Temperature Dependence of the Reversible and Irreversible Polarization Contributions in Ferroelectric Thin Films
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the total polarization into reversible and irreversible contributions that has long been appreciated in the study of ferromagnetic materials[4-6] might facilitate the understanding of ferroelectric polarization mechanisms[7-10]. Especially, the irreversible processes would be important for ferroelectric memory devices, since the reversible processes cannot be used to store information. For ferroelectrics, mainly two conceivable mechanisms for irreversible processes exist. 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"). 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. Reversible contributions in ferroelectrics are due to ionic and electronic displacements and to domain wall motions with a small amplitude. These mechanisms are very fast. As already mentioned the reorientation of dipoles and/or defect or free charges also contributes to the total polarization. These mechanisms are usually much slower, but they also might be reversible (relaxation contribution). Since the electronic and ionic contributions are only weakly temperature dependent the analysis of the temperature dependence of the reversible and irreversible contributions might give new insights into the domain wall dynamics. 301 Mat. Res. Soc. Symp. Proc. Vol. 596 ©2000 Materials Research Society
EXPERIMENT Sample Preparation SBT thin films were prepared using a Metal-Organic-Deposition (MOD) process. First, a 625 nm wet oxide was grown on Si-substrates with electrodes with thickness of 10 nm Ti and 100 nm Pt which were sputtered in-situ using a DC magnetron sputtering system. The electrodes were then annealed in 02. SBT was deposited using a spin-on process with an MOD solution. After bake and RTP steps the films underwent a crystallization anneal at 800 'C in 02. After this anneal, a top Pt electrode of 200 nm was sputtered, followed by the backend processing of patterning and final anneal. The SBTN films were prepared in an analogous manner. All tested capacitors had an area of 1000 pm2 . Both SBT and SBTN films were 180 nm thick. The PbZr0 .3Ti0 703 films (thickness 200 nm) were deposited by a 2-butoxyethanol based sol-gel spin coating (16% excess Pb, 4 layers; pyrolysis for each layer: 400 'C, 2 min in air;
crystallization 700 'C RTP, 5 min in 02) on commercial platinized Si-wafers (Si/Si0 2/TiO 2/Pt(100nm)). Pt top electrodes of 100 nm were sputtered and structured with a photolithography /lift-off process. A post-anneal was performed at 700'C in 02. Electrical Characterization The continuous, as well as the step-wise hysteresis loops reported in this paper have been recorded by means of the FE module of the aixACCT Thin Film Analyzer 2000. The continuous hystere
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