Asymmetrical leakage currents as a possible origin of the polarization offsets observed in compositionally graded ferroelectric films

Offsets of hysteresis loops along the polarization axis have been observed on a step graded $Pb(Zr,Ti)O3$ (PZT) thin film using a Sawyer–Tower (ST) circuit. However, the same effect may be artificially reproduced by adding adequate resistors and diodes in parallel with a nongraded PZT thin film. The hypothesis that the offsets were mainly due to the asymmetrical charging up of the standard capacitor used in the ST circuit, allows us to establish that the graded ferroelectric sample behaves as a kind of rectifying device. It is concluded that the presence of asymmetrical leakage currents in compositionally graded devices may allow the elucidation of the origin of the offsets often observed in these structures. Correlatively, it is demonstrated that such offsets do not represent an abnormal static polarization but a dc voltage. The $Em4$ power law dependence of the offsets (where $Em$ is the amplitude of the electric field) was found to be attributable to the nonlinear increase of the net leakage current.