  Room temperature ferroelectricity is observed in lattice-matched ~18%
ScAlN/GaN heterostructures grown by molecular beam epitaxy on single-crystal
GaN substrates. The epitaxial films have smooth surface morphologies and high
crystallinity. Pulsed current-voltage measurements confirm stable and
repeatable polarization switching in such ferroelectric/semiconductor
structures at several measurement conditions, and in multiple samples. The
measured coercive field values are Ec~0.7 MV/cm at room temperature, with
remnant polarization Pr~10 {\mu}C/cm2 for ~100 nm thick ScAlN layers. These
values are substantially lower than comparable ScAlN control layers deposited
by sputtering. Importantly, the coercive field of MBE ScAlN is smaller than the
critical breakdown field of GaN, offering the potential for low voltage
ferroelectric switching. The low coercive field ferroelectricity of ScAlN on
GaN heralds the possibility of new forms of electronic and photonic devices
with epitaxially integrated ferroelectric/semiconductor heterostructures that
take advantage of the GaN electronic and photonic semiconductor platform, where
the underlying semiconductors themselves exhibit spontaneous and piezoelectric
polarization.
