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

Abstract

Multiferroic response of nanocrystalline lithium niobate J. Appl. Phys. 111, 07D907 (2012) Non-radiative complete surface acoustic wave bandgap for finite-depth holey phononic crystal in lithium niobate Offsets of hysteresis loops along the polarization axis have been observed on a step graded Pb͑Zr,Ti͒O 3 ͑PZT͒ thin film using a Sawyer-Tower ͑ST͒ 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 E m 4 power law dependence of the offsets ͑where E m is the amplitude of the electric field͒ was found to be attributable to the nonlinear increase of the net leakage current.

Explicit expressions for the dynamic polarization behavior in ferroelectrics with symmetric/asymmetric electrical conductivity

“…The superscript ''Ϯ'' denotes f may take different values for E f Ͼ0 ͑represented by ''ϩ''͒ and E f Ͻ0 ͑represented by ''Ϫ''͒. This usage is consistent with the notation adopted by Zheng et al 15 and Bouregba et al 30 Using Eqs. ͑1͒-͑3͒, we obtain, after some manipulation,…”

“…A composite possesses effective dielectric and ferroelectric properties, and we imagine this ''effective'' sample is placed in the Sawyer-Tower circuit. Figure 5 ϩ (sat) generally obeys a power law, 30,34,35 i.e., D o f f (sat)ϰE A ϩ (sat) n , 2ՇnՇ5, which differ from the present results. The possible reason is that, in our calculation, f ϩ and f Ϫ are assumed to be constants independent of E A ϩ (sat).…”

“…The effects of the applied ac field, electric, dielectric, and ferroelectric properties on the various important phenomena will be systemically examined. Discussions on the experimental data of Miller et al 3 and Bouregba et al 30 will also be made.…”

“…29 Recently, Bouregba et al demonstrated this idea by setting diodes and resistors in parallel with a nongraded structure. 30 Without considering the role of ferroelectric properties ͑in their theoretical analysis, the ferroelectric sample is treated as a series connection of a linear resistor and a diode͒, they obtained an expression for the dynamic behavior of polarization offset. All in all, previous analytical models treated the ferroelectric sample at most as a linear dielectric to study the polarization offset.…”

Effects of polarization and permittivity gradients and other parameters on the anomalous vertical shift behavior of graded ferroelectric thin films

“…This is also the range of experimental measurements where notable vertical shifts of the hysteresis loop are reported in the literature. [12][13][14][15] In our previous study of compositionally graded ferroelectric films, we have already demonstrated that the polarization gradient across a ferroelectric film can lead to asymmetric conduction, which we believe provides a mechanism for the polarization offset phenomenon. Now it is worth further examining the asymmetric conduction current.…”