Vapor phase self-assembly of molecular gate dielectrics for thin film transistors

Article abstract of DOI:10.1021/ja801309g

Organic-inorganic films grown entirely via a vapor-phase deposition process and composed of highly polarizable molecular structures are investigated as gate dielectrics in organic field-effect transistors (OFETs). Molecules 1 and 2 form self-ordered thin films via hydrogen bonding, and these organic-inorganic structures exhibit large capacitances and large pentacene OFET mobilities. Copyright

Full text of DOI:10.1021/ja801309g

Published on Web 05/23/2008
Vapor Phase Self-Assembly of Molecular Gate Dielectrics for Thin Film Transistors
Sara A. DiBenedetto, David Frattarelli, Mark A. Ratner,* Antonio Facchetti,* and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208
Received February 21, 2008; E-mail: ratner@northwestern.edu; a-facchetti@northwestern.edu; t-marks@northwestern.edu
Small molecule and polymeric dielectric thin films with large
permittivities (k) have recently found applications in organic field
effect transistors (OFETs), where replacing the traditional SiO2 gate
dielectric with high capacitance materials enables lower operating
voltages, reduced power consumption, and improved performance.1
The gate dielectric capacitance Ci in an OFET can be described by
a parallel plate capacitor, where Ci ) ꢀok/d (d ) the thickness, and
ꢀo ) the permittivity of vacuum). To achieve large capacitances, it
is necessary to reduce the thickness and/or increase the dielectric
film permittivity. This approach offers an intriguing materials
challenge not only because small molecule and polymeric materials
with large permittivities (i.e., π-conjugated) are usually poor
insulators but also because as the insulator film thickness is reduced,
concurrent leakage current increases can degrade OFET perfor-
mance.2
Figure 1. (A) Chemical structures of 1 and 2, and (B) OFET device
Dielectric materials composed of molecular components are ideal
candidates for OFETs because small molecules have nanometer
length scales, tunable electronic structures, processability on flexible
substrates, and are compatible with organic semiconductors.3
Previously, we demonstrated the application of molecular self-
assembled nanodielectrics (SANDs) as gate dielectrics. SAND films
are fabricated via a layer-by-layer solution phase self-assembly of
σ-π organosilane precursors to yield hybrid organic-inorganic
multilayers.4 Here the highly polarizable π-conjugated component
is a stilbazolium group (Stb). An interesting question is whether
similar hybrid films based on highly π-polarizable components can
be fabricated from the vapor phase in a “dry” process that does
not involve multistep “wet” organosilane self-assembly. In this
contribution, we demonstrate that SAND-like organic-inorganic
films for OFET gate dielectrics can be fabricated entirely via vapor-
phase processing (v-SAND). Pentacene OFETs based on v-SANDs
exhibit excellent performance at low operating voltages, demon-
strating a new approach to high-k organic gate dielectrics.
fabrication: (i) vapor deposition of 1 or 2, (ii) exposure of F1 or F2 to
Cl3Si-O-SiCl3 vapors, completing the v-SAND structure, (iii) vapor
deposition of pentacene (50 nm), and (iV) vapor deposition of Au electrodes
through a shadow mask to complete the OFET device.
Figure 2. (A) Leakage current density versus voltage plots for F1 (red
dotted line), F2 (blue dotted line), F1-cap (red solid line), and F2-cap (blue
solid line) films. (B) Capacitance versus voltage plots for F1-cap (red) and
F2-cap (blue) films. Inset (top): AFM image of F1 on Si. Inset (bottom):
AFM image of F1-cap. The scale bar is 2 µm.
phase versus solution-deposited film growth techniques should
New v-SAND π-conjugated building blocks 1 and 2 were
designed to form robust self-ordered thin films via head-to-tail
intramolecular hydrogen-bonding (Figure 1A). In addition, the
π-conjugated organic donor-acceptor functionalities should en-
hance the electronic polarizability needed to achieve a high-k bulk
layer. The molecular geometries of 1 and 2 were optimized using
DFT/B3LYP density functional theory and a 6-31G** basis set in
Jaguar v5.00.22 using the Maestro Molecular Modeling Interface
c5.10.20. Molecular polarizabilities in the z-direction (Rz, parallel
to the molecular long axis, Figure 1A) were calculated using
INDO(SOS).5 The computed polarizabilities of 1 (176 × 10-24
cm3) and 2 (70.2 × 10-24 cm3) are, respectively, larger than and
comparable to that of Stb in SAND (91.3 × 10-24 cm3). Classic
dielectric theory relates molecular polarizability to bulk dielectric
permittivity through the Clausius-Mossotti relationship, k (3 +
2RzΝ)/(3 - RzΝ) where N is the molecular density (molecules/
cm3).6 Accordingly, it is expected that the larger polarizabilities
of 1 and 2 and the greater molecular densities afforded by vapor
translate into higher k’s.7
Compounds 1 and 2 were synthesized following procedures
reported in Supporting Information and were characterized by
conventional analytical/spectroscopic techniques. v-SAND films are
fabricated according to the scheme in Figure 1B and are composed
of a bottom layer of either 1 or 2 and capped with a top SiOx
“capping layer.” Films of 1 and 2 (F1 and F2, respectively) were
vapor-deposited at a rate of 0.2 Å/s onto solvent-cleaned8 heavily
doped n+-Si substrates at 10-6 Torr. To deposit the SiOx capping
layer, F1 and F2 were exposed to hexachlorodisiloxane
(Cl3Si-O-SiCl3) vapors under N2 for 20 min and then annealed
under vacuum (20 Torr) at 110 °C to afford F1-cap and F2-cap,
respectively (see Figure S1 for the apparatus). AFM images (Figures
2 and S2) of the uncapped and capped films demonstrate smooth
surface morphologies (maximum rms roughness ∼ 0.5 nm) that is
important for optimal OFET performance.9 Optical spectra of the
films on glass before and after SiOx deposition (Figure S3) exhibit
9
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10.1021/ja801309g CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
have larger mobilities at lower operating voltages than OFETs with
pentacene fabricated on 300 nm SiO2 (Figure S5). To understand the
origin of the large carrier mobilities, AFM images and XRD spectra
of the pentacene films on top of v-SANDs were acquired (Figure 3).
Very large pentacene crystal grains (∼2-3 µm) are observed by AFM,
which are comparable to or larger than those in other high-mobility
OFETs.12 In addition, the XRD spectra demonstrate highly textured
pentacene films having the thin film phase crystal structure (d spacing
) 15.4 Å on both v-SANDs).12
In summary, we have demonstrated a straight forward, vapor-
phase approach for fabricating self-assembled nanodielectrics (v-
SANDs) exhibiting substantial capacitances and excellent insulating
properties. Pentacene OFETs based on F1-cap and F2-cap exhibit
very large mobilities.
Acknowledgment. This work was supported by the NSF
MRSEC program (DMR-0520513) at the Materials Research Center
of Northwestern University, and by the ONR MURI Program
(N00014-02-1-0909).
Figure 3. Transfer (A and D) and output (B and E) plots for pentacene
OFETs based on v-SANDs of F1-cap (top) and F2-cap (bottom). AFM
images and XRD spectra of the corresponding 50 nm thick pentacene films
grown on F1-cap (C) and F2-cap (F).
Supporting Information Available: Synthesis of 1 and 2, film/
device fabrication details, and AFM/optical/XRR/OFET data. This
material is available free of charge via the Internet at http://pubs.acs.org.
only minor differences, arguing that the capping reagent does not
react with the underlying F1 or F2 molecular layer but is simply
deposited on top.
To assess v-SAND dielectric properties, metal-insulator-semi-
conductor devices were fabricated by depositing Au electrodes (200
× 200 µm2) on both the all-organic F1 and F2 films as well as on
the organic-inorganic F1-cap and F2-cap films. As in the case of
solution processed SANDs,4a a substantial leakage current density
(Jleak) reduction is observed (10-10-2 f 10-5-10-7 A/cm2 at 2
V) after capping layer deposition (F1, F2 f F1-cap, F2-cap). The
suppressed leakage current indicates that the capping material
functions as an efficient electron tunneling barrier.10
References
(1) (a) Katz, H. E.; Hong, M.; Dodabalapur, A. J. Appl. Phys. 2002, 91, 1572.
(b) Podzorov, V.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett.
2003, 82, 1739. (c) Majewski, L. A.; Schroeder, R.; Grell, M. J. Phys. D:
Appl. Phys. 2004, 37, 21. (d) Park, Y. D.; Kim, D. H.; Jang, Y.; Hwang,
M.; Lim, A. L.; Cho, K. Appl. Phys. Lett. 2005, 87, 243509. (e) Yang,
S. Y.; Kim, S. H.; Shin, K.; Jeon, H.; Park, C. E. Appl. Phys. Lett. 2006,
88, 173507. (f) Onoue, T.; Nakamura, I.; Sakabe, Y.; Yasuda, T.; Tsutsui,
T. Jpn. Soc. Appl. Phys. 2006, 45, L770. (g) Singh, B.; Sariciftci, N. S.
J. Appl. Phys. 2006, 100, 024514. (h) Klauk, H.; Zschieschang, U.; Pflaum,
J.; Halik, M. Nature 2006, 445, 745. (i) McDowell, M.; Hill, I. G.;
McDermott, J. E.; Bernasek, S. L.; Schwartz, J. Appl. Phys. Lett. 2006,
88, 073505. (j) Kim, C. S.; Jin, S.; Lee, S. W.; Kim, W. J.; Bail, H. K.;
Lee, S. J.; Hwang, D. K.; Im, S. Appl. Phys. Lett. 2006, 88, 243515. (k)
Jeong, Y. T.; Dodabalapur, A. Appl. Phys. Lett. 2007, 91, 193509. (l) Lee,
J.; Panzer, M. J.; He, Y.; Lodge, T. P.; Frisbie, C. D. J. Am. Chem. Soc.
2007, 129, 4532.
(2) Devine, R. A. B.; Busani, T. Appl. Phys. Lett. 2005, 86, 062902.
(3) (a) Veres, J.; Ogier, S.; Lloyd, G.; de Leeuw, D. Chem. Mater. 2004, 16,
4543. (b) Weitz, R. T.; Zschieschang, U.; Effenberger, F.; Klauk, H.;
Burghard, M.; Kern, K. Nano. Lett. 2007, 7, 22. (c) Collet, J.; Tharaud,
O.; Chapoton, A.; Vuillaume, D. Appl. Phys. Lett. 2000, 76, 1941.
(4) (a) Yoon, M.-H.; Facchetti, A.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A.
2005, 102, 4678. (b) Ju, S.; Lee, K.; Janes, D. B.; Yoon, M.-H.; Facchetti,
A.; Marks, T. J. Nano. Lett. 2005, 5, 2281. (c) Hur, S.-H.; Yoon, M.-H.;
Gaur, A.; Facchetti, A.; Marks, T. J.; Rogers, J. A. J. Am. Chem. Soc.
2005, 127, 13808. (d) Ju, S.; Lee, K.; Yoon, M.-H.; Facchetti, A.; Marks,
T. J.; Janes, D. B. Appl. Phys. Lett. 2006, 89, 073510. (e) Ju, S.; Lee, K.;
Yoon, M.-H.; Facchetti, A.; Marks, T. J.; Janes, D. B. Nanotechnol. 2007,
18, 155201. (f) Lin, H. C.; Ye, P. D.; Xuan, Y.; Lu, G.; Facchetti, A.;
Marks, T. J. Appl. Phys. Lett. 2006, 89, 14210. (g) Ju, S.; Janes, D. B.; Lu,
G.; Facchetti, A.; Marks, T. J. Appl. Phys. Lett. 2006, 89, 193506. (h) Wang,
L.; Yoon, M.-H.; Facchetti, A.; Marks, T. J. AdV. Mater. 2007, 19, 3252.
(5) Keinan, S.; Ratner, M. A.; Marks, T. J. Chem. Phys. Lett. 2006, 417, 293.
(6) (a) Rohleder, J. W.; Munn, R. W. Magnetism and Optics of Molecular
Crystals; Wiley: New York, 1992. (b) Yamada, K.; Saiki, A.; Sakaue, H.;
Shingubara, S.; Takahagi, T. Jpn. J. Appl. Phys. 2001, 40, 4829.
(7) (a) Typical N values of self-assembled siloxane-based chromophores are
∼1020 molecules/cm3 (ref 7b), while vapor deposition N values are typically
∼1021 molecules/cm3 (ref 7c). (b) Roscoe, S. B.; Yitzchaik, S.; Kakkar,
A. K.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. Langmuir
1996, 12, 5338. (c) Choubey, A.; Kwon, O.-P.; Jazbinsek, M.; Gu¨nter, P.
Cryst. Growth Des. 2007, 7, 402.
Because of the large F1 and F2 Jleak values, meaningful
capacitances are only measurable for F1-cap and F2-cap and are
found to be ∼400 nF/cm2 at 2 V (Figure 2). Permittivities of the
v-SAND organic components can be estimated by modeling F1-
cap and F2-cap dielectric layers as three parallel plate capacitors
in series (Si native oxide + F1 or F2 + SiOx). The capping layer
thickness is ∼5.9 nm, and the organic layer thicknesses are ∼3.6
nm (by X-ray reflectivity, Figure S4). Using the accumulation
regime capacitances of F1-cap (400 nF/cm2) and F2-cap (390 nF/
cm2), k values of ∼11 and ∼9 are estimated for F1 and F2,
respectively (dnative oxide ) 1.5 nm, and knative oxide ) kcapping layer
)
3.9). The larger k of F1 versus that of F2 is anticipated by the
INDO(SOS) computation of the molecular Rz values in the context
of the aforementioned Clausius-Mossotti equation and can be
explained by the greater acidity of 1, which enhances the molecular
zwitterionic character upon intermolecular hydrogen bonding.
Given the excellent v-SAND dielectric properties, bottom-gate top-
contact OFETs were fabricated by vapor deposition of pentacene on
the v-SAND films, and the I-V characteristics were analyzed by
standard procedures.11 Using the above capacitances and channel
dimensions (L ) 200 µm and W ) 5000 µm), these OFETs exhibit
excellent performance with hole mobilities of 1.9 ( 0.3 and 2.4 (
0.3 cm2/V·s for the F1-cap and F2-cap based devices, respectively,
Ion/Ioff ∼ 105, and VT ∼1 V. Note that I-V output plots at each gate
voltage intersect at the origin, indicating very low gate leakage during
device operation (Figure 3). OFETs tested after 4 months from
fabrication exhibit small performance erosion with µ’s decreased by
∼10 and ∼30% for the devices based on F2-cap and F1-cap,
respectively (see Figure S6). These results demonstrate that vapor-
phase deposition can profitably utilize and probe the dielectric response
properties of high-k molecular layers without compromising the
insulating properties of the resulting hybrid films and, furthermore,
(8) Kim, C.; Facchetti, A.; Marks, T. J. AdV. Mater. 2007, 19, 2561.
(9) Steudel, S.; Vusser, S. D.; Jonge, S. D.; Janssen, D.; Verlaak, S.; Genoe,
J.; Heremans, P. Appl. Phys. Lett. 2004, 85, 4400.
(10) (a) McBrayer, J. D.; Swanson, R. M.; Sigmon, T. W. J. Electrochem. Soc.
1986, 133, 1242. (b) Huntley, F. A.; Willoughby, A. F. W. Solid-State
Electron. 1970, 14, 641. (c) Bal, J. K.; Hazra, S. Phys. ReV. B 2007, 75,
205411. (d) Dubois, G.; Volksen, W.; Magbitang, T.; Miller, R. D.; Gage,
D. M.; Dauskardt, R. H. AdV. Mater. 2007, 19, 3989.
(11) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bredas, J.-L.; Ewbank,
P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436.
(12) (a) Yang, H.; Shin, T. J.; Ling, M.-M.; Cho, K.; Ryu, C. Y.; Bao, Z. J. Am.
Chem. Soc. 2005, 127, 11542. (b) Kim, C.; Facchetti, A.; Marks, T. J.
Science 2007, 318, 76.
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