  It is generally accepted that oxygen vacancies play a central role in the
emergence of ferroelectricity for HfO2-based materials, but the underlying
mechanism still remains elusive. Herein, starting from the basic
characterization circuit, we propose that the observed ferroelectricity is
extrinsic. A key finding is that charged oxygen vacancies oscillate within the
sample under repeated electric pulses, yielding a nonlinear current which
behaves similarly to the polarization current for a normal ferroelectric. This
unwanted current signal results in a ferroelectric-like hysteresis loop with
both remnant polarization and coercive field in good agreements with
experimental values, given a charged oxygen vacancy concentration in the
vicinity of 1*10^20/cm^3. Moreover, it is possible to exploit this mechanism to
reproduce the effects of wake-up, split-up and limited endurance that are of
crucial relevance for the device applications.
