Switching and Scaling Effects in Ferroelectric Capacitors

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)

Direct Observation of Capacitor Switching Using Planar Electrodes

Ferroelectric polarization switching in epitaxial (110) BiFeO3 films is studied using piezoresponse force microscopy of a model in-plane capacitor structure. The electrode orientation is chosen such that only two active domain variants exist. Studies of the kinetics of domain evolution allows clear visualization of nucleation sites, as well as forward and lateral growth stages of domain formation. It is found that the location of the reverse-domain nucleation is correlated with the direction of switching in a way that the polarization in the domains nucleated at an electrode is always directed away from it. The role of interface charge injection and surface screening charge on switching mechanisms is explored, and the nucleation is shown to be controllable by the bias history of the sample. Finally, the manipulation of domain nucleation through domain structure engineering is illustrated. These studies pave the way for the engineering and design of the ferroelectric device structures through control of individual steps of the switching process

Size effects in ultrathin epitaxial ferroelectric heterostructures

In this letter we report on the effect of thickness scaling in model PbZr0.2Ti0.8O3(PZT)/SrRuO3 heterostructures. Although theoretical models for thickness scaling have been widely reported, direct quantitative experimental data for ultrathin perovskite (<10 nm) films in the presence of real electrodes have still not been reported. In this letter we show a systematic quantitative experimental study of the thickness dependence of switched polarization in (001) epitaxial PZT films, 4 to 80 nm thick. A preliminary model based on a modified Landau Ginzburg approach suggests that the nature of the electrostatics at the ferroelectric-electrode interface plays a significant role in the scaling of ferroelectric thin films. (C) 2004 American Institute of Physics

Epitaxial BiFeO3 thin films on Si

BiFeO3 was studied as an alternative environmentally clean ferro/piezoelectric material. 200-nm-thick BiFeO3 films were grown on Si substrates with SrTiO3 as a template layer and SrRuO3 as bottom electrode. X-ray and transmission electron microscopy studies confirmed the epitaxial growth of the films. The spontaneous polarization of the films was ~45μC/cm2. Retention measurement up to several days showed no decay of polarization. A piezoelectric coefficient (d33) of ~60 pm/V was observed, which is promising for applications in micro-electro-mechanical systems and actuators.Published versio