Influence of gate dielectrics on the electrical properties of F8T2 polyfluorene thin-film transistors James Swensen, Jerzy Kanicki, and Alan J. Heeger Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106-5090, USA ABSTRACT The electrical properties of polymeric thin film transisitors (P-TFTs) based on poly(9,9-dioctylfluorene-co- bithiophene) alternating copolymer (F8T2) have been studied. Device performance was compared for amorphous silicon nitride deposited by LPCVD and PECVD techniques, aluminum oxide deposited by sputtering, titanium oxide deposited by sputtering, and thermal silicon oxide gate dielectrics. A heavily n-type doped crystalline silicon wafer coated with the desired gate dielectric was used. Photolithographic patterning of source/drain electrodes directly on top of the F8T2 layer is also discussed. The main conclusion from this work is that traps within the F8T2 define the conduction process within the device. Keywords: F8T2, polymer thin-film transistor, gate dielectrics, titanium oxide 1. INTRODUCTION The gate dielectric layer is one of the critical materials in polymeric thin-film transistors (P-TFTs), since the electrical characteristics and the density of carriers in the conduction channel of the P-TFTs are controlled by the gate insulator capacitance. The drain current of the P-TFTs is linearly proportional to the capacitance of the dielectric material. Also the gate dielectric-polymer semiconductor interface can influence the measured mobility and therefore the P-TFT properties. Various studies have been done on both high dielectric constant ( ) materials 1, 2, 3 , and low- materials 4, 5, 6 . Usually these studies report on each material independently. Dimitrakopoulos et al. compared barium zirconate titonate (BZT) films, a high- material, to thermal SiO 2 3 . They showed equivalent device performance and mobility at lower voltages due to the higher charge that was present across the higher dielectric constant material. The main purpose of this paper is to report on the effect that different gate dielectrics have on the electrical properties of P- TFTs. Poly(9,9-dioctylfluorene-co-bithiophene) alternating co-polymer (F8T2) was used as the polymer semiconductor in the P-TFTs, as shown in Figure 1a. The following dielectrics were used as the gate insulator: titanium oxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), low pressure chemical vapor deposition amorphous silicon nitride (LPCVD -Si 3 N 4 :H), plasma enhanced chemical vapor deposition amorphous silicon nitride (PECVD -SiN x :H), and thermal silicon oxide (SiO 2 ). b) a) Top Contact Bottom Contact Gold Electrodes F8T2 Film Thermal SiO 2 n ++ Si Wafer Figure 1. a) F8T2 chemical structure. b) Schematic showing top contact and bottom contact device geometries. Proceedings of SPIE Vol. 5217 Organic Field Effect Transistors II, edited by Christos D. Dimitrakopoulos, Ananth Dodabalapur (SPIE, Bellingham, WA, 2003) · 0277-786X/03/$15.00 159