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2007, 111, 12508-12511 Published on Web 08/08/2007
Temperature-Dependent Pentacene Nanostructures on Hydrophobic Gate-Dielectrics Correlated with Charge Carrier Mobilities Hoichang Yang,*,† Mang-Mang Ling,‡ and Lin Yang§ Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180, Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305, and National Synchrotron Light Source, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: June 29, 2007; In Final Form: July 27, 2007
Crystalline nanostructures of pentacene thin films on octadecyltrichlorosilane-treated dielectrics, held at various substrate deposition temperatures (TDs), have been correlated with charge mobilities in the 60 nm thick pentacene-based field-effect transistors. On the gate-dielectrics held at TD e 60 °C, the first pentacene seeding crystals tend to grow with layer-by-layer mode, while at TD > 60 °C they start to grow with island mode, affecting the subsequently growing crystal structure. Grazing-incidence X-ray diffraction shows that with an increase in TD a surface-induced “thin film phase” competes with a triclinic “bulk phase”, which significantly interferes lateral π-conjugation of pentacene due to different polymorphic structures.
1. Introduction Organic field-effect transistors (OFETs) are being considered for potential applications, such as liquid crystal displays, organic light-emitting diodes, smart cards, and gas sensors.1-4 Pentacene-based OFETs have shown great performance with charge carrier mobility, ∼3 cm2 V-1 s-1, and on/off ratio >106, comparable to hydrogenated amorphous silicon-based FETs.5-10 The relationship between crystalline morphology, structure, and charge mobility of pentacene films has been extensively studied.5,8-11 In general, vacuum-deposited semiconducting thin films grow in one of the three modes: layer-by-layer (Frankvan del Merwe mode), layer-plus-island (Stranski-Krastannov mode), and island mode (Volmer Weber). Rigid-rod type pentacene has been reported to have crystalline phases even in its bulk phase with a triclinic unit cell (the spacing between the (001)* interlayer lattice planes, d(001)* of ∼14.1 Å).8,12,13 Vacuum-deposited pentacene thin films, however, tend to have a metastable or substrate-induced phase that has been called “thin film phase” with a pseudocentered rectangular8 or orthorhombic crystal unit (15.0 < d(001) < 15.5 Å)12 containing nearly edge-on molecules with respect to a flat inert substrate. In addition, the bulk phase has been reported to coexist with the thin film phase in pentacene thin films beyond a certain critical thickness, depending on the deposition temperature (TD).12-18 The thin film phase appears in thermally evaporated films of thickness generally less than 100 nm grown onto substrates held at room temperature or colder. Crystal nanostructures of pentacene on a dielectric substrate strongly depend on the molecule-substrate interaction, which can be controlled by modifying the substrate surface chemistry. We previously reported that there was a direct correlation * To whom correspondence should be addressed. E-mail:
[email protected]. † Rensselaer Polytechnic Institute. ‡ Stanford University. § Brookhaven National Laboratory.
10.1021/jp075074c CCC: $37.00
between the crystalline structure of the first seeding pentacene layer and the charge mobility of the corresponding thick films;8 grain size and morphology of the first seeding islands on the substrates are considerably affected by the dielectric surface properties, resulting in generating different density in charge trap on charge-carrier transport parallel to the insulator. Specifically, because the thin film phase has been found to grow in intimate contact with the substrate, the molecule-substrate interaction, dependent on TD, can critically change the thickness of the polymorphic structural transition in the pentacene film.12 Using the advantage of two-dimensional grazing-incidence X-ray diffraction (2D GIXD) for parallel data collection of crystal reflections at different scattering angles, we have recently found that in pentacene thin films-stacked molecular layers (15.0 < d(001) < 15.5 Å) of thin film phase are oriented parallel to the substrate, while those ones of bulk phase are not oriented parallel to the substrate anymore, showing slightly tilted peaks with respect to the film normal along (00l)* Debye rings in 2D GIXD patterns.8 This unexpected orientation in pentacene thin films implies that the existence of the bulk phase has been underestimated in most of works studied with typical 1D outof-plane X-ray diffraction, leading to an unsatisfactory correlation between microscale crystalline structures and charge mobilities of pentacene thin films. Here, we report temperature-dependent crystalline structures of pentacene in thin films deposited on octadecyltrichlorosilane (OTS, CH3-termini)-treated SiO2/Si substrates held at various TDs (25, 40, 60, and 80 °C), by using 2D GIXD and atomic force microscopy, and further correlate these structural transitions with field-effect mobilities of top-contact electrode OFETs containing 60 nm thick pentacene films. 2. Experimental Section 2.1. Materials and Sample Preparation. Pentacene was obtained from Aldrich Chemical Co. and then purified through © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12509
Figure 1. AFM topographs (a-c) and 2D GIXD patterns (d-f) of 60 nm thick pentacene films deposited on the OTS-treated SiO2/Si substrates held at different TDs: (a, d) 25 °C; (b, e) 60 °C; and (c, f) 80 °C.
Figure 2. Typical current-voltage characteristics: (a) IDS-VDS for various gate voltage, VG for top-contact electrode OFET device with 60 nm thick pentacene film deposited at TD ) 60 °C. (b) IDS-VG and x(-IDS)-VG for VDS ) -100 V. (The channel width/length is about 20. The charge mobility in the pentacene-based OFET was calculated from the linear slope of x(-IDS)-VG curve)
a solvent extraction with toluene, chloroform, and acetone. Highly doped n-type (100) Si wafers were used as gate substrates. The 300 nm thick SiO2 layers (capacitance per unit area, Ci ) 10.0 nF cm-2) were thermally grown on the Si substrates. Then, the SiO2 surface was chemically treated with hydrophobic OTS self-assembled layer (SAM). Various thicknesses of pentacene thin films (from submonolayer (sub-ML) to 50 nm) were vacuum-deposited onto the OTS-treated SiO2/ Si substrates held at 25, 40, 60, and 80 °C at a rate of 0.5 Å/s and vacuum condition of 10-6 Torr, respectively. Top-contact gold source and drain electrodes were vacuum-deposited through a shadow mask (channel length (L) and width/length (W/L) of 150 µm and ca. 20, respectively). 2.2. Characterization. The electrical characteristics of OFET devices were measured in the saturation regime (drain-source voltage, VDS ) -100 V) using a semiconductor parameter analyzer (Keithley 4200-SCS). Field-effect mobilities were calculated from transfer curves swept in a gate-voltage range of VG ) + 20 to -100 V at 1 V steps. Atomic force microscopy (AFM)experiments for pentacene films were performed using a Multimode Nanoscope IIIa (Veeco Metrology Group), operat-
ing in a tapping mode with a Si tip (MikroMasch, resonant frequency ∼300 kHz, curvature radius ∼5 nm). Synchrotronbased 2D GIXD experiments on pentacene films of varying thickness were performed at beam line X21 of the National Synchrotron Light Source (NSLS) at Brookhaven national Laboratory. The sample was mounted on a two-axis goniometer on top of an x-z stage and the scattered intensity was recorded by a 2D Mar charge-coupled device detector. The incident-beam angle was ∼0.3° for all GIXD patterns. 3. Results and Discussion Figure 1 represents typical AFM topographic images and 2D GIXD patterns of nominal 50 nm thick pentacene thin films vacuum-deposited on OTS-treated SiO2/Si substrates held at 25, 60, and 80 °C, respectively. Overall grain sizes (G) of pentacene crystals with a terracelike texture tend to increase gradually with an increase in TD (Figure 1a-c). In the 25 °C sample (Figure 1a), comblike crystals on the topmost film can be estimated as a pentacene bulk formed by flat-lying molecules.19 At TD ) 80 °C, multilayered pentacene crystals (Figure 1c) show more
12510 J. Phys. Chem. C, Vol. 111, No. 34, 2007 TABLE 1: Average grain size (G) and Field-Effect Mobility (µ) of OFET Devices Containing 60 nm Thick Pentacene Thin Films Deposited at Various Substrate Deposition Temperatures (TDs) TD, °C
G, nm
µ, cm2 V-1 s-1
25 40 60 80
610 ( 50 830 ( 45 1020 ( 100 1150 ( 150
0.625 ( 0.05 0.850 ( 0.12 1.120 ( 0.06 0.725 ( 0.08
faceted crystalline shape when compared to those in the 25 and 60 °C samples. It has been known that increasing TD or lowering the deposition rates decreases the nucleation density, resulting in increases in G.17 The electrical characteristics of OFET devices containing 60 nm thick pentacene films on OTS-treated dielectrics were measured using a Keithley 4200-SCS semiconductor parameter analyzer in ambient lab environment. The field-effect mobility (µ) was calculated from the drain-source current (IDS)-VG characteristics as shown in Figure 2, where Figure 2a,b represens the typical output and transfer curves of 50 nm thick pentacene films deposited at TD ) 60 °C, respectively. From Table 1, it can be seen that the pentacene film deposited at TD ) 60 °C gives the highest µ value, ∼1.12 cm2 V-1 s-1. For a polycrystalline film, most defects are likely to condense in crystal grain boundary (GB). A model,20 which assumes tunneling current due to high-charge carrier density at the conducting channel through a back-to-back Schottky barrier induced by GB, predicts that µ is proportional to G. The G-dependent µ for rigid-rod type pentacene11,21-24 and octithiophene (8T) has been presented.20 As seen in Table 1, G-dependent mobility of pentacene thin films, except for the
Letters 80 °C sample, is in good agreement with previously published data of organic semiconducting thin films.12,17,21-24 For example, G in pentacene films 3D grown on hydrophilic aluminum oxide or Si substrates tends to increase linearly with TD, showing high µ of these films grown at elevated TD.17 In terms of the crystalline phase of pentacene, higher TD starts to grow a bulk phase, which has a different structure with the thin film phase, in the vacuum-deposited films. The 2D GIXD pattern of the 25 °C sample shows preferentially oriented (00l) reflected peaks (d(001) ) 15.5 Å) along the qz axis (Figure 1d) and many reflection spots in the qz axis at a given qxy,8 suggesting that terracelike-textured crystal layers consist of thin film phase, that is, multilayered structure of laterally π-conjugated edge-on molecule with respect to the substrate. In particular, the in-plane X-ray reflection at qxy ) 1.964 Å-1 corresponded to the second-order peak of the degenerate (1 ( 2) reflection,25 suggesting that the pentacene crystal has a herringbone packing structure.8 On the basis of the X-ray analysis for these 60 nm thick pentacene films, we suggest that, at TD e 60 °C, most growing pentacene crystals adopt thin film phase (a ) 5.946 Å, b ) 7.558 Å, γ ) 89.77°, d(001) ) 15.5 Å) in these pentacene films deposited on the hydrophobic OTStreated substrates (static water contact angle ≈ 100°), although weak X-ray reflections able to be indicated as a bulk phase are monitored in 2D GIXD patterns (arrow-marked peak). However, the 80 °C sample clearly shows the existence of a bulk phase, where (001)* crystal reflection (d(001)* ) 14.1 Å) are preferentially tilted ∼5° (along the q(001)* axis) with respect to the film normal along (00l)* Debye rings, as proved by 1D X-ray profile (Figure 1f). In this case, the peak portion of the bulk phase is about 6% as calculated from the ratio of (001) and (001)* reflection peaks in the inset of Figure 1f.
Figure 3. AFM topographs and 2D GIXD patterns of nominally ∼2 ML thick pentacene films deposited on the OTS-treated SiO2/Si substrates held at different TDs: (a, b) 25 °C; (c, d) 40 °C; (e, f) 60 °C; and (g, h) 80 °C.
Letters As a result, because 2D GXID can provide information on not only the vertical molecular ordering, but also the lateral π-conjugated structure in thermally evaporated thin films, it is an ideal tool to study the intra and internal self-assembled structure of π-conjugated molecules used for OFET applications. The coexistence of crystalline phases in the pentacene thin films can increase crystalline mismatch between grains, causing poor connection of lateral π-conjugated pathway between the pentacene crystals. This speculation can explain lower charge mobility (∼0.73 cm2 V-1 s-1) of the 80 °C sample containing multilayered pentacene crystals with single crystal-like-faceted texture and larger G, compared to that (∼1.12 cm2 V-1 s-1) of the 60 °C sample. Ye et al. reported that both the bulk phase and G in pentacene films grown on SiO2/Si substrates increased with an increase in TD, and the highest mobility was obtained for pentacene film optimized at TD ) 40 °C.18 Figure 3 represents typical AFM topographic images and 2D GIXD patterns of nominally ∼2 ML thick pentacene films deposited at different TDs. As TD increases, initial layer-by-layer growth mode of the first seeding molecule is changed to islandby-island, specifically above TD ) 60 °C, showing 3D grown pentacene islands (insets in Figure 3e,g). Accordingly, at TD ) 80 °C, the 2 ML thick film still shows disconnected pentacene islands on the substrates (Figure 3g). For these ultrathin films, GIXD analysis clearly indicates crystalline-phase transition of pentacene from thin film phase to bulk phase with an increase in TD. In particular, as TD increases, a full width at halfmaximum of crystal reflection peak becomes narrower, suggesting that the molecular ordering of pentacene can be improved by increasing TD during vacuum deposition. Interestingly, the 25, 40, and 60 °C samples have only the thin film phase, while the bulk phase (asterisk marked peaks in Figure 3h) can grow even in the 2 ML film on the substrate held at 80 °C. It is clear that these films contain mixed structures, which may contribute to an increase in a charge trap density from (1) the intergrain and interdomain structural mismatch between two crystalline phases and (2) change of initial crystal growth mode from layer-by-layer to island-by-island, even though the mechanism through which the two crystalline phases form different layer orientations with respect to the substrate cannot be proved. In pentacene crystals, thinner d(001)* ) 14.1 Å, compared to d(001) ) 15.5 Å also implies that the pentacene molecules in the thinner structure are more tilted, which is also likely to translate into a difference in charge transport within a single crystal domain. However, because pentacene films consist of crystalline layers with herring bone packing of the molecules, which involve both face-to-face and face-to-edge π-π interactions,26 precise prediction of the field-effect mobility is complicated. 4. Conclusion We have reported temperature-dependent crystalline structures of pentacene deposited on hydrophobic octadecyltrichlorosilane (OTS)-treated SiO2/Si substrates held at various TDs (25, 40, 60, and 80 °C) and further correlated these structural transitions with field-effect mobilities of top-contact electrode OFETs containing 60 nm thick pentacene films. The size of the overall pentacene crystal grains in the films on hydrophobic OTS-treated
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12511 dielectric increases gradually with an increase in TD, while at a higher TD such as 80 °C a triclinic bulk phase starts to grow and compete with substrate-induced thin film phase with a pseudo-orthorhombic structure (a ) 5.946 Å, b ) 7.558 Å, γ ) 89.77°, d(100) ) 15.5 Å). The crystalline phase transition affects lateral π-conjugated connections between pentacene grains as charge-transfer pathways, resulting in decreasing the charge mobility in OFET devices. As a result, 2D GIXD and AFM analysis for these thin films strongly supports that molecular orientation and nanomorphology of pentacene in the films can be finely tuned through controlling TD during vacuumdeposition, resulting in high field-effect mobility in OFETs. Acknowledgment. This work was supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation (DMR-0117792). References and Notes (1) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99-117. (2) Ling, M. M.; Bao, Z. N. Chem. Mater. 2004, 16, 4824-4840. (3) Newman, C. R.; Frisbie, C. D.; da Silva, D. A.; Bredas, J. L.; Ewbaank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436-4451. (4) Klauk, H.; Schmid, G.; Radlik, W.; Weber, W.; Zhou, L. S.; Sheraw, C. D.; Nichols, J. A.; Jackson, T. N. Solid-State Electron. 2003, 47, 297301. (5) 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, 4926-4928. (6) Fritz, S. E.; Kelley, T. W.; Frisbie, C. D. J. Phys. Chem. B 2005, 109, 10574-10577. (7) Nickel, B.; Barabash, R.; Ruiz, R.; Koch, N.; Kahn, A.; Feldman, L. C.; Haglund, R. F.; Scoles, G. Phys. ReV. B 2004, 70, 125401. (8) Yang, H. C.; Shin, T. J.; Ling, M. M.; Cho, K.; Ryu, C. Y.; Bao, Z. N. J. Am. Chem. Soc. 2005, 127, 11542-11543. (9) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084-4085. (10) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. Y. P. J. Phys. Chem. B 2003, 107, 5877-5881. (11) Knipp, D.; Street, R. A.; Volkel, A.; Ho, J. J. Appl. Phys. 2003, 93, 347-355. (12) Drummy, L. F.; Martin, D. C. AdV. Mater. 2005, 17, 903-907. (13) Mattheus, C. C.; Dros, A. B.; Baas, J.; Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Acta Crystallogr., Sect. C 2001, 57, 939-941. (14) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson, S. F.; Schlom, D. G. IEEE Electron DeVice Lett. 1997, 18, 87-89. (15) Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996, 80, 2501-2508. (16) Bouchoms, I. P. M.; Schoonveld, W. A.; Vrijmoeth, J.; Klapwijk, T. M. Synth. Met. 1999, 104, 175-178. (17) Lee, J.; Kim, J. H.; Lm, S. J. Appl. Phys. 2004, 95, 3733. (18) Ye, R.; Baba, M.; Ohishi, Y.; Mori, K.; Suzuki, K. Mol. Cryst. Liq. Cryst. 2003, 407, 147-155. (19) Pernstich, K. P.; Hass, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431-6438. (20) Horowitz, G.; Hajlaoui, M. E. Synth. Met. 2001, 122, 185-189. (21) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Chem. Mater. 1996, 8, 2542-2544. (22) Ruiz, R.; Nickel, B.; Koch, N.; Feldman, L. C.; Haglund, R. F.; Kahn, A.; Family, F.; Scoles, G. Phys. ReV. Lett. 2003, 91, 136102. (23) Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Appl. Phys. Lett. 2002, 81, 268-270. (24) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270271. (25) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Chem. ReV. 2001, 101, 1659-1696. (26) Cornil, J.; Calbert, J. P.; Bredas, J. L. J. Am. Chem. Soc. 2001, 123, 1250-1251.
