Switching and Scaling Effects in Ferroelectric Capacitors
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Ferroelectric materials are one of the most attractive candidates for the next generation of nonvolatile memories due to their unique properties such as spontaneous polarization, low operating voltage, and high fatigue resistance. In the development of high density ferroelectric memories, the density must be increased by reducing both lateral and thickness dimensions occupied by a memory capacitor. At the same time, sufficient and stable ferroelectric properties must be maintained. Therefore, it is desirable to investigate size effects, which can disturb the ferroelectric properties, both experimentally and theoretically, and evaluate the fundamental aspects governing the scaling. In this dissertation, the switching behavior of ferroelectric materials was investigated as a function of capacitor lateral size and film thickness. To investigate the properties, a novel method of atomic force microscopy and pulse switching measurement was used, where an atomic force microscope was used for making an electrical contact with a submicron capacitor top electrode, and the pulse switching setup was used to provide the testing conditions simulating the operation of real ferroelectric memory devices. The thesis begins with a look at results of lateral scaling of ferroelectric capacitors from the micron to the submicron range. In the study of lateral size scaling, the switching properties of lead zirconium titanate (PbZrTiO3) were evaluated. The second half of this dissertation focuses on thickness scaling of the ultra thin films of thicknesses less than 100 nm. The study on the lateral size effects revealed that there was no change in the switching polarization for the capacitor sizes investigated. However, the intrinsic size effects were observed as the lateral size scaled down from the micron to the submicron range. The study on the thickness scaling showed the suppression of polarization at a critical thickness and an increase of coercive field as the film thickness decreased. This work was supported by the National Science Foundation - Materials Research Science and Engineering Center (NSF-MRSEC).