  Wurtzite-type Al$_{1-x}$Sc$_x$N solid solutions grown by metal organic
chemical vapour deposition are for the first time confirmed to be
ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive
field of 5.5 MV/cm at a measurement frequency of 1.5 kHz. Single crystal
quality and homogeneous chemical composition of the film was confirmed by X-ray
diffraction spectroscopic methods such as time of flight secondary ion mass
spectrometry. Annular bright field scanning transmission electron microscopy
served to proof the ferroelectric polarization inversion on unit cell level.
The single crystal quality further allowed to image the large-scale domain
pattern of a wurtzite-type ferroelectric for the first time, revealing a
predominantly cone-like domain shape along the c-axis of the material. As in
previous work, this again implies the presence of strong polarization
discontinuities along this crystallographic axis, which could be suitable for
current transport. The domains are separated by narrow domain walls, for which
an upper thickness limit of 3 nm was deduced, but which could potentially be
atomically sharp. We are confident that these results will advance the
commencing integration of wurtzite-type ferroelectrics to GaN as well as
generally III-N based heterostructures and devices.
