Probing Local Origins of Nonlinearity in Ferroelectric Films

NON-TECHNICAL DETAILS: Ferroelectric materials (materials that have a switchable spontaneous polarization) are at the heart of ultrasonic imaging systems for fetal and cardiac monitoring, the multilayer capacitors used in virtually every handheld device and computer, as well as in high precision positioning systems for advanced microscope systems. There are a number of unanswered questions surrounding the properties of the materials under high alternating electric fields; these are becoming increasingly more important as we continue to miniaturize devices. This program attempts to address open questions associated with the field dependence of the properties by investigating the role that defects play in influencing the mobility of ferroelectric domain walls. The insights gained here will be utilized to help design next generation components. The educational outreach program will utilize workshops directed at elementary school students taught by the principal investigators and their graduate students. These will engage ~ 80 students per year in a series of hands-on experimental activities designed to teach fundamentals of materials science. The graduate student will spend time both at Penn State University and at the Center for Nanoscale Materials Science at Oak Ridge National Laboratories.

TECHNICAL DETAILS: Ferroelectric materials are at the heart of ultrasonic imaging systems for fetal and cardiac monitoring, the multilayer capacitors used in virtually every handheld device and computer, as well as in high precision positioning systems. It is known that defects contribute to domain wall pinning in ferroelectric materials, and so influence the dielectric and piezoelectric response. There is a growing need to understand the interplay between domain wall mobility and microstructure as devices continue to scale down in dimensions. Thus, this program is addressing the following fundamental questions:

What is the potential depth associated with any pinning center?

What concentration of defects is required to pin a domain wall?

How do particular defect types influence the volume of material participating in a domain wall cascade?

How do macroscopic nonlinearities develop from local responses?

To address these critical questions, model ferroelectric films with controlled defect concentrations are being grown, including epitaxial ferroelectric films on bicrystal substrates with known twist and tilt angles. Large-grained polycrystalline films allow a wider distribution of grain boundaries to be probed. Point defect concentrations are tailored through aliovalent doping on cation sublattices, or through controlled levels of reduction to create defects on the anion sublattice. The resulting films are being probed by band excitation piezoelectric force microscopy to provide a quantitative measurement of the domain wall mobility at a very fine spatial scale, so that we can understand the relative importance of each type of defect in controlling domain wall mobility.