  The first epitaxial ferroelectric wurtzite film with clear
polarization-electric field hysteresis behavior is presented. The coercive
field of this epitaxial Al0.7Sc0.3N film on W/c-sapphire substrate is 0.4 +-
0.3 MV cm-1 (8 %) smaller than that of a conventional fiber textured film on a
Pt/TiOx/SiO2/Si substrate, attributed to the 0.01 +- 0.007 {\AA} smaller c-axis
lattice parameter in the epitaxial film. The strain and decrease of the
coercive field most likely originate from epitaxial strain rather than the
mismatch in thermal coefficient of expansion. These results provide an insight
for further coercive field reduction of novel wurtzite ferroelectrics using
epitaxial mismatch strain.
