecu-
(U = 5 nm/min) at the same substrate temperature (Fig. 1b) leads
to smaller and more numerous grains, indicating a larger number
of nucleation sites. The maximum peak-to-valley value is 130 nm
À6
lar oxygen (partial pressure 1 Â 10 mbar) at RT for 1 min. This
x
gives a very thin oxide layer (denoted SiO in the following) in
(
average roughness = 7 nm). Further increase of the deposition rate
which more than one sub-oxide component is present [29]. The
preparation of the silicon dioxide on Si(1 0 0) was carried out by
to 17 nm/min gives a smooth and homogeneous film (the maxi-
mum peak-to-valley height and the average roughness are 67
and 7 nm, respectively).
exposing the substrate to molecular oxygen (partial pressure
À3
1
 10 mbar) at 870 K for 30 min. This method gives oxide layers
AFM images are different when the substrate is kept at RT
2
that are about 2 nm thick (denoted SiO in the following). Each step
(Fig. 1d–f). The grain size is relevantly larger in comparison with
of the substrate preparation was checked by XPS, LEED, and AFM.
Perylene (Aldrich) was purified using thermal gradient sublima-
the previous preparation and the film growth shows a different
behavior. The film grown with 1.6 nm/min deposition rate
tion and deposited in situ under UHV conditions on SiO
x
/Si(1 0 0)
(Fig. 1d) is characterized by a maximum peak-to-valley variation
and SiO /Si(1 0 0) by organic molecular beam deposition (OMBD).
2
of 1268 nm, while this value decreases to 1181 and 585 nm for
evaporation rates of 5 and 11.6 nm/min, respectively (Fig. 1e and
f). In the film shown in Fig. 1f, the island density is significantly
higher than for the other two films prepared at RT.
The AFM micrograph of perylene thin films deposited on a
heated substrate (400 K) are shown in Fig. 1g and h. At first glance,
it seems that the growth at lower deposition rate (Fig. 1g)
The deposition was monitored by XPS and the nominal thickness
was determined using the attenuation of the Si 2p substrate signal
and calibrating the Knudsen cell. Furthermore, the homogeneity of
the film thickness was probed by XPS, checking simultaneously the
C 1s and the Si 2p core level spectra. The deposition rate (U) was
varied in the range between 0.8 and 16 nm/min. The substrate
temperature (Tsub) was varied between 200 K and 400 K in order
Fig. 1. Ten square micron AFM images of 120 nm-thick perylene films on SiO
min, respectively; at Tsub = 300 K and deposition rates of: (d) 1.6 nm/min, (e) 5 nm/min, (f) 11.6 nm/min, respectively; at Tsub > 300 K and deposition rates of: (g) 0.8 nm/min,
h) 2.5 nm/min, respectively. The grey scale corresponds to the first derivative to better represent the grain structure. The z-axis is expanded in comparison to the x- and y-
axes.
x
/Si(1 0 0), grown at Tsub = 200 K with deposition rates of: (a) 1 nm/min, (b) 5 nm/min, (c) 17 nm/
(

7
8
M.B. Casu et al. / Chemical Physics Letters 479 (2009) 76–80
a
b
c
1
.0
.5
1.0
0.5
1.0
0.5
1
5
nm/min
nm/min
17 nm/min
0
1
1
.6 nm/min
5
nm/min
0.8 nm/min
2.5 nm/min
1.6 nm/min
0
20
40
60
80
100 120
0
40
80
120
160
0
40
80
120
160
Nominal Thickness (nm)
Nominal Thickness (nm)
Nominal Thickness (nm)
Fig. 2. Mg K
a x
XPS Si 2p signal attenuation versus film thickness for perylene on SiO /Si(1 0 0): (a) Tsub = 200 K, (b) Tsub = 300 K, and (c) Tsub > 300 K. The relative deposition
rates are indicated in each figure. All graphs have the same ordinate scale. The lines connecting the data points are intended as a guide to the eye.
resembles the mechanism of the films shown in Fig. 1d and e.
However, the details are actually quite different: the grains are lar-
ger; their average lateral size is several lm. The grains grown with
deposition rate of 2.5 nm/min exhibit flat top facets (the maximum
peak-to-valley value is 1000 nm), while the grains in Fig. 1d and e
do not.
XPS measurements performed on the films grown at 200 K cor-
roborate these observation (Fig. 2a). The substrate signals are rap-
idly and completely attenuated for higher deposition rates, i.e., for
nominal film thicknesses of 18 and 25 nm, respectively. The
growth mode is Stranski–Krastanov, as will be discussed below.
The relative Si 2p line intensity versus the film thickness ob-
tained from the XPS measurements performed on the films grown
at RT is shown in Fig. 2b. The Si 2p intensity decreases as the per-
ylene film covers the substrate. The decrease is quite gradual in
comparison with the case of the substrate kept at 200 K, indicating
a strong tendency for three-dimensional (3D) growth, in agree-
ment with the respective AFM micrographs. The same holds true
for the films grown at higher substrate temperature. Also in this
case, the intensity of the Si 2p signal decreases very gradually
path (k, IMFP) of the Si 2p photoelectrons is about 3.7 nm and the
sampling depth is about 11.1 nm. In case of layer-by-layer growth,
95% of the collected photoelectrons come from a thickness within
3k below the surface. Thus, the signal should be completely atten-
uated above the thickness corresponding to three times the IMFP.
None of the films exhibits such XPS signal attenuation, indicating
that only Stranski–Krastanov and 3D island growth modes are
present in our study. The different (‘slower’) exponential decay
for the films grown at substrate temperatures P RT indicates a
strong tendency towards 3D growth with increasing substrate
temperature.
3.2. Perylene thin films deposited on SiO /Si(1 0 0)
2
Fig. 3 shows 10
l
m  10
lm AFM images of 100 nm thick per-
2
ylene films deposited on SiO /Si(1 0 0) varying the substrate tem-
perature and using different deposition rates. The films grown at
a substrate temperature of 200 K are smooth and homogeneous.
The film grown with 0.8 nm/min deposition rate (Fig. 3a) is charac-
terized by a maximum peak-to-valley variation of 70 nm, while
this value decreases to 40 nm when the evaporation rate is
10 nm/min (Fig. 3b). The relative Si 2p line intensities versus the
film thickness are shown in Fig. 4a. The Si 2p intensity decreases
to zero as the perylene film covers the substrate. For the higher
(Fig. 2c), indicating strong island growth. Comparing the attenua-
tion behavior of the substrate signal as a function of thickness,
we observe a different decay for the three substrate temperatures.
Under the present experimental conditions, the inelastic mean free
2
Fig. 3. Ten square micron AFM images of 120 nm-thick perylene films deposited on SiO /Si(1 0 0) at Tsub = 200 K with deposition rates of: (a) 0.8 nm/min, (b) 10 nm/min,
respectively; at Tsub = 300 K and deposition rates of: (c) 3.1 nm/min, (d) 13.3 nm/min, (e) 16 nm/min, respectively; at Tsub > 300 K and deposition rates of: (f) 0.8 nm/min, (g)
0 nm/min, respectively. The grey scale corresponds to the first derivative to better represent the grain structure. The z-axis is expanded in comparison to the x- and y-axes.
1

M.B. Casu et al. / Chemical Physics Letters 479 (2009) 76–80
79
a
b
c
1
.0
.5
1.0
0.5
1.0
0.5
0
1
.8 nm/min
0 nm/min
0
3
.1 nm/min
0
1
.8 nm/min
0 nm/min
1
1
3.3 nm/min
6 nm/min
0
40
80
120
160
0
40
80
120
160
0
40
80
120
160
Nominal Thickness (nm)
Nominal Thickness (nm)
Nominal Thickness (nm)
a 2
Fig. 4. Mg K XPS Si 2p signal attenuation versus film thickness for perylene on SiO /Si(1 0 0): (a) Tsub = 200 K, (b) Tsub = 300 K, and (c) Tsub > 300 K. The relative deposition
rates are indicated in each figure. All graphs have the same ordinate scale. The lines connecting the data points are intended as a guide to the eye.
deposition rate, the process is quite fast, leading to an almost com-
plete extinction of the Si 2p signal when the adlayer is 17 nm thick.
ory of nucleation when looking at the thermodynamic aspects of
thin film growth [30,31]. In particular, they affect the change in
the free energy of the adsorbate. This influence has several conse-
quences: (1) the higher the substrate temperature, the bigger the
size of the critical nucleus, i.e., the size at which the island becomes
stable with the addition of only one more molecule. (2) A nucle-
ation barrier may exist at higher substrate temperature. (3) The
number of supercritical nuclei decreases with temperature, i.e.,
there are fewer nuclei at higher temperature. (4) Increasing the
deposition rate results in smaller islands. (5) A continuous film
can be grown at lower film thicknesses [30,31]. These are exactly
the phenomena shown by the sequence of micrographs in Figs. 1
and 3. The growth mechanisms in the case of perylene deposited
x
As in the case of perylene deposited on SiO /Si(1 0 0), AFM
images are significantly different when the substrate is kept at
RT (Fig. 3c–e). Fig. 3c shows a typical AFM image of a film grown
with a deposition rate of 3.1 nm/min. Perylene forms large grains,
their lateral size extends to several
valley value is 1800 nm. Increasing the deposition rate to
3.3 nm/min (Fig. 3d) leads to a size reduction of the voids, while
the grain size is also smaller (in average about 1.7 m). As in the
case of the SiO /Si(1 0 0) substrate, this again indicates a larger
lm. The maximum peak-to-
1
l
x
number of nucleation sites. The maximum peak-to-valley height
is 1300 nm. Further increase of the deposition rate up to 16 nm/
min gives a smoother (the average roughness is 225 nm) but again
grain-like film (Fig. 3e). XPS measurements performed on these
films confirm these coverage observations (Fig. 4b). For all deposi-
tion rates the substrate signal remains very strong after the various
deposition steps, despite the coexistent presence of a strong C 1s
signal (not shown here). This is in agreement with the AFM images
that indicate an island growth mode.
AFM micrographs of perylene films deposited on the heated
substrate are shown in Fig. 3f and g. Also in this case, it seems that
the growth at lower deposition range (Fig. 3f) resembles the mech-
anisms of the film shown in Fig. 3c. Once more, the details are quite
different: the grains are smaller; they exhibit sharp edges and flat
x 2
on SiO /Si(1 0 0) and SiO /Si(1 0 0) have different energy barriers
to overcome in order to reach the stage of the film formation.
The associated inhomogeneous growth is clearly evidenced by
the different attenuation of the Si 2p signal versus the film thick-
ness for the films grown at the different temperatures (see Figs.
2 and 4).
In addition to the thermodynamic and kinetic aspects, the sub-
strate–molecule interaction has to be taken into account. Perylene
growth on SiO
x
/Si(1 0 0) resembles that of perylene adsorbed on
Si(1 0 0) [19]. The low exposure (45 L) used to prepare the SiO
x
/
Si(1 0 0) surface gives rise to a two-dimensional island nucleation
of the oxide layer and, therefore, the Si(1 0 0) is not completely
covered [29]. Thus, we may reckon that the Si(1 0 0) regions play
the major role in the film growth, under these conditions. By inter-
preting the attenuation behavior of the Si 2p substrate signal in
Figs. 2 and 4, it can be seen how it is apparently energetically more
top facets. Their average lateral size is about 1.4 lm. The XPS Si 2p
signal does not decrease very fast (Fig. 4c), in agreement with an
island growth mode. Increasing the deposition rate up to 10 nm/
min leads to a higher nucleation density. The grains again show flat
top facets and sharp edges. It is also possible to distinguish the for-
mation of long grains that grow along two preferential directions.
favorable to obtain a closed film on SiO
deposition on SiO /Si(1 0 0). This is due to the different interaction
strengths between molecule and substrate in the two cases. In per-
ylene films deposited on SiO /Si(1 0 0), the influence of the
x
/Si(1 0 0), compared to
2
The size of the grains ranges from 1.3 to 2.7 lm.
x
Si(1 0 0), although not so strong to constitute a template for the
film, is still sensible, lowering the barrier necessary for two-dimen-
sional growth. On the other hand, the strength of the interaction
molecule–substrate is definitely quenched by the closed oxide
4
. Discussion
The present results give a detailed picture of the growth mech-
anisms of perylene thin films on SiO /Si(1 0 0) and SiO /Si(1 0 0).
x
2
2
layer in SiO /Si(1 0 0). This leads to a higher probability for a
The AFM images show the film morphology as a function of the
preparation parameters. Looking at Figs. 1 and 3, it is evident that
for substrate temperatures equal to or above RT, the typical growth
mode is substantially an island-type growth mode (Volmer–Weber
and/or Stranski–Krastanov) with formation of large islands. In con-
trast, for a cooled substrate, the films are homogeneous and com-
pletely closed. By changing the deposition rate it is possible to
modify the film morphology, as is easily seen comparing Fig. 1d–
f. Here, the morphology slowly changes from a film with a strong
three-dimensional character to a film that, still grain-like, presents
large domains with the substrate completely covered.
three-dimensional growth over a larger range of preparation
conditions.
The findings presented in this Letter are important also from a
technological viewpoint. By using an appropriate set of preparation
conditions for the perylene thin films, the morphology can be var-
ied, ranging from the growth of homogeneous smooth films to big
single grains. This indicates that, in principle, it is possible to grow
high quality organic thin films matching the exact requirements of
specific devices, opening the way towards a film-engineering ap-
proach for the optimization of organic-based devices. We can pre-
dict the growth of perylene on technologically relevant substrates
as: (i) homogeneous smooth films at lower substrate temperature
(6200 K), and (ii) single large grains, with dimensions big enough
Substrate temperature and deposition rate are among the
parameters that affect the deposition process in the classical the-

8
0
M.B. Casu et al. / Chemical Physics Letters 479 (2009) 76–80
to build a single nanodevice for higher Tsub. Consequently, this rec-
ipe has recently been applied to build an OFET based on a micro-
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Voigt (University of Bonn, Germany) for supplying highly purified
perylene. Financial support by the DFG through the OFET-
Schwerpunktprogramm Um 6/8-1 and Um 6/8-2 and by the BMBF
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