Raman, XRD and positron study of ferroelectric films

Ferroelectric thin films have important applications in electronic memory, infrared sensor, piezoelectric sensors and actuators. In this work, ferroelectric properties of metalorganic decomposition derived PbZrxTi 1-xO3 (PZT), Bi4Ti 3O12 (BIT) and Bi4-xLa xTi3O12 (LBIT) thin films deposited onto Pt(111)/Ti/SiO2/Si(100) substrates were studied. The thin films were studied using x-ray diffraction, scanning electron microscopy, Raman scattering, positron annihilation spectroscopy and x-ray photoelectron spectroscopy methods.;The ferroelectric properties and the orientation dependence of these properties in PZT and LBIT thin films were determined by hysteresis measurements. The polarization and coercive field increase with (111) orientation in PZT and are due to the orientation of the unit cell. The dielectric constant and polarization increase with non-c-orientation in LBIT thin films. This is due to the major polarization vector of LBIT which lies along the a/b plane. Through controlling heating rate the orientation and the orientation-dependent polarization properties can be controlled. This is because increase of heating rate increases (111) orientation in PZT and c-orientation of LBIT. Also, doping (increase of Zr content in PZT) tends to increase (111) orientation, and La doping in LBIT tends to increase non-c-orientation. In addition, grain size can affect the polarization properties. Small grains possess better polarization properties than large grains which usually have defects. Ferroelectricity can be affected by very small grains due to the internal stress. The Raman study showed an orientation and grain-size dependence of the Raman modes.;The XRD study shows that the PZT has a change of strain after fatigue. The change of strain is due to the change between the 90° and 180° domains. Raman study shows that fatigue reduces the Raman intensity and crystallinity. Positron annihilation depth-profile indicates a change of defect concentration at the surface. The defect can be attributed to uncompensated oxygen vacancies. The removal of oxygen after fatigue is suggested by XPS study. Thus fatigue in PZT can be due to oxygen vacancies and strain which is due to the change of 90° to 180° domains.;For LBIT thin films, Raman study shows that La doping changes the O-Ti-O bending symmetry which is important for ferroelectric polarization. Positron annihilation study shows the change in defect concentration with La doping is not significant. XPS study does not show significant removal of oxygen. Thus the improvement in ferroelectric properties by La doping is due to the lattice symmetry and not defect concentrations. This is consistent with the fact that isovalent doping does not induce a defect concentration change.