  Thickness scaling of ferroelectricity in AlScN is a determining factor for
its potential application in neuromorphic computing and memory devices. In this
letter, we report on ultrathin (10 nm) Al0.72Sc0.28N films that are
ferroelectrically switchable at room temperature. All-epitaxial
Al0.72Sc0.28N/Pt heterostructures are grown by magnetron sputtering onto
GaN/sapphire substrates followed by an in situ Pt capping approach to avoid
oxidation of the Al0.72Sc0.28N film surface. Structural characterization by
X-ray diffraction and transmission electron microscopy reveals the established
epitaxy. The thus obtained high-quality interfaces in combination with the in
situ capping is expected to facilitate ferroelectric switching of AlScN in the
ultrathin regime. The analysis of the relative permittivity and coercive field
dependence on the Al0.72Sc0.28N film thicknesses in the range of 100 nm down to
10 nm indicates only moderate scaling effects, suggesting that the critical
thickness for ferroelectricity is not yet approached. Furthermore, the
deposited layer stack demonstrates the possibility of including ultrathin
ferroelectric AlScN into all-epitaxial GaN-based devices using sputter
deposition techniques. Thus, our work highlights the integration and scaling
potential of all-epitaxial ultrathin AlScN offering high storage density paired
with low voltage operation desired for state of the art ferroelectric memory
devices.
