  Analog switching in ferroelectric devices promises neuromorphic computing
with highest energy efficiency, if limited device scalability can be overcome.
To contribute to a solution, we report on the ferroelectric switching
characteristics of sub-5 nm thin Al$_{0.74}$Sc$_{0.26}$N films grown on
Pt/Ti/SiO2/Si and epitaxial Pt/GaN/sapphire templates by sputter-deposition. In
this context, we focus on the following major achievements compared to
previously available wurtzite-type ferroelectrics: 1) Record low switching
voltages down to 1 V are achieved, which is in a range that can be supplied by
standard on-chip voltage sources. 2) Compared to the previously investigated
deposition of thinnest Al$_{1-x}$Sc$_x$N films on epitaxial templates, a
significantly larger coercive field to breakdown field ratio is observed for
Al$_{0.74}$Sc$_{0.26}$N films grown on silicon substrates, the technologically
most relevant substrate-type. 3) The formation of true ferroelectric domains in
wurtzite-type materials is for the first time demonstrated on the atomic scale
by scanning transmission electron microscopy investigations of a sub-5 nm thin
partially switched film. The direct observation of inversion domain boundaries
within single nm-sized grains supports the theory of a gradual domain-wall
motion limited switching process in wurtzite-type ferroelectrics. Ultimately,
this should enable the analog switching necessary for mimicking neuromorphic
concepts also in highly scaled devices.
