Nanoscale studies of switching behavior of ferroelectric thin films by using piezoresponse force microscopy

The work presented in this dissertation is focused on the study of ferroelectric thin films using the method of Piezo-response Force Microscopy (PFM) with several modifications specific to ferroelectrics. In this research, the main motivation is the study of polarization-reversal mechanisms for different sizes of very small-scale (0.5-5 mum size) ferroelectric capacitors in possible applications to ferroelectric random-access-memory devices (FeRAM). In order to make such FeRAM devices more competitive with other types of nonvolatile memory technologies such as phase-change random access memory (PRAM) and magneto-resistive random access memory (MRAM), etc., it is necessary to increase the integration density and therefore reduce bit-cell size. This in turn requires a detailed understanding of (and therefore studies of) the switching properties of small-scale ferroelectric capacitors at the micrometer and sub-micrometer size scale. With traditional methods such as the polarization-hysteresis-loop measurement and the transient-switching-current measurement, such switching properties at the sub-micrometer or nanometer scale are difficult to obtain. This is due to the difficulty of electrically contacting each individual capacitor and also due to the drastically reduced electric signal at such a small scale. In addition, these methods do not provide needed spatially-resolved information about local switching. By using different experimental approaches based on PFM, all of these problems were solved and now one can directly study the switching behavior of these ferroelectric capacitors (as shown in this thesis) through observing and quantifying their PFM images. Also this thesis also presents a detailed description of PFM theory as well as the modified PFM experimental setup.;In this thesis we present studies of ferroelectric thin films of two different types: polycrystalline and epitaxial. Each film has a different texture and therefore different interface defects which will affect the characteristic switching behavior. It is found from these studies that for the polycrystalline PZT thin film capacitors there is a significant capacitor-size effect. In both the time-dependent and bias-dependent studies for larger-size polycrystalline PZT capacitors the switching is dominated by nucleation. For smaller-size PZT capacitors the switching is dominated by domain-wall motion. These experimental data have been fitted with two different theoretical models: the Nucleation-Limited-Switching model (NLS model) with a Lorentz distribution of the characteristic nucleation time and Kolmogorov-Avrami-Ishibashi model (KAI model).;For studies of epitaxial PZT capacitors, the time-dependent switching kinetic behavior both for larger square-shape and for smaller circular-shape capacitors is also investigated. The epitaxial-capacitor-experimental data has been interpreted as due to a theoretical model based on the Kolmogorov-Avrami-Ishibashi model (KAI model). Finally a series of ultra-thin BaTiO3 films have been investigated by using PFM and conducting-AFM to study polarization-dependent resistance. In addition, several new ideas are discussed for future experiments to further extend our knowledge in this area.