Nanoscale Scanning Force Imaging of Polarization Phenomena in Ferroelectric Thin Films
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MRS BULLETIN/JANUARY 1998
complex domain structure in the polarizable ferroelectric layer and its dynamics under high-speed switching conditions. The miniaturization of NVFRAMs requires understanding of granularity in polarization reversal dynamics, fatigue, and retention characteristics. In this respect, theoretical models and electrical measurements (e.g., polarization hysteresis loops and transient currents) have provided substantial insights into the nature of the switching processes. However, the models (phenomenological in nature) and the electrical measurements provide only a global or macroscopic view of the switching process. Application of high-resolution techniques such as scanning force microscopy (SFM), using recently developed nanoscale imaging methods,2^7 provides an opportunity to achieve unique insight at the nanoscale level into the polarization processes that occur in ferroelectric thin films. Scanning force microscopy can provide nanoscale imaging resolution of ferroelectric domains, which in conjunction with conventional electrical experimental methods, can provide valuable information to understand and control the macroscopic properties of ferroelectric capacitors and their potential function as core elements of NVFRAMs. A comparatively low voltage (slO V) applied on a ferroelectric layer between the
SFM tip—acting as a movable top electrode—and an electrode layer underneath the ferroelectric film can induce a strong, localized electric field—depending on film thickness—to produce polarization at the nanometer scale. Scanning-force-microscopy characterization methods facilitate the study of a wide spectrum of ferroelectric material properties via mapping of dielectric constants and polarization forces, surface potentiometric measurements, evaluation of local electromechanical properties, and imaging of polarization domains. Due to space limitations however, this article focuses on advances in the application of SFM imaging methods to investigate ferroelectric domains and their dynamics—an important issue in the physics of ferroelectric thin films for memory applications. SFM Noncontact Mode Domain Imaging A natural way of imaging domains in ferroelectric materials, particularly thin films, involves polarization charges on the surface of the material associated with the permanent built-in electrical polarization. This method was pioneered by Saurenback and Terris2 during study of domains in ferroelectric-ferroelastic crystals of gadolinium molybdate [Gd2(MoO4)3]. In this method, the SFM cantilever is oscillated just above its resonance frequency using a piezoelectric bimorph. Polarization charges of opposite sign to that of the ferroelectric surface charges are induced on the tip of the SFM as the latter is scanned over the ferroelectric surface. The tip charges induce an additional Coulombic attractive force gradient between the tip and the ferroelectric surface, which is proportional to the product of the electric field produced by the ferroelectric surface polarization and the charge induc
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