Influence of Metal Electrodes on the Ferroelectric Responses of Poly(vinylidene fluoride-trifluoroethylene) Copolymer Th
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Influence of Metal Electrodes on the Ferroelectric Responses of Poly(vinylidene fluoridetrifluoroethylene) Copolymer Thin Films Feng Xia and Q. M. Zhang Department of Electrical Engineering and Materials Research Institute The Pennsylvania State University, University Park, PA 16802 ABSTRACT Ferroelectric polymer thin films have been investigated for applications such as sensors, MEMS, and memory devices, just name a few. In these thin film devices, it is anticipated that the interface effect will play an important role in determining the device performance. In this paper, we present the results of a recent study on the influence of metal electrodes on ferroelectric switching behavior of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) polymer thin films. The results show that the influence of metal electrodes on the polarization response can be divided into two effects, the bulk effect and interface effect. The bulk effect manifests itself as the built-in bias field when metal electrodes with different work functions were used on the two surfaces of the P(VDF-TrFE) film. The interface effect is more complicated but is directly related to the metal work function. For a metal I/insulator (ferroelectric film)/metal II (MIM) sandwich structure in which the metal I and metal II possess different work functions, the low frequency polarization hysteresis loop shows asymmetric response (different switching fields). The polarization switching time also depends on whether the applied voltage is in parallel or anti-parallel to the built-in bias field. In the fast polarization switching process, it was observed that the interface effect plays a dominating role and the switching time is mainly limited by the charge injection from metal electrodes to the polymer film. For metal electrodes with higher work function, higher injection currents and hence faster polarization switching were observed. The results from I-V studies also show that the charge injection process is a Schottky type and the barrier height estimated from the temperature dependence of the I-V curves is consistent with the metal work functions used. INTRODUCTION Ferroelectric polymer thin films have attracted a great deal of attention due to their emerging applications including sensors, MEMS, and memory devices [1-2]. Among ferroelectric polymers, poly(vinylidene-trifluoroethylene) (P(VDF-TrFE)) copolymer is the best-known and most widely used ferroelectric polymer and its bulk ferroelectric properties have been well studied [3]. Recently, we systematically investigated the ferroelectric behaviors of the P(VDFTrFE) copolymer thin films. It is found that the crystallinity [4], surface layer [5], thin film morphology [6] and metal electrodes all have profound effects on the ferroelectric responses of the polymer thin and ultrathin films. Here we reported the effect of metal electrodes. In many applications, polymer thin films need to be metalized to form a metal/insulator (ferroelectric film)/metal (MIM) structure. It is expected that the interfa
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