  In hafnia-based thin-film ferroelectric devices, chemical phenomena during
growth and processing such as oxygen vacancy formation and interfacial
reactions appear to strongly affect device performance. However, the nanoscale
structure, chemistry, and electrical potentials in these devices are not fully
known, making it difficult to understand their influence on device properties.
Here, we directly image the composition and electrostatic potential with
nanometer resolution in the cross section of a nanocrystalline W /
Hf$_{0.5}$Zr$_{0.5}$O$_{2-\delta}$ (HZO) / W ferroelectric capacitor using
multimodal electron microscopy. This reveals a 1.4 nm wide tungsten sub-oxide
interfacial layer formed at the bottom interface during fabrication which
introduces a potential dip and leads to asymmetric switching fields.
Additionally, the measured inner potential in HZO is consistent with the
presence of about 20% oxygen vacancies and a negative built-in potential in
HZO. These chemical and electrostatic details are important to characterize and
tune to achieve high performance ferroelectric devices.
