Coupling of Defect Fields to Domains and Phase Transition Characteristics of Ferroelectric Thin Films with Charged Defec
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Coupling of Defect Fields to Domains and Phase Transition Characteristics of Ferroelectric Thin Films with Charged Defects Ibrahim B. Misirlioglu1, Hale N. Cologlu1 and Mehmet Yildiz1 1
Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla/Orhanli, 34956 Istanbul, Turkey ABSTRACT We analyze the effect of charged defects on the electrical domains, phase transition characteristics and electrical properties of ferroelectric thin films with thin dead layers using a non-linear thermodynamic model. Depending on their density and field strength, defects can pin and couple to electrical domains in the film. For ultrathin films, depolarizing effects dominate and the transition from the paraelectric state is into the multidomain ferroelectric state during cooling and is strongly smeared. The competition between defect induced extrinsic effects and the dead layer related limit is demonstrated. INTRODUCTION Phenomena related to inhomogeneities and defects in ferroelectric thin films have been an interest for the scientific community since more than the last two decades. The impact of defect fields on the physical properties of ferroelectric films have been the focus of numerous studies including dedicated book chapters [1-10]. The motivation is to understand the structure-electrical property relation in these materials that are strong candidates for several electronic applications. Recently, with the emergence of techniques capable of fabricating very thin films, wherein the distances through which defect fields permeate become comparable to film thickness, related effects have gained a special importance. Defects with charges near the surfaces have also been treated as centers strongly influencing switching characteristics or hinder domain wall motion [9, 18, 19]. An example to the macroscopic effect of a network of defects is the discussions on whether the disappearance of ferroelectricity in ultrathin films is an intrinsic behavior set by atomistic mechanisms or caused by interfacial inhomogeneities that put an extrinsic limit on the film thickness. However, how such an intrinsic limit trend would be modified in the presence of even a low density of charged defects remains a very interesting question. In this article, we study the behavior of ferroelectric thin films with charged defects that are introduced into the system as point potentials, sandwiched between metallic electrodes with dead layers. To probe the strength of the defect effects we use the Landau-Ginzburg-Devonshire (LGD) formalism for ferroelectric materials coupled with the interface conditions and defect fields. Following the simulation of P at 25϶C, we study the phase transition characteristics of the films both for the chosen thicknesses.
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THEORY As the first step, we first construct a two dimensional grid with a sandwich type capacitor geometry that is 200n x kn cells where k (200) is the number of cells along the film thickness (width) and each cell, n, has a dimension of 0.4 nm, imitating the unit cell dimensions of PZT. The LGD v
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