Substrate Influence on the Reversible and Irreversible Polarization Contributions in Ferroelectric Thin Films
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Substrate Influence on the Reversible and Irreversible Polarization Contributions in Ferroelectric Thin Films Dierk Bolten1), Ulrich Böttger1), Julio Rodriguez2), Oliver Lohse1), Michael Grossmann1), Theodor Schneller1), and Rainer Waser1,2) 1) IWE II, RWTH Aachen University of Technology, D-52056 Germany 2) IFF, Research Center Juelich, Germany ABSTRACT In this contribution, the influence of different substrates and textures on the reversible and irreversible polarization in Pb(Zr,Ti)O3 (PZT) thin films will be presented. One possible scenario to explain the origin of the ferroelectric hysteresis is the notion that the domain walls move through a potential generated by their interaction with randomly distributed defects of the matrix. This potential then gives rise to reversible and irreversible changes in the ferroelectric polarization. The exact features of the interaction potential also depend on the stress state of the material which can be influenced by a suitable choice of the substrate. To study the substrate influence, PZT thin films have been deposited on commercial Si wafers, MgO and SrTiO3 single crystals. Electrical characterization methods (hysteresis and small signal capacitance measurements) have been used to extract information on reversible and irreversible polarization contributions. INTRODUCTION The origin of the ferroelectric hysteresis is the existence of irreversible polarization processes, i.e. the irreversible polarization reversal of a single ferroelectric lattice cell as explained by the Landau-Devonshire theory [1]. However, the exact interplay between this fundamental process, domain walls, defects and the overall appearance of the ferroelectric hysteresis is still not precisely known, but is of special interest for the development of nonvolatile ferroelectric memory devices (FeRAMs) [2,3]. The separation of the total polarization into reversible and irreversible contributions that has long been appreciated in the study of ferromagnetic materials [4] might facilitate the understanding of ferroelectric polarization mechanisms albeit the differences that exist between ferroelectric and ferromagnetic materials, e.g. the presence of defect dipoles or space charges that drastically complicate ferroelectric materials and for which no ferromagnetic analogue exists. For ferroelectrics, mainly two mechanisms for irreversible processes are conceivable. First, lattice defects which interact with a domain wall and hinder it from returning into its initial position after removing the electric field that initiated the domain wall motion (“pinning”) [5,6]. Second, the nucleation and growth of new domains which do not disappear after the field is removed again. In ferroelectric materials the matter is further complicated by defect dipoles and free charges that also contribute to the measured polarization and can also interact with domain walls. The exact features of the interaction potential depend on the microstructure of the material. In principle, it is conceivable that a domain wall can interact wit
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