Role of Charge Interaction in the Behavior of Organic Thin Film Transistors
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Role of Charge Interaction in the Behavior of Organic Thin Film Transistors Christoph Erlen1, Francesca Brunetti1,2, Matthias Fiebig3, Giuseppe Scarpa1, Bert Nickel3, Aldo Di Carlo2, and Paolo Lugli1 1 Institute for Nanoelectronics, Technische Universitaet Muenchen, Munich, Germany 2 University of Rome "Tor Vergata", Rome, Italy 3 Ludwig-Maximilians-Universitaet, Munich, Germany ABSTRACT Strong hysteresis in the I-V characteristics of organic thin film transistors are a severe obstacle for the implementation of large circuits. It therefore is a key success factor for the optimization and widespread application of organic electronics to understand the underlying principles. We report the fabrication of two types of pentacene transistors with either polyvinyl alcohol (PVA) or SiO2 as gate dielectric. These devices respond to transient measurement sweeps with a fundamentally different I-V hysteresis. A self-contained model is presented, which associates this behavior with the influence of traps at the SiO2/pentacene interface and polarization in the PVA layer. Simulations employing the commercial drift-diffusion tool SENTAURUSTM are performed to verify our models. INTRODUCTION Organic semiconductors and associated fabrication techniques have witnessed considerable improvements over the past years. Carrier mobility used to be some orders of magnitude lower than those encountered in amorphous silicon. But recent advances have resulted in values of more than 1 cm2/(Vs) [1]. In combination with the potential for cost-efficient production on large area substrates, organic semiconductors are increasingly interesting for commercial applications. But a frequently observed property, which adds to the difficulties in the design of large circuits, is a pronounced hysteresis behavior in the I-V characteristics. Being a fingerprint of time dependent processes, it is often encountered in form of transfer and output curve hysteresis in organic thin film transistors (OTFT). Up to now, research has mainly targeted device descriptions in terms of static models [2]. In this contribution, we show that both polarization and traps can effectively influence the dynamic behavior. The employed materials and fabrication processes define to which extent they are present in a particular device. In order to demonstrate our model, we have realized and simulated two different types of pentacene OTFTs dominated either by trapping or polarization effects. By analyzing dynamic measurement sweeps, characteristic signatures of the two mechanisms are identified. DEVICE FABRICATION In both devices the semiconductor layer consists of pentacene as it exhibits good hole mobility. The transistors mainly differ with respect to the choice of the dielectric material. The trap-dominated transistor is built in bottom gate configuration on an n-doped silicon substrate, which also serves as back gate [3]. The insulating dielectric is a 200 nm layer of thermal silicon
dioxide. Gold source and drain contacts are defined using optical lithography. Oxygen
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