  The unconventional Si-compatible ferroelectricity in hafnia-based systems,
which becomes robust only at nanoscopic sizes, has attracted a lot of interest.
While a metastable polar orthorhombic (o-) phase (Pca21) is widely regarded as
the responsible phase for ferroelectricity, a higher energy polar rhombohedral
(r-) phase is recently reported on epitaxial HfZrO4 (HZO) films grown on (001)
SrTiO3 (R3m or R3), (0001) GaN (R3), and Si (111). Armed with results on these
systems, here we report a systematic study leading towards identifying
comprehensive global trends for stabilizing r-phase polymorphs in epitaxially
grown HZO thin films (6 nm) on various substrates (perovskites, hexagonal and
Si).
