Effect of Interface and Crystallinity on the Ferroelectric Properties of Poly(Vinylidene Fluoride-Trifluoroethylene) Cop
- PDF / 93,617 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 37 Downloads / 224 Views
Effect of Interface and Crystallinity on the Ferroelectric Properties of Poly(vinylidene fluoride-trifluoroethylene) Copolymer Thin Films Feng Xia, H.S. Xu, Babak Razavi, Q. M. Zhang Materials Research Laboratory and Department of Electric Engineering The Penn State University, University Park, PA 16802, U.S.A. ABSTRACT Ferroelectric polymer thin films are attractive for a wide range of applications such as MEMS, IR sensors, and memory devices. We present the results of a recent investigation on the thickness dependence of the ferroelectric properties of poly(vinylidene fluoridetrifluoroethylene) copolymer spin cast films on electroded Si substrate. We show that as the film thickness is reduced, there exist two thickness regions. For films at thickness above 100 nm, the thickness dependence of the ferroelectric properties can be attributed to the interface effect. However, for thinner films, there is a large change in the ferroelectric properties such as the polarization level, the coercive field, and polarization switching speed, which is related to the large drop of the crystallinity in the ultrathin film region (below 100 nm). The results from Xray, dielectric measurement, and AFM all indicate that there is a threshold thickness at about 100 nm below which the crystallinity in the film reduces abruptly. INTRODUCTION Ferroelectric polymer thin films are attractive for a wide range of applications such as MEMS, IR sensors, and memory devices [1-2]. In these applications, the issues of how the functional properties change as the film thickness is reduced and what is responsible for these changes are of great interest and concern. As the polymer film thickness is reduced, finite size effects where the system dimension approaches one of the intrinsic dimensions of the polymer will influence the structures of these films. Furthermore, it is expected that the surface and interface will play a significant role in the development of morphology and chain conformation of the films [3-4]. Recently, we investigated the ferroelectric behaviors in the P(VDF-TrFE) spin cast thin films [5-6]. We will show that the variation in the ferroelectric functional behavior with film thickness can be divided into two thickness regions, those above 100 – 120 nm and those below that thickness. For films thicker than 100-120 nm (referred to here as thin films), the change in the switching behavior can be understood by assuming an effective interface layer with lower dielectric constant, which is in series with the film. While for thinner films (referred to as ultra-thin films), there exists an additional effect, i.e., the large reduction of the crystallinity in the film that causes a large increase in the switching time of the film. In other words, there is a threshold thickness under which the crystalline process is strongly hindered. EXPERIMENTAL DETAILS The 75/25 VDF/TrFE copolymer thin films were prepared by solution spin cast. The solvent used is dimethylformide (DMF) and the solution concentrations ranging from 4 wt% to 15 wt% were used
Data Loading...
