TiO2 nanocomposite gate insulator

Thin Solid Films 518 (2009) 588–590

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Pentacene thin-film transistor with poly(methyl methacrylate-co-methacrylic acid)/ TiO2 nanocomposite gate insulator Jaehoon Park a, Jong Won Lee b, Dong Wook Kim b, Bong June Park c, Hyoung Jin Choi c, Jong Sun Choi b,⁎ a b c

School of Electrical Engineering, Seoul National University, Seoul 151-600, South Korea School of Electronic and Electrical Engineering, Hongik University, Seoul 121-791, South Korea Dept. of Polymer Science and Engineering, Inha University, Incheon 402-751, South Korea

a r t i c l e

i n f o

Available online 10 July 2009 Keywords: Organic thin-film transistors Insulator Nanocomposite Gate-leakage current

a b s t r a c t A poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MAA) and titanium dioxide (TiO2) composite was fabricated to use as a gate insulator in pentacene-based organic thin-film transistors (OTFTs). The dispersion stability was confirmed by observing the sedimentation time of TiO2 nanoparticles in the PMMA-co-MAA solution, which is essential to avoid a severe gate-leakage current in OTFTs. From the measured capacitancefrequency characteristics, a dielectric constant value of 4.5 was obtained for the composite film and 3.3 for the PMMA-co-MAA film. Consequently, we could enhance the field-induced current and reduce the threshold voltage of OTFT by adopting the composite insulator, without augmenting the gate-leakage current. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Organic thin-film transistors (OTFTs) are envisaged to pave the way for flexible and rugged electronics owing to simple and lowtemperature processabilities [1–3]. Considering its targeted portable applications, such as rollable displays and disposable chips, OTFTs should be operated at low driving voltage to reduce power consumption. It is considered as the primary solution for low-voltage operation of OTFTs to increase the capacitance of gate insulator [4,5]. In principle, reducing the insulator thickness can increase the capacitance value [6,7]. But minimizing insulator thickness often results in large gate-leakage currents because the potential for pinhole defects increases as the film gets thinner. On this account, organic/ inorganic composites are embossed as an alternative to provide the increased capacitance. Previous works have demonstrated low-voltage OTFTs using solution-processable organic/inorganic composite insulators with high dielectric constant [8–12]. Nevertheless, most of these works have pointed out critical issues associated with the aggregation and non-uniform dispersion of inorganic particles, resulting in serious gate-leakage currents. Hence, uniform dispersion of inorganic particles in the polymer solution is a subject of great consequence in the field of composite insulator research. This report presents the characteristics of poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MAA)/titanium dioxide (TiO2) nanocomposite and its application as a gate insulator in pentacenebased OTFTs. It is shown that the composite insulator contributes

⁎ Corresponding author. 72-1 Sangsu-dong, Mapo-gu, Hongik Univ. P-419, Seoul 121-791, South Korea. Tel.: +82 2 320 1488; fax: +82 2 320 1193. E-mail address: [email protected] (J.S. Choi). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.07.047

to improving the electrical characteristics of OTFTs mainly due to its enhanced dielectric property. The significance of the fabricated composite insulator is discussed in terms of the dispersion stability of TiO2 nanoparticles and the gate-leakage currents. 2. Experimental details For the preparation of the composite solutions, PMMA-co-MAA (2 wt.%, Aldrich) was dissolved in chloroform and blended with different concentrations (1 and 3 wt.%) of TiO2 nanoparticles (50 nm in diameter, Ishihara). Then, the composite solutions were dispersed using a homomixer for 24 h. To fabricate OTFTs, a 150-nm-thick Al gate was deposited on a glass substrate using the first shadow mask. From the pristine PMMA-co-MAA and composite solutions, a 600-nmthick gate insulator was formed by spin-coating, and baked at 80 °C for 30 min and consecutively at 100 °C for 1 h in a vacuum dry oven. Then, a 60-nm-thick pentacene layer (without purification, Aldrich,) was evaporated through the second shadow mask. The deposition rate was 0.1 nm/s and the substrate was kept at room temperature. Top-contact OTFTs were constructed by depositing 50-nm-thick Au source/drain electrodes using the third mask. The channel length (L) and width (W) of our TFT were 50 and 300 μm, respectively. The dispersion stability of TiO2 nanoparticles in the PMMA-coMAA solution was monitored using an optical analyzer (Turbiscan Expert®, Formulaction) at room temperature for 50 h. The topography of the fabricated films was observed with scanning electron microscope (SEM) (S4300, Hitachi) and atomic force microscopy (AFM) (XE150, PSIA Inc.). The dielectric property of the gate insulators and the electrical characteristics of OTFTs were measured with impedance analyzer (HP 4192A, Agilent Technologies) and semiconductor analyzer (EL 421C, Elecs Co.), respectively.

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3. Results and discussion In order to examine the dispersion stability of TiO2 nanoparticles in the PMMA-co-MAA solution, turbidity of the composite aqueous solution was measured with time. Fig. 1 shows the light retrodiffusion through a fixed position of the composite solutions. It is observed that the value of retrodiffusion for the PMMA-co-MAA/TiO2 composite solution maintained its level with small variance for about 50 h, indicating that TiO2 particles are uniformly dispersed in the PMMAco-MAA solution. On the other hand, the value for the poly(methyl methacrylate) (PMMA)/TiO2 composite solution abruptly decreased with time, representing the coalescence of TiO2 particles in the solution. The presented dispersion stability of the PMMA-co-MAA/ TiO2 composite can be attributed to the strong interaction of TiO2 particles with the carboxylic acid group (i.e. −COOH) of PMMA-coMAA. Beek et al. reported that nano-sized TiO2 particles can form heterosupramolecular assemblies in the presence of π-conjugated materials containing a carboxylic acid functional group [13], which is the reason for using PMMA-co-MAA for the composite material in this work. As a consequence, the homogeneous dispersion of TiO2 nanoparticles in the PMMA-co-MAA solution can be achieved, even without surfactants generally used for preventing nanoparticles from aggregations in the organic/inorganic composite materials. The frequency-dependent dielectric constants of films with different TiO2 concentrations are shown in Fig. 2(a). For the composite film with 3 wt.% of TiO2 nanoparticles, the dielectric constant was estimated to be about 6.7 at 100 kHz , which is larger than those for the composite film with 1 wt.% of nanoparticles (4.5) and the pristine PMMA-co-MAA film (3.3). This result shows that TiO2 nanoparticles (dielectric constant of about 50) increase the dielectric constant of the PMMA-co-MAA/TiO2 composite film with their amount blended into the composite material. However, the composite film with 3 wt.% of TiO2 nanoparticles exhibits a wavy surface as shown in Fig. 2(b), which is presumably induced by a viscosity change of the composite solution. The surface morphology of gate insulator is required to be smooth because the conducting channel in OTFT is formed at the interface between the organic active layer and the gate insulator [14], and the surface roughness of gate insulator is one of decisive factors determining the performance of OTFTs. From the results, we have selected the composite material with 1 wt.% of TiO2 nanoparticles for a gate insulator of our OTFTs. It is suggested that rheology study can be an important subject in the field of organic/inorganic composite materials for OTFT applications, in addition to the current issue related with uniform dispersion of inorganic particles. The surface morphology of the fabricated films was observed with AFM. As shown in Fig. 3(a) and (b), the PMMA-co-MAA/TiO2 (1 wt.%)

Fig. 1. Turbiscan images of PMMA/TiO2 nanoparticle composite and PMMA-co-MAA/ TiO2 nanoparticle composite solutions.

Fig. 2. (a) Dielectric constant versus frequency plots of the fabricated insulators. (b) SEM micrograph of the PMMA-co-MAA/TiO2 (3 wt.%) composite film.

composite film exhibits a little rough surface with a root-mean-square (rms) roughness of 3.3 nm, whereas that for the PMMA-co-MAA film without TiO2 nanoparticles is about 1.5 nm. This suggests that the rough surface of the composite insulator can be a limiting factor for

Fig. 3. AFM images (3 μm × 3 μm) of (a) the PMMA-co-MAA film, (b) the PMMA-coMAA/TiO2 (1 wt.%) composite film, (c) the pentacene film on the PMMA-co-MAA film, and (d) the pentacene film on the PMMA-co-MAA/TiO2 (1 wt.%) composite film.

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the characteristics of OTFTs. However, it should be mentioned that serious aggregations of TiO2 nanoparticles were not observed for the presented composite film, while our previous work has reported a critical aggregation of TiO2 nanoparticles in the poly(4-vinylphenol) solution in spite of remarkable improvement in their dispersion state using a surfactant [11]. Fig. 3(c) and (d) shows that the surface of the pentacene film deposited onto the composite film (rms roughness of about 5.6 nm) is slightly rougher than that on the PMMA-co-MAA film (rms roughness of about 5.1 nm), which is mainly influenced by the surface roughness of gate insulator. Fig. 4(a) shows the drain current (ID) versus drain voltage (VD) curves of the fabricated OTFTs with different gate insulators, operating at a negative gate voltage (VG) of −30 V. It is observed that ID is higher for the OTFT with the PMMA-co-MAA/TiO2 (1 wt.%) composite insulator than for the device with the PMMA-co-MAA insulator. This enhancement is due to the high dielectric property of the composite insulator because the field-induced current is proportional to the field-induced charge density. Important device parameters were obtained from the transfer characteristics at a negative VD of −30 V, shown in Fig. 4(b). The field-effect mobility (μeff) was calculated in the saturation region with the following equation: ID =

WCi μeff 2 ðVG −VT Þ 2L

where Ci is the capacitance of gate insulator per unit area and VT is the threshold voltage [15]. And VT was extracted from the corresponding plot of (ID)1/2 versus VG by extrapolating to ID = 0. For the OTFT with the PMMA-co-MAA/TiO2 (1 wt.%) composite insulator, the field-effect

mobility and the threshold voltage were 0.53 cm2/Vs and −13.9 V, respectively. The subthreshold swing, the inverse of the subthreshold slope [∂logID/∂VG], was about 1.7 V/decade, which represents the change in VG needed to change ID by a factor of 10. And the on-off ratio was about 5.7 × 105. These properties are superior to the characteristics (i.e. 0.48 cm2/Vs, −16.8 V, 2.1 V/decade, and 3.9 × 105 respectively) for the device with the PMMA-co-MAA insulator. Consequently, it is confirmed that the fabricated composite insulator is conducive to reduce the threshold voltage and subthreshold. Most importantly, the gate-leakage currents for both devices are almost comparable as shown in Fig. 4(c), which can be ascribed to the dispersion stability of TiO2 nanoparticles in the PMMA-co-MAA solution. Previous works have suffered from aggregations of TiO2 particles and concomitant increase in gate-leakage currents [9–11]. In this regard, the presented PMMA-co-MAA/TiO2 composite is expected to allow for a more efficient application for low-voltage OTFTs without augmenting the gate-leakage currents. We believe that the performance of OTFT with the PMMA-co-MAA/TiO2 composite insulator can be further improved by optimizing the composition ratio and fabrication process of the composite material keeping pace with investigating on its rheological properties. 4. Conclusion We have fabricated nanocomposite materials of PMMA-co-MAA and high dielectric constant TiO2 nanoparticles in order to provide a gate insulator with an increased capacitance for low-voltage OTFTs. Aggregation of TiO2 nanoparticles in the composite solution could be prohibited by the interaction between nanoparticles and carboxylic acid group of PMMA-co-MAA. As a result, we could reduce the threshold voltage and enhance the field-induced current of OTFT without augmenting the gate-leakage current. For future works, rheology study is suggested to optimize the synthetic and processing conditions of the PMMA-co-MAA/TiO2 composites, thereby achieving further improvements in the device performance. Acknowledgement This research was supported by a grant (F0004022-2008-31) from Information Display R&D Center, one of the 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean Government. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

Fig. 4. (a) Output and (b) Transfer curves of the fabricated OTFTs according to the gate insulators. (c) Gate-leakage current versus VG curves obtained during measuring the transfer characteristics.

J.H. Burroughes, C.A. Jones, R.H. Friend, Nature. 335 (1988) 137. Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl Phys Lett. 69 (1996) 4108. C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. S.J. Kang, K.B. Chung, D.S. Park, H.J. Kim, Y.K. Choi, M.H. Jang, M. Noh, C.N. Whang, Synth. Met. 146 (2004) 351. J.M. Lee, K.T. Kim, C.I. Kim, Thin Solid Films 447/448 (2004) 322. A. Maliakal, In: Z. Bao, J. Locklin (Eds.), Organic Field-Effect Transistors, CRC Press, New York, 2007, p. 233. A. Sathyapalan, S.C. Ng, A. Lohani, T.T. Ong, H. Chen, S. Zhang, Y.M. Lam, S.G. Mhaisalkar, Thin Solid Films 516 (2008) 5645. A.L. Deman, J. Tardy, Org. Electron. 6 (2005) 78. F.C. Chen, C.W. Chu, J. He, Y. Yang, J.L. Lin, Appl. Phys. Lett. 85 (2004) 3295. C.H. Kim, J.H. Bae, S.D. Lee, J.S. Choi, Mol. Cryst. Liq. Cryst. 471 (2007) 147. K.H. Lee, B.J. Park, H.J. Choi, J. Park, J.S. Choi, Mol. Cryst. Liq. Cryst. 471 (2007) 173. B.J. Park, J.H. Sung, J.H. Park, J.S. Choi, H.J. Choi, J. Nanosci. Nanotechnol. 8 (2008) 2676. W.J.E. Beek, R.A.J. Janssen, Adv. Funct. Mater. 12 (2002) 519. S. Fritz, T. Kelley, C. Frisbie, J. Phys. Chem. B. 109 (2005) 10574. S.M. Sze, K.K. NG, Physics of Semiconductor Devices, John Wiley & Sons, New Jersey, 2007, p. 306.