Size and texture effects on ferroelectrics

Sarah E Leach, Purdue University

Abstract

Analytical and numerical theories are proposed to describe the polarization hysteresis behavior of individual PZT grains as a function of epitaxial strain and crystallographic orientation for polycrystalline ferroelectrics. 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, h_f, to mesa width, w, ratios h_f/w less than 0.0001. For h_f/w greater than 0.01, 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, where h_f/ w approaches 0. For films with h_f/w = 0.0011, the remnant polarization decreases from 0.69 C/m ² for highly textured films to 0.66 C/m² for untextured films. Results show that as h_f/w approaches 1, the average value of remnant polarization decreases to approximately 0.64 C/m². This means that small mesas will have a reduced value of remnant polarization compared with infnitely wide films, but the remnant polarization does not decrease to zero. For an individual grain in a polycrystalline ferroelectric, enhanced or inhibited hysteresis behavior is observed, depending on proximity to the grain boundaries, the orientation if 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.^

Degree

Ph.D.

Advisors

R. Edwin Garcia, Purdue University.

Subject Area

Engineering, Materials Science

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