Stress and orientation effects in ferroelectric thin films

Ferroelectric thin films have higher energy densities, larger strain capabilities, and more rapid response times than their bulk counter parts. The present work investigates the residual stress and preferred crystallographic orientation effects on the electromechanical behavior of ferroelectric thin films from both experimental and theoretical standpoints.; A high resolution laser Doppler heterodyne interferometer is developed to quantify the piezoelectric response and field-induced strains in lead zirconate titanate (PZT) thin films ranging from 0.5 to 2.0 μm in thickness. The d₃₃ coefficients of the films are calculated via finite element analysis and compared with existing analytical estimates. PZT films with (100) preferred orientation display superior ferroelectric properties than films with (111) preferred orientation. In addition, the properties of the films are found to be strongly dependent on the film thickness. The dependence on thickness is correlated with the residual stress state in the film.; A numerical study based on classic lamination theory is performed to study the development of residual stress induced by thermal expansion mismatch between the film and substrate during the fabrication process. Comparison of numerical results with experimental data reveals the importance of factors such as drying of gel, grain formation and interaction, and phase transformation in the prediction of residual stress in ferroelectric thin films.; The effects of stress on the response of ferroelectric thin films are measured directly by a beam bending experiment in a unique double-beam laser interferometer. Application of a compressive bending stress to relieve the tensile residual stress in PZT (52/48) film increases the field-induced strains. The opposite effect is observed for application of a tensile stress. To gain further insight into these observations, a previously developed micro-electro-mechanical model is applied to numerically simulate the response of ferroelectrics under a general state of stress and electric field. Material parameters obtained by fitting simulation results with available experimental data are used to predict effects of the preferred crystallographic orientation and biaxial tensile stress applied transverse to the poling direction. The simulation results are consistent with experimental observations of the preferred orientation and stress effects in PZT (52/48) thin films.