Japanese Journal of Applied Physics 49 (2010) 08LB08

REGULAR PAPER

Effect of Defects Buried in Pentacene/Alkanethiol Self-Assembled Monolayer/Au Filmon Its Electronic Properties Visualized by Scanning Tunneling Microscopy/Spectroscopy
Yasuhiko Terada, Noriaki Takeuchi, Shoji Yoshida, Atsushi Taninaka, Osamu Takeuchi, and Hidemi Shigekawa
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba, Ibaraki 307-8573, Japan
Received January 15, 2010; accepted February 18, 2010; published online August 20, 2010
We have used scanning tunneling microscopy/spectroscopy to visualize the spatial correlation between buried structural defects and observed
electronic properties in organic device structures. As a typical structure of an organic field-effect transistor, we have prepared pentacene/alkanethiol self-assembled monolayer (SAM)/Au samples with or without defects associated with a gap state at the molecule/Au interface. The
effect of the defects, which were hidden behind the pentacene overlayers, on the electronic properties of the SAM was clearly observed. The
method used in this study has potential for evaluating the nanoscale correlation between electrical properties and hidden defects inside organic
devices. # 2010 The Japan Society of Applied Physics
DOI: 10.1143/JJAP.49.08LB08

1.

Introduction

Designing nanometer-sized devices requires a well-deﬁned
fabrication technique using ultrathin dielectric layers.
Downsizing and speeding up of ﬁeld-eﬀect transistors, forexample, have been achieved by reducing the thicknessof gate insulator ﬁlms to the nanoscale limit. Inorganic
materials are commonly used for insulator ﬁlms, butultrathin-ﬁlm processing gives rise to complex issues, suchas a large leakage current via tunneling. On the other hand,organic materials have an intrinsically low conductivity andare the best candidates for dielectric ﬁlms.1) Additionally,advances in chemical synthesis and fabrication have promoted the use of organic materials in electronic devices.
In particular, the use of a self-assembled monolayer(SAM) as a gate dielectric in organic transistors was
pioneered by Vuillaume et al.2) and has attracted considerable attention. Previous studies revealed a leakage currentof 108 A cm2 ,2) which is suﬃciently low for practical use.
Despite their high potential,3–5) SAMs still have criticalissues, such as low ﬁlm quality and packing order. SAMs
have intrinsic and extrinsic defects, through which semiconductor materials may penetrate into the substrate. Such
penetration reduces the eﬀective dielectric thickness andyields current paths with a low resistivity. These imperfections are undesirable not only because they may trigger alarge leakage current but also because the charging arisingfrom the leakage results in the disorder in interfacialpotential, which greatly aﬀects carrier transport.6) Therefore,
signiﬁcant eﬀort has been made to fabricate defect-free
SAMs with suﬃciently high robustness and insulating
quality.7)
In this context, there is increasing demand for thequantitative investigation of the correlation between structural disorders in SAM dielectrics, such as defects andpinholes, and the overall electronic transport propertiesof the devices. Structural defects have been evaluated byspatially averaged techniques, including the evaluation oftilt, order, and gauche defects by Fourier transform infrared
spectrometer (FT-IR),8) electron transport analysis by electrochemistry,8) and the investigation of structural defects
such as pinholes.8,9) Local probe techniques, such as
scanning tunneling microscopy/spectroscopy (STM/STS),

E-mail address: hidemi@ims.tsukuba.ac.jp

atomic force microscopy, lateral force microscopy, and
Kelvin probe force microscopy, have also been used forthe real-space analysis of, for example, the self-assembly
mechanism,10,11) defect characterization,11) and interfacial
charging.12) Although these techniques provide much valuable information, little is known of the direct microscopic
correlation between structural defects in buried dielectric
layers and electronic transport properties of device structures.
Here, we present the potential of STM/STS to visualize
the eﬀect of defects formed at a molecule/metal interface
and hidden behind the molecular overlayers on the electronic
properties of thin ﬁlms. STS spectra are, in general, obtainedby plotting the tunneling current I or tunneling conductance
dI=dV versus voltage V between an STM tip and a sample.
dI=dV corresponds to the local density of states (LDOS) of
the sample, and its mapping gives the spatial distribution of
the electronic structure. If nanoscale defects associated with
gap states exist in a bulk or on a surface, they appear in the
dI=dV image as localized spots. We artiﬁcially introduced
defects into SAM dielectric layers and investigated the eﬀectof the defect-induced electronic states hidden beyond theoverlayers using STM/STS. As a typical system, we useda pentacene thin ﬁlm deposited on alkanethiol SAM/Au
samples.
2.

Experimental Procedure

The nature of electronic states of the hybrid orbital between
a single pentacene molecule and a gold atom was investigated by Repp et al.13) They observed a new peak of dI=dV
close to the lowest unoccupied molecular orbital (LUMO) of
the original pentacene and conﬁrmed that the peak is due tothe hybrid orbital between the pentacene LUMO and theAu 6s state by density-functional calculations. On the other
hand, X-ray photoemission and STM studies14) showed thatthe oxidation of the alkanethiol terminus results in thedesorption of alkanethiol, which is characterized by thepresence of striped phases in SAMs at the initial stage ofoxidation. In each striped phase, alkanethiol molecules areoriented with the chain axis parallel to the surface, and theSAM molecules in the striped phase no longer act as aphysical barrier and allow direct contact between pentacene
and Au. Therefore, when pentacene molecules are adsorbedon the SAM formed by the oxidized alkanethiol molecules,
pentacene molecules in striped-phase regions are expected to

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form gap states. On this basis, we prepared samples with orwithout buried defects associated with gap states.
Au(111) substrates were prepared by thermal evaporation
on mica in vacuum. SAMs at saturation coverage wereprepared by immersing Au(111) for 12 h into either
octanethiol (C8) or dodecanethiol (C12) solution in toluene(1 mM) and subsequent rinsing. Pentacene powder wasplaced in a ceramic crucible and thermally deposited on a
SAM in vacuum (5:0  105 Torr) at a rate of 0.002 nm/s.
The coverage was monitored using a quartz oscillator.
SAMs with defects were prepared by promoting theoxidation of alkanethiol molecules during the SAM formation on Au; prior to the SAM formation, the alkanethiol
solutions were exposed to air for more than a month toincrease the concentration of dissolved oxygen. Referential
SAMs were prepared with alkanethiol solutions that were
deoxygenated and dehydrated to suppress the oxidation.
STM and STS measurements were performed in ultrahighvacuum. I–V curves were taken at grid points covering thescanning area. dI=dV images were obtained by plotting thenumerically calculated value of dI=dV at each grid point.
3.

Results and Discussion

3.1 Introduction of defects by oxidized alkanethiol
molecules

First, we examined the introduction of defect into SAMs. To
conﬁrm their introduction, we observed a C8-SAM that had
been subjected to oxidation promotion. Figure 1(a) shows
an example STM image. Although most of the surface iscoveredpﬃﬃﬃ pﬃﬃﬃby a saturation phase, a cð42Þ superlattice of a
ð 3 3ÞR 30 lattice [inset in Fig. 1(a)],11) the following
two types of defect were observed: One type comprises
pitlike defects [marked as A in Fig. 1(a)], which are commonly observed for alkanethiol SAMs at saturation. Since
they have the depth of the Au(111) single step height of
2.5 Å, they are assigned to islands of Au vacancies and not
the defects in the SAM itself. The other type comprises
stripe-phase (marked as B) in which alkanethiol molecules
are oriented with the chain axis parallel to the surface.
Figures 1(b) and 1(c) show wide-area images obtained for anoxidized sample and an oxidation-suppressed sample, where
the stripe-phase defects, etch pits, and domain boundaries
are drawn in red, blue, and green, respectively. As shown in
Fig. 1(b), stripe phases (drawn in red) surround and connect
etch pits or steps. On the other hand, as expected, the stripe
phase was hardly observed in the reference SAM preparedwith its oxidation suppressed [Fig. 1(c)], although etch pits
and domain boundaries were still observed.
3.2 STM/STS results of pentacene/alkanethiol/Au

Next, we show results for a pentacene ﬁlm on a C8-SAM
with striped defects. The STS results revealed interesting
features diﬀerent from the topographic results. In Fig. 2(a),stacked pentacene layers with atomically ﬂat terraces canbe observed. The step height is 1.6 nm, which corresponds
to the interlayer distance of pentacene (001).15) Figure 2(b)shows I–V and dI=dV curves obtained for diﬀerent
pentacene layers in order of increasing height (from A to
F). The dI=dV curves exhibit diﬀerent gaps, regardless ofthe energy band gap of pentacene (2.2 eV). Onset voltages(or band-edge voltages) are almost the same in the negative-

Fig. 1. STM images of (a), (b) oxidized C8SAM/Au and (c) unoxidized
C12SAM/Au. Stripe-phase defects, etch pits, and domain boundaries are
drawn in red, blue, and green, respectively, in (b) and (c). Inset in(a)pﬃﬃﬃshows
pﬃﬃﬃ a magnified area consisting of a cð42Þ superlattice of a
ð 3 3ÞR 30 lattice.

voltage region, while they are diﬀerent in the positivevoltage region; the positive onset voltage increases with thelayer thickness. These features can be explained by tipinduced band bending (TIBB).16,17) Owing to the presenceof TIBB in the undoped pentacene, the bias voltage applied
between the tip and the sample drops in the pentacene layer,
and the voltage drop in the tunneling gap becomes lower,
resulting in a reduction in tunneling current. Consequently, ahigher sample voltage is required for the same amount of
current to ﬂow, compared with the case where TIBB does
not occur. This causes the onset-voltage shift and apparent
gap opening. As the thickness of the pentacene layer islarger, the voltage drop in the layer becomes higher, and theshift in onset voltage increases. Thus, the apparent energy
gap is larger than the highest occupied molecular orbital–
lowest unoccupied molecular orbital (HOMO–LUMO) gap
of pentacene, depending on the layer height of pentacene.
Tunneling current starts to ﬂow when the Fermi energy of
the tip is equal to the HOMO of pentacene in the positivesample-voltage region, and to the LUMO of pentacene in thenegative-sample-voltage region. Since the Fermi energyof the W tip (work function, 4.8 eV) is much closer to the
LUMO than the HOMO of pentacene (aﬃnity, 2.9 eV), theadditional voltage required to allow the tunneling current to
ﬂow is smaller in the negative-sample-voltage region than

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images obtained over a wide area [Figs. 2(e) and 2(f)]. In theﬁgures, the areas where localized gap states are observed inFig. 2(f) (gap-state areas) are marked by circles. The gapstate area density is estimated to be 1:5  1010 cm2 fromthe dI=dV images. This value is close to the stripe-phasedensity of 2:5  1010 cm2 and diﬀerent from the etch-pit
density of 4:1  1011 cm2 . This reveals that the gap state isclosely related to the stripe defect.
Here, the gap-state areas have no relation with thestructures in the topographic image shown in Fig. 2(e).
The eﬀect of the defects, which were hidden behind thepentacene overlayers, on the electronic properties of the
SAM was clearly observed.
3.3 Discussions on origin of gap stateFig. 2. STS results for pentacene/C8SAM (oxidized)/Au. (a) STM
image. (b) I–V and dI=d V curves obtained for different areas marked in(a). (c) dI=d V image at 1 V. (d) Overlay of (a) with (c). (e, f) Wide-area
STM image and dI=d V image at 1.3 V. The gap-state areas are marked
by circles in (e) and (f).

in the positive-sample-voltage region. This leads to theasymmetry of onset voltages observed in Fig. 2(b).
These features are the same as those observed in aprevious paper on pentacene on SAMs prepared without
oxidation promotion.18) However, the diﬀerence arisingfrom oxidation was clearly observed in the mapping of
dI=dV. Figure 2(c) shows a dI=dV image at the sample bias
voltage of 1 V. A localized region (with a diameter of about
100 nm) exhibiting much higher values of dI=dV is observedin the upper left of the ﬁgure. An image overlaid with the
topography [Fig. 2(d)] shows the diﬀerence in the dI=dV
value from the surface geometry; the dI=dV values measured in the D2 region are diﬀerent from those measured in
the D1 region, although they were measured on the sameterrace. As is clear in Fig. 2(b), in the D2 region, the onset
voltage shifts toward zero, and a broad peak is observed at
1 V (marked by an arrow in the ﬁgure). Since the energy gap
shown in the dI=dV curve for the D1 region corresponds tothe apparent HOMO–LUMO gap at this layer height, thebroad peak is a state in the pentacene band gap (gap state).
To examine the spatial correlation between the gap stateand striped defects in SAMs, we compared STM and dI=dV

Let us consider the origin of the gap state. The gap state may
originate from defects in pentacene crystalline layers, but
this is unlikely for the following reasons. There have been
many studies on the electronic structures of defects inorganic semiconductors.3) Vacancies do not form gap statesin general. Structural defects accompanied by local latticestrains (such as point defects, dislocations, or grain boundaries) could be the origin of the gap state. However, these areshallow states (located within 0.2 eV from the band edge),and here, we observed no correlation between the topographic features and the gap-state distribution. Thus, these
defects could not be the origin of the gap state. Chemical
defects arising from a chemical reaction with oxygen or
moisture also form gap states. For example, gap statesoriginating from oxygen-related chemical defects are formed
at 0.28,19) 0.38,20) and 0.6 eV21) away from the valence bandedge of pentacene. These states are located close to the
HOMO, which is diﬀerent from the present case. Thus, theobserved gap state is unlikely to originate from defects inpentacene crystalline layers.
On the other hand, in the striped region, alkanethiol
molecules are oriented parallel to the surface and do not
suﬃciently act as a physical isolator, resulting in theformation of a hybrid orbital between pentacene and Au,
which could be the origin of the gap state. To conﬁrm thismechanism, we performed STS measurements for a pentacene ﬁlm on a reference C12-SAM with few striped defects.
Figure 3(a) shows an STM image obtained for a pentacene/
C12SAM (unoxidized)/Au sample and Fig. 3(b) shows the
I–V and dI=dV curves obtained for diﬀerent areas marked inFig. 3(a). As discussed in §3.2, the shift in positive onset
voltage, which depends on the layer height, was observed.
Similarly, a gap state at 2.5 V was observed in region C2,
which was not observed in region C1 on the same terrace.
In the dI=dV image at 2.5 V [Fig. 3(c)], the gap state isobserved only in region C2 and its density is low, in contrast
to the case where the SAM with many striped defects wasused [Fig. 2(f)]. The other regions exhibiting high dI=dV
values are observed (in, for example, region A) because thepentacene layer is thin and the onset voltage is lower than
2.5 V in these regions. For regions with thick layers (around,for example, region D), however, even if the gap state exists,
it might not be observed in the dI=dV image at 2.5 V
because of the onset voltage shifts due to the large amount of
TIBB that occurs on a thick layer. To examine this point, we
performed dI=dV measurement at 4 V [Fig. 3(d)]. As is clear

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Fig. 4. STS results for petantacene/C12SAM (oxidized)/Au. (a) STM
image. (b) d I=dV image at 3.1 V. The gap-state areas are marked bycircles.

Fig. 3. (a) STS results for pentacene/C12SAM (unoxidized)/Au.
(b) I–V and dI=d V curves obtained for different areas marked in (a).
(c, d) d I=dV images for (c) Vs ¼ 2:5 and (d) 4 V. The gap-state area ismarked by a circle in (c) and (d).

from the dI=dV image, no localized gap states wereobserved in D region. In this manner, we investigated theexistence of the gap state over the entire surface andconﬁrmed that no gap sates exist except in region C2. Fromthese results, the gap-state density was estimated to be
8:0  108 cm2 , which is signiﬁcantly reduced, as expected.
In addition, the reference SAM has many pits and domainboundaries [Fig. 1(c)], and it is conﬁrmed that these pits andboundaries are not related to the formation of the gap state.
Before concluding on the origin of the gap state, we
examined the eﬀect of the chain length of the alkanethiol.
The crystallinity and resistance to ion penetration increase
with SAM thickness.22) In analogy with this, when a SAM is
not suﬃciently thick, pentacene molecules may penetrate
into it, resulting in the formation of a hybrid orbital. That is,such penetration may occur in the thin C8-SAM but not in
the thick C12-SAM, resulting in the diﬀerence in gap state
density. To exclude this possibility, we examine a pentacene
ﬁlm on a C12-SAM with many striped defects. Figures 4(a)and 4(b) show the STM and dI=dV images at 3.1 V,
respectively. Many gap states are observed in Fig. 4(b). The
gap-area density is estimated to be 8:3  109 cm2 , which
is an order of magnitude higher than that in the case where
the C12-SAM with few striped defects was used [Figs. 3(c)and 3(d), note the diﬀerence in scale]. Thus, it is conﬁrmedthat the gap state is mainly due to striped defects, rather
than the penetration of pentacene molecules into the SAM
crystalline layer.
4.

Conclusions

We visualized the spatial correlation between structuraldefects associated with a gap state in SAM dielectrics andthe electronic properties of pentacene/SAM/Au systems on

the nanoscale using STM/STS. The gap state was introduced
using oxidized alkanthiol molecules based on the formationof a hybrid orbital between pentacene molecules and Au
substrate. The gap state hidden behind the pentacene overlayers was clearly observed in dI=dV images for a SAM
with striped defects, with the distribution unrelated to the
topographic features, as expected. These ﬁndings suggestthat the technique used in this study is applicable to thenanoscale real-space evaluation of the correlation between
structural defects in buried dielectrics and the overall
performance of organic devices.

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