  By performing standard PUND (positive-up-negative-down), hysteresis-loop and
dielectric measurements on the ferroelectric lead zirconate titanate (PZT)
thin-film capacitors subject to bipolar/unipolar electrical cycling, we show
that unipolar fatigue is evident though still less severe than bipolar fatigue
conducted at the same voltage. That has been attributed to polarization
retention (backswitching) induced by the residual depolarization field between
the monopolar pulses where the applied field is lower than the depolarization
field, and explained using the LPD-SICI model (LPD-SICI stands for local phase
decomposition caused by switching-induced charge injection). The conventional
view that switching does not occur during unipolar electrical cycling may need
to be corrected. PUND results recorded using the pulses of the same voltage as
those for repetitive fatigue cycling are not reliable if the voltage is lower
than 2Vc (Vc is the saturated coercive voltage). Dielectric measurements or
hysteresis-loop measurements at higher voltages (e.g. 4Vc) are more reliable
ways to evaluate the degree of fatigue and could provide more valuable
information in such situations. Finally, dielectric results have been used to
estimate the effective thickness di of the fatigue-induced degraded
(pyrochlorelike) interfacial layer after bipolar/unipolar fatigue, which has
not been done so far to our best knowledge. The fact that di is still much less
than the film thickness even after the most severe bipolar fatigue strongly
suggests that polarization fatigue in ferroelectrics is an interface effect,
not a bulk one.
