Characterization of poly(vinylidene fluoride-trifluoroethylene) 50/50 copolymer films as a gate dielectric Soojin Wi Æ N. Senthilkumar Æ Shi-Woo Rhee Received: 2 January 2007 / Accepted: 12 March 2007 / Published online: 7 April 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Thin films of poly(vinylidene fluoride- trifluoroethylene) (P(VDF-TrFE)) 50/50 copolymer were prepared by spin coating on p-Si substrate. Thermal behavior of the film was observed by measuring the film thickness with ellipsometry as a function of the temperature and abrupt volume expansion was observed at 130–150 °C. Capacitance-voltage (C-V) and current-voltage (I-V) behavior of the aluminum/P(VDF-TrFE)/p-Si MIS (metal- insulator-semiconductor) structures were studied and dielectric constant of the P(VDF-TrFE) film was measured to be about 15.3 at optimum condition. No hysteresis was observed in the C-V curve for films as deposited and annealed (70–200 °C). Films annealed at temperatures higher than the volume expansion temperature showed substantial surface roughness due to the crystallization. Flat band voltage (V FB ) of the MIS structure with as deposited films was about –0.3 V and increased up to –2.0 V with annealing. This suggested that positive charges were gen- erated in the film. Electronic properties of the annealed P(VDF-TrFE) film at above melting temperature were degraded substantially with larger shift in flat band voltage, low dielectric constant and low breakdown voltage. Organic thin film transistor with pentacene active layer and P(VDF-TrFE) as a gate dielectric layer showed a mobility of 0.31 cm 2 /VÁs and threshold voltage of –0.45 V. 1 Introduction The development of organic thin film transistors using polymers as a gate dielectric and organic semiconducting materials has raised interest for their use in the field of electronics [1, 2]. The use of polymeric materials is most attractive for low cost devices fabricated by printing techniques on large area, flexible substrates [3–6]. There are numerous papers about the polymeric thin films as a gate dielectric in organic thin film transistors (OTFTs) [7– 11]. The dielectric material needs to have very high resistivity (greater than 10 12 W cm) and high breakdown strength (greater than 0.2 MV/cm) to prevent the leakage between gate metal and semiconductor channel. It should have the highest possible dielectric constant (greater than 4) to have enough capacitance for channel current flow [12]. High dielectric constant insulators result in low switching voltage of the OTFTs. Polymeric dielectric films are deposited by spin coating and the surface roughness should be as low as possible (less than 10 nm in root mean square roughness) because it has an effect on the mor- phology of the organic semiconductor grown on it and also on the mobility of the carrier. The film should be easily formed at low temperature with high thermal stability and the thermal expansion coefficient should match the sub- strate [1]. Poly(vinylidene fluoride-trifluoroethylene) copolymer [P(VDF-TrFE): –(CH 2 –CF 2 ) m –(CHF–CF 2 ) n –] is one of the promising polymeric materials and draws much attention as a high dielectric constant polymeric insulator and as a ferroelectric material [7–11, 13–16]. P(VDF-TrFE) has a dielectric constant of 12–40 which is much greater than most other polymers (3–4) due to the high polarity from fluorine with high electronegativity. S. Wi Á N. Senthilkumar Á S.-W. Rhee (&) Department of Chemical Engineering, Laboratory for Advanced Molecular Processing (LAMP), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea e-mail: srhee@postech.ac.kr 123 J Mater Sci: Mater Electron (2008) 19:45–50 DOI 10.1007/s10854-007-9266-9