msepostdoc-list REMINDER: Seminar Notice: S. Leach PhD Final Exam Sept. 4 2014

[Stacey, Lisa A]

MATERIALS SCIENCE AND ENGINEERING SEMINAR

MSE PhD Final Examination

Size and Texture Effects on Ferroelectrics

by:
Sarah E. Leach

Advisor:
Prof. R. E. Garcia

ABSTRACT

Thin film ferroelectric materials have generated a great deal of interest for use as low-voltage non-volatile memory, sensors, and actuators. The commercial market for non-volatile memory alone is projected to exceed 20 billion dollars in 2014. Ferroelectric materials are characterized by a reorientable spontaneous electric dipole moment, or polarization domain. A detailed understanding of the underlying domain switching processes in a ferroelectric material is of critical importance for the engineering of memories, where the total switching time affects the write/read time of the device, and the polarization values affect the difference between memory states. An imposed temperature or stress field alters the material’s crystallographic symmetry, which controls the electrostrictive, ferroelectric, or piezoelectric behavior. This study presents analytical and numerical theories to describe the polarization hysteresis behavior for three different configurations: regions within grains of a polycrystalline thin film, a single crystal, and individual grains in a polycrystalline thin film. First, the polarization hysteresis behavior for regions within individual PZT (PbTiO3) grains as a function of epitaxial strain and crystallographic orientation is modeled. In agreement with published experimental results, enhanced or inhibited hysteresis behavior is described, depending on proximity to the grain boundaries, the orientation of the grain to adjacent grains, and the orientation of the grain with respect to the geometry of the thin film. Grain boundaries with different misorientation angles create different local strain fields that couple to the local electric field via the piezoelectric effect, and change the switching behavior of adjacent regions. For 100 nm diameter grains, the coercive field required for polarization switching can vary approximately ±10% between a region near a grain boundary and one near the grain center. Secondly, for single-crystal thin-film devices formed into discrete mesas, the film stresses and average remnant polarization are shown to be controlled by the aspect ratio of the mesa. Calculations demonstrate that the stresses at the edges are relaxed for film height, hf , to width, w, ratios hf /w  ≤ 1 x 10-4. For hf /w ≥ 1 x 10-2, the effective in-plane stress is relaxed throughout the deposited film. Moreover, the effective stresses at the center of the mesa are 15% of the stresses of an infinitely wide film, hf /w → 0. Finally, the overall texture, or orientation distribution, of the grains in a polycrystalline film is shown to influence the polarization hysteresis behavior through coupling of the mechanical and electrical fields. For films with hf /w = 0.0011, the remnant polarization decreases from 0.69 C/m2 to 0.66 C/m2 as the orientation distribution becomes more random. Results show that as hf /w → 1, the average value of remnant polarization decreases to approximately 0.64 C/m2 which means that small devices will have a reduced value of remnant polarization compared with infinitely wide films, but the remnant polarization does not decrease to zero.  These modeling results are useful for optimizing the performance of PZT devices, but can also be applied to other ferroelectric materials, including lead-free formulations such as Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3.



Date:     Thursday, September 4, 2014
Time:    1:30 PM
Place:    ARMS 3115




Lisa Stacey
Secretary/Development Assistant
Purdue University
School of Materials Engineering
765/494-4100

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