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Physics Procedia 14 (2011) 172–176

1875-3892 © 2011 Published by Elsevier Ltd.doi:10.1016/j.phpro.2011.05.035

9th International Conference on Nano-Molecular Electronics

Flexible Pentacene Thin Film Transistors with Cyclo-Olefin Polymer as a Gate Dielectric

Rongbin Ye*, Tomohiro Oyama, Koji Ohta, Mamoru Baba

Faculty of Engineering, Iwate Univisty, 4-3-5 Ueda, Morioka 0208551, Japan

Abstract

Flexible pentacene thin film transistors were fabricated and characterized with a hydroxyl-free and amorphous low-k cyclo-olefin polymer (COP) as a gate dielectric. The maximum processing temperature was 60 oC. The spin-coated COP thin films had excellent electrical insulating properties, and the pentacene TFTs showed negligible current hysteresis, a field-effect mobility of 1.60 cm2 /Vs, a subthreshold swing of 0.56 V/decade and an on/off ratio of > 106. Furthermore, a small threshold voltage shift below 1.83 V was obtained despite unpassivation of the pentacene layer after a gate bias stress of 104 s. These excellent air stabilities were attributed to the insulator with hydroxyl-free and low water absorbency.

© 2010 Published by Elsevier B.V. Keywords: Organic thin film transistor; Cyclo-olefin polymer; Insulatior; Air stable; Bias stress; Threshold voltage shift; Hysteresis

1. Introduction

Organic thin film transistors (OTFTs) offer a promising technology for low-cost and large-area electronic applications such as active-matrix displays, electronic papers, flexible microelectronics and chemical sensor arrays [1-3]. Generally, the deposition of inorganic dielectrics would be difficult on top of organic semiconductors. The surface treatments employed on inorganics would be practically impossible to use in top gate devices. Organic dielectrics offer the freedom to build both top and bottom gate devices more easily by the use of solution coating techniques and printing. Thus polymeric dielectrics have been considered as preferable gate dielectric materials due to their numerous advantages over inorganic materials: i.e., flexible, hydrophilic, easy process, and low cost [4]. However, the hydroxyl-like polar functional groups of polymers are responsible for these factors such as hysteresis behavior, threshold voltage shift and serious obstacle to n-channel conduction of OTFTs because these groups combine with water molecules in air and act as trap sites [5-9]. On the other hand, the thermal curing processes have been used to reduce the number of hydroxyl groups in these popular polymers such as poly(4-vinylphenol) (PVP) and poly(vinyl alcohol) (PVA). The thermal curing complicates the manufacturing process since it requires a high processing temperature and/or additional curing agents [9-13].

* Corresponding author. Tel.: +81-19-621-6364, Fax: +81-19-621-6978

E-mail address: ye@iwate-u.ac.jp

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It has been known that ZENON’s cyclo-olefin polymer (COP) is an amorphous and hydroxyl-free polyolefin, which exhibits such properties as high transparency, low specific gravity, low water absorbency, excellent insulating property, low birefringence, high heat resistance and excellent precision molding and have widely been used as materials for optical components such as optical lenses, prisms, optical cells for the analysis [14]. Furthermore, its volatile chemical content, which causes molecular contamination, is extremely low at 1/200 ~ 1/300 compared to conventional polymers and extremely low residual metal. These excellent properties imply that ZENON’s COP is an idea choice of gate dielectrics in OTFTs except for low dielectric constant. In this paper, we report on COP was used as a gate dielectric to confer the transfer characteristics of flexible pentacene TFTs.

2. Experimental

OTFTs were fabricated using the top contact structure shown in Fig. 1(a). Al thin films of 100 nm were deposited on 100 µm flexible substrates (ZF14-100, ZEON) to be used as the gate electrodes. ZEONEX 480R (5 wt. %, ZEON) were spun on Al electrodes at 3000 rpm. To ensure that cyclohexane solvents were removed from the ZEONEX 480R films, these dielectric films were annealed at 60 oC in air atmosphere for 1 h. Pentacene was purchased commercially (98%, Tokyo Kasei Co., Ltd.) and used without further purification. Pentacene thin films of ca. 50 nm were deposited at 60 oC under a base pressure of less than 4.0 × 10 - 4 Pa. Film thickness and growth rates were monitored by a thickness and rate monitor (CRTM-6000, ULVAC). Finally, the source and drain electrodes of 20 nm Au films were deposited through a shadow mask. The channel width is 200 µm and the channel length is 50 µm. The characteristics of OTFTs were measured with three channel voltage/current sources (Keithley 2400) under ambient laboratory air conditions. Furthermore, we also fabricated MIM (Metal-Insulator-Metal) capacitors shown in Fig. 1(b) to analyze the electrical properties of ZEONEX 480R thin films, and a device area was 2 mm × 2 mm. C-f characteristics were measured using a LCR meter (4284A, HP).

Figure 1 (a) Schematic structures of (a) OTFT, (b) chemical structure of ZEONEX 480R and (c) MIM (Al/COP/Au) capacitor.

3. Results and discussion

Figure 2 (a) shows the frequency dependence of the capacitance (Ci) of ZEONEX 480R thin films coated on a flexible substrate. At 1k Hz, Ci of the COP thin films was 5.66 nF/cm2 and was not changed very much in range of frequencies from 20Hz to 1 MHz. COP is a low-k polymer and its dielectric constant ( ) is 2.3 [14], thus the film thickness of the COP films coated on the flexible substrates was about 360 nm. Fig. 2(b) shows the leakage current (J) through the dielectric layer as a function of electric field applied across the capacitor. Any breakdown was not observed under 1.67 MV/cm. At an applied electric field of 1.67 MV/cm, the conductivity of COP thin films was smaller than 6.0 x 10-14 S/cm. These excellent insulating properties are due to no polar dipoles and few impurities in the dielectric films [14].

Figure 3 shows the output and transfer characteristics of a pentacene TFTs on a flexible substrate. The device typically worked in a p-channel accumulation mode and showed good saturation behavior. As shown in Fig 3(b), the transfer characteristics exhibit negligible hysteresis despite the operation in ambient air, which will be discussed in

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later. Generally, the device performance of OTFTs is evaluated with field-effect mobility (µ) and threshold voltage (VT) extracted using the saturated drain current ID vs. VG relation [15]:

2)()2/( TGiD VVCLWI , (1)

where W, L and Ci are channel width, channel length and gate dielectric capacitance per unit area, respectively. From the transfer characteristics (shown in Fig. 3(b)) expressed as the square root of source current (IS) with VG, we found µ=1.60 cm2 /Vs and VT = -21.79 V for the device. The flexible device shows mobility that is about 3 times greater than that of the reported high-performance TFTs only with 400 nm thick SiO2 gate insulator at the same gate electrical field [16]. The energetic disorder induced by gate dielectrics plays an important role in determining the field-effect mobility in OTFTs [17, 18]. Reduced energetic disorder (or low polarity interfaces) could be contributing to increased mobility. Thus COP is an attractive choice of gate dielectric polymers in OTFTs. Furthermore, from Fig. 3(b) we calculated that the current on/off ratio is > 106 and a subthreshold swing of 0.56 V/decade for the device when VG changes from 0 to -60 V.

Figure 2 (a) Frequency dependence of Ci of an MIM on flexible substrates and (b) Leakage current characteristic of the MIM structure as a function of the applied voltage.

Figure 3 (a) Output and (b) transfer characteristics of a flexible pentacene TFT prepared with the COP dielectric.

Figure 4 (a) shows the transfer characteristics (VD = -60 V) of another device before and after gate bias stress of VG = -60 V (VD = 0 V) for 500, 1000, 5000 and 10000 s under laboratory air conditions. During the stress period, VD

was held 0 V to ensure a homogeneous electric stress field was applied through the channel. The transfer characteristics shifted slightly in the negative direction with the negative gate bias stress. The variation of VT (ΔVT)

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as a function of stress time is plotted in Fig. 4 (b). Although the electric field (1.67 MV/cm) at the gate bias voltage of -60 V was high and the OTFT was stressed without any passivation layer over the semiconducting layer, theΔVT

was only 1.83 V at 10000 s. The instabilities in OTFTs such as the hysteresis and VT shifts induced by gate bias stress are generally

considered to be due to charges trapped between the dielectric and the semiconductor. In the polymer gate dielectrics such as poly(methyl methacrylate) (PMMA), PVP and PVA, the hydroxyl-like polar functional groups of the polymer are mainly responsible for these factors because these groups combine with water molecules in air and act as trap sites [8, 13]. Although the hydroxyl groups are substituted by crosslink agents, it is difficult to completely substitute all hydroxyl groups in polymers. The amorphous COP used in our devices, on the other hand, does not have hydroxyl groups in its chemical structure as seen in Fig. 1(b); the COP films have low water absorbency (<0.01%, 23oC, 24 h), which is 1/20 ~ 1/30 smaller than these of optical grade polycarbonate and PMMA[14], and crosslikes are not required. Therefore, the negligible hysteresis and the small VT shifts were attributed to the hydroxyl-free and low water absorbent insulator.

Figure 4 (a) Transfer characteristics (VD = -60 V) of another device before and after gate bias stress of VG = -60 V (VD = 0 V) for 500, 1000, 5000 and 10000 s and (b) Variations in VT as an function of stress time for gate bias voltage of -60 V.

Figure 5 Transfer characteristics of an inverter and the inverter gains at VD of -60 V for a voltage sweep from 0 to -60 V.

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Furthermore, we fabricated an inverter circuit using an OTFT and a resistor. The transfer characteristics of the inverter scanned for voltages of from 0 to –60V and then from -60 to 0V with VD = -60 V are shown in Fig. 5. There is almost no hystersis in the full voltage sweep, which implies the drive transistor still properly functions. The inverter gain is defined as the absolute value of (dVOUT/dVIN) and is plotted on the right side of Fig. 5. The maximum gain reached 4.5, which is low when compared to those recently reported for two OTFTs inverters [13].

4. Conclusions

We demonstrated that the flexible pentacene thin film transistors were fabricated and characterized with an amorphous and hydroxyl-free low-k COP as a gate dielectric. The maximum processing temperature was 60 oC. The spin-coated COP thin films had excellent electrical insulating properties, and the pentacene TFTs showed negligible current hysteresis, a field-effect mobility of 1.60 cm2 /Vs, a subthreshold swing of 0.56 V/decade and an on/off ratio of > 106. Furthermore, a small threshold voltage shift below 1.83 V was obtained despite unpassivation of the pentacene layer after a gate bias stress of 104 s. These excellent air stabilities were attributed to the hydroxyl-free and low water absorbent insulator.

.

Acknowledgements

The authors would like to thank ZEON for its contribution of ZEONEX 480R and ZF14 -100 flexible films.

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