Structural Characterization and Electromechanical Response of Epitaxial Bilayer Lead Zirconate Titanate Thin Films

This thesis investigates the crystallographic structure, domain morphology and electrical properties of single crystalline epitaxial (001)-oriented perovskitic lead zirconate titanate, Pb(ZrxTi(1-X))O3, (PZT) thin films. The particular goal of the thesis was to study the domain evolution and ferroelastic domain switching in epitaxial (001) bilayered PZT heterostructures as a function of layer thickness. The films were fabricated by pulsed laser deposition. Firstly, single layered PZT films were fabricated to set a benchmark for the bilayer film studies and to gain fundamental understanding of the lattice dependence on thickness. Systematic x-ray diffraction studies provide direct evidence of the change in lattice tetragonality (or c/a ratio) as a function of thickness. Results confirm the film relaxes with increasing thickness where the degree of relaxation is commensurate with the thickness. As a consequence, domain formation varies as a function of thickness. Having set a benchmark and gained understanding of the structure of single-layer epitaxial PZT thin films, bilayered thin films consisting of a tetragonal PZT layer either 30 nm or 100 nm in thickness were deposited on a 100 nm thick rhombohedral PZT layer, which in turn was PbZr0.30Ti0.70O3 and PbZr0.55Ti0.45O3. The domain morphology of the tetragonal layer was found not only to change as a function of thickness but also had a domain state totally different than of a single layer of similar thickness. The domain structure changes from a fully a1/a2 domain to a three domain stress-free c/a1/c/a2 state in the 30 nm to 100 nm thick tetragonal PZT layer respectively. We show that the a-axis orientated domains are formed not simply due to the presence of a mechanical strain but also from an imposed electrostatic interaction between the ferroelectric layers due to the interlayer coupling. Finally, the ferroelectric and electromechancial properties were investigated. Polarization switching experiments confirm large polarization and enhanced piezoelectric coefficient attributed to the motion of nanoscale ferroelastic domains. Piezoresponse force microscopy images also demonstrate gross movement for the ferroelastic domains under local bias. The results presented here thus provide fundamental insight into domain-engineered ferroelectric thin film devices and furthermore, report a practical approach to achieve thin films with giant piezoelectric coefficient without the use of complicated fabrication metho