  Hafnium oxide (HfO2)-based ferroelectrics offer remarkable promise for memory
and logic devices in view of their compatibility with traditional silicon CMOS
technology, high switchable polarization, good endurance and thickness
scalability. These factors have led to steep rise in research on this class of
materials over the past number of years. At the same time, only a few reports
on the direct sensing of nanoscale ferroelectric properties exist, with many
questions remaining regarding the emergence of ferroelectricity in these
materials. While piezoresponse force microscopy (PFM) is ideally suited to
probe piezo- and ferro-electricity on the nanoscale, it is known to suffer
artifacts which complicate quantitative interpretation of results and can even
lead to claims of ferroelectricity in materials which are not ferroelectric. In
this paper we explore the possibility of using an improved PFM method based on
interferometric displacement sensing (IDS) to study nanoscale ferroelectricity
in bare Si doped HfO2. Our results indicate a clear difference in the local
remnant state of various HfO2 crystallites with reported values for the
piezoelectric coupling in range 0.6-1.5 pm/V. In addition, we report unusual
ferroelectric polarization switching including possible contributions from
electrostriction and Vegard effect, which may indicate oxygen vacancies or
interfacial effects influence the emergence of nanoscale ferroelectricity in
HfO2.
