Structural and abnormal electrical properties of excess PbO-doped lead lanthanum titanate thin films

Abstract

Lead lanthanum titanate (PLT) thin films with excess PbO (from 0 to 20 mol%) were prepared by a metal-organic decomposition process. The ferroelectric properties and current-voltage (C -V ) characteristics of PLT films were investigated as a function of the excess PbO. Abnormal ferroelectric and C -V properties were observed in PLT films with excess PbO. The polarization against applied electric field (P -E ) hysteresis loops were pinched before saturation of polarization of the films, and C -V curves had four peaks instead of the two peaks found in the normal C -V curves. The abnormal level of the hysteresis loops and C -V curves deteriorate with increasing concentrations of excess PbO in the films. Electron probe microanalysis has revealed that there is excess Pb in PLT thin films. Auger electron spectroscopy has detected that the Pb accumulates at the interfaces between the thin film and the bottom electrode. Meanwhile, transmission electron microscopy has found that PbO nanocrystals on the interface between the PLT thin film and the bottom electrode, and clusters of vacancies and interstitials, in particular, exist in the PLT grains. Therefore, a part of the excess PbO may accumulate at the domain wall of the grains and the grain boundaries and the interface between the bottom electrode and film during the thermal annealing process of the films. Meanwhile, the oxygen vacancies of the grains will increase with the increasing concentration of the excess PbO in the films. The excess PbO and oxygen vacancies act as pinning centres and have a strong pinning effect on the domains. When the poling voltage is not large enough, part of the domains can overcome the force of the pinning, and abnormal ferroelectric and C -V properties were observed.