Investigation of ferroelectricity and piezoelectricity in ferroelectric thin film capacitors using synchrotron x-ray microdiffraction

Ferroelectric materials have a wide range of applications in nonvolatile memory devices, sensors, and actuators due to their properties such as remnant polarization and piezoelectricity. It is necessary to investigate those properties to understand physical phenomena in ferroelectric materials and to have better device performance. Since ferroelectricity and piezoelectricity are directly related to structure at the scale of a single unit cell, we have investigated those properties in Pb(Zr,Ti)O3 (PZT) thin films using synchrotron x-ray microdiffraction, a structural probe.; In order to understand the mechanisms of polarization switching and polarization fatigue, images of polarization domains were made by mapping the intensity of the x-ray reflections of a ferroelectric thin film as a function of position. We found that the degree of polarization switching observed using x-ray measurements agreed with the total switched polarization measured electrically. X-ray microdiffraction experiments also revealed that polarization fatigue in the PZT thin films occurred differently in two regimes depending on the magnitude of the applied electric fields.; Piezoelectricity in PZT thin films was investigated by measuring the changes in the lattice constant under an electric field. Because x-ray diffraction is directly sensitive to the change in lattice constant, it is possible to perform precise measurements of piezoelectric coefficients. We found that the longitudinal piezoelectric coefficient (d33) in Pb(Zr0.45Ti0.55)O3 thin film capacitors was 53 pm V-1. However the values of d 33 measured using bipolar electric field pulses were different in the two opposite electric field directions. The difference in d 33 values can be explained using a model based on the coexistence of switchable and nonswitchable polarization domains. This effect leads to a reduction in the piezoelectric coefficient and a broadening of the x-ray reflections of the PZT thin film. The transverse piezoelectric coefficient (d 31) was measured to study the clamping effect induced by a substrate. We found that these films did not show the clamping effect and d31 was -17 pin V-1.