Enhanced Electrical Percolation Due to Interconnection of Three

Sep 12, 2006 - Hadayat Ullah Khan , Ruipeng Li , Yi Ren , Long Chen , Marcia M. Payne , Unnat S. Bhansali , Detlef-M. Smilgies , John E. Anthony , and...
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J. Phys. Chem. B 2006, 110, 20302-20307

Enhanced Electrical Percolation Due to Interconnection of Three-Dimensional Pentacene Islands in Thin Films on Low Surface Energy Polyimide Gate Dielectrics Sang Yoon Yang,† Kwonwoo Shin,† Se Hyun Kim,† Hayoung Jeon,† Jin Ho Kang,‡ Hoichang Yang,*,§ and Chan Eon Park*,† Polymer Research Institute, Department of Chemical Engineering, Pohang UniVersity of Science and Technology (POSTECH), Pohang, 790-784, Korea, National Institute of Aerospace (NIA), NASA Langley Research Center (NASA LaRC), Hampton, Virginia 23681, and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180 ReceiVed: July 22, 2006

The role of lateral interconnections between three-dimensional pentacene islands on low surface energy polyimide gate dielectrics was investigated by the measurement of the surface coverage dependence of the charge mobility and the use of conducting-probe atomic force microscopy (CP-AFM). From the correlation between the electrical characteristics and the morphological evolution of the three-dimensionally grown pentacene films-based field-effect transistors, we found that during film growth, the formation of interconnections between the three-dimensional pentacene islands that are isolated at the early stage contributes significantly to the enhancement process of charge mobility. The CP-AFM current mapping images of the pentacene films also indicate that the lateral interconnections play an important role in the formation of good electrical percolation pathways between the three-dimensional pentacene islands.

1. Introduction The use of vapor-deposited pentacene films as organic semiconductor films has actively been studied and has resulted in the highest charge mobility (∼3 cm2/Vs) achieved in organic field-effect transistors (OFETs).1-3 It has been found that pentacene molecules on hydrogen-passivated Si and thermally grown SiO2,4-6 hexamethyldisilazane (HMDS)-treated SiO2,3 and polymer7,8 dielectric substrates undergo two-dimensional (2-D) growth, in which the subsequent layers grow after the first monolayer covers the dielectric substrate. In contrast, several monolayer-thick pentacene islands can build up on dielectric substrates with a low surface energy, such as poly(imide-siloxane) (PIS)8 and octadecyltrichlorosilane self-assembled monolayer treated SiO29-11 (or eicosanoic selfassembled monolayer treated Al2O312) even at the initial stage of thin film growth. Such three-dimensional (3-D) growth is mainly due to the weak interactions between pentacene molecules and low surface-energy dielectrics, which mean that many molecules are required to form a stable nucleus.13,14 In general, it has been reported that 2-D nucleation is better than 3-D nucleation when a continuous layer capable of transporting charge carriers efficiently is required because 3-D growth may result in ill-connected grains and discontinuous films.14 Despite this drawback for charge transport of 3-D growth, pentacene films resulting from 3-D growth have been found to exhibit high charge mobilities in OFETs comparable to films resulting from the 2-D growth.8,10-12 We have previously suggested that the formation of interconnection that results in improved contact * Address correspondence to these authors. Chan Eon Park: phone +8254-279-2269, fax +82-54-279-8298, e-mail [email protected]. Hoichang Yang: phone 518-276-2298, fax 518-276-6540, e-mail [email protected]. † POSTECH. ‡ NASA Langley Research Center. § Rensselaer Polytechnic Institute.

between 3-D pentacene grains can contribute to achieving high charge mobility in pentacene FETs.8 In the case of layer-by-layer (2-D) growth of vacuumdeposited small molecule organic semiconductor films, there have been studies of the dependence of the charge mobility in OFETs on semiconductor film thickness to determine how many molecular layers are required next to the dielectric interface for effective charge transport.15-18 By investigating 3-D grown pentacene FETs in this way, it is possible to determine the influence of lateral interconnections between islands on the charge mobility because the interconnection formation occurs as the nominal thickness of pentacene increases. In this study, we found that the formation of lateral interconnections between locally isolated pentacene islands resulting from 3-D growth on low surface-energy (30.2 mJ/m2) PIS dielectrics is directly correlated to the enhancement process of charge mobility in pentacene FETs. In particular, our research focused on determining how the interconnections influence the formation of lateral conducting pathways between 3-D grown pentacene islands by using conducting-probe atomic force microscopy (CPAFM). 2. Experimental Section A poly(imide-siloxane) with R,ω-(aminophenoxypropyl)poly(dimethylsiloxane) (SDA) moieties with a number-average molecular weight of 10 000 g/mol in the polyimide backbone was polymerized according to published procedure.8 The amount of SDA relative to the amount of diamines was 20 wt %. The reaction proceeded for 8 h with stirring at 10 °C in an N2 atmosphere. Poly(amic acid-siloxane) solution (with a solid content of 7 wt %) was filtered with poly(tetrafluoroethylene) syringe filters (pore size ) 1 µm), spin-coated onto a heavily doped silicon wafer, and then imidized in an N2-purged furnace (150 °C for 30 min, 200 °C for 30 min, and 250 °C for 60

10.1021/jp0646527 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006

Electrical Percolation by Interconnection of 3D Pentacene Islands

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Figure 1. Tapping mode-AFM (TM-AFM) topographic images of 3-D grown pentacene films on PIS gate dielectrics for θ ranging from 0.63 to 1.00. All images are 2.5 µm × 2.5 µm in size and represent a z-range of 20 nm.

min). The surface energy of the poly(imide-siloxane) (PIS) films was found to be 30.2 mJ m-2, as determined with contact angle measurement using two test liquids, water and diiodomethane. The polar and dispersion component of the surface energy were found to be 0.8 and 29.4 mJ m-2, respectively. The capacitance of PIS dielectric films was measured with a HP 4284A Precision LCR meter. Ellipsometry was used to measure the thickness of PIS dielectrics. Heavily doped n-type silicon wafers (F ) 0.002-0.003 Ω cm) were used as the substrate and the gate electrode. On this substrate, PIS films were prepared by imidization of spin-coated poly(amic acid-siloxane) as described above. Pentacene was purchased from Aldrich and used without any further purification. Pentacene active layers with various nominal thicknesses, as determined with a quartz crystal microbalance (QCM), were deposited at a deposition rate of 0.3-0.4 Å/s onto PIS surfaces with a substrate temperature of room temperature under a base pressure of 10-6 Torr by using organic molecular beam deposition. Pentacene FETs with top-contact geometries were then obtained by evaporating gold through a shadow mask to define the source and drain contacts. The channel length (L) and the channel width (W) were 100 and 1500 µm, respectively. The electrical characteristics of the devices were measured in an air atmosphere, using Keithley 2400 and 236 source/measure units. The morphologies of the various thick-pentacene films on the PIS dielectrics were characterized by using an atomic force microscope (AFM, Multimode IIIa, Digital Instrument) operating in tapping mode. Conducting-probe AFM (CP-AFM) was also used to determine the distribution of current flow in the 3-D grown pentacene thin films. The samples investigated with CP-AFM scans had the same structure as the top contact OFET devices except that only a source contact was deposited onto the pentacene films. The sample surface was scanned with a Pt/Cr-coated conductive tip (MikroMasch, diameter ∼30 nm) in a contact mode. The CP-AFM images were recorded simultaneously with contact topographies by applying a bias to the source contact to measure the lateral current flow. Synchrotron X-ray diffraction analysis for 3-D grown pentacene thin films was performed at the 10C1 beam line (wavelength ∼1.54 Å) at the Pohang Accelerator Laboratory (PAL).

3. Results and Discussion Figure 1 shows AFM topographic images of 3-D pentacene films vapor-deposited onto the PIS dielectrics for various surface coverages (θ), as determined from the ratio of the area of the pentacene crystals to the substrate in the AFM image. θ was used here instead of the nominal pentacene thickness (d) because charge transport phenomena are closely related to the surface coverage in 3-D grown organic semiconductor films. Cicoira et al. have reported that for 3-D grown tetracene films on SiO2, the uniformity of the substrate coverage is more important than the density of the grain boundaries in determining the charge mobility of OFETs.19 For 3-D pentacene films with θ < 0.63, it was found that isolated pentacene islands were present over the entire PIS surface.8 However, the formation of interconnections between these isolated islands was observed for θ > 0.63, and the 3-D pentacene grains became completely interconnected for θ > 0.92, resulting in continuous film formation over the entire area of the PIS dielectric surface. X-ray diffraction experiments were performed on the thin pentacene films formed on the low surface energy PIS dielectrics to check the structure and arrangement of the pentacene molecules in the 3-D grown films. Figure 2a shows the outof-plane X-ray diffraction profiles of the 3-D grown pentacene films. The Bragg reflection peaks corresponding to (00l) planes indicate that only a thin film phase characterized by an interplanar spacing of 15.6 Å is observed in the pentacene films for both θ ) 0.81 and 0.93. Synchrotron one-dimensional grazing incident angle X-ray diffraction (GIXD) studies were also carried out with a fixed grazing incidence angle of 0.18° to determine the in-plane structures of these thin films (Figure 2b). (110), (200), and (210) reflections were observed in the pentacene films for both θ ) 0.81 and 0.93. The appearance of these reflection peaks indicates that the pentacene molecules in the 3-D grown crystals are packed with a herringbone (edge-to-face) geometry along the in-plane direction.20,21 These X-ray diffraction results confirm that the 3-D pentacene islands consist of layers of pentacene molecules with an interplanar spacing of 15.6 Å and pentacene molecules in each layer are arranged with herringbone packing, which indicates that the pentacene molecules have a tendency to self-

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Figure 2. One-dimensional X-ray diffraction patterns of 3-D grown pentacene thin films on low surface-energy PIS gate dielectrics: (a) out-ofplane profiles and (b) in-plane profiles with a fixed grazing incidence angle of 0.18°.

Figure 3. (a) The relationship between nominal thickness and surface coverage of the pentacene films. (b) Charge mobility of top-contact OFETs with 3-D grown pentacene films on PIS gate dielectrics as a function of θ. (c) Transfer characteristics of 3-D grown pentacene FETs as a function of θ. (d) Plot of eq 3 and its derivative as a function of θ.

assemble, resulting in the formation of crystalline domains on inert and flat surfaces.22 The crystalline structure of the 3-D pentacene films indicates that pentacene crystals have a molecular arrangement that is favorable to the lateral charge transport in pentacene FETs. Panels a and b of Figure 3 show the relationship between the nominal thickness (d) and θ for 3-D pentacene thin films and that between the charge mobility of pentacene FETs and θ, respectively. The charge mobility of pentacene FET with each θ was extracted from the slope of a plot of the square root of the drain current versus gate voltage in the saturation regime by fitting the data to the following equation,

IDS ) (WCi /2L) µ(VG - Vth)2

(1)

where IDS is the drain current, µ is the charge mobility, W and L are the channel width and length, respectively, and Vth is the threshold voltage (Figure 3c). The gate voltage (VG) was swept from 20 to -40 V at a constant drain voltage of -50 V, and the capacitances per unit area (Ci) of the PIS dielectrics were found to be about 9.5 nF/cm2 at 1 MHz with 310 nm thick PIS dielectrics. The charge mobility rises from 5.5 × 10-3 cm2/Vs at θ ) 0.63 and increases rapidly for θ > 0.90. The charge mobility is saturated when the PIS surface is completely covered with pentacene film (θ ∼ 0.99). The empirical formulation in

Electrical Percolation by Interconnection of 3D Pentacene Islands

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Figure 4. (a) Scheme of the CP-AFM observations, in which a Pt/Cr-coated AFM tip was used to detect current- flow across pentacene films connected to a top-contact Au electrode. Direct current bias (Vbias) was applied through the Au electrode from a sample stage. (b) TM-AFM topograph of a sample device used in the CP-AFM. With a contact load of ∼100 nN, the CP-AFM scan was performed for a local area about 5 µm away from the electrode. Simultaneously recorded contact AFM topographs (left) and CP-AFM current mapping images (right) of 3-D grown pentacene thin films with θ ) 0.70 (c) and 0.82 (d) on PIS gate dielectrics.

eq 2 has been used to describe the dependence of the charge mobility in OFETs on semiconductor thickness for vapordeposited organic semiconductors such as R-hexathienylene15 and pentacene,16 which were grown layer-by-layer (2-D growth) for the first few layers.

µ ) µsat(1 - exp[-(d/d0)R])

(2)

Here, µsat is the saturated value of the charge mobility, d0 is the crossover thickness of pentacene, and R is the exponent. By replacing the nominal film thickness d with the fitting equation d ) -5.26552 ln(1.00185 - θ) (see Figure 3a), we obtained the following relationship between the charge mobility and θ:

µ ) µsat(1 - exp[-(ln(1.00185 - θ)/ln(1.00185 - θc))R]) (3) where θc denotes the crossover surface coverage of the pentacene film. The solid line in Figure 3b is the fit with eq 3 of the variation in the charge mobility with θ; the parameters µsat, θc, and R were obtained from the fitting process and found to be 0.458 ( 0.024 cm2/Vs, 0.970 ( 0.005, and 4.397 ( 0.867, respectively. It is interesting that the increase in the charge mobility with increasing θ in 3-D grown pentacene FETs is also satisfactorily described by eq 3, which is obtained by modification of eq 2. The crossover coverage (θc) is the point at which the rate of increase in the charge mobility with the surface coverage (dµ/dθ) reaches a maximum. dµ/dθ increases for θ < θc, which indicates that the contribution of the increment in θ to the rate of the increase in charge mobility become larger for θ < θc (Figure 3d). Note that the term “crossover coverage” as used by Dinelli et al.15 and the term “saturation thickness” as used by Ruiz et al.16 have the same meanings as in the above discussion. The evolution of the charge mobility with θ in 3-D grown pentacene FETs mainly depends on the formation of the interconnections between the 3-D islands. Interconnections between the 3-D islands start to appear at θ ) 0.63, at which value the charge mobility of the 3-D grown pentacene FETs becomes apparent. Above θ ) 0.90, the interconnections are highly enhanced, which results in sufficient electrical pathways

in the pentacene film and leads to an exponential increase in the charge mobility. The saturation of the charge mobility arises when tight packing of the grains is achieved (θ ∼ 0.99). This correlation between the surface coverage dependent mobility and the morphological evolution strongly indicates that the formation of interconnection is crucial to the enhancing process of the charge mobility in 3-D grown pentacene FETs because they provide efficient percolation pathways for charge carriers injected from the source electrode. To confirm the presence of percolation pathways across the 3-D pentacene films as a result of interconnection formation, CP-AFM analysis was carried out for pentacene films with different coverages. Direct current bias (Vbias) was applied through a gold (Au) electrode from a sample stage and the contact current was recorded with a Pt/Cr-coated tip at the virtual ground (Figure 4a). A TM-AFM topograph for a sample device used in the CP-AFM is shown in Figure 4b. With a contact load of ∼100 nN, the CP-AFM scan was performed for a local area about 5 µm away from the Au electrode. After charge injection into the pentacene thin films through the positively biased Au electrode, stored charges in the thin films are detected by the conducting tip.3 In panels c and d of Figure 4, the left and right images show the contact AFM topograph and the CPAFM current image, respectively. The CP-AFM image of a pentacene film with θ ) 0.70 at Vbias ) 2.0 V (on the right in Figure 4c) indicates that there is no current signal; the charges injected from the Au electrode are not transported across the film due to the immature interconnections between the 3-D islands. In contrast, the variation in current in the wellinterconnected film could be clearly mapped at Vbias ) 1.0 V for θ ) 0.82 because the injected charges can be delocalized and transported along the interconnected 3-D pentacene islands into the tip (Figure 4d).23-25 The low current regions (the dark features) in Figure 4d are due to small stored charges in the islands, which indicate that less interconnected paths can be present between the Au electrode and the tip. From the CPAFM image, it can also be inferred that the interconnection points consist of pentacene crystals with molecular packing sufficient for charge delocalization.23 This is because nearly equivalent current levels for adjacent pentacene islands are

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Figure 5. Schematic diagram of the formation of interconnections between 3-D grown pentacene islands on low surface-energy PIS dielectrics: (a) the formation of several monolayers thick 3-D pentacene islands at the initial growth stage, (b) the formation of a multilayer on top of a 3-D island during growth, (c) the limitation of vertical growth that arises from the reduced topmost site area, which results in the interconnection and lateral growth of the 3-D pentacene islands until the substrate is completely covered with pentacene films, and (d) a TM-AFM topographic image of 3-D pentacene at θ ) 0.82. The arrows indicate the formation of a multilayer on top of the 3-D islands.

observed at the interconnection points. The variation with θ of the charge mobilities and the CP-AFM results for the 3-D pentacene films strongly support the idea that the formation of interconnections between the isolated 3-D islands contributes significantly to percolation, which makes charge transport easier during OFET operation. The formation of the interconnections can be accounted for by the island-edge barrier effect (Figure 5). A pentacene molecule landing on top of an existing island may encounter a potential energy barrier (Schwoebel barrier)26 as it tries to hop down the island to the lower layer. In this situation, the incoming molecule can be confined to the top of the island, increasing the density of molecules there and thus the probability that a new island is nucleated on top of the existing island. The hopdown probability increases as the size of existing islands becomes smaller.27 In our system, a Schwoebel barrier seems to be present on the 3-D island-edge, in contrast to previous reports for 2-D pentacene island growth.28 When θ exceeds 0.6, both the formation of interconnections between islands and vertical growth by the formation of multilayers on the 3-D pentacene islands were observed. In other words, when the lateral size of the 3-D islands approached 300-500 nm, multilayers were formed on top of the islands, which resulted in reductions in the topmost site area. There is a high probability that incoming pentacene molecules landing on this topmost site will hop down at the island-edge before they find another pentacene molecule. These processes result in both vertical growth of each 3-D island and the formation of interconnections between the existing 3-D islands. 4. Conclusion We have demonstrated a direct correlation between the formation of interconnections during the 3-D growth of pentacene films and the enhancement process of charge mobility in 3-D grown pentacene FETs by combining our results for the surface coverage dependent mobility and the morphological evolution of 3-D pentacene films. The formation of interconnections between the initially isolated 3-D islands that consist of layers of pentacene molecules (with an interplanar spacing

of 15.6 Å, thin film phase) packed in a herringbone geometry provides efficient percolation pathways for the injected charge carriers, as demonstrated by our CP-AFM observations. From these results, we conclude that interconnection formation can dominate charge transport and is therefore a critical factor to determine the performance of OFETs when 3-D growth occurs during organic semiconductor film growth. Acknowledgment. This work was supported by the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE, No. RTI04-01-04) and the Nanoscale Science and Engineering Center of the National Science Foundation (No. DMR 0117792). References and Notes (1) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys. Chem. B 2003, 107, 5877. (2) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259. (3) Yang, H.; Shin, T. J.; Ling, M.; Cho, K.; Ryu, C. Y.; Bao, Z. J. Am. Chem. Soc. 2005, 127, 11542. (4) Mayer, A. C.; Ruiz, R.; Headrick, R. L.; Kazimirov, A.; Malliaras, G. G. Org. Electron. 2004, 5, 257. (5) Ruiz, R.; Nickel, B.; Koch, N.; Feldman, L. C.; Haglund, R. F.; Kahn, A.; Family, F.; Scoles, G. Phys. ReV. Lett. 2003, 91, 136102. (6) Pratontep, S.; Brinkmann, M. Phys. ReV. B 2004, 69, 165201. (7) Pratontep, S.; Nu¨esch, F.; Zuppiroli, L.; Brinkmann, M. Phys. ReV. B 2005, 72, 085211. (8) Yang, S. Y.; Shin, K.; Park, C. E. AdV. Funct. Mater. 2005, 15, 1806. (9) Shankar, K.; Jackson, T. N. J. Mater. Res. 2004, 19, 2003. (10) Knipp, D.; Street, R. A.; Volkel, A.; Ho, J. J. Appl. Phys. 2003, 93, 347. (11) Gundlach, D. J.; Kuo, C. S.; Sheraw, C. D.; Nichols, J. A.; Jackson, T. N. Proc. SPIE-Int. Soc. Opt. Eng. 2001, 4466, 54. (12) Kalb. W.; Lang, P.; Mottaghi, M.; Aubin, H.; Horowitz, G.; Wuttig, M. Synth. Met. 2004, 146, 279. (13) Steudel, S.; Janssen, D.; Verlaak, S.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2004, 85, 5550. (14) Verlaak, S.; Steudel, S.; Heremans, P.; Janssen, D.; Deleuze, M. S. Phys. ReV. B 2003, 68, 195409. (15) Dinelli, F.; Murgia, M.; Levy, P.; Cavallini, M.; Biscarini, F.; de Leeuw, D. M. Phys. ReV. Lett. 2004, 92, 116802. (16) Ruiz, R.; Papadimitratos, A.; Mayer, A. C.; Malliaras, G. G. AdV. Mater. 2005, 17, 1795.

Electrical Percolation by Interconnection of 3D Pentacene Islands (17) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270. (18) Kiguchi, M.; Nakayama, M.; Fujiwara, K.; Ueno, K.; Shimada, T.; Saiki, K. Jap. J. Appl. Phys. 2003, 42, L1408. (19) Cicoira, F.; Santato, C.; Dinelli, F.; Murgia, M.; Loi, M. A.; Biscarini, F.; Zamboni, R.; Heremans, P.; Muccini, M. AdV. Funct. Mater. 2005, 15, 375. (20) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084. (21) Ruiz, R.; Mayer, A. C.; Malliaras, G. G.; Nickel, B.; Scoles, G.; Kazimirov, A.; Kim, H.; Headrick, R. L.; Islam, Z. Appl. Phys. Lett. 2004, 85, 4926.

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20307 (22) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R. L.; Iannotta, S.; Malliaras, G. G. Chem. Mater. 2004, 16, 4497. (23) Heim, T.; Lmimouni, K.; Vuillaume, D. Nano Lett. 2004, 4, 2145. (24) Kelly, T. W.; Frisbie, C. D. J. Vac. Sci. Technol. B 2000, 18, 632. (25) Loiacono, M. J.; Granstrom, E. L.; Frisbie, C. D. J. Phys. Chem. B 1998, 102, 1679. (26) Schwoebel, R. L.; Shipsey, E. J. J. Appl. Phys. 1966, 37, 3682. (27) Zhang, Z.; Lagally, M. G. Science 1997, 276, 377. (28) Ruiz, R.; Nickel, B.; Koch, N.; Feldman, L. C.; Haglund, R. F.; Kahn, A.; Scoles, G. Phys. ReV. B 2003, 67, 125406.